Dipole interactions with random anisotropy in a frozen ferrofluid

Glassy behavior (including hysteresis, irreversibility, a peak in the zero-field-cooled magnetization, and nonexponential relaxation) is observed in a quenched ferrofluid system consisting of 50-Å magnetite particles. An Arrott plot, ${\mathit{M}}^2$ versus H/M, shows clear features of random anisotropy similar to what is found in amorphous ferromagnets. We discuss the glassy behavior in terms of both the random anisotropy and the dipole interactions, and we constrast the unusual response of our system with canonical spin glasses.