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Magnetic, structural, and electronic properties of the multiferroic compound FeTe 2O 5Br with geometrical frustration
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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 086001 (8pp)
doi:10.1088/0953-8984/26/8/086001
Magnetic, structural, and electronic properties of the multiferroic compound FeTe2O5Br with geometrical frustration K-Y Choi1 , I H Choi1 , P Lemmens2 , J van Tol3,4 and H Berger5 1 2 3 4 5
Department of Physics, Chung-Ang University, Seoul 156-756, Republic of Korea Institute for Condensed Matter Physics, TU Braunschweig, D-38106 Braunschweig, Germany Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306, USA National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA Institute de Physique de la Matiere Complexe, EPFL, CH-1015 Lausanne, Switzerland
E-mail: [email protected] Received 29 September 2013, revised 24 November 2013 Accepted for publication 4 December 2013 Published 6 February 2014
Abstract
We report electron spin resonance (ESR), Raman scattering, and interband absorption measurements of the multiferroic FeTe2 O5 Br with two successive magnetic transitions at TN1 = 11.0 K and TN2 = 10.5 K. ESR measurements show all characteristics of a low-dimensional frustrated magnet: (i) the appearance of an antiferromagnetic resonance (AFMR) mode at 40 K, a much higher temperature than TN1 , and (ii) a weaker temperature dependence of the AFMR linewidth than in classical magnets, 1Hpp (T ) ∝ T n with n = 2.2–2.3. Raman spectra at ambient pressure show a large variation of phonon intensities with temperature while there are no appreciable changes in phonon numbers and frequencies. This demonstrates the significant role of the polarizable Te4+ lone pairs in inducing multiferroicity. Under pressure at P = 2.12–3.04 GPa Raman spectra undergo drastic changes and absorption spectra exhibit an abrupt drop of a band gap. This evidences a pressure-induced structural transition related to changes of the electronic states at high pressures. Keywords: multiferroics, frustrated systems, electron spin resonance, Raman spectroscopy (Some figures may appear in colour only in the online journal)
1. Introduction
invoked to account for the simultaneous occurrence of ferroelectric and magnetic transitions [3–8]. Transition metal (TM) oxohalides provide genuine schemes for researching novel magnetoelectric materials because they easily host lone-pair (LP) cations (Te4+ , Se4+ , As3+ , and Sb3+ ). Both lone-pair and halide ions terminate some exchange paths, leading to magnetic frustration which provides a favorable condition for ICM magnetic ordering [9–13]. Nevertheless, the complex exchange network involving both TM–O–TM and TM–O–LP–O–TM exchange interactions has prohibited until now an understanding of the underlying magnetoelastic coupling mechanism. The geometrically frustrated chain compound FeTe2 O5 Br is regarded as a new class of multiferroic material [12]. It has
Recently, magnetism-induced ferroelectricity has revived research interest in magnetoelectric multiferroics that display large cross-coupling between ferroelectric and magnetic order, that is, the switching of electric polarization by a magnetic field or the induction of magnetization with an applied electric field [1, 2]. This holds promise for multifunctional devices in spintronics and memory areas. These new multiferroics commonly share an incommensurate (ICM) magnetic structure with broken inversion symmetry. Depending on detailed (spiral or collinear) spin arrangements, exchange striction, inverse Dzyaloshinskii–Moriya or spin current mechanisms are 0953-8984/14/086001+08$33.00
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a monoclinic layered structure with space group P21 /c, a = 13.346, b = 6.590, c = 14.242 Å, and β = 108.204◦ [14]. The layers are composed of edge-sharing FeO6 octahedra forming [Fe4 O16 ]20− tetramer spin clusters bridged by Te4+ ions. They are stacked along the a axis and separated by Br− ions. Density functional calculations reveal that FeTe2 O5 Br is described by alternating Fe3+ (S = 5/2) spin chains with frustrated interchain interactions within the bc layers [15]. Magnetic susceptibility follows a Curie–Weiss law for temperatures above 100 K, yielding the Curie–Weiss temperature TCW = −110 K, and shows a broad maximum at Tmax = 41.8 K, characteristic of low-dimensional spin systems. The reduced dimensionality and frustration effects are confirmed by a µSR study [16]. FeTe2 O5 Br undergoes two successive magnetic transitions at TN1 = 11.0 K and TN2 = 10.5 K [12, 17]. The first transition at TN1 is a paramagnetic to a high-temperature (HT) ICM state with a wavevector k = (0.5, 0.466, 0.0). The second transition into the low-temperature (LT) incommensurately modulated phase is described by eight almost collinear sublattices with a wavevector k = (0.5, 0.463, 0) and ¯ [12]. magnetic moments oriented predominately along (110) Both phases exhibit ICM transverse amplitude modulated magnetic order, leading to the coexistence of persistent spin dynamics and long-range order as T −→ 0 K [18]. Noticeably, the spontaneous electric polarization develops only below TN2 with the largest component along the c-axis, P(c) = 8.5(2) µC m−2 [12]. The inversion symmetry is broken already in the HT-ICM phase but the ferroelectricity occurs only in the LT-ICM phase. This indicates that the magnetoelectric mechanism of FeTe2 O5 Br is distinctly different from that of the cycloidal multiferroic compounds [2]. The exotic magnetoelastic mechanism is evidenced also by the magnetic field effect on the electric properties. The electric polarization disappears when an external magnetic field of 4.5 T is applied along the incommensurate direction [19]. The difference between the HT- and the LT-ICM phases lies in the orientation of the magnetic moments and phase shift of the amplitude modulated waves, as well as in the presence of tiny displacements of the Te4+ ions only in the LT-ICM phase [20]. This suggests that the ferroelectricity might be driven by exchange striction of the interchain Fe–O–Te–O– Fe exchange paths, which involve the Te4+ LP ions [21]. However, there is no hint of structural anomalies near TN2 , except detectable changes of the Fe–O and Fe–Te distances. An optical study shows no noticeable field and temperature dependence of phonon spectra through the magnetic ordering temperature [22]. This points to subtle lattice distortions. To gain further insight into the complex magnetic, structural, and electronic properties, electron spin resonance (ESR), Raman and interband absorption spectroscopy are employed as experimental tools. Pressure can add one parameter which produces large changes in cell volume and bond lengths. In this paper, we demonstrate that FeTe2 O5 Br has all of the characteristics of a low-dimensional frustrated magnet with respect to ESR. Furthermore, Raman and optical absorption measurements under pressure give evidence of a pressure-induced structural transition at P = 2.12–3.04 GP. By investigating phonon anomalies we will address the origin of magnetoelastic effects.
