Magnetic Interactions And Domain Structures In Rare-Earth-based Magnetic Multilayers

Nd-Fe-B based rare-earth permanent materials have excellent magnetic properties,which are widely used in many fields,such as computer,vehicle,electrical devices and aerospace industry,providing important support for the rapid development of modern society.Recently,with the vigorous development of rare-earth permanent magnet industry,the consumption of Nd and Pr elements also increases.In the meantime,the intrinsic magnetic properties of magnets based on La2Fe14B,Ce2Fe14B and Y2Fe14B are poor,which makes difficulties to meet the actual application requirements,resulting in a large backlog of high abundant rare-earth elements La,Ce and Y.For the purpose of balancing the utilization of rare-earth resources and the long-term development of rare-earth permanent magnet industry,the development of high abundance rare-earth permanent magnets has great economic benefits.The interactions between the magnetic phases can be changed by tailoring the microstructure,so as to improve the magnetic properties of high abundant rare-earth permanent magnets.Rare-earth elements have strong spin orbit coupling and it is easy to tune the magnetization of rare-earth-transition-metal ferrimagnets by changing temperature or composition.These two features make rare-earth elements play an important role in the research of spintronics.In order to promote the application of high abundant rare-earth permanent magnets,especially expand the research of rare earth elements in the study of spintronics,such as spin orbit torque(SOT)and chiral domain wall motion,this paper includes the following two parts:(1)In the traditional Nd-Fe-B system,the coercivity,squareness and temperature stability of high abundant rare-earth permanent magnetic films can be improved by tailoring the microstructure.In this work,the(La,Nd,Dy)-Fe-Co-B single layer and multilayers with Ta spacer-layer were prepared by magnetron sputtering.Combining with the initial magnetization curves,magnetic domain observations by magnetic force microscopy(MFM)and micromagntic analysis,it is found that the coercivity mechanism is a kind of mixed mechanism dominated by nucleation mechanism.The existence of Ta spacer-layer can provide more pinning centers,reduce nucleation centers,weaken the exchange coupling between Nd-Fe-B grains and enhance the long-ranged magnetostatic interactions among different 2:14:1-type grains,leading to the higher coercivity in multilayers.In the perpendicularly magnetized La-Nd-Fe-B/Ta/Co multilayers and disks,it is found that the disk samples have better squareness.By using first order reversal curves(FORC)method and MFM,we study the magnetization reversal behaviors and magnetic interactions.It is determined that the nucleation and propagation of new domains mainly lead to the irreversible magnetization process.The shape-dependent anisotropy of Co disks can enhance the dipole interaction,and then lead to stronger interlayer interaction,which helps to improve the squareness.By comparing the coercivity and remanent magnetic polarization of the structures of multilayer and disk at 300 K and 150 K,it is found that the change of the two values of disk sample is smaller,meaning that the La-Nd-Fe-B(film)/Ta/Co(disk)composite structure has higher temperature coefficient and stability.(2)We prepared Pt/Co/Ho(wedge)multilayers and GdCo/Ru/GdCo synthetic antiferromagnet(SAF)structure by magnetron sputtering and micro-nanofabrication technologies,and systematically studied the Dzyaloshinskii Moriya interaction(DMI)and spin orbit torque(SOT)effect.Based on the method of current-induced shifts of anomalous Hall loops,it is found that both the DMI exchange constant and spin torque efficiency decrease with increasing the thickness of Ho,meaning that Ho has a positive spin Hall angle similar to that of Pt.In the assist of in-plane field,deterministic current-induced magnetization switching from one magnetic state to another magnetic state can be realized by sweeping applied current.If the in-plane field is weaker than DMI effective field,the current-induced asymmetric motion of the Neel-type domain wall with a left-handed chirality can be observed.In GdCo/Ru/GdCo SAF structure,the coupling between two GdCo layers can be tuned by changing the thickness of Ru spacer-layer.The antiferromagnetic coupling between two GdCo layers can significantly enhance the SOT effect.In the strongly ferromagnetically and antiferromagnetically coupled samples,in the case of collaboration between interlayer exchange coupling and SOT effect,the current-induced magnetization switching between two parallel or antiparallel states can be achieved.IEC becomes weak when the thickness of Ru spacer-layer exceeds to 0.57 nm,and the current-induced multi-state switching can be observed.