Spintronic anisotropy: proximity-induced superparamagnetism

Superparamagnetism of molecular magnets, i.e. the preferential alignment of their spins along an easy axis, is a useful effect for nanoscale applications as it prevents undesired spin reversals. In these systems such a stabilization of axial spin states is ensured by the magnetic anisotropy barrier stemming from intrinsic spin-orbit coupling. Here we demonstrate that any spin-isotropic high-spin quantum dot coupled to ferromagnets can in fact acquire such superparamagnetic properties in a spintronic way [1], even though spin-orbit interaction is negligible. We predict a proximity-induced spin-anisotropy barrier, which has hallmarks of a spintronic exchange-field of quadrupolar nature: it is highly localized, electrically controllable, increases with tunnel coupling and spin-polarization. Such a field is a generalization of the dipolar exchange field that relates to a current-induced spin-torque, effect well established in spintronics [1-3].\newline [1] M. Misiorny, M. Hell and M. Wegewijs, Nature Phys. advanced online publication, 6 October 2013.\newline [2] J. Martinek et al., Phys. Rev. Lett. 91, 127203 (2003); Phys. Rev. B 72, 121302 (2005).\newline [3] J. Hauptmann et al., Nature Phys. 4, 373 (2008).\newline [4] M. Gaass et al., Phys. Rev Lett. 107, 176808 (2011)