2. Experimental details
FeTe2 O5 Br single crystals were grown using chemical transport in sealed evacuated silica tubes [14]. The crystal quality was confirmed by single crystal x-ray diffraction and magnetization measurements. High-frequency ESR experiments were performed at 240 GHz using the quasi-optical spectrometer that has been developed at the National High Magnetic Field Laboratory with a sweepable 12 T superconducting magnet. The spectrometer employs a superheterodyne detection scheme with high-frequency Schottky diode mixers and a lock-in amplifier for field modulation. Thus, the field derivative of a microwave absorption signal was recorded under a sweep of an external magnetic field. Polarized Raman scattering experiments at ambient pressure were performed in quasi-backscattering geometry using a solid state laser (λ = 532.1 nm, P = 1 mW). The samples were installed into a He-cooled closed cycle cryostat with a temperature range of 3–300 K. The spectra were collected via a Dilor-XY 500 triple spectrometer. For hydrostatic pressure measurements we used a diamond anvil high-pressure cell (DAC) with a methanol/ethanol mixture as a pressure medium. The pressure inside the DAC was determined by comparing the peak positions of the R1 fluorescence line of a ruby chip placed inside and outside the DAC, respectively. Samples for the high-pressure experiment were exfoliated with a thickness of 30 µm. Unpolarized Raman spectra at high pressure were measured in a backscattering geometry using a micro-Raman system equipped with a single grating monochromator and an Ar laser (λ = 514.5 nm). 3. Results and discussion 3.1. Electron spin resonance
Pregelj et al [15] have reported the detailed frequency-field dispersions of antiferromagnetic resonance (AFMR) modes for H k b, namely for the field perpendicular to the chain direction. However, temperature dependence of ESR spectra has not been fully investigated. To address the development of magnetic correlations, we performed high-frequency ESR measurements in two different crystallographic orientations. As figure 1(a) shows, at ν = 222.4 GHz and T = 4 K the AFMR mode is observed at 2.0(8) T for H k b. This result is in good agreement with the reported one (compare to figure 3 of [15]). However, in a whole temperature range of T = 4–300 K we can obtain a complete set of the ESR data only for H k a. Hereafter, we thus focus on the ESR spectra for H k a. Figure 1(b) exhibits the temperature dependence of the ESR spectra measured at ν = 240 GHz for H k a. At room temperature the ESR signal consists of a single Lorentzian line, which originates from paramagnetic Fe3+ ions. An exchange-narrowed resonance is usually expected for dense insulating spin systems due to fast electronic fluctuations induced by an exchange interaction between the Fe3+ ions. The g-factor is determined by the relation gc = hν/µB Hres = 2.01(5), where ν = 240 GHz, µB is the Bohr magneton, and 2
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Figure 2. Temperature dependence of the ESR linewidth 1Hpp (T ) (upper panel) and the resonance field Hres (T ) (lower panel). The open triangle and the full circle symbols stand for Hres (T ) and 1Hpp (T ) for the A1 and the A2 signals, respectively. Inset: a zoom of 1Hpp (T ) at low temperatures. The solid (dashed) lines are a fit
of the main (extra) signal to a power law. See the text for details.
where 1Hpp (θ, ∞) is the linewidth in an uncorrelated paramagnetic limit. A T-independent linewidth is characteristic of a high-temperature exchange narrowed paramagnetic state. Upon cooling from 100 K to T ∗ = 20 K, 1Hpp (T ) exhibits a critical-like broadening. This is ascribed to a development of short-range spin correlations. The persistence of strong spin correlations up to the Curie–Weiss temperature of TCW = 110 K is a hallmark of frustrated magnets. The broadening is fitted to a critical power law, 1Hpp (T ) ∝ (T − TN1 )− p with the exponent of p ≈ 0.75(5) (see the solid line in the inset of figure 2). The observed critical exponent is comparable to the values of p ≈ 0.8(1) − 0.8(4) reported in the triangular S = 5/2 antiferromagnet Ba3 NbFe3 Si2 O14 [23]. In the temperature interval between TN1 and T ∗ , 1Hpp (T ) shows a rounded maximum, suggesting the competition between a broadening and a narrowing process. Hres (T ) undergoes a large shift to a lower field and then a tiny dip, and finally shifts towards higher fields. This points to the development of an internal magnetic field. We note that the onset temperature, T ∗ = 20 K, is much higher than the magnetic ordering temperature, TN1 = 11.0 K. Thus, it is assigned to the short-range ordering temperature, being consistent with the µSR experiments [18]. The shift, being related to the formation of the short-ranged magnetic ordering, is a characteristic feature of frustrated magnets [24]. For temperatures below 40 K we observe the new A1 peak in addition to the A2 resonance line. The former is assigned to a precursor of the AFMR mode. For a classical spin system, AFMR modes, which arise from spin wave excitations by a microwave at Q = 0, ±qICM , are detected only in the long-ranged ordered state. However, it is wellknown that in the case of low-dimensional frustrated magnets, the AFMR modes are visible even at temperatures where
Figure 1. (a) Derivative of the ESR absorption spectrum of the FeTe2 O5 Br crystal measured at ν = 222.4 GHz and T = 4 K for H kb. (b) Temperature dependence of the derivative of the ESR absorption spectra measured at ν = 240 GHz for H ka. The spectra are vertically shifted for clarity. The asteroids denote impurity signals. A1 and A2 stand for two AFMR modes.
Hres = 8.05(9) T is the resonance field. The obtained g-value, being close to a free spin value, is what is expected for a half-filled ion with a quenched orbital moment. With decreasing temperature the spectrum undergoes a broadening and a shift. The remarkable feature is the appearance of an additional resonance line (designated as A1 ) for temperatures below 40 K. Since there is no evidence for a structural phase transition at the corresponding temperature, it might be associated with the development of short-range magnetic correlations. This interpretation is supported by the formation of a broad maximum in magnetic susceptibility at the respective temperature [14]. To detail their behavior, the resonance field (Hres ) and the peak-to-peak linewidth (1Hpp ) are extracted by fitting to a Lorentzian profile. Hres (T ) and 1Hpp (T ) of the A2 (full circle) and the A1 (open triangle) signals are plotted in figure 2. For temperatures above 100 K, both the linewidth and the resonance field are constant. The temperature dependence of the linewidth is given by 1Hpp (θ, T ) = α(θ, T )1Hpp (θ, ∞) 3
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short-range ordering is well developed [25, 26]. Indeed, neutron diffraction and thermal expansion measurements give evidence for the persistence of short-range magnetic ordering up to 50 K. We note that our ESR and neutron diffraction measurements probe the short-range ordering at much higher temperatures than µSR. The inconsistency might be partially due to the different frequency windows of the employed experimental techniques. We further note that the numbers and resonance fields of AFMR modes are different between H k b and H k a, corroborating anisotropic magnetic interactions (compare figures 1(a) to (b)). Next, we will turn to the temperature dependence of the AFMR linewidth. The AFMR linewidth is determined by the population of magnons and is described by a power law 1Hpp (T ) ∝ T 4 for a classical magnet [27]. Enhanced quantum fluctuations in frustrated magnets lead to a weaker temperature dependence. As to the A1 mode, the linewidth is described by 1Hpp (T ) ∝ T n with n = 2.2(2) for temperatures below TN2 and 1Hpp (T ) ∝ A + (T − TN2 )n with A = 0.9(5) T and n = 1.8(8) between T ∗ and TN2 . The weaker T-dependence of 1Hpp (T ) observed for temperatures above TN2 is due to a dimensional crossover of spin correlations from a three to a quasi-one dimension. The A2 mode shows a comparable T-dependence with n = 2.3(0) to the A1 mode for T < TN . We note that the obtained exponent is larger than that of geometrically frustrated magnets [28], but it is smaller than classical magnets without frustration. Consistently, a relaxation rate λL (T ) determined by µSR measurements shows a different T-dependence from the conventional magnon modes [18]. This was ascribed to fluctuations of the disordered spin component at each Fe site. Therefore, our result lends support for persisting spin fluctuations, which are a natural consequence of the IC amplitude modulated magnetic order.
Figure 3. Comparison of Raman spectra measured at 3 and 285 K in
(cc) and (cb) polarizations, respectively.
