Bulk Magnetocrystalline Anisotropy Research Articles

Square plate shaped magnetite nanocrystals

Square plate shaped magnetite nanocrystals have been synthesized by chemical precipitation from solution using arabinogalactan. A high crystal quality was observed in the plate plane while, across the plate, there is some stratification. The magnetic hysteresis in such particles is determined by the bulk magnetocrystalline anisotropy, plate shape anisotropy, and surface magnetic anisotropy. It is shown using the micromagnetic simulation that the ferromagnetic square nanoplates exhibit the extraordinary magnetization switching anisotropy, which should be taken into account for understanding the hysteretic properties of the particles.

Prediction of room-temperature half-metallicity in layered halide double perovskites

Half-metallic ferromagnets (HMFs) that possess intriguing physical properties with completely spin-polarized current are key candidates for high-efficiency spintronic devices. However, HMFs that could simultaneously have high Curie temperature (Tc), wide half-metallic gap (ΔHM), and large bulk magnetocrystalline anisotropy energy (MAE) are very rare, which significantly restrict their room-temperature (RT) applications. In this article, through materials screening in layered halide double perovskites (LHDPs), we have theoretically identified that Cs4FePb2Cl12, which has good crystallographic, dynamic and thermal stabilities, possesses an intrinsic half-metallic ground-state with a high Tc ~ 450 K. Interestingly, the long-range ferromagnetic ordering in bulk Cs4FePb2Cl12 is contributed by the strong super-superexchange interactions between the neighboring Fe d orbitals mediated by different anionic Cl p orbitals. The high Tc of layered Cs4FePb2Cl12 can be well maintained even in the monolayer limitation, i.e., Tc ~ 370 K for Cs4FePb2Cl12 monolayer, which is critical for nanoscale device applications. Moreover, both bulk and monolayer Cs4FePb2Cl12 can exhibit wide ΔHM ~ 0.55 eV and large MAE >320 μeV/Fe, comparable to that of the best HMFs reported in the literature. Our findings can significantly extend the potentials of LHDPs for high-temperature spintronic applications.

Structural, magnetic, and electron-transport properties of epitaxial Mn2PtSn films

The growth of new magnetic materials on suitable insulating substrates is an important part of the development of spin-electronics devices for memory or information processing. Epitaxial thin films of Mn2PtSn were grown on a MgO [001] substrate by magnetron co-sputtering of the constituents. Structural, magnetic, and electron-transport properties were investigated. The epitaxial Mn2PtSn film has an inverse tetragonal structure with the c-axis aligned in the plane of the MgO substrate. The lattice constants determined using XRD and TEM analysis are c = 6.124 Å and a = b = 4.505 Å. The orientation of Mn2PtSn c-axis which is 45° away from the a-axis of MgO has resulted in a small lattice mismatch of about 2.8%. The measured saturation magnetization is 5.3 μB/f.u., which is smaller than the first-principles calculated value of 6.4 μB/f.u. for ferromagnetic spin arrangement. Magnetization measurements determined the bulk magnetocrystalline anisotropy constant Kv of about 11.3 Merg/cm3 (1.13 MJ/m3). The electron-transport behavior is similar to that of normal magnetic metals. These results indicate that Mn2PtSn may have promising applications in spintronic devices.

Interfacial magnetic anisotropy from a 3-dimensional Rashba substrate.

We study the magnetic anisotropy which arises at the interface between a thin film ferromagnet and a 3-d Rashba material. We use a tight-binding model to describe the bilayer, and the 3-d Rashba material characterized by the spin-orbit strength α and the direction of broken bulk inversion symmetry n̂. We find an in-plane uniaxial anisotropy in the ẑ × n̂ direction, where ẑ is the interface normal. For realistic values of α, the uniaxial anisotropy is of a similar order of magnitude as the bulk magnetocrystalline anisotropy. Evaluating the uniaxial anisotropy for a simplified model in 1-d shows that for small band filling, the in-plane easy axis anisotropy scales as α4 and results from a twisted exchange interaction between the spins in the 3-d Rashba material and the ferromagnet. For a ferroelectric 3-d Rashba material, n̂ can be controlled with an electric field, and we propose that the interfacial magnetic anisotropy could provide a mechanism for electrical control of the magnetic orientation.

