Superior Magnetic Performance in FePt L10 Nanomaterials

The discovery of the high maximum energy product of 59 MGOe for NdFeB magnets is a breakthrough in the development of permanent magnets with a tremendous impact in many fields of technology. This value is still the world record, for 40 years. This work reports on a reliable and robust route to realize nearly perfectly ordered L1₀‐phase FePt nanoparticles, leading to an unprecedented energy product of 80 MGOe at room temperature. Furthermore, with a 3 nm Au coverage, the magnetic polarization of these nanomagnets can be enhanced by 25% exceeding 1.8 T. This exceptional magnetization and anisotropy is confirmed by using multiple imaging and spectroscopic methods, which reveal highly consistent results. Due to the unprecedented huge energy product, this material can be envisaged as a new advanced basic magnetic component in modern micro and nanosized devices.     

Probing magnetic anisotropy and spin polarization in spintronic materials

Magnetic anisotropy and spin polarization are fundamental parameters in ferromagnetic materials that have use in spintronic device applications. As the need for screening properties of new magnetic materials rises, it is important to have measurement probes for quantities such as anisotropy and spin polarization. We have developed two unconventional yet powerful techniques to study these parameters. A resonant RF transverse susceptibility method is used to map the characteristic anisotropy and switching fields over a wide range in temperature and magnetic fields. For studies of spin polarization, the phenomenon of Andreev reflection across ferromagnet-superconductor junctions is used to extract values of the transport spin polarization. The effectiveness of these approaches is demonstrated in candidate spintronic materials such as half-metallic CrO/sub 2/ thin films and arrays of monodisperse, single-domain Fe nanoparticles.     

Transition-metal dimers and physical limits on magnetic anisotropy

Recent advances in nanoscience have raised interest in the minimum bit size required for classical information storage. This bit size is determined by the necessity for bistability with suppressed quantum tunnelling and energy barriers that exceed ambient temperatures. In the case of magnetic information storage, much attention has centred on molecular magnets with bits consisting of about 100 atoms, magnetic uniaxial anisotropy energy barriers of about 50 K and very slow relaxation at low temperatures. Here, we draw attention to the remarkable magnetic properties of some transition-metal dimers, which have energy barriers approaching 500 K with only two atoms. The spin dynamics of these ultrasmall nanomagnets is strongly affected by a Berry phase, which arises from quasi-degeneracies at the electronic highest occupied molecular orbital energy. In a giant-spin approximation, this Berry phase makes the effective reversal barrier thicker.     

Permanent magnets based on materials with high crystal anisotropy

The properties of Alnicos and elongated single-domain fine-particle materials, both interpreted as due to shape anisotropy of fine particles, are still far short of those predicted by simple theory. The hard ferrites, with properties due to magnetocrystalline anisotropy, come much closer, but are restricted by their low magnetization. The search for a combination of high magnetization and high magnetocrystalline anisotropy has led to the investigation of a variety of intermetallic compounds. Some of them, particularly the cobalt-rare earths, appear quite promising. Mechanical grinding often has an adverse effect on the magnetic properties of crystal anisotropy materials. Chemical stability is also often a problem. Crystal anisotropy materials in general have high values of intrinsic coercive force. This makes them especially suitable for applications involving widely varying dynamic conditions. Their proper evaluation may be best done using criteria other than the usual maximum energy product.     

Epitaxial garnet ferrite films with anisotropy of the inclined easy plane type

The character of the magnetic anisotropy in epitaxial (112)-oriented garnet ferrite films has been studied. It is shown that at a certain ratio of the constants of uniaxial and orthorhombic anisotropy in these films, the anisotropy of the “inclined easy plane” type can appear. The possible application of epitaxial garnet ferrite films with the inclined easy-plane anisotropy in highly sensitive transducers of inhomogeneous magnetic fields is substantiated.     

