Study On The Triggering And Mechanism Of Spontaneous Exchange Bias In MnNiSnTi And NiMnInAl Heusler Alloys

As a new type of multifunctional magnetic materials,Heusler alloy has been paid more and more attention by researchers.Due to its excellent and rich physical effects,it has been playing a more and more important role in aerospace,medical,communications,new energy and other fields.Exchange bias effect in Heusler alloys becomes a research hotspot at present,due to its potential application in spin valve,magnetic recording and permanent magnet.Compared with the conventional exchange bias,the spontaneous exchange bias(SEB)effect does not require cooling with external magnetic field or preparation under external magnetic field,making the application of exchange bias effect more extensive and convenient.Therefore,the research on SEB will be a new and meaningful work.SEB in Heusler alloys is systematically studied in this thesis.The precursor ingots of MnsoNi40Sn10-xTix(x=0-4)and Ni50Mn34In15-xAlx(x=0-15)were prepared by arc melting under an argon atmosphere.In order to ensure the homogeneousness of the samples,each sample was repeatedly melted three times.All samples were annealed at 1123 K under high vacuum for 72 h.The crystal structure was characterized by X-ray diffraction(XRD).The magnetic measurements including M(H),M(T)and the AC susceptibility χ’(T),were performed using a physical property measurement system(PPMS-9,Quantum Design,Inc.).Two conclusions are drawn,as follows:1.In order to obtain the conditions of triggering SEB in Heusler alloys,the data of several typical systems in Heusler family are collected and analyzed.It is found that the molecular magnetic moment(Ms)is no larger than 1μB/f.u.in typical SEB systems.Based on this commonality,a method is put forw ard by us to trigger SEB in the materials:tuning the relative strength of the ferromagnetic(FM)and the antiferromagnetic(AFM)interactions.In order to verify this view,MnsoNi40Sn10 with stronger FM interaction and with no SEB effect is chosen as the mother composition,and Sn is substituted with valence-isoelectronic Ti to enhance AFM interaction.It is found that AFM interaction is continuously enhanced with the increase in the Ti content,and SEB is generated and regulated.2.SEB is closely related to the magnetic properties of the samples.In order to successfully control and explain its mechanism,we choose the Ni50Mn35In15 alloy with stronger FM interaction and no SEB effect as the research platform.Al is substituted for In,ensuring that the valence electron number remains unchanged.A series of Ni50Mn35In15-xAlx(x=0-15)alloys is obtained.The substitution of Al in the system introduces the negative chemical pressure,which makes the cell volume decrease.With the increase of Al content,the interaction of AFM increases,and the spontaneous switching bias field(HSEB)is continuously adjustable from 0 to 1456 Oe.