| Magnetization switching driven by current-induced spin-orbit torque（SOT） in the field of spintronics has drawn plenty of attention,because SOT offers a fast and efficient way to manipulate magnetization.Based on SOT effect,this doctoral thesis studies the spin currents of heavy metal and ferromagnetic materials（FM）,controlling and driving the magnetization switching in magnetic nano heterostructures,and how to influence the transport properties.The contents are as follows:（1）By using two orthogonal spin-orbit torques,we could non-volatilely switch magnetization in the perpendicular Pt/Co/MgO system among such multistates as the spin-up,spin-down and spin-random states.And the chirality of the SOT switching（clockwise or counterclockwise）could be reversed by electric method by using the newly-introduced random state,which used to be realized by a direction-reversible magnetic field.Furthermore,the main switching characteristics were well reproduced by a nucleation switching model.In addition,the other intermediate states between spin-up and spin-down state were also addressable by accurately controlling on the scanning current and route.（2）We employed spin-torque ferromagnetic resonance（ST-FMR） technique to study the M-independent spin current in the Y3Fe5O12/NiFe system,which excluded the interference of anomalous Hall effect（AHE）and anisotropic magnetoresistance（AMR）effect.The key of this experiment relies on different FMR frequencies of the YIG and the NiFe film at the same field.Thus at the FMR mode of the YIG,a rectification voltage Vdc could be detected due to spin Hall magnetoresistance（SMR）.And the AMR of NiFe had no contribution to the measured Vdc signal because NiFe was not in its the precession state.Secondly,NiFe was always parallel toYIG during the ST-FMR measurement.The spin current of NiFe induced by AHE could not contribute to the dc voltage(YIG×AHE=0).Therefore,we obtained the M-independent dampinglike spin torque efficiency of NiFe was about 25%of Pt,and its spin Hall angle(SH)is the same sign of Pt.（3）We investigated the angular dependence of spin current generated by CoFeB on magnetization during the SOT magnetization switching process in Ir Mn/CoFeB/W/CoFeB system,where the two CoFeB layers had perpendicular magnetic anisotropy（PMA）and in-plane magnetic anisotropy（IMA）,respectively.The direction of in-plane CoFeB magnetization was determined by SMR and AMR.Besides,the angular dependence of spin current generated by CoFeB on magnetization was characterized by critical switching current and spin orbit efficiency on the same device which excluded accidental interference.We discovered that the contribution from AHE was negligible during the switching process.In other words,the SOT efficiency generated by CoFeB was M-independent.And it further demonstrated that the optimal easy magnetization direction of the in-plane FM was parallel to the current I in the T type structure.（4）We realized the field-free magnetization switching in a T-type structure（CoFeB/W/CoFeB） and discovered that the direction of symmetry-breaking field was parallel to the magnetization of the bottom CoFeB（IMA）,which was opposite to the direction of the stray field provided by this layer.Moreover,by placing a 2.5-nm thick insulating layer of MgO between the bottom CoFeB and W layer（CoFeB/MgO/W/CoFeB）to block the interlayer exchange coupling and the spin current from the bottom CoFeB,the field-free SOT switching was still achieved,indicating that the field-free SOT switching was primarily caused by the Ne?el orange-peel effect in our T type structure（CoFeB/W/CoFeB）.Furthermore,the results of experiments were confirmed and reproduced by micromagnetic simulations.In addition,we obtained the effectiveSH of CoFeB(-0.024)by the current-induced hysteresis loop shift method,and the contribution of the spin current from the bottom CoFeB was accounted for about 26%of the total in the current-induced SOT switching process. |
