Ferromagnetic Bulk Research Articles

Inverse proximity effect and influence of disorder on triplet supercurrents in strongly spin-polarized ferromagnets

We discuss the Josephson effect in strongly spin-polarized ferromagnets where triplet correlations are induced by means of spin-active interface scattering, extending our earlier work [Phys. Rev. Lett. 102, 227005 (2009)] by including impurity scattering in the ferromagnetic bulk and the inverse proximity effect in a fully self-consistent way. Our quasiclassical approach accounts for the differences of Fermi momenta and Fermi velocities between the two spin bands of the ferromagnet, and thereby overcomes an important short-coming of previous work within the framework of Usadel theory. We show that non-magnetic disorder in conjunction with spin-dependent Fermi velocities may induce a reversal of the spin-current as a function of temperature.

Surface antiferromagnetism and incipient metal-insulator transition in strained manganite films

Using first-principles calculations, we show that the (001) surface of the ferromagnet La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$ under an epitaxial compressive strain favors antiferromagnetic (AF) order in the surface layers, coexisting with ferromagnetic (FM) bulk order. Surface antiferromagnetism is accompanied by a very marked surface-related spectral pseudogap, signaling an incomplete metal-insulator transition at the surface. The different relaxation and rumpling of the MnO${}_{2}$ and LaO surface planes in the two competing magnetic phases cause distinct work-function changes, which are of potential diagnostic use. The AF phase is recognized as an extreme surface-assisted case of the combination of in-plane AF super-exchange and vertical FM double-exchange couplings that rules magnetism in manganites under in-plane compression.

Weyl problem and Casimir effects in spherical shell geometry

We compute the generic mode sum that quantifies the effect on the spectrum of a harmonic field when a spherical shell is inserted into vacuum. This encompasses a variety of problems including the Weyl spectral problem and the Casimir effect of quantum electrodynamics. This allows us to resolve several long-standing controversies regarding the question of universality of the Casimir self-energy; the resolution comes naturally through the connection to the Weyl problem. Specifically we demonstrate that in the case of a scalar field obeying Dirichlet or Neumann boundary conditions on the shell surface the Casimir self-energy is cutoff-dependent while in the case of the electromagnetic field perturbed by a conductive shell the Casimir self-energy is universal. We additionally show that an analog non-relativistic Casimir effect due to zero-point magnons takes place when a non-magnetic spherical shell is inserted inside a bulk ferromagnet.

Structural analysis of nanocrystals and their role in the coercivity mechanism of Nd–Fe–Al–Dy bulk amorphous ferromagnets

The magnetic property and microstructure of Nd60−xFe30Al10Dyx (x=0, 2, 4) bulk ferromagnets have been investigated. At room temperature, each alloy sample shows hard magnetic behavior, and the intrinsic coercivity increases significantly with increasing Dy content. Transmission electron microscopy (TEM) and Atom probe tomography (APT) results show nanocrystals of width 5–20nm embedded in the Fe-rich amorphous matrix. The coercivity mechanism of the Nd60−xFe30Al10Dyx (x=0, 2, 4) amorphous alloys is discussed in the context of Gaunt's strong pinning model of domain walls. The crystallographic phases of nanocrystals identified by high-resolution TEM can act as pinning centers. Our findings give further insight into the coercivity mechanism of Nd-based bulk amorphous alloys through a better understanding of the nanostructural environment.

Ferromagnetic Fe-based bulk metallic glasses with high thermoplastic formability

New multi-component Fe-based bulk metallic glasses with excellent thermoplastic formability and good soft magnetic and mechanical properties were developed based on an Fe 75 P 10 C 10 B 5 alloy. These glasses also exhibit a wide supercooled liquid region of ∼89 K, a low glass transition temperature of ∼705 K, low viscosity of ∼10 7 Pa s in the supercooled liquid state and high glass-forming ability, with critical sample diameters exceeding 4 mm.

Carrier density dependence of the magnetic properties in iron-doped Bi2Se3 topological insulator

The electronic and magnetic properties of iron-doped topological insulator Bi1.84−xFe0.16CaxSe3 single crystals were studied. By co-doping Fe and Ca atoms, ferromagnetic bulk states with different carrier density (from n-type to p-type) were obtained. Effective magnetic moments for each Fe atom was estimated as small as about 0.07μB. Magnetic and non-magnetic phases separation was observed in all samples. Our results suggest that the bulk ferromagnetism in Fe-doped Bi2Se3 is not intrinsic and regardless of carrier density.

