Abstract
The CoFeB thickness, t dependence of the effective first- and second-order magnetic anisotropy, K1eff and K2, for MgO/(Co1-xFex)80B20/Ta films (x=0.3-1.0) is investigated. As Co40Fe40B20 thickness decreases, K1eff increases and shows a perpendicular magnetic anisotropy for t=1.2 nm. On the other hand, in-plane magnetic anisotropy is observed for t≥1.4 nm. Also, a 1.3-nm-thick CoFeB sample demonstrates an easy-cone behavior, which suggests that the magnitude of K1eff and K2 becomes comparable. By plotting the product of K2 and t-td as a function of t-td, where td is a magnetic dead layer thickness, linear dependences with negative y-axis intercepts are displayed for all ranges of x. The extracted interfacial K2, Ki(2) are varied depending on the compositions in the range of-0.024 to −0.042 erg/cm2 for x=100% and 30, 50%, respectively. A magnetic phase diagram summarizing the results of K1-2πMs2 and K2 suggests that the ratio of K2 against K1-2πMs2 is varied depending on the compositions. These results give us a guideline to achieve the desired magnetic properties of CoFeB for spintronic applications.
Highlights
Magnetic tunnel junctions (MTJs) using CoFeB/MgO/CoFeB are a building block for spintronic devices due to their large tunnel magnetoresistance (TMR) ratios up to 604% at room temperature,[1,2] which enables MTJs to be used in practical applications such as magnetoresistive random access memory (MRAM) and magnetic field sensor devices
Since an interfacial magnetic anisotropy is mostly generated on the interface of Fe/MgO by orbital hybridization,[3,4] a manipulation of the K1 in CoFeB has been established by means of thickness, compositions, and voltage controls[2,5,6,7] and contribute to the development of low-power-consumption MRAM and high-sensitive magnetic sensor devices.[8,9]
An improved linear conductance output of MTJ has been demonstrated by reducing the K2 of the free layer, and complete linear conductance has been predicted when K2=0.14 For these purposes, as both large and small K2 are significantly profitable for spintronic devices, a controlling method of K2 should be established
Summary
Magnetic tunnel junctions (MTJs) using CoFeB/MgO/CoFeB are a building block for spintronic devices due to their large tunnel magnetoresistance (TMR) ratios up to 604% at room temperature,[1,2] which enables MTJs to be used in practical applications such as magnetoresistive random access memory (MRAM) and magnetic field sensor devices. Referring to the K2 study for other ferromagnets, an evident linear relationship between K2 against 1/t has been observed in a CoFe/Ni multilayer, where t is the thickness of the CoFe/Ni layer and the magnitude of interfacial K2 was found to be one order smaller than that of K1.15 a sizable voltage controlled magnetic anisotropy (VCMA) effect has been found in the K2 of epitaxial Cr/Fe/MgO films.[16]. These results may ensure the existence of an interfacial contribution in K2 as is the case with well-defined K1. In this paper, we studied the thickness dependence of K2 for MgO/CoFeB/Ta films with different CoFeB compositions and report the composition dependence of interfacial magnetic anisotropy of K2
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