Demonstration and quantitative characterization of effective random exchange fields in ferromagnet/antiferromagnet bilayers*

It was conjectured over 30 years ago that some of the unusual magnetic properties of ferromagnet/antiferromagnet bilayers are associated with the effective random field arising due to the frustration of exchange interaction at their interface [1]. This conjecture was supported by recent measurements demonstrating a correlated spin glass state in these systems [2], but has not yet been directly confirmed.
We utilize magnetoelectronic measurements of anisotropic magnetoresistance in Permalloy(Py)/CoO bilayers to confirm the predicted random-field effects and quantify the random field. In particular, we show that the component of magnetization of F perpendicular to the external field scales as a power-law with the exponent dependent on the thickness of Py. Scaling analysis and micromagnetic simulations confirm that these dependences are consistent with the expected effects of random field, and allow us to determine its magnitude. Our results open a route for analyzing and controlling magnetic frustration in heterostructures.

[1] A. Malozemoff, Phys. Rev. B 37, 7673 (1988).
[2] T. Ma, S. Urazhdin, Phys. Rev. B 97, 054402 (2018); S. Urazhdin, W. Li, L. Novozhilova, JMMM 476, 75 (2019).