Study On Laser-induced Ultrafast Magnetic Dynamics Of Co/Ni Multilayers

Recently,with the development of the information revolution,such as Artificial Intelligence(AI),the Internet of Things et al.,the demands of ultrahigh density and speed magnetic recording for big data has increased dramatically.Since the traditional storage has meet an intrinsic physical scaling limit,spintronic devices based on through spin-coherent channels can be gradually developed.These devices can not only solve the problem of high power consumption,but also break through the current bottleneck of speed and integration.At present,the Magnetic Random Access Memory(MRAM)has several advantages over the spin devices,such as ultra-fast read and write speed,low power consumption,non-volatility and et al.As the second generation of MRAM,Spin-transfer Torque(STT-MRAM)reduces the additional write information line and the switching current,leading to the lower power consumption and cost,even the ultrahigh density of magnetic storage.As we know,the proper perpendicular magnetic anisotropy(PMA),high spin polarizability and low magnetic damping constant are the keys to increase the STT-recording density and reduce the power consumption of STT-MRAM.Based on the above background,this thesis systematically studied the physical origins of PMA,magnetization precession frequency and Gilbert damping of[Co/Ni]_nmultilayer film system.The two main works of this thesis are shown as following:In the first part,a series of Co/Ni multilayers(Ta(3)/Pt(t _Pt)/[Co(0.3)/Ni(0.4)]_n/Ta(3)(Unit:nm),here n is the number of bilayer periods)with various PMA were systematically prepared by using magnetron DC sputtering.The PMA of the Co/Ni multilayers could be modulated by changing n,t _Ptand annealing temperature T,respectively.The magnetic parameters of all the samples such as magnetization M_s,magnetic anisotropy field H_keffand et al,were obtained by vibrating the sample magnetometer(VSM),and the perpendicular magnetic anisotropy constant K_u could also be calculated.And then,the relationship among K_u,magnetization precession frequency f and intrinsic Gilbert damping?₀ of[Co/Ni]_n multilayers were studied by the time-resolved magneto-optic Kerr effect(TR-MOKE).(i)With a fixed t _Pt,at beginning both K_u and?₀ increase with increasing n,and then decrease with further increasing n,ascribing to the positive correlation relationship among K_u,?₀and the spin-orbit coupling constant,respectively.(ii)For a fixed n,with increasing t _Pt,K_u exhibits a non-monotonic change behavior,but?₀ increases monotonously,attributed to the spin pumping effect and the enhancement of the interface roughness.(iii)For fixed n and t,?₀ increases with increasing T,attributing to the increase of grain size,which shows a different trend of that for K_u.In the second part,with different buffer layers as Cu and Pt respectively,two series of Co/Ni multilayers with PMA on different substrates(Si single crystal,Glass,ITO Glass,Sapphire single crystal and Mg O single crystal)were prepared.The two series of the films are Ta/Cu(t _Cu)/[Co/Ni]₆/Ta(nm)(I)and Ta/Pt(6)/[Co/Ni]₆/Ta(nm)(II)respectively.By systematical research works,we have obtained the following main conclusions:(i)For the series I with a fixed t _Cu,the samples grown on the different substrates shows different strengths of PMA.The samples grown on glass and Si substrates own the largest K_u,moreover,other samples show in-plane magnetic anisotropy(IMA)component,which could be ascribed to substrate roughness and stress caused by lattice mismatch.We use TR-MOKE system to measure the magnetization dynamic process of the samples,and find the ultrafast demagnetization time?_D almost remains a constant.However,the transient magnetization recovery time and recovery rate are obviously affected by the substrate,which relate to the thermal conductivity of the substrate and the lattice mismatching between the substrate and the film.Furthermore,for different substrates,?₀has a strong correlation with the change of the strength of PMA and IMA component caused by the variation of t _Cu.(ii)For the series II,except for Mg O substrate,the samples grown on other substrates show completely PMA.The relationship between PMA strength and substrates are the similar with that of the same t _Cuin the series I?Since almost all samples have complete PMA,it clearly proves that the?_Dand₀ are not affected by the substrates,but the transient magnetization recovery time and recovery rate are strongly modulated by the substrate materials.The above experimental results may be helpful to the potential application of[Co/Ni]_nmultilayer system in STT-MRAM with high density,low power consumption and high stability.