Spin Transport In Cr₂O₃ Based Antiferromagnetic Heterostructures

Antiferromagnetic spintronics can be described simply as the spintronics phenomenon in antiferromagnetic materials.This research field has attracted much attention and a number of spintronics phenomena have been found in antiferromagnetic materials.It is known that Cr2O3 is the earliest magnetoelectric coupling material,it is also a G-type antiferromagnetic material and its Neel temperature is approximately 307K,slightly higher than the room temperature.So far,few attempts have been made about Cr2O3 in the field of spintronics,though the unique antiferromagnetic structure of Cr2O3 deserves further exploration and study.In this thesis,Cr2O3 thin films with different magnetic structure characteristics are obtained by changing the growth method.On this basis,we combine them with non-magnetic heavy metals to explore the generation mechanism of spin Hall magnetoresistance(SMR),and research the internal relationship by comparing SMR with anomalous Hall effect(AHE).Finally,we construct Cr2O3 based perpendicular magnetic anisotropy(PMA)systems and study the influence of Cr2O3 with different magnetic structures on the spin-orbit torque(SOT),as a result,a SOT without the assistance of the external magnetic field is obtained successfully obtained.The main research contents of my thesis are listed below:(1)The(0001)-oriented Cr2O3 thin films were epitaxial grown by pulsed laser deposition(PLD),and the antiferromagnetism of Cr2O3 was confirmed by the measurements of structure and magnetic characterization.In addition,the Cr2O3/W heterostructure was prepared,and the magnetoresistence ratio of nearly 0.1%by electrical test was obtained.According to the signal characteristics of the angular-dependence magnetoresistence,the influence of anisotropic magnetoresistence (AMR)can be excluded,confirming the existence of the SMR in the Cr2O3/W system.Finally,through comparing Cr2O3 films with different roughness,the influence of interface on the SMR was confirmed.(2)The epitaxial Cr2O3/Ta heterostructures by PLD and magnetron sputtering were prepared,and the SMR and AHE in this system were researched.By comparing the variation trend of the two effects with temperature,as well as according to their respective physics mechanisms,the spin accumulation generated by the AHE at the interface was believed to contributes to the signal of the SMR in the form of a spin current.On the other hand,through changing the types of non-magnetic heavy metals attached with Cr2O3,the intrinsic relationship between SMR and AHE was revealed again.(3)Amorphous Cr2O3 film with TaOx buffered was prepared,and the existence of amorphous phase was confirmed by X-ray diffraction.The differences of the SMR signals between the amorphous and epitaxial Cr2O3/Ta heterostructures were compared and studied.According to the analysis of Cr2O3 with different magnetic structures,the cause of the reversed SMR signal can be explained by the competition mechanism of the spin current in different magnetic regions of epitaxial Cr2O3.While in the amorphous Cr2O3/Ta heterostructure,the variation of antiferromagnetic coupling strength and the occurrence of magnetic frustration at low temperature are responsible for the two changes of SMR sign.(4)Cr2O3/Pt/Co/Pt heterostructures were prepared by means of PLD and magnetron sputtering.Three different Cr2O3structures were obtained by changing the crystal orientation of substrate Al2O3 and inserting TaOx buffer layer.Firstly,the crystal structures of the three structures were confirmed by X-ray diffraction,and the magnetic moment distribution of Cr2O3 were described.Then the perpendicular magnetic anisotropy of several structures by the magnetic hysteresis loop and Hall curve tests were compared.Next,the measurements of SOT were carried out and a nearly 70%field-free magnetization switching driven by spin Hall effect in(1120)-oriented epitaxial Cr2O3 heterostructure was obtained.Finally,the effective field of SOT and the effective spin Hall angle were calculated by harmonic measurements,and the origin of the field-free SOT was discussed.