High-temperature magnetic properties of noninteracting randomly oriented single-domain Fe 3 − δO 4 nanoparticles

Magnetic measurements have been performed on 40-nm sphere-like Fe 3 − δ O 4 ( δ = 0.043 ) nanoparticles using a Quantum Design vibrating sample magnetometer. Coating Fe 3 − δ O 4 nanoparticles with SiO 2 effectively eliminates magnetic interparticle interactions so that the coercive field H C in the high-temperature range between 300 K and the Curie temperature (855 K) can be well fitted by an expression for noninteracting randomly oriented single-domain particles. From the fitting parameters, the effective anisotropy constant K is found to be ( 1.38 ± 0.11 ) × 10 5 erg / cm 3 , which is very close to the bulk magnetocrystalline anisotropy constant of 1.35 × 10 5 erg / cm 3 . Moreover, the inferred mean particle diameter from the fitting parameters is in quantitative agreement with that determined from transmission electron microscope. Such a quantitative agreement between data and theory suggests that the ensemble of our SiO 2-coated sphere-like Fe 3 − δ O 4 nanoparticles represents a good system of noninteracting randomly-oriented single-domain particles.

Selective Synthesis of Peapodlike Ni/Ni3S2 Nanochains and Nickel Sulfide Hollow Chains and Their Magnetic Properties

AbstractPeapodlike Ni/Ni3S2 chains of about 30 nm in outer diameter, with Ni cores of 10–15 nm, can be synthesized by a sacrificial template route. The Ni3S2 shell exhibits paramagnetic properties with a mass susceptibility of χ ≈ 5 × 10−5 emu (gOe)−1, while the ferromagnetic Ni cores show a superparamagnetic behavior with a blocking temperature of TB ≈ 130 K. The shape anisotropy of the chainlike structure is determined as 5.0 × 104 J·m−3, which is larger than the bulk magnetocrystalline anisotropy by one order of magnitude. The demagnetization factor is determined as ΔN = 0.29. The sample provides an ideal structure for studying the magnetization reversal property by the chain‐of‐sphere model. The observations on the formation of the peapod structure verify a growth mechanism of the nanoscale Kirkendall effect. Based on the preparation of peapod chains, a series of nickel sulfide hollow chains with average diameters of 25, 50, and 100 nm are fabricated. In addition, the phase transition for hollow chains from Ni3S2 to NiS is studied.

Surfactant effects in magnetite nanoparticles of controlled size

Magnetite Fe 3O 4 nanoparticles of controlled size within 6 and 20 nm in diameter were synthesised by thermal decomposition of an iron organic precursor in an organic medium. Particles were coated with oleic acid. For all samples studied, saturation magnetisation M s is size-independent, and reaches a value close to that expected for bulk magnetite, in contrast to results in small particle systems for which M s is usually much smaller due to surface spin disorder. The coercive field for the 6 nm particles is in agreement with coherent rotation, taking the bulk magnetocrystalline anisotropy into account. Both results suggest that the oleic acid molecules covalently bonded to the nanoparticle surface yield a strong reduction in the surface spin disorder. However, although the saturated state may be similar, the approach to saturation is different and, in particular, the high-field differential susceptibility is one order of magnitude larger than in bulk materials. The relevance of these results in biomedical applications is discussed.

Enhancement of the exchange-bias onset temperature in a columnar nanocrystalline Ni80Fe20∕Co3O4 thin film

Typically, exchange coupling can only occur when ferromagnetic spins couple to antiferromagnetic spins below the Néel temperature. We present results on a Ni80Fe20∕Co3O4 thin film that is composed of ∼10nm diameter nanocrystalline columns. Field cooling the film reveals low-temperature exchange-bias hysteresis loop shifts that are the same magnitude as those measured in a similar Ni80Fe20∕CoO system, although the bulk magnetocrystalline anisotropy is a factor of 5 smaller. The coercivity and exchange-bias loop shift exhibit the same temperature dependence with individual “blocking” temperatures. Most surprising is that the onset temperature of the exchange-bias is nearly four times higher than the bulk Néel temperature.

