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High pressure induced spin state crossover in Sr2CaYCo4O10.5
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 J. Phys.: Condens. Matter 27 046005 (http://iopscience.iop.org/0953-8984/27/4/046005) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 198.91.37.2 This content was downloaded on 21/02/2015 at 04:18
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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 27 (2015) 046005 (5pp)
doi:10.1088/0953-8984/27/4/046005
High pressure induced spin state crossover in Sr2CaYCo4O10.5 V Sikolenko1,3,6 , I Troyanchuk2 , M Bushinsky2 , V Efimov3 , L Keller4 , J S White4,5 , F R Schilling6 and S Schorr1 1
Helmholtz-Zentrum Berlin, 14109 Berlin, Germany Scientifical-Practical Material Science Centre of NAS of Belarus, 220072 Minsk, Belarus 3 Joint Institute for Nuclear Research, 141980 Dubna, Russia 4 Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland 5 ´ Laboratory for Quantum Magnetism, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), 1015 Lausanne, Switzerland 6 Institute of Applied Geosciences, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany 2
E-mail: [email protected] Received 7 October 2014, revised 30 November 2014 Accepted for publication 15 December 2014 Published 8 January 2015 Abstract
The layered cobaltite Sr2 CaYCo4 O10.5 with formal average cobalt oxidation state close to 3+ has been studied as functions of both temperature and pressure up to 4 GPa by neutron powder diffraction (NPD). The crystal structure is shown to have tetragonal symmetry (space group I4/mmm; 2ap × 2ap × 4ap superstructure), and the magnetic structure at ambient pressure is found to be G-type antiferromagnetic with TN close to 310 K. The magnetic moments within the CoO6 octahedral layers and anion-deficient CoO4.5 layers are 1.2µB and 2.8µB , respectively. At 25 K, and applied pressure of 3.5 GPa is sufficient to completely suppress a long-range magnetic order. This result is interpreted in terms of a pressure-induced high-to-low spin state crossover of the Co3+ ions. (Some figures may appear in colour only in the online journal) Keywords: neutron diffraction, high pressure, spin state
above 600 K in a tetragonal structural phase with I4/mmm symmetry, and with lattice parameters a = 2ap and c = 4ap (ap —is the lattice parameter of the primitive perovskite cell). The crystal structure contains anion-deficient CoO4+δ pyramidal layers alternating with stacked CoO6 octahedral layers in the direction of the c-axis. This class of compounds is characterized by high magnetic ordering temperatures, and a ferromagnetic component that is observable in magnetization measurements [6, 7]. In addition, the magnetic ordering is √ √ accompanied by the appearance of a 4 2ap × 2 2ap × 4ap superstructure (space group A2/m) [7]. In [8, 9] it was found that magnetic structure of Sr3 YCo4 O10.5 can be formally described as a G-type antiferomagnetic. The magnetic moments reside on two distinct Co sites. The Co(1) atom is in the oxygen-deficient layer and has a higher magnetic moment compared to the Co(2) atom which resides in the octahedral layer. These compounds can also exhibit a firstorder antiferromagnet–ferromagnet transition similar to that
1. Introduction
Complex cobalt oxides attract great interest due to their various and diverse physical properties; giant negative magnetoresistance, insulator-metal transitions and spin state magnetic transitions being some of the examples [1]. The nature of the magnetic interactions depends on the spin state 6 of the Co3+ ion. It can be in a low spin state (LS) t2g 5 (S = 0), intermediate spin (IS) state t2g eg (S = 1) or 4 2 high spin (HS) state t2g eg (S = 2) because the energy of the crystal field splitting of the Co 3d states (Ecf ) and the Hund’s rule exchange energy (Eex ) are comparable. For instance, La1−x Bax CoO3 [2] and La1−x Sr x CoO3 [3] are ferromagnetic over a wide temperature range, while in layered perovskites LnBaCo2 O5.5 (Ln=lantanide), metal-insulator and antiferromagnet–ferromagnet transitions are observed [4]. Recently, new perovskite-related cobaltites Sr 3 LnCo4 O10.5+δ were synthesized [5, 6]. These compounds crystallize 0953-8984/15/046005+05$33.00
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Figure 1. (a) Temperature dependences of magnetization (FC at 100 Oe) for Sr 3 YCo4 O10.5+δ and Sr2 CaYCo4 O10.57 samples. (b) Magnetization versus magnetic field dependences for both Sr 3 YCo4 O10.5+δ and Sr2 CaYCo4 O10.57 , and at different temperatures.
observed in the LnBaCo2 O5.5 series [4, 10]. The origin of the room-temperature ferromagnetism has been attributed to orbital ordering [11], ferrimagnetism [12], the formation of ferromagnetic spin bags in the oxygen-rich perovskite layers [13] or a canting of the magnetic structure in aniondeficient layers [9, 10]. Importantly, cobaltites are very sensitive to external pressures, because the crystal field splitting energy strongly depends on Co–O bond lengths. According to [14], the population of the IS state depends not only on the competition between Ecf and the exchange interaction Jex , but on the eg -electron bandwidth W resulting from the σ -bonding of the Co–O–Co hybridization. A broadening of W was found to stabilize the IS state. The authors of [15] performed a high pressure study of the Sr2.8 Y1.2 Co4 O10.5 . They have found the suppression of the initial G-type antiferromagnetic structure and a formation of the new antiferromagnetic state with a propagation vector (1/2 1/2 1). According to [15] the HS state of both the tetrahedral and five-coordinated Co3+ ions in oxygen-deficient layers remained stable under pressure. The substitution of Sr2+ ions by significantly smaller Ca2+ should stabilize the low spin state of Co3+ ions. In this work, we report a neutron diffraction study of the Sr2 CaYCo4 O10.5 at high pressure.