54Bg (ac, bc). In the present scattering geometry the Ag symmetry modes are expected in the (cc) polarization and the Bg modes are in the (cb) polarization. For the (cc) and (cb) polarizations, we count a sum of 49 and 48 modes, respectively. This number is pretty reasonable, given the low symmetry of the atomic positions leading to a merging of several phonon modes together and the huge variation of the scattering intensities. We can group the phonon modes into four spectral regimes in accordance with the frequency separation: 40–120 (I), 150–280 (II), 330–500 (III), and 600–850 cm−1 (IV). The high-energy IV modes correspond to the vibrational modes of the FeO6 blocks. The low- and middle-energy modes are much more intense than the high-energy modes. This points to the participation of the LP Te atoms [30]. The group III (II) modes involve the motions of Te, Fe, and O atoms where the lightest O (heaviest Te) atoms have the largest net atomic displacements. In the case of the group I, the Br atoms, having a weak interaction with the other atoms, are responsible for the lowest frequency modes. Figure 4 zooms the temperature dependence of the phonon modes into the low-, medium-, and high-energy regimes. With increasing temperature the scattering intensity is reduced enormously, while both the frequencies and the linewidths of the phonon modes undergo a moderate softening and damping due to anharmonicities. In addition, all modes show a similar variation of integrated intensity with temperature. Here we note that the phonon linewidths of 2–18 cm−1 are bigger than our spectrometer spectral resolution of 2 cm−1 . Thus, we rule out any instrument influences on the linewidth and intensity. This suggests that the integrated intensity gain is due to a change in the electronic states involved in the electron–hole pair excitation process and not to a specific displacement of the normal mode. A detailed analysis of the frequency, line width, and intensity can shed light on these processes. In figure 5 we plot the frequency and normalized integrated intensity of the representative modes at 89, 255,
3.2. Raman spectra at ambient pressure
In a search for magnetoelastic effects Raman spectroscopy is chosen to probe possible phonon anomalies and structural instabilities of the compound induced by the coupling of lattice and spin degrees of freedom. Figure 3 presents ambient-pressure Raman spectra of FeTe2 O5 Br measured at T = 3 K and 295 K in (cc) and (cb) polarization. In (cc) scattering geometry, the incident and scattered light polarization are both parallel to the c axis, while in the (cb) scattering configuration the incident (scattered) light is polarized parallel to the c (b) axis. For frequencies below 850 cm−1 , we observe a large number of partially overlapping phonon modes. At low temperatures we find no additional phonon modes, which rules out any structural transitions. Instead, a strong enhancement of the Raman scattering cross section is observed as reported also for other LP compounds [13, 29]. This is due to the enormous electronic polarizability of their electronic states. According to the factor group analysis of the monoclinic space group, P21 /c, all atoms occupy the same Wyckoff position 4e and thereby yield 3Ag + 3Bg Raman-active modes. Subtracting the acoustic modes, we obtain a total of 108 Raman-active modes: 0Raman = 54Ag (aa, bb, cc, ab) + 4
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Figure 4. A zoom of the temperature dependence of Raman spectra to the low-, medium-, and high-energy regimes respectively, measured in the (cc) polarization.
Figure 5. Temperature dependence of the peak position and the normalized integrated intensity of phonon modes at 89, 255, 440, and 666 cm−1 on a semilogarithmic scale. The solid lines represent a fit to equation (1).
440, and 666 cm−1 as a function of temperature. The total hardening of the frequencies tends to decrease from 2.3% to 0.6% as the phonon frequency increases. The temperature dependence of the frequency is described by a model based on phonon–phonon decay processes [31], ωph (T ) = ω0 +
C , 1 + 2/(ex − 1)
of the whole temperature data gives a reasonable description (see the solid lines). The frequencies of nearly all modes do not substantially vary with temperature, indicating that anharmonicities are not strong. The small effect seen at the magnetic ordering temperature is due to magnetoelastic couplings. This anomaly is not pronounced although ferroelectric anomalies in FeTe2 O5 Br associated with magnetic ordering have been related to strong spin–lattice interactions. In contrast such a weak effect may be expected taking into account the long
(1)
where x = h¯ ω0 /2kB T and ω0 and C are constants. The fitting 5
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Figure 7. Frequency versus pressure plot for the low- and
Figure 6. Unpolarized Raman spectra of FeTe2 O5 Br measured at
high-pressure phases. The dotted lines are linear fits of the data to ω(P) = ω0 + α P.
T = 300 K and under pressure of P = 0 − 6.62 GPa.
Fe–O–Te–O–Fe exchange paths. These exchange paths also contain easily polarizable Te4+ LP electrons which are the basis of a sizable magnetoelectric effect without accompanying strong lattice anomalies. The Raman scattering intensity is reduced by almost one order of magnitude upon heating from 3 K to room temperature. Since the intensity is determined by a square of the partial derivative of the dielectric function with respect to the amplitude of the normal mode, the strong decrease of the integrated intensity points to a substantial variation of dielectric properties with temperature. As mentioned above, the change of the intensity is ubiquitous on all phonon modes. This suggests that a change of the band energies is mainly responsible for the phonon intensity anomalies [32].
the Raman modes, which show the large intensity increase, and (iii) a slight increase in the number of the observed modes. The overall changes in the Raman spectra can be evaluated by analyzing the pressure coefficients (dω/dP) shown in figure 7. For all peaks the ω(P) behavior can be described by a linear function, ω(P) = ω0 + α P with α = dω/dP. For P < 2.11 GPa, all modes show positive pressure coefficients of α = 1.1(0) − 8.9(8) cm−1 GPa−1 except for the 285 cm−1 mode with α = −0.9(5) cm−1 GPa−1 . The negative α points to the instability of the ambient-pressure structure. For P > 3.04 GPa, we find the moderate positive α = 0.2(8) − 5.3(1) cm−1 GPa−1 . These results suggest that the crystal experiences a relatively weak structural evolution through the phase transition. Further x-ray studies under pressure are needed to determine the crystal symmetry of the high-pressure phase. Since the phonon spectra of the high-pressure phase are distributed more evenly than those of the ambient-pressure phase below 2.11 GPa and their phonon intensities are selectively enhanced, the transition might be associated with the compression between the (Fe4 O16 ) layer and the (Te4 O10 Br2 ) layer. This might lead to an increase in electrostatic repulsion between oxygen atoms and consequently to a structural instability. We note that the copper analog Cu2 Te2 O5 Br2 of FeTe2 O5 Br has been reported to show remarkable stability of the ambient-pressure tetragonal crystal structure (space group P4) up to 14 GPa [33]. Under hydrostatic pressure phonon parameters show a continuous and nearly linear variation while a magnetic mode exhibits a weak first-order quantum phase transition at Pc = 1 GPa. The contrasting pressure behaviors
3.3. Raman spectra under pressure
We next examine the effects of hydrostatic pressure on the structural instability and vibrational properties of this compound. The unpolarized Raman spectra of FeTe2 O5 Br crystal measured under pressure and at room temperature are shown in figure 6. Upon increasing pressure, the Raman spectra retain the same form up to 2.12 GPa, except for blueshifts of all peaks due to the compression. For pressures higher than 3.04 GPa, we observe a drastic change of the phonon frequency and intensity. This indicates that FeTe2 O5 Br undergoes a structural phase transition between 2.12 and 3.04 GPa. Remarkable features of this phase transition are: (i) strong enhancements of the phonon intensity of the 162 (a), 410 (m), and 645 (r) cm−1 mode, (ii) significant and discontinuous frequency shifts of 6
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Figure 8. A log–log plot of the frequency dependence of the mode-Gr¨uneisen parameters, γi ∝ 1/ωi (dωi /dP), for the low- (full triangles) and high-pressure (open squares) phases. The horizontal line is a guide corresponding to a constant γi . Figure 9. A plot of (αhν)2 versus photon energy hν near the band edge under pressure where α is an optical absorption coefficient. Inset: pressure dependence of the direct band gap.
