Competition between magnetic relaxation mechanisms in exchange-coupled CoO∕Co bilayers

Transverse, ac-susceptibility measurements, χ(T), of polycrystalline [Co(8nm)∕CoO(xnm); 1⩽x⩽10] bilayers are used to derive the magnetic anisotropy in an exchange-coupled ferromagnetic(FM)/antiferromagnetic(AFM) system. χ(T) was measured using the anisotropic magnetoresistance (AMR) induced by a small (≲5Oe) applied field at frequencies ranging from 40to400Hz. This low-field technique is sensitive to the full anisotropy field and to the distinctions between rotatable and nonrotatable anisotropy, reversible and dissipative contributions, and to the difference between the Néel and blocking temperatures, TN and TB. The imaginary response shows three distinct mechanisms of relaxation, each with a different characteristic activation energy. These mechanisms are modelled succesfully using a simple magnetic free energy and thermal activation. Near TB the sample changes from a saturated FM state to an unpinned, multidomain FM state with no exchange bias. Near TN, the sublattice magnetization of individual crystalline AFM grains switches between easy axes and the interfacial exchange anisotropy field disappears. At low temperature (∼70K), ferromagnetic exchange coupling of individual FM grains to the surrounding FM film can be bistable, creating soft FM degrees of freedom. This mechanism produces a rotatable anisotropy which dynamically inhibits these grains from contributing to the AFM interfacial exchange coupling. As these motions are frozen out at very low temperature, there is a large increase in the measured magnetic anisotropy which gives an important contribution to the difference in the exchange anisotropy as measured by low- and high-field techniques.