Stabilization of the easy-cone magnetic state in free layers of magnetic tunnel junctions

The influence of temperature, FeCoB layer thickness, and insertion of ultrathin metal spacers (Ta and W) on the magnetic anisotropy of MgO/FeCoB/MgO free layers has been explored by means of ferromagnetic resonance. The second-order contribution to the perpendicular magnetic anisotropy (PMA), stemming from the fluctuations of the first-order term, accounts for the onset of an easy-cone magnetic state in the course of the transition from the in-plane to out-of-plane magnetization. Since the second-order term is small, the easy-cone state is stable only within a narrow range of free-layer thicknesses and temperatures, where the interfacial first-order PMA term gets counterbalanced by the magnetostatic term. We have found that the insertion of metallic spacers in the middle of the FeCoB layer noticeably widens the range of thicknesses and temperatures for obtaining the easy-cone state. We show that the W spacer outperforms its Ta counterpart in this enhancement, albeit increasing the magnetization damping due to a higher degree of alloying with FeCoB. A physical mechanism of the easy-cone stabilization is proposed. It considers a variation of the local saturation magnetization (MS) and, notably, Curie temperature (TC) near the MgO/FeCoB interface versus the distance to the metal spacer (or the capping) layer. The larger MS and TC values near MgO provide more thermal stability to the first-order PMA (ks1), while the lower MS and TC ones near the spacer layer result in a faster drop of the magnetostatic term with temperature. As a result, the effective PMA field exhibits a thermal stabilization effect that can be exploited to stabilize the easy-cone anisotropy. Alongside the improved conditions for setting an easy cone in the MgO/FeCoB/MgO free layers, we demonstrate that an easy-cone configuration with an almost temperature-independent opening angle can be obtained using a MgO/FeCoB(1.6 nm)/Ta free layer.