Measurement And Study On Ultrafast Spin Dynamics Of Magnetic Exchange Coupling Structure And Heusler Alloy Film

With the continuous miniaturization of semiconductor technology,a series of technical issues caused by high power consumption has become more and more important constraints for performance improvement in large-scale integrated circuits.Thus,spin-wave devices based on novel magnetic materials are a new type of device proposed under the roadmap beyond Moore to make use of spin waves to transport and process information and replace the traditional charge transport method.Spin waves are usually excited and propagated in various magnetic materials.The choice of materials is the key factor in determining the basic performance of spin devices.Among these materials,the Cobalt-based full-Heusler alloys are very promising candidates for spin-wave devices due to their high spin polarization and Curie temperature but extremely low magnetic damping.In addition,because of the unique spin dynamics characteristics of the magnetic interface exchange coupling multilayer films,it has received extensive attention in quantum information processing and storage.On the other hand,the development of femtosecond laser technology provides a powerful boost to the study of ultrafast spin dynamics on the femtosecond scale and promotes the development of spintronic technology.The study of ultrafast spin dynamics in magnetic films has been a focused area in condensed matter physics.At present,there are still many spin-related mechanisms of magnetic materials at ultrafast time scales that are worth exploring.For example,the process of spin dynamic in the heusler alloys and magnetic interface exchange coupling multilayer films involves rich and varied physical phenomena,and further experimental research is needed to provide a reference for revealing the mechanisms of spin dynamic.The main work of this thesis is arranged as follows:Firstly,based on the pump-probe magneto-optical measurement technology,a time-revolved multi-component magneto-optical Kerr effect spectroscopy system was established,which has a time resolution capability of the order of hundreds of femtoseconds.We have optimized the setup in terms of the optical path design about multi-component magneto-optical Kerr measurement,background noise suppression,and weak signal acquisition,etc.After experimental testing,the measurement of spin dynamics can achieve a good SNR.In addition,different magneto-optical effect components can be measured according to requirements,and fast switching of multi-component observations can be realized.Secondly,external field-dependent and excitation fluence-dependent spin-wave dynamics in full-Heusler Co₂FeAl_0.5Si_0.5 films are studied.Volume magnetostatic spin-wave(VMSW)and perpendicular standing spin-wave modes(PSSW)are excited in the films with thicknesses of 60 and 100 nm,while only the Kittel mode is observed in the films with thicknesses of 150and 200 nm.Among three spin-wave modes,the effective damping of VMSW and Kittel modes presents strong field dependences that are significantly different from each other,which can be attributed to the field-dependent group velocity and magnetic inhomogeneity,respectively.In addition,the effective damping of VMSW and PSSW modes increases with increasing fluence,showing a dominant mechanism of electron–phonon scattering enhancement.However,that of the Kittel mode decreases with increasing fluence and further demonstrates the dominant factor from magnetic inhomogeneity in this case,which is inferred to suppress the excitation of the VMSW mode.Thirdly,under the perpendicular external field,the ultrafast spin dynamics of the synthetic antiferromagnet CoFeB/Ir/CoFeB were studied.The optical branches and acoustic branches of the synthetic antiferromagnet were clearly observed by time-domain spin-wave dynamics signal.According to the fitting analysis under the time-domain signal with the dependence of the magnetic field,the strong magnon-magnon coupling between the acoustic mode and the optical mode in synthetic antiferromagnets is demonstrated,verifying that it is feasible to change the perpendicular external field to tune the magnon-magnon coupling.On the other hand,when comparing our results to those of older studies,it further shows the influence of the asymmetric synthetic antiferromagnetic structure on the magnon-magnon coupling.