Dilute ferromagnetic oxides having Curie temperatures far in excess of 300 K and exceptionally large ordered moments per transition-metal cation challenge our understanding of magnetism in solids. These materials are high-k dielectrics with degenerate or thermally activated n-type semiconductivity. Conventional super-exchange or double-exchange interactions cannot produce long-range magnetic order at concentrations of magnetic cations of a few percent. We propose that ferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band. The Curie temperature in the mean-field approximation varies as (xdelta)(1/2) where x and delta are the concentrations of magnetic cations and donors, respectively. High Curie temperatures arise only when empty minority-spin or majority-spin d states lie at the Fermi level in the impurity band. The magnetic phase diagram includes regions of semiconducting and metallic ferromagnetism, cluster paramagnetism, spin glass and canted antiferromagnetism.

It is generally accepted that magnetic order in an insulator requires the cation to have partially filled shells of d or f electrons. Here we show that thin films of hafnium dioxide (HfO2), an insulating oxide better known as a dielectric layer for nanoscale electronic devices, can be ferromagnetic even without doping. This discovery challenges our understanding of magnetism in insulators, because neither Hf4+ nor O2- are magnetic ions and the d and f shells of the Hf4+ ion are either empty or full.

Abstract. Room-temperature ferromagnetism is observed in (110) oriented ZnO films containing 5 at % of Sc, Ti, V, Fe, Co or Ni, but not Cr, Mn or Cu ions. There are large moments, 1.9 and 0.5µ B /atom for Co-and Ti-substituted oxides, respectively. Scsubstituted ZnO shows also a moment of 0.3 µ B /Sc. Magnetization is very anisotropic, with variations of up to a factor three depending on the orientation of the applied field relative to the R-cut sapphire substrates. Results are interpreted in terms of a spin-split donor impurity band model, which can account for ferromagnetism in insulating or conducting high-k oxides with concentrations of magnetic ions that lie far below the percolation threshold. The variation of the ferromagnetism with oxygen pressure used during film growth is evidence of a link between ferromagnetism and defect concentration.PACS Numbers: 75.50.Pp; 75.30.Hx;75.30.Gw;75.70 [2][3][4][5][6] or another transition element [7][8][9][10]. The results are sensitive to the form of the sample and preparation method. Other studies found lower magnetic ordering temperatures [11][12][13][14], or no ferromagnetism at all above 3 K for any 3d dopant [15]. In the absence of an exchange mechanism which could account for a high Curie temperature at doping levels far below the percolation threshold, these reports have been received with skepticism, and the belief that the ferromagnetism must somehow be associated with clustering or incipient formation of secondary phases. But there is spectroscopic evidence that divalent cobalt does indeed substitute on the tetrahedral sites of the wurtzite structure [1,11,17], with a wide solid solubility range [15]. A search by Rode et al [4] revealed no evidence for phase segregation in Co-doped ZnO films.Nevertheless, until a clear connection between the magnetic properties and electronic structure can be shown, doubts that doped zinc oxide is truly a magnetic semiconductor will persist.We recently proposed a model for high-temperature ferromagnetism in dilute n-type

Thin films grown by pulsed-laser deposition from targets of Sn 0.95 Fe 0.05 O 2 are transparent ferromagnets with Curie temperature and spontaneous magnetization of 610 K and 2.2 A m 2 kg Ϫ1 , respectively. The 57 Fe Mössbauer spectra show the iron is all high-spin Fe 3ϩ but the films are magnetically inhomogeneous on an atomic scale, with only 23% of the iron ordering magnetically. The net ferromagnetic moment per ordered iron ion, 1.8 B , is greater than for any simple iron oxide with superexchange interactions. Ferromagnetic coupling of ferric ions via an electron trapped in a bridging oxygen vacancy (F center͒ is proposed to explain the high Curie temperature. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1650041͔ First generation spin electronics 1 was based on magnetoresistive sensors and memory elements using electrodes made from alloys of the ferromagnetic 3d metals Fe, Co and Ni. There is an ongoing quest for ferromagnetic semiconductors with a Curie temperature well above room temperature, which could be used for a second generation of spin electronics, as well as a search for transparent ferromagnets which can add an optoelectronic dimension. Much recent interest has been generated by high temperature ferromagnetism in oxide and nitride materials such as ZnO with Co or Mn doping, 2-4 TiO 2 ͑anatase͒ with Co, 5 GaN with Mn, 6 AlN with Cr, 7 and SnO 2 with Mn ͑Ref. 8͒ or Co. 9 Doubts linger as to whether these are homogeneous, single-phase materials, particularly since the well-accepted mechanisms for ferromagnetic coupling, via spin-polarized p-band holes like those in Ga 1Ϫx Mn x As, 10 or via double exchange as in mixed valence manganites, 11 do not seem to apply in these oxides and nitrides.Following a recent report by Ogale et al. 9 of high temperature ferromagnetism with a giant cobalt moment in Codoped SnO 2 , we undertook an investigation of the magnetism of Fe-doped SnO 2 . We find atomic-scale inhomogeneity and remarkably strong ferromagnetism, for which a novel ferromagnetic exchange mechanism is suggested.Ceramic targets of Sn 0.95 Fe 0.05 O 2 were first prepared by solid-state reaction of SnO 2 and FeO or 57 Fe 2 O 3 at 1150°C. Rietveld analysis of the x-ray diffraction patterns of the targets showed SnO 2 with a trace of ␣-Fe 2 O 3 ͑Fig. 1͒. Elemental maps of the targets obtained by energy-dispersive x-ray ͑EDAX͒ diffraction indicated a nonuniform iron distribution, with some tendency to accumulate iron in regions 2-4 m in size which were identified as Sn-doped hematite. The ceramics were ferromagnetic, with magnetization at 5 K of 2.3 A m 2 kg Ϫ1 (Ϸ1.2 B /Fe), and a Curie temperature of 360 K. Mössbauer spectra showed that all the iron was highspin Fe 3ϩ , and 88% of it was magnetically ordered with a hyperfine field of 53.3 T at 19 K. The ferromagnetism cannot be attributed to the Sn-doped hematite, which is a canted antiferromagnet with a weak net moment. 12 The thin films were deposited on R-cut sapphire substrates using a KrF excimer laser operating at 248 nm and 10 Hz. Laser ...

Thin films of HfO 2 produced by pulsed-laser deposition on sapphire, yttria-stabilized zirconia, or silicon substrates show ferromagnetic magnetization curves with little hysteresis and extrapolated Curie temperatures far in excess of 400 K. The moment does not scale with film thickness, but in terms of substrate area it is typically in the range 150-400 B nm −2 . The magnetization exhibits a remarkable anisotropy, which depends on texture and substrate orientation. Pure HfO 2 powder develops a weak magnetic moment on heating in vacuum, which is eliminated on annealing in oxygen. Lattice defects are the likely source of the magnetism.