Magnetic anisotropy control in amorphous FeCoSiB films by stress annealing

It is important to control magnetic anisotropy of ferromagnetic materials. In this work, magnetic anisotropy of amorphous FeCoSiB films is controlled by stress annealing. FeCoSiB films are deposited on glass substrate and annealed with stress in vacuum. When the annealed films are released from clamp, permanent tensile or compressive strain can be introduced in the films. Influences of both tensile and compressive strain on the magnetic properties of FeCoSiB films have been studied. The results show that FeCoSiB films by stress annealing exhibit strong magnetic anisotropy while the samples by normal annealing exhibit magnetic isotropy. Easy axis along the stress is induced in the films with tensile stress, while easy axis perpendicular to the stress is induced in the samples with compressive stress. It has also been found that the magnetic anisotropy increases with the increase of the strain. The effects of strain on the magnetic properties of FeCoSiB films have been interpreted by stress induced anisotropy via magnetoelastic coupling.     

Effect of induced shape anisotropy on magnetic properties of ferromagnetic cobalt nanocubes

We report on the synthesis of ferromagnetic cobalt nanocubes of various sizes using thermal pyrolysis method and the effect of shape anisotropy on the static and dynamic magnetic properties were studied. Shape anisotropy of approximately 10% was introduced in nanocubes by making nanodiscs using a linear chain amine surfactant during synthesis process. It has been observed that, ferromagnetism persisted above room temperature and a sharp drop in magnetic moment at low temperatures in zero-field cooled magnetization may be associated with the spin disorder due to the effective anisotropy present in the system. Dynamic magnetic properties were studied using RF transverse susceptibility measurements at different temperatures and the singularities due to anisotropy fields were probed at low temperatures. Symmetrically located broad peaks are observed in the frozen state at the effective anisotropy fields and the peak structure is strongly affected by shape anisotropy and temperature. Irrespective of size the shape anisotropy gave rise to higher coercive fields and larger transverse susceptibility ratio at all temperatures. The role of shape anisotropy and the size of the particles on the observed magnetic behaviour were discussed.     

Hard magnetic cobalt nanomaterials as an electrocatalyst for oxygen evolution reaction

Magnetic cobalt nanomaterials, due to the high magneto-crystalline anisotropy, are favorable candidates for hard magnets. Herein, we improved the polyol process, which replaces α-diol with cost-effective 1,4-butanediol, and explored the best conditions for the synthesis of hard magnetic cobalt materials. The synthesized hexagonal cobalt nano-urchins exhibited a high coercivity of 2891.5 Oe. Further, we also studied the influence of the structure and morphology of cobalt nanomaterials on the oxygen evolution reaction (OER) activity. This combination of highly crystalline hexagonal close-packed nanostructure and special branch morphology can effectively enhance OER performance.

     

Annealing behavior of magnetic anisotropy in CoNbZr films

The effect of magnetic and nonmagnetic annealing on the magnetic anisotropy in CoNbZr films, formed by a DC opposing-targets sputtering method, was investigated. It was revealed that the origin of the magnetic anisotropy is the directional ordering of the magnetic atoms. The anisotropy fields and the direction of the easy axis obtained when the films are annealed in zero magnetic field are almost the same as those for the magnetic field parallel to the easy axis of the as-deposited films. When the films are annealed in a magnetic field perpendicular to the easy axis, the anisotropy field induced in parallel with the magnetic field, H_k(t), is well represented by the following formula: ln {1-H_k(t )/H_k(∞)∝-√Tt, where H_k(∞) is the thermal equilibrium value of the anisotropy field and D is the diffusion constant. The activation energy of the as-deposited film is 0.86 eV. Annealing the film increases the activation energy which is 2.1 eV when the film is annealed at a temperature of 450°C for 2 h     

纳米硬磁晶粒间的交换耦合相互作用和有效各向异性

以纳米Nd2Fe14B永磁材料为例,研究了硬磁晶粒间交换耦合相互作用对磁体有效各向异性的影响。结果表明晶粒间交换耦合相互作用随晶粒尺寸的减小而增强,材料的有效各向异性常数Keff随晶粒尺寸的减小而逐渐下降,Keff随晶粒尺寸的变化与矫顽力的变化规律相似。纳米单相永磁材料有效各向异性的减小是矫顽力降低的主要原因,交换耦合系数口aex实际上是各向异性的减小量。为保证纳米Nd2Fe14B材料具有较高的各向异性和矫顽力,晶粒尺寸应不小于30nm。     

Anisotropic and high magnetization rare earth transition metal compounds containing metalloids

