Bulk Magnetic Materials Research Articles

X rays peer inside a magnet

Submicron spin textures in bulk magnetic materials have been stubbornly hard to detect.

Skyrmions in chiral magnets with Rashba and Dresselhaus spin-orbit coupling

Skyrmions are topological spin textures of interest for fundamental science and applications. Previous theoretical studies have focused on systems with broken bulk inversion symmetry, where skyrmions are stabilized by easy-axis anisotropy. We investigate here systems that break surface inversion symmetry, in addition to possible broken bulk inversion. This leads to two distinct Dzyaloshinskii-Moriya (DM) terms with strengths $D_\perp$, arising from Rashba spin-orbit coupling (SOC), and $D_\parallel$ from Dresselhaus SOC. We show that skyrmions become progressively more stable with increasing $D_\perp/D_\parallel$, extending into the regime of easy-plane anisotropy. We find that the spin texture and topological charge density of skyrmions develops nontrivial spatial structure, with quantized topological charge in a unit cell given by a Chern number. Our results give a design principle for tuning Rashba SOC and magnetic anisotropy to stabilize skyrmions in thin films, surfaces, interfaces and bulk magnetic materials that break mirror symmetry.

Preparation and evaluation of magnetic nanocomposite fibers containing α″-Fe16N2 and α-Fe nanoparticles in polyvinylpyrrolidone via magneto-electrospinning

Two kinds of ferromagnetic nanocomposite fiber comprising α″-Fe16N2 and α-Fe nanoparticles (NPs), which have the highest magnetic moments as hard and soft magnetic materials, respectively, embedded in polyvinylpyrrolidone (PVP) have been synthesized via the magneto-electrospinning method. Both α″-Fe16N2 and α-Fe were single-domain core–shell NPs with an average outer diameter of 50 nm and Al2O3 as the shell. Ferrofluid precursors used for the electrospinning were prepared by dispersing these NPs in a PVP-toluene-methanol solution. The results show that applying the magnetic field in the same direction as the electric field resulted in smaller and more uniform fiber diameters. Nanocomposite fibers containing α″-Fe16N2 had smaller diameters than those containing α-Fe NPs. These magnetic-field effects on the fiber formation were explained by referring to the kinetic energy of the moving jet in the electrospinning process. In addition, magnetic hysteresis curves showed an enhancement of the magnetic coercivity (Hc) and remanence (Mr) by 22.9% and 22.25%, respectively. These results imply a promising possibility of constructing bulk magnetic materials using α″-Fe16N2 NPs, which furthermore reveals attractive features for many other magnetic applications, such as magnetic sensors.

Preparation and characterization of magnetic films of well-dispersed single domain of core–shell α″-Fe16N2/Al2O3 nanoparticles

Ferromagnetic thin films of single domain core–shell α″-Fe16N2/Al2O3 nanoparticles (NPs) with average size of 47nm were prepared by spin coating of the NPs which were well-dispersed to the primary NPs size in toluene by low energy bead-mill dispersion. The magnetic orientation (perpendicular to the Si substrate) was set by applying a magnetic field of 1.2T during the film formation with a thickness of ∼1μm followed by fixation of the NPs with resin. The SEM and XRD results showed that densely packed assemblies of the NPs were formed and aligned perpendicularly in the film. Based on the magnetic hysteresis curves of the NPs film, the application of the magnetic field during film formation increase the magnetic coercivity (Hc) and remanence (Mr) values of the resulted films by 22.7% and 55.2%. These results demonstrate the potential for nanostructuring bulk magnetic materials comprised of single domain core–shell α″-Fe16N2/Al2O3 NPs.

Record High Single-Ion Magnetic Moments Through 4f(n)5d(1) Electron Configurations in the Divalent Lanthanide Complexes [(C5H4SiMe3)3Ln]⁻.

