Tuning the inplane anisotropy of CoFeB films

Abstract

Soft magnetic films consisting of CoFeB alloys are widely applied in magnetic sensor [1] and MRAM devices [2], but also used as cores for microfabricated transformers used to power specific subunits on microprocessors [3]. These applications typically require a well-defined in-plane uniaxial anisotropy axis which aligns the magnetic moments and permits operation modes where a field is applied either along the anisotropy axis to obtain a domain wall motion easy axis or a magnetization rotation governed magnetization process.Several implementations have been discussed in literature to obtain a well-defined uniaxial in-plane anisotropy axis and to tune the anisotropy. Apart from the geometry of a sensor element which can be chosen to obtain a shape anisotropy , oblique sputtering of seed layers for the successively deposited magnetic layer(s) [4], sputter deposition or post-deposition annealing in a magnetic field [5] have been employed.Here, we deposit CoFeB layers on Ta seed layers grown by oblique sputtering and compare the resulting anisotropies to those obtained by deposition of the magnetic layer in an applied field. Moreover, for the CoFeB layers on the oblique-sputtered seeds we compare the anisotropies obtained for different CoFeB-thicknesses to distinguish between a surface or interface induced anisotropy or a bulk anisotropy term.Specifically, we deposited Ta(2nm)/CoFeB(6nm)/Ta(Xnm)-seed films onto a Si-wafer covered by its native oxide using angles of 40°, 50° and 60° between the sample normal and the Ta source, and Ta-seed layer thicknesses from 2nm to 10nm. For an angle of 60° the Ta-seed layer was additionally deposited with a thickness of 15nm and 20nm. In addition, we deposited 4nm, 8nm, 16nm and 32nm CoFeB films on 4nm-thick Ta-seed layer deposited at an angle of 50°. Selected systems were deposited in fields of 4mT and 40mT, applied parallel to the substrate surface. VSM was then used to obtain the magnetization of the CoFeB films. Hard- and easy axis magnetization loops were measured with a Kerr Microscope to determine the in-plane anisotropy (see figure 1 & figure 2).We find that the in-plane anisotropy increases with the deposition angle and also the seed layer thickness. For a deposition angle of 60° the anisotropy increases from 4kJ/m3 to 25kJ/m3 with thicknesses from 2nm to 20nm. For smaller deposition angles, we measure a much smaller increase of in-plane anisotropy. Concerning the scientific question whether the anisotropy arises from a surface or a bulk effect, our analysis of CoFeB films with thicknesses from 4nm to 32nm revealed that the anisotropies stemming from the oblique-sputtered Ta-seed layers is well modeled by a surface anisotropy term.While with oblique sputtering, a wide range of in-plane anisotropies can be covered, the anisotropies remain much smaller when films are deposited in fields. However, it has already been demonstrated that with CoSm-alloys much larger anisotropies have been obtained [5]. To correlate the seed-layer morphology with the in-plane anisotropy, high resolution AFM imaging on the Ta-seed layer was performed.Our work shows that in-plane anisotropies in magnetically soft systems can be tuned over a wide range by oblique seed-layer depositions. However, the anisotropy arises from an interface effect and thus decays as 1/t with CoFeB film layers thickness "t". The anisotropy of CoFeB films deposited in an applied field does not depend on film thickness and remains rather small. We propose that alloying Sm into the CoFeB films may increase the field-induced anisotropy, while keeping the soft magnetic properties of the CoFeB films. **