Spin Dynamics In Magnetic Heusler Alloys And Non-magnetic Topological Semimetals

Since the discovery of giant magnetoresistance effect(GMR),spintronics has developed as a new subject rapidly.Spintronics is a science that studies the rule which the electron spin follows,and to realizes the injection,manipulation and detection of the electron spin in electronic materials,so as to apply the electron spin degrees of freedom in information storage and operation.In spintronics application,there are some new requirements for new magnetic materials,such as large spin polarizability and low damping constant.Among various kinds of magnetic materials,Heusler alloys has attracted much attention because of its diverse spintronic properties.Some Heusler alloys,for example Co₂FeAl and Co₂Mn Si,have been proved to possess half metallic properties,which means they can theoretically provide 100%spin polarized current.Moreover,such materials often have low damping constant,which means that spintronic devices based on these materials can have lower energy consumption.For the half-metallic Heusler alloy Co₂Fe Al,it is of great significance to modulate its damping factor via adjusting its composition for understanding its damping mechanism and optimizing its damping properties.In the first work of this dissertation,we studied the composition modulation of the damping constant of Heusler alloy Co_2+xFe_1-xAl(x=-0.4,-0.2,0.2,0.4)epitaxial films by using time-resolved magneto-optical Kerr effect(TR-MOKE).By adjusting the atomic component ratio of Co and Fe in Co_2+xFe_1-xAl alloy,the damping constant varied from 0.0065 to 0.0156,and the minimum value appeared when x=-0.2.After that,we explain the component modulation of damping constant using the relative movement between the Fermi level and the band gap of spin-down electrons caused by the component modulation.In Heusler alloy,there is also a special kind of spintronic materials,which is called compensated ferrimagnetic materials,represented by Mn₂Ru Ga and Ti₂Mn Al.In recent years,the first principle calculation studies show that in this kind of materials,different magnetic atoms have different atomic magnetic moments,and the arrangement direction is also opposite,but the net residual magnetic moment of all atoms is zero(ideal case).Because they have zero net magnetization,they also have zero stray field,and their magnetization state is not easy to change with external field.Moreover,many Heusler type compensated ferrimagnets also have half-metallic property,which increases their application value in spintronics.However,the arrangement of the magnetic moments in this kind of ferrimagnetic material has not been observed directly in the experiment,which brings difficulties to their property control and application.In the second work of this dissertation,we have investigated the atomic magnetic moment arrangement of the B2 phase Heusler alloy Mn₂Co Al by X-ray magnetic circular dichroism(XMCD).We have directly observed that the atomic magnetic moments of Mn and Co atoms in B2 Mn₂Co Al are arranged in antiparallel.And the B2 Mn₂Co Al has only a very small net magnetic moment of 0.34?B/f.u.These characteristics are all similar to those of the compensated ferrimagnet.In addition,we find that the Mn XMCD peak at L₃absorption edge shows an obvious split,which indicates the spin localization caused by Mn-Co 3d orbital hybridization,and means a half metallic property of this material.This work reveals an application value of B2 phase Mn2Co Al as a potential compensated ferrimagnetic half metal,and provides a new way for the research of magnetic moment arrangement of compensated ferrimagnet.Beside the magnetic materials,topological materials are also beginning to show great potential application in spintronic devices.On the one hand,topologically protected surface states can transmit information with extremely high speed and without dissipation;on the other hand,they often have large spin orbit coupling and can be used as providers of pure spin current.WTe₂,as a new topological material,Weyl semimetal,has attracted much attention because of its novel and diverse physical properties.However,the spin dynamics of WTe₂ is still lack of study.In the third work of this dissertation,we use circularly polarized pump light to inject spin polarization into WTe₂,and use TR-MOKE measurement to observe the spin relaxation process in this material.In order to clarify the spin relaxation mechanism of WTe₂,we studied the relationship between the spin relaxation process and temperature,external magnetic field and pump flux.It is found that the spin relaxation time does not change with temperature,external field and pump flux.Therefore,we propose that the main mechanism of spin relaxation in WTe₂ is the scattering of impurities inside the material.