Anisotropic magnetocaloric effect in HoAlGa polycrystalline compound

In this work, a nonnegligible anisotropic magnetocaloric effect (MCE) in HoAlGa polycrystalline compounds has been observed. With temperature increasing, the HoAlGa compound undergoes two kinds of magnetic transitions at 19 K and 31 K, respectively, the later has been recognized as an ordinary antiferromagnetic to paramagnetic (AFM-PM) transition. The -ΔS peak of HoAGa reaches 5.4 J/kg K and 1.5 J/kg K at 35 K along parallel and perpendicular texture directions respectively, for a field change of 0-5 T. The result indicates that the HoAlGa polycrystalline compounds with excellent anisotropic MCE can be expected to have effective magnetic refrigeration applications in low temperature range.In this work, a nonnegligible anisotropic magnetocaloric effect (MCE) in HoAlGa polycrystalline compounds has been observed. With temperature increasing, the HoAlGa compound undergoes two kinds of magnetic transitions at 19 K and 31 K, respectively, the later has been recognized as an ordinary antiferromagnetic to paramagnetic (AFM-PM) transition. The -ΔS peak of HoAGa reaches 5.4 J/kg K and 1.5 J/kg K at 35 K along parallel and perpendicular texture directions respectively, for a field change of 0-5 T. The result indicates that the HoAlGa polycrystalline compounds with excellent anisotropic MCE can be expected to have effective magnetic refrigeration applications in low temperature range.


I. INTRODUCTION
It is well known that magnetic materials impact almost every aspect in our life from household appliances to aerospace field.In recent years, magnetic refrigeration based on the magnetocaloric effect (MCE) has been demonstrated to be a new-style application of functional magnetic materials.][3] The research and development of magnetic refrigeration utilized at low temperature is very important for the fuel industry and space science. 4Large anisotropic magnetocaloric effect (MCE) has been acquired in some single crystals, such as ErGa 2 and HoGa 2 , by taking the different entropy change from easy and hard magnetization directions. 5This idea was first used to study a singlecrystal of DyAlO 3 by Kuzmin and Tishin. 6][9][10] However, the high cost and complexity of single-crystals growth baffle the widely applications of this kind of magnetic refrigeration model.Zhang Hu et.observed giant rotating MCE in textured DyNiSi polycrystalline material in 2015, for the first time. 113][14] Complex field-temperature magnetic phase diagrams of RGa 2 below T N , characterized by additional transitions, and multistep metal-magnetic processes at low temperature have been reported detailedly.
a Corresponding author Electronic mail: shenbaogen@yeah.net2158-3226/2018/8(5)/056427/5 8, 056427-1 © Author(s) 2018 Considering the highly preferred crystallographic orientation, a large magnetic anisotropy along different crystalline orientation can be expected in this textured HoAlGa compound.In this work, a nonnegligible anisotropic MCE in HoAlGa polycrystalline compound has been observed due to the texture structure.

II. EXPERIMENTAL DETAILS
Polycrystalline HoAlGa compound was synthesized by arc-melting the stoichiometric mixture of constituent elements Ho, Al, and Ga with high-purity under purified argon atmosphere.3 at% excessive rare earths were added to compensate the weight loss during the arc-melting.These ingots have been re-melted several times to obtain their homogeneity.Then these samples were annealed in a quartz tube filled with high-purity argon atmosphere for one week at 900 K. Phase purity and crystal structure of annealed samples were checked by Powder X-ray diffraction (XRD) using Cu Ka radiation at room temperature.Magnetizations were carried out on a commercial MPMS SQUID VSM magnetometer (Quantum Design).

