Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films

Abstract

The origin of ferromagnetism in ZnO-based systems was investigated using Co-doped ZnO thin films as prototypical examples of II–VI-based diluted magnetic semiconductors. In spite of the atomic-scale dissolution of Co ions in wurtzite ZnO, both the magnetization-temperature curve and the magnetization-field curve demonstrated that Zn1−xCoxO thin films were paramagnetic for x⩽0.12. On the other hand, Zn1−xCoxO films with x greater than 0.12 were characterized by the Co-metal clustering and apparently showed room-temperature ferromagnetism. The discrepancy between the zero-field cooling and the field cooling curves further indicates that Co-doped ZnO films (for x>0.12) are superparamagnetic and the observed ferromagnetism originates from the nanometer-sized Co clusters.

Chemical Manipulation of High-TCFerromagnetism in ZnO Diluted Magnetic Semiconductors

“…1(b) shows statistics from 10 films prepared using O-capped nanocrystals [M S (300 K) 0:045 0:030 B =Co 2 ] and 10 films prepared using N-capped nanocrystals [M S (300 K) 0:00 0:00 B =Co 2 ]. Notably, the oxidative synthesis and spin coating conditions preclude the formation of cobalt metal, and these data therefore refute suggestions that ferromagnetism in Co 2 :ZnO arises only from precipitation of cobalt metal [19]. Instead, the data in Figs.…”

Probing the Local Coordination Environment for Transition Metal Dopants in Zinc Oxide Nanowires

“…It is known that much of the controversy has stemmed from the fact that clustering or phase separation of the magnetic dopant ions can result in magnetic data that is misleading or unreliable. [3][4][5] Even in systems that appear to be quite pure, reported magnetic behavior can vary widely, ranging from room-temperature ferromagnetism 6 to no magnetic ordering at all. 7,8 This variance in magnetism is attributed to the multitude of synthetic methods used for the production of DMS materials, which can result in differences in dopant environment, structural disorder, or carrier concentration.…”

Cobalt substituted ZnO thin films: a potential candidate for spintronics

“…The RTFM observed in these oxide semiconductors has opened a new window for potential application of these materials in realization of spintronic devices. However, the experimental findings on the room temperature ferromagnetic ordering in transition metal doped oxide semiconductors reported so far are at large variance and the nature (i.e., extrinsic or intrinsic) and origin of room temperature ferromagnetism is yet far from being clearly understood [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. In our earlier study on chemically synthesized nanocrystalline ZnO:Co powder samples [3][4][5], we have established the presence of RTFM in these samples as an intrinsic effect.…”

“…In case of thin films, though there exist many reports of RTFM in Co doped ZnO films [6][7][8][9][10][11][12][13][14][15]. In one case, giant magnetic moment of 6.1 l B /Co has been reported in the insulating ZnO:Co thin films [6], whereas in some other cases, RTFM is either claimed to be carrier mediated [7,8] or attributed to the presence of Co clusters [13][14][15]. Contrary to the above, ZnO:Co films are reported to be paramagnetic [16][17][18].…”