Manipulation Of Perpendicular Magnetic Anisotropy Of CoFeB Films By B Content And Spin Hall Effect Of CrAl Films

Magnetic random access memory(MRAM),as a burgeoning spintronic device,promises to bring non-volatile,high density and high speed memory to compete with FLASH and other available memories.Magnetic tunnel junctions(MTJ)with perpendicular magnetic anisotropy(PMA)have been demonstrated to be superior to those with in-plane anisotropy for MRAM.It is urgently demanded to improve the perpendicular magnetic anisotropy and thermal stability of the CoFeB/MgO MTJ.So far,the perpendicularly magnetized CoFeB/MgO films and the corresponding perpendicular MTJs have been extensively explored with the B content of the CoFe B layer mostly fixed at about 20 atomic percent.In this thesis,the effect of B content on the perpendicular magnetic properties of the Ta/CoFeB/MgO trilayers and the samples with the inverted structure has been systematically investigated.Two kinds of multilayered films Ta/(Co0.5Fe0.5)1-xBx/MgO(x = 0.1,0.2,0.3)and MgO/(Co0.5Fe0.5)1-xBx/Ta(x = 0.1,0.2,0.3)annealed at different temperatures were characterized magnetically.With normal B content of 20% for the CoFeB layer,the Ta/Co0.4Fe0.4B0.2 /MgO annealed at 300? shows PMA when the CoFeB layer is no thicker than 1.2 nm;The inverted structure gives PMA with the CoFeB thickness from 1.2 nm to 1.4 nm.When the B content decreases to 10%,the interfacial PMA appreciably falls off.PMA is achieved only in the sample with a 0.8 nm CoFeB layer for Ta/Co0.45Fe0.45B0.10/MgO and 1.0 nm CoFeB layer for MgO/Co0.45Fe0.45B0.10/Ta under the same annealing condition.On the other hand,when the B content increases to 30%,both PMA and its temperature stability are appreciably improved.The structure Ta/Co0.35Fe0.35B0.30/MgO annealed at 300? exhibits PMA with the CoFeB layer thickness 1.4 nm,and the corresponding interfacial anisotropy Ks increases from 1.7 erg/cm2 for the normal B content case to 1.9 erg/cm2.When the annealing temperature increases to 325?,the structure Ta/Co0.35Fe0.35B0.30/MgO still shows PMA with the CoFeB thickness less than 1.3 nm,in contrast,the normal 20% B content samples annealed at this temperature no longer exhibits PMA.Moreover,the structure MgO/Co0.35Fe0.35B0.30/Ta shows optimum annealing temperature of about 350?,at which Ks reaches 2.0 erg/cm2 and PMA is realized in the samples with the CoFeB thickness up to 1.5 nm.In contrast,the same structure with 20% B is magnetically destroyed completely after 350? annealing.In a word,we find greatly improved PMA and temperature stability for the CoFeB/MgO system with excess B,our results suggest that one should take into account the B content effect to optimize the spintronic devices based on the perpendicularly magnetized CoFeB/MgO system.As is known,the 3d metal Cr is antiferromagnetic with a spin-density wave incommensurate with the lattice,it has a large spin Hall angle,comparable to that of 5d metals such as W;the addition of nonmagnetic light metal Al into Cr leads to a simple commensurate antiferromagnetic spin structure for the Cr-Al alloy,moreover,the Cr1-xAlx(x?0.26)with a very high Neel temperature exhibits semiconducting behavior.In this thesis,the spin Hall effect of the highly resistive Cr0.72 Al0.28 alloy has also been explored.From the two sets of heterostructure films YIG/Cr0.72 Al0.28 and YIG/Cr,the resistivity of the Cr0.72 Al0.28 is found to be much higher than that of Cr,especially for the relatively thick metallic layers.Using longitudinal spin Seebeck configuration with a perpendicular temperature gradient,the inverse spin Hall voltage of the YIG/Cr0.72 Al0.28 is measured and found to be significantly larger than that of the YIG/Cr with the same metallic layer thickness.The spin diffusion length and spin Hall angle are further characterized and compared.Although the electronic structure and transport properties are greatly modified by alloying of Cr with Al,the spin diffusion length changes very little,whereas the spin Hall angle decreases by a factor of 3,possibly because of the extremely small spin-orbit interaction of Al.Considering the significantly larger inverse spin Hall voltage of YIG/Cr0.72 Al0.28 because of its very high resistivity,Cr0.72 Al0.28 still could be an efficient spin current detector in spintronic applications.