Tb_(3)NiSi_(2)合金磁相变与磁热性能研究

Magnetic Phase Transition and Magnetocaloric Effect of TbNiSi Alloy

依据X射线衍射(XRD)与等温磁化曲线和等磁场变温磁化曲线,主要研究了Tb_(3)NiSi_(2)合金相结构与磁性相变和磁热性能。XRD表明,采用800℃保温14天,然后炉冷至室温的热处理方法制备的R3NiSi2(R=Tb,Dy,Ho,Er)合金中,主相均为Gd3NiSi2型正交结构(空间群:Pnma,No.62)相,但杂相R5Si3含量存在差异,其规律是从Er到Tb,含量依次减少,Tb_(3)NiSi_(2)合金样品基本为一个单相,其相应晶格常数分别为a=1.1240(8)nm,b=0.41009(8)nm,c=1.12058(1)nm。等温磁化曲线显示在50~300 K温度范围内,Tb_(3)NiSi_(2)合金仅展现出铁磁-顺磁相变,并没有在130,82,66,53 K等观察到相关文献报道的多重的反常反铁磁态-铁磁态(AFM-FM)相变。0.01 T磁场下的磁化强度对温度求导曲线(d M/d T)和0~2 T磁场下的Arrott图结果证实合金铁磁-顺磁二级磁相变居里温度(Tc)=88 K。居里外斯定理拟合表明合金中Tb^(3+)粒子的有效磁矩为9.90μB(μB为玻尔磁子),同期望值μeff/Tb^(3+)=g(J(J+1))1/2=9.72μB基本一致。在磁热性方面,Tb_(3)NiSi_(2)合金在0~2 T磁场范围内,低场响应性较差,铁磁态分子的有效磁矩远低于顺磁分子有效磁矩,最大磁熵变(-ΔSM_(max))为3.2 J·kg^(-1)·K^(-1);在对应的半高宽温跨(δTFWHM)=35.5K范围内,相对制冷量为113 J·kg^(-1)。

In R_(3)NiSi_(2) alloys,all atoms occupy 4C position,which may lead to abundant magnetic phenomena.R3NiSi2 alloys have attracted wide attention of researchers.In this work,the phase formation and crystal structure of R3NiSi2 alloys(R=Tb,Dy,Ho and Er)were investigated using X-ray powder diffraction(XRD),and the magnetic phase transition and magnetocaloric effect(MCE)of Tb_(3)NiSi_(2)alloy were studied based on magnetic susceptibility and magnetization measurements.The room temperature XRD patterns showed that the main phases were Gd_(3)NiSi_(2)-type orthogonal structure phase(space group:Pnma,No.62)in R_(3)NiSi_(2) alloys(R=Tb,Dy,Ho and Er)annealed at 800℃for 14 d and cooled down to room temperature in furnace,but the purity amounts of R5 Si3 phase gradually increased from Tb to Er.At the same time,the Rietveld refinement of the powder XRD pattern showed the crystal parameters a,b and c of R_(3)NiSi_(2) alloys decreased linearly in turn from R=Tb to Er.Because the Tb_(3)NiSi_(2)alloy was almost a single phase with a-1.1240(8)nm,b=0.41009(8)nm and c=1.12058(1)nm.The temperature dependences of DC magnetization of Tb_(3)NiSi_(2)alloy(M-T curve)measured by the zero-field-cooled(ZFC)heating and field-cooled cooling(FC)methods in different magnetic fields showed that there was only a ferromagnetic-paramagnetic transition in the temperature range from 50 to 300 K,and the reported abnormal antiferromagnetic(AFM)-ferromagnetic(FM)phase transitions were observed at 130,82,66 and 53 K,respectively.The derivative curve of magnetiation versus temperature(dM/dT)data in a field of 0.1 T and Arrott plots in a field change of 0-2 T indicated that the magnetic order transition in Tb_(3)NiSi_(2)was a typical second order with Curie temperature(Tc)=88 K.The M-T curve of ZFC measurement mode did not coincide with that of FC measurement mode,especially in 0.01 T magnetic field.This phenomenon was similar to that in TbNi4 Si alloy,and the main source was the competition between AFM and FM in the ground state alloy when the temperature is lower than Tc.With the increasing in magnetic field,the difference decreased.The ZFC and FCM-T curves coincided basically in a 1 T field,which indicated that the AFM state in Tb_(3)NiSi_(2)alloy was easily induced to FM state by magnetic field.The inverses of magnetic susceptibility dependence of temperature in different fields derived from M-T data of ZFC in the high temperature PM region(150~300 K)obeyed the Curie-Weiss law.The fitting result showed the molecule effective magnetic moments of Tb_(3)NiSi_(2)alloy were 17.06 and17.28 in 0.01 and 0.2 T fields.Assuming that the Ni atom was nonmagnetic,the effective magnetic moments of Tb3+ions were 9.85μB and 9.97μB,which were consistent with the expected value 9.72μB.This result confirmed that nonmagnetic or weak magnetic properties of Ni atoms in Tb_(3)NiSi_(2)alloy.In fact,in many R-Ni-Si(R=heavy rare earth elements)ternary alloys,Ni atoms exhibited non-magnetic or weak magnetic properties,but the physical mechanism was still not fully clear.For example,in Gd3 NiSi2,Tb_(3)NiSi_(2),DyNi2 Si,Dy3 Ni8 Si,and Dy3 Ni2 Si4 alloys,the effective magnetic moments of R ions were 7.98μB,9.41μB,10.75μB,10.59μB and 10.73μB,respectively.For studying the magnetocaloric properties,the isothermal magnetization curves of Tb_(3)NiSi_(2)alloy was measured by a continuous method(firstly cooling down from room temperature to 60 K in zero field,and then adding field measurement from 0-2-0 T,following zero field heating to 62 K and then adding field continuous measurement again,repeating this process to 114 K with a temperature interval of 2 K).The isothermal magnetization curves of 60 K under 2 T field showed that the low field response of Tb_(3)NiSi_(2)alloy was poor,and there was no obvious characteristic horizontal line of FM saturation(PM region).The maximum magnetization was 114.9 A·mm·kg-1at 60 K in a field of 2 T and the effective magnetic moment of Tb_(3)NiSi_(2)alloy molecule(12.2μB)was much lower than that of PM molecule 17.10μB.The positive slope of the Arrott plot constructed from the isothermal magnetization curves near TC confirmed the occurrence of a second-order phase transition from FM to FM in Tb_(3)NiSi_(2)alloy.This result also showed no structural phase change or volume mutation during the magnetic transition process.The maximum value of the magnetic entropy change(-ΔS_(max))of Tb_(3)NiSi_(2)alloy calculated by Maxwell equation based on the isothermal magnetization curves near Tc was only-3.2 J·kg^(-1)·K^(-1) in a field change 0-2 T.This value was almost the same as-ΔSmax of TbNi4 Si and Gd3 NiSi2 under 0-2 T field,but lower than that of GdNi4 Si alloy(6.6 J·kg^(-1)·K^(-1))in R-Ni-Si alloys with FM-PM transition.In addition,the relative cooling power(RCP)of Tb_(3)NiSi_(2)alloy was about 113 J·kg-1 with 35.5 K temperature half-high width(TFWHM)of maximum magnetic entropy,which was much lower than that of Gd3 NiSi2 alloy at 200 J·kg^(-1) under the same conditions.