beamline (X04SA) at SLS PSI. Neutron diffraction data at ambient pressure and high resolution were collected using the powder diffractometer HRPT [16] at SINQ, with a wavelength λ = 1.494 Å and over a wide temperature range. Neutron diffraction measurements with the sample under pressure were carried out with the DMC diffractometer [17] at SINQ spallation source with a Paris-Edinburgh press in its dedicated cryostat [18]. The pressure range explored was from ambient to 4 GPa, and the temperatures 25, 100 and 300 K. The sample was mixed with a small amount of NaCl which was used as a pressure calibrant [20]. The diffraction patterns were collected with a cold neutron wavelength of λ = 2.45 Å. The experimental data were analyzed by the Rietveld method using the FullProf software package [19]. 3. Results and discussion
Bulk magnetization data for both Sr2 CaYCo4 O10.5 and the parent compound Sr3 YCo4 O10.5 are presented in figure 1. The parent compound exhibits a pronounced ferromagnetic component above 200 K, with the spontaneous magnetization disappearing with the decreasing of temperature. A large thermal hysteresis (around 13 K) indicates a first-order magnetic transition, which is in agreement with previous work [10]. The substitution of part of Sr2+ by Ca2+ leads to an almost complete suppression of the spontaneous magnetization (figure 1). In contrast to the parent compounds, anomalous behavior in the magnetization below 300 K is not observed in the Ca-doped material. In this instance, the magnetization anomaly around 310 K can be attributed to the N´eel point. Synchrotron x-ray data were collected in the temperature range 100–400 K. The diffraction peaks can be indexed on a tetragonal cell with the I4/mmm symmetry (2ap × 2ap × 4ap superstructure), and they showed a small amount of the Y2 O3 impurity phase (∼1 wt%). In refinement, we have used the model, described in detail in [8, 9] and [12]. The Rietveld refinement made at 300 K is shown in figure 2. The refined unit cell parameters and their temperature-dependence are shown in figure 3. The thermal expansion along the c-axis is significantly larger than that along the a-axis, such
2. Experimental
Sr2 CaYCo4 O10.5 and Sr2 CaYCo4 O10.57 polycrystalline samples were synthesized using conventional ceramic synthesis technology as described in [3]. Stoichiometric amounts of Y2 O3 , CoO oxides and SrCO3 (CaCO3 ) carbonates were mixed for 30 min in a Retsch planetary ball mill (300 rpm). The preliminary burning was performed at 1000 ◦ C for 2 h, and the obtained product was ground. A final synthesis was performed at 1150 ◦ C for 8 h. The sample cooling rate from 1150 to 300 ◦ C was 200 ◦ C h−1 . The magnetic properties of the samples were investigated with a physical properties measurement system (PPMS) set-up with magnetic fields up to 14 T, and temperatures in the range 5–320 K (Cryogenic Ltd). Synchrotron x-ray diffraction data have been collected using the powder diffraction station of the Materials Sciences 2
J. Phys.: Condens. Matter 27 (2015) 046005
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Figure 2. SLS-powder diffraction pattern for Sr2 CaYCo4 O10.5
Figure 4. NPD patterns recorded at 2 K on HRPT for Sr2 CaYCo4
recorded at 300 K. The line and points refer to calculated and observed profiles. The bottom line represents their difference. The upper row of vertical ticks marks the Bragg reflections of the phase Sr2 CaYCo4 O10.5 while the bottom row denotes the Bragg reflections of the impurity phase Y2 O3 .
O10.5 . The black line and red points denote the calculated and observed profiles, respectively. The bottom blue line represents their difference. The upper row of vertical ticks marks the Bragg reflections of the structural phase while the bottom row denotes magnetic reflections. Table 1. Crystallographic parameters of Y0.25 Ca0.25 Sr0.5 CoO3 at T = 2 K obtained from the Rietveld fit of the HRPT data.
Space group: ´ a, b (Å) ´ c (Å) ´ 3) V (Å Y1, Sr1, Ca1, site 4e (0, 0, z) Y2, Sr2, Ca2, site 4e (0, 0, z) Y3, Sr3, Ca3, site 8 g (0, 1/2, z) Co1, site 8 h (x, x, 0) Co2, site 8f (1/4, 1/4, 1/4) O1, site 16n (0, y, z) O2, site 16 m (x, x, z) O3, site 8i (x, 0, 0) Fractional occupation O3 O4, site 8j (x, 1/2, 0) ´ Co2-O1 (Å) ´ Co2-O2 (Å) Co2-O1-Co2 (deg.) Co1-O2-Co2 (deg.) µ1, µB µ2, µB Rp /Rwp χ2
Figure 3. Temperature dependence of unit cell parameters for Sr2 CaYCo4 O10.5 obtained from the Rietveld fits of the SLS data.
