Nonthermal viscosity in magnets: Quantum tunneling of the magnetization (invited)

In this article we present experimental results on the magnetic relaxation in different systems (single domain particles, magnetic grains, and random magnets). The existence of two relaxation regimes is demonstrated. At high temperatures, the magnetic viscosity S≡1/M₀∂M/∂ ln(t) is proportional to temperature in accordance with theoretical expectation for thermally activated processes. At low temperatures, the viscosity is independent of temperature, providing evidence to quantum tunneling of magnetization. Qualitative agreement between theory and experiment is found.