Enhanced intergrain tunneling magnetoresistance in half-metallic CrO2 films

Magnetic transitions of hydrogenated HxCrO2(x= 0-2) monolayer from a ferromagnetic half-metal to antiferromagnetic insulator

Two-dimensional (2D) transition metal oxide monolayers are currently attracting great interest in materials research due to their versatility and tunable electronic and magnetic properties. In this study, we report the prediction of magnetic phase changes in H_xCrO₂(0 ⩽x⩽ 2) monolayer on the basis of first-principles calculations. As the H adsorption concentrationxincreases from 0 to 0.75, H_xCrO₂monolayer transforms from a ferromagnetic (FM) half-metal to a small-gap FM insulator. Whenx= 1.00 and 1.25, it behaves as a bipolar antiferromagnetic (AFM) insulator, and eventually becomes an AFM insulator asxincreases further up to 2.00. The results suggest that the magnetic properties of CrO₂monolayer can be effectively controlled by hydrogenation, and that H_xCrO₂monolayers have the potential for realizing tunable 2D magnetic materials. Our results provide a comprehensive understanding of the hydrogenated 2D transition metal CrO₂and provide a research method that can be used as a reference for the hydrogenation of other similar 2D materials.

Realization of a Half Metal with a Record-High Curie Temperature in Perovskite Oxides

Half metals, in which one spin channel is conducting while the other is insulating with an energy gap, are theoretically considered to comprise 100% spin-polarized conducting electrons, and thus have promising applications in high-efficiency magnetic sensors, computer memory, magnetic recording, and so on. However, for practical applications, a high Curie temperature combined with a wide spin energy gap and large magnetization is required. Realizing such a high-performance combination is a key challenge. Herein, a novel A- and B-site ordered quadruple perovskite oxide LaCu₃ Fe₂ Re₂ O₁₂ with the charge format of Cu²⁺ /Fe³⁺ /Re^4.5+ is reported. The strong Cu²⁺ (↑)Fe³⁺ (↑)Re^4.5+ (↓) spin interactions lead to a ferrimagnetic Curie temperature as high as 710 K, which is the reported record in perovskite-type half metals thus far. The saturated magnetic moment determined at 300 K is 7.0 μ_B f.u.^-1 and further increases to 8.0 μ_B f.u.^-1 at 2 K. First-principles calculations reveal a half-metallic nature with a spin-down conducting band while a spin-up insulating band with a large energy gap up to 2.27 eV. The currently unprecedented realization of record Curie temperature coupling with the wide energy gap and large moment in LaCu₃ Fe₂ Re₂ O₁₂ opens a way for potential applications in advanced spintronic devices at/above room temperature.

Origins of Thermal Spin Depolarization in Half-Metallic Ferromagnet CrO_{2}

Using high-resolution spin-resolved photoemission spectroscopy, we observe a thermal spin depolarization to which all spin-polarized electrons contribute. Furthermore, we observe a distinct minority spin state near the Fermi level and a corresponding depolarization that seldom contributes to demagnetization. The origin of this depolarization has been identified as the many-body effect characteristic of half-metallic ferromagnets. Our investigation opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.

Lithium permeation within lithium niobate multilayers with ultrathin chromium, silicon and carbon spacer layers

Li permeation through ultrathin Cr, Si and C layers and interfaces is of interest in the optimization of lithium ion batteries with respect to the control of Li flux. Twenty-one LiNbO3 layers (9 nm), which serve as solid state Li reservoirs, were sputter deposited in an alternating sequence of enriched 6Li or 7Li isotope fractions spaced with (8 nm) thin Cr, Si and C layers. The Li isotope contrast was used to measure Li permeation using depth profiling by secondary ion mass spectrometry and neutron reflectometry on a nanometer scale. Extremely low Li permeation for Cr and Si at room temperature exemplifies the effective blocking of Li movement at least for five years. However, Li permeation through C layers was found to be faster than through Cr and Si layers. With temperature, the Li permeation is enhanced through Cr as compared to that through Si layers. Furthermore, material characterisation shows amorphous LiNbO3, C and Si layers and polycrystalline Cr layers (with 80% elemental bcc chromium and 20% chromium-oxide situated at Cr/LiNbO3 interfaces). Annealing in air at 100 °C (373 K) does not oxidize the Cr layers any further. A stress of 12 GPa, which was measured in Cr spacer layers at room temperature, remains unchanged upon annealing. The origin of a weak ferromagnetic order measured at room temperature (300 K) was attributed to some traces of Cr and Si inside LiNbO3.

