Home
Search
Collections
Journals
About
Contact us
My IOPscience
Evidence of antiferromagnetic and ferromagnetic superexchange interactions in bulk TbMn1−x Crx O3
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 046005 (http://iopscience.iop.org/0953-8984/26/4/046005) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.151.40.2 This content was downloaded on 24/01/2014 at 21:11
Please note that terms and conditions apply.
Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 046005 (6pp)
doi:10.1088/0953-8984/26/4/046005
Evidence of antiferromagnetic and ferromagnetic superexchange interactions in bulk TbMn1−xCrxO3 M Staruch1 and M Jain1,2 1 2
Department of Physics, University of Connecticut, Storrs, CT, USA Institute of Materials Science, University of Connecticut, Storrs, CT, USA
E-mail: [email protected] Received 16 August 2013, revised 18 November 2013 Accepted for publication 4 December 2013 Published 8 January 2014 Abstract
Powder samples of solid solution TbMn1−x Crx O3 (0 6 x 6 1) were synthesized via a facile solution route. The substitution of non-Jahn–Teller active Cr3+ for Mn3+ in TbMnO3 was found to decrease the unit cell volume and orthorhombic distortion. TbMn1−x Crx O3 with low Cr content (x 6 0.33) exhibited magnetic behavior similar to the pure TbMnO3 sample. However, ferromagnetic-like Mn–Cr interactions were introduced in these samples and maximum magnetic field coercivity and remanence were found at x ∼ 0.33. For x > 0.5, signatures of a canted G-type antiferromagnetic ordering similar to pure TbCrO3 were observed. The Mn3+ /Cr3+ spins rotate from parallel to the a-axis to parallel to the c-axis with increasing Cr content. Based on the magnetization results, a magnetic phase diagram for bulk solid solution TbMn1−x Crx O3 has been proposed for the first time. Keywords: manganites, multiferroics, magnetic measurements (Some figures may appear in colour only in the online journal)
1. Introduction
studies of TbMn1−x Scx O3 revealed a cluster glass behavior with no long-range magnetic order for samples with x > 0.2 [11]. In DyMn1−x Fex O3 , static orbital ordering and thus magnetic behavior similar to DyMnO3 was observed for x 6 0.2 that switched to a G-type antiferromagnetic (AFM) structure similar to DyFeO3 for x > 0.5 [12]. Similar effects have been reported for Co- or Ru-doped TbMnO3 (TMO), where after about 10% to 20% substitution of Mn3+ , the NSS ordering is suppressed resulting in the disappearance of ferroelectricity [11–14]. However, in Cr-doped orthorhombic YMnO3 , FM interactions were induced while still showing ferroelectric polarization [15]. Recently, rare-earth chromites (RCrO3 ), which are also orthorhombically distorted perovskites, have also been identified as potential MFs. The RCrO3 exhibit AFM ordering where both the G-type AFM spin configuration and TN (between 125 and 240 K) depends on R [16–18]. For example, in TbCrO3 the Cr3+ moments are aligned along the c-axis and canted towards the a-axis due to DM interaction, whereas in GdCrO3 the Gd3+ spins align antiparallel to the canted Cr3+
Single phase magnetoelectric (ME) multiferroics (MFs) with coupling between ferroelectric and ferro- or antiferromagnetic orders have been a focus of research in the last decade due to their potential in high efficiency non-volatile memory [1, 2]. In the MF rare-earth manganites RMnO3 with orthorhombically distorted perovskite (ABO3 ) structure, such as TbMnO3 or DyMnO3 , ferroelectricity is induced due to a magnetoelastically induced lattice modulation giving rise to strong magnetoelectric coupling at low temperatures [3–5]. The ferroelectric polarization in these materials arises due to an antisymmetric (SEi × SEj ) Dzyaloshinskii–Moriya (DM) exchange interaction resulting from the noncollinear spiral spin (NSS) ordering of the Mn3+ moments, at a temperature Tlock (∼28 K for TbMnO3 and ∼18 K for DyMnO3 ) below the N´eel temperature (TN , 41 K for TbMnO3 and 39 K for DyMnO3 ) [6–9]. In order to increase the maximum theoretical ME coupling in RMnO3 , the introduction of ferromagnetic (FM) interactions would be desirable [10]. However, magnetic 0953-8984/14/046005+06$33.00
1
c 2014 IOP Publishing Ltd Printed in the UK
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
moments and at low temperatures there is a spin reorientation where the Cr3+ moments flip from along the a-axis to along the c-axis [17–19]. Anomalies in the dielectric constant and pyrocurrent measurements have indicated that RCrO3 compounds are relaxor ferroelectrics at high temperatures (>400 K) [20, 21]. In contrast, in RCrO3 in which the R ion has a non-zero magnetic moment, FE behavior has recently been revealed below TN possibly due to the poling electric field allowing for a symmetric exchange striction between the R3+ and Cr3+ moments leading to ferroelectricity (and hence multiferroicity) [22]. Further, since TN and the ferroelectric ordering temperature are higher in the RCrO3 systems as compared to their manganite counterparts (e.g. TN ∼157 K for TbCrO3 versus ∼41 K for TbMnO3 ) [6, 17], the substitution of Cr for Mn in MF RMnO3 could not only increase TN but also the lowest temperature at which they are FE, which would be useful for device applications. It should be noted that the solid solutions, in which both the end members are MF, have not yet been explored. Thus it is of great significance to elucidate the effect of cation disorder at the B-site of TbMnO3 on its magnetic properties. Understanding how the magnetic interactions are modulated with the addition of Cr3+ will also provide essential insight into the evolution of the multiferroic properties. In this work, we present the detailed structural and magnetic properties of powder samples of solid solution TbMnO3 –TbCrO3 (TbMn1−x Crx O3 ) synthesized via the citrate route. Samples with x 6 0.33 reveal magnetic transitions similar to pure TbMnO3 but with a ferromagnetic component that is most likely due to Mn–Cr interactions. Samples with x > 0.5 show behavior consistent with a G-type AFM order similar to pure TbCrO3 .
Figure 1. Lattice parameters (closed symbols) and total cell volume
(open symbols) as a function of Cr content in TbMn1−x Crx O3 samples.
