Magnetic And Electronic Transport Properties Of Noncoplanar Ferromagnets RMn₂Ge₂（R=La,Ce,Pr And Nd） Single Crystals

In recent years,many new materials,such as magnetic semiconductors,topological semimetals,topological insulators and so on,have been extensively studied in order to meet the needs of spintronic devices.Topological materials have rich electronic and magnetic properties,such as giant magnetoresistance effect,anomalous Hall effect,topological Hall effect and so on,which have great application potential.RMn₂Ge₂（R=La,Ce,Pr and Nd）is a kind of rare earth Mn-based compounds with noncoplanar ferromagnetic structure.The magnetic properties of RMn₂Ge₂（R=La,Ce,Pr and Nd）are rich with the difference of interlayer Mn-Mn atom spacing,and there exists skyrmion topologic structure near room temperature.Therefore,RMn₂Ge₂（R=La,Ce,Pr and Nd）compounds exhibit rich and novel physical properties.In this paper,the electronic transport properties of RMn₂Ge₂（R=La,Ce,Pr and Nd）single crystals were systematically studied by preparing high-quality single crystal samples.The topological transport phenomenon caused by noncoplanar ferromagnetic structure was found,and the anisotropic magnetoresistance,planar Hall effect and planar topological Hall effect were studied.The main research contents are as follows:（1）RMn₂Ge₂（R=La,Ce,Pr and Nd）single crystal was successfully prepared by using low melting point metal In as flux and optimizing growth conditions.The results show that the single crystals have good quality with large size,uniform composition,and regular morphology.The most naturally exposed surface is ab plane.The magnetic properties of RMn₂Ge₂（R=La,Ce and Pr）single crystals have been studied.The results show that they all exhibit non-coplanar ferromagnetism and uniaxial anisotropy below room temperature.The magnetic phase transition process is relatively simple,and only the ferromagnetic phase transition is obvious.Nd Mn₂Ge₂ has complex magnetic behavior,and there are five magnetic phase transitions including spin reorientation transition.（2）The magnetoresistance behavior of RMn₂Ge₂（R=La,Ce,Pr and Nd）single crystal was systematically analyzed,and the physical mechanism of magnetoresistance was elucidated.The reason for the emergence of anisotropic magnetoresistance and its relationship with magnetocrystalline anisotropy were analyzed.Because AMR is the main contribution of MR,the in-plane MR of Ce Mn₂Ge₂ and Nd Mn₂Ge₂crystals firstly shows positive at low field and then turn to be negative.The saturation fields of RMn₂Ge₂（R=La,Ce,Pr and Nd）single crystal are consistent with the corresponding saturation fields in magnetization process.This means that the magnetoresistance characteristics below saturation field reflect the change of domain structure under the applied magnetic field.（3）The anomalous Hall effect in RMn₂Ge₂（R=La,Ce,Pr and Nd）single crystals was systematically analyzed and its physical mechanism was elucidated respectively.The planar Hall effect of Ce Mn₂Ge₂ and Nd Mn₂Ge₂ was tested and the anomalous Hall signal is found.The variation curve of Hall resistivity with magnetic field is correlated with magnetization curve,indicating that this phenomenon is anomalous Hall effect related to magnetization intensity M.The anomalous Hall effect in plane is directly related to the anisotropy of magnetic structure.Generally,this phenomenon is attributed to spin reversal of noncoplanar spin structure.（4）Through analyzing Hall resistance data,it is found that in addition to ordinary Hall effect and anomalous Hall effect,there are topological Hall effect parts in Hall resistance data,which may come from noncoplanar spin structure or nontrivial topological texture.The relationship between topological Hall effect and magnetization process is analyzed by phase diagram.Combined with the ac susceptibility analysis,no signal of Skyrmion generation and annihilation was found,suggesting that topological Hall effect in RMn₂Ge₂ should come from noncoplanar spin structure and topological trivial spin structure would change to topological nontrivial spin structure with the changing of the thickness of sample.Berry curvature contributed from noncoplanar magnetic structure can be equivalent to virtual magnetic field.When current and magnetic field are coplanar or even collinear,topological Hall effect is also observed,namely,in-plane topological Hall effect.The in-plane topological Hall effect resulting from the inversion of noncoplanar spin structure is closely related to the spin topological structure and crystal axis of magnetic materials.