Temperature-and magnetic-field-induced magnetization reversal in perovskite YFe 0 . 5 Cr 0 . 5

Perovskite YFe0.5Cr0.5O3 exhibits magnetization reversal at low applied fields due to the competition between the single ion magnetic anisotropy and the antisymmetric Dzyaloshinsky–Moriya interaction. Below a compensation temperature (Tcomp), a tunable bipolar switching of magnetization is demonstrated by changing the magnitude of the field while keeping it in the same direction. The present compound also displays both normal and inverse magnetocaloric effects above and below 260 K, respectively. These phenomena coexisting in a single magnetic system can be tuned in a predictable manner and have potential applications in electromagnetic devices.

2][3][4] As early as in 1948, Néel predicted that this phenomenon can occur in some ferrimagnetic materials such as spinel oxides, 5 when the two antiferromagnetically coupled magnetic substructures exhibit a different temperature dependence of the magnetization.Recent studies have focused on some transition metal oxides with the perovskite structure, e.g., La 0.2 Ce 0.8 CrO 3 , Sr 2 YbRuO 6 , and YbCrO 3 , [6][7][8] while the mechanisms of the magnetization reversal are different from that expected for ferrimagnetic systems.More interestingly, the coexistence of sign reversal of both magnetization and exchange bias field has been achieved in a single compound. 6,7Combining magnetization reversal effect with magnetoelectronics can exploit tremendous technological potential for device applications, for example, thermally assisted magnetic random access memories, thermomagnetic switches and other multifunctional devices, in a preselected and convenient manner.Unfortunately, such compounds are rare, and the low temperature operation is the main hindrance for their potential applications.Therefore, it is necessary to explore materials exhibiting magnetization reversal at higher temperature.
Very recently, the presence of magnetization reversal in BiFe 0.5 Mn 0.5 O 3 with high compensation temperature T comp ͑the characteristic temperature where the material exhibits a net zero magnetization͒ of 208 K arouses much interest in this area. 9It highlights the possibility of designing materials with higher T comp in perovskites with two transition metal magnetic ions at the B site.Actually, Azad et al. 10 have reported the negative field-cooled ͑FC͒ magnetization below 210 K in LaFe 0.5 Cr 0.5 O 3 with the Fe and Cr cations randomly positioned at the B sublattice, whereas the origin of the magnetization reversal remains an open question.In order to explore materials exhibiting magnetization reversal at higher temperature and understand the origin of negative magnetization in perovskites with two magnetic ions at the B site, the temperature-and magnetic-filed-induced magnetization reversal in YFe 0.5 Cr 0.5 O 3 was investigated in this letter.Perovskite YFe 0.5 Cr 0.5 O 3 has the orthorhombic structure but does not display the 1:1 ordering of the Fe 3+ and Cr 3+ ions characteristics of the double perovskite.The observation of magnetization reversal in this system can be ascribed to the competition between the single ion magnetic anisotropy and the antisymmetric Dzyaloshinsky-Moriya ͑DM͒ interaction.Moreover, a bipolar switching of magnetization and both normal and inverse magnetocaloric effects ͑MCEs͒ were observed under low magnetic fields, indicating the potential applications of this material in spintronics.
YFe 0.5 Cr 0.5 O 3 ceramics were synthesized by the standard solid-state reaction.X-ray diffraction ͑XRD͒ powder patterns were collected using a Bede D 1 XRD unit with Ni-filtered Cu K␣ radiation at room temperature.Magnetic measurements were carried out using a vibrating sample magnetometer in a physical properties measurement system ͑PPMS-9T͒ of Quantum Design.
Figure 1 shows the experimental and calculated XRD patterns of YFe 0.5 Cr 0.5 O 3 .No trace of any impurity phases is discernible for the studied sample.The XRD data were refined by the General Structure Analysis System Rietveld refinement program.The best fit of the observed diffraction peaks is obtained with the orthorhombic structure with space group Pnma, which is the same as that of the end members YFeO 3 and YCrO 3 . 11,12The calculated lattice parameters are a = 5.4987͑1͒ Å, b = 7.9786͑5͒ Å, and c = 5.5142͑3͒ Å, respectively.The crystallographic data are similar to the neutron diffraction data as Azad et al. 10 reported in LaFe 0.5 Cr 0.5 O 3 , which also did not show any signal of ordering between Fe 3+ and Cr 3+ cations in these perovskite oxides.
Figure 2 illustrates the temperature dependence of dc magnetization under FC mode at applied fields of Ϯ100 Oe.Intriguing magnetic behaviors were observed as the temperature decreases.Both these two curves exhibit the onset of magnetic ordering at 274 K ͑see top inset of Fig. 2͒.On further cooling the compound under the external field +100 Oe, the magnetization increases, reaches a maximum at 260 K, then drops down and crosses the zero value of magnetization at T comp = 248 K. Afterward, the magnetization shows negative values, indicating that the direction of magnetization is against that of the applied field.On the other hand, a mirror image is seen if a field of Ϫ100 Oe is applied, namely, the magnetization at high temperature is negative and reverts to positive below T comp .Compared to other compounds, [1][2][3][4][5][6][7][8][9][10]13 YFe 0.5 Cr 0.5 O 3 exhibits higher T comp . T cmp decreases with increasing external field, and the negative magnetization is suppressed and vanishes at 3 kOe, as shown in Fig. 4͑a͒.It suggests that the net magnetic moments initially in the opposite direction of the field are realigned in the same direction as the specimen in a higher magnetic field.
To understand the origin of magnetization reversal in YFe 0.5 Cr 0.5 O 3 , it is essential to ascertain the magnetic ground state.The isothermal magnetization M͑H͒ at 5 K is depicted in the bottom inset of Fig. 2.There exhibits an obvious hysteresis loop between Ϯ2 kOe.No saturation is found even up to 50 kOe, instead the magnetization increases linearly in the region of higher magnetic field.This kind of magnetization loop and the linear dependence at high field are usually attributed to the weak ferromagnetism caused by a departure from the collinearity of the moments in an antiferromagnet. 8,12,13The high field part of M͑H͒ curves can be represented as M = AFM H + S , where AFM H is the antiferromagnetic ͑AFM͒ contribution and S is the saturation magnetization of the weak ferromagnetism.Thus, the ferromagnetic component in YFe 0.5 Cr 0.5 O 3 can be obtained by subtracting the AFM contribution from the total magnetization.The remanent magnetization M r is 0.0055 B / f.u. and the coercive field H c is 2.2 kOe.
As discussed above, YFe 0.5 Cr 0.5 O 3 shows AFM ordering with weak ferromagnetism and the Fe 3+ and Cr 3+ ions are distributed randomly at the B site.Evidently, the model proposed by Néel for ferrimagnetic systems is not applicable to the present compound.Normally, there are two mechanisms giving rise to a canting of the AFM alignment of moments: the single ion magnetic anisotropy and the antisymmetric DM interaction. 14,15YFeO 3 is a canted antiferromagnet with weak ferromagnetism due to DM interaction, 11 while in orthochromites the Cr 3+ ions exhibit the single ion magnetic anisotropy arising from the effect of t-e hybridization owing to the local site distortion and cooperative octahedral-site rotation. 16Hence, the substitution of Cr at the Fe site brings out the competition between the single ion magnetic anisotropy and the antisymmetric DM interaction.Noting the equal proportion of Fe and Cr in YFe 0.5 Cr 0.5 O 3 , such competition is more prominent.As reported by Kadomtseva et al., 17 the net moments produced by these two factors are in opposite direction and they have different temperature dependence of magnetization.Consequently, as temperature decreases negative magnetization takes place at low fields.If the applied field is large enough, the average magnetic anisotropy is overcome by the energy induced by the field, the net magnetic moment is orientated parallel to the field direction, and the negative magnetization disappears.This can be demonstrated clearly by a tunable bipolar switching of magnetization at a given temperature below T comp .
Figure 3 shows the switching of FC magnetization between negative and positive values in various magnetic fields at 230 K.The sample was cooled to 230 K under 100 Oe and then the external filed was increased to 1120 Oe in order to switch the magnetization to a positive value of equal magnitude.The magnetization polarity switching can be cycled several times and shows good reproducibility of this field- induced magnetization reversal.This phenomenon is also observed by varying the field to obtain other magnetization values, for example, as per sequence of 300 Oe-910 Oe-300 Oe-¯.Once the external field is switched off, the corresponding magnetization state ceases to exist rapidly without any noticeable decay in their values.As shown in Fig. 3, the alternation of the positive and negative magnetization states can be actuated by only changing the magnitude of the field while keeping it in the same direction.It can be tuned in a predictable way.This behavior can find application in magnetic data storage and switches devices such as nonvolatile magnetic memory, which facilitates two distinct states of magnetization.Compared with similar demonstration in a Prussian blue type molecular magnet and oxide BiFe 0.5 Mn 0.5 O 3 , 4,9 YFe 0.5 Cr 0.5 O 3 displays a bipolar switching of magnetization at higher temperature.
The temperature-induced normal and inverse MCEs were also observed at low magnetic fields.According to the magnetic entropy change ⌬S M of the material with the variation in temperature and an external magnetic field, we can obtain ⌬S M from the temperature dependence of magnetization ͑M-T͒ at different fields, ⌬S M = ͚͓͑‫ץ‬M / ‫ץ‬T͒ H i + ͑‫ץ‬M / ‫ץ‬T͒ H i+1 ͔͑1 / 2͒⌬H i , where ͑‫ץ‬M / ‫ץ‬T͒ H i and ͑‫ץ‬M / ‫ץ‬T͒ H i+1 are the experimental values obtained from the M-T curves in magnetic fields H i and H i+1 , respectively. 18As a direct consequence of the change in the sign of ⌬M / ⌬T, 4 the polarity reversal of −⌬S M can be observed in Fig. 4͑b͒.The increase in −⌬S M emerges at the onset of magnetic ordering temperature 274 K, and then it reaches a maximum at 262 K, where the −⌬S M values start to decrease.On cooling below 262 K, the decrease in −⌬S M continues and it crosses the −⌬S M =0 axis at 260 K.Then, −⌬S M moves to negative values ͑i.e., the inverse MCE͒ with further lowering the temperature.Obviously, an extraordinary coexistence of normal and inverse MCEs has been realized in YFe 0.5 Cr 0.5 O 3 .It is well known that −⌬S M Ͼ 0 results in cooling in the conventional magnetic refrigeration cycle while −⌬S M Ͻ 0 induces a heating under the same refrigeration cycle.This phenomenon can be applied to a constant temperature bath proposed by Yusuf et al. 4 If YFe 0.5 Cr 0.5 O 3 is used as the working medium, at T Ͼ 260 K, the normal MCE will cause the temperature of the system to decrease, while at T Ͻ 260 K, the temperature of the system will increase due to the inverse MCE.Accordingly, in the vicinity of 260 K, any temperature fluctuation will be balanced by these two opposite MCEs.Therefore, a magnetic cooling/heating based constant temperature bath of 260 K can be realized in YFe 0.5 Cr 0.5 O 3 .
In conclusion, we have demonstrated that perovskite YFe 0.5 Cr 0.5 O 3 exhibits magnetization reversal arising from the competition between the single ion magnetic anisotropy and the antisymmetric DM interaction.A tunable bipolar switching of magnetization and both normal and inverse MCEs at low applied fields were observed in this system above 230 K whereas the room temperature operation has not been realized yet.However, the present work shows a promising way of designing materials exhibiting magnetization reversal at higher temperature and proposes the potential applications of this effect in electromagnetic and magnetocaloric devices, such as nonvolatile memories and magnetic cooling/heating based constant temperature bath.To explore materials exhibiting magnetization reversal at room temperature, further theoretical and experimental work is needed.

FIG. 1 .
FIG. 1. ͑Color online͒ Results of Rietveld analysis of the XRD pattern for YFe 0.5 Cr 0.5 O 3 .The observed and calculated patterns are denoted by open circles and solid lines, respectively.The solid lines at the bottom of the panel show the difference in the patterns.The Bragg peak positions are marked by vertical lines.
This work is supported by the National Natural Science Foundation of China ͑Grant No. 10804024͒.