Negative spin polarization of Fe3O4 in magnetite/manganite-based junctions

Exploring unconventional ferromagnetism in hole-doped LaCrAsO: insights into charge-transfer and magnetic interactions

Itinerant ferromagnetism due to the canonical double exchange (CDE) mechanism always occurs at low doping concentrations. Here we demonstrate the occurrence of robust itinerant ferromagnetism that can persist high doping concentrations. Using experimentally synthesized LaCrAsO as an illustrative example, we study the effects of hole doping via first-principles calculations and observe that the parent G-type antiferromagnetism vanishes quickly at a low doping concentration (∼0.20) and the system becomes a ferromagnetic metal due to the CDE mechanism. As the doping concentration continues to increase, the As 4p orbitals are gradually pushed up to the Fermi level and doped with holes. These ligand holes participate in the exchange interactions and drive the system toward ferromagnetism. Therefore, itinerant ferromagnetism doesn't terminate at an intermediate doping concentration as the CDE mechanism usually predicts. Furthermore, our results reveal that both the nearest and the next-nearest ferromagnetic exchange coupling strengths keep growing with doping concentration monotonically, showing that the emergent ferromagnetism mediated by As 4p orbitals is "stronger" than that of the CDE picture. Our work unlocks a new mechanism of itinerant ferromagnetism and potentially paves the way towards novel magneto-transport properties.

Anomalous low temperature magnetic properties in bulk magnetite

We systematically investigated the magnetic properties of a single crystal bulk Fe₃O₄ at low temperatures. Below Verwey transition, the magnetization versus temperature curves under zero-field cooling present anomalous behaviors for [1 0 0], [1 0 1] and [0 0 1] directions, respectively. Furthermore, at Verwey temperature, a high sensitivity of relative magnetization change (ΔM/M) to small magnetic field and, in particular, a reversal of ΔM/M at moderate magnetic field range for [0 0 1] direction is observed. Our work demonstrates a very important role of magnetic axis reorientation effect in low temperature magnetic properties of magnetite, which will stimulate further research on the intricate microscopic physics mechanisms in this classic material.

Large magnetoresistance effects in Fe3O4

We investigated the magnetoresistance (MR) of a single crystal of magnetite, Fe₃O₄. In an effort to distinguish between different contributions to the MR the samples were prepared in two different initial magnetic states, i.e. by either zero-field or by field cooling from room temperature. The different magnetic structures in this sample have a dramatic effect on the magnetoresistance: for initially zero-field-cooled conditions a negative MR of about  -20% is observed just below the Verwey transition at [Formula: see text] K. For decreasing temperature the MR increases, changes sign at  ∼78 K and reaches a record positive value of  ∼45% at around 50 K. This behavior is completely absent in the field-cooled sample. Magnetization measurements corroborate an alignment of the easy magnetization direction in applied magnetic fields below [Formula: see text] as a cause of the strong effects observed in both, magnetization and MR. Our results point to a complex interplay of structural and magnetocrystalline effects taking place upon cooling Fe₃O₄ through [Formula: see text].

Deposition of Magnetite Nanofilms by Pulsed Injection MOCVD in a Magnetic Field

This report is on the growth of Fe₃O₄ nanofilms on Al₂O₃(0001) and MgO(001) substrates with and without the presence of an external magnetic field using a pulsed injection metallorganic chemical vapour deposition (PI MOCVD) technique. The effects of growing magnetic oxide nanofilms in a 1 T field have been examined using various instrumental methods. It was found that the application of a magnetic field during PI MOCVD does not drastically alter the crystalline texture, surface morphology, and film thickness, but it significantly modifies the Fe₃O₄ film magnetisation and coercive field. Moreover, it was shown that the application of a 1 T field during the cooling of the sample also improves the magnetic properties. We believe that the large external field orients the magnetic spin structure at high temperatures (during growth or the initial stages of cool down) and that cooling through local magnetic ordering temperatures at Fe₃O₄ defect sites subsequently favours a ferromagnetic spin alignment. The control of magnetic properties of magnetite nanofilms by the application of magnetic fields during growth opens up new routes towards the fabrication and application of magnetic thin film devices.

