Non-classical orbital magnetism in ultrathin transition metal films

Fe, Co, and Ni are the "classical" transition metal ferromagnets usually described in terms of the Stoner or Curie-Weiss model of magnetism. We utilize anomalous Hall effect (AHE) measurements to demonstrate that as a function of temperature T, ultrathin films of Co/Ni exhibit two phase transitions at Tc1 and Tc2>Tc1. The transition at Tc1 is identified as the onset of the usual spin ferromagnetism. The transition at Tc2, in the paramagnetic spin state, is characterized by the opposite sign of AHE, and the corresponding order parameter freezes out above Tc2 before the corresponding symmetry-broken state becomes stable. We analyze the dependence on the field direction, the relative thicknesses of Co and Ni, dc and microwave electric current, and magneto-optic response to show that the transition at Tc2 is associated with the onset of a metastable orbitally-ordered state of 3d electrons. This state is non-classical, it cannot have in-plane magnetic components, and does not exhibit precessional dynamics. Calculations based on the Hubbard model accurately reproduce the observed effects. Our results shed light on the mechanisms of magnetism in multiorbital systems and other electron correlation effects including unconventional superconductivity.