(with space group Pbnm). The lattice parameters and unit cell volume (V) of the samples were calculated from the XRD data and are plotted as a function of Cr content in figure 1. The in-plane lattice parameters a and b were found to generally decrease with increasing values of x, resulting in decreasing unit cell volume and reducing orthorhombic distortion (b/a)√with x. An increase in the tolerance factor, t = (rA − rO )/ 2(rB + rO ), is expected with Cr substitution ˚ and rMn = 0.645 A) ˚ [23] similar to the effect (rCR = 0.615 A of increasing the ionic radius of the rare-earth ion in RMnO3 . However, in RMnO3 , increased values of a, c, and V were observed as rR was increased that were not consistent with the present results [24]. The behavior observed in the present TbMn1−x Crx O3 series is instead similar to that reported in Tb1−x Cax MnO3 , where as Ca2+ content (and therefore Mn4+ content) increased the value of b decreased significantly with only slight variations in the values of a and c [25]. The introduction of non-Jahn–Teller (JT) active Mn4+ was considered to be the most important factor in the modification of the lattice parameters. Thus, it can be inferred that the observed structural modulations in the present samples occur primarily through the replacement of JT active Mn3+ (t32g e1g ) with non-JT active Cr3+ (t32g e0g ), and are not due to an increase in tolerance factor. To further examine the effect of Cr substitution on the structure of TbMn1−x Crx O3 , room temperature Raman measurements were performed as shown in figure 2(a). The 24 Raman active modes of space group Pbnm are 0 = 7Ag + 5B1g + 7B2g + 5B3g [26–28]. Comparison of the Raman spectra with previous studies on RMnO3 and RCrO3 revealed (i) a B2g symmetric oxygen stretching mode at 608 cm−1 , (ii) Ag /B2g MO6 (M = Mn, Cr) bending modes at 503/528 cm−1 , (iii) an Ag MO6 bending mode ∼480 cm−1 , (iv) an Ag out-of-phase rotation of the MO6 octahedra ∼380 cm−1 , and (v) an Ag in-phase rotation of the MO6 octahedra ∼270 cm−1 . The two modes at lower frequencies (∼139 cm−1 and ∼157 cm−1 ) correspond to the vibrations of the Tb3+ ions [26, 29] and exhibit a negligible shift with Cr substitution, indicating that the reduction in lattice
2. Experiment
Bulk samples with molar ratios Tb(1)Mn(1 − x)Cr(x) (with x = 0, 0.1, 0.33, 0.5, 0.7, and 1) were synthesized via the citrate route using high purity precursors of Tb/Mn/Cr, which after drying were annealed at 900◦ C for 2 h in oxygen. X-ray diffraction (XRD) θ –2θ scans were performed to confirm the phase purity and to evaluate the structure of the TbMn1−x Crx O3 samples. Room temperature micro-Raman measurements were carried out with a Renishaw Ramascope employing a 514.5 nm excitation line. The temperature dependent zero-field cooled (ZFC) and field cooled (FC) dc susceptibility (χdc ) data with 50 Oe (0.005 T) applied magnetic field and magnetic field dependent dc magnetization data at several temperatures were measured with a vibrating sample magnetometer attached to an Evercool physical property measurement system (PPMS, by Quantum Design). The ac susceptibility versus temperature data at several frequencies were measured with an ac measurement system attached to the PPMS. 3. Results and discussion
XRD θ –2θ scans (not shown) confirmed that all the present TbMn1−x Crx O3 samples were phase pure and orthorhombic 2
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
Figure 2. (a) Room temperature Raman spectra and (b) shift of
phonon frequencies in TbMn1−x Crx O3 samples.
Figure 4. The temperature dependent real (χ 0 , closed symbols) and
complex (χ 00 , open symbols) ac susceptibility data for (a) TbMn0.9 Cr0.1 O3 , (b) TbMn0.67 Cr0.33 O3 , (c) TbMn0.5 Cr0.5 O3 , (d) TbMn0.3 Cr0.7 O3 , and (e) TbCrO3 samples.
for YMnO3 and YCrO3 [28] and can be explained by the reduction of JT active B-site ions, which both weakens the intensity of peaks resulting from the JT distortion and reduces the unit cell volume which results in a change of the M–O bond lengths (M = Mn, Cr). Bulk dc and ac susceptibility measurements were performed to probe the evolution of the magnetic structure of TbMn1−x Crx O3 and the results are displayed in figures 3 and 4, respectively. The dc susceptibility (χdc ) data of the samples were fitted with the Curie–Weiss law (from 300 K to 60 K for x 6 0.33 and from 300 K to 90 K, 150 K, or 170 K for x = 0.5, x = 0.7, and x = 1 respectively): χdc = C/(T − 2), where C is the Curie constant and 2 is the Curie–Weiss temperature. The effective moment (µeff ) represents contributions from the Tb3+ and Mn3+ /Cr3+ sublattices. It should be noted that the value of 2 is negative (see table 1) for all the solid solution samples studied here, indicating predominantly AFM interactions in these materials. Previous reports on a pure TbMnO3 polycrystalline bulk sample revealed an anomaly at ∼7 K due to Tb3+ ordering and no anomalies due to Mn3+ ordering were observed due the large paramagnetic contribution from Tb3+ (9.72 µB ) [30]. However, as shown in figure 3(a), for the present TbMn0.9 Cr0.1 O3 (TMCO10)
Figure 3. The temperature dependent zero-field cooled (ZFC) and
field cooled (FC) dc susceptibility data for (a) TbMn0.9 Cr0.1 O3 , (b) TbMn0.67 Cr0.33 O3 , (c) TbMn0.5 Cr0.5 O3 , (d) TbMn0.3 Cr0.7 O3 , and (e) TbCrO3 samples.
volume does not strongly affect the phonon modes of the rare-earth ion. The modes related to the M–O bonds and MO6 octahedra, however, systematically vary (in frequency) with Cr content as shown in figure 2(b). The observed shifts in phonon frequencies are consistent with previous results 3
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
Table 1. The effective moment (µeff ), Curie–Weiss temperature (2), and Curie constant as determined from the Curie–Weiss fit of χdc in
TbMn1−x Crx O3 samples. TbMnO3 2 (K) C (emu K/mol Oe) µeff (µB )
−33.94 15.84 11.25
TMCO10 −39.36 14.64 10.83
sample, a bifurcation of the ZFC and FC χdc data is observed at ∼27 K. Further, frequency dependence in the value of the real component of ac susceptibility (χ 0 ) develops below 27 K and a peak in the complex component of ac susceptibility (χ 00 ) appears with an onset at 27 K (as determined from dχ 00 /dT) (figure 4(a)). These results together with the bifurcation suggest FM-like ordering in the TMCO10 sample. A similar weak FM ordering was reported in LaMn1−x Crx O3 and orthorhombic YMn1−x Crx O3 systems [31, 32]. In Cr-doped LaMnO3 , the nature of the Mn3+ –Cr3+ interactions remains a subject of debate [31–36]. These interactions can be FM or AFM depending on whether the σ -bonding eg orbital of Mn3+ is half-filled (e1g –e0g FM exchange) or empty (e0g –e0g AFM semicovalence) according to the Goodenough–Kanamori rules [37–39]. In the present TbMn1−x Crx O3 samples, this will depend significantly on the orthorhombic distortion as well as the JT distortion. The B-site disorder in doped TbMn1−x Crx O3 may give rise to Cr-rich (canted AFM) and Mn-rich (AFM) regions, which would result in cluster-glass-like behavior (as revealed by the frequency dependence of the peak positions in χ 0 (T) and χ 00 (T) plots). Thus, in order to evaluate this possibility, the relative change of the frequencydependent peak temperature, Tpeak , in χ 0 with frequency (f ), i.e. 1Tpeak /{Tpeak 1 log(f )} was calculated to be ∼0.005 for the TMCO10 sample. This value is lower than in typical spin-glass systems and two orders of magnitude below what is expected for superparamagnetism [40]. Hence, the absence of glassy behavior in the present sample suggests that the FM ordering in the present TMCO10 sample is not due to the Cr-rich clusters but rather due to the introduction of Mn–Cr interactions. In TbMn0.67 Cr0.33 O3 (TMCO33), two peaks with onsets at ∼27 and ∼46 K are revealed in ZFC χdc data as well as χ 00 (T) data (determined by peaks in the dχdc /dT and dχ 00 /dT plots) as shown in figures 3(b) and 4(b). Three possibilities for the presence of two FM-like peaks in the x = 0.33 sample are (i) Cr3+ –Cr3+ interactions occurring at TN ∼46 K with an additional FM interaction at 27 K analogous to that in TMCO10, (ii) a ferrimagnetic response due to nonequivalent spins (4.9 µB for Mn3+ and 3.8 µB for Cr3+ ), or (iii) a local canting of the moments induced by the introduction of Mn3+ –Cr3+ interactions where the overall Mn3+ /Cr3+ magnetic structure is similar to pure TbMnO3 (sinusoidal ordering below TN and a noncollinear spiral spin transition at Tlock ∼ 27 K). The first possibility (Cr3+ –Cr3+ interactions) is again disregarded as both peaks are relatively frequency-independent. The observation of two peaks at TN and Tlock in TMCO33 but only one peak at Tlock in TMCO10 is not consistent with ferrimagnetic ordering of nonequivalent
TMCO33 −32.18 15.72 11.22
TMCO50 −15.83 12.72 10.09
TbCrO3 −23.13 11.96 9.79
Figure 5. Projection of the sinusoidal Mn/Cr ordering onto the b–c plane in (a) TbMn0.9 Cr0.1 O3 (Tlock < T < TN ) and (b) TbMn0.67 Cr0.33 O3 (Tlock < T < TN ). (c) Local canting of the spiral spin ordering as a result of ferromagnetic Mn–Cr interactions (T < Tlock ). Nearest neighbors in this projection are located at c/2 and b/2, where the neighbors at b/2 represent two atoms ferromagnetically aligned along the a-axis.