The recently reported series of divalent lanthanide complex salts, namely [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm; Cp' = C5H4SiMe3) and the analogous trivalent complexes, Cp'3Ln, have been characterized via dc and ac magnetic susceptibility measurements. The salts of the complexes [Cp'3Dy](-) and [Cp'3Ho](-) exhibit magnetic moments of 11.3 and 11.4 μB, respectively, which are the highest moments reported to date for any monometallic molecular species. The magnetic moments measured at room temperature support the assignments of a 4f(n+1) configuration for Ln = Sm, Eu, Tm and a 4f(n)5d(1) configuration for Ln = Y, La, Gd, Tb, Dy, Ho, Er. In the cases of Ln = Ce, Pr, Nd, simple models do not accurately predict the experimental room temperature magnetic moments. Although an LS coupling scheme is a useful starting point, it is not sufficient to describe the complex magnetic behavior and electronic structure of these intriguing molecules. While no slow magnetic relaxation was observed for any member of the series under zero applied dc field, the large moments accessible with such mixed configurations present important case studies in the pursuit of magnetic materials with inherently larger magnetic moments. This is essential for the design of new bulk magnetic materials and for diminishing processes such as quantum tunneling of the magnetization in single-molecule magnets.

Giant magnetostriction in magnetite nanoparticles

Typically the value of the magnetostrictive coefficient (λ) observed for bulk magnetic materials such as cubic ferrites is 10−6. However, giant magnetostriction (λ ≤ 10−3) is only observed in a few bulk intermetallic materials based on alloys of rare earth and iron such as TbFe, TbFe2, DyFe2 and Terefenol-D. While giant magnetostriction is known in nanostructured films, we show for the first time, this phenomenon occurs in magnetic nanoparticles. By using in-field small angle X-ray scattering (SAXS) as a tool, we demonstrate that a 4% relative change in dimension of the particle can be observed in 5.0 nm Fe3O4 nanoparticles at room temperature with 1 kG magnetic field. Also, we propose that the observed values are due to interaction effects and magnetoelastic coupling of particle magnetic moments and external magnetic field.

Three-dimensional (3D) Ising universality in magnets and critical indices at fluid–fluid phase transition

Experimental data for critical exponents in some magnetic materials are compared with recent theoretical results on the three-dimensional (3D) Ising model, as derived by one of us (ZDZ) based on two conjectures [Z.D. Zhang, Conjectures on the exact solution of three-dimensional (3D) simple orthorhombic Ising lattices, Phil. Mag. 87 (2007), pp. 5309–5419]. It is found that critical exponents in some bulk magnetic materials indeed form a 3D Ising universality. Our attention is then focused on the critical indices at fluid–fluid phase transition. We suggest to use Zhang's exponent β = 3/8 to fit the experimental data over the wider asymptotic region near the critical point of a fluid–fluid phase transition. The 3D Ising universality should exist for critical indices in a certain class of magnets and at fluid–fluid phase transition.

Stable π–π dependent electron conduction band of TPP[M(Pc)L2]2 molecular conductors (TPP = tetraphenylphosphonium; M = Co, Fe; Pc = phthalocyaninato; L = CN, Cl, Br)

The partially-oxidized TPP[M(Pc)L(2)](2) molecular conductors exhibit variable electronic and magnetic transport bulk materials properties due to central metal and axial ligand molecular modifications. The controllable electrical conductivity and giant negative magnetoresistance can be mainly attributable to the varying ligand field energy and physical bulkiness of the axial ligands which cause modulation in the intra-molecular π-d (Pc-M) and inter-molecular π-π (Pc-Pc) interactions in the TPP[M(Pc)L(2)](2) system, respectively. Characterization of the electronic conduction band utilizing one-dimensional (1-D) tight-binding approximation from infrared reflectance and thermoelectric power profile reveal consistent band widths of 0.43 eV-0.62 eV for the Co series (L = Br < Cl < CN) and 0.44-0.56 eV for the Fe series (L = Br < Cl < CN). The fixed band width suggests that stable electron conduction bands (transport pathway) can be constructed which can withstand the molecular π-d interaction modifications that severely alter the bulk electronic and magnetic materials properties of the TPP[M(Pc)L(2)](2) molecular conductors.

