Abstract
In order to provide insight into the magnetic properties of semiconductors with ferromagnetic inclusions, we present a thorough quantitative analysis of (Zn,Co)O films with high Co concentration, which give rise to high saturation magnetization values (similar to 110 kA m(-1)) at room temperature. From the orientation-, temperature-, and field-dependent responses of the magnetization and of the Q-band ferromagnetic resonance modes, we give evidence of two types of nanosized ferromagnetic Co clusters and derive their respective contributions. The first type are spherical with diameters about 5 nm and blocking temperatures T(B) about 100 K. The second type are elongated along the surface normal and crystallographically oriented with respect to the ZnO host. Some are nanocolumns about 4 nm wide, whose height may reach up to 60 nm, leading to T(B) >= 300 K. These results are confirmed by high-resolution transmission electron microscopy analysis. In addition to provide transition from ferromagnetism to superparamagnetism above 300 K, the nanocolumns lead to strong anisotropic magnetic properties. We believe that they could find applications in spintronic devices.
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