Mn3+ /Cr3+ moments, as this should result in an FM-like contribution at both transition temperatures. However, in the third possibility, the discrepancy between the magnetic results of TMCO10 and TMCO33 samples may arise from FM Mn3+ –Cr3+ interactions that could affect the orientation of neighboring ions resulting in a local canting of the magnetic ordering and thus the presence of a net magnetic moment. For TN < T < Tlock (sinusoidal ordering of Mn/Cr moments), it is more probable that the moments of the majority of Cr3+ nearest neighbors (NNs) are aligned parallel (as shown for TMCO10 in figure 5(a)), which would significantly reduce local distortion due to an FM interaction and therefore decrease the net magnetic moment. With further addition of Cr (figure 5(b)), more sites with a majority of antiparallel NNs are expected to be filled which could result in a larger number of canted Mn3+ /Cr3+ moments. This possibility would explain the observation of a peak at TN in the present TMCO33 sample and not TMCO10. A local canting would be induced in the NSS ordering irrespective of the location of the Cr ion in the spiral ordering (figure 5(c)), resulting in the peak at Tlock in TMCO10 and TMCO33. This suggests that e1g –e0g FM Mn3+ –Cr3+ interactions are introduced with substitution of Cr3+ and a net canted moment dominates the susceptibility for TbMn1−x Crx O3 samples with lower Cr content. In the TbMn0.5 Cr0.5 O3 (TMCO50) sample, a large bifurcation of the ZFC and FC data with a peak in the ZFC data at slightly lower temperatures (as shown in figure 3(c)) is a signature of AFM ordering at TN ∼ 84 K (as determined by a peak in dχdc /dT). A similar peak at TN is also observed in χ 00 (T) data (figure 4(c)). Separate peaks related to Mn3+ –Mn3+ and Cr3+ –Cr3+ interactions are not observed as there is only one peak in both the dc and ac susceptibility 4
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
Figure 6. Temperature dependent coercive field (HC ) values for
Figure 7. The proposed magnetic phase diagram for the solid
TbMn1−x Crx O3 samples.
solution TbMn1−x Crx O3 . Paramagnetic (PM) and antiferromagnetic (AFM) regions are noted. Dotted lines indicate approximately where the transitions between AFM configurations are expected to occur. Transition temperatures for TbMnO3 are taken from [6]. TN for x = 0.1 (open symbol) is assumed from the trend.
data, suggesting that the Mn and Cr moments are equivalent in the overall magnetic order. The anomalies observed at TN in temperature dependent χdc and χ 00 are similar to those observed in the present TbCrO3 sample (figures 3(e) and 4(e)) in which TN is ∼158 K. The value of TN in the present TbCrO3 sample is consistent with previous reports and therefore the magnetic structure in the present sample is expected to be G-type AFM with canting along the a-axis (02 representation following Bertaut) [16, 17]. It should be noted that in LaMn1−x Crx O3 or NdMn1−x Crx O3 , the addition of only 40% Cr resulted in G-type ordering similar to that observed in RCrO3 [31, 41]. With 50% Cr addition, the FM Mn3+ –Cr3+ interactions observed for x 6 0.33 are suppressed and the e0g –e0g AFM semicovalent exchange dominates the magnetic structure. Thus, the magnetic ordering in TMCO50 is also envisaged to be canted G-type AFM. Preliminary neutron diffraction results and first-principles calculations have revealed that the moments are aligned along the a-axis with canting along the c-axis, and will be presented separately. As the Cr content is increased further, the TbMn0.3 Cr0.7 O3 (TMCO70) sample shows a peak at TN ∼ 122 K in the ZFC χdc (T) and χ 00 (T) data (figures 3(d) and 4(d)) and a lower temperature peak with an onset of ∼90 K. In the FC mode with cooling below TN , magnetization first increases, but starts to decrease below 90 K and passes through χdc = 0 at the compensation temperature (Tcomp ) of 37 K, which indicates ferrimagnetic behavior. Ferrimagnetism has previously been noticed in the manganites and chromites when the ferromagnetically ordered B-site ions impose an internal magnetic field that rotates the paramagnetic rare-earth moments antiparallel to the net B-site moment [42–44]. Further, at 25 K, an upturn in the susceptibility is observed and eventually results in a second compensation temperature at Tcomp2 ∼ 14 K, which is a signature of a spin reorientation. This behavior has previously been observed in GdCrO3 and TmCrO3 , where the rare-earth and net Cr3+ moments are aligned antiparallel before the Cr moments rotate in the a–c plane as the magnetic structure switches from 04 (Gx Fz ) to 02
(Gz Fx ) [42, 43]. In polycrystalline GdCrO3 and TmCrO3 or the present TMCO70 sample, the continuous rotation of the moments could result in a point with zero net magnetization at Tcomp2 . The spin reorientation in TMCO70 may arise due to competition between Gx Fz (TMCO50) and Gz Fx (TbCrO3 ) spin configurations. The field-dependent magnetization curves were measured in a range of temperatures for each sample. The Arrott plots (not shown) reveal no critical point for TbMnO3 , TMCO10, and TMCO33 samples as well as a critical point at low temperatures (below TN ) for TMCO50, TMO70, and TbCrO3 samples, further suggesting similar magnetic order for x 6 0.33 and for x > 0.5. The coercive field (HC ) was calculated from a series of isothermal magnetization versus applied magnetic curves and is plotted versus temperature for TbMn1−x Crx O3 in figure 6. Significant values of coercive field are observed at 5 K for all samples with x > 0.1. This also implies that the Mn/Cr ordering in TMCO10 and TMCO33 is in fact similar to TbMnO3 , however with large magnetic field coercivity and remanence (not present in pure TMO) due to the introduction of Cr3+ moments. However, there is a distinct change in the HC versus temperature behavior for x > 0.5. Instead of reducing significantly above 5 K, HC reaches a maximum and then diminishes ∼TN . This peak has previously been observed in pure TbCrO3 [45] and is most likely associated with the alignment of Tb3+ to the net canted moment, suggesting that TMCO50 and TMCO70 exhibit similar magnetic structure to TbCrO3 . The peak may arise due to the reduction of magnetocrystalline anisotropy resulting from the Tb3+ –Mn3+ /Cr3+ exchange interaction [46]. Similar behavior in the present TMCO50 and TMCO70 samples would be consistent with G-type AFM ordering in these materials. Based on the above discussion, a magnetic phase diagram of the TbMn1−x Crx O3 solid solution is proposed 5
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
in figure 7. For TbMn0.9 Cr0.1 O3 and TbMn0.67 Cr0.33 O3 samples, the magnetic structures (and likely the FE properties) are similar to pure TbMnO3 , although with significantly enhanced coercivity and remanence not present in the pure sample. This is thought to be due to a reduction in the orthorhombic distortion and the introduction of FM Mn3+ –Cr3+ interactions. In TbMn0.5 Cr0.5 O3 , the magnetic properties are similar to those of pure TbCrO3 and thus the proposed magnetic structure for x > 0.5 is canted G-type AFM (possibly with moments aligned along the a-axis and with a net moment along the c-axis). It is also expected that similar to TbCrO3 , the present TMCO50 sample will be ferroelectric. In TbMn0.3 Cr0.7 O3 , a rotation in the magnetic configuration 04 (Gx Fz ) → 02 (Gz Fx ) is observed below TSR ∼ 25 K, and this sample is expected to lie near the phase boundary between 04 and 02 ordering with the competition between the two magnetic structures driving the spin reorientation.