Anisotropic sensor and memory device with a ferromagnetic tunnel barrier as the only magnetic element

Multiple spin functionalities are probed on Pt/La₂Co_0.8Mn_1.2O₆/Nb:SrTiO₃, a device composed by a ferromagnetic insulating barrier sandwiched between non-magnetic electrodes. Uniquely, La₂Co_0.8Mn_1.2O₆ thin films present strong perpendicular magnetic anisotropy of magnetocrystalline origin, property of major interest for spintronics. The junction has an estimated spin-filtering efficiency of 99.7% and tunneling anisotropic magnetoresistance (TAMR) values up to 30% at low temperatures. This remarkable angular dependence of the magnetoresistance is associated with the magnetic anisotropy whose origin lies in the large spin-orbit interaction of Co²⁺ which is additionally tuned by the strain of the crystal lattice. Furthermore, we found that the junction can operate as an electrically readable magnetic memory device. The findings of this work demonstrate that a single ferromagnetic insulating barrier with strong magnetocrystalline anisotropy is sufficient for realizing sensor and memory functionalities in a tunneling device based on TAMR.

Direct observation of cation distributions of ideal inverse spinel CoFe2O4 nanofibres and correlated magnetic properties

Low-dimensional spinel ferrites have recently attracted increasing attention because their tunable magnetic properties make them attractive candidates as spin-filtering tunnel barriers in spintronic devices and as magnetic components in artificial multiferroic heterostructures. Although we know that the distribution of cations (Fe³⁺ and Co²⁺) in a spinel structure governs its magnetic properties, their distribution in the so-called ideal inverse spinel structure of a ferrite, CoFe₂O₄, has not yet been imaged with sub-ångstrom resolution. In this work, we fill this gap in evidence by reporting a direct observation of the distribution of cations in an ideal inverse spinel structure of CoFe₂O₄ nanofibres using aberration-corrected transmission electron microscopy (TEM). The ordering of Co²⁺ and Fe³⁺ at the octahedral sites imaged along either [001], [011] or [-112] orientation was identified as 1 : 1, in accordance with the ideal inverse spinel structure. The saturation magnetisation calculated based on the crystal structure as determined from the TEM image is in good agreement with that measured experimentally on the spinel CoFe₂O₄ nanofibres, further confirming results from TEM.

Inverse Tunnel Magnetocapacitance in Fe/Al-oxide/Fe3O4

Magnetocapacitance (MC) effect, observed in a wide range of materials and devices, such as multiferroic materials and spintronic devices, has received considerable attention due to its interesting physical properties and practical applications. A normal MC effect exhibits a higher capacitance when spins in the electrodes are parallel to each other and a lower capacitance when spins are antiparallel. Here we report an inverse tunnel magnetocapacitance (TMC) effect for the first time in Fe/AlO_x/Fe₃O₄ magnetic tunnel junctions (MTJs). The inverse TMC reaches up to 11.4% at room temperature and the robustness of spin polarization is revealed in the bias dependence of the inverse TMC. Excellent agreement between theory and experiment is achieved for the entire applied frequency range and the wide bipolar bias regions using Debye-Fröhlich model (combined with the Zhang formula and parabolic barrier approximation) and spin-dependent drift-diffusion model. Furthermore, our theoretical calculations predict that the inverse TMC effect could potentially reach 150% in MTJs with a positive and negative spin polarization of 65% and -42%, respectively. These theoretical and experimental findings provide a new insight into both static and dynamic spin-dependent transports. They will open up broader opportunities for device applications, such as magnetic logic circuits and multi-valued memory devices.

Extremely low coercivity in Fe3O4 thin film grown on Mg2TiO4 (001)

The Fe₃O₄ film grown on Mg₂TiO₄ (001) shows an extremely small coercivity as low as around 7 Oe from the Verwey transition to room temperature. This low coercivity is close to that of the single-crystal bulk but several times smaller than that of the sample on MgO (001).

