Study On Electrical Transport And Magnetic Properties Of Non-collinear Antiferromagnets

Antiferromagnetic materials,as an alternative material for traditional ferromagnetic semiconductor electronic devices,have been popular among device development researchers because of their advantages of zero macro net magnetic moment,insensitive to magnetic field disturbance and more stable data storage.However,due to its special structure,it is difficult for ordinary detection equipment to detect the signal output.Mn₃X（X=Ga,Ge,Sn,etc）series materials are a kind of inclined non-collinear antiferromagnet,which can not only be detected by the external magnetic moment signal,but also show good electromagnetic transport behavior through electromagnetic regulation,and have broad application prospects.At the same time,non-collinear antiferromagnetic Mn₃X series materials also belong to Weyl semimetal,which provides a good research platform for researchers in the current condensed matter physics research.Researchers found that there are many novel physical effects in these materials,such as Anomalous Hall effect,Anomalous Nernst effect,Exchange bias effect,magneto-optical Kerr effect and so on,which opens the door for subsequent researchers to study antiferromagnetic materials.In this paper,Mn₃Ge crystal materials were prepared by flux method.The electrical and magnetic transport properties of single crystal samples and the magnetic properties of polycrystalline powders were taken as the main research objects.The transport properties and the physical mechanism behind were discussed and analyzed comprehensively.The specific work includes the following seven chapters:In the first chapter,the introduction mainly introduces the research background,research progress,research significance of non-collinear antiferromagnetic materials and some basic physical effects mainly involved in the study of Mn₃Ge materials in this paper.Through the introduction of background knowledge,this paper clarifies the significance of the research.Through the introduction of several physical effects,it not only knows where the physical phenomenon analysis comes from,but also points out the direction for the next step.As a representative member of noncollinear antiferromagnetic materials,Mn₃Ge materials have many interesting areas that can be excavated for research.In particular,there is still much room for development in the highly characteristic anisotropy.The second chapter mainly introduces the preparation method of non-collinear antiferromagnetic material Mn₃Ge and some instruments involved in the characterization process,such as flux method,XRD powder diffractometer,energy spectrometer,comprehensive physical property measurement system and ultra-low temperature SQUID magnetic measurement system,and its working principle is briefly described.The third chapter mainly introduces the preparation and structural characterization of single crystal sample Mn₃Ge.Single crystal samples were prepared by flux method;the structure of the single crystal powder was characterized by XRD powder diffractometer,and the cell parameters were obtained.Energy dispersive X-ray detector was used for component analysis to confirm the sample quality;then,the electrical and magnetic transport properties of the samples were tested by the integrated physical property measurement system and the ultra-low temperature SQUID magnetic measurement system.The fourth chapter mainly introduces the magnetic properties of Mn₃Ge single crystal.The irreversibility and splitting behavior of the material were found by testing the magnetization versus temperature curves under field cooling mode and zero field cooling mode.By testing the curves of magnetization versus magnetic field along three directions,it is found that the magnetization of the material is obviously affected by temperature and external magnetic field.At the same time,it is found that the magnetization in XY plane is higher than that in Z axis at low temperature,which is also consistent with the structural characteristics of the material.In the test process,it is also found that the exchange bias effect in the three directions of the material is relatively obvious,and the value of the magnetic field parallel to the Z direction is greater than that in the other two directions,which may correspond to the direction of inclination of the antiferromagnetic material and the direction of resultant force of internal exchange coupling.Finally,the negative magnetoresistance data at low temperature are used to further verify that the material belongs to the outer half metal,which is not only an important experimental evidence for the theoretical prediction of predecessors,but also has important significance for the later research progress.The fifth chapter mainly studies the electrical transport properties of Mn₃Ge single crystal.The test of longitudinal resistivity shows that the sample has strong anisotropy,and the upwarping of the low-temperature region of longitudinal resistivity indicates that there may be Kondo effect caused by impurity scattering in the material.The intrinsic mechanism of its physical source is further verified through the test and analysis of the anomalous Hall effect.The value of the additional spontaneous Hall effect is obtained by solving the overall Hall effect formula.It is found that most of the Hall effect in the material comes from the spontaneous Hall effect caused by the Bailey curvature.Moreover,when the spontaneous Hall effect is adjusted by a weak magnetic field,the positive and negative symbols will change,which has important potential application value for the subsequent magnetic memory devices with 0 and 1 as the control switches.In the sixth chapter,the magnetic properties of Mn₃Ge polycrystalline powders were studied.This paper mainly analyzes the magnetic exercise effect which causes the exchange bias effect gradually weakened with the increase of the number of cycles.After several cycles,the manganese magnetic moment partially frozen by the external field is gradually restored to the initial state.The pinning effect of the antiferromagnetic component on the ferromagnetic component in the material is becoming weaker and weaker,and the mutual coupling effect is becoming weaker and weaker.Macroscopically,the exchange bias field is continuously reduced until the normal state is restored.The seventh chapter summarizes and prospects the above contents.In this thesis,noncollinear antiferromagnetic Mn₃Ge single crystal and polycrystalline powder are studied.The electrical and magnetic transport properties of the material and the magnetic properties of polycrystalline powder are studied.The test data are analyzed and discussed,and the physical mechanism behind is studied.Subsequently,the future work is prospected and summarized,which also provides a reference direction for the next work.