Abstract
Diamagnetic oxides can, under certain conditions, become ferromagnetic at room temperature and therefore are promising candidates for future material in spintronic devices. Contrary to early predictions, doping ZnO with uniformly distributed magnetic ions is not essential to obtain ferromagnetic samples. Instead, the nanostructure seems to play the key role, as room temperature ferromagnetism was also found in nanograined, undoped ZnO. However, the origin of room temperature ferromagnetism in primarily non–magnetic oxides like ZnO is still unexplained and a controversial subject within the scientific community. Using low energy muon spin relaxation in combination with SQUID and TEM techniques, we demonstrate that the magnetic volume fraction is strongly related to the sample volume fraction occupied by grain boundaries. With molecular dynamics and density functional theory we find ferromagnetic coupled electron states in ZnO grain boundaries. Our results provide evidence and a microscopic model for room temperature ferromagnetism in oxides.
Highlights
Diamagnetic oxides can, under certain conditions, become ferromagnetic at room temperature and are promising candidates for future material in spintronic devices
Using low energy muon spin relaxation in combination with superconducting quantum interference device (SQUID) and TEM techniques, we demonstrate that the magnetic volume fraction is strongly related to the sample volume fraction occupied by grain boundaries
The idea was that a slight (, 5%) transition metal (TM) doping of a semiconductor like ZnO would lead to ferromagnetic coupling of these ions within the host material, even at room temperature (RT)
Summary
Diamagnetic oxides can, under certain conditions, become ferromagnetic at room temperature and are promising candidates for future material in spintronic devices. The idea was that a slight (, 5%) transition metal (TM) doping of a semiconductor like ZnO would lead to ferromagnetic coupling of these ions within the host material, even at room temperature (RT). By analyzing a large number of experimental publications we found earlier, that the grain size, in particular the grain area to volume fraction, i.e. the specific grain boundary area sGB, plays an important role for ferromagnetism in doped and undoped ZnO nanostructures[14,15]. Low energy positive muons are implanted into the host material and come to rest at interstitial lattice site due to their positive charge. They act as highly sensitive probes of magnetic fields originating from magnetic moments in their close proximity. Two 10 3 10 3 1 mm[3] and 5 3 5 3 1 mm[3] commercial ZnO single crystals (Mateck Company, Germany) were used as nearly grain boundary free, nonmagnetic reference system for mSR and SQUID measurements
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