Inversion of Spin Signal and Spin Filtering in Ferromagnet|Hexagonal Boron Nitride-Graphene van der Waals Heterostructures

Two dimensional atomically thin crystals of graphene and its insulating isomorph hexagonal boron nitride (h-BN) are promising materials for spintronic applications. While graphene is an ideal medium for long distance spin transport, h-BN is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. Here, we demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization in graphene at room temperature. Through nonlocal pure spin transport and Hanle precession measurements performed on devices with different interface barrier conditions, we associate the negative spin polarization with high resistance few layer h-BN|ferromagnet contacts. Detailed bias and gate dependent measurements reinforce the robustness of the effect in our devices. These spintronic effects in two-dimensional van der Waals heterostructures hold promise for future spin based logic and memory applications.

In Situ Tuning of Magnetization and Magnetoresistance in Fe3O4 Thin Film Achieved with All-Solid-State Redox Device

An all-solid-state redox device composed of Fe3O4 thin film and Li(+) ion conducting solid electrolyte was fabricated for use in tuning magnetization and magnetoresistance (MR), which are key factors in the creation of high-density magnetic storage devices. Electrical conductivity, magnetization, and MR were reversibly tuned by Li(+) insertion and removal. Tuning of the various Fe3O4 thin film properties was achieved by donation of an electron to the Fe(3+) ions. This technique should lead to the development of spintronics devices based on the reversible switching of magnetization and spin polarization (P). It should also improve the performance of conventional magnetic random access memory (MRAM) devices in which the ON/OFF ratio has been limited to a small value due to a decrease in P near the tunnel barrier.

Tuning Electron-Conduction and Spin Transport in Magnetic Iron Oxide Nanoparticle Assemblies via Tetrathiafulvalene-Fused Ligands

We report a strategy to coat Fe3O4 nanoparticles (NPs) with tetrathiafulvalene-fused carboxylic ligands (TTF-COO-) and to control electron conduction and magnetoresistance (MR) within the NP assemblies. The TTF-COO-Fe3O4 NPs were prepared by replacing oleylamine (OA) from OA-coated 5.7 nm Fe3O4 NPs. In the TTF-COO-Fe3O4 NPs, the ligand binding density was controlled by the ligand size, and spin polarization on the Fe3O4 NPs was greatly improved. As a result, the interparticle spacing within the TTF-COO-Fe3O4 NP assemblies are readily controlled by the geometric length of TTF-based ligand. The shorter the distance and the better the conjugation between the TTF's HOMO and LUMO, the higher the conductivity and MR of the assembly. The TTF-coating further stabilized the Fe3O4 NPs against deep oxidation and allowed I2-doping to increase electron conduction, making it possible to measure MR of the NP assembly at low temperature (<100 K). The TTF-COO-coating provides a viable way for producing stable magnetic Fe3O4 NP assemblies with controlled electron transport and MR for spintronics applications.

Epitaxial growth of highly-crystalline spinel ferrite thin films on perovskite substrates for all-oxide devices

The potential growth modes for epitaxial growth of Fe₃O₄ on SrTiO₃ (001) are investigated through control of the energetics of the pulsed-laser deposition growth process (via substrate temperature and laser fluence). We find that Fe₃O₄ grows epitaxially in three distinct growth modes: 2D-like, island, and 3D-to-2D, the last of which is characterized by films that begin growth in an island growth mode before progressing to a 2D growth mode. Films grown in the 2D-like and 3D-to-2D growth modes are atomically flat and partially strained, while films grown in the island growth mode are terminated in islands and fully relaxed. We find that the optimal structural, transport, and magnetic properties are obtained for films grown on the 2D-like/3D-to-2D growth regime boundary. The viability for including such thin films in perovskite-based all-oxide devices is demonstrated by growing a Fe₃O₄/La_0.7Sr_0.3MnO₃ spin valve epitaxially on SrTiO₃.

Atomic-scale structure and properties of highly stable antiphase boundary defects in Fe3O4

The complex and intriguing properties of the ferrimagnetic half metal magnetite (Fe₃O₄) are of continuing fundamental interest as well as being important for practical applications in spintronics, magnetism, catalysis and medicine. There is considerable speculation concerning the role of the ubiquitous antiphase boundary (APB) defects in magnetite, however, direct information on their structure and properties has remained challenging to obtain. Here we combine predictive first principles modelling with high-resolution transmission electron microscopy to unambiguously determine the three-dimensional structure of APBs in magnetite. We demonstrate that APB defects on the {110} planes are unusually stable and induce antiferromagnetic coupling between adjacent domains providing an explanation for the magnetoresistance and reduced spin polarization often observed. We also demonstrate how the high stability of the {110} APB defects is connected to the existence of a metastable bulk phase of Fe₃O₄, which could be stabilized by strain in films or nanostructures.

