Heisenberg domain state in thin-film ferromagnet/antiferromagnet bilayers is a magnetic glass*

Random exchange interaction at the interface between ferromagnetic (F) and antiferromagnetic (AF) films results in the formation of a multidomain state. It was predicted by Malozemoff that for sufficiently thin films, domains become smaller than the domain wall width, forming a “Heisenberg domain state” (HDS) [1]. The nature and properties of HDS remain unexplored.
We utilize magnetoelectronic and magneto-optical measurements of relaxation and dynamical characteristics in F/AF bilayers with AF=CoO, NiO, and FeMn to show that HDS is a magnetic glass. Our observations include nearly temperature-independent universal magnetic aging below a certain temperature T_G, transitioning to a strong temperature dependence at T>T_G. The transition is associated with a qualitative change in the dependence on the magnetic history. Transverse ac susceptibility measurements indicate that the magnetic viscosity varies by 4 orders of magnitude when T~T_G is varied by 20 K, indicative of magnetic freezing consistent with the glass transition. Our results demonstrate that thin-film F/AF bilayers are a readily realizable and experimentally accessible quasi-2d magnetic glass system with controllable characteristics.

[1] A. Malozemoff, Phys. Rev. B 37, 7673 (1988).