Nonmagnetic Oxides Research Articles

Achieving Isolated Fe100−x Pt x Nanoparticles with High Magnetic Coercivity

Monodisperse Fe100−xPtx (x = 37, 41, 47, 54, 61, 66, 74) nanoparticles with an average size of 4.5 nm were successfully synthesized using the chemical polyol process. As-synthesized Fe100−xPtx nanoparticles have the chemi- cally disordered face-centered cubic (fcc) structure with A1 phase. To achieve an ordered structure L10 phase for FePt and L12 phase for FePt3 particles, high-temperature anneal- ing is required. In this work, with optimizing effective pa- rameters of annealing and also by changing the stoichiomet- ric of Fe100−xPtx nanoparticles, we were able to achieve co- ercivity of 16,500 Oe for Fe53Pt47, which is heat treated at 650 ◦ C for 60 min with 20 ◦ C/min (annealing heating rate). It is obvious that the annealing procedure in this tempera- ture leads to destruction of surfactant and sintering. In this work, chemically synthesized Fe53Pt47 nanoparticles were coated by a nonmagnetic CoO oxide shell to prevent them from sintering. Results show that the size of the core/shell (Fe53Pt47/CoO) nanoparticles after the annealing at a tem- perature of 650 ◦ C has not changed compared to the size of the as-synthesized state. Meanwhile, the coercivity of about 5580 Oe is obtained for this nanocomposite.

The effect of capped layer thickness on switching behavior in perpendicular CoCrPt based coupled granular/continuous media

A systematic investigation of magnetic switching behavior of CoCrPt based capped media (perpendicularly coupled granular/continuous (CGC) media consisting of granular CoCrPt:SiO2TiO2Ta2O5/capped CoCrPt(B)) is performed by varying the thickness of the capped layer from 0 to 9nm. The microscopic structures of CGC media with different thickness of capped layer are examined by transmission electron microscope. We find out that CoCrPt magnetic grains are separated by nonmagnetic oxide grain boundaries. Grain size and grain boundary are about 8.9nm and 2nm, respectively. The nonmagnetic oxide grain boundaries in the granular layer do not disappear immediately at the interface between the granular and capped layers. The amorphous grain boundary phase in the granular layer propagates to the top surface of the capped layer. After capping with the CoCrPt(B) layer, the grain size at the surface of CGC structure increases and the grain boundary decreases. Both coercivity and intergranular exchange coupling of the CGC media are investigated by Polar magneto-optic Kerr effect magnetometer and alternating gradient force magnetometer. Although Hc apparently decreases at thicker capped layer, no obvious variation of macroscopic switching field distribution (SFD/Hc) is observed. We separate intrinsic switching field distribution from intergranular interactions. The investigation of reduced intrinsic SFD/Hc and increased hysteresis loop slope at coercivity, suggests that improvement of absolute switching field distribution (SFD) is caused by both strong intergranular exchange coupling and uniform grain size. Micromagnetic simulation results further verify our conclusion that the capped layer in CGC media is not uniformly continuous but has some granular nature. However, grains in the CoCrPt(B) capped layer is not absolutely isolated, strong exchange coupling exists between grains.

Ferromagnetism induced by magnetic vacancies as a size effect in thin films of nonmagnetic oxides

A theory of magnetization induction in thin films of binary nonmagnetic oxides has been developed. We show that the origin and the main peculiarities of magnetization experimentally observed in thin films of such nonmagnetic oxides as SnO2, CeO2, Al2O3, ZnO, MgO, and HfO2 can be explained if oxygen vacancies are considered as magnetization sources. Our calculations have shown that the oxygen vacancies become magnetic at the film–substrate interface, and a long-range ferromagnetic order appears in thin films at room and higher temperatures. The role of substrate turns out to be extremely important for the accumulation of magnetic vacancies taking place owing to the film-substrate misfit stress. The vacancy accumulation has been shown to result in the film magnetization increase, which agrees with experimental data.

