Studies On The Growth And Properties Of Magnetic Topological Materials

Topological insulators and topological semi-metals that can be described by topological invariants are the focus of recent research.It is found that if the material has intrinsic magnetism,or through other methods such as magnetic doping and magnetic proximity,the ideal magnetic topological material can be obtained,which has many exotic physical properties.such as negative magnetoresistance,anomalous Hall effect,anomalous Nernst effect,and non-dissipative edge states,driving the development of low-energy devices.A giant nonsaturating magnetoresistance was observed at low temperatures,reaching～7×10⁴%at 2 K in a magnetic field of 9 T in Eu Ga₄.In the antiferromagnetic（AFM）state,Eu Ga₄ undergoes a series of metamagnetic transitions in an applied magnetic field,clearly manifested in its field-dependent electrical resistivity.Below T_N,in the～4–7 T field range,we observe also a clear hump-like anomaly in the Hall resistivity which is part of the anomalous Hall resistivity.We attribute such a hump-like feature to the topological Hall effect,usually occurring in noncentrosymmetric materials known to host topological spin textures（as e.g.,magnetic skyrmions）.In addition,the magnetic properties of Eu Ga₄ have been studied by means of by means ofR spectroscopy,and strong spin fluctuations have been found in Eu Ga₄.Therefore,the family of materials with a tetragonal Ba Al₄-type structure,to which Eu Ga₄ and Eu Al₄ belong,seems to comprise suitable candidates on which one can study the interplay among correlated-electron phenomena（such as charge-density wave or exotic magnetism）with topological spin textures and topologically nontrivial bands.Materials with a colossal magnetoresistance effect（CM）have broad application prospects in spintronics devices,magnetic sensors and hard disk drives.We measured the transport properties of magnetic Weyl semi-metal Eu Cd₂P₂ and observed colossal magnetoresistance（10⁴%）in both in-plane and out-of-plane directions.Eu Cd₂P₂ does not have manganese,oxygen,a mixed valence,a double-exchange（DE）interaction,a perovskite structure,or a Jahn–Teller JT distortion.It has an AFM order at low temperature（T_N=11 K）,instead of the ferromagnetic（FM）order at high temperature as seen in manganates.The appearance of Eu Cd₂P₂ has broken our understanding of traditional CM materials.We plan to further analyze the magnetic order and the relationship between spin fluctuation and magnetoresistance in Eu Cd₂P₂ withR spectroscopy,so as to explain the mechanism of the colossal magnetoresistance effect in Eu Cd₂P₂.Non-collinear antiferromagnetic D0₁₉-Mn₃X（X=Ga,Ge,Rn,etc.）with kagome structure is another typical class of magnetic topological materials,whose momentum space shows nonzero Berry curvature acting as an effective magnetic field.Taking D0₁₉-Mn3Ge as the starting point,we have grown three kinds of Manganese-germanium films with different magnetic structures by adjusting the manganese-germanium ratio in epitaxial films.Firstly,the transformation temperature of ferromagnetic film Mn₅Ge₃ is near room temperature,and the orientation of the epitaxial film is changed by adjusting the growth temperature.By increasing the proportion of Mn in the films,the low-temperature ferromagnetic films D0₂₂-Mn₃Ge and D0₁₉-Mn₃Ge films with noncollinear antiferromagnetic structure（polycrystalline films containing low-temperature Mn₃Ge）were epitaxial grown.Our exploration of the growth conditions of manganese-germanium thin films will pave the way for the subsequent research and application of spintronics devices.