Crystal Growth,Magnetic Properties And Magnetoresistance Effect Of Rare-Earth Nickel-Based Alloys

The rare-earth(R)-transition metal(T)intermetallic compounds have always attracted a great deal of interest in fundamental properties and practical applications.Unique magnetic properties of these compounds are related to the localized magnetic moments,originating from the rare-earth 4f electrons.In the R-T intermetallics,the crystal electric field(CEF)and the Rudermen-Kittel-Kasuya-Yosida(RKKY)interaction are dominant,which give rise to a variety of properties,like complex magnetic structures,metamagnetism,magnetocaloric effect(MCE),magnetoresistance(MR),magnetostriction and so on.Great attentions recently have been paid to the materials with magnetocaloric effect in consequence of the potential application in efficient and friendly cooling technologies.The R-T compounds came to the limelight recently in view of the large magnetocaloric effect and magnetoresistance.Meanwhile,due to the large magneto-crystalline anisotropy,the magnetocaloric effect is anisotropic in single crystals and highly textured polycrystalline of R-T compounds.By simply rotating the materials in a certain field,a caloric effect termed as rotating magnetocaloric effect(RMCE)can be acquired.In this thesis,RNi₃Al₉,RNiGe₃,and ErNi single crystals have been grown by flux method.The anisotropic magnetocaloric effect and rotating magnetocaloric effect of these single crystals have been studied to explore novel low temperature magnetic refrigeration materials.The anisotropic magnetoresistance(MR)effects of single crystals have been systematically studied,and the relationship between the MR and the complex magnetic structures was discussed.The main contents and results are as follows:Firstly,the anisotropic magnetocaloric effect and magnetoresistance have been investigated in RNiGe₃(R=Ho and Er)single crystals grown by Ge self-flux method.It is found a transition from antiferromagnetic to paramagnetic state at 10.5 and 6 K for HoNiGe₃ and ErNiGe₃,respectively.The easy magnetization direction is the a axis for HoNiGe₃ and the c axis for ErNiGe₃.With the field increasing,RNiGe₃ undergoes the metamagnetic transition induced by the magnetic field along the easy magnetization axis below Néel temperature,while the spin-flop transition occurs for the field along the other orientations.The maximum magnetic entropy changes are-16.2 and-17.2 J/kg K under the magnetic field change of 0-80 kOe for HoNiGe₃ and ErNiGe₃,respectively.The maximum rotating magnetic entropy changes are-12.13 and-12.5 J/kg K under 50 kOe.Besides,anisotropic transport properties have been observed in HoNiGe₃ single crystal,which are closely related to spin-flip or spin-flop transitions.Secondly,we present a detailed study on the magnetic properties and rotating magnetocaloric effect of RNi₃Al₉(R=Ho,Er and Tm)single crystals grown by Al self-flux method.In HoNi₃Al₉,the easy magnetization direction is along the ab plane,and no obvious CEF effect is observed.The rotating magnetocaloric effect in HoNi₃Al₉ is smaller than its magnetocaloric effect.In ErNi₃Al₉ and TmNi₃Al₉,the easy magnetization direction is along the c axis.Due to the CEF effect,positive magnetic entropy change is obtained above the T_N for the field along the ab plane.Therefore,a giant rotating magnetocaloric effect is obtained in ErNi₃Al₉,which is even larger than the conventional one,suggesting that the CEF effect should have a significant effect on RMCE.The maximum rotating magnetic entropy change is 14.7 J/kg K under 50 kOe for ErNi₃Al₉.The giant rotating adiabatic temperature change is obtained in TmNi₃Al₉,reaching 20 K at 5 K under 50 kOe.Besides,the magnetoresistance of TmNi₃Al₉ single crystal has been studied.Large positive and negative magnetoresistance are obtained at low temperature.Thirdly,we have synthesized ErNi single crystal by the Er-Ni self-flux method,and the magnetic and transport properties have been studied in detail.With decreasing temperature,ErNi single crystal undergoes two successive magnetic transitions:a paramagnetic to ferromagnetic transition at T_C=11 K and a spin-reorientation transition at T_SR=5 K.Meanwhile,a sharp field-induced metamagnetic transition is observed below the T_C along the a axis.Giant magnetocaloric effect is obtained along the three crystalline directions at T_C.The maximum magnetic entropy change is-36.1 J/kg K along the a axis under the field change of 0-50 kOe.Meanwhile,a large rotating magnetocaloric effect is acquired under a relatively low constant magnetic field(9.3 J/kg K under13 kOe).Thus,ErNi single crystal could be an attractive candidate for application in magnetic refrigeration.Besides,a large negative magnetoresistance is also observed around transition temperatures.A considerable large magnetoresistance with the value of-34.5%is acquired at 8 K under 50 kOe when the field is along the a axis.