III. RESULTS AND DISCUSSION
X-ray diffraction patterns of HoAlGa polycrystalline samples were obtained at room temperature.Almost all the diffraction peaks can be indexed to a hexagonal AlB 2 -type structure (Space group P6/mmm), as shown in Fig. 1.The lattice parameters are determined to be a=4.435Å, b=4.435Å, and c=3.545Å.It has been reported that R(AlxGa 1-x ) 2 (R=rare earth) compounds crystallize in the simple AlB 2 -type hexagonal structure.3][14] The peak position at 31K has been recognized as an ordinary antiferromagnetic to paramagnetic (AFM-PM) transition.The ZFC and FC curves are in good consistent with each other, suggesting an excellent thermal reversibility.The inverse susceptibility 1/χ as a function of temperature under 0.01 T is plotted in the inset of Fig. 2. The effective magnetic moment is estimated to be 11.01 µ B , by simulating the inversed susceptibility vs temperature curve to the Curie-Weiss Law at temperature above 50 K, which is close to the free Ho 3+ ion value (10.6 µB).
The magnetization isotherms taken below T N temperature from 0 T to 7 T field are shown in Fig. 3.The significant difference of the slope along parallel and perpendicular texture directions is expected to result in a large anisotropy of MCE. Figure 3(b) shows schematic for the formation of texture structure in HoAlGa compound during arc-melting.The formation of texture structure is due to the large temperature gradient between the bottom and top of button during the arc-melting.6][17] It is also expected to obtain large anisotropic MCE by rotating the HoAlGa polycrystal sample from perpendicular (90 • ) to parallel (0 • ) direction.Along parallel direction, the magnetization curve at 10K shows a linear increase and then a sudden jump with increasing magnetic field, indicating the metamagnetic transition from AFM to FM phase, as shown in figure 3(a).However, there is no magnetic phase change induced by magnetic field along perpendicular direction", suggesting a strong antiferromagnetic coupling.
Magnetic entropy changes of HoAlGa compound in different magnetic fields were calculated using the Maxwell relation ∆S M = ∫  of ∆S M were found near 25 K, as shown in Fig. 4, corresponding to the AFM nature of HoAlGa in temperature range from 19 K to 31 K.It is obvious that negative values were also appearing in the latter temperature range, which is reported by neutron diffraction experiments and Quantitative analysis. 13,14However, we don't observe any abnormality at M-T curves mentioned above.A. R. Ball et.thought it a transition toward the ferromagnetic state.Between 18.5 K and 6 K, the intermediate field-induced phase lead to a two-step metamagnetic process.The maximum values of ∆S M reach 5.5 J/kg K along parallel texture directions, twice as large as the value of 2 J/kg K along perpendicular texture direction, for the field changes of 0-7 T. The D-value can be compared with the difference between magnetic field applied parallel to the c-axis (a and c) and perpendicular to the c-axis of RGa 2 (R=Ho, Er) single crystal. 5Above all, a strong magnetic anisotropy for HoAlGa polycrystalline compounds has been observed.

IV. CONCLUSION
AlB2-type hexagonal polycrystalline HoAlGa compound has been synthesized with texture structure in it.With the increasing of magnetic field, HoAlGa compound undergoes two magnetic transitions at 19 K and 31 K, respectively, of which the latter one has been proved to be AFM-PM type magnetic transition.Obvious anisotropy on magnetizing process and magnetic entropy changing was observed due to the strong uniaxial magneto crystalline anisotropy and highly texture structure in HoAlGa compound.The maximum values of -∆S M for 0-7 T were calculated to be 5.5 and 2 J/kgK along texture-parallel direction and texture-perpendicular direction, respectively.Large magnetic anisotropy of textured polycrystalline HoAlGa compound indicates the underlying applications on rotating magnetic refrigeration.

FIG. 1 .
FIG. 1. X-ray diffraction patterns of HoAlGa polycrystalline samples at room temperature.The short vertical lines indicate the angular positions of the Bragg peaks of HoAlGa.
FIG. 3. (a) Magnetization isotherms below T N along parallel and perpendicular texture directions, respectively.(b) The schematic describes the rotation of sample with texture structure forming during arc-melting from perpendicular (90 • ) to parallel (0 • ) direction in magnetic field.