Local and Global Magnetic Properties of Zn$_1-x$Co$_x$O and Mn-Doped GaAs Thin Films

Amorphous and crystalline thin films of Mn-doped (0.5%-10%) GaAs and crystalline thin films of Zn
_1-x
Co
_x
O(x= 3%-20%) were investigated by means of magnetic susceptibility and electron spin resonance (ESR). For the Mn-doped GaAs samples, our results show the absence of ferromagnetic ordering for the amorphous films in the 300>T>2 K temperature range, in contrast to the ferromagnetism found in crystalline films for T
_C
< 110 K. A single ESR line with a temperature independent g-value (g~2) is observed for the amorphous films, and the behavior of this ESR linewidth depends on the level of crystallinity of the film. For the Mn-doped GaAs crystalline films, only a ferromagnetic mode is observed for T_C
when the film is ferromagnetic. Turning now the Zn
_1-x
Co
_x
O films, ferromagnetic loops were observed at room temperature for these films. The magnetization data show an increasing of the saturation magnetization M
_s
as a function of x reaching a maximum value for xap10%. ESR experiments at T= 300 K in the same films show a strong anisotropic ferromagnetic mode (FMR) for x=0.10