Glassy dynamics in Ferromagnet/Antiferromagnet bilayers Open Access

Ma, Tianyu (2016)

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We demonstrate that thin polycrystalline films of antiferromagnet CoO and NiO, in bilayers with ferromagnetic Permalloy, exhibit slow power-law aging of their magnetization state. The aging characteristics are remarkably similar to those previously observed in thin epitaxial Fe(50) Mn(50) films, indicating that these behaviors are likely generic to ferromagnet/antiferromagnet bilayers. For our CoO samples, aging is observed over a wide temperature range in very thin film. In thicker CoO, aging effects become reduced at low temperatures. Aging entirely disappears for large CoO thicknesses. We also investigate the dependence of aging characteristics on temperature and magnetic history. Analysis shows that the observed behaviors are inconsistent with the Neel-Arrhenius model of thermal activation, and are instead indicative of cooperative aging of the antiferromagnet. Our NiO sample exhibits power law aging over a wide range of temperatures. The temperature dependence of the temporal evolution rate exhibits two qualitatively different regimes, with a crossover at the exchange bias blocking temperature. Our results show that the exchange bias blocking temperature represents a transition between two different aging dynamics, reminiscent of the glass transition in amorphous materials. Our results provide new insights into the mechanisms controlling the stationary states and dynamics of ferromagnet/antiferromagnet bilayers, and potentially other frustrated magnetic systems.

Table of Contents

I. Introduction. 1

II. Experiment. 4

III. Summary and Conclusions. 15

Appendix A. 17

Appendix B. 18

Reference. 24

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