Origin of ferromagnetism in ZnO∕CoFe multilayers: Diluted magnetic semiconductor or clustering effect?

Abstract

Epitaxial growth of (0001) oriented ͓ZnO͑20 Å͒ /Co 0.7 Fe 0.3 ͑x Å͔͒ 25 multilayers (MLs) with nominal thickness x = 1, 2 and 5 has been prepared on ␣-Al 2 O 3 (0001) substrate by ion-beam sputtering. The magnetic properties over a temperature range of 6-350 K and structures probing by x-ray absorption spectroscopy (XAS) are reported. Above room-temperature ferromagnetism has been observed for x = 1 and x = 2 MLs, while superparamagnetic behavior dominates for x = 5 ML. The field-cooled magnetization-temperature M͑T͒ curves of x = 1 and x = 5 MLs can be fitted by a standard three-dimensional (3D) spin-wave and a Curie-Weiss model, respectively. For x = 2 ML, however, neither a 3D spin-wave nor a Curie-Weiss model, but a combination of the two fits the M-T curve. The XAS studies together with the magnetic measurements further reveal that x =1 sample behaves as a diluted magnetic semiconductor (DMS) ML, while x = 2 ML shows a mixed structure consisting of a minor component of DMS and a major component of CoFe clusters. A predominant clustering phase appears for x = 5 ML.

Enhanced ferromagnetism in single crystalline Co-doped ZnO thin films by Al codoping

“…Theoretically, Dietl et al first predicted that p-type Mn doped ZnO would show ferromagnetism with T C well above RT [3]. In contrast, Co-doped ZnO was theoretically predicated to become ferromagnetic with n-type doping [4] and a flood of experimental studies of this subject have been reported upon [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. However, the presented data were plagued by instability and a lack of reproducibility.…”

“…Zn0.96Co0.04O (ZnCoO) and Zn0.95Co0.04Al0.01O (ZnCoAlO) thin films with a thickness of 500 Å were prepared in an MBE system with a base pressure of 5 × 10 −10 Torr on Al2O3 (1 12 0) substrate at 400 • C using the ␦-doping technique [23,24]. A 50 Å pure ZnO film was grown on the substrate as a buffer layer before the film was deposited.…”

Room temperature ferromagnetism in Mn-doped zinc oxide nanorods prepared by hybrid wet chemical route

“…This is in agreement with the results reported by others [42,43] which led to the suggestion of room temperature ferromagnetism. A model [44] combining both paramagnetic and ferromagnetic components may account for the initial decrease of magnetization from 2 K to 35 K and remaining non-zero up to 330 K. No signature of superparamagnetic blocking temperature (T B ) peak could be observed in the ZFC magnetization data. The blocking may be present in the nanorods but may not be observed because of very small particle size.…”

Room-temperature ferromagnetism in Co-doped ZnO bulk induced by hydrogenation

“…But some groups also failed to found ferromagnetism in ZnO-based semiconductors doped with ferromagnetic transition metal [4,5]. The origin of ferromagnetism in Co-doped ZnO material is still not clear [6]. Recently, Martinez et al proposed the relation between the microstructure of the Co-doped ZnO particle and the RTFM [7].…”