that the primitive unit cell becomes pseudo-cubic near room temperature. NPD patterns were obtained in the temperature range 2–450 K using the HRPT diffractometer. The pattern at 2 K is shown in figure 4. Across the entire temperature range, all data can be indexed within the I4/mmm tetragonal cell, and the magnetic contribution starts to develop close to 310 K. The unit cell parameters at room temperature have been refined to be a = 7.6003(2) Å and c = 15.2003(4) Å. These values are significantly smaller than those of a = 7.65 and c = 15.35 Å obtained for the parent compound [8]. The refined structural parameters at 2 K are presented in table 1. The obtained oxygen content corresponds to the chemical formula Sr2 CaYCo4 O10.57 . The analysis of the magnetic structure was done using the model proposed in [8] and [9]. In agreement with the previously determined magnetic structure (see [8]), the magnetic structure can be described as G-type antiferromagnetic. There are two types of cobalt ions:
I4/mmm 7.5866(2) 15.1382(4) 871.31(4) 0.14957(41) 0.62159(49) 0.13485(30) 0.24459(80) 0.24585(50), 0.26017(27) 0.21621(33), 0.11918(24) 0.10177(214) 0.288(8) 0.21711(82) 1.9031(2) 2.0133(2) 170.527(2) 160.047(4) ±2.835 ±1.231 5.48/7.13 6.17
Co(1) located in oxygen-deficient CoO4.5 layers and Co(2) located in CoO6 octahedral layers. These layers are alternately stacked in the direction of the c-axis. Figure 5 shows the temperature dependence of the Co magnetic moments at ambient pressure in Sr2 CaYCo4 O10.5 . The values of the magnetic moment on the Co(1) site is similar to that obtained in the parent compound [8, 9]. The magnetic moment on the Co(2) sites (octahedral coordination) is slightly smaller than that found in pure Sr3 YCo4 O10.5 [8]. The tetragonal crystal symmetry remains unchanged across the whole temperature and pressure ranges. Figure 6 shows the pressure- and temperature-dependence of the unit cell volume. By fitting the Birch-Murnagham equation of 3
J. Phys.: Condens. Matter 27 (2015) 046005
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Figure 7. Rietveld refined magnetic moments as a function of pressure at T = 100 K and T = 25 K.
Figure 5. Temperature dependence of the Rietveld refined magnetic moment sizes at ambient pressure.
Figure 8. (1 1 0) and (1 1 2) Brag peaks, measured at ambient pressure (upper red curve) and at 4 GPa (lower blue curve) at 100 K.
Figure 6. Unit cell volume as a function of pressure for Sr2 CaYCo4 O10.5 . The error bars are smaller than the data symbols.
pressure, nor was any structural distortion observed near the N´eel temperature. This implies that orbital ordering is not realized in spite of the nominal 3+ valence state of the cobalt ions in both layers and an antiferromagnet–ferromagnet transition seems to be incompatible with orbital ordering in the octahedral layers. It is more likely that orbital ordering occurs within the anion-deficient layers of the Ca-free parent compounds. The magnetic moment value in the CoO4.5 sublattice of Sr2 CaYCo4 O10.5 is refined to be 2.8µB per ion at 2 K and at ambient pressure. This value is significantly larger than the 2 µB expected for an IS state. However, the magnetic moment expected for the high-spin state (S = 2) is about 4µB , which is significantly greater than the observed value. In contrast, the size of the magnetic moments in the octahedral layer (1.2µB ) is much lower than expected for the IS state (S = 1). Therefore, we assume that the present data might not be explained by only a pure IS or HS state for Co3+ . We can, however, suppose that a significant fraction of Co3+ is in an LS state. This is because the ionic radius of Co3+ in the LS state is ´ and thus smaller than the radius of Co3+ in either the 0.685 Å,
state [20] we obtain an experimental value V0 = 871, which subsequently gives a value of the bulk modulus B0 = −V (dP /dV )T equal to B0 = 103.5 that is very close to that reported in [15] for undoped Sr3 YCo4 O10.5 . The intensities of the (1 1 0) and (1 1 2) magnetic Bragg peaks decrease with increasing pressure (see figures 7 and 8) and completely disappears at P = 3.5 GPa. At these high pressures, no additional diffraction peaks were observed, implying that applied pressure suppresses the long-range magnetic ordering. The strong suppression of the ferromagnetic component in Sr2 YCaCo4 O10.5 in comparison with the parent compound is readily explained in terms of structural changes. Indeed, the √ √ Ca-free compound is characterized by a 4 2ap ×2 2ap ×4ap superstructure that arises from oxygen vacancies ordering within anion-deficient layers according to [7]. Substitution of Sr2+ with Ca2+ leads to vacancy disorder within the aniondeficient layer, and the crystal structure becomes close to pseudo-cubic. We did not observe any indication of a structural transition within the temperature range (2–450) K at ambient 4
