Study Of Magnetic Functional Thin Film Materials Based On Temperature Control

Magnetic functional thin film materials are promising in modern information technology for their applications in storage,logic or sensor devices.However,most studies in magnetic thin films only cares their performances and applications under electric inputs:On one side,nearly all devices that employ the spin-dependent transport properties of a magnetic thin film are driven by electric sources;On the other side,main effective methods to control magnetization of a thin film are also based on electrical control,including magnetization reversals via ferroelectric substrates or via spin transfer torque.These electrical relied studies will not only limit the functions of magnetic thin films but also create Joule heat which causes heat dissipation and energy consumption problems in the devices.Focus on the above issues,we suggested that "temperature control" could be an alternative way.Particularly,from the point of view of the "transport properties under a temperature gradient" and the "magnetization changes under temperature controls" of magnetic thin films,we carried out systematically studies of magnetic functional thin film materials based on temperature control.The main researches are as below:(1)The spin-dependent thermoelectric transport behavior of a NiFe thin film under temperature gradient was studied.Firstly,the effects of a NiFe/MgO interface on the thermoelectric transport behavior of spin-polarized electrons along with the temperature gradient was studied by measuring the Seebeck coefficient(Sxxavg)and anisotropic magneto-thermopower(AMT)of a thin film sample with the structure of MgO(3)/Ni81Fe19(10)/MgO(2)/SiO2(1)(in nm).Thermally annealing this thin film was found to increase its Sxxavg(343%at 450℃)through crystallization of interfacial MgO,which allows conductive electrons to achieve better specular scattering at the interface.Its AMT was enhanced by annealing(139%at 450℃),too,but this is ascribed to a chemical metallization of interfacial Fe in which the deposition-induced oxygen atoms migrate out of the NiFe layer.Secondly,along with a perpendicular direction to the temperature gradient,the Planar Nernst Effect(PNE)of the thin film was also found to be enhanced,for which a transverse Seebeck coefficient(Sxy)as large as 73.1nV/K was observed after 450℃annealing.(2)A thermally enhanced magnetoresistance(ThMR)was designed and obtained by simultaneously applying charge and heat currents to a NiFe thin film.From the measurement,we observed that the magnetoresistance value was as high as-22600%when the input charge current and applied temperature gradient was 0.966μA and 2.5℃/mm,respectively.This ThMR value can be controllable by adjusting the relative values of the input charge and heat currents.In this particular study,for example,on increasing the input charge current from 0.85 to 1.05μA by fixing the temperature gradient at 2.5℃/mm,the ThMR value first increased from 9%to 183%and then decreased from-259%to-13%,at intervals of-0.96μA;Vise versa,similar change of the ThMR value was also obtained by applying various temperature gradient while inputting a fixed charge current.This can be explained by the spin-dependent transport phenomenon i.e.,scattering induced sign difference between magnetoresistance(MR)and magnetothermopower(MT)in NiFe.(3)Enhancement of post-annealing stability in Co/Ni multilayers with perpendicular magnetic anisotropy(PMA)was obtained by inserting Au layers into Ni/Co interfaces.After 350℃annealing,the effective magnetic anisotropy density(Keff)for Ta(3)/Pt(2)/[Co(0.3)/Ni(0.6)/Au(0.3)]x3/Co(0.3)/Pt(1)/Ta(3)(in nm)sample kept at 0.48×105 J/m3.STEM-HAADF analysis showed that the diffusion between Ni and Co layers was obstructed by the Au insertion layers among them,which was responsible for the multilayers’ post-annealing stability enhancement.Multilayers with Pt insertion layers were also investigated as reference samples in this work.Compared with Pt-layer-inserted Co/Ni multilayers,the Au insertion layers were found to bring seldom interfacial PMA to the multilayers,making it competitive in being employed to enhance the post-annealing stability of PMA Co/Ni multilayers which were used for magnetic random access memory devices.(4)A series of NiTi shape memory alloy substrate/Fe thin film composite materials as well as their magnetic anisotropy changes under temperature control was designed and studied.The in-plane magnetic anisotropy of one-way NiTi substrate/Fe(5nm)thin film increased after being heated up to 200℃and then cooled down to room temperature.X-ray photoelectron spectroscopy(XPS)analysis indicated a compressive strain in Fe film due to the heating induced size reduction of NiTi substrate.According to ab initio calculations,this compressive strain could reduce the spin-orbit interaction(SOI)along z direction of a Fe crystal lattice,resulting an in-plane magnetic anisotropy increase of the Fe thin film.Similarly,two-way NiTi substrate/Fe(5nm)thin films were found to exhibit reversible in-plane magnetic anisotropy changes under temperature loops of"heating,freezing,heating" or "freezing,heating,freezing".