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.

Donor impurity band exchange in dilute ferromagnetic oxides.

A review of recent results on transition metal doping of electronic oxides such as ZnO, TiO2, SnO2, BaTiO3, Cu2O, SrTiO3 and KTaO3 is presented. There is interest in achieving ferromagnetism with Curie temperatures above room temperature in such materials for applications in the field of spintronic devices, in which the spin of the carriers is exploited. The incorporation of several atomic per cent of the transition metals without creation of second phases appears possible under optimized synthesis conditions, leading to ferromagnetism. Pulsed laser deposition, reactive sputtering, molecular beam epitaxy and ion implantation have all been used to produce the oxide-based dilute magnetic materials. The mechanism is still under debate, with carrier-induced, double-exchange and bound magnetic polaron formation all potentially playing a role depending on the conductivity type and level in the material.

Dilute magnetic semiconducting oxides

There has been considerable recent interest in the design of diluted magnetic semiconductors, with a particular focus on the exploration of different semiconductor hosts. Among these, the oxide-based diluted magnetic semiconductors are attracting increasing attention, following reports of room temperature ferromagnetism in anatase TiO2 and wurtzite ZnO doped with a range of transition metal ions. In this review we summarize the current status of oxide-based diluted magnetic semiconductors, and discuss the influence of growth method, substrate choice, and temperature on the microstructure and subsequent magnetic properties of thin films. We outline the experimental conditions that promote large magnetization and high ferromagnetic Curie temperature. Finally, we review the proposed mechanisms for the experimentally observed ferromagnetism and compare the predictions to the range of available data.

https://iopscience.iop.org/article/10.1088/0953-8984/17/27/R01/pdf

Transition metal-doped TiO2 and ZnO?present status of the field

Oxide-diluted magnetic semiconductors (O-DMS) have attracted a great deal of interest in recent years due to the possibility of inducing room temperature ferromagnetism These materials are of particular interest for spintronic devices such as spin valves This review describes the experimental status of the O-DMS including the recent results on ZnO- and TiO2-based systems

https://iopscience.iop.org/article/10.1088/0953-8984/15/37/R01/pdf

Oxide-diluted magnetic semiconductors: a review of the experimental status

Concomitant with the development of metal-based spintronics in the late 1980s and 1990s, important advances were made on the growth of high-quality oxide thin films and heterostructures. While this was at first motivated by the discovery of high-temperature superconductivity in perovskite Cu oxides, this technological breakthrough was soon applied to other transition-metal oxides and, notably, mixed-valence manganites. The discovery of colossal magnetoresistance in manganite films triggered intense research activity on these materials, but the first notable impact of magnetic oxides in the field of spintronics was the use of such manganites as electrodes in magnetic tunnel junctions, yielding tunnel magnetoresistance ratios that are one order of magnitude larger than what had been obtained with transition-metal electrodes. Since then, research on oxide spintronics has been intense, with the latest developments focused on diluted magnetic oxides and, more recently, on multiferroics. In this paper, the most important results on oxide spintronics was reviewed, emphasizing materials physics as well as spin-dependent transport phenomena, and finally give some perspectives on how the flurry of new magnetic oxides could be useful for next-generation spintronics devices

Oxide Spintronics

We have investigated Co-doped TiO2 thin films grown by reactive co-sputtering. X-ray diffraction showed a single phase polycrystalline rutile structure, without any segregation of Co into particulates within the instrumental resolution limit. The atomic content of Co ranged from 1% to 12%. The temperature dependence of resistivity showed an extrinsic semiconducting behavior. From optical absorption measurements, the band gap Eg≈3.25±0.05 eV was found, independent of the Co concentration, and in agreement with a literature value. Room temperature M-H loops showed a ferromagnetic behavior for Co content higher than 3%. The magnetic moment per Co atom was estimated to be about 0.94 μB, suggesting a low spin configuration of Co ions. The temperature dependence of remanent magnetization revealed a Curie temperature higher than 400 K for Co content of 12%.

Semiconducting and ferromagnetic behavior of sputtered Co-doped TiO2 thin films above room temperature

We report detailed studies of magnetization (M) as a function of magnetic field H (up to 50 kOe) and temperature T (5–380 K) in x% Co/TiO2 (rutile) films for x=1, 4, 6, 10, and 12. These films, prepared by sputtering, show room temperature ferromagnetism (RTFM). Measurements of M vs T at H=50 Oe reported here in zero-field-cooled (ZFC) and field-cooled (FC) cases show M peaking at Tp≃30 K for 1% Co to Tp≃125 K for 12% Co doped films whereas bifurcation of the FC and ZFC data is observed near 300 K. For T<Tp, the coercivity Hc rises sharply. These observations and the electron magnetic resonance data suggest that the observed RTFM is at least partly due to magnetic nanoparticles, most likely of Co, that are undetected in standard X-ray diffraction (XRD) for lightly doped samples. XRD studies show that a fraction of Co does substitute for Ti in CoxTi1−xO2. However, because of interference from Co nanoparticles, the RTFM of CoxTi1−xO2 in these samples could not be established.

On the room temperature ferromagnetism in Co-doped TiO2 films

Low-frequency voltage fluctuations in magnetic and nonmagnetic tunnel junctions have been investigated. Tunneling magnetoresistance ratio of junctions ranged from 8.6% to 50% at room temperature. The origin of voltage noise was observed to be nonmagnetic. It was found that the resistance fluctuations lead to 1/f noise at room temperature, whereas the white noise was dominant at 77 K. A phenomenological noise parameter, defined for the comparison of noise levels in different junctions, was shown to scale with the junction resistance–area product. This observation suggests that low values of resistance×area are required for a low noise operation as well as for the integration of magnetic tunnel junctions.

Noise properties of magnetic and nonmagnetic tunnel junctions

The time dependences, up to 200 s, and the noise power spectrum (0.005-10 Hz) in the electron transport response at bias up to 300 mV of Co/Al2O3/Ni80Fe20 magnetic tunnel junctions (MTJs) and of Co/Al2O3δ(Fe)/Ni80Fe20 (with Fe δ dopants of thickness 1.8 A inside the barrier) were investigated. The magnetic field was changed between +100 and -100 G in steps of 1 G. The measurements were carried out at different temperatures between 77 and 300 K for the samples with large tunnel magnetoresistance (exceeding 14% at 300 K). We found that the magnetization reversal of the Co and permalloy electrodes, as detected from the time response near the coercive field, occurs via relaxation on the timescale of about 102 s with sudden jumps in the resistance (ΔR/R~10-2-10-3). We link this noise to the depinning of the domain walls. In addition to the magnetic noise, in some of the studied MTJs with Fe δ dopants, we observed a two-level-system telegraph-type noise, which was independent of the magnetic field, indicating its relation to the trapped charges inside the insulating barrier. For MTJs, the noise power spectrum has 1/fα character for a wide frequency range below a few Hz. At low bias and parallel state the exponent α is close to 1-1.5, but at higher bias or in the antiparallel state the exponent increases to 2. We link these effects to non-equilibrium noise in magnetic structure of the electrodes in the antiparallel state and to non-equilibrium transport inside the barrier at high bias.

https://iopscience.iop.org/article/10.1088/0022-3727/35/14/317/pdf

Wan Kyu Park

Papers

Semiconducting and ferromagnetic behavior of sputtered Co-doped TiO2 thin films above room temperature

On the room temperature ferromagnetism in Co-doped TiO2 films

Noise properties of magnetic and nonmagnetic tunnel junctions

Low frequency noise in Co/Al2O3⟨δ(Fe)⟩/Ni80Fe20 magnetic tunnel junctions