Short-Range Correlated Magnetic Core-Shell CrO₂/Cr₂O₃ Nanorods: Experimental Observations and Theoretical Considerations

With the evolution of synthesis and the critical characterization of core-shell nanostructures, short-range magnetic correlation is of prime interest in employing their properties to develop novel devices and widespread applications. In this regard, a novel approach of the magnetic core-shell saturated magnetization (CSSM) cylinder model solely based on the contribution of saturated magnetization in one-dimensional CrO₂/Cr₂O₃ core-shell nanorods (NRs) has been developed and applied for the determination of core-diameter and shell-thickness. The nanosized effect leads to a short-range magnetic correlation of ferromagnetic core-CrO₂ extracted from CSSM, which can be explained using finite size scaling method. The outcome of this study is important in terms of utilizing magnetic properties for the critical characterization of core-shell nanomagnetic materials.

Synthesis of nanobelt-like 1-dimensional silver/nanocarbon hybrid materials for flexible and wearable electroncs

Most synthetic processes of metallic nanostructures were assisted by organic/inorganic or polymeric materials to control their shapes to one-dimension or two-dimension. However, these additives have to be removed after synthesis of metal nanostructures for applications. Here we report a straightforward method for the low-temperature and additive-free synthesis of nanobelt-like silver nanostructures templated by nanocarbon (NC) materials via bio-inspired shape control by introducing supramolecular 2-ureido-4[1H]pyrimidinone (UPy) groups into the NC surface. The growth of the Ag nanobelt structure was found to be induced by these UPy groups through observation of the selective formation of Ag nanobelts on UPy-modified carbon nanotubes and graphene surfaces. The synthesized NC/Ag nanobelt hybrid materials were subsequently used to fabricate the highly conductive fibres (>1000S/cm) that can function as a conformable electrode and highly tolerant strain sensor, as well as a highly conductive and robust paper (>10000S/cm after thermal treatment).

Effects of dynamic diffraction conditions on magnetic parameter determination in a double perovskite Sr2FeMoO6 using electron energy-loss magnetic chiral dichroism

Electron energy-loss magnetic chiral dichroism (EMCD) spectroscopy, which is similar to the well-established X-ray magnetic circular dichroism spectroscopy (XMCD), can determine the quantitative magnetic parameters of materials with high spatial resolution. One of the major obstacles in quantitative analysis using the EMCD technique is the relatively poor signal-to-noise ratio (SNR), compared to XMCD. Here, in the example of a double perovskite Sr₂FeMoO₆, we predicted the optimal dynamical diffraction conditions such as sample thickness, crystallographic orientation and detection aperture position by theoretical simulations. By using the optimized conditions, we showed that the SNR of experimental EMCD spectra can be significantly improved and the error of quantitative magnetic parameter determined by EMCD technique can be remarkably lowered. Our results demonstrate that, with enhanced SNR, the EMCD technique can be a unique tool to understand the structure-property relationship of magnetic materials particularly in the high-density magnetic recording and spintronic devices by quantitatively determining magnetic structure and properties at the nanometer scale.

