Macrospin behavior and superparamagnetism in (Ga,Mn)As nanodots

An array of small independent and quasimonodisperse nanodots of the dilute magnetic semiconductor (Ga,Mn)As with out-of-plane easy axis has been patterned by electronic lithography and ion-beam etching from a strained epitaxial film. The thermal dependence (2<T<120 K) of the field-induced magnetic loops, coercivity, and anisotropy of the nanodots, have been measured by sensitive magneto-optical magnetometry. Below the superparamagnetic blocking temperature TB, all results are consistent with a quasicoherent thermally activated and field-induced spin-rotation process. In spite of the nonconventional nature of the ferromagnetism mediated by holes and of the high dilution in manganese ions, TB is well accounted for by the usual N\'eel-Brown model.