Efficient spin–orbit torque in magnetic trilayers using all three polarizations of a spin current

Spin–orbit coupling can convert a charge current into a spin current, thereby generating a spin–orbit torque (SOT). Energy-efficient, commercially viable SOT technology requires field-free switching of perpendicular magnetization at low current. In heterostructures incorporating ferromagnets, the polarization of spin current consists, in general, of three vectors: \(( {{{{\hat{\mathrm z}}}} \times {{{\hat{\mathrm E}}}}} )\), \({{{\hat{\mathrm m}}}}\) and \({{{\hat{\mathrm m}}}} \times ( {{{{\hat{\mathrm z}}}} \times {{{\hat{\mathrm E}}}}} )\), where \({{{\hat{\mathrm z}}}}\) is the film normal, \({{{\hat{\mathrm E}}}}\) is the electric-field direction and \({{{\hat{\mathrm m}}}}\) is the magnetization direction. Previous studies on SOT have used only part of all the three polarizations, because the two \({{{\hat{\mathrm m}}}}\)-dependent polarizations are mutually orthogonal. Here we show that all the three polarizations can be exploited in systems with ferromagnet/non-magnet/ferromagnet trilayers, having a bottom epitaxial ferromagnet layer with a tilted magnetic easy axis. The approach reduces the field-free SOT switching current compared with approaches that exploit only part of all the three polarizations. We also show that this technique can be used with a sputtered polycrystalline trilayer, illustrating its potential applicability to mass production.