Enhanced room-temperature magnetoresistance in self-assembled Ag-coated multiphasic chromium oxide nanocomposites

Self-assembled Ag-coated multiphasic diluted magnetic chromium oxide nanocomposites were developed by a facile chemical synthesis route involving a reaction of CrO3 in the presence of Ag(+) ions in an aqueous solution of poly-vinyl alcohol (PVA) and sucrose. The tiny ferromagnetic single domains of tetragonal and orthorhombic CrO2 (t-CrO2 and o-CrO2) embedded in a dominantly insulating matrix of antiferromagnetic Cr2O3 and Cr3O8, and paramagnetic CrO3 and Cr2O, with a correlated diamagnetic thin and discontinuous shell layer of Ag efficiently tailor useful magnetic and room-temperature magnetoresistance (RTMR) properties. The t-CrO2, o-CrO2, possible canted ferromagnetism due to spin disorder in the matrix components, and the associated exchange interactions are the elements responsible for the observed ferromagnetism in the composite structure. The chain of ferromagnetic centers embedded in the composite matrix constitutes a type of magnetic tunnel junction through which spin-polarized electrons can effectively move without significant local interruptions. Electrical transport measurements showed that the spin-dependent tunneling (SDT) mechanism in the engineered microstructure of the nanocomposites exists even at room temperature (RT). A typical sample unveils a markedly enhanced RTMR-value, e.g., -80% at an applied field (H) of 3 kOe, compared to the reported values for compacted CrO2 powders or composites. The enhanced RTMR-value observed in the Coulomb blockade regime appears not only due to the considerably suppressed spin flipping at RT but primarily due to a highly effective SDT mechanism through an interlinked structure of Ag-coated multiphasic chromium oxide nanocomposites.

Spin-dependent Transport Properties of CrO2 Micro Rod

The CrO₂ micro rod powder was synthesized by decomposing the CrO₃ flakes at a specific temperature to yield precursor and annealing such a precursor in a sealed glass tube. The magneto-transport properties have been measured by a direct current four-probe method using a Cu/CrO₂ rods/colloidal silver liquid electrode sandwich device. The largest magnetoresistance (MR) around ~72 % was observed at 77 K with applied current of 0.05 μA. The non-linear I-V curve indicates a tunneling type transport properties and the tunneling barrier height is around 2.2 ± 0.04 eV at 77 K, which is obtained with fitting the non-linear I-V curves using Simmons' equation. A mixing of Cr oxides on the surface of CrO₂ rod observed by X-ray photoemission spectroscopy provides a tunneling barrier rather than a single phase of Cr₂O₃ insulating barrier. The MR shows strong bias voltage dependence and is ascribed to the two-step tunneling process.

An Inexpensive Route to Synthesize High-Purity CrO2 for EMI Shielding in X-Band Frequencies

Rod-shaped high-purity samples of CrO2 have been synthesized by an inexpensive and simplified procedure. Here, we have prepared pure CrO2 without applying any external pressure or control it during synthesis. The sample prepared exhibited an improvement in saturation magnetization values, 68 emu/g at 300 K, 136 emu/g at 80 K, and uniform grained microstructure. The complex permittivity, permeability, and microwave absorption properties of high-purity CrO2 sample were investigated in the 8.2–12.2 GHz (X-band) microwave frequency range. Microwave measurements have shown the high shielding effectiveness due to absorption (SEA) of 20.3 dB. The high value of SEA suggests that CrO2 can be used as a promising electromagnetic shielding, EMI, material in 8.2–12.2 GHz (X-band) microwave frequency range.

Oxide nanowires for spintronics: materials and devices

Spintronics, or spin-based data storage and manipulation technology, is emerging as a very active research area because of both new science and potential technological applications. As the characteristic lengths of spin-related phenomena naturally fall into the nanometre regime, researchers start applying the techniques of bottom-up nanomaterial synthesis and assembly to spintronics. It is envisaged that novel physics regarding spin manipulation and domain dynamics can be realized in quantum confined nanowire-based devices. Here we review the recent breakthroughs related to the applications of oxide nanowires in spintronics from the perspectives of both material candidates and device fabrication. Oxide nanowires generally show excellent crystalline quality and tunable physical properties, but more efforts are imperative as we strive to develop novel spintronic nanowires and devices.