Broadband Characterization of Bulk and Thin Magnetic Composites Using Stripline Structures

We present two broadband characterization techniques to extract the RF (50 MHz-4.5 GHz) permeability of bulk and thin magnetic composite materials. Both techniques are based on broadband stripline structures with the permeability determined from -parameter measurements. For bulk samples, a tapered stripline fixture, supporting transverse electromagnetic (TEM) waves, is employed. As this approach is not suitable for thin material samples (thickness t <; 1 mm), a microstrip line fixture is proposed along with a new permeability de-embedding technique. Compared with conventional microstrip line methods, this approach is found to be accurate for thin samples with small surface areas (less than 1 × cm × 1 cm). The validity of the techniques is demonstrated by measuring a bulk ferrite slab and a thin patterned ferrite composite with in-plane anisotropy.

Anomalous Hall effect in the high-temperature ferrimagnetic semiconductorsBaFe2±xRu4∓xO11

Field and temperature dependences of the magnetic and transport properties of single crystals of ${\text{BaFe}}_{2+x}{\text{Ru}}_{4\ensuremath{-}x}{\text{O}}_{11}$ $(x=0.5,0.9,1.4)$ R-type ferrites are reported. A wide homogeneity range of Ru/Fe compositions allows to tune the strength of the anomalous Hall effect (AHE), the width of the semiconducting gap $(\ensuremath{\Delta})$, and the temperature of magnetic ordering $({T}_{C})$ within one structural family. These parameters are enhanced for higher Fe content. A large Hall resistivity ${\ensuremath{\rho}}_{xy}=77\text{ }\ensuremath{\mu}\ensuremath{\Omega}\text{ }\text{cm}$ observed in single-crystal ${\text{BaFe}}_{3.4}{\text{Ru}}_{2.6}{\text{O}}_{11}$ at 300 K is several orders of magnitude large than that observed for bulk magnetic materials, and comparable to the Hall resistivity of thin films of magnetite. The anomalous Hall coefficient ${R}_{s}$ changes sign at a characteristic temperature ${T}^{\ensuremath{\ast}}$ that depends on the Fe/Ru ratio, which we attribute to variations in the electronic band structure. The ${R}_{s}$ vs longitudinal resistivity $({\ensuremath{\rho}}_{xx})$ dependence of ${\text{BaFe}}_{3.4}{\text{Ru}}_{2.6}{\text{O}}_{11}$ can be described in terms of negative skew and positive side-jump scattering contributions that differ above and below ${T}^{\ensuremath{\ast}}$. The very small side jump $\ensuremath{\Delta}y$ estimated from the contribution to the AHE that is quadratic in ${\ensuremath{\rho}}_{xx}$ suggests that the AHE is caused by a Karplus-Luttinger (Berry phase) mechanism.

Refraction indices of spatially nonuniform magnetic systems and nanostructures from the first principles

An equilibrium two-time temperature Green’s function (TTGF)-based, quantum statistical mechanical approach has been used to derive from the first principles an explicit expression for the tensor of “local” refraction indices of spatially nonuniform systems in weak external electromagnetic (EM) fields in the linear approximation with regard to the field magnitudes. Written in terms of the TTGF-based, first-principle tensorial dielectric and magnetic susceptibilities, the obtained formula for the local tensor of refraction indices (TRI) is applicable to any system, including individual nanoscale objects, such as quantum dots and wires, magnetic nanostructures, composite materials, or spatially nonuniform, bulk magnetic materials. An explicit expression for the space-time Fourier transform (STFT) of the dielectric susceptibility tensor used in TRI is derived in terms of STFTs of the charge density—charge density TTGFs, while the corresponding STFT of the magnetic susceptibility tensor also includes STFTs of the microcurrent—microcurrent TTGFs. The STFTs of the equilibrium TTGFs featuring in the susceptibilities, and thus necessary to calculate TRI, can be obtained by equilibrium quantum statistical mechanical means, modeling and simulations, or from experimental data. Two TRI regimes of significant interest for applications that can be realized in spatially inhomogeneous magnetic systems have been identified.

