J. M. D. Coey

Bio: J. M. D. Coey is an academic researcher from Trinity College, Dublin. The author has contributed to research in topics: Oxide & Composite number. The author has an hindex of 5, co-authored 9 publications receiving 1435 citations.

Thin films: unexpected magnetism in a dielectric oxide.

Abstract: 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.

Magnetism in dilute magnetic oxide thin films based on SnO2

Abstract: Thin films of $\mathrm{Sn}{\mathrm{O}}_{2}$ prepared by pulsed-laser deposition on R-cut sapphire substrates exhibit ferromagnetic properties at room temperature when they are doped with Cr, Mn, Fe, Co, or Ni, but not with other $3d$ cations. Extrapolated Curie temperatures are generally in excess of $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The moment of the films is roughly independent of doping level, from $0.1--15\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$, with a value per unit substrate area of $200\ifmmode\pm\else\textpm\fi{}100\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}\phantom{\rule{0.3em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}2}$. When magnetization is expressed as a moment per $3d$ dopant ion, it varies from more than the spin-only value at low concentrations to less than $0.2\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$/ion near the percolation threshold. Greatest values are found for iron. The magnetization of the films is highly anisotropic with values when the field is applied perpendicular to the substrate more than double the in-plane values. There is little hysteresis except at high doping levels. The oxides are degenerate $n$-type semiconductors with a Hall mobility of $100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and $1.4\ifmmode\times\else\texttimes\fi{}{10}^{19}$ carriers ${\mathrm{cm}}^{\ensuremath{-}3}$ in a one-band model, but no anomalous Hall effect or magnetoresistance was observed at room temperature. The data are discussed in relation to (a) the donor impurity-band model of ferromagnetism in semiconductors and (b) the magnetic defect model.

Critical behavior of La 0.75 Sr 0.25 MnO 3

Abstract: Magnetization, susceptibility, and specific heat measurements were made on a single crystal of ${\mathrm{La}}_{0.75}{\mathrm{Sr}}_{0.25}{\mathrm{MnO}}_{3}.$ The ferromagnetic-to-paramagnetic phase transition was found at 346 K and four critical exponents were measured as: $\ensuremath{\alpha}=0.05\ifmmode\pm\else\textpm\fi{}0.07,$ $\ensuremath{\beta}=0.40\ifmmode\pm\else\textpm\fi{}0.02,$ $\ensuremath{\gamma}=1.27\ifmmode\pm\else\textpm\fi{}0.06,$ and $\ensuremath{\delta}=4.12\ifmmode\pm\else\textpm\fi{}0.33.$ The values of critical exponents are all between mean-field values and three-dimensional-(3D)-Ising-model values. The scaling behavior is well obeyed for all measurements, and the associated exponent relations are well satisfied, validating the critical analysis. Although the cubic crystal structure of this material makes the 3D Heisenberg the expected model, uniaxial magnetic anisotropy arising from the shape of the sample causes the 3D Ising model to be important within the experimental temperature range.

Magnetization and 57Fe hyperfine fields in Y2Fe17Z3- delta (Z=H, C, or N) interstitial compounds.

Abstract: Measured magnetization and 57 Fe hyperfine fields at T∼0 K for Y 2 Fe 17 and Y 2 Fe 17 Z 3-δ , with Z=H, C, or N and δ∼0. 5, are analyzed to determine the influence of the interstitial atoms on the 3d magnetism. All are weak ferromagnets with a nearly-full 3d subband, although when Z=N, the magnetic moment of 38.1 μg is very close to the fully saturated value of 39.4 μg

Hydrogen in R 2 Fe 17 intermetallic compounds: Structural, thermodynamic, and magnetic properties