[10] Brown W, Hornreich R and Shtrikman S 1968 Phys. Rev. 168 574 [11] Cuartero V, Blasco J, Garc´ıa J, Stankiewicz J, Sub´ıas G, Rodriguez-Velamaz´an J A and Ritter C 2013 J. Phys.: Condens. Matter 25 195601 [12] Hong F, Cheng Z, Zhao H, Kimura H and Wang X 2011 Appl. Phys. Lett. 99 092502 [13] Guo Y Y, Guo Y J, Zhang N, Lin L and Liu J-M 2011 Appl. Phys. A 106 113 [14] Cuartero V, Blasco J, Garc´ıa J, Lafuerza S, Sub´ıas G, Rodriguez-Velamaz´an J A and Ritter C 2012 J. Phys.: Condens. Matter 24 455601 [15] Li S Z, Wang T T, Han H Q, Wang X Z, Li H, Liu J and Liu J-M 2012 J. Phys. D: Appl. Phys. 45 055003 [16] Bertaut E, Mareschal J, de Vries G, Aleonard R, Pauthenet R, Rebouillat J and Zarubicka V 1966 IEEE Trans. Magn. 2 453 [17] Gordon J D, Hornreich R M, Shtrikman S and Wanklyn B M 1976 Phys. Rev. B 13 3012 [18] Hornreich R M 1978 J. Magn. Magn. Mater. 7 280 [19] Cooke A, Martin D and Wells M 1974 J. Phys. C: Solid State Phys. 7 3133 [20] Lal H B, Dwivedi R D and Gaur K 1996 J. Mater. Sci. Mater. Electron. 7 35 [21] Ramesha K, Llobet A, Proffen T, Serrao C R and Rao C N R 2007 J. Phys.: Condens. Matter 19 102202 [22] Rajeswaran B, Khomskii D I, Zvezdin A K, Rao C N R and Sundaresan A 2012 Phys. Rev. B 86 214409 [23] Shannon R D 1976 Acta Crystallogr. A 32 751 [24] Alonso J A, Mart´ınez-Lope M J, Casais M T and Fern´andez-D´az M T 2000 Inorg. Chem. 39 917 [25] Blasco J, Ritter C, Garc´ıa J, de Teresa J M, P´erez-Cacho J and Ibarra M R 2000 Phys. Rev. B 62 5609 [26] Iliev M, Abrashev M, Laverdi`ere J, Jandl S, Gospodinov M, Wang Y-Q and Sun Y-Y 2006 Phys. Rev. B 73 064302 [27] Weber M C, Kreisel J, Thomas P A, Newton M, Sardar K and Walton R I 2012 Phys. Rev. B 85 054303 [28] Todorov N D, Abrashev M V, Ivanov V G, Tsutsumanova G G, Marinova V, Wang Y-Q and Iliev M N 2011 Phys. Rev. B 83 224303 [29] Mart´ın-Carr´on L, de Andr´es A, Mart´ınez-Lope M, Casais M and Alonso J 2002 Phys. Rev. B 66 174303 [30] Staruch M, Lawes G, Kumarasiri A, Cotica L F and Jain M 2013 Appl. Phys. Lett. 102 062908 [31] Bents U 1957 Phys. Rev. 106 225 [32] Li S Z, Wang T T, Han H Q, Wang X Z, Li H, Liu J and Liu J-M 2012 J. Phys. D.: Appl. Phys. 45 055003 [33] Deisenhofer J, Paraskevopoulos M, Krug von Nidda H-A and Loidl A 2002 Phys. Rev. B 66 054414 [34] Cabeza O, Long M, Severac C, Bari M, Muirhead C, Francesconi M and Greaves C 1999 J. Phys.: Condens. Matter 11 2569 [35] Sun Y, Tong W, Xu X and Zhang Y 2001 Phys. Rev. B 63 174438 [36] Kimura T, Tomioka Y, Kumai R, Okimoto Y and Tokura Y 1999 Phys. Rev. Lett. 83 3940 [37] Goodenough J B 1955 Phys. Rev. 100 564 [38] Kanamori J 1959 J. Phys. Chem. Solids 10 87 [39] Anderson P W 1959 Phys. Rev. 115 2 [40] Mydosh J A 1993 Spin Glasses: An Experimental Introduction (London: Taylor and Francis) [41] Troyanchuk I O, Bushinsky M V and Karpinsky D V 2006 J. Exp. Theor. Phys. 103 580 [42] Yoshii K 2001 J. Solid State Chem. 159 204 [43] Yoshii K 2012 Mater. Res. Bull. 47 3243 [44] Snyder G J, Hiskes R and Dicarolis S 1997 Phys. Rev. B 55 6453 [45] Bertaut E F, Mareschal J and De Vries G F 1967 J. Phys. Chem. Solids 28 2143 [46] Tiwari B, Surendra M K and Ramachandra Rao M S 2013 J. Phys.: Condens. Matter 25 216004
4. Conclusions
In summary, bulk samples of solid solution TbMn1−x Crx O3 (0 6 x 6 1) were synthesized via the citrate route. The substitution of non-Jahn–Teller active Cr3+ for Mn3+ was found to reduce the unit cell volume and the orthorhombic distortion of the sample. For samples with lower Cr content (x 6 0.33), peaks resulting from a ferromagnetic distortion of the Mn/Cr sinusoidal and noncollinear spiral spin ordering were observed. This ferromagnetic-like contribution resulted in an enhanced magnetic field coercivity and remanence, with maximum values for the sample with x ∼ 0.33. For x > 0.5, signatures of a canted G-type antiferromagnetic ordering similar to pure TbCrO3 were observed in the dc and ac susceptibility data as well as the temperature dependent coercive field plot. Based on the bulk magnetization results, a magnetic phase diagram for the solid solution TbMn1−x Crx O3 has been proposed herein. Acknowledgments
This work is based in part upon the work supported by the National Science Foundation grants DMR #1310149 and DMR #1105975. References [1] Bibes M and Barth´el´emy A 2008 Nature Mater. 7 425 [2] Binek C and Doudin B 2005 J. Phys.: Condens. Matter 17 L39 [3] Goto T, Kimura T, Lawes G, Ramirez A P and Tokura Y 2004 Phys. Rev. Lett. 92 257201 [4] Lorenz B, Wang Y, Sun Y and Chu C 2004 Phys. Rev. B 70 212412 [5] Wang L J, Chai Y S, Feng S M, Zhu J L, Manivannan N, Jin C Q, Gong Z Z, Wang X H and Li L T 2012 J. Appl. Phys. 111 114103 [6] Kimura T, Goto T, Shintani H, Ishizaka K, Arima T and Tokura Y 2003 Nature 426 55 [7] Kimura T, Lawes G, Goto T, Tokura Y and Ramirez A P 2005 Phys. Rev. B 71 224425 [8] Malashevich A and Vanderbilt D 2008 Phys. Rev. Lett. 101 037210 [9] Jia C, Onoda S, Nagaosa N and Han J 2006 Phys. Rev. B 74 224444 6
Search
Collections
Journals
About
Contact us
My IOPscience
Evidence of antiferromagnetic and ferromagnetic superexchange interactions in bulk TbMn1−x Crx O3
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 046005 (http://iopscience.iop.org/0953-8984/26/4/046005) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.151.40.2 This content was downloaded on 24/01/2014 at 21:11
Please note that terms and conditions apply.
Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 046005 (6pp)
doi:10.1088/0953-8984/26/4/046005
Evidence of antiferromagnetic and ferromagnetic superexchange interactions in bulk TbMn1−xCrxO3 M Staruch1 and M Jain1,2 1 2
Department of Physics, University of Connecticut, Storrs, CT, USA Institute of Materials Science, University of Connecticut, Storrs, CT, USA
E-mail: [email protected] Received 16 August 2013, revised 18 November 2013 Accepted for publication 4 December 2013 Published 8 January 2014 Abstract
Powder samples of solid solution TbMn1−x Crx O3 (0 6 x 6 1) were synthesized via a facile solution route. The substitution of non-Jahn–Teller active Cr3+ for Mn3+ in TbMnO3 was found to decrease the unit cell volume and orthorhombic distortion. TbMn1−x Crx O3 with low Cr content (x 6 0.33) exhibited magnetic behavior similar to the pure TbMnO3 sample. However, ferromagnetic-like Mn–Cr interactions were introduced in these samples and maximum magnetic field coercivity and remanence were found at x ∼ 0.33. For x > 0.5, signatures of a canted G-type antiferromagnetic ordering similar to pure TbCrO3 were observed. The Mn3+ /Cr3+ spins rotate from parallel to the a-axis to parallel to the c-axis with increasing Cr content. Based on the magnetization results, a magnetic phase diagram for bulk solid solution TbMn1−x Crx O3 has been proposed for the first time. Keywords: manganites, multiferroics, magnetic measurements (Some figures may appear in colour only in the online journal)
1. Introduction
studies of TbMn1−x Scx O3 revealed a cluster glass behavior with no long-range magnetic order for samples with x > 0.2 [11]. In DyMn1−x Fex O3 , static orbital ordering and thus magnetic behavior similar to DyMnO3 was observed for x 6 0.2 that switched to a G-type antiferromagnetic (AFM) structure similar to DyFeO3 for x > 0.5 [12]. Similar effects have been reported for Co- or Ru-doped TbMnO3 (TMO), where after about 10% to 20% substitution of Mn3+ , the NSS ordering is suppressed resulting in the disappearance of ferroelectricity [11–14]. However, in Cr-doped orthorhombic YMnO3 , FM interactions were induced while still showing ferroelectric polarization [15]. Recently, rare-earth chromites (RCrO3 ), which are also orthorhombically distorted perovskites, have also been identified as potential MFs. The RCrO3 exhibit AFM ordering where both the G-type AFM spin configuration and TN (between 125 and 240 K) depends on R [16–18]. For example, in TbCrO3 the Cr3+ moments are aligned along the c-axis and canted towards the a-axis due to DM interaction, whereas in GdCrO3 the Gd3+ spins align antiparallel to the canted Cr3+
Single phase magnetoelectric (ME) multiferroics (MFs) with coupling between ferroelectric and ferro- or antiferromagnetic orders have been a focus of research in the last decade due to their potential in high efficiency non-volatile memory [1, 2]. In the MF rare-earth manganites RMnO3 with orthorhombically distorted perovskite (ABO3 ) structure, such as TbMnO3 or DyMnO3 , ferroelectricity is induced due to a magnetoelastically induced lattice modulation giving rise to strong magnetoelectric coupling at low temperatures [3–5]. The ferroelectric polarization in these materials arises due to an antisymmetric (SEi × SEj ) Dzyaloshinskii–Moriya (DM) exchange interaction resulting from the noncollinear spiral spin (NSS) ordering of the Mn3+ moments, at a temperature Tlock (∼28 K for TbMnO3 and ∼18 K for DyMnO3 ) below the N´eel temperature (TN , 41 K for TbMnO3 and 39 K for DyMnO3 ) [6–9]. In order to increase the maximum theoretical ME coupling in RMnO3 , the introduction of ferromagnetic (FM) interactions would be desirable [10]. However, magnetic 0953-8984/14/046005+06$33.00
1
c 2014 IOP Publishing Ltd Printed in the UK
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
moments and at low temperatures there is a spin reorientation where the Cr3+ moments flip from along the a-axis to along the c-axis [17–19]. Anomalies in the dielectric constant and pyrocurrent measurements have indicated that RCrO3 compounds are relaxor ferroelectrics at high temperatures (>400 K) [20, 21]. In contrast, in RCrO3 in which the R ion has a non-zero magnetic moment, FE behavior has recently been revealed below TN possibly due to the poling electric field allowing for a symmetric exchange striction between the R3+ and Cr3+ moments leading to ferroelectricity (and hence multiferroicity) [22]. Further, since TN and the ferroelectric ordering temperature are higher in the RCrO3 systems as compared to their manganite counterparts (e.g. TN ∼157 K for TbCrO3 versus ∼41 K for TbMnO3 ) [6, 17], the substitution of Cr for Mn in MF RMnO3 could not only increase TN but also the lowest temperature at which they are FE, which would be useful for device applications. It should be noted that the solid solutions, in which both the end members are MF, have not yet been explored. Thus it is of great significance to elucidate the effect of cation disorder at the B-site of TbMnO3 on its magnetic properties. Understanding how the magnetic interactions are modulated with the addition of Cr3+ will also provide essential insight into the evolution of the multiferroic properties. In this work, we present the detailed structural and magnetic properties of powder samples of solid solution TbMnO3 –TbCrO3 (TbMn1−x Crx O3 ) synthesized via the citrate route. Samples with x 6 0.33 reveal magnetic transitions similar to pure TbMnO3 but with a ferromagnetic component that is most likely due to Mn–Cr interactions. Samples with x > 0.5 show behavior consistent with a G-type AFM order similar to pure TbCrO3 .
Figure 1. Lattice parameters (closed symbols) and total cell volume
(open symbols) as a function of Cr content in TbMn1−x Crx O3 samples.