Uniaxial anisotropy in magnetite thin film—Magnetization studies

Magnetization and electrical resistivity measurements have been performed on a stoichiometric single crystalline magnetite Fe 3O 4 thin film (thickness of ca. 500 nm) MBE deposited on MgO (1 0 0) substrate. The aim of these studies was to check the influence of preparation method and sample form (bulk vs. thin film) on magnetic anisotropy properties in magnetite. The film magnetization along 〈0 0 1〉 versus applied magnetic field has been determined both in the direction parallel and perpendicular to the film surface, and at temperatures above and below the Verwey transition. We have found, in agreement with published results, that the in-plane field of 10 kOe was not sufficient to saturate the sample. This can be understood if some additional factor, on top of the bulk magnetocrystalline anisotropy, is taken into account.

Magnetic behaviour of iron oxide nanoparticles dispersed in a silica matrix

At room temperature the magnetic behaviour of non-interacting γ-Fe 2O 3 nanoparticles dispersed in a silica matrix is superparamagnetic. At a lower temperature (77 K) hysteresis was observed. The appearance of the hysteresis loop is attributed to the Néel magnetic relaxation processes; the easy magnetisation axes of the nanoparticles being randomly oriented, the magnetic moments will rotate in an external magnetic field with a relaxation time τ, which depends on the effective anisotropy constant K eff. A supplementary anisotropy, e.g. stress anisotropy and the surface anisotropy leads to a total value ( K eff), approximately 10 times higher than the bulk magnetocrystalline anisotropy K. At the surface of embedded nanoparticles a modification of the superexchange interaction takes place—due to specific particle-matrix interactions—on decreasing the temperature from 300 to 77 K; the magnetic moment of the particles increases 1.6 times. This behaviour can be attributed to the increase of the nanoparticles magnetic core volume of aligned spins and is not due to the variation of the spontaneous magnetisation with temperature.

Surface anisotropy in ferromagnetic nanoparticles

The effect of surface anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a “throttled” spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surface anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a “hedgehog” configuration with no net magnetization. Implications for FePt nanoparticles are discussed.

Spin waves and magnetic anisotropy in ultrathin (111)-oriented cubic films

The dispersions of dipolar (Damon-Eshbach modes) and exchange-dominated spin waves are calculated for in-plane-magnetized thin and ultrathin cubic films with (111) crystal orientation and the results are compared with those obtained for the other principal planes. The properties of these magnetic excitations are examined from the point of view of Brillouin light scattering experiments. Attention is paid to the study of the spin-wave frequency variation as a function of the magnetization direction in the film plane for different film thicknesses. Interface anisotropies and the bulk magnetocrystalline anisotropy are considered in the calculation. A quantitative comparison between an analytical expression obtained in the limit of small film thickness and wave vector and the full numerical calculation is given.

Magnetic properties of monodomain Nd-Fe-B-C nanoparticles

Nanoparticles of Nd-Fe-B-C with their crystal structure similar to the Nd2Fe14B phase were generated in a carbon arc. With an average diameter of less than 40 nm, they are monodomain. They have a smaller room-temperature coercivity than would be predicted from the bulk magnetocrystalline anisotropy. However, their coercivity is greater than was previously observed in particles <5 μm prepared by spark erosion. While the carbon arc process is useful for making small carbon-coated particles which resist oxidation, here dispersion of excess Nd in the carbon matrix results in a significant paramagnetic signal. The dc demagnetization curves enable the paramagnetic and ferromagnetic contributions to be distinguished.

Mössbauer spectrometry study of magnesioferrite particles

Fine particles of ferrimagnetic MgFe2O4 were synthesized through a sol-gel supercritical drying procedure, then cold-pressed and heated at 500 and 800 °C. These materials were characterized by Mössbauer spectrometry in the temperature range 25–298 K, and in the presence of a small applied magnetic field. The Mössbauer measurements show that heat treatment results in the formation of a small percentage of α-Fe2O3, an increase in the mean particle size and hyperfine magnetic field, and a decrease in the inversion parameter of the magnesioferrite. An average anisotropy constant equivalent to the bulk magnetocrystalline anisotropy is estimated for particles with diameter D=14 nm.

Perpendicular anisotropy in Co/Ru epitaxial superlattices

We present recent results on the magnetic properties of Co/Ru hcp superlattices. From magnetization and FMR measurements, we determine the rather strong uniaxial anisotropy, which tends to align the moments along the normal to the layers for Co thickness < 15 Å. This anisotropy is analyzed in terms of interface anisotropy and magneto-elastic anisotropy of the strained Co lattice which adds to the bulk magnetocrystalline anisotropy of the hcp oriented cobalt layers.