Quantification of the contribution of pedogenic magnetic minerals to magnetic susceptibility of loess and paleosols on Chinese Loess Plateau: Paleoclimatic implications

This study investigates the grain size–dependent properties of magnetic minerals in Chinese loess samples with an aim to better understand the contribution of pedogenesis to magnetic susceptibility enhancement of the paleosols. Samples of loess and paleosol were divided into two fractions, magnetic extracts and residues, by extraction in a weak magnetic field. Both the raw samples of loess and paleosol (i.e., before magnetic extraction) and the samples of residues after magnetic extraction were then separated into five grain size fractions by gravitational settling and centrifugation. Low‐field magnetic susceptibility (MS) and the frequency‐dependent MS of these samples were measured, and samples treated with citrate‐bicarbonate‐dithionite (CBD) to remove the iron oxides were also analyzed. For loess samples, coarse grains (≥2μm) contribute more than 50% of the magnetic signal. The ≤0.3 μm fraction, considered to be of pedogenic origin, contributes 22% (before magnetic extraction) or 36% (after magnetic extraction) of the magnetic signal in loess. A different pattern is found in the paleosol samples. For these samples, the ≤0.3 μm fraction contributes &gt;60% (before magnetic extraction) or 71% (after extraction) to the bulk MS. In addition, 91% of the magnetic signal in paleosol S5 is CBD extractable. The CBD technique is able to extract pedogenic superfine‐grained magnetic particles and coarse dust magnetic particles. These results confirm that pedogenesis leads to enhanced MS and helps to quantify the multiple origins of the bulk magnetic material that defines loess and paleosol MS. This approach provides a reliable estimate of the contribution of pedogenic magnetic materials to MS. Our study indicates that multiple methods should be employed in analyzing paleoclimatic changes recorded by MS in the Chinese loess‐paleosol sequences.

Sensors based on bulk soft magnetic materials: Advances and challenges

Sensors with cores, yokes or field concentrators made of bulk magnetic material are more sensitive and stable than thin-film sensors. Non-linearity and temperature dependence of sensitivity is often suppressed by a feedback. The common problem of these sensors is remanence, cross-field sensitivity and temperature stability of offset. The long-time effort to miniaturize the fluxgates led only to a few practical designs. For flat sensors (either pcb or CMOS) the core etched from amorphous tape gives better properties then electrodeposited or sputtered core. We compare traditional miniature fluxgates using wire cores based on longitudinal fluxgate effect with sensors using transverse fluxgate effect and GMI effect. Well-designed field concentrators or yokes can improve the parameters of any magnetic sensor. The achievable stable amplification factor is 10–100. Having a means to demagnetize the field concentrator is desirable. Overview of magnetic sensors for mechanical quantities is also given with special focus on torque sensors.

Towards Hall effect spintronics

Major efforts in the current exploration of spintronics are focused on the giant magnetoresistance (GMR) phenomenon in metallic, semiconducting and tunnel junction magnetic heterostructures. I wish to present a different approach based on the extraordinary Hall effect (EHE). Since its discovery more than a century ago, the EHE was not considered seriously for technological applications because of its relatively small value in bulk magnetic materials. Several techniques were recently developed to significantly enhance the effect. Field sensitivity of tens to hundreds Ω/T has been obtained. We argue that EHE-based sensors and memory devices promise a number of valuable advantages, including high sensitivity, thermal stability and simplicity and low cost manufacture, and can become an alternative to the GMR.

Méthode de caractérisation en hyperfréquences de matériaux magnétiques et diélectriques

Some microwave passive components (such as circulators, isolators,... etc) still use bulk magnetic materials. A method for the dielectric and magnetic characterization of multi-layer materials in the microwave range is presented as well as its reliability and its domain of validity. It allows to determine the elements of the permeability tensor of gyromagnetic slab. This method of material characterization can be implemented at various microwave bands by changing the geometrical dimensions of the rectangular waveguide.