(with space group Pbnm). The lattice parameters and unit cell volume (V) of the samples were calculated from the XRD data and are plotted as a function of Cr content in figure 1. The in-plane lattice parameters a and b were found to generally decrease with increasing values of x, resulting in decreasing unit cell volume and reducing orthorhombic distortion (b/a)√with x. An increase in the tolerance factor, t = (rA − rO )/ 2(rB + rO ), is expected with Cr substitution ˚ and rMn = 0.645 A) ˚ [23] similar to the effect (rCR = 0.615 A of increasing the ionic radius of the rare-earth ion in RMnO3 . However, in RMnO3 , increased values of a, c, and V were observed as rR was increased that were not consistent with the present results [24]. The behavior observed in the present TbMn1−x Crx O3 series is instead similar to that reported in Tb1−x Cax MnO3 , where as Ca2+ content (and therefore Mn4+ content) increased the value of b decreased significantly with only slight variations in the values of a and c [25]. The introduction of non-Jahn–Teller (JT) active Mn4+ was considered to be the most important factor in the modification of the lattice parameters. Thus, it can be inferred that the observed structural modulations in the present samples occur primarily through the replacement of JT active Mn3+ (t32g e1g ) with non-JT active Cr3+ (t32g e0g ), and are not due to an increase in tolerance factor. To further examine the effect of Cr substitution on the structure of TbMn1−x Crx O3 , room temperature Raman measurements were performed as shown in figure 2(a). The 24 Raman active modes of space group Pbnm are 0 = 7Ag + 5B1g + 7B2g + 5B3g [26–28]. Comparison of the Raman spectra with previous studies on RMnO3 and RCrO3 revealed (i) a B2g symmetric oxygen stretching mode at 608 cm−1 , (ii) Ag /B2g MO6 (M = Mn, Cr) bending modes at 503/528 cm−1 , (iii) an Ag MO6 bending mode ∼480 cm−1 , (iv) an Ag out-of-phase rotation of the MO6 octahedra ∼380 cm−1 , and (v) an Ag in-phase rotation of the MO6 octahedra ∼270 cm−1 . The two modes at lower frequencies (∼139 cm−1 and ∼157 cm−1 ) correspond to the vibrations of the Tb3+ ions [26, 29] and exhibit a negligible shift with Cr substitution, indicating that the reduction in lattice
2. Experiment
Bulk samples with molar ratios Tb(1)Mn(1 − x)Cr(x) (with x = 0, 0.1, 0.33, 0.5, 0.7, and 1) were synthesized via the citrate route using high purity precursors of Tb/Mn/Cr, which after drying were annealed at 900◦ C for 2 h in oxygen. X-ray diffraction (XRD) θ –2θ scans were performed to confirm the phase purity and to evaluate the structure of the TbMn1−x Crx O3 samples. Room temperature micro-Raman measurements were carried out with a Renishaw Ramascope employing a 514.5 nm excitation line. The temperature dependent zero-field cooled (ZFC) and field cooled (FC) dc susceptibility (χdc ) data with 50 Oe (0.005 T) applied magnetic field and magnetic field dependent dc magnetization data at several temperatures were measured with a vibrating sample magnetometer attached to an Evercool physical property measurement system (PPMS, by Quantum Design). The ac susceptibility versus temperature data at several frequencies were measured with an ac measurement system attached to the PPMS. 3. Results and discussion
XRD θ –2θ scans (not shown) confirmed that all the present TbMn1−x Crx O3 samples were phase pure and orthorhombic 2
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
Figure 2. (a) Room temperature Raman spectra and (b) shift of
phonon frequencies in TbMn1−x Crx O3 samples.
Figure 4. The temperature dependent real (χ 0 , closed symbols) and
complex (χ 00 , open symbols) ac susceptibility data for (a) TbMn0.9 Cr0.1 O3 , (b) TbMn0.67 Cr0.33 O3 , (c) TbMn0.5 Cr0.5 O3 , (d) TbMn0.3 Cr0.7 O3 , and (e) TbCrO3 samples.
for YMnO3 and YCrO3 [28] and can be explained by the reduction of JT active B-site ions, which both weakens the intensity of peaks resulting from the JT distortion and reduces the unit cell volume which results in a change of the M–O bond lengths (M = Mn, Cr). Bulk dc and ac susceptibility measurements were performed to probe the evolution of the magnetic structure of TbMn1−x Crx O3 and the results are displayed in figures 3 and 4, respectively. The dc susceptibility (χdc ) data of the samples were fitted with the Curie–Weiss law (from 300 K to 60 K for x 6 0.33 and from 300 K to 90 K, 150 K, or 170 K for x = 0.5, x = 0.7, and x = 1 respectively): χdc = C/(T − 2), where C is the Curie constant and 2 is the Curie–Weiss temperature. The effective moment (µeff ) represents contributions from the Tb3+ and Mn3+ /Cr3+ sublattices. It should be noted that the value of 2 is negative (see table 1) for all the solid solution samples studied here, indicating predominantly AFM interactions in these materials. Previous reports on a pure TbMnO3 polycrystalline bulk sample revealed an anomaly at ∼7 K due to Tb3+ ordering and no anomalies due to Mn3+ ordering were observed due the large paramagnetic contribution from Tb3+ (9.72 µB ) [30]. However, as shown in figure 3(a), for the present TbMn0.9 Cr0.1 O3 (TMCO10)
Figure 3. The temperature dependent zero-field cooled (ZFC) and
field cooled (FC) dc susceptibility data for (a) TbMn0.9 Cr0.1 O3 , (b) TbMn0.67 Cr0.33 O3 , (c) TbMn0.5 Cr0.5 O3 , (d) TbMn0.3 Cr0.7 O3 , and (e) TbCrO3 samples.
volume does not strongly affect the phonon modes of the rare-earth ion. The modes related to the M–O bonds and MO6 octahedra, however, systematically vary (in frequency) with Cr content as shown in figure 2(b). The observed shifts in phonon frequencies are consistent with previous results 3
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
Table 1. The effective moment (µeff ), Curie–Weiss temperature (2), and Curie constant as determined from the Curie–Weiss fit of χdc in
TbMn1−x Crx O3 samples. TbMnO3 2 (K) C (emu K/mol Oe) µeff (µB )
−33.94 15.84 11.25
TMCO10 −39.36 14.64 10.83
sample, a bifurcation of the ZFC and FC χdc data is observed at ∼27 K. Further, frequency dependence in the value of the real component of ac susceptibility (χ 0 ) develops below 27 K and a peak in the complex component of ac susceptibility (χ 00 ) appears with an onset at 27 K (as determined from dχ 00 /dT) (figure 4(a)). These results together with the bifurcation suggest FM-like ordering in the TMCO10 sample. A similar weak FM ordering was reported in LaMn1−x Crx O3 and orthorhombic YMn1−x Crx O3 systems [31, 32]. In Cr-doped LaMnO3 , the nature of the Mn3+ –Cr3+ interactions remains a subject of debate [31–36]. These interactions can be FM or AFM depending on whether the σ -bonding eg orbital of Mn3+ is half-filled (e1g –e0g FM exchange) or empty (e0g –e0g AFM semicovalence) according to the Goodenough–Kanamori rules [37–39]. In the present TbMn1−x Crx O3 samples, this will depend significantly on the orthorhombic distortion as well as the JT distortion. The B-site disorder in doped TbMn1−x Crx O3 may give rise to Cr-rich (canted AFM) and Mn-rich (AFM) regions, which would result in cluster-glass-like behavior (as revealed by the frequency dependence of the peak positions in χ 0 (T) and χ 00 (T) plots). Thus, in order to evaluate this possibility, the relative change of the frequencydependent peak temperature, Tpeak , in χ 0 with frequency (f ), i.e. 1Tpeak /{Tpeak 1 log(f )} was calculated to be ∼0.005 for the TMCO10 sample. This value is lower than in typical spin-glass systems and two orders of magnitude below what is expected for superparamagnetism [40]. Hence, the absence of glassy behavior in the present sample suggests that the FM ordering in the present TMCO10 sample is not due to the Cr-rich clusters but rather due to the introduction of Mn–Cr interactions. In TbMn0.67 Cr0.33 O3 (TMCO33), two peaks with onsets at ∼27 and ∼46 K are revealed in ZFC χdc data as well as χ 00 (T) data (determined by peaks in the dχdc /dT and dχ 00 /dT plots) as shown in figures 3(b) and 4(b). Three possibilities for the presence of two FM-like peaks in the x = 0.33 sample are (i) Cr3+ –Cr3+ interactions occurring at TN ∼46 K with an additional FM interaction at 27 K analogous to that in TMCO10, (ii) a ferrimagnetic response due to nonequivalent spins (4.9 µB for Mn3+ and 3.8 µB for Cr3+ ), or (iii) a local canting of the moments induced by the introduction of Mn3+ –Cr3+ interactions where the overall Mn3+ /Cr3+ magnetic structure is similar to pure TbMnO3 (sinusoidal ordering below TN and a noncollinear spiral spin transition at Tlock ∼ 27 K). The first possibility (Cr3+ –Cr3+ interactions) is again disregarded as both peaks are relatively frequency-independent. The observation of two peaks at TN and Tlock in TMCO33 but only one peak at Tlock in TMCO10 is not consistent with ferrimagnetic ordering of nonequivalent
TMCO33 −32.18 15.72 11.22
TMCO50 −15.83 12.72 10.09
TbCrO3 −23.13 11.96 9.79
Figure 5. Projection of the sinusoidal Mn/Cr ordering onto the b–c plane in (a) TbMn0.9 Cr0.1 O3 (Tlock < T < TN ) and (b) TbMn0.67 Cr0.33 O3 (Tlock < T < TN ). (c) Local canting of the spiral spin ordering as a result of ferromagnetic Mn–Cr interactions (T < Tlock ). Nearest neighbors in this projection are located at c/2 and b/2, where the neighbors at b/2 represent two atoms ferromagnetically aligned along the a-axis.
