In R3NiSi2 alloys, all atoms occupy 4C position, which may lead to abundant magnetic phenomena. R3NiSi2 alloys have at-tracted 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 Tb3NiSi2 alloy were studied based on magnetic susceptibility and magnetization measurements. The room temperature XRD patterns showed that the main phases were Gd3NiSi2-type orthogonal structure phase (space group: Pnma, No.62) in R3NiSi2 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 R5Si3 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 R3NiSi2 alloys decreased linearly in turn from R=Tb to Er. Because the Tb3NiSi2 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 Tb3NiSi2 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 Tb3NiSi2 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 TbNi4Si 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 FC M-T curves coincided basically in a 1 T field, which indicated that the AFM state in Tb3NiSi2 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 Tb3NiSi2 alloy were 17.06 and 17.28 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 Tb3NiSi2 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 Gd3NiSi2, Tb3NiSi2, DyNi2Si, Dy3Ni8Si, and Dy3Ni2Si4 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 Tb3NiSi2 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 Tb3NiSi2 alloy was poor, and there was no obvious characteristic horizontal line of FM saturation (PM region). The maximum magnetization was 114.9 A•m2•kg-1 at 60 K in a field of 2 T and the effective magnetic moment of Tb3NiSi2 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 Tb3NiSi2 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 (-ΔSmax) of Tb3NiSi2 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 TbNi4Si and Gd3NiSi2 under 0-2 T field, but lower than that of GdNi4Si 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 Tb3NiSi2 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 Gd3NiSi2 alloy at 200 J•kg-1 under the same conditions. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.
