Physics Enabled by the Discovery of Spin-Transfer Torques
The magnetization of a magnetic material can be reversed by using electric currents that transport spin angular momentum. This was predicted in magnetic tunnel junctions—two thin metallic ferromagnetic layers separated by a thin insulating barrier—in 1989 and in magnetic multilayers by Luc Berger and John Slonczewski in 1996 and demonstrated experimentally several years later. This discovery has had an enormous impact on magnetism research and technology, as prior to this the primary means to reorient the magnetization of a magnet was by applying magnetic fields (dating to 1819 and Oersted!). In this talk I will highlight some of the new physics enabled by the discovery of spin-transfer torques. This includes recent experiments in my group to create spin-wave excitations (magnons) and “Bose” condensates of magnons in thin films with uniaxial magnetic anisotropy. Spin-transfer torques also permit study of magnetic analogues of superconductivity, superfluidity and the Josephson effect that promise to increase our understanding of collective quantum effects.