Spin Transport Properties In Fe Based Alloy And Multilayer Films

With the rapid development of spintronics,using the spin properties of electrons has gradually become a new generation of information storage and transmission means.Spintronics devices based on the giant magnetoresistance(GMR)and tunneling magnetoresistance(TMR)effects have been widely used in hard disk read heads and magnetic random access memories.As people's requirements for spintronics devices become higher and higher,more and more researchers are interested in the study of new spintronics devices based on the spinorbit torque(SOT)effect.Different from the GMR and TMR devices,SOT devices use electric current to replace magnetic field as a novel method to manipulate the magnetic moment of thin films and the resistive state of devices.Compared with the magnetic field,the electric current manipulation eliminates the process of using electromagnet to convert the current into magnetic field.And the overall structure of the device is simpler,with higher energy efficiency and smaller thermal effect.SOT devices have a very wide prospect of application in the electronics industry.Therefore,SOT devices has become one of the research hotspots of spintronics.In this paper,we have systematically studied the spin transport properties of FePt single layer films and Fe based multilayer films.Including the SOT and anomalous Hall effect(AHE)in L10 FePt films with different chemical order and thickness,and spin Hall magnetoresistance(SMR)and SOT in Fe/Pt and Fe/CuOx bilayers with the different thickness of Fe layer.The main conclusions are as follows:(1)The 3-nm-thick FePt films were epitaxially deposited on MgO substrates with different growth temperatures and fabricated them into SOT devices.We have studied the current induced magnetization switching in ultra-thin L10 FePt single layer with different disorder.An enhanced Ll0 ordering has been induced as the growth temperatures increased from 350? to 450?.We found that SOT can only induce partial magnetization switching in L10 FePt films,and magnetization switching ratio is closely related to the disorder.Nearly full magnetization switching only happens in more disordered films,and the magnetization switching ratio becomes smaller as increasing L10 ordering.At the same time,the method for deriving effective spin torque fields in the previous studies cannot fully explain the spin current generation and self-induced SOT in L10 FePt single layer.Combined with Magneto-Optical Kerr Effect(MOKE)and AHE measurements,we found that the disorder should make a significant contribution to the spin transport of L10 FePt films.A larger spin current should be generated in more disordered FePt films,which may be represented by skew scattering.Disorder should determine the formation of domain walls,as well as the spin current generation.At the same time,we investigated the current induced magnetization switching in 3-nm-thick L10 FePt films grown on SrTiO3(STO)substrate by the same methods.The conclusions are the same as those of L10 FePt grown on MgO substrate.(2)We have investigated the thickness dependent SOT in Ll0 FePt single layers.SOT-induced magnetization switching ratio in Ll0 FePt films with higher chemical ordering becomes smaller as increasing thickness from 8 nm to 16 nm have been discovered.After analysis,we believe that this is due to different interfacial strain and grain sizes of L10 FePt film with varying thickness.The reduced grain size in relatively thinner films could leads to lower energy barrier for domain nucleation,thereby enhancing the switching ratio.It indicates that more factors should be taken into consideration for SOT of L10 FePt single film.In addition,we prepared 8-nm-thick L10 FePt films with different growth temperatures.The results show that more magnetization can be switched in 8-nm-thick L10 FePt films with increasing chemical ordering,which is opposite to the change trend of 3-nm-thick L10 FePt samples.When the FePt film is thick enough,the SOT in L10 FePt is closely related to the L10-ordered structure,which indicates a bulk nature.Therefore,the disorder plays an important role in the magnetization switching only for the ultra-thin L10 FePt films,while the composition gradient may play a leading role for thicker films.Although many factors influence SOT in L10 FePt film,the bulk nature of strong spin-orbit coupling in L10 FePt have been emphasized through density functional theory calculations,which should generate large spin current due to spin Hall effect.(3)SOT devices were prepared by Fe/Pt(5),Fe/CuOx(5)and Fe/AlO3(5)bilayers with different thickness of Fe layer.We have compared the Fe-thicknessdependent SMR and SOT in Fe/Pt(5)and Fe/CuOx(5)bilayers.A larger SMR in Fe/CuOx than that in Fe/Pt has been found when the thickness of Fe layer is 3 nm.As the thickness of Fe increased from 3 nm to 10 nm,the SMR of the two bilayers both decrease due to the shunting current effect,but the value in Fe/CuOx bilayers dropt more sharply.It can be considered that the origin of spin current in Fe/CuOx bilayers is more sensitive with the current flow path than that in Fe/Pt bilayers.For the Fe/A12O3 bilayers,almost no spin current is generated due to the negligible spinorbit coupling of Al2O3.Through harmonic measurements,we have observed considerably large SOT in both the Fe/Pt and Fe/CuOx bilayers,which proves that there exists spin current in the Fe/CuOx bilayers.We have discussed the origin of the strong spin current in the Fe/CuOx bilayer film.The interface-generated spin current should be ruled out.The mixed phase of CuOx has been confirmed including CuO,Cu2O and Cu,which performs strong spin-orbit coupling and produce large spin current.