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´ or HS states (0.750 Å). ´ Therefore, the population IS (0.717 Å), of the LS state might be correlated with the unit cell volume, and is smaller in the case of Ca doping in comparison to that of the pure Sr3 YCo4 O10.5 . The superexchange interaction between cobalt ions in the HS state is expected to be both strong, and negative according to the Goodenough–Kanamori rules [21]. Therefore, our data can be readily described in terms of a mixture of Co with different spin states. Indeed, an oxygen deficiency will promote a stabilization of the HS state while full covalency would stabilize the IS state of Co3+ ions [22, 23]. Therefore, one can expect that the application of external pressure on the sample leads to a favoring of the relative populations of the LS or IS states with smaller ionic radii in comparison with the HS one. It is well known that applied pressure stabilizes the LS state in LaCoO3 [24], while an external pressure induces an HS to IS state crossover in La0.5 Ba0.5 CoO2.8 [23]. The latter compound has a much ´ compared to LaCoO (a ∼ larger unit cell (ap = 3.92 Å) 3 p ´ 3.805 Å), while the primitive unit cell parameter of LaCoO3 ´ is very close to that of Sr YCaCo O (a = 3.800 Å). 2
4
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This work is based on experiments performed at the Swiss Spallation Neutron Source SINQ, Paul Scherrer Institute, Villigen, Switzerland. References [1] Raveau B and Seikh M M 2012 Cobalt Oxides: From Crystal Chemistry to Physics (Weinheim, Germany: Wiley) [2] Sazonov A P, Troyanchuk I O, Gamari-Seale H, Sikolenko V V, Stefanopoulos K L, Nicolaides G K and Atanassova Y K 2009 J. Phys.: Condens. Matter 21 156004 [3] Sikolenko V V, Sazonov A P, Troyanchuk I O, Tobbens D M, Zimmermann U, Pomjakushina E V and Szymczak H 2004 J. Phys.: Condens. Matter 16 7313 [4] Troyanchuk I O, Kasper N V, Khalyavin D D, Szymczak H and Baran M 1998 Phys. Rev. Lett. 80 3380 [5] Istomin S Ya, Grins J, Swensson G, Drozhzhin O A, Kozhevnikov V L, Antipov E V and Attfield J P 2003 Chem. Mater. 15 4012 [6] James M, Avdeev M, Barnes P, Morales L, Wallwork K and Withers R 2007 J. Solid State Chem. 180 2233 [7] Ishiwata S, Kobayashi W, Terasaki I, Kato K and Takata M 2007 Phys. Rev. B 75 220406 [8] Sheptyakov D, Pomjakushin V, Drozhzhin O, Istomin S, Antipov E, Bobrikov I and Balagurov A 2009 Phys. Rev. B 80 024409 [9] Troyanchuk I O, Karpinsky D V, Sazonov A P, Sikolenko V, Efimov V and Senyshyn A 2009 J. Mater. Sci. 44 5900 [10] Troyanchuk I O, Bushinsky M V, Dobryanskii V M and Pushkarev N V 2011 JETP Lett. 94 849 [11] Nakao H, Murata T, Bizen D, Murakami Y, Ohoyama K, Yamada K, Ishiwata S, Kobayashi W and Terasaki I 2011 J. Phys. Soc. Japan 80 023711 [12] Khalyavin D D, Chapon L C, Suard E, Parker J E, Thompson S P, Yaremchenko A A and Kharton V V 2011 Phys. Rev. B 83 140403 [13] Bettis J L, Xiang H and Whangbo M-H 2012 Chem. Mater. 24 3117 [14] Fita I, Szymczak R, Puzniak R, Wisnievwski A, Troyanchuk I, Karpinsky D, Markovich V and Szymczak H 2011 Phys. Rev. B 83 064414 [15] Golosova N O, Kozlenko D P, Dubrovinsly L S, Drohzhin O A, Istomin S Ya and Savenko B N 2009 Phys. Rev. B 79 104431 [16] Fisher P et al 2000 Physica B 276–278 146 [17] Fisher P, Keller L, Schefer J and Kohlbrecher J 2000 Neutron News 11 19 [18] Klotz S, Str¨assle T, Rousse G, Hamel G and Pomjakushin V 2005 Appl. Phys. Lett. 86 031917 [19] Rodr´ıguez-Carvajal J 1993 Physica B 192 55 [20] Birch F 1986 J. Geophys. Res. 91 4949 [21] Goodenough J B 1963 Magnetism and Chemical Bond (New York: Interscience) [22] Troyanchuk I O, Karpinsky D V, Bushinsky M V, Sikolenko V V, Efimov V, Cervellino A and Raveau B 2012 J. Appl. Phys. 112 013916 [23] Troyanchuk I, Bushinsky M, Sikolenko V, Efimov V, Ritter C, Hansen T and T¨obbens D M 2013 Eur. Phys. J. B 86 435 [24] Vogt T, Hriljac J A, Hyatt N C and Woodward P 2003 Phys. Rev. B 67 140401(R)
p
Besides, the anion-deficient Sr2 YCaCo4 O10.5 is much less covalent than the stoichiometric LaCoO3 . Hence, one can conclude that external pressure should induce the HS-LS state crossover in Sr2 YCaCo4 O10.5 . The observed disappearance of the magnetic contribution to the neutron diffraction patterns under pressure is in agreement with the stabilization of the LS state in both anion deficient and octahedral layers. 4. Conclusion
We have investigated the effect of applied pressure on the magnetic order in anion-deficient Sr2 YCaCo4 O10.5 . This compound is characterized by a G-type antiferromagnetic structure below TN = 310 K at ambient pressure, and the magnetic moments in the CoO6 octahedral layers and the CoO4.5 anion-deficient layers are found to be 1.2µB and 2.8µB respectively. It was found that an applied pressure of 4 GPa is sufficient to completely suppress the long-range antiferromagnetic ordering. The observed pressure-driven collapse of the magnetic order is attributed to a pressureinduced high-low spin state transition of the Co ions. Acknowledgments
We would like to thank Dr Thierry Str¨assle and Dr Denis Sheptyakov for fruitful discussions and help with experiments. This work has been supported by the Belarus Foundation for Basic Research, project F14R-040, the Russian Foundation for Basic Research, project 13-02-00699, and MaNEP.
