A Distribution of Tunnel Splittings in Mn${}_{12}$-Acetate

Abstract

In magnetic fields applied parallel to the anisotropy axis, the relaxation of the magnetization of Mn${}_{12}$ measured for different sweep rates is shown to collapse onto a single scaled curve. The form of the scaling implies that the dominant symmetry-breaking process that gives rise to tunneling is a locally varying second-order anisotropy, forbidden by tetragonal symmetry in the perfect crystal, which gives rise to a broad distribution of tunnel splittings in a real crystal of Mn${}_{12}$-acetate. Different forms applied to even and odd-numbered steps provide a distinction between even step resonances (associated with crystal anisotropy) and odd resonances (which require a transverse component of magnetic field).

4 pages, 5 figures. New title; text more clearly written

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