Ferromagnetism as a universal feature of nanoparticles of the otherwise nonmagnetic oxides

Abstract

Room-temperature ferromagnetism has been observed in nanoparticles ͑7 -30 nm diam͒ of nonmagnetic oxides such as CeO 2 , Al 2 O 3 , ZnO, In 2 O 3 , and SnO 2 . The saturated magnetic moments in CeO 2 and Al 2 O 3 nanoparticles are comparable to those observed in transition-metal-doped wideband semiconducting oxides. The other oxide nanoparticles show somewhat lower values of magnetization but with a clear hysteretic behavior. Conversely, the bulk samples obtained by sintering the nanoparticles at high temperatures in air or oxygen became diamagnetic. As there were no magnetic impurities present, we assume that the origin of ferromagnetism may be the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of nanoparticles. We suggest that ferromagnetism may be a universal characteristic of nanoparticles of metal oxides.Integration of semiconductor with ferromagnetic functionality of electrons has been the focus of recent research in the area of spintronics because of the difficulties associated with the injection of spins into nonmagnetic semiconductors in conventional spintronic devices. Ferromagnetism in semiconductors and insulators is rare, the well-known ferromagnetic semiconductors being the chalcogenides EuX ͑X =O, S, and Se͒ ͑T C Ͻ 70 K͒ and CdCr 2 X 4 ͑X = S and Se͒ ͑T C Ͻ 142 K͒ with the rocksalt and spinel structure, respectively. 1,2 Following the theoretical prediction of Dietl et al. that Mn-doped ZnO and GaN could exhibit ferromagnetism above room temperature, 3 several studies have focused on films and bulk samples of metal oxides such as TiO 2 , ZnO, In 2 O 3 , SnO 2 , and CeO 2 doped with Mn, Co, and other transition metal ions. [4][5][6][7][8] While the existence of ferromagnetism in transitionmetal-doped semiconducting oxides remains controversial, 9 thin films of the band insulator HfO 2 have been reported to exhibit ferromagnetism at room temperature in the absence of any doping. 10 This is puzzling, since pure HfO 2 does not have any magnetic moment and the bulk sample is diamagnetic. Similar ferromagnetism has been reported in other nonmagnetic materials such as CaB 6 , CaO, and SiC where the origin of ferromagnetism is believed to be due to intrinsic defects. 11-13 It has been suggested that ferromagnetism in thin films of HfO 2 may be related to anion vacancies. 14 It has been reported very recently that thin films of undoped TiO 2 and In 2 O 3 also show ferromagnetism at room temperature, 15 the corresponding bulk forms of these materials being diamagnetic. Thin films of these oxides might have defects or oxygen vacancies that could be responsible for the observed ferromagnetism. Ab initio electronic structure calculations using density functional theory in HfO 2 have shown that isolated halfnium vacancies lead to ferromagnetism. 16 Meanwhile, there is a conflicting report attributing the ferromagnetism in HfO 2 to possible iron contamination while using stainless-steel tweezers in handling thin films. 17 In this Rapid Communication, ...

Magnetism in Dopant-Free ZnO Nanoplates

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“…Recently, in Ref. 67, surface-vacancy-induced ferromagnetism was obtained for a series of oxide materials including ZnO. In the present situation also, defects may play important roles in obtaining ferromagnetism.…”

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Room-temperature ferromagnetism in Li-dopedp-type luminescent ZnO nanorods

“…Dilute ferromagnetism in oxide semiconductors generally has been reported with 3d dopants in ZnO particularly with Mn, Co, Ni, Fe, Cu, and V dopings. [1][2][3] Unexpected magnetization without transition metal has been reported in dielectric oxides such as HfO 2 , ZrO 2 , 4 and nonmagnetic oxides, e.g., ZnO, 5 hexaborides, 6 carbon-doped ZnO, 7 and in irradiated graphite. 8 The theory of Dietl et al 9 suggests that carrier-induced ferromagnetism in Mn-doped p-type material may be observed at higher temperature.…”