The Modulation Of Phase Transitions And Magnetism In Heusler Alloys

Ni-Mn-based ferromagnetic shape memory alloys have exhibited complex physical properties due to the martensitic transformation and the coupling between structure, magnetism, and electricity. These alloys have been found to demonstrate inverse magnetocaloric effect, magnetoresistance effect, metamagnetic shape memory effect, exchange bias, and baracaloric effect. These phenomena make these alloys have abundant physics and potential technological applications, such magnetic refrigerator, magnetic sensor, and magnetic drivers. In this paper, we mainly focus on the modulation of phase transitions and magnetism in Ni-Mn-based Heusler alloys, and studied their magnetocaloric effect and exchange bias effect. The article is mainly divided into three parts:?1? Intermartensitic Transformation and Enhanced Exchange Bias in Pd?Pt?-doped Ni-Mn-Sn alloysWe studied the phase transitions and exchange bias of Ni_50-x Mn₃₆Sn₁₄Tx?T= Pd, Pt; x=0, 1, 2, 3? alloys. An intermartensitic transition?IMT?, not observed in Ni₅₀Mn₃₆Sn₁₄ alloy, was induced by the proper application of negative chemical pressure by Pd?Pt? doping in Ni_50-x Mn₃₆Sn₁₄Tx?T=Pd, Pt? alloys. IMT weakened and was suppressed with the increase of applied field; it also disappeared with further increase of Pd?Pt? content?x=3 for Pd and x=2 for Pt?. Another striking result is the enhancement of exchange bias effect by Pd?Pt? addition. This phenomena may be mainly due to the increase of unidirectional anisotropy associated with the stronger spin-orbital coupling of Pd?Pt? atoms than Ni.?2?The phase transitions, magnetocaloric effect in Mn-rich Mn₅₀Ni_50-x Sn_x alloysThe magnetism in Ni-Mn-based Heusler alloys is mainly from Mn, therefore the increase of Mn content can raise the magnetism in these alloy. A series of Mn₅₀Ni_50-x Sn_x?x=5-25? alloys were prepared by arc melting method, and their phase transitions and magnetocaloric effect were investigated. With the increase of Sn content, the martensitic transformation temperature decrease gradually. The alloys with x?15 show spin glass behavior at low temperature, while for x>15, the alloys exhibit typical ferromagnetic behavior. The alloy with x=9.5 undergoes a transition from weak-magnetic martensitic to ferromagnetic austenitic phase transition, resulting a large magnetocaloric effect.?3? Large low-field magnetic entropy changes in as-cast Hf_0.83-x Zr_x Ta_0.17Fe₂ compoundsSeries of Hf_0.83-x Zr_x Ta_0.17Fe₂?x=0, 0.1, 0.2, 0.3? compounds have been prepared by arc-melting method without further annealing. As-cast samples crystallize in single phase with hexagonal Mg Zn^2-type structure, and exhibit a first-order transition from ferromagnetic to antiferromagnetic state for x<0.3. With the increase of Zr content?x<0.3?, the transition shifts towards lower temperature with sharper magnetization change and enhanced magnetization. Field-induced metamagnetic transition from antiferromagnetic to ferromagnetic state was observed around the transition, giving rise to the large magnetocaloric effect. The adjustable transition temperature and large magnetocaloric effect make the as-cast Hf_0.83-x Zr_x Ta_0.17Fe₂ compounds potential candidate as magnetic refrigerants.