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High pressure induced spin state crossover in Sr2CaYCo4O10.5
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 J. Phys.: Condens. Matter 27 046005 (http://iopscience.iop.org/0953-8984/27/4/046005) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 198.91.37.2 This content was downloaded on 21/02/2015 at 04:18
Please note that terms and conditions apply.
Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 27 (2015) 046005 (5pp)
doi:10.1088/0953-8984/27/4/046005
High pressure induced spin state crossover in Sr2CaYCo4O10.5 V Sikolenko1,3,6 , I Troyanchuk2 , M Bushinsky2 , V Efimov3 , L Keller4 , J S White4,5 , F R Schilling6 and S Schorr1 1
Helmholtz-Zentrum Berlin, 14109 Berlin, Germany Scientifical-Practical Material Science Centre of NAS of Belarus, 220072 Minsk, Belarus 3 Joint Institute for Nuclear Research, 141980 Dubna, Russia 4 Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland 5 ´ Laboratory for Quantum Magnetism, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), 1015 Lausanne, Switzerland 6 Institute of Applied Geosciences, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany 2
E-mail: [email protected] Received 7 October 2014, revised 30 November 2014 Accepted for publication 15 December 2014 Published 8 January 2015 Abstract
The layered cobaltite Sr2 CaYCo4 O10.5 with formal average cobalt oxidation state close to 3+ has been studied as functions of both temperature and pressure up to 4 GPa by neutron powder diffraction (NPD). The crystal structure is shown to have tetragonal symmetry (space group I4/mmm; 2ap × 2ap × 4ap superstructure), and the magnetic structure at ambient pressure is found to be G-type antiferromagnetic with TN close to 310 K. The magnetic moments within the CoO6 octahedral layers and anion-deficient CoO4.5 layers are 1.2µB and 2.8µB , respectively. At 25 K, and applied pressure of 3.5 GPa is sufficient to completely suppress a long-range magnetic order. This result is interpreted in terms of a pressure-induced high-to-low spin state crossover of the Co3+ ions. (Some figures may appear in colour only in the online journal) Keywords: neutron diffraction, high pressure, spin state
above 600 K in a tetragonal structural phase with I4/mmm symmetry, and with lattice parameters a = 2ap and c = 4ap (ap —is the lattice parameter of the primitive perovskite cell). The crystal structure contains anion-deficient CoO4+δ pyramidal layers alternating with stacked CoO6 octahedral layers in the direction of the c-axis. This class of compounds is characterized by high magnetic ordering temperatures, and a ferromagnetic component that is observable in magnetization measurements [6, 7]. In addition, the magnetic ordering is √ √ accompanied by the appearance of a 4 2ap × 2 2ap × 4ap superstructure (space group A2/m) [7]. In [8, 9] it was found that magnetic structure of Sr3 YCo4 O10.5 can be formally described as a G-type antiferomagnetic. The magnetic moments reside on two distinct Co sites. The Co(1) atom is in the oxygen-deficient layer and has a higher magnetic moment compared to the Co(2) atom which resides in the octahedral layer. These compounds can also exhibit a firstorder antiferromagnet–ferromagnet transition similar to that
1. Introduction
Complex cobalt oxides attract great interest due to their various and diverse physical properties; giant negative magnetoresistance, insulator-metal transitions and spin state magnetic transitions being some of the examples [1]. The nature of the magnetic interactions depends on the spin state 6 of the Co3+ ion. It can be in a low spin state (LS) t2g 5 (S = 0), intermediate spin (IS) state t2g eg (S = 1) or 4 2 high spin (HS) state t2g eg (S = 2) because the energy of the crystal field splitting of the Co 3d states (Ecf ) and the Hund’s rule exchange energy (Eex ) are comparable. For instance, La1−x Bax CoO3 [2] and La1−x Sr x CoO3 [3] are ferromagnetic over a wide temperature range, while in layered perovskites LnBaCo2 O5.5 (Ln=lantanide), metal-insulator and antiferromagnet–ferromagnet transitions are observed [4]. Recently, new perovskite-related cobaltites Sr 3 LnCo4 O10.5+δ were synthesized [5, 6]. These compounds crystallize 0953-8984/15/046005+05$33.00
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Figure 1. (a) Temperature dependences of magnetization (FC at 100 Oe) for Sr 3 YCo4 O10.5+δ and Sr2 CaYCo4 O10.57 samples. (b) Magnetization versus magnetic field dependences for both Sr 3 YCo4 O10.5+δ and Sr2 CaYCo4 O10.57 , and at different temperatures.
observed in the LnBaCo2 O5.5 series [4, 10]. The origin of the room-temperature ferromagnetism has been attributed to orbital ordering [11], ferrimagnetism [12], the formation of ferromagnetic spin bags in the oxygen-rich perovskite layers [13] or a canting of the magnetic structure in aniondeficient layers [9, 10]. Importantly, cobaltites are very sensitive to external pressures, because the crystal field splitting energy strongly depends on Co–O bond lengths. According to [14], the population of the IS state depends not only on the competition between Ecf and the exchange interaction Jex , but on the eg -electron bandwidth W resulting from the σ -bonding of the Co–O–Co hybridization. A broadening of W was found to stabilize the IS state. The authors of [15] performed a high pressure study of the Sr2.8 Y1.2 Co4 O10.5 . They have found the suppression of the initial G-type antiferromagnetic structure and a formation of the new antiferromagnetic state with a propagation vector (1/2 1/2 1). According to [15] the HS state of both the tetrahedral and five-coordinated Co3+ ions in oxygen-deficient layers remained stable under pressure. The substitution of Sr2+ ions by significantly smaller Ca2+ should stabilize the low spin state of Co3+ ions. In this work, we report a neutron diffraction study of the Sr2 CaYCo4 O10.5 at high pressure.