between the copper and the iron analogs may be due to the difference in electronic properties of Fe3+ in FeO6 octahedral environments and Cu2+ in O3 Br distorted square. Another useful way of expressing the pressure effect on each phonon mode i is provided by the mode-Gr¨uneisen parameter γi , defined as γi = −(∂ ln ωi /∂ ln V ) = (1/βωi ) (∂ωi /∂ P). Here V is the crystal volume, P the pressure, and β the isothermal compressibility. In the Gr¨uneisen approximation, all phonon frequencies vary uniformly with pressure, implying that all γi are constant. This approximation is valid for three-dimensional crystals with a single type of bonding force. Figure 8 shows the frequency dependence of the modeGr¨uneisen parameters in a log–log plot together with a constant γi . Far from being frequency independent, γi (ωi ) varies in a non-monotonic way with frequency. We find that the values of γi (ωi ) drop abruptly through the different phonon groups. According to an elementary vibrational model [34], a frequency dependence of γi is a consequence of disparate bonding forces. In this regard, the lack of systematic behaviors of γi (ωi ) can be interpreted as an intriguing admixture of external and internal modes. Despite their complex pattern, there exists some tendency. In the low-pressure phase γi (ωi ) shows a very large change while in the high-pressure phase γi (ωi ) becomes more or less constant within each phonon group. The latter means that bonding forces become more uniform due to the admixture of them under compression, being consistent with the evenly distributed Raman spectra (see figure 6).
that the absorption band in the lower-energy region is well described by the relation, αhν ∼ (hν − E g )2 (not shown here). This is due to an indirect band-to-band transition, implying that the lowest energy gap corresponds to band extrema located at different points in the Brillouin zone. The direct band gap is extracted by extrapolating the linear part of the plot of (αhν)2 versus hν (see the solid line). The result is summarized in the inset of figure 9. At ambient pressure the bandgap is E g ≈ 2.4(8) eV at 300 K. With increasing pressure up to 2.11 GPa, the energy gap increases substantially to E g ≈ 2.5(7) eV and then undergoes an abrupt drop by 0.1 eV through the structural phase transition. For pressure above 2.5 GPa, the energy gap is approximately constant. The strong pressure dependence of E g in the low-pressure phase might be associated with the layer structure. Under compression between the two different layers, anisotropic Fe(3d) orbitals will be admixed with Br(4p) and O(2p) ones. This leads to a rather three-dimensional electronic structure in the high-pressure phase and thus the pressure effects on E g are minute. 4. Conclusion
The multiferroic FeTe2 O5 Br with a frustrated spin chain structure has been investigated using ESR, Raman scattering, and optical absorption studies at both ambient and high pressure. The temperature dependence of the ESR signals shows persisting spin fluctuations in the ordered state as well as the appearance of an AFMR mode at about 40 K (≈4TN ), confirming frustrated, low-dimensional spin dynamics. Raman scattering at ambient pressure shows a large and continuous variation of integrated phonon intensities with temperature. This is attributed to substantial variation of the band energies
3.4. Optical absorption under pressure
Figure 9 shows the absorption coefficient (α) spectra of FeTe2 O5 Br recorded under pressure at room temperature. (αhν)2 is plotted as a function of a photon energy hν. In the higher-energy region, the spectra follow a linear relation, (αhν)2 ∼ hν − E g , indicative of a direct band gap. We find 7
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with temperature. No noticeable phonon anomalies are detected for temperatures in proximity to TN1 and TN2 . Therefore multiferroicity is not related to strong spin–lattice but to the large polarizability of lone-pair electron distributions of the Te ions. Raman and optical absorption spectra under pressure give evidence for a structural phase transition at P = 2.12–3.04 GPa leading into a high-pressure phase. Effects of very disparate bonding forces and a strongly pressure dependent admixture of anisotropic Fe(3d) orbitals with Br(4p) and O(2p) are resolved in this phase. A detailed characterization of the high-pressure phase might therefore bring in-depth understanding of the electron–phonon and spin–phonon coupling of this compound.
[13] Zhang D, Kremer R K, Lemmens P, Choi K-Y, Liu J, Whangbo M-H, Berger H, Skourski Y and Johnsson M 2011 Inorg. Chem. 50 12877 [14] Becker R, Johnsson M, Kremer R K, Klauss H-H and Lemmens P 2006 J. Am. Chem. Soc. 128 15469 [15] Pregelj M et al 2012 Phys. Rev. B 86 054402 [16] Zorko A, Pregelj M, Berger H and Arcon D 2010 J. Appl. Phys. 107 09D906 [17] Zaharko O, Pregelj M, Arcon D, Brown P J, Chernyshov D, Stuhr U and Berger H 2010 J. Phys.: Conf. Ser. 211 012002 [18] Pregelj M, Zorko A, Zaharko O, Arcon D, Komelj M, Hillier A D and Berger H 2012 Phys. Rev. Lett. 109 227202 [19] Pregelj M et al 2010 Phys. Rev. B 82 144438 [20] Pregelj M, Jeglic P, Zorko A, Zaharko O, Apih T, Gradisek A, Komelj M, Berger B and Arcon D 2013 Phys. Rev. B 87 144408 [21] Chakraborty J, Ganguli N, Saha-Dasgupta T and Dasgupta I 2013 Phys. Rev. B 88 094409 [22] Pfuner F, Degiorgi L, Berger H and Forro L 2009 J. Phys.: Condens. Matter 21 375401 [23] Choi K-Y, Wang Z, Ozarowski A, van Tol J, Zhou H D, Wiebe C R, Skourski Y and Dalal N S 2012 J. Phys.: Condens. Matter 24 246001 [24] Viticoli S, Fiorani D, Nogues M and Dormann J L 1982 Phys. Rev. B 26 6085 [25] Tuchendler J, Magarinno J and Renard J P 1981 Phys. Rev. B 24 5363 [26] Park J S, Ponomaryov A N, Choi K-Y, Wang Z, van Tol J, Ok K M, Jang Z H, Yoon S W and Suh B J 2011 J. Korean Phys. Soc. 58 270 [27] Rezende S M and White R M 1976 Phys. Rev. B 14 2939 [28] Wulferding D, Choi K-Y, Lemmens P, Ponomaryov A N, van Tol J, Nazmul Islam A T M, Toth S and Lake B 2012 J. Phys.: Condens. Matter 24 435604 [29] Choi K-Y, Nojiri H, Dalal N S, Berger H, Brenig W and Lemmens P 2009 Phys. Rev. B 79 024416 [30] Miller K H, Xu X S, Berger H, Craciun V, Xi X, Martin C, Carr G L and Tanne D B 2013 arXiv:1301.5881 unpublished [31] Balkanski M, Wallis R F and Haro E 1983 Phys. Rev. B 28 1928 [32] Sherman E Ya, Misochko O V and Lemmens P 2001 Spectroscopical Studies of High Temperature Superconductors ed N M Plakida (London: Gordon Breach) chapter 2 [33] Wang X, Syassen K, Johnsson M, Moessnader R, Choi K-Y and Lemmens P 2011 Phys. Rev. B 83 134403 [34] Zallen R 1974 Phys. Rev. B 9 4485
Acknowledgments
This work was supported by NTH ‘Contacts in Nanosystems’ and DFG. KYC acknowledges financial support from the Alexander-von-Humboldt Foundation and Korea NRF Grant (No. 2009-0093817 and 2012-046138). This work of IHC was supported by a Center for Inorganic Photovoltaic Materials (No. 2012-0001168) grant funded by the Korean government (MEST). The National High Magnetic Field Laboratory is supported by NSF Cooperative Agreement No. DMR-0654118, and by the State of Florida. References [1] See for review,Cheong S-W and Mostovoy M 2007 Nature Mater. 6 13 [2] Fiebig M 2005 J. Phys. D: Appl. Phys. 38 R123 [3] Kimura T, Goto T, Shintani H, Ishizaka K, Arima T and Tokura Y 2003 Nature 426 55 [4] Hur N, Park S, Sharma P A, Ahn J S, Guha S and Cheong S-W 2004 Nature 429 392 [5] Sergienko I A and Dagotto E 2006 Phys. Rev. B 73 094434 [6] Katsura H, Nagaosa N and Balatsky A V 2005 Phys. Rev. Lett. 95 057205 [7] Mostovoy M 2006 Phys. Rev. Lett. 96 067601 [8] Harris A B, Yildirim T, Aharony A and Entin-Wohlman O 2006 Phys. Rev. B 73 184433 [9] Lawes G, Ramirez A P, Varma C M and Subramanian M A 2003 Phys. Rev. Lett. 91 257208 [10] Zaharko O et al 2006 Phys. Rev. B 73 064422 [11] Bos J-W G, Colin C V and Palstra T T M 2008 Phys. Rev. B 78 094416 [12] Pregelj M, Zaharko O, Zorko A, Kutnjak Z, Jeglic P, Brown P J, Jagodic M, Jaglicic Z, Berger H and Arcon D 2009 Phys. Rev. Lett. 103 147202