Although Fe₃O₄ is widely investigated for a variety of applications, the relation between some defects and its properties remains poorly understood. Here, the authors use high-resolution transmission electron microscopy and simulations to determine the atomic structure of the common antiphase boundary defects.

Self-polarized spin-nanolasers

Besides adding a new functionality to conventional lasers, spin-polarized lasers can, potentially, offer lower threshold currents and reach higher emission intensities. However, to achieve spin-polarized lasing emission a material should possess a slow spin relaxation and a high propensity to be injected with spin-polarized currents. These are stringent requirements that, so far, have limited the choice of candidate materials for spin-lasers. Here we show that these requirements can be relaxed by using a new self-polarized spin mechanism. Fe3O4 nanoparticles are coupled to GaN nanorods to form an energy-band structure that induces the selective charge transfer of electrons with opposite spins. In turn, this selection mechanism generates the population imbalance between spin-up and spin-down electrons in the emitter's energy levels without an external bias. Using this principle, we demonstrate laser emission from GaN nanorods with spin polarization up to 28.2% at room temperature under a low magnetic field of 0.35 T. As the spin-selection mechanism relies entirely on the relative energy-band alignment between the iron oxide nanoparticles and the emitter and requires neither optical pumping with circularly polarized light nor electrical pumping with magnetic electrodes, potentially a wide range of semiconductors can be used as spin-nanolasers.

Efficient spin-light emitting diodes based on InGaN/GaN quantum disks at room temperature: a new self-polarized paradigm

A well-behaved spin-light emitting diode (LED) composed of InGaN/GaN multiple quantum disks (MQDs), ferromagnetic contact, and Fe3O4 nanoparticles has been designed, fabricated, and characterized. The degree of circular polarization of electroluminescence (EL) can reach up to a high value of 10.9% at room temperature in a low magnetic field of 0.35 T, which overcomes a very low degree of spin polarization in nitride semiconductors due to the weak spin-orbit interaction. Several underlying mechanisms play significant roles simultaneously in this newly designed device for the achievement of such a high performance. Most of all, the vacancy between nanodisks can be filled by half-metal nanoparticles with suitable energy band alignment, which enables selective transfer of spin polarized electrons and holes and leads to the enhanced output spin polarization of LED. Unlike previously reported mechanisms, this new process leads to a weak dependence of spin relaxation on temperature. Additionally, the internal strain in planar InGaN/GaN multiple quantum wells can be relaxed in the nanodisk formation process, which leads to the disappearance of Rashba Hamiltonian and enhances the spin relaxation time. Our approach therefore opens up a new route for the further research and development of semiconductor spintronics.

High-T C fully compensated ferrimagnetic semiconductors as spin-filter materials: the case of CrVXAl (X = Ti, Zr, Hf) Heusler compounds

We extend our recent work on spin-filter materials (Galanakis et al 2013 Appl. Phys. Lett.103 142404) to the case of CrVXAl (X = Ti, Zr, Hf) compounds, for which, using ab initio electronic structure calculations, we show that p-d hybridization leads to the formation of a fully compensated ferrimagnetic semiconducting state with moderate exchange splitting. The magnetism is of covalent-type and the very strong antiferromagnetic Cr-V exchange interactions lead to extremely high Curie temperature, TC, values. Furthermore, all three compounds are thermodynamically and magnetically stable. The combination of very high TC values with a zero total net magnetization makes them promising materials for spintronics applications.

