Ultrafast Spin Dynamics On Complex Magnetic Structures

With the progress of science and technology,the storage density of magnetic storage and the limit of read and write speeds of storage information have been broken through.As the most practical storage medium of magnetic storage,magnetic thin films are attracting more and more attention of researchers.The new Magnetic Random Access Memory?MRAM?is predefined as"ideal storage".Selecting the most suitable magnetic film for MRAM application is the key to developing high-density,low power and high stability memory devices.At present,a lot of researches focus on how to improve the performance of MRAM based on Spin Transfer Torques?STT?.An important research direction is to reduce the critical switching current J_c of STT-MRAM by reducing the damping constant of magnetic thin films.The time-resolved magneto-optical Kerr effect?TR-MOKE?is one of the most important ways to study ultrafast spin dynamics.The influence factors of damping constant are discussed by analyzing the precession of dynamics.This paper focuses on the ultrafast spin dynamics of two complex magnetic thin film systems and explores the dependence between damping constant and perpendicular anisotropy,composition and temperature of samples.The main results are summarized as follows:In the first part,we have studied the exchange bias and ultrafast spin dynamics of[Pt/Co]₃/MnIr multilayers.The exchange bias was manipulated by varying thicknesses of the nonmagnetic layer Pt,magnetic layer Co and antiferromagnetic layer MnIr.By using means of the TR-MOKE measurements,we found that pumping laser energy can significantly affect the perpendicular magnetic anisotropy of the sample and the effective damping constant.In addition,it is found that the saturation damping constant increases with the increase of perpendicular anisotropy when the thickness of Pt is 0.5 to 1.5 nm and the thickness of Co is0.8 to 2.4 nm,respectively.Although the exchange bias is not directly related to the precession damping,we assume that the exchange bias is not conducive to the reduction of the damping constant.In the second part,laser-induced ultrafast spin dynamics in amorphous alloy TbFe Co is investigated by pump-probe TR-MOKE experiments.In the range of Tb composition from 10%to 33%in our samples,the amplitude of demagnetization shows a slight increasing trend with the increase of Tb composition.Moreover,we have proved that,if a strong magnetic field is applied non-collinearly to the easy axis,the reduction of polar MOKE signal occurring in tens of ps is a magnetization precession of FeCo spins with large damping constant.By further reducing the composition of Tb to 6%,we observed more periodic oscillations,indicating that the damping of the sample has a direct dependence on the compositon of Tb.In addition,we simultaneously measured the ultrafast dynamics of FeCo and Tb sublattice with 800 nm and400 nm probe beam.It was found that the start of demagnetization of the Tb spin is more than200 fs later than that of the FeCo spin,implying the FeCo demagnetization contributes to the demagnetization of Tb spin.The demagnetization of Tb spin still goes on when the FeCo spin recovers.In the last part,by changing the measurement temperature,we have found that the temperature played an important role in adjusting the precession damping constant.We observed precession oscillations presenting in TbFeCo alloys with less Tb compositions at higher temperatures.The precession amplitude has maximum value when the external magnetic field is equal to the perpendicular anisotropy field?the external magnetic field parallel to the sample plane?.The phase shift and the different trend of precession frequency in positive and negative magnetic field are resulted from different precession models.In addition,we found that the influence of magnetic anisotropy of TbFeCo in a composite structure of PMN-PT/TbFe Co.