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Magnetic, structural, and electronic properties of the multiferroic compound FeTe 2O 5Br with geometrical frustration
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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 086001 (8pp)
doi:10.1088/0953-8984/26/8/086001
Magnetic, structural, and electronic properties of the multiferroic compound FeTe2O5Br with geometrical frustration K-Y Choi1 , I H Choi1 , P Lemmens2 , J van Tol3,4 and H Berger5 1 2 3 4 5
Department of Physics, Chung-Ang University, Seoul 156-756, Republic of Korea Institute for Condensed Matter Physics, TU Braunschweig, D-38106 Braunschweig, Germany Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306, USA National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA Institute de Physique de la Matiere Complexe, EPFL, CH-1015 Lausanne, Switzerland
E-mail: [email protected] Received 29 September 2013, revised 24 November 2013 Accepted for publication 4 December 2013 Published 6 February 2014
Abstract
We report electron spin resonance (ESR), Raman scattering, and interband absorption measurements of the multiferroic FeTe2 O5 Br with two successive magnetic transitions at TN1 = 11.0 K and TN2 = 10.5 K. ESR measurements show all characteristics of a low-dimensional frustrated magnet: (i) the appearance of an antiferromagnetic resonance (AFMR) mode at 40 K, a much higher temperature than TN1 , and (ii) a weaker temperature dependence of the AFMR linewidth than in classical magnets, 1Hpp (T ) ∝ T n with n = 2.2–2.3. Raman spectra at ambient pressure show a large variation of phonon intensities with temperature while there are no appreciable changes in phonon numbers and frequencies. This demonstrates the significant role of the polarizable Te4+ lone pairs in inducing multiferroicity. Under pressure at P = 2.12–3.04 GPa Raman spectra undergo drastic changes and absorption spectra exhibit an abrupt drop of a band gap. This evidences a pressure-induced structural transition related to changes of the electronic states at high pressures. Keywords: multiferroics, frustrated systems, electron spin resonance, Raman spectroscopy (Some figures may appear in colour only in the online journal)
1. Introduction
invoked to account for the simultaneous occurrence of ferroelectric and magnetic transitions [3–8]. Transition metal (TM) oxohalides provide genuine schemes for researching novel magnetoelectric materials because they easily host lone-pair (LP) cations (Te4+ , Se4+ , As3+ , and Sb3+ ). Both lone-pair and halide ions terminate some exchange paths, leading to magnetic frustration which provides a favorable condition for ICM magnetic ordering [9–13]. Nevertheless, the complex exchange network involving both TM–O–TM and TM–O–LP–O–TM exchange interactions has prohibited until now an understanding of the underlying magnetoelastic coupling mechanism. The geometrically frustrated chain compound FeTe2 O5 Br is regarded as a new class of multiferroic material [12]. It has
Recently, magnetism-induced ferroelectricity has revived research interest in magnetoelectric multiferroics that display large cross-coupling between ferroelectric and magnetic order, that is, the switching of electric polarization by a magnetic field or the induction of magnetization with an applied electric field [1, 2]. This holds promise for multifunctional devices in spintronics and memory areas. These new multiferroics commonly share an incommensurate (ICM) magnetic structure with broken inversion symmetry. Depending on detailed (spiral or collinear) spin arrangements, exchange striction, inverse Dzyaloshinskii–Moriya or spin current mechanisms are 0953-8984/14/086001+08$33.00
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a monoclinic layered structure with space group P21 /c, a = 13.346, b = 6.590, c = 14.242 Å, and β = 108.204◦ [14]. The layers are composed of edge-sharing FeO6 octahedra forming [Fe4 O16 ]20− tetramer spin clusters bridged by Te4+ ions. They are stacked along the a axis and separated by Br− ions. Density functional calculations reveal that FeTe2 O5 Br is described by alternating Fe3+ (S = 5/2) spin chains with frustrated interchain interactions within the bc layers [15]. Magnetic susceptibility follows a Curie–Weiss law for temperatures above 100 K, yielding the Curie–Weiss temperature TCW = −110 K, and shows a broad maximum at Tmax = 41.8 K, characteristic of low-dimensional spin systems. The reduced dimensionality and frustration effects are confirmed by a µSR study [16]. FeTe2 O5 Br undergoes two successive magnetic transitions at TN1 = 11.0 K and TN2 = 10.5 K [12, 17]. The first transition at TN1 is a paramagnetic to a high-temperature (HT) ICM state with a wavevector k = (0.5, 0.466, 0.0). The second transition into the low-temperature (LT) incommensurately modulated phase is described by eight almost collinear sublattices with a wavevector k = (0.5, 0.463, 0) and ¯ [12]. magnetic moments oriented predominately along (110) Both phases exhibit ICM transverse amplitude modulated magnetic order, leading to the coexistence of persistent spin dynamics and long-range order as T −→ 0 K [18]. Noticeably, the spontaneous electric polarization develops only below TN2 with the largest component along the c-axis, P(c) = 8.5(2) µC m−2 [12]. The inversion symmetry is broken already in the HT-ICM phase but the ferroelectricity occurs only in the LT-ICM phase. This indicates that the magnetoelectric mechanism of FeTe2 O5 Br is distinctly different from that of the cycloidal multiferroic compounds [2]. The exotic magnetoelastic mechanism is evidenced also by the magnetic field effect on the electric properties. The electric polarization disappears when an external magnetic field of 4.5 T is applied along the incommensurate direction [19]. The difference between the HT- and the LT-ICM phases lies in the orientation of the magnetic moments and phase shift of the amplitude modulated waves, as well as in the presence of tiny displacements of the Te4+ ions only in the LT-ICM phase [20]. This suggests that the ferroelectricity might be driven by exchange striction of the interchain Fe–O–Te–O– Fe exchange paths, which involve the Te4+ LP ions [21]. However, there is no hint of structural anomalies near TN2 , except detectable changes of the Fe–O and Fe–Te distances. An optical study shows no noticeable field and temperature dependence of phonon spectra through the magnetic ordering temperature [22]. This points to subtle lattice distortions. To gain further insight into the complex magnetic, structural, and electronic properties, electron spin resonance (ESR), Raman and interband absorption spectroscopy are employed as experimental tools. Pressure can add one parameter which produces large changes in cell volume and bond lengths. In this paper, we demonstrate that FeTe2 O5 Br has all of the characteristics of a low-dimensional frustrated magnet with respect to ESR. Furthermore, Raman and optical absorption measurements under pressure give evidence of a pressure-induced structural transition at P = 2.12–3.04 GP. By investigating phonon anomalies we will address the origin of magnetoelastic effects.