Metal-Insulator Transition from Graphene to Graphane

Graphane is obtained by perfectly hydrogenating graphene. There exists an intermediate material, partially hydrogenated graphene (which we call hydrographene), interpolating from pure graphene to pure graphane. Employing a graph theoretical approach to the site-percolation model, we present an intuitive and physical picture revealing a percolation transition from graphene to graphane. It is demonstrated that hydrographene shows a bulk ferromagnetism based on the Lieb theory. We also propose a weighted percolation model in order to take into account the tendency of hydrogenation to cluster.

Calculation of Gilbert damping in ferromagnetic films

The Gilbert damping constant in the phenomenological Landau-Lifshitz-Gilbert equation which de- scribes the dynamics of magnetization, is calculated for Fe, Co and Ni bulk ferromagnets, Co films and Co/Pd bilayers within a nine-band tight-binding model with spin-orbit coupling included. The calculational e! ciency is remarkably improved by introducing finite temperature into the electronic occupation factors and subsequent summation over the Matsubara frequencies. The calculated dependence of Gilbert damping constant on scattering rate for bulk Fe, Co and Ni is in good agreement with the results of previous ab initio calculations. Calculations are reported for ferromagnetic Co metallic films and Co/Pd bilayers. The dependence of the Gilbert damping constant on Co film thickness, for various scattering rates, is studied and compared with recent experiments.

Interface mediated ferromagnetism in bulk CuO/Cu2O composites

In this letter, we report the interface-related room temperature ferromagnetism in bulk CuO/Cu2O composites which were synthesized by heat-decomposing or oxidation methods with the CuO and Cu2O as the precursors, respectively. Magnetic measurement results indicate that all the CuO/Cu2O composites show obvious room temperature ferromagnetism, while the samples with the pure CuO and Cu2O phases are paramagnetic. What is more, the ferromagnetism can be tuned by changing the phase rations of the CuO and Cu2O in the composites. The observed ferromagnetism in CuO/Cu2O composites is considered to be related to the interface between CuO and Cu2O phases.

Establishing the Glass Forming Ability of Ferromagnetic Bulk Amorphous Alloys Using a Mathematical Model Based on the Chemical Composition

This paper presents a mathematical model that describes the influence of the chemical composition on the glass forming ability of ferromagnetic alloys. Glass forming ability is given by the difference between the glass transition temperature and the primary crystallization temperature of the alloy. The glass forming ability is better as long this difference has a higher value. These temperatures were determined using differential thermal analysis.

Preparation of thin ribbon and bulk glassy alloys in CoFeBSiNb(Ga) using planar flow casting and suction casting methods

Co-based ferromagnetic bulk glassy alloy (BGA) system is promising for future applications as new structural and functional materials. In the present paper as-cast Co47Fe20.9B21.2Si4.6Nb6.3 bilayer, ribbon, rods with diameter up to 5mm and [Co47Fe20.9B21.2Si4.6Nb6.3]98Ga2 as-cast ribbon as well as rod with 4mm diameter were investigated. Co based master ingots with the composition Co47Fe20.9B21.2Si4.6Nb6.3 have been prepared in a vacuum furnace. The as-prepared master ingots were then purified by fluxing. Small amount of gallium (2at.%) was added into one part of the purified master alloy. Ribbons, bilayer and bulk samples in form of rods were prepared by subsequent planar flow casting and suction casting method, respectively. Glassy structure of as-cast samples was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermal stability associated with glass transition temperature (Tg), crystallization temperature (Tx) and supercooled liquid region (∆Tx=Tx–Tg) were examined by differential scanning calorimetry (DSC). The Curie temperature of the investigated ribbons and rods was determined by magnetic thermogravimetry analysis (TGA). Using special disc-shaped samples field dependencies of magnetostriction in parallel and perpendicular directions of the applied magnetic field were obtained by direct measurement.

Development of quaternary Fe-based bulk metallic glasses with high saturation magnetization above 1.6 T

Quaternary Fe-based ferromagnetic bulk metallic glasses (BMGs) with saturation magnetization above 1.6T were successfully fabricated in Fe–Si–B–P alloy system by copper mold casting. These BMGs exhibit low coercive force of 1.6–1.9A/m, high effective permeability of 16,500–17,200 and low core loss. In additional, these BMGs exhibit good mechanical properties as well, i.e., high strength of 3200MPa and plasticity of 1.1%. They are promising to be used as magnetic functional and structural materials in the future.