Magnetization-Induced Third-Harmonic Generation in Nanostructures and Thin Films

This paper reports on the results of research into low-dimensional magnetic structures that have been intensively studied over previous decades due to the discovery of novel effects that are exhibited by these structures but not observed in bulk magnetic materials. A nonlinear optical analog of the magnetooptical Kerr effect is revealed in the optical third-harmonic generation in thin magnetic metallic films and nanogranular structures. It is shown that the magnetic nonlinear optical Kerr effect observed in the third harmonic exceeds the magnetooptical analog by more than one order of magnitude. © 2005 Pleiades Publishing, Inc.

Humidity-induced magnetization and magnetic pole inversion in a cyano-bridged metal assembly

In general, magnetic properties of bulk magnetic materials are independent of the humidity of the environment. To obtain a magnetic material that has humidity-sensitive characteristics, water vapour must penetrate the lattice and act on spin sites. Nanoporous materials composed of metal-assembled complexes may be expected to display some humidity response because materials in this category can show functionalities such as gas storage and molecular recognition. Here, we demonstrate humidity-induced reversible variations in the magnetic properties of cyano-bridged cobalt(II)-manganese(II)-chromium(III) metal assemblies. The observed magnetic humidity response is due to adsorption and desorption of a ligand water molecule on the cobalt ion, which changes cobalt (II) between a 6- and 4-fold coordination geometry and switches the magnetic interaction between ferromagnetic coupling and antiferromagnetic coupling.

Simulation of Surface Heating Effects and Effective Permeabilities Using a Jiles–Atherton Model

The hysteretic, nonlinear character of ferromagnetic steels, coupled with pronounced stress and temperature sensitivities, make detailed magnetoelastic sensor design and calibration very difficult at present, particularly under possible high repetition rates during disaster scenarios, and in large cables (/spl delta//R<1,Bi<0.1, where /spl delta/ is the magnetic skin depth and Bi is the thermal Biot number) with inhomogeneous stress and thermal fields. The desire also exists to associate bulk magnetic material parameters with microstructural features to enable generally useful correlations to be developed. The Jiles-Atherton class of thermodynamic models potentially fit these requirements, and we explore their use for investigating field thermomagnetic effects in A36/1018 steels. Model development is supported by microstructural characterization of the anhysteretic via magnetic force microscopy and tunneling electron microscope imaging.

The possibility of utilizing the high permeability magnetic materials in construction of magnetoelastic stress and force sensors

The paper presents new possibilities in the field of construction of the magnetoelastic stress and force sensors. Two methods of applying mechanical stress to the core of the magnetoelastic sensing elements were presented. The first one is suitable for bulk magnetic materials, such as ferrites. The second method can be utilized in a case of both ribbon and bulk ring cores. Such ring-shaped cores can be made of amorphous alloys as well as ferrites. Both these methods enable achieving uniform distribution of stresses in the samples with closed magnetic circuit. The experiment was performed on high permeability Mn–Zn ferrite and Co-rich amorphous alloy. The results confirm, that magnetic properties of both ferrite and amorphous alloys change significantly under compressive stress from the external forces. The relative changes of the permeability exceed 100% for stresses up to 25 MPa. Paper presents also functional characteristics of the magnetoelastic sensor which operate in the resonant circuit configuration. Output frequency signal from the sensor operating in this configuration is suitable for further digital processing.

Study of internal permanent magnet rotor made of 0.6C–13Cr–Fe dual state magnetic material

We have successfully developed an internal permanent magnet (IPM) rotor using dual state bulk magnetic material to increase usable magnetic flux dramatically. The most significant benefit of the IPM rotor is its mechanical reliability, because permanent magnets are inserted in slots of soft magnetic material. On the other hand, there is significant leakage flux between adjoining permanent magnets in the soft magnetic rotor core, reducing the usable magnetic flux flowing into the stator core. To solve this problem, we used a dual state magnetic material, 0.6C–13Cr–Fe alloy. This soft magnetic material could locally be changed into nonmagnetic material by localized heat treatment. By changing the material at leakage flux path into nonmagnetic, we can reduce the leakage flux, while keeping the rotor mechanically sound. By applying the dual state magnetic material to an experimental eight pole IPM rotor, the useful flux flowing in the stator core differs by 8% when compared to an all soft magnetic rotor core.