Spin-glass-like behavior of Ge:Mn

We present a detailed study of the magnetic properties of low-temperature molecular-beam-epitaxy-grown Ge:Mn dilute magnetic semiconductor films. We find strong indications for a frozen state of Ge1-x Mnx, with freezing temperatures of Tf =12 K and Tf =15 K for samples with x=0.04 and x=0.2, respectively, determined from the difference between field-cooled and zero-field-cooled magnetization. For Ge0.96 Mn0.04, ac susceptibility measurements show a peak around Tf, with the peak position Tf′ shifting as a function of the driving frequency f by Δ Tf′ [Tf′ Δ log f] ≈0.06, whereas for sample Ge0.8 Mn0.2 a more complicated behavior is observed. Furthermore, both samples exhibit relaxation effects of the magnetization after switching the magnitude of the external magnetic field below Tf which are in qualitative agreement with the field- and zero-field-cooled magnetization measurements. These findings consistently show that Ge:Mn exhibits a frozen magnetic state at low temperatures and that it is not a conventional ferromagnet.