The structural and magnetic behavior is presented for selected metalloid (B,C) containing hexagonal and tetragonal rare earth-transition metal compounds and compound series. Focus is on materials with high Fe content and resulting high magnetizations. The unusual axial ratios and features of the sheet type structures of these materials have pronounced consequences on such properties as magnetic anisotropy and magnetic hardness. Individual site anisotropy contributions are studied by temperature dependence of magnetization along easy and hard magnetic axes. As an example it is found that tetragonal Nd₂Fe₁₄B has axial anisotropy with H_A = 76 kOe at 300 K but shows tendencies for a spin reorientation around 150 K. Y₂Fe₁₄B has axial anisotropy with H_A = 25 kOe but does not exhibit a similar spin reorientation. This indicates that the two crystallographic Nd sites (4f and 4g) have axis and plane preference respectively, with different temperature dependencies. Axial Nd anisotropy is a consequence of the lack of Nd coordination along the z axis due to intervening thick Fe layers. Both extrinsic (fine particle) and intrinsic magnetic hardness is observed. Crystallographically disordered materials show intrinsic hardness based on domain wall pinning by local fluctuations of magnetic parameters. Strong nucleation phenomena are characteristic for ordered materials in bulk and powder form. The unusually high achievable ratios of extrinsic coercivities to anisotropy fields in the metalloid stabilized materials are related to their chemically relatively inert layer structure. This appears to lead to less corrugated surface structures and is so responsible for the characteristic domain wall nucleation processes.     

Rotatable magnetic anisotropy in epitaxial ferrite layers

Magnetic domain structure and magnetic anisotropy were studied in monocrystalline epilayers of Mg0.9Mn0.3Fe1.8O4ferrite. The layers, several micrometers thick, were obtained by a CVD method on monocrystalline MgO substrates. Domain observations were performed by the Bitter's method. Magnetic anisotropy measurements were performed by torque and FMR methods. In the demagnetized state, a typical stripe structure of 2.0 to 2.8 μm period was observed. From the domains behavior in the in-plane magnetic fields it was found that in these epilayers the rotatable anisotropy was present. The existence of this anisotropy was confirmed by torque measurements in small in-plane fields. The magnetic parameters characterizing these layers are: 4ΠM = 3500 Gs, K1= - 2.2 × 10⁴ergs/cc, KN= 2.3 × 10⁵ergs/cc.     

The remarkable difference between surface and step atoms in the magnetic anisotropy of two-dimensional nanostructures

The original magnetic properties of nanometre-sized particles are due to the distinct contributions of volume, surface and step atoms. To disentangle these contributions is an ongoing challenge of materials science. Here we introduce a method enabling the identification of the remarkably different contributions of surface and perimeter atoms to the magnetic anisotropy energy of two-dimensional nanostructures. Our method uses the generally nonlinear relationship between perimeter length and surface area. Atomic-scale characterization of the morphology of ensembles of polydisperse nanostructures, combined with in situ measurements of their temperature-dependent magnetic susceptibility, gives access to the role played by the differently coordinated atoms. We show for Co nanostructures on a Pt(111) surface that their uniaxial out-of-plane magnetization is entirely caused by edge atoms having 20 times more anisotropy energy than their bulk and surface counterparts. Identification of the role of perimeter and surface atoms opens up unprecedented opportunities for materials engineering. As an example, we separately tune magnetic hardness and moment in bimetallic core-shell nanostructures.     

Magnetically Enhanced Thin-Film Coarsening by a Magnetic XPFC Model Allowing to Decouple Magnetic Anisotropy and Magnetostriction

External magnetic fields provide a macroscopic control mechanism to influence the microstructure of polycrystalline materials. Herein, the influence of strong magnetic fields on grain growth in thin films is modeled with a magnetic extended phase field crystal model. The magneto–structural effects are incorporated into the correlation function in reciprocal space. With this approach, magnetic anisotropy, magnetostriction, and mobility of grain boundary can be controlled and a variety of geometrical and topological properties consistent with experimental results can be determined.     