Mn3+ /Cr3+ moments, as this should result in an FM-like contribution at both transition temperatures. However, in the third possibility, the discrepancy between the magnetic results of TMCO10 and TMCO33 samples may arise from FM Mn3+ –Cr3+ interactions that could affect the orientation of neighboring ions resulting in a local canting of the magnetic ordering and thus the presence of a net magnetic moment. For TN < T < Tlock (sinusoidal ordering of Mn/Cr moments), it is more probable that the moments of the majority of Cr3+ nearest neighbors (NNs) are aligned parallel (as shown for TMCO10 in figure 5(a)), which would significantly reduce local distortion due to an FM interaction and therefore decrease the net magnetic moment. With further addition of Cr (figure 5(b)), more sites with a majority of antiparallel NNs are expected to be filled which could result in a larger number of canted Mn3+ /Cr3+ moments. This possibility would explain the observation of a peak at TN in the present TMCO33 sample and not TMCO10. A local canting would be induced in the NSS ordering irrespective of the location of the Cr ion in the spiral ordering (figure 5(c)), resulting in the peak at Tlock in TMCO10 and TMCO33. This suggests that e1g –e0g FM Mn3+ –Cr3+ interactions are introduced with substitution of Cr3+ and a net canted moment dominates the susceptibility for TbMn1−x Crx O3 samples with lower Cr content. In the TbMn0.5 Cr0.5 O3 (TMCO50) sample, a large bifurcation of the ZFC and FC data with a peak in the ZFC data at slightly lower temperatures (as shown in figure 3(c)) is a signature of AFM ordering at TN ∼ 84 K (as determined by a peak in dχdc /dT). A similar peak at TN is also observed in χ 00 (T) data (figure 4(c)). Separate peaks related to Mn3+ –Mn3+ and Cr3+ –Cr3+ interactions are not observed as there is only one peak in both the dc and ac susceptibility 4
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
Figure 6. Temperature dependent coercive field (HC ) values for
Figure 7. The proposed magnetic phase diagram for the solid
TbMn1−x Crx O3 samples.
solution TbMn1−x Crx O3 . Paramagnetic (PM) and antiferromagnetic (AFM) regions are noted. Dotted lines indicate approximately where the transitions between AFM configurations are expected to occur. Transition temperatures for TbMnO3 are taken from [6]. TN for x = 0.1 (open symbol) is assumed from the trend.
data, suggesting that the Mn and Cr moments are equivalent in the overall magnetic order. The anomalies observed at TN in temperature dependent χdc and χ 00 are similar to those observed in the present TbCrO3 sample (figures 3(e) and 4(e)) in which TN is ∼158 K. The value of TN in the present TbCrO3 sample is consistent with previous reports and therefore the magnetic structure in the present sample is expected to be G-type AFM with canting along the a-axis (02 representation following Bertaut) [16, 17]. It should be noted that in LaMn1−x Crx O3 or NdMn1−x Crx O3 , the addition of only 40% Cr resulted in G-type ordering similar to that observed in RCrO3 [31, 41]. With 50% Cr addition, the FM Mn3+ –Cr3+ interactions observed for x 6 0.33 are suppressed and the e0g –e0g AFM semicovalent exchange dominates the magnetic structure. Thus, the magnetic ordering in TMCO50 is also envisaged to be canted G-type AFM. Preliminary neutron diffraction results and first-principles calculations have revealed that the moments are aligned along the a-axis with canting along the c-axis, and will be presented separately. As the Cr content is increased further, the TbMn0.3 Cr0.7 O3 (TMCO70) sample shows a peak at TN ∼ 122 K in the ZFC χdc (T) and χ 00 (T) data (figures 3(d) and 4(d)) and a lower temperature peak with an onset of ∼90 K. In the FC mode with cooling below TN , magnetization first increases, but starts to decrease below 90 K and passes through χdc = 0 at the compensation temperature (Tcomp ) of 37 K, which indicates ferrimagnetic behavior. Ferrimagnetism has previously been noticed in the manganites and chromites when the ferromagnetically ordered B-site ions impose an internal magnetic field that rotates the paramagnetic rare-earth moments antiparallel to the net B-site moment [42–44]. Further, at 25 K, an upturn in the susceptibility is observed and eventually results in a second compensation temperature at Tcomp2 ∼ 14 K, which is a signature of a spin reorientation. This behavior has previously been observed in GdCrO3 and TmCrO3 , where the rare-earth and net Cr3+ moments are aligned antiparallel before the Cr moments rotate in the a–c plane as the magnetic structure switches from 04 (Gx Fz ) to 02
(Gz Fx ) [42, 43]. In polycrystalline GdCrO3 and TmCrO3 or the present TMCO70 sample, the continuous rotation of the moments could result in a point with zero net magnetization at Tcomp2 . The spin reorientation in TMCO70 may arise due to competition between Gx Fz (TMCO50) and Gz Fx (TbCrO3 ) spin configurations. The field-dependent magnetization curves were measured in a range of temperatures for each sample. The Arrott plots (not shown) reveal no critical point for TbMnO3 , TMCO10, and TMCO33 samples as well as a critical point at low temperatures (below TN ) for TMCO50, TMO70, and TbCrO3 samples, further suggesting similar magnetic order for x 6 0.33 and for x > 0.5. The coercive field (HC ) was calculated from a series of isothermal magnetization versus applied magnetic curves and is plotted versus temperature for TbMn1−x Crx O3 in figure 6. Significant values of coercive field are observed at 5 K for all samples with x > 0.1. This also implies that the Mn/Cr ordering in TMCO10 and TMCO33 is in fact similar to TbMnO3 , however with large magnetic field coercivity and remanence (not present in pure TMO) due to the introduction of Cr3+ moments. However, there is a distinct change in the HC versus temperature behavior for x > 0.5. Instead of reducing significantly above 5 K, HC reaches a maximum and then diminishes ∼TN . This peak has previously been observed in pure TbCrO3 [45] and is most likely associated with the alignment of Tb3+ to the net canted moment, suggesting that TMCO50 and TMCO70 exhibit similar magnetic structure to TbCrO3 . The peak may arise due to the reduction of magnetocrystalline anisotropy resulting from the Tb3+ –Mn3+ /Cr3+ exchange interaction [46]. Similar behavior in the present TMCO50 and TMCO70 samples would be consistent with G-type AFM ordering in these materials. Based on the above discussion, a magnetic phase diagram of the TbMn1−x Crx O3 solid solution is proposed 5
J. Phys.: Condens. Matter 26 (2014) 046005
M Staruch and M Jain
in figure 7. For TbMn0.9 Cr0.1 O3 and TbMn0.67 Cr0.33 O3 samples, the magnetic structures (and likely the FE properties) are similar to pure TbMnO3 , although with significantly enhanced coercivity and remanence not present in the pure sample. This is thought to be due to a reduction in the orthorhombic distortion and the introduction of FM Mn3+ –Cr3+ interactions. In TbMn0.5 Cr0.5 O3 , the magnetic properties are similar to those of pure TbCrO3 and thus the proposed magnetic structure for x > 0.5 is canted G-type AFM (possibly with moments aligned along the a-axis and with a net moment along the c-axis). It is also expected that similar to TbCrO3 , the present TMCO50 sample will be ferroelectric. In TbMn0.3 Cr0.7 O3 , a rotation in the magnetic configuration 04 (Gx Fz ) → 02 (Gz Fx ) is observed below TSR ∼ 25 K, and this sample is expected to lie near the phase boundary between 04 and 02 ordering with the competition between the two magnetic structures driving the spin reorientation.
[10] Brown W, Hornreich R and Shtrikman S 1968 Phys. Rev. 168 574 [11] Cuartero V, Blasco J, Garc´ıa J, Stankiewicz J, Sub´ıas G, Rodriguez-Velamaz´an J A and Ritter C 2013 J. Phys.: Condens. Matter 25 195601 [12] Hong F, Cheng Z, Zhao H, Kimura H and Wang X 2011 Appl. Phys. Lett. 99 092502 [13] Guo Y Y, Guo Y J, Zhang N, Lin L and Liu J-M 2011 Appl. Phys. A 106 113 [14] Cuartero V, Blasco J, Garc´ıa J, Lafuerza S, Sub´ıas G, Rodriguez-Velamaz´an J A and Ritter C 2012 J. Phys.: Condens. Matter 24 455601 [15] Li S Z, Wang T T, Han H Q, Wang X Z, Li H, Liu J and Liu J-M 2012 J. Phys. D: Appl. Phys. 45 055003 [16] Bertaut E, Mareschal J, de Vries G, Aleonard R, Pauthenet R, Rebouillat J and Zarubicka V 1966 IEEE Trans. Magn. 2 453 [17] Gordon J D, Hornreich R M, Shtrikman S and Wanklyn B M 1976 Phys. Rev. B 13 3012 [18] Hornreich R M 1978 J. Magn. Magn. Mater. 7 280 [19] Cooke A, Martin D and Wells M 1974 J. Phys. C: Solid State Phys. 7 3133 [20] Lal H B, Dwivedi R D and Gaur K 1996 J. Mater. Sci. Mater. Electron. 7 35 [21] Ramesha K, Llobet A, Proffen T, Serrao C R and Rao C N R 2007 J. Phys.: Condens. Matter 19 102202 [22] Rajeswaran B, Khomskii D I, Zvezdin A K, Rao C N R and Sundaresan A 2012 Phys. Rev. B 86 214409 [23] Shannon R D 1976 Acta Crystallogr. A 32 751 [24] Alonso J A, Mart´ınez-Lope M J, Casais M T and Fern´andez-D´az M T 2000 Inorg. Chem. 39 917 [25] Blasco J, Ritter C, Garc´ıa J, de Teresa J M, P´erez-Cacho J and Ibarra M R 2000 Phys. Rev. B 62 5609 [26] Iliev M, Abrashev M, Laverdi`ere J, Jandl S, Gospodinov M, Wang Y-Q and Sun Y-Y 2006 Phys. Rev. B 73 064302 [27] Weber M C, Kreisel J, Thomas P A, Newton M, Sardar K and Walton R I 2012 Phys. Rev. B 85 054303 [28] Todorov N D, Abrashev M V, Ivanov V G, Tsutsumanova G G, Marinova V, Wang Y-Q and Iliev M N 2011 Phys. Rev. B 83 224303 [29] Mart´ın-Carr´on L, de Andr´es A, Mart´ınez-Lope M, Casais M and Alonso J 2002 Phys. Rev. B 66 174303 [30] Staruch M, Lawes G, Kumarasiri A, Cotica L F and Jain M 2013 Appl. Phys. Lett. 102 062908 [31] Bents U 1957 Phys. Rev. 106 225 [32] Li S Z, Wang T T, Han H Q, Wang X Z, Li H, Liu J and Liu J-M 2012 J. Phys. D.: Appl. Phys. 45 055003 [33] Deisenhofer J, Paraskevopoulos M, Krug von Nidda H-A and Loidl A 2002 Phys. Rev. B 66 054414 [34] Cabeza O, Long M, Severac C, Bari M, Muirhead C, Francesconi M and Greaves C 1999 J. Phys.: Condens. Matter 11 2569 [35] Sun Y, Tong W, Xu X and Zhang Y 2001 Phys. Rev. B 63 174438 [36] Kimura T, Tomioka Y, Kumai R, Okimoto Y and Tokura Y 1999 Phys. Rev. Lett. 83 3940 [37] Goodenough J B 1955 Phys. Rev. 100 564 [38] Kanamori J 1959 J. Phys. Chem. Solids 10 87 [39] Anderson P W 1959 Phys. Rev. 115 2 [40] Mydosh J A 1993 Spin Glasses: An Experimental Introduction (London: Taylor and Francis) [41] Troyanchuk I O, Bushinsky M V and Karpinsky D V 2006 J. Exp. Theor. Phys. 103 580 [42] Yoshii K 2001 J. Solid State Chem. 159 204 [43] Yoshii K 2012 Mater. Res. Bull. 47 3243 [44] Snyder G J, Hiskes R and Dicarolis S 1997 Phys. Rev. B 55 6453 [45] Bertaut E F, Mareschal J and De Vries G F 1967 J. Phys. Chem. Solids 28 2143 [46] Tiwari B, Surendra M K and Ramachandra Rao M S 2013 J. Phys.: Condens. Matter 25 216004
4. Conclusions
In summary, bulk samples of solid solution TbMn1−x Crx O3 (0 6 x 6 1) were synthesized via the citrate route. The substitution of non-Jahn–Teller active Cr3+ for Mn3+ was found to reduce the unit cell volume and the orthorhombic distortion of the sample. For samples with lower Cr content (x 6 0.33), peaks resulting from a ferromagnetic distortion of the Mn/Cr sinusoidal and noncollinear spiral spin ordering were observed. This ferromagnetic-like contribution resulted in an enhanced magnetic field coercivity and remanence, with maximum values for the sample with x ∼ 0.33. For x > 0.5, signatures of a canted G-type antiferromagnetic ordering similar to pure TbCrO3 were observed in the dc and ac susceptibility data as well as the temperature dependent coercive field plot. Based on the bulk magnetization results, a magnetic phase diagram for the solid solution TbMn1−x Crx O3 has been proposed herein. Acknowledgments
This work is based in part upon the work supported by the National Science Foundation grants DMR #1310149 and DMR #1105975. References [1] Bibes M and Barth´el´emy A 2008 Nature Mater. 7 425 [2] Binek C and Doudin B 2005 J. Phys.: Condens. Matter 17 L39 [3] Goto T, Kimura T, Lawes G, Ramirez A P and Tokura Y 2004 Phys. Rev. Lett. 92 257201 [4] Lorenz B, Wang Y, Sun Y and Chu C 2004 Phys. Rev. B 70 212412 [5] Wang L J, Chai Y S, Feng S M, Zhu J L, Manivannan N, Jin C Q, Gong Z Z, Wang X H and Li L T 2012 J. Appl. Phys. 111 114103 [6] Kimura T, Goto T, Shintani H, Ishizaka K, Arima T and Tokura Y 2003 Nature 426 55 [7] Kimura T, Lawes G, Goto T, Tokura Y and Ramirez A P 2005 Phys. Rev. B 71 224425 [8] Malashevich A and Vanderbilt D 2008 Phys. Rev. Lett. 101 037210 [9] Jia C, Onoda S, Nagaosa N and Han J 2006 Phys. Rev. B 74 224444 6
