Magnetic Configuration And Transport Properties Of Hexagonal MnMx And Fe₃GeTe₂ Alloys

Unique magnetic structures,magnetic domain structures,topological electronic structures and related magnetic transport effects in materials,such as magnetic skyrmions and topologic Wely semimetals,have attracted considerable research interests in condensed matter physics due to their novel physical states and significant potential in the application of spintronics.In this paper,we focus on the hexagonal MnMX alloys hosting skyrmions and two-dimensional ferromagnetic Fe₃GeTe₂ semimetal with topological nodal line.On the one hand,the formation mechanism of peculiar noncollinear magnetic structure is explored by theoretical calculation,providing theoretical guidance for the tune of magnetic and transport properties.On the other hand,the transport behavior under the magnetic field is studied by experimental measurements,and the magnetic or transport effect is tuned by means such as composition changing.The main results are as follows:（1）Since the discovery of biskyrmion states in a wide temperature range in the Ni₂In-type MnNiGa alloy,the formation mechanism of the skyrmion state in centrosymmetric magnets is still need to be clarified.Therefore,in this work,we systematically investigate the magnetic configuration,magnetic domains and transport properties in MnNiGa and related Ni₂In-type MnMX alloysby first principles calculation and experimental characterization.（i）First-principles calculations were carried out for the magnetic ground states of hexagonal MnNiGa,MnFeGe,and MnCoSn with typical Ni₂In structure.We found the minimum energy difference of ferromagnetic and ferrimagnetic states and identified that the shortest Mn-Mn distance d₁ along the c axis plays an important role in determining the magnetic ground state.The calculations of the exchange parameters J_ij clearly reveal the competing FM and AFM interaction along the c axis and the calculation of the stability of the canted magnetic structure directly verified the existence of noncollinear magnetic state.Moreover,the experimental characterization on MnFeGe_1-xSn_x（x=0,0.05 and 0.1）alloys also indicates the possible noncollinearity of the magnetic structure and its tunability.（ii）Magnetic and transport properties of Ni₂In-type structure Mn_1+xCu_1-xGa（x=0.3,0.4,0.5）alloys are systematically investigated.A cluster spin-glass phase is identified below the Curie temperature.Large exchange bias effect and coercivity are established in Mn_1.5Cu_0.5Ga samples,which results from the exchange interaction between Mn-Mn clusters.The exchange bias effect and coercivity,as well as the anomalous Hall effect can be greatly tuned via field cooling process,making them show great potential in the application in rare-earth-free magnets and spintronic devices.（iii）We investigated the variation of spin reorientation transition and magnetocrystalline anisotropy by the introduction of nonmagnetic Cu into Ni site in MnNiGa.By composition changing,the Curie temperature,spin reorientation transition temperature and magnetocrystalline anisotropy field were tuned simlutaneously.Lorentz transmission electron microscopy revealed the formation of stacked biskyrmions in x=0-0.3 samples.The conditions for forming biskyrmions were analyzed combined with the uniaxial anisotropy.Our work further offered a guideline to tune the topological structure of skyrmion.（2）Hexagonal van der Waals ferromagnet Fe₃GeTe₂ with large perpendicular magnetocrystals anisotropy,which is also known as a topological nodal line semimetal,exhibits large anomalous Hall conductivity and anomalous Hall angle when the magnetic field is applied perpendicular to the sample plane.In this work,we systematically studied the electrical transport properties in Fe₃GeTe₂ single crystals under in-plane magnetic field,and explored the methods to further improve its anomalous Hall conductivity and Curie temperature.（i）We have observed an abnormal turning peak of anomalous Hall effect in quasi-two-dimensional ferromagnet Fe₃GeTe₂ single crystals when the magnetic field is nearby the ab plane.The abnormal peak of anomalous Hall effect emerged when the external field tilted from easy magnetized c axis to easily cleavable ab plane,and maintained when the external magnetic field rotated in the ac（or bc）plane in the whole temperature region below Curie temperature.The perpendicular magnetocrystals anisotropy determines the final trends of the angular-dependent Hall effect,which indicates the existence of skyrmions.Thus,our work provides an alternative tool to detect the skyrmions.（ii）First-principles calculations were carried out to study the effect of element doping on the atomic magnetic moment,band structure,Berry curvature and anomalous Hall conductivity of Fe₃GeTe₂.It is found that when Fe atoms are substituted with Mn,Co,and Cu atoms,extra small band gaps would emerge near the Fermi energy,resulting in the non-zero distribution of Berry curvature,which greatly enhances the anomalous Hall conductivity.（iii）First-principles calculations were carried out to inveatigate the influence of strain on the magnetic ground state and exchange interactions in Fe₃GeTe₂.Under a large strain of over2%compression and 4%expansion along the a axis,or over 7%compression along the c axis,the magnetic ground states undergo a transition from the AFM-stable to the FM-stable state.By calculating the inter-atom exchange interaction constant and Curie temperature,it was found that when isotropic strain is applied,the increase of Curie temperature is limited in Fe₃GeTe₂.While when uniaxial strain is applied along the a axis or c axis,the Curie temperature can be increased efficiently.Under 5%a axis strain,the Curie temperature of Fe₃GeTe₂ can be successfully increased to room temperature.