ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5K at 60kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.
In this paper we show that ferromagnetism can be induced in pure TiO2 single crystals by oxygen ion irradiation. By combining x-ray diffraction, Raman-scattering, and electron spin resonance spectroscopy, a defect complex, i.e. Ti 3+ ions on the substitutional sites accompanied by oxygen vacancies, has been identified in irradiated TiO2. This kind of defect complex results in a local (TiO6−x) stretching Raman mode. We elucidate that Ti 3+ ions with one unpaired 3d electron provide the local magnetic moments.Recently, ferromagnetism has been observed in nonmagnetically doped, but defective oxides, including TiO 2 1,2,3,4 . This kind of observation challenges the conventional understanding of ferromagnetism, which is rather due to spin-split states or bands. Thus, one fundamental question must be answered: where are the moments located? Intensive theoretical work has been performed to understand the ferromagnetism in defective oxides 5,6,7 . In these papers, the triplet states of p-like electrons, located at cation or oxygen vacancies, yield the local moments, leading to a kind of ferromagnetism without the involvement of 3d electrons. Experimentally the ferromagnetism in undoped TiO 2 has been found to relate with oxygen vacancies (O V ) 2,3 , however, its mechanism remains unclear. It is worth to note that Ti 3+ ions with one 3d electron are usually generated in slightly reduced TiO 2 . When O is removed, the excess electrons are unpaired 8 . They can occupy the nearby localized Ti 3d orbit and therefore convert Ti 4+ ions to Ti 3+ ions. In a reduced rutile TiO 2 (110) surface, such a defect complex, Ti 3+ -O V , has been well studied by first-principles calculations 9,10 and experimentally by resonant photoelectron diffraction 11 . Therefore, experimental work is needed to clarify whether the magnetic moments in defective TiO 2 is due to unpaired 3d electrons localized on Ti 3+ ions.Ion irradiation is a non-equilibrium and reproducible method of inducing defects. Energetic ions displace atoms from their equilibrium lattice sites, thus creating mainly vacancies and interstitials. The amount of defects can be controlled by the ion fluence and energy. In this paper, we irradiated rutile TiO 2 single crystals with 2-MeV O ions, resulting in a projected range of 1.52 µm and a longitudinal straggling of 0.16 µm as calculated by SRIM code (The Stopping and Range of Ions in Matter) 12 . As a result of this irradiation, the formation of Ti/O vacancies/interstitials is expected 12 . We selected high-energy oxygen ions as projectiles to avoid the introduction of foreign elements. Moreover, from a ballistic point of view, the creation of oxygen vacancies is more efficient, e.g., by a factor of 1.5 larger than the Ti-vacancy creation. From SRIM calculations it is also evident that, at the given energy, the maximum atomic concentration of the implanted oxygen ions is by a factor of 500 smaller than the concentration of oxygen recoils. For the region of maximum defect creation, i.e., at the end of the ion range, those project...
Ferromagnetism in certain alloys consisting of magnetic and nonmagnetic species can be activated by the presence of chemical disorder. This phenomenon is linked to an increase in the number of nearest-neighbor magnetic atoms and local variations in the electronic band structure due to the existence of disorder sites. An approach to induce disorder is through exposure of the chemically ordered alloy to energetic ions; collision cascades formed by the ions knock atoms from their ordered sites and the concomitant vacancies are filled randomly via thermal diffusion of atoms at room temperature. The ordered structure thereby undergoes a transition into a metastable solid solution. Here we demonstrate the patterning of highly resolved magnetic structures by taking advantage of the large increase in the saturation magnetization of Fe60Al40 alloy triggered by subtle atomic displacements. The sigmoidal characteristic and sensitive dependence of the induced magnetization on the atomic displacements manifests a sub-50 nm patterning resolution. Patterning of magnetic regions in the form of stripes separated by ∼ 40 nm wide spacers was performed, wherein the magnet/spacer/magnet structure exhibits reprogrammable parallel (↑/spacer/↑) and antiparallel (↑/spacer/↓) magnetization configurations in zero field. Materials in which the magnetic behavior can be tuned via ion-induced phase transitions may allow the fabrication of novel spin-transport and memory devices using existing lateral patterning tools.
In the last decade, transition metal doped ZnO has been intensively investigated as a route to room temperature diluted magnetic semiconductors (DMS). However the origin for the reported ferromagnetism in ZnO based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition or secondary phases. In order to clarify this question, we have performed a thorough characterization of the structural and magnetic properties of Co and Ni implanted ZnO single crystals. Our measurements reveal that Co or Ni nanocrystals (NCs) are the major contribution of the measured ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed, and they exhibit superparamagnetic properties. The Co or Ni NCs are crystallographically oriented with respect to the ZnO matrix. Their magnetic properties, e.g. the anisotropy and the superparamagnetic blocking temperature can be tuned by annealing. We discuss the magnetic anisotropy of Ni NCs embedded in ZnO concerning the strain anisotropy.
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