Abstract: The absorption and diffusion behavior of hydrogen in iron-rich rare-earth intermetallics of the type ${\mathrm{R}}_{2}$${\mathrm{Fe}}_{17}$ (R=Y, Sm, Gd) and the resulting changes of their magnetic properties are investigated. Intermediate hydrogen concentration indicate the solid-solution character of the hydrides. From equilibrium and nonequilibrium absorption experiments net reaction energies, bulk diffusion constants, and surface barrier energies for ${\mathrm{Sm}}_{2}$${\mathrm{Fe}}_{17}$ are determined and discussed. The low-temperature quantum diffusion is discussed for the hydride and the corresponding nitride. The interstitial modification changes drastically the crystalline electric field in the rare-earth intermetallics. For ${\mathrm{Sm}}_{2}$${\mathrm{Fe}}_{17}$${\mathrm{H}}_{4}$ the crystalline electric-field parameter ${\mathrm{A}}_{20}$=110 ${\mathrm{Ka}}_{0}^{\mathrm{\ensuremath{-}}2}$ is estimated from the anisotropy constants ${\mathrm{K}}_{1}$ and ${\mathrm{K}}_{2}$.

Donor impurity band exchange in dilute ferromagnetic oxides.

Abstract: 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.

Magnetic Properties of ZnO Nanoparticles

Abstract: We experimentally show that it is possible to induce room-temperature ferromagnetic-like behavior in ZnO nanoparticles without doping with magnetic impurities but simply inducing an alteration of their electronic configuration. Capping ZnO nanoparticles ( approximately 10 nm size) with different organic molecules produces an alteration of their electronic configuration that depends on the particular molecule, as evidenced by photoluminescence and X-ray absorption spectroscopies and altering their magnetic properties that varies from diamagnetic to ferromagnetic-like behavior.

Ferromagnetism as a universal feature of inorganic nanoparticles

Abstract: Summary Room-temperature ferromagnetism is exhibited by nanoparticles of a variety of inorganic materials although they are intrinsically non-magnetic. Typical of such nanomaterials are the oxides, CeO 2 , TiO 2 , Al 2 O 3 , and MgO. Nanoparticles of nitrides such as GaN and chalcogenides such as CdS and CdSe also exhibit ferromagnetism. Ferromagnetism of the nanoparticles is confined to the surface. This phenomenon has been utilized to render the classic ferroelectric BaTiO 3 to be multiferroic wherein surface ferromagnetism coexists with bulk ferroelectricity. Interestingly, nanoparticles of superconducting YBa 2 Cu 3 O 7 show surface ferromagnetism. It is possible that surface ferromagnetism of inorganic nanoparticles can be usefully exploited.

Dilute doping, defects, and ferromagnetism in metal oxide systems.

Abstract: Over the past decade intensive research efforts have been carried out by researchers around the globe on exploring the effects of dilute doping of magnetic impurities on the physical properties of functional non-magnetic metal oxides such as TiO2 and ZnO. This effort is aimed at inducing spin functionality (magnetism, spin polarization) and thereby novel magneto-transport and magneto-optic effects in such oxides. After an early excitement and in spite of some very promising results reported in the literature, this field of diluted magnetic semiconducting oxides (DMSO) has continued to be dogged by concerns regarding uniformity of dopant incorporation, the possibilities of secondary ferromagnetic phases, and contamination issues. The rather sensitive dependence of magnetism of the DMSO systems on growth methods and conditions has led to interesting questions regarding the specific role played by defects in the attendant phenomena. Indeed, it has also led to the rapid re-emergence of the field of defect ferromagnetism. Many theoretical studies have contributed to the analysis of diverse experimental observations in this field and in some cases to the predictions of new systems and scenarios. In this review an attempt is made to capture the scope and spirit of this effort highlighting the successes, concerns, and questions.

Observation of ferromagnetism at room temperature in ZnO thin films

Abstract: Room-temperature ferromagnetism (FM) has been observed in laser-ablated ZnO thin films. The FM in this type of compound does not stem from oxygen vacancies as in the case of TiO2 and HfO2 films, but from defects on Zn sites. Magnetization of very thin films is much larger than that of the thicker films, showing that defects must be located mostly at the surface and/or the interface between the film and the substrate. Results on Fe-doped ZnO and Mn-doped ZnO films reveal clearly that the metal-transition doping does not play any essential role in introducing the magnetism in ZnO.