Significant room temperature ferromagnetism in PAA thin films

Room-temperature ferromagnetism has been observed in Porous Anodic Aluminum (PAA) thin films, with an out-of-plane saturation magnetization of 60 emu cm−3. The significant out-of-plane saturation magnetization is associated with the porous structure of the film. As there were no magnetic impurities present, the origin of the ferromagnetism is most likely due to exchange interactions between localized electron spin moments resulting from oxygen vacancies in the films. A similar hysteretic behavior is observed in porous thin films of nonmagnetic TiO2 and ZnO. This suggests that ferromagnetism may be a universal characteristic of porous thin films of nonmagnetic metal oxides. Our present results suggest a promising direction for research in spintronic devices.

Influence of Nonmagnetic Oxides on Low-Temperature Resistivity Minima in La2/3Sr1/3MnO3-ZrO2 Matrix Composites

The unusual low-temperature resistivity upturn of composites La2/3Sr1/3MnO3 doped with nonmagnetic-insulator oxide ZrO2 is presented in this paper. The depth of the resistance minima was found for all samples below 30 K, and the resistance minima were suppressed under the applied magnetic field. At the same time, it is remarkable that the temperature of the resistivity of the samples and the temperature of the resistance minima (T min) are changed from increasing to decreasing as the applied magnetic field increases continuously. Therefore, there are at least two dominant physical mechanisms in the (1−x)La2/3Sr1/3MnO3-xZrO2 system; the grand boundaries tunneling effect will promote the electrical transport but it will be suppressed by the external magnetic fields; on the other hand, the grand boundaries scattering effect will suppress the electrical transport. We indicate that the grand boundaries tunneling effect was the main factor in low doping, and the grand boundaries scattering effect began to play a leading role in the low-temperature upturn with the increase of nonmagnetic oxides quantity by theoretical analyzing; in the meantime, we also consider that it will improve spin-order of the electrons in the system at low temperature after La2/3Sr1/3MnO3 has been doped with a trace of ZrO2.

PREVENTION OF SINTERING DURING ANNEALING PROCESS OF FePt NANOPARTICLES COATED WITH ZnO SHELL

Monodispersed 4.1 nm FePt nanoparticles with narrow size distribution were successfully synthesized by the chemical polyol process with co-reduction of Fe (acac)3 and Pt (acac)2 in the presence of 1,2-hexadecanediol as a reducing agent. To achieve hard ferromagnetic behavior with L10 phase and face center tetragonal (fct) structure, high temperature annealing is performed. Annealing causes the surfactant surrounding particles to decompose and agglomeration of particles occurs. In the present work, chemically synthesized FePt nanoparticles were coated with nonmagnetic ZnO oxide shell to prevent them from sintering. Coercivity of FePt and FePt/ZnO nanoparticles increases from 5 kOe to 10 kOe and 1.8 kOe to 6 kOe respectively, with the increasing annealing temperatures from 650 to 750°C.

Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface

LaAlO3 and SrTiO3 are insulating, nonmagnetic oxides, yet the interface between them exhibits a two-dimensional electron system with high electron mobility,1 superconductivity at low temperatures,2-6 and electric-field-tuned metal-insulator and superconductorinsulator phase transitions.3,6-8 Bulk magnetization and magnetoresistance measurements also suggest some form of magnetism depending on preparation conditions5,9-11 and suggest a tendency towards nanoscale electronic phase separation.10 Here we use local imaging of the magnetization and magnetic susceptibility to directly observe a landscape of ferromagnetism, paramagnetism, and superconductivity. We find submicron patches of ferromagnetism in a uniform background of paramagnetism, with a nonuniform, weak diamagnetic superconducting susceptibility at low temperature. These results demonstrate the existence of nanoscale phase separation as suggested by theoretical predictions based on nearly degenerate interface sub-bands associated with the Ti orbitals.12,13 The magnitude and temperature dependence of the paramagnetic response suggests that the vast majority of the electrons at the interface are localized, and do not contribute to transport measurements.3,6,7 In addition to the implications for magnetism, the existence of a 2D superconductor at an interface with highly broken inversion symmetry and a ferromagnetic landscape in the background suggests the potential for exotic superconducting phenomena.