2. Experimental details
FeTe2 O5 Br single crystals were grown using chemical transport in sealed evacuated silica tubes [14]. The crystal quality was confirmed by single crystal x-ray diffraction and magnetization measurements. High-frequency ESR experiments were performed at 240 GHz using the quasi-optical spectrometer that has been developed at the National High Magnetic Field Laboratory with a sweepable 12 T superconducting magnet. The spectrometer employs a superheterodyne detection scheme with high-frequency Schottky diode mixers and a lock-in amplifier for field modulation. Thus, the field derivative of a microwave absorption signal was recorded under a sweep of an external magnetic field. Polarized Raman scattering experiments at ambient pressure were performed in quasi-backscattering geometry using a solid state laser (λ = 532.1 nm, P = 1 mW). The samples were installed into a He-cooled closed cycle cryostat with a temperature range of 3–300 K. The spectra were collected via a Dilor-XY 500 triple spectrometer. For hydrostatic pressure measurements we used a diamond anvil high-pressure cell (DAC) with a methanol/ethanol mixture as a pressure medium. The pressure inside the DAC was determined by comparing the peak positions of the R1 fluorescence line of a ruby chip placed inside and outside the DAC, respectively. Samples for the high-pressure experiment were exfoliated with a thickness of 30 µm. Unpolarized Raman spectra at high pressure were measured in a backscattering geometry using a micro-Raman system equipped with a single grating monochromator and an Ar laser (λ = 514.5 nm). 3. Results and discussion 3.1. Electron spin resonance
Pregelj et al [15] have reported the detailed frequency-field dispersions of antiferromagnetic resonance (AFMR) modes for H k b, namely for the field perpendicular to the chain direction. However, temperature dependence of ESR spectra has not been fully investigated. To address the development of magnetic correlations, we performed high-frequency ESR measurements in two different crystallographic orientations. As figure 1(a) shows, at ν = 222.4 GHz and T = 4 K the AFMR mode is observed at 2.0(8) T for H k b. This result is in good agreement with the reported one (compare to figure 3 of [15]). However, in a whole temperature range of T = 4–300 K we can obtain a complete set of the ESR data only for H k a. Hereafter, we thus focus on the ESR spectra for H k a. Figure 1(b) exhibits the temperature dependence of the ESR spectra measured at ν = 240 GHz for H k a. At room temperature the ESR signal consists of a single Lorentzian line, which originates from paramagnetic Fe3+ ions. An exchange-narrowed resonance is usually expected for dense insulating spin systems due to fast electronic fluctuations induced by an exchange interaction between the Fe3+ ions. The g-factor is determined by the relation gc = hν/µB Hres = 2.01(5), where ν = 240 GHz, µB is the Bohr magneton, and 2
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Figure 2. Temperature dependence of the ESR linewidth 1Hpp (T ) (upper panel) and the resonance field Hres (T ) (lower panel). The open triangle and the full circle symbols stand for Hres (T ) and 1Hpp (T ) for the A1 and the A2 signals, respectively. Inset: a zoom of 1Hpp (T ) at low temperatures. The solid (dashed) lines are a fit
of the main (extra) signal to a power law. See the text for details.
where 1Hpp (θ, ∞) is the linewidth in an uncorrelated paramagnetic limit. A T-independent linewidth is characteristic of a high-temperature exchange narrowed paramagnetic state. Upon cooling from 100 K to T ∗ = 20 K, 1Hpp (T ) exhibits a critical-like broadening. This is ascribed to a development of short-range spin correlations. The persistence of strong spin correlations up to the Curie–Weiss temperature of TCW = 110 K is a hallmark of frustrated magnets. The broadening is fitted to a critical power law, 1Hpp (T ) ∝ (T − TN1 )− p with the exponent of p ≈ 0.75(5) (see the solid line in the inset of figure 2). The observed critical exponent is comparable to the values of p ≈ 0.8(1) − 0.8(4) reported in the triangular S = 5/2 antiferromagnet Ba3 NbFe3 Si2 O14 [23]. In the temperature interval between TN1 and T ∗ , 1Hpp (T ) shows a rounded maximum, suggesting the competition between a broadening and a narrowing process. Hres (T ) undergoes a large shift to a lower field and then a tiny dip, and finally shifts towards higher fields. This points to the development of an internal magnetic field. We note that the onset temperature, T ∗ = 20 K, is much higher than the magnetic ordering temperature, TN1 = 11.0 K. Thus, it is assigned to the short-range ordering temperature, being consistent with the µSR experiments [18]. The shift, being related to the formation of the short-ranged magnetic ordering, is a characteristic feature of frustrated magnets [24]. For temperatures below 40 K we observe the new A1 peak in addition to the A2 resonance line. The former is assigned to a precursor of the AFMR mode. For a classical spin system, AFMR modes, which arise from spin wave excitations by a microwave at Q = 0, ±qICM , are detected only in the long-ranged ordered state. However, it is wellknown that in the case of low-dimensional frustrated magnets, the AFMR modes are visible even at temperatures where
Figure 1. (a) Derivative of the ESR absorption spectrum of the FeTe2 O5 Br crystal measured at ν = 222.4 GHz and T = 4 K for H kb. (b) Temperature dependence of the derivative of the ESR absorption spectra measured at ν = 240 GHz for H ka. The spectra are vertically shifted for clarity. The asteroids denote impurity signals. A1 and A2 stand for two AFMR modes.
Hres = 8.05(9) T is the resonance field. The obtained g-value, being close to a free spin value, is what is expected for a half-filled ion with a quenched orbital moment. With decreasing temperature the spectrum undergoes a broadening and a shift. The remarkable feature is the appearance of an additional resonance line (designated as A1 ) for temperatures below 40 K. Since there is no evidence for a structural phase transition at the corresponding temperature, it might be associated with the development of short-range magnetic correlations. This interpretation is supported by the formation of a broad maximum in magnetic susceptibility at the respective temperature [14]. To detail their behavior, the resonance field (Hres ) and the peak-to-peak linewidth (1Hpp ) are extracted by fitting to a Lorentzian profile. Hres (T ) and 1Hpp (T ) of the A2 (full circle) and the A1 (open triangle) signals are plotted in figure 2. For temperatures above 100 K, both the linewidth and the resonance field are constant. The temperature dependence of the linewidth is given by 1Hpp (θ, T ) = α(θ, T )1Hpp (θ, ∞) 3
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short-range ordering is well developed [25, 26]. Indeed, neutron diffraction and thermal expansion measurements give evidence for the persistence of short-range magnetic ordering up to 50 K. We note that our ESR and neutron diffraction measurements probe the short-range ordering at much higher temperatures than µSR. The inconsistency might be partially due to the different frequency windows of the employed experimental techniques. We further note that the numbers and resonance fields of AFMR modes are different between H k b and H k a, corroborating anisotropic magnetic interactions (compare figures 1(a) to (b)). Next, we will turn to the temperature dependence of the AFMR linewidth. The AFMR linewidth is determined by the population of magnons and is described by a power law 1Hpp (T ) ∝ T 4 for a classical magnet [27]. Enhanced quantum fluctuations in frustrated magnets lead to a weaker temperature dependence. As to the A1 mode, the linewidth is described by 1Hpp (T ) ∝ T n with n = 2.2(2) for temperatures below TN2 and 1Hpp (T ) ∝ A + (T − TN2 )n with A = 0.9(5) T and n = 1.8(8) between T ∗ and TN2 . The weaker T-dependence of 1Hpp (T ) observed for temperatures above TN2 is due to a dimensional crossover of spin correlations from a three to a quasi-one dimension. The A2 mode shows a comparable T-dependence with n = 2.3(0) to the A1 mode for T < TN . We note that the obtained exponent is larger than that of geometrically frustrated magnets [28], but it is smaller than classical magnets without frustration. Consistently, a relaxation rate λL (T ) determined by µSR measurements shows a different T-dependence from the conventional magnon modes [18]. This was ascribed to fluctuations of the disordered spin component at each Fe site. Therefore, our result lends support for persisting spin fluctuations, which are a natural consequence of the IC amplitude modulated magnetic order.