Origin of anomalous magnetite properties in crystallographic matched heterostructures: Fe3O4(111)/MgAl2O4(111)

Magnetite films grown on crystallographically matched substrates such as MgAl2O4 are not expected to show anomalous properties such as negative magnetoresistance and high saturation fields. By atomic resolution imaging using scanning transmission electron microscopy we show direct evidence of anti-phase domain boundaries (APB) present in these heterostructures. Experimentally identified 1/4<101> shifts determine the atomic structure of the observed APBs. The dominant non-bulk superexchange interactions are between 180° octahedral-Fe/O/octahedral-Fe sites which provide strong antiferromagnetic coupling across the defect interface resulting in non-bulk magnetic and magnetotransport properties.

Adsorption of amino acids on the magnetite-(111)-surface: a force field study

Magnetite (Fe(3)O(4)) is an important biomineral, e.g., used by magnetotactic bacteria. The connection between the inorganic magnetite-(111)-surface and the organic parts of the bacteria is the magnetosome membrane. The membrane is built by different magnetosome membrane proteins (MMPs), which are dominated by the four amino acids glycine (Gly), aspartic acid (Asp), leucine (Leu) and glutamic acid (Glu). Force field simulations of the interaction of the magnetite-(111)-surface and the main amino acid compounds offer the possibility to investigate if and how the membrane proteins could interact with the mineral surface thus providing an atomistic view on the respective binding sites. In a force field simulation the four amino acids were docked on the Fe-terminated magnetite-(111)-surface. The results show that it is energetically favorable for the amino acids to adsorb on the surface with Fe-O-distances between 2.6 Å and 4.1 Å. The involved O-atoms belong to the carboxyl-group (Asp and Glu) or to the carboxylate-group (Gly, Leu and Glu). Electrostatic interactions dominate the physisorption of the amino acids. During the simulations, according to the frequency of the best results, the global minimum for the docking interaction could be attained for all amino acids analyzed.

BaFe12O19 single-particle-chain nanofibers: preparation, characterization, formation principle, and magnetization reversal mechanism

BaFe(12)O(19) single-particle-chain nanofibers have been successfully prepared by an electrospinning method and calcination process, and their morphology, chemistry, and crystal structure have been characterized at the nanoscale. It is found that individual BaFe(12)O(19) nanofibers consist of single nanoparticles which are found to stack along the nanofiber axis. The chemical analysis shows that the atomic ratio of Ba/Fe is 1:12, suggesting a BaFe(12)O(19) composition. The crystal structure of the BaFe(12)O(19) single-particle-chain nanofibers is proved to be M-type hexagonal. The single crystallites on each BaFe(12)O(19) single-particle-chain nanofibers have random orientations. A formation mechanism is proposed based on thermogravimetry/differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM) at six temperatures, 250, 400, 500, 600, 650, and 800 °C. The magnetic measurement of the BaFe(12)O(19) single-particle-chain nanofibers reveals that the coercivity reaches a maximum of 5943 Oe and the saturated magnetization is 71.5 emu/g at room temperature. Theoretical analysis at the micromagnetism level is adapted to describe the magnetic behavior of the BaFe(12)O(19) single-particle-chain nanofibers.

Tunneling magnetoresistance with sign inversion in junctions based on iron oxide nanocrystal superlattices

Magnetic tunnel junctions sandwiching a superlattice thin film of iron oxide nanocrystals (NCs) have been investigated. The transport was found to be controlled by Coulomb blockade and single-electron tunneling, already at room temperature. A good correlation was identified to hold between the tunnel magnetoresistance (TMR), the expected magnetic properties of the NC arrays, the charging energies evaluated from current-voltage curves, and the temperature dependence of the junction resistance. Notably, for the first time, a switching from negative to positive TMR was observed across the Verwey transition, with a strong enhancement of TMR at low temperatures.

Orbital order in ZnV(2)O(4)

In view of recent controversy regarding the orbital order in the frustrated spinel ZnV(2)O(4), we analyze the orbital and magnetic ground state of this system within an ab initio density functional theory approach. While local density approximation+Hubbard U calculations in the presence of a cooperative Jahn-Teller distortion stabilize an A-type staggered orbital order, the consideration of relativistic spin-orbit (SO) effects unquenches the orbital moment and leads to a uniform orbital order with a net magnetic moment close to the experimental one. Our results show that ab initio calculations are able to resolve the existing discrepancies in previous theories and that it is the SO coupling along with electronic correlations which play a significant role in determining the orbital structure in these materials.