Surface ferromagnetism in non-magnetic and dilute magnetic oxides

We explore hole-doping as an efficient route to develop magnetism in simple nonmagnetic oxides. Based on ab-initio calculations of the prototypical material ZnO, we show that local spin polarization can be obtained even in the absence of magnetic dopants both in the bulk or at surfaces. However, the onset of long-range magnetic order additionally requires extended states, that can be achieved in surfaces without distorting the host lattice. We propose different possibilities which either profit the spontaneous polarization of Oxygen p states under defective charge conditions, or require the presence of adsorbates.

Magnetism of ZnO nanoparticles: Dependence on crystallite size and surfactant coating

Many recent reports on magnetism in otherwise nonmagnetic oxides have demonstrated that nanoparticle size, surfactant coating, or doping with magnetic ions produces room-temperature ferromagnetism. Specifically, ZnO has been argued to be a room-temperature ferromagnet through all three of these methods in various experimental studies. For this reason, we have prepared a series of 1% Fe doped ZnO nanoparticle samples using a single forced hydrolysis coprecipitation synthesis method from the same precursors, while varying size (6–15 nm) and surface coating concentration to study the combined effects of these two parameters. Size was controlled by modifying the water concentration. Surfactant coating was adjusted by varying the concentration of polyacrylic acid in solution. Samples were characterized by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, optical absorbance spectroscopy, and magnetometry. No clear systematic effect on magnetization was observed as a function of surfactant coating, while evidence for a direct dependence of magnetization on the crystallite size is apparent.

Magnetism in C- or N-doped MgO and ZnO: A Density-Functional Study of Impurity Pairs

It is shown that substitution of C or N for O recently proposed as a way to create ferromagnetism in otherwise nonmagnetic oxide insulators is curtailed by formation of impurity pairs, and the resultant C2 spin=1 dimers as well as the isoelectronic N2(2+) interact antiferromagnetically in p-type MgO. For C-doped ZnO, however, we demonstrate using the Heyd-Scuseria-Ernzerhof hybrid functional that a resonance of the spin-polarized C2 ppπ* states with the host conduction band results in a long-range ferromagnetic interaction. Magnetism of open-shell impurity molecules is proposed as a possible route to d(0)-ferromagnetism in oxide spintronic materials.

ChemInform Abstract: Dilute Doping, Defects, and Ferromagnetism in Metal Oxide Systems

AbstractReview: 184 refs.

Dilute Doping, Defects, and Ferromagnetism in Metal Oxide Systems

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 TiO(2) 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.

On the Defect Origin of the Room‐Temperature Magnetism Universally Exhibited by Metal‐Oxide Nanoparticles

The occurrence of ferromagnetism in nanoparticles of otherwise non-magnetic oxides seems to be well established. It is, however, necessary to understand the origin of ferromagnetism in these materials. Herein, we present a combined study of the magnetic properties and photoluminescence (PL) behavior of nanoparticles of ZnO, ZrO(2), and MgO annealed at different temperatures (and therefore of different sizes). We find that the magnetization and the intensity of the bands due to defects vary parallel in all these materials. The adsorption of ethanol leads to a decrease in the magnetization and to a reduced intensity of the defect PL band of ZnO nanoparticles whereas UV irradiation has the opposite effect. We have also examined the effect of the morphology of the ZnO on the properties.

Origin of FM Ordering in Pristine Micro- and Nanostructured ZnO

An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpoint its origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly we find that annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initio calculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions. For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations.

High-frequency Properties of Sub-micron-sized Fe-Co Particles

We report the high-frequency properties of sub-micron-sized Fe-Co particles synthesized with a polyol process, and what influence hard magnetic and non-magnetic oxide layers had on those high-frequency properties. The Fe-Co particles exhibited a resonance peak in the range of a few gigahertz, and the resonance peak shifted to a higher region for decreasing particle size. Furthermore, the presence of a multi-resonance peak in the gigahertz range was recorded. The permeability of surface-oxidized particles decreased and the resonant frequency shifted to a higher region for increasing CoFe2O4 layer thickness on the surface of the particles. However, SiO2 coating did not influence the resonant frequency even though the permeability decreased due to the decrease in the volume fraction of FeCo particles in the sample.

