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-xMnx, with freezing temperatures of T-f=12 K and T-f=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 Ge0.96Mn0.04, ac susceptibility measurements show a peak around T-f, with the peak position T-f ' shifting as a function of the driving frequency f by Delta T-f '/[T-f ' Delta log f]approximate to 0.06, whereas for sample Ge0.8Mn0.2 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 T-f 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.