The Magnetoelectric Transport Properties And Its Microscopic Mechanism Of Mn-Co-Al Heusler Alloy Single Crystal Thin Films

With the continuous shrinking of electronic components,traditional semiconductor devices are facing the limits of classical physics,and human beings must develop new technologies to break through the current development bottleneck.Unlike traditional semiconductor devices that only use the charge property of electrons,spintronics also use the spin properties of electrons for information manipulation and storage.Exploring magnetic semiconductor materials with high spin polarization is of great significance for the development of a new generation of spintronic devices with multiple functions,high performance and low power consumption.As a typical spin-gapless semiconductor,the bulk Mn₂Co Al Heusler alloy is reported to have the characteristics of low carrier concentration(10²⁰/cm³),100%spin polarization and high Curie temperature（720 K）.Therefore,it promises broad applications in semiconductor spintronics.However,due to the induced atomic disorder and lattice strain when fabricated into thin films,it may have a significant impact on the spin-gapless semiconductor properties.Studying the magnetoelectric transport properties and its physical mechanism of Mn₂Co Al single crystal thin films under the influence of atomic disorder and strain relaxation can provide a physical basis and experimental guidance for the realization of semiconductor materials with high spin polarizability.On the other hand,the non-collinear magnetic structures with topological properties,such as magnetic skyrmions,can be formed in topological magnetic materials.The study of spin-dependent topological transport phenomena is not only of great value for fundamental research,but also of great significance for the development of a new generation of topological electronic devices.In this thesis,Mn₂Co Al Heusler alloy single crystal thin films were first prepared by molecular beam epitaxy,and the effects of B2 disorder and strain relaxation on their magnetoelectric transport properties were studied.Then,we studied the topological Hall effect in the off-stoichiometric Mn_0.74Co_0.57Al_0.69 single crystal thin films,and the formation mechanism of the topological magnetic structure were clarified.The main contents and innovations are as follows:（1）Based on the first-principles calculations and electrical transport properties measurements,we investigated the effect of B2 disorder on the transport properties of Mn₂Co Al thin films.We found that the B2 disorder destroys the band structure of its spin-gapless semiconductor,making the carrier concentration in the metallic range(10²²/cm³).Meticulous low-temperature resistance and magnetoresistance analyzes reveal that the low-temperature resistance and magnetoresistance of B2-disordered Mn₂Co Al films are dominated by the disorder-enhanced electron-electron interactions.Our results elucidate the physical mechanism underlying the transport properties of B2-ordered Mn₂Co Al single crystal thin films at low temperatures.To achieve the predicted spin-gapless semiconducting properties,the atomic disorder needs to be overcome.（2）The strain relaxation of Mn₂Co Al epitaxial single crystal thin film is controlled by photolithography and ion irradiation.We found that the lattice distortion caused by stress relaxation makes the anomalous Hall conductance of Mn₂Co Al film gradually reverse sign,which can be attributed to the Fermi surface shift generated by lattice distortion.Therefore,the small anomalous Hall conductance itself cannot be regarded as a signature of the spin-gapless semiconductor of Mn₂Co Al.In particular,we found topological-Hall-effect-like signals in strain-relaxed Mn₂Co Al.Careful analysis of the electrical transport properties reveals that these signals originate from the inhomogeneous anomalous Hall effect produced by the strain relaxation effect in the lateral and thickness directions of the film.（3）The topological Hall effect and its physical mechanism in Mn_0.74Co_0.57Al_0.69single crystal thin films were studied.Careful crystal structure analysis and transport measurements reveal that the topological Hall effect is independent of the material’s magnetic inhomogeneity.Further,through the ferromagnetic resonance measurements and micromagnetic simulations,we prove that the topological Hall effect originates from the topological magnetic structures produced by the competition between the tetragonal magnetocrystalline anisotropy and demagnetizing field,such as magnetic skyrmions and bi-merons,.This work helps to elucidate the stabilization mechanism of emerging topological magnetic structures in topological magnetic materials and provides a material platform for studying the interesting magnetic skyrmion dynamics.