Figure 3. Comparison of Raman spectra measured at 3 and 285 K in
(cc) and (cb) polarizations, respectively.
54Bg (ac, bc). In the present scattering geometry the Ag symmetry modes are expected in the (cc) polarization and the Bg modes are in the (cb) polarization. For the (cc) and (cb) polarizations, we count a sum of 49 and 48 modes, respectively. This number is pretty reasonable, given the low symmetry of the atomic positions leading to a merging of several phonon modes together and the huge variation of the scattering intensities. We can group the phonon modes into four spectral regimes in accordance with the frequency separation: 40–120 (I), 150–280 (II), 330–500 (III), and 600–850 cm−1 (IV). The high-energy IV modes correspond to the vibrational modes of the FeO6 blocks. The low- and middle-energy modes are much more intense than the high-energy modes. This points to the participation of the LP Te atoms [30]. The group III (II) modes involve the motions of Te, Fe, and O atoms where the lightest O (heaviest Te) atoms have the largest net atomic displacements. In the case of the group I, the Br atoms, having a weak interaction with the other atoms, are responsible for the lowest frequency modes. Figure 4 zooms the temperature dependence of the phonon modes into the low-, medium-, and high-energy regimes. With increasing temperature the scattering intensity is reduced enormously, while both the frequencies and the linewidths of the phonon modes undergo a moderate softening and damping due to anharmonicities. In addition, all modes show a similar variation of integrated intensity with temperature. Here we note that the phonon linewidths of 2–18 cm−1 are bigger than our spectrometer spectral resolution of 2 cm−1 . Thus, we rule out any instrument influences on the linewidth and intensity. This suggests that the integrated intensity gain is due to a change in the electronic states involved in the electron–hole pair excitation process and not to a specific displacement of the normal mode. A detailed analysis of the frequency, line width, and intensity can shed light on these processes. In figure 5 we plot the frequency and normalized integrated intensity of the representative modes at 89, 255,
3.2. Raman spectra at ambient pressure
In a search for magnetoelastic effects Raman spectroscopy is chosen to probe possible phonon anomalies and structural instabilities of the compound induced by the coupling of lattice and spin degrees of freedom. Figure 3 presents ambient-pressure Raman spectra of FeTe2 O5 Br measured at T = 3 K and 295 K in (cc) and (cb) polarization. In (cc) scattering geometry, the incident and scattered light polarization are both parallel to the c axis, while in the (cb) scattering configuration the incident (scattered) light is polarized parallel to the c (b) axis. For frequencies below 850 cm−1 , we observe a large number of partially overlapping phonon modes. At low temperatures we find no additional phonon modes, which rules out any structural transitions. Instead, a strong enhancement of the Raman scattering cross section is observed as reported also for other LP compounds [13, 29]. This is due to the enormous electronic polarizability of their electronic states. According to the factor group analysis of the monoclinic space group, P21 /c, all atoms occupy the same Wyckoff position 4e and thereby yield 3Ag + 3Bg Raman-active modes. Subtracting the acoustic modes, we obtain a total of 108 Raman-active modes: 0Raman = 54Ag (aa, bb, cc, ab) + 4
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Figure 4. A zoom of the temperature dependence of Raman spectra to the low-, medium-, and high-energy regimes respectively, measured in the (cc) polarization.
Figure 5. Temperature dependence of the peak position and the normalized integrated intensity of phonon modes at 89, 255, 440, and 666 cm−1 on a semilogarithmic scale. The solid lines represent a fit to equation (1).
440, and 666 cm−1 as a function of temperature. The total hardening of the frequencies tends to decrease from 2.3% to 0.6% as the phonon frequency increases. The temperature dependence of the frequency is described by a model based on phonon–phonon decay processes [31], ωph (T ) = ω0 +
C , 1 + 2/(ex − 1)
of the whole temperature data gives a reasonable description (see the solid lines). The frequencies of nearly all modes do not substantially vary with temperature, indicating that anharmonicities are not strong. The small effect seen at the magnetic ordering temperature is due to magnetoelastic couplings. This anomaly is not pronounced although ferroelectric anomalies in FeTe2 O5 Br associated with magnetic ordering have been related to strong spin–lattice interactions. In contrast such a weak effect may be expected taking into account the long
(1)
where x = h¯ ω0 /2kB T and ω0 and C are constants. The fitting 5
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Figure 7. Frequency versus pressure plot for the low- and
Figure 6. Unpolarized Raman spectra of FeTe2 O5 Br measured at
high-pressure phases. The dotted lines are linear fits of the data to ω(P) = ω0 + α P.
T = 300 K and under pressure of P = 0 − 6.62 GPa.
Fe–O–Te–O–Fe exchange paths. These exchange paths also contain easily polarizable Te4+ LP electrons which are the basis of a sizable magnetoelectric effect without accompanying strong lattice anomalies. The Raman scattering intensity is reduced by almost one order of magnitude upon heating from 3 K to room temperature. Since the intensity is determined by a square of the partial derivative of the dielectric function with respect to the amplitude of the normal mode, the strong decrease of the integrated intensity points to a substantial variation of dielectric properties with temperature. As mentioned above, the change of the intensity is ubiquitous on all phonon modes. This suggests that a change of the band energies is mainly responsible for the phonon intensity anomalies [32].