Composition controlled spin polarization in Co(1-x)Fe(x)S(2) alloys

The transition metal (TM) chalcogenides of the form TMX(2) (X = S or Se) have been studied for decades due to their interesting electronic and magnetic properties such as metamagnetism and metal-insulator transitions. In particular, the Co(1-x)Fe(x)S(2) alloys were the subject of investigation in the 1970s due to general interest in itinerant ferromagnetism. In recent years (2000-present) it has been shown, both by electronic structure calculations and detailed experimental investigations, that Co(1-x)Fe(x)S(2) is a model system for the investigation of highly spin polarized ferromagnetism. The radically different electronic properties of the two endpoint compounds (CoS(2) is a narrow bandwidth ferromagnetic metal, while FeS(2) is a diamagnetic semiconductor), in a system forming a substitutional solid solution allows for composition control of the Fermi level relative to the spin split bands, and therefore composition-controlled conduction electron spin polarization. In essence, the recent work has shown that the concept of 'band engineering' can be applied to half-metallic ferromagnets and that high spin polarization can be deliberately engineered. Experiments reveal tunability in both sign and magnitude of the spin polarization at the Fermi level, with maximum values obtained to date of 85% at low temperatures. In this paper we review the properties of Co(1-x)Fe(x)S(2) alloys, with an emphasis on properties of relevance to half-metallicity. Crystal structure, electronic structure, synthesis, magnetic properties, transport properties, direct probes of the spin polarization, and measurements of the total density of states at the Fermi level are all discussed. We conclude with a discussion of the factors that influence, or even limit, the spin polarization, along with a discussion of opportunities and problems for future investigation, particularly with regard to fundamental studies of spintronic devices.

Large Low-Field Magnetoresistance in Nanocrystalline Magnetite Prepared by Sol−Gel Method

Nanocrystalline magnetite Fe3O4 samples with a grain size of about 40 nm have been synthesized by an optimized sol-gel method. The single phase of spinel magnetite was confirmed by both X-ray diffraction and transmission electron microscopy. It has been found that the magnetoresistance of the samples at low field (LFMR) is relatively large, and with the decrease of temperature its value at a field of 0.5 T changes dramatically from -2.5% at 300 K to -17.0% at 55 K. With the further decrease of temperature a sharp drop occurs for the magnitude of the magnetoresistance (MR), regarded as a spin (cluster) glass transition in the surface region of the grains that can be confirmed by the zero-field-cooled and field-cooled magnetization and ac susceptibility measurement. The mechanism of the magnetic and transport properties was discussed.

Spin-filter effect in magnetite nanowire

Spin-dependent electron transport in individual magnetite (Fe3O4) nanowires contacted with normal metallic electrodes was investigated. Such a configured device demonstrated a spin-filter effect, that is, only the minority spin carriers can transport through the magnetite nanowire due to its negative spin polarization. An anomalous positive magnetoresistance approximately 7.5% is observed at room temperature. Moreover, the magnetoresistance can be controlled via bias voltage.

Co1-xFexS2: a tunable source of highly spin-polarized electrons

In the emerging field of spin-electronics ideal ferromagnetic electron sources would not only possess a high degree of spin polarization, but would also offer control over the magnitude of this polarization. We demonstrate here that a simple scheme can be utilized to control both the magnitude and the sign of the spin polarization of ferromagnetic CoS2, which we probe with a variety of techniques. The position of the Fermi level is fine-tuned by solid solution alloying with the isostructural diamagnetic semiconductor FeS2, leading to tunable spin polarization of up to 85%.

Atomically resolved spin-dependent tunneling on the oxygen-terminated Fe3O4(111)

We employ spin-polarized STM to study the spin-dependent tunneling between a magnetite (111) sample and an antiferromagnetic tip through a vacuum barrier at room temperature. Atomic scale STM images show significant magnetic contrast corresponding to variations in the local surface states induced by oxygen vacancies. The estimated variations in tunneling magnetoresistance of 250% suggest that the spin-transport properties are significantly altered locally by the presence of surface defects.