beamline (X04SA) at SLS PSI. Neutron diffraction data at ambient pressure and high resolution were collected using the powder diffractometer HRPT [16] at SINQ, with a wavelength λ = 1.494 Å and over a wide temperature range. Neutron diffraction measurements with the sample under pressure were carried out with the DMC diffractometer [17] at SINQ spallation source with a Paris-Edinburgh press in its dedicated cryostat [18]. The pressure range explored was from ambient to 4 GPa, and the temperatures 25, 100 and 300 K. The sample was mixed with a small amount of NaCl which was used as a pressure calibrant [20]. The diffraction patterns were collected with a cold neutron wavelength of λ = 2.45 Å. The experimental data were analyzed by the Rietveld method using the FullProf software package [19]. 3. Results and discussion
Bulk magnetization data for both Sr2 CaYCo4 O10.5 and the parent compound Sr3 YCo4 O10.5 are presented in figure 1. The parent compound exhibits a pronounced ferromagnetic component above 200 K, with the spontaneous magnetization disappearing with the decreasing of temperature. A large thermal hysteresis (around 13 K) indicates a first-order magnetic transition, which is in agreement with previous work [10]. The substitution of part of Sr2+ by Ca2+ leads to an almost complete suppression of the spontaneous magnetization (figure 1). In contrast to the parent compounds, anomalous behavior in the magnetization below 300 K is not observed in the Ca-doped material. In this instance, the magnetization anomaly around 310 K can be attributed to the N´eel point. Synchrotron x-ray data were collected in the temperature range 100–400 K. The diffraction peaks can be indexed on a tetragonal cell with the I4/mmm symmetry (2ap × 2ap × 4ap superstructure), and they showed a small amount of the Y2 O3 impurity phase (∼1 wt%). In refinement, we have used the model, described in detail in [8, 9] and [12]. The Rietveld refinement made at 300 K is shown in figure 2. The refined unit cell parameters and their temperature-dependence are shown in figure 3. The thermal expansion along the c-axis is significantly larger than that along the a-axis, such
2. Experimental
Sr2 CaYCo4 O10.5 and Sr2 CaYCo4 O10.57 polycrystalline samples were synthesized using conventional ceramic synthesis technology as described in [3]. Stoichiometric amounts of Y2 O3 , CoO oxides and SrCO3 (CaCO3 ) carbonates were mixed for 30 min in a Retsch planetary ball mill (300 rpm). The preliminary burning was performed at 1000 ◦ C for 2 h, and the obtained product was ground. A final synthesis was performed at 1150 ◦ C for 8 h. The sample cooling rate from 1150 to 300 ◦ C was 200 ◦ C h−1 . The magnetic properties of the samples were investigated with a physical properties measurement system (PPMS) set-up with magnetic fields up to 14 T, and temperatures in the range 5–320 K (Cryogenic Ltd). Synchrotron x-ray diffraction data have been collected using the powder diffraction station of the Materials Sciences 2
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Figure 2. SLS-powder diffraction pattern for Sr2 CaYCo4 O10.5
Figure 4. NPD patterns recorded at 2 K on HRPT for Sr2 CaYCo4
recorded at 300 K. The line and points refer to calculated and observed profiles. The bottom line represents their difference. The upper row of vertical ticks marks the Bragg reflections of the phase Sr2 CaYCo4 O10.5 while the bottom row denotes the Bragg reflections of the impurity phase Y2 O3 .
O10.5 . The black line and red points denote the calculated and observed profiles, respectively. The bottom blue line represents their difference. The upper row of vertical ticks marks the Bragg reflections of the structural phase while the bottom row denotes magnetic reflections. Table 1. Crystallographic parameters of Y0.25 Ca0.25 Sr0.5 CoO3 at T = 2 K obtained from the Rietveld fit of the HRPT data.
Space group: ´ a, b (Å) ´ c (Å) ´ 3) V (Å Y1, Sr1, Ca1, site 4e (0, 0, z) Y2, Sr2, Ca2, site 4e (0, 0, z) Y3, Sr3, Ca3, site 8 g (0, 1/2, z) Co1, site 8 h (x, x, 0) Co2, site 8f (1/4, 1/4, 1/4) O1, site 16n (0, y, z) O2, site 16 m (x, x, z) O3, site 8i (x, 0, 0) Fractional occupation O3 O4, site 8j (x, 1/2, 0) ´ Co2-O1 (Å) ´ Co2-O2 (Å) Co2-O1-Co2 (deg.) Co1-O2-Co2 (deg.) µ1, µB µ2, µB Rp /Rwp χ2
Figure 3. Temperature dependence of unit cell parameters for Sr2 CaYCo4 O10.5 obtained from the Rietveld fits of the SLS data.