Electron-phonon scattering dynamics in ferromagnets on ultrafast timescales: Influence of the phonon temperature

The magnetization response of bulk ferromagnets after excitation by an ultrashort optical pulse is calculated using a dynamical model of the Elliott-Yafet type that includes the effects of the spin-orbit interaction in the ab-initio ferromagnetic band structure, the electron-phonon interaction at the level of Boltzmann scattering integrals, and dynamical changes in the temperature of the phonon bath. Using realistic parameters for the ultrashort optical pulse, the computed maximum magnetization quenching achievable with electron-phonon scattering in a fixed band structure is much smaller than the quenching observed in experiments. Heating of the phonon bath is found to not appreciably change the magnetization dynamics on ultrashort timescales.

The Effect of Co Addition on Glassy Forming Ability and Soft Magnetic Properties of Fe-Si-B-P Bulk Metallic Glass

We Successfully Prepared the Rod Glassy Samples of (Fe1-xCox)76Si9B10P5(x= 0~0.4) Bulk Metallic Glass (BMG) with the Diameters up to 3.0 mm by Substituting Fe for a Small Amount of Co Element. A Certain Amount of Co Substitution for Fe Contributes to the Increase of the Glass-Forming Ability (GFA) while Maintaining Good Mechanical Properties (the Fracture Strength up to 3700 MPa). This Co-Added Ferromagnetic Bulk Glassy Alloy System Also Exhibits a Higher Saturation Magnetization of 1.49 T and Lower Coercive Force (Hc, 1.2 A/m). The Fe-Based BMGs with Alloying a Small Amount of Co Element Demonstrate Excellent Combination of High GFA, Good Soft-Magnetic Properties as Well as High Strength.

Soft magnetic properties of bulk FeCoMoPCBSi glassy core prepared by copper mold casting

Bulk Fe66Co10Mo3.5P10C4B4Si2.5 glassy core of 10 mm in outer diameter, 6 mm in inner diameter, and 1 mm in thickness was successfully prepared by copper mold casting. The effects of annealing treatments on magnetic properties and microstructure of these cores were investigated. After an optimum annealing treatment, the resulting bulk glassy core exhibits good magnetic properties, i.e., high saturation magnetic flux density of 1.23 T, low coercive force of 1.0 A/m, high maximum permeability of 450 000, respectively. In addition, the glassy core also shows low core loss of 0.4 W/kg at 50 Hz and at maximum magnetic flux density of 1 T. The synthesis of bulk glassy core with excellent magnetic properties is encouraging for enlarging the application field of ferromagnetic bulk glassy alloys.

On the structural analysis and magnetic behavior of heavily doped Zn1−xMnxO nanopowders synthesized by wet chemistry method

Nanocrystalline Zn1−xMnxO(x=0−0.1) powders are prepared by polymeric precursor method and their structural and magnetic properties carefully studied. X-ray diffraction studies and Raman spectroscopy reveal that Mn2+ ions have substituted the Zn2+ ion without changing the würtzite structure of pristine ZnO up to Mn concentrations x≤0.05. The presence of a secondary phase, related to the solubility of Mn in ZnO is evident for higher Mn-doping concentrations. The negative value obtained for the Curie–Weiss temperature indicates that the interactions between the Mn ions are predominantly antiferromagnetic. Thus, no bulk ferromagnetism is evident in any of the studied samples.

Slow magnetic dynamics and hysteresis loops of the bulk ferromagnet Co7(TeO3)4Br6

Magnetic dynamics of a bulk ferromagnet, a new single crystalline compound Co7(TeO3)4Br6, was studied by ac susceptibility and the related techniques. Very large Arrhenius activation energy of 17.2 meV (201 K) and long attempt time (2x10^(-4)s) span the full spectrum of magnetic dynamics inside a convenient frequency window, offering a rare opportunity for general studies of magnetic dynamics. Within the experimental window the ac susceptibility data build almost ideally semicircular Cole-Cole plots. Comprehensive study of experimental dynamic hysteresis loops of the compound is presented and interpreted within a simple thermal-activation-assisted spin lattice relaxation model for spin reversal. Quantitative agreement between the experimental results and the model's prediction for dynamic coercive field is achieved by assuming the central physical quantity, the Debye relaxation rate, to depend on frequency, as well as on the applied field strength and sample temperature. Cross-over between minor- to major hysteresis loops is carefully analyzed. Low-frequency limitations of the model, relying on domain wall pinning effects, are experimentally detected and appropriately discussed.

EFFECT OF COPPER ADDITIVES ON IRREVERSIBLE MELTING IN [{(Fe0.5Co0.5)0.75B0.2Si0.05}96Nb4]100-xCux, x ≤ 3, ALLOYS

Iron and cobalt based ferromagnetic bulk amorphous alloys have received considerable interest nowadays in view of their useful properties for wide spread applications in magnