SCIENTIFIC REPpRTS

OPEN Anisotropic magnetic entropy change in RFeO3 single crystals(R=Tb, Tm, or Y)

Received: 19 June 2015 Accepted: 20 October 2015 Published: 25 January 2016

Ya-Jiao Ke, Xiang-Qun Zhang, Yue Ma & Zhao-Hua Cheng

Compared with traditional gas-compression/expansion refrigeration, magnetic refrigeration based on magnetocaloric effect (MCE) exhibits the advantages of high energy efficiency and environment friendliness. Here, we created large MCE in RFeO3 (R=Tb or Tm) single crystals by the magnetization vector rotation of single crystal with strong magnetocrystalline anisotropy (MCA), rather than merely via the order-disorder magnetic phase transition or magnetic structural transition. Owing to the difference in charge distribution of 4/-electrons between Tb3+ and Tm3+ ions, the rotating field entropy with different signs, -ASMR = 17.42 J/kg K, and -ASMR = -9.01 J/kg K are achieved at 9 K and 17 K for TbFeO3 and TmFeO3 single crystals from b axis to c axis, at 50 kOe, respectively. The finding of the large anisotropic MCE not only advances our understanding of the anisotropy of MCE, but also extends the application for single crystals to magnetic refrigeration.

Magnetocaloric effect (MCE), which describes the temperature change of magnetic materials in an adiabatic process caused by magnetic entropy change ASM under external magnetic field, has been extensively investigated. In comparison with traditional gas-compression/expansion refrigeration, magnetic refrigeration based on MCE exhibits the advantages of high energy efficiency and environment friendliness. The giant or very large magnetic entropy change was obtained in various kinds of magnetic materials, including Gd-based alloys Gd5(Si%Ge1-%)1-2, Mn-based Ni-Mn-Ga(Sn) alloys3-4 and MnFeP045As0 555, Fe-based LaFe13-%(Si, Al)/-7, as well as rare-earth perovskite-type manganites (La1-xMx)MnO3 (M = Ca, Sr, and Ba etc.)8'9. Although numerous studies on MCE have been concentrated on exploring new materials with giant MCE near room temperature for domestic applications, giant MCE in the low-temperature region from about 30 K down to sub-Kelvin temperatures is also essential for utilization in certain fields, such as liquid hydrogen economy and space application10.

The magnetic, barocaloric and electrocaloric effects can be tuned or created by element substitution11, pressure12-15, strain16,17, electric field18,19, or elastic force20. The giant magnetic entropy change in the vicinity of magnetic ordering temperature is usually accompanied by a field-induced or temperature-induced magnetic phase transition with the changes in either crystal symmetry or volume21. In addition to magnetic entropy change, mechanical properties and chemical stability are key issues for the practical use of magnetic refrigerator22. The material will definitely become very brittle and even break into smaller grains if its crystal symmetry or volume is changed very frequently, and consequently the corrosion resistance and the lifetime of a magnetic refrigerator will be deteriorated. Therefore, it is interesting to explore whether the giant MCE can be created by the magnetization vector rotation of single crystal with strong magnetocrystalline anisotropy (MCA), rather than merely via the order-disorder magnetic phase transition or magnetic structural transition.

Although the anisotropic MCE, which was discovered in Ni single crystal more than 70 years ago23, is lower than that from the paramagnetic-ferromagnetic phase transition, it should be large for materials with high values of derivatives of the MCA with respect to temperature24-35. Here, we explore the anisotropic magnetic entropy change of RFeO3 single crystals with R = Tb, Tm or Y. The reasons for choosing RFeO3(R = Tb, Tm, or Y) single crystals are three-fold. Firstly, RFeO3 show a complex magnetic transformation and spin-reorientation transitions36. The magnetoelectric properties and superfast optomagnetic effect of RFeO3 single crystal have been extensively investigated37,38. Unfortunately, the effect of the complex magnetic transformation on MCE is not understood yet. Secondly, the magnetic moments of Tb3+ ion and Tm3+ ion are large, and we can achieve a larger

magnetic entropy change in RFeO3(R = Tb, Tm) single crystal according to the relation of — ASM

= Rln(2/ + 1),

State Key Laboratory of Magnetism and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. Correspondence and requests for materials should be addressed to Z.-H.C. (email: zhcheng@iphy.ac.cn)

0 50 100 150 200 250 300

Figure 1. Zero-field-cooled (ZFC) and field-cooled (FC) thermal magnetization curves. (a) of TbFeO3 along a and c axis; and (b) of TmFeO3 along a and c axis from 2 K to 300 K under a magnetic field of 100 Oe. Insets: thermal magnetization curves along a and c axis of TbFeO3.

where R is the gas constant and J is the total angular momentum of the magnetic ion. Thirdly, the 4f shell of Tb3+, Tm3+ and Y3+ has an oblate, a prolate, and a spherical shape, respectively, a different anisotropy of MCE would be expected between the TbFeO3 and TmFeO3 single crystals on the basis of single-ion-anisotropy model39. The rotating field entropy with different signs, -ASMR = 17.42 J/kg K, and -ASMR = -9.01 J/kg K are achieved at 9 K and 17 K for TbFeO3 and TmFeO3 single crystals from b axis to c axis, respectively. The finding not only advances our understanding of the MCE anisotropy in magnetic single crystals, but also opens a new arena for magnetic refrigerator by rotating its magnetization vector.

X-ray diffraction (XRD) patterns and back-reflection Laue XRD patterns demonstrate that RFeO3(R = Tb, Tm or Y) single crystals have an orthorhombically distorted pervoskite structure with Pbnm symmetry (not shown). Figure 1(a,b) display the zero-field-cooled (ZFC) and field-cooled (FC) thermal magnetization curves along a, c axes from 2 K to 300 K under a magnetic field of 100 Oe for TbFeO3 and TmFeO3 single crystals, respectively. The kink point at 3 K indicated by the arrows in inset of Fig. 1(a) corresponds to the ordering temperature of Tb3+ moments (TNTb). From the inset thermal magnetization curves of a and c axes, two spin-reorientation transitions are observed in the temperature range from 8.5 K to 6 K and 3.5 K to 2.5 K, corresponding to the spin-reorientation of Fe3+ moments from r4(GxAy,Fz) configuration to r2(Fx,Cy,Gz) configuration, and then back to the high temperature configuration r4(Gx,Ay,Fz)36,37,40. From the thermal magnetization curves of a and c axes of TmFeO3 single crystal shown in Fig. 1(b), a spin-reorientation transitions is observed in the temperature range from 85 K to 95 K, corresponding to the spin-reorientation of Fe3+ moments rotate from r4(Gx,Ay,Fz) configuration to r2(Fx,Cy,Gz) configuration36.

Figure 2(a-c) illustrate the isothermal magnetization curves along a, b and c axes of TbFeO3 single crystal in the temperature range of 2-40 K with an interval of 2 K, respectively. The magnetization curves for these three directions are different either in shape or in magnetization values. Data for increasing and decreasing the magnetic field at 2 K to 10 K along all the three directions are given, for better viewing we enlarged the data along b axis in the inset , which demonstrates a little hysteresis loss in the cycling process. A spin-flip phenomenon can be observe along a, b and c axis of the TbFeO3 single crystal at 2 K due to the antiferromagnetic interaction of Tb-Tb ions40,41. Form the data we can see that the easy magnetization direction (EMD) lies in ab plane for TbFeO3 single crystal. The significant difference in the isothermal magnetization curves along a, b and c axis of TbFeO3 single crystal implies that an anisotropic MCE can be expected.

At temperature T, the magnetic entropy change due to applied field H can be calculated from the isothermal curves by the Maxwell relation

rH (dM) H

ASMT H)=J0 (f [dH = ?

t+at t-at

(T + AT) - (T - AT)

where the slope of two adjacent data points is approximatively used for the numerical calculation of the gradient of (BM/BT)H.

By selecting AT = 1 K and AH = 2 kOe, the calculated -ASMvs temperature is shown in Fig. 2(d-f) for fields along a, b and c axis, respectively. A large anisotropy of MCE is observed in TbFeO3 single crystal along ab plane and c axis. The maximum values of -ASM are achieved of 24.05 J/kg K and 20.18 J/kg K in a field of 70 kOe at 11 K along a axis and 9 K along b axis, respectively. The values of -ASM along c axis are smaller than those along a and b axis above the ordering temperature of Tb3+ moments (TNTb ~ 3K). Around TNTb ~ 3K, a field-induced transition from antiferromagnetic configuration of Tb3+ moments to ferromagnetic one results in —A SM = 10.55 J/kg K.

Figure 3(a-c) illustrate the isothermal magnetization curves along a, b and c axis of TmFeO3 single crystal in the temperature range of 2-40 K with an interval of 2 K, respectively. In contrast to TbFeO3 single crystal, TmFeO3 single crystal exhibits a uniaxial magnetic anisotropy with EMD along c-axis. The magnetic entropy

0 10 20 30 40 50 60 70 0 5 10 15 20 25 30 35 40

H (kOe) T (K)

Figure 2. Isothermal magnetization curves and magnetic entropy change of TbFeO3 single crystal.

(a) magnetization curves along a axis, (b) magnetization curves along b axis, (c) magnetization curves along c axis; (d) magnetic entropy change along a axis, (e)magnetic entropy change along b axis, and (f) magnetic entropy change along c axis.

Figure 3. Isothermal magnetization curves and magnetic entropy change of TmFeO3 single crystal. (

a) magnetization curves along a axis, (b) magnetization curves along b axis, (c) magnetization curves along c axis, (d) magnetic entropy change along a axis, (e) magnetic entropy changes along b axis, and (f) magnetic entropy change along c axis.

-5? 2.5

■(a) H//a

■ 40 K

(AT=2 K)

■(b) H//b

■ 2 K

am* laTJU^'ifo5^!

^ ^ ............. 40 K

■ (AT=2 K)

.(c) 2 K

- gf***^0 K

^M***9^ (AT=2 K)

0 10 20 30 40 50 60 70

H (kOe)

YFeCL H//a

70 kOe

OkOe^1^

70 kOe

70 kOe

OkOe (AH=10 kOe)

0 5 10 15 20 25 30 35 40

-0.4^ 0.4^:

0.2 j?

-0.2 2

-0.4^ 0.4 1

Figure 4. Isothermal magnetization curves and magnetic entropy change of YFeO3 single crystal.

(a) magnetization curves along a axis, (b) magnetization curves along b axis, (c) magnetization curves along c axis, (d) magnetic entropy change along a axis, (e) magnetic entropy change along b axis, and (f) magnetic entropy change along c axis.

change —A SM calculated from the isothermal curves using the equation (1) is shown in Fig. 3(d-f) for fields along the a, b and c axis, respectively. The maximum values of —ASM are achieved of 11.93 J/kg K in a field of 70 kOe at 17 K along c axis, whereas —ASM for a and b axes are about one order of magnitude smaller than those along c axis in the whole temperature range.

The anisotropy of magnetic entropy change results from the MCA. In general, the overall MCA of RFeO3 single crystal is the sum of R3+ sublattice anisotropy and Fe3+ sublattice one, as similar with RMnO3 series29. In order to separate the individual contribution from R3+ ion sublattice, we measured the magnetization curves and magnetic entropy change of YFeO3 single crystal for comparison. Since Y ion has non-magnetic moments, and consequently makes no contribution to the overall MCA. Therefore, it affords a separate investigation of the Fe3+ sublattice anisotropy. Isothermal magnetization curves along a, b and c axis of YFeO3 single crystal are shown in Fig. 4(a-c), respectively. The magnetization curves indicate that the magnetic anisotropy among a, b and c axis is not significant. Furthermore, the magnetic entropy change of YFeO3 are nearly zero (Fig. 4(d-f)), suggesting that the anisotropy of magnetic entropy change in TbFeO3 and TmFeO3 single crystals is arisen mainly from the contribution of Tb3+ and Tm3+ ions sublattice anisotropy.

In the first approximation, the MCA constant K1R can be described as42.

Klfr = - 23«A0

< rf > (3jrz

- Jr (Jr

where aJ is the second-order Stevens coefficient, and A20 is the second-order crystalline electrical field (CEF) coefficient. <r4f > is the squared 4f shell radius. JR is the Hund's rules angular moment of R ion.

Since the sign of A20 for orthorhombically distorted pervoskite structure RFeO3(R = Tb, Tm or Y) single crystals is the same and negative43, the easy magnetization directions of these single crystals are governed by the sign of the second-order Stevens factor aj of rare earth ions. The signs of aj for Tb3+ and Tm3+ are negative and positive, respectively. Therefore, the MCA constants K1Tb < 0, and K1Tm > 0, suggesting that the easy magnetization direction of TbFeO3 and TmFeO3 single crystals aligns in ab plane and c axis, respectively. Similar results were also observed in DyFeO3 and ErFeO3 single crystals33,34.

The connection between anisotropic magnetic entropy change and magnetic anisotropy is evident from the field-dependence of —ASM for TbFeO3 single crystal at 9 K and TmFeO3 single crystal at 17 K along different axis (Fig. 5(a,c)). For magnetic refrigeration application, not only a large entropy change value, but also a large refrigeration capacity (RC) is required. RC is defined as

Figure 5. Calculated -ASM and refrigeration capacity (RC). (a) field dependence of magnetic entropy changes of TbFeO3 along a, b and c axis at 9 K, and (b) refrigeration capacity; (c) field dependence of magnetic entropy change of TmFeO3 along a, b and c axis at 17 K, and (d) refrigeration capacity.

where T1 and T2 are the temperatures corresponding to both sides of the half-maximum value of —ASM peak. The RC is the measure of the amount of heat transfer between the cold and hot reservoirs in an ideal refrigerator as a function of field. The field-dependent refrigeration capacity of TbFeO3 and TmFeO3 single crystals is shown in Fig. 5(b) and Fig. 5(d). The three directions manifest obvious anisotropy with values of RC in a field of 70 kOe are 504.8 J/kg, 319.9 J /kg and 11.4 J/kg for a, b and c axes for TbFeO3 single crystal, respectively. For TmFeO3 single crystal, we also see obvious anisotropy with values of RC in a field of 70 kOe are 34.8 J/kg , 47.8 J/kg and 279.2 J/ kg for a, b and c axis. It is interesting that both TmFeO3 single crystal and TbFeO3 single crystal exhibit a strong magnetocrystalline anisotropy between ab plane and c axis, and almost magnetic isotropy in ab plane.

The rotating magnetic entropy change —ASM can be obtained by rotating the crystal from b to c axis and measuring the corresponding isothermal magnetization curves. Figure 6(a,b) indicate the representative isothermal magnetization curves at different angles for temperatures of 8 K and 10 K for TbFeO3 single crystal and of16 K and 18 K for TmFeO3 single crystal, respectively. Taking b axis as the starting angle, we can get the rotating magnetic entropy change —ASMR as a function of angle by using Eq. (1). As is shown in Fig. 7(a,b), the largest values of -A SMR = 17.42 J/kg K can be achieved at temperature of 9 K for TbFeO3 and —A SMR = - 9.01 J/kg K can be achieved at temperature of 17 K for TmFeO3 both under a magnetic field of 50 kOe from b to c axis. Since RFeO3 (R = Tb, Tm) single crystals exhibit almost magnetic isotropy in ab plane and a strong magnetocrystalline anisotropy between ab plane and c axis, Fig. 7(c-d) display the "expected" magnetic entropy change — ASMR. As proposed by Kuz'min and Tishin24, the large and reversible anisotropic magnetic entropy change with broad temperature span suggests that a promising candidate for new type magnetic refrigeration can be achieved by simply rotating the RFeO3 (R = Tb, or Tm) single crystals or magnet.

In conclusion, we investigated the MCE of RFeO3 single crystals among a, b and c axis. The large MCE with broad temperature span and little hysteresis loss is ideal for the application of magnetic refrigeration operated in a wide temperature window. The detailed analysis of magnetization data shows that both TbFeO3 single crystal and TmFeO3 single crystal exhibit a strong magnetocrystalline anisotropy between ab plane and c axis and almost magnetic isotropy in ab plane. Owing to the difference in charge distribution of 4/-elctrons between Tb3+ and Tm3+ ions, the rotating field entropy with different signs, —ASMR = 17.42 J/kg K, and —ASMR = - 9.01 J/ kg K are achieved at 9 K and 17 K for TbFeO3 and TmFeO3 single crystals from b axis to c axis, respectively. This discovery not only gives us a deeper insight into the understanding of the MCE anisotropy in spin canting anti-ferromagnetic single crystal, but also opens a new arena for rotary magnetic refrigerator by rotating its magnetization vector.

Methods

TbFeO3, TmFeO3 and YFeO3 ceramic were prepared with the starting material Tb4O7 (>99.9%), Tm2O3(>99.9%), Y2O3(> 99.9%) and Fe2O3(> 99.9%) with the ratio of stoichiometric. Then, they were pressed into pellets and sintered in air atmosphere for 48 hours using the solid state reaction method at 1250 °C, 1300 °C and 1300 °C.

Figure 6. Representative isothermal magnetization curves at different angles in bc plane. (a) of TbFeO3 single crystal at 8 K and 10 K; (b) of TmFeO3 single crystal at 16 K and 18 K. 0 and 90 correspond to the b and c directions, respectively.

Figure 7. Rotating field entropy changes -ASR(a) from b axis to c axis vs magnetic field. (a) of TbFeO3 single crystal at 9 K; (b) of TmFeO3 single crystal at 17 K; (c) "expected" anisotropy of magnetic entropy change of TbFeO3 single crystal; and (d) "expected" anisotropy of magnetic entropy change of TmFeO3 single crystal.

X-ray diffraction (XRD) patterns showed the prepared samples were single-phase with Pbnm crystallographic symmetry. The ceramics were compressed into rods under the hydrostatic pressure and sintered at 1400 °C for 48 hours. TbFeO3, TmFeO3 and YFeO3 single crystals were grown with four ellipsoidal mirrors (Crystal Systems Inc,

FZ-T-10000-H-VI-VP) by the floating zone method. X-ray diffraction (XRD) patterns were collected by Rigaku D/MAX 2400 x-ray diffractometer with Cu-Ka radiation (X = 1.5406A). Back-reflection Laue x-ray diffraction measurements were carried out to determine the crystallographic direction. Magnetization measurements were performed on commercial superconducting quantum interference device (SQUID) magnetometer (Quantum design MPMS-XL).

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Acknowledgements

This work was supported by the National Basic Research Program of China (973 program, Grant Nos. 2011CB921801, 2012CB933102, 2015CB921403), and the National Natural Sciences Foundation of

China (1117435, 11274360, and 51427801) and by the Wuhan National High Magnetic Field Center(No. PHMFF2015009).

Author Contributions

Z.H.C. designed the experiments. Y.J.K., X.Q.Z. and Y.M. grew the single crystal. Y.J.K. carried out the magnetic entropy changes experiments and calculation. All the co-authors contributed to the analysis and discussion for the results. Z.H.C. wrote the paper with the input from all the co-authors.

Additional Information

Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Ke, Y.-J. et al. Anisotropic magnetic entropy change in RFeO3 single crystals(R=Tb, Tm, or Y). Sci. Rep. 6, 19775; doi: 10.1038/srep19775 (2016).

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