that the primitive unit cell becomes pseudo-cubic near room temperature. NPD patterns were obtained in the temperature range 2–450 K using the HRPT diffractometer. The pattern at 2 K is shown in figure 4. Across the entire temperature range, all data can be indexed within the I4/mmm tetragonal cell, and the magnetic contribution starts to develop close to 310 K. The unit cell parameters at room temperature have been refined to be a = 7.6003(2) Å and c = 15.2003(4) Å. These values are significantly smaller than those of a = 7.65 and c = 15.35 Å obtained for the parent compound [8]. The refined structural parameters at 2 K are presented in table 1. The obtained oxygen content corresponds to the chemical formula Sr2 CaYCo4 O10.57 . The analysis of the magnetic structure was done using the model proposed in [8] and [9]. In agreement with the previously determined magnetic structure (see [8]), the magnetic structure can be described as G-type antiferromagnetic. There are two types of cobalt ions:
I4/mmm 7.5866(2) 15.1382(4) 871.31(4) 0.14957(41) 0.62159(49) 0.13485(30) 0.24459(80) 0.24585(50), 0.26017(27) 0.21621(33), 0.11918(24) 0.10177(214) 0.288(8) 0.21711(82) 1.9031(2) 2.0133(2) 170.527(2) 160.047(4) ±2.835 ±1.231 5.48/7.13 6.17
Co(1) located in oxygen-deficient CoO4.5 layers and Co(2) located in CoO6 octahedral layers. These layers are alternately stacked in the direction of the c-axis. Figure 5 shows the temperature dependence of the Co magnetic moments at ambient pressure in Sr2 CaYCo4 O10.5 . The values of the magnetic moment on the Co(1) site is similar to that obtained in the parent compound [8, 9]. The magnetic moment on the Co(2) sites (octahedral coordination) is slightly smaller than that found in pure Sr3 YCo4 O10.5 [8]. The tetragonal crystal symmetry remains unchanged across the whole temperature and pressure ranges. Figure 6 shows the pressure- and temperature-dependence of the unit cell volume. By fitting the Birch-Murnagham equation of 3
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Figure 7. Rietveld refined magnetic moments as a function of pressure at T = 100 K and T = 25 K.
Figure 5. Temperature dependence of the Rietveld refined magnetic moment sizes at ambient pressure.
Figure 8. (1 1 0) and (1 1 2) Brag peaks, measured at ambient pressure (upper red curve) and at 4 GPa (lower blue curve) at 100 K.
Figure 6. Unit cell volume as a function of pressure for Sr2 CaYCo4 O10.5 . The error bars are smaller than the data symbols.
pressure, nor was any structural distortion observed near the N´eel temperature. This implies that orbital ordering is not realized in spite of the nominal 3+ valence state of the cobalt ions in both layers and an antiferromagnet–ferromagnet transition seems to be incompatible with orbital ordering in the octahedral layers. It is more likely that orbital ordering occurs within the anion-deficient layers of the Ca-free parent compounds. The magnetic moment value in the CoO4.5 sublattice of Sr2 CaYCo4 O10.5 is refined to be 2.8µB per ion at 2 K and at ambient pressure. This value is significantly larger than the 2 µB expected for an IS state. However, the magnetic moment expected for the high-spin state (S = 2) is about 4µB , which is significantly greater than the observed value. In contrast, the size of the magnetic moments in the octahedral layer (1.2µB ) is much lower than expected for the IS state (S = 1). Therefore, we assume that the present data might not be explained by only a pure IS or HS state for Co3+ . We can, however, suppose that a significant fraction of Co3+ is in an LS state. This is because the ionic radius of Co3+ in the LS state is ´ and thus smaller than the radius of Co3+ in either the 0.685 Å,
state [20] we obtain an experimental value V0 = 871, which subsequently gives a value of the bulk modulus B0 = −V (dP /dV )T equal to B0 = 103.5 that is very close to that reported in [15] for undoped Sr3 YCo4 O10.5 . The intensities of the (1 1 0) and (1 1 2) magnetic Bragg peaks decrease with increasing pressure (see figures 7 and 8) and completely disappears at P = 3.5 GPa. At these high pressures, no additional diffraction peaks were observed, implying that applied pressure suppresses the long-range magnetic ordering. The strong suppression of the ferromagnetic component in Sr2 YCaCo4 O10.5 in comparison with the parent compound is readily explained in terms of structural changes. Indeed, the √ √ Ca-free compound is characterized by a 4 2ap ×2 2ap ×4ap superstructure that arises from oxygen vacancies ordering within anion-deficient layers according to [7]. Substitution of Sr2+ with Ca2+ leads to vacancy disorder within the aniondeficient layer, and the crystal structure becomes close to pseudo-cubic. We did not observe any indication of a structural transition within the temperature range (2–450) K at ambient 4