the Raman modes, which show the large intensity increase, and (iii) a slight increase in the number of the observed modes. The overall changes in the Raman spectra can be evaluated by analyzing the pressure coefficients (dω/dP) shown in figure 7. For all peaks the ω(P) behavior can be described by a linear function, ω(P) = ω0 + α P with α = dω/dP. For P < 2.11 GPa, all modes show positive pressure coefficients of α = 1.1(0) − 8.9(8) cm−1 GPa−1 except for the 285 cm−1 mode with α = −0.9(5) cm−1 GPa−1 . The negative α points to the instability of the ambient-pressure structure. For P > 3.04 GPa, we find the moderate positive α = 0.2(8) − 5.3(1) cm−1 GPa−1 . These results suggest that the crystal experiences a relatively weak structural evolution through the phase transition. Further x-ray studies under pressure are needed to determine the crystal symmetry of the high-pressure phase. Since the phonon spectra of the high-pressure phase are distributed more evenly than those of the ambient-pressure phase below 2.11 GPa and their phonon intensities are selectively enhanced, the transition might be associated with the compression between the (Fe4 O16 ) layer and the (Te4 O10 Br2 ) layer. This might lead to an increase in electrostatic repulsion between oxygen atoms and consequently to a structural instability. We note that the copper analog Cu2 Te2 O5 Br2 of FeTe2 O5 Br has been reported to show remarkable stability of the ambient-pressure tetragonal crystal structure (space group P4) up to 14 GPa [33]. Under hydrostatic pressure phonon parameters show a continuous and nearly linear variation while a magnetic mode exhibits a weak first-order quantum phase transition at Pc = 1 GPa. The contrasting pressure behaviors
3.3. Raman spectra under pressure
We next examine the effects of hydrostatic pressure on the structural instability and vibrational properties of this compound. The unpolarized Raman spectra of FeTe2 O5 Br crystal measured under pressure and at room temperature are shown in figure 6. Upon increasing pressure, the Raman spectra retain the same form up to 2.12 GPa, except for blueshifts of all peaks due to the compression. For pressures higher than 3.04 GPa, we observe a drastic change of the phonon frequency and intensity. This indicates that FeTe2 O5 Br undergoes a structural phase transition between 2.12 and 3.04 GPa. Remarkable features of this phase transition are: (i) strong enhancements of the phonon intensity of the 162 (a), 410 (m), and 645 (r) cm−1 mode, (ii) significant and discontinuous frequency shifts of 6
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Figure 8. A log–log plot of the frequency dependence of the mode-Gr¨uneisen parameters, γi ∝ 1/ωi (dωi /dP), for the low- (full triangles) and high-pressure (open squares) phases. The horizontal line is a guide corresponding to a constant γi . Figure 9. A plot of (αhν)2 versus photon energy hν near the band edge under pressure where α is an optical absorption coefficient. Inset: pressure dependence of the direct band gap.
between the copper and the iron analogs may be due to the difference in electronic properties of Fe3+ in FeO6 octahedral environments and Cu2+ in O3 Br distorted square. Another useful way of expressing the pressure effect on each phonon mode i is provided by the mode-Gr¨uneisen parameter γi , defined as γi = −(∂ ln ωi /∂ ln V ) = (1/βωi ) (∂ωi /∂ P). Here V is the crystal volume, P the pressure, and β the isothermal compressibility. In the Gr¨uneisen approximation, all phonon frequencies vary uniformly with pressure, implying that all γi are constant. This approximation is valid for three-dimensional crystals with a single type of bonding force. Figure 8 shows the frequency dependence of the modeGr¨uneisen parameters in a log–log plot together with a constant γi . Far from being frequency independent, γi (ωi ) varies in a non-monotonic way with frequency. We find that the values of γi (ωi ) drop abruptly through the different phonon groups. According to an elementary vibrational model [34], a frequency dependence of γi is a consequence of disparate bonding forces. In this regard, the lack of systematic behaviors of γi (ωi ) can be interpreted as an intriguing admixture of external and internal modes. Despite their complex pattern, there exists some tendency. In the low-pressure phase γi (ωi ) shows a very large change while in the high-pressure phase γi (ωi ) becomes more or less constant within each phonon group. The latter means that bonding forces become more uniform due to the admixture of them under compression, being consistent with the evenly distributed Raman spectra (see figure 6).
that the absorption band in the lower-energy region is well described by the relation, αhν ∼ (hν − E g )2 (not shown here). This is due to an indirect band-to-band transition, implying that the lowest energy gap corresponds to band extrema located at different points in the Brillouin zone. The direct band gap is extracted by extrapolating the linear part of the plot of (αhν)2 versus hν (see the solid line). The result is summarized in the inset of figure 9. At ambient pressure the bandgap is E g ≈ 2.4(8) eV at 300 K. With increasing pressure up to 2.11 GPa, the energy gap increases substantially to E g ≈ 2.5(7) eV and then undergoes an abrupt drop by 0.1 eV through the structural phase transition. For pressure above 2.5 GPa, the energy gap is approximately constant. The strong pressure dependence of E g in the low-pressure phase might be associated with the layer structure. Under compression between the two different layers, anisotropic Fe(3d) orbitals will be admixed with Br(4p) and O(2p) ones. This leads to a rather three-dimensional electronic structure in the high-pressure phase and thus the pressure effects on E g are minute. 4. Conclusion
The multiferroic FeTe2 O5 Br with a frustrated spin chain structure has been investigated using ESR, Raman scattering, and optical absorption studies at both ambient and high pressure. The temperature dependence of the ESR signals shows persisting spin fluctuations in the ordered state as well as the appearance of an AFMR mode at about 40 K (≈4TN ), confirming frustrated, low-dimensional spin dynamics. Raman scattering at ambient pressure shows a large and continuous variation of integrated phonon intensities with temperature. This is attributed to substantial variation of the band energies
3.4. Optical absorption under pressure
Figure 9 shows the absorption coefficient (α) spectra of FeTe2 O5 Br recorded under pressure at room temperature. (αhν)2 is plotted as a function of a photon energy hν. In the higher-energy region, the spectra follow a linear relation, (αhν)2 ∼ hν − E g , indicative of a direct band gap. We find 7
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with temperature. No noticeable phonon anomalies are detected for temperatures in proximity to TN1 and TN2 . Therefore multiferroicity is not related to strong spin–lattice but to the large polarizability of lone-pair electron distributions of the Te ions. Raman and optical absorption spectra under pressure give evidence for a structural phase transition at P = 2.12–3.04 GPa leading into a high-pressure phase. Effects of very disparate bonding forces and a strongly pressure dependent admixture of anisotropic Fe(3d) orbitals with Br(4p) and O(2p) are resolved in this phase. A detailed characterization of the high-pressure phase might therefore bring in-depth understanding of the electron–phonon and spin–phonon coupling of this compound.
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Acknowledgments
This work was supported by NTH ‘Contacts in Nanosystems’ and DFG. KYC acknowledges financial support from the Alexander-von-Humboldt Foundation and Korea NRF Grant (No. 2009-0093817 and 2012-046138). This work of IHC was supported by a Center for Inorganic Photovoltaic Materials (No. 2012-0001168) grant funded by the Korean government (MEST). The National High Magnetic Field Laboratory is supported by NSF Cooperative Agreement No. DMR-0654118, and by the State of Florida. References [1] See for review,Cheong S-W and Mostovoy M 2007 Nature Mater. 6 13 [2] Fiebig M 2005 J. Phys. D: Appl. Phys. 38 R123 [3] Kimura T, Goto T, Shintani H, Ishizaka K, Arima T and Tokura Y 2003 Nature 426 55 [4] Hur N, Park S, Sharma P A, Ahn J S, Guha S and Cheong S-W 2004 Nature 429 392 [5] Sergienko I A and Dagotto E 2006 Phys. Rev. B 73 094434 [6] Katsura H, Nagaosa N and Balatsky A V 2005 Phys. Rev. Lett. 95 057205 [7] Mostovoy M 2006 Phys. Rev. Lett. 96 067601 [8] Harris A B, Yildirim T, Aharony A and Entin-Wohlman O 2006 Phys. Rev. B 73 184433 [9] Lawes G, Ramirez A P, Varma C M and Subramanian M A 2003 Phys. Rev. Lett. 91 257208 [10] Zaharko O et al 2006 Phys. Rev. B 73 064422 [11] Bos J-W G, Colin C V and Palstra T T M 2008 Phys. Rev. B 78 094416 [12] Pregelj M, Zaharko O, Zorko A, Kutnjak Z, Jeglic P, Brown P J, Jagodic M, Jaglicic Z, Berger H and Arcon D 2009 Phys. Rev. Lett. 103 147202
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