Characterization of iron oxide films prepared by laser irradiation in oxygen atmosphere

Iron oxide films were produced by pulsed laser irradiation of iron metal in pure oxygen gas at 0.1 MPa pressure. A XeCl excimer laser was used for this purpose. Different energy densities were applied for the laser irradiation in order to investigate the laser–plasma–metal interaction and the phase formation mechanisms. The oxygen depth profiles as obtained by Rutherford backscattering spectrometry show an increase in oxygen content with increasing laser energy density. Different magnetic and non-magnetic oxide phases were formed, as revealed by x-ray diffraction and Mössbauer analysis. A possible mechanism for the formation of oxides is presented. Subsequent irradiation of the oxide film with the laser beam in an Ar atmosphere reduces the oxide layer back to pure α-Fe.

Dilute-defect magnetism: Origin of magnetism in nanocrystallineCeO2

We investigate experimentally and theoretically the origin of magnetism in nanocrystalline-undoped ceria films. Our results are consistent with density-functional calculations showing that both oxygen and cerium vacancies lead to ferromagnetism. Although the number of cerium vacancies is smaller than that of oxygen vacancies, the relatively high magnetic moment of the former might result in a substantial contribution to the magnetic properties. Diluted defect magnetism is suggested as a universal feature of ferromagnetism in nonmagnetic oxides without magnetic impurities.

Ab initio study of a TiO2/LaAlO3 heterostructure

In this work we explore the origin of the ferromagnetism appearing when a TiO2 film is grown on another non-magnetic oxide as a substrate such as LaAlO3 (001), concentrating on the role played by the oxygen vacancies in this phenomenon. Using Density Functional Theory ab-initio methods, we study the free-standing anatase film as well as the interfaces with either the LaO or AlO2 planes of LaAlO3. Our results show that the interface LaO/TiO2 is favored against the AlO2/TiO2 one if no oxygen vacancies are present in the interface whereas the contrary happens when there are oxygen vacancies. In both cases, the cohesive energy is of the same order of magnitude but only at AlO2/TiO2 we found a magnetic solution.

Room-temperature ferromagnetism in undopedGaNandCdSsemiconductor nanoparticles

Room-temperature ferromagnetism has been observed in undoped $\mathrm{GaN}$ and $\mathrm{CdS}$ semiconductor nanoparticles of different sizes with the average diameter in the range $10--25\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. These nanoparticles were synthesized by simple routes and thoroughly characterized by various techniques. Magnetization measurements at room temperature show that these nanoparticles are ferromagnetic with a saturation magnetization of $\ensuremath{\sim}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\mathrm{emu}∕\mathrm{gm}$, which is comparable to that observed in nanoparticles of nonmagnetic oxides. On the other hand, agglomerated particles of $\mathrm{GaN}$ and $\mathrm{CdS}$ exhibit diamagnetism or ferromagnetism with small saturation moments. Furthermore, the saturation magnetic moment decreases with the increase in particles size, suggesting that ferromagnetism is due to the defects confined to the surface of the nanoparticles, while the core of the particles remains diamagnetic. The observation of ferromagnetism is consistent with the prediction that Ga vacancies in $\mathrm{GaN}$ give rise to a ferromagnetic ground state.

Magnetic effects at the interface between non-magnetic oxides

The electronic reconstruction at the interface between two insulating oxides can give rise to a highly conductive interface. Here we show how, in analogy to this remarkable interface-induced conductivity, magnetism can be induced at the interface between the otherwise non-magnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance. At low temperatures, the sheet resistance reveals magnetic hysteresis. Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguing many-body effects such as Ruderman-Kittel-Kasuya-Yosida interactions, the Kondo effect and carrier-induced ferromagnetism in diluted magnetic semiconductors. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise non-magnetic materials.