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´ or HS states (0.750 Å). ´ Therefore, the population IS (0.717 Å), of the LS state might be correlated with the unit cell volume, and is smaller in the case of Ca doping in comparison to that of the pure Sr3 YCo4 O10.5 . The superexchange interaction between cobalt ions in the HS state is expected to be both strong, and negative according to the Goodenough–Kanamori rules [21]. Therefore, our data can be readily described in terms of a mixture of Co with different spin states. Indeed, an oxygen deficiency will promote a stabilization of the HS state while full covalency would stabilize the IS state of Co3+ ions [22, 23]. Therefore, one can expect that the application of external pressure on the sample leads to a favoring of the relative populations of the LS or IS states with smaller ionic radii in comparison with the HS one. It is well known that applied pressure stabilizes the LS state in LaCoO3 [24], while an external pressure induces an HS to IS state crossover in La0.5 Ba0.5 CoO2.8 [23]. The latter compound has a much ´ compared to LaCoO (a ∼ larger unit cell (ap = 3.92 Å) 3 p ´ 3.805 Å), while the primitive unit cell parameter of LaCoO3 ´ is very close to that of Sr YCaCo O (a = 3.800 Å). 2
4
10.5
This work is based on experiments performed at the Swiss Spallation Neutron Source SINQ, Paul Scherrer Institute, Villigen, Switzerland. References [1] Raveau B and Seikh M M 2012 Cobalt Oxides: From Crystal Chemistry to Physics (Weinheim, Germany: Wiley) [2] Sazonov A P, Troyanchuk I O, Gamari-Seale H, Sikolenko V V, Stefanopoulos K L, Nicolaides G K and Atanassova Y K 2009 J. Phys.: Condens. Matter 21 156004 [3] Sikolenko V V, Sazonov A P, Troyanchuk I O, Tobbens D M, Zimmermann U, Pomjakushina E V and Szymczak H 2004 J. Phys.: Condens. Matter 16 7313 [4] Troyanchuk I O, Kasper N V, Khalyavin D D, Szymczak H and Baran M 1998 Phys. Rev. Lett. 80 3380 [5] Istomin S Ya, Grins J, Swensson G, Drozhzhin O A, Kozhevnikov V L, Antipov E V and Attfield J P 2003 Chem. Mater. 15 4012 [6] James M, Avdeev M, Barnes P, Morales L, Wallwork K and Withers R 2007 J. Solid State Chem. 180 2233 [7] Ishiwata S, Kobayashi W, Terasaki I, Kato K and Takata M 2007 Phys. Rev. B 75 220406 [8] Sheptyakov D, Pomjakushin V, Drozhzhin O, Istomin S, Antipov E, Bobrikov I and Balagurov A 2009 Phys. Rev. B 80 024409 [9] Troyanchuk I O, Karpinsky D V, Sazonov A P, Sikolenko V, Efimov V and Senyshyn A 2009 J. Mater. Sci. 44 5900 [10] Troyanchuk I O, Bushinsky M V, Dobryanskii V M and Pushkarev N V 2011 JETP Lett. 94 849 [11] Nakao H, Murata T, Bizen D, Murakami Y, Ohoyama K, Yamada K, Ishiwata S, Kobayashi W and Terasaki I 2011 J. Phys. Soc. Japan 80 023711 [12] Khalyavin D D, Chapon L C, Suard E, Parker J E, Thompson S P, Yaremchenko A A and Kharton V V 2011 Phys. Rev. B 83 140403 [13] Bettis J L, Xiang H and Whangbo M-H 2012 Chem. Mater. 24 3117 [14] Fita I, Szymczak R, Puzniak R, Wisnievwski A, Troyanchuk I, Karpinsky D, Markovich V and Szymczak H 2011 Phys. Rev. B 83 064414 [15] Golosova N O, Kozlenko D P, Dubrovinsly L S, Drohzhin O A, Istomin S Ya and Savenko B N 2009 Phys. Rev. B 79 104431 [16] Fisher P et al 2000 Physica B 276–278 146 [17] Fisher P, Keller L, Schefer J and Kohlbrecher J 2000 Neutron News 11 19 [18] Klotz S, Str¨assle T, Rousse G, Hamel G and Pomjakushin V 2005 Appl. Phys. Lett. 86 031917 [19] Rodr´ıguez-Carvajal J 1993 Physica B 192 55 [20] Birch F 1986 J. Geophys. Res. 91 4949 [21] Goodenough J B 1963 Magnetism and Chemical Bond (New York: Interscience) [22] Troyanchuk I O, Karpinsky D V, Bushinsky M V, Sikolenko V V, Efimov V, Cervellino A and Raveau B 2012 J. Appl. Phys. 112 013916 [23] Troyanchuk I, Bushinsky M, Sikolenko V, Efimov V, Ritter C, Hansen T and T¨obbens D M 2013 Eur. Phys. J. B 86 435 [24] Vogt T, Hriljac J A, Hyatt N C and Woodward P 2003 Phys. Rev. B 67 140401(R)
p
Besides, the anion-deficient Sr2 YCaCo4 O10.5 is much less covalent than the stoichiometric LaCoO3 . Hence, one can conclude that external pressure should induce the HS-LS state crossover in Sr2 YCaCo4 O10.5 . The observed disappearance of the magnetic contribution to the neutron diffraction patterns under pressure is in agreement with the stabilization of the LS state in both anion deficient and octahedral layers. 4. Conclusion
We have investigated the effect of applied pressure on the magnetic order in anion-deficient Sr2 YCaCo4 O10.5 . This compound is characterized by a G-type antiferromagnetic structure below TN = 310 K at ambient pressure, and the magnetic moments in the CoO6 octahedral layers and the CoO4.5 anion-deficient layers are found to be 1.2µB and 2.8µB respectively. It was found that an applied pressure of 4 GPa is sufficient to completely suppress the long-range antiferromagnetic ordering. The observed pressure-driven collapse of the magnetic order is attributed to a pressureinduced high-low spin state transition of the Co ions. Acknowledgments
We would like to thank Dr Thierry Str¨assle and Dr Denis Sheptyakov for fruitful discussions and help with experiments. This work has been supported by the Belarus Foundation for Basic Research, project F14R-040, the Russian Foundation for Basic Research, project 13-02-00699, and MaNEP.
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