When measured spin polarization is not spin polarization

Spin polarization is an unusually ambiguous scientific idiom and, as such, is rarely well defined. A given experimental methodology may allow one to quantify a spin polarization but only in its particular context. As one might expect, these ambiguities sometimes give rise to inappropriate interpretations when comparing the spin polarizations determined through different methods. The spin polarization of CrO(2) and Cr(2)O(3) illustrate some of the complications which hinders comparisons of spin polarization values.

Unusual field dependence of remanent magnetization in granular CrO2: the possible relevance of piezomagnetism

We present low field thermoremanent magnetization (TRM) measurements in granular CrO(2) and composites of ferromagnetic (FM) CrO(2) and antiferromagnetic (AFM) Cr(2)O(3). TRM in these samples is seen to display two distinct timescales. A quasi-static part of remanence, appearing only in the low field regime, exhibits a peculiar field dependence. TRM is seen to first rise and then fall with increasing cooling fields, eventually vanishing above a critical field. Similar features in TRM have previously been observed in some antiferromagnets that exhibit the phenomenon of piezomagnetism. Scaling analysis of the TRM data suggest that presumably piezomoments generated in the AFM component drive the FM magnetization dynamics in these granular systems in the low field regime.

Low field magnetotransport in manganites

The perovskite manganites with generic formula RE(1-x)AE(x)MnO(3) (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator-metal/paramagnetic-ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g. structure, phase diagram, double-exchange mechanism, Jahn-Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g. pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

Intergranular magnetoresistance in Sr2FeMoO6 from a magnetic tunnel barrier mechanism across grain boundaries

We present magnetization (M) and magnetoresistance (MR) data for a series of Sr2FeMoO6 samples with independent control on antisite defect and grain-boundary densities, which reveal several unexpected features, including a novel switching-like behavior of MR with M. These, in conjunction with model calculations, establish that the MR in Sr2FeMoO6 is dominantly controlled by a new mechanism, derived from the magnetic polarization of grain-boundary regions acting like spin valves, leading to behavior qualitatively different from that usually encountered in tunneling MR. We show that a simple and useful experimental signature for the presence of this spin-valve-type MR (SVMR) is a wider hysteresis in MR compared to that in M.

Supercritical Self-Assembled Monolayer Growth

The growth of octadecyltrimethylammonium bromide (C(18)TAB) monolayers on mica was investigated using atomic force microscopy and infrared spectroscopy. A critical temperature was identified below which the monolayer formed via an "islanding" mechanism, that is, nucleation and growth of densely packed two-dimensional (2D) islands within a matrix of a disordered dilute phase. However, above the critical temperature, there was no coexistence of 2D phases during film formation. Instead, the monolayer gradually became better ordered, remaining laterally homogeneous throughout. We show that this corresponds to a critical point in a 2D phase diagram of the monolayer. Additional evidence is provided by the in situ observation of 2D phase separation upon cooling an incomplete monolayer from the one-phase to the two-phase region. The lack of coexisting domains (and domain boundaries) during growth above the critical point provides a possible route for the preparation of essentially defect-free monolayers.

Giant room-temperature magnetoresistance in polycrystalline Zn(0.41)Fe(2.59)O4 with alpha-Fe2O3 grain boundaries

A tunneling-type magnetoresistance (MR) as large as 158% is observed at T = 300 K in a polycrystalline Zn0.41Fe2.59O4 sample, in which the Zn0.41Fe2.59O4 grains are separated by insulating alpha-Fe2O3 boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn(0.41)Fe(2.59)O4 grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating alpha-Fe2O3 boundary. The MR exhibits strong temperature dependence below 100 K and its magnitude is enhanced to reach 1280% at 4.2 K, which may arise from the Coulomb blockade effect.