Control Of Spin Order And Magnetotransport By Dimensionality And Domains In SrRuO₃ Films

Over the past few decades,silicon-based semiconductor chips have maintained an astonishing rate of iteration,until in recent years,when the iteration rate began to slow down after the process entered the 3 nm node.The main challenge comes from the enhanced quantum effect after the chip size is reduced.Therefore,the need to find alternative materials for silicon-based semiconductors is increasingly urgent.At this time,people once again turn their attention to transition metal oxides because they have strong correlation interactions and a variety of degrees of freedom that can be adjusted.Actually,the spintronics research on transition metal oxides has made remarkable progress,discovering many new quantum states that do not exist in conventional metals/alloys and semiconductors,and is expected to play an important role in the field of information storage.As a very important transition metal oxide,SrRuO3 is not only widely used as the bottom electrode of capacitors and tunnel junctions because of its chemical inertness,but also one of the foundmental materials in the field of anomalous Hall effect research.In recent years,the control of the Berry curvature of SrRuO3 in real space and k-space has become a hot topic in the field of condensed matter.The discovery of the topological Hall effect in SrRuO3 reveals the existence of the magnetic skyrmions which is expected to become a new type of memory cell as a granular spin magnetic structure can be independent manipulated and arbitrary arranged.However,due to the special band structure of SrRuO3,it is very sensitive to various order parameters(such as temperature,thickness and chemical defects),resulting in inhomogenous spatial distribution of magnetic structure in the film,which interferes with the detection and even formation of magnetic skyrmions.In addition,just as the decrease of dimensionality in siliconbased semiconductors will introduce stronger and stronger quantum effects and greatly increase the difficulty of manufacturing,the decrease of dimensionality in SrRuO3 films will also lead to dead-layer behavior and inhibit its ferromagnetism and metallicity.In order to realize the application of SrRuO3-based memory devices,it is a challenge to explore the influence of dead-layer behavior and magnetic structure inhomogeneity on the spin order and magnetic transport of SrRuO3.To address this challenge,our thesis has focused on the following work:1.In this thesis,the physical origin of dead layer behavior in SrRuO3 thin films was investigated by magnetic transport measurements and first-principles calculations.In this chapter,ultrathin SrRuO3 films were grown with a thickness down to the monolayer limit.Transport and magnetic characterizations show that the films remain ferromagnetism metallic down to 4 unit cell(u.c.),but the 3 and 2 u.c.films become antiferromagnetism insulating with<110>magnetic easy axes.Density functional theory calculations further elucidate these experimental findings and reveal that the orbitalselective quantum confinement effect and out-of-plane OOR lead to the reconstruction of Ru 4d orbitals,resulting in an AFM insulating state and reorienting Ru magnetic moment.Furthermore,this chapter excludes the possible role of structural defects in driving the magnetic phase transition by controlling the oxygen deficiency and Ru/Ti intermixing in the films.These findings provide insights in how quantum confinement tailors the metal-insulator transition and magnetic states in complex oxide heterostructures.2.This thesis demonstrates that scanning the minor loops of the Hall resistivity is a simple way to quantitatively distinguish the inhomogeneous AHE from the THE.Ultrathin SrRuO3 films with thickness from 5 to 15 u.c.were grown on SrTiO3 substrates and comprehensive Hall measurements on these samples were performed.We have observed both types of Hall humps as mentioned above in the SrRuO3 films without any post-treatments.Furthermore,by scanning the minor loops of the Hall resistivity,it is revealed that the AHE anomalies is caused by multi-channel AHE due to the thickness inhomogeneities,rather than by THE.This part of work is mainly shown in the fourth chapter of this paper.3.In this thesis,the non-defect magnetic structural inhomogeneity caused by the twin domains in SrRuO3 and its significant influence on the magnetic anisotropy and AHE are deeply studied.In this chapter,SrRuO3 thin films with single-domain and double-domain structures were grown on substrates with miscut angles of 0.3° and 0.03°,respectively.The magnetic anisotropy of single-domain and double-domain SrRuO3 films was analyzed by characterizing the angle dependence of Hall resistivity,and it was found that the direction of the magnetic anisotropy axis corresponds to the phase structure.In this chapter,a very significant signal similar to the THE was observed in the Hall resistivity curve of a thick single SrRuO3 film in an external magnetic field in a specific angle range.Transport and magnetic force microscope measurements visually confirm that the signal similar to the THE in the two-domain film is caused by the two-channel AHE induced by the misaligned dual magnetic hard axes in the film,which is a very interesting two-channel AHE induced by non-chemical defects.This part of work is mainly shown in the fifth chapter of this paper.