Effects of MgO and MgO/Pd seed-layers on perpendicular magnetic anisotropy of CoPd thin films

We studied the effects of MgO and MgO/Pd seed-layers on perpendicular magnetic anisotropy in co-sputtered CoPd films. CoPd films with the MgO seed-layer showed perpendicular magnetic properties that were superior to those with another after annealing. The loop squareness was unity, indicating strong perpendicular magnetic anisotropy, when the MgO seed-layer was thicker than 2 nm. We observed that the out-of-plane CoPd (111) texture was strongly developed, as well as the in-plane tensile stress in the CoPd films. The magnetoelastic anisotropy coming from a negative magnetostriction λ₁₁₁ under the in-plane tensile stress dominating over other anisotropies is likely responsible for creating such strong perpendicular magnetic anisotropy. In the case of the MgO/Pd seed-layer, the CoPd films showed mixed anisotropy having both in-plane and out-of-plane magnetic anisotropy components after annealing. The appearance of the strong (100) texture of the CoPd films with the MgO/Pd seed-layer is believed to have caused the decrease in the perpendicular magnetic anisotropy that originated from the magnetoelastic anisotropy due to the additional contribution from the positive magnetostriction λ₁₀₀ but less contribution from the negative magnetostriction λ₁₁₁ when the CoPd films are under in-plane tensile stress.     

纳米软、硬磁性晶粒间的交换耦合及其有效各向异性

以软磁性相α-Fe和硬磁性相Nd2Fe14B为例,研究了软、硬磁性晶粒间的交换耦合作用和有效各向异性常数〈Ksh〉随晶粒尺寸的变化关系。由于晶粒间的交换耦合作用,晶粒可分为晶粒内部无界面交换耦合作用影响和晶粒表面有界面交换耦合作用影响两部分,其各向异性常数为两部分的统计平均值。计算结果表明：对固定的软磁性晶粒尺寸Ds,〈Ksh〉随硬磁性晶粒尺寸Dh一致增加;对固定的Db,〈Ksj〉随Ds一致减小。为使软、硬磁性晶粒间的有效各向异性常数墨。保持较高的值,应控制硬磁性晶粒大于35nm,软磁性晶粒在10nm左右。     

Observation of giant exchange anisotropy

Exchange anisotropy, an interface phenomena in layered magnetic films that produces a hysteresis loop centered at a finite field, has been observed in multilayered films of GdCo/sub 2/ and Co. It is found that the effective bias field diverges at the compensation temperature of the film. The data reported were for films with single-layer thicknesses of 35 AA. Multilayered films are deposited at room temperature and are, therefore, highly disordered or amorphous. To induce exchange anisotropy in these films, they were heated rapidly (<1 min) to 500 degrees C, well above the ordering temperature of the GdCo/sub 2/, and allowed to cool quickly (<3 min to 200 degrees C). The resulting hysteresis loop is shown. Even in the absence of an applied field, this annealing induces exchange anisotropy, presumably due to the presence of a stray magnetic field.<>     

Strain Anisotropy and Magnetic Domains in Embedded Nanomagnets

Nanoscale modifications of strain and magnetic anisotropy can open pathways to engineering magnetic domains for device applications. A periodic magnetic domain structure can be stabilized in sub‐200 nm wide linear as well as curved magnets, embedded within a flat non‐ferromagnetic thin film. The nanomagnets are produced within a non‐ferromagnetic B2‐ordered Fe₆₀Al₄₀ thin film, where local irradiation by a focused ion beam causes the formation of disordered and strongly ferromagnetic regions of A2 Fe₆₀Al₄₀. An anisotropic lattice relaxation is observed, such that the in‐plane lattice parameter is larger when measured parallel to the magnet short‐axis as compared to its length. This in‐plane structural anisotropy manifests a magnetic anisotropy contribution, generating an easy‐axis parallel to the short axis. The competing effect of the strain and shape anisotropies stabilizes a periodic domain pattern in linear as well as spiral nanomagnets, providing a versatile and geometrically controllable path to engineering the strain and thereby the magnetic anisotropy at the nanoscale.     

纳米晶软磁合金环形试样有效磁各向异性的研究

采用趋近饱和定律首次测定了纳米晶合金环形试样有效磁各向异性常数〈K〉.为了对比测量的准确度,同时测试了传统的晶态坡莫合金环形试样的磁晶各向异性.结果表明,用环形试样可以完成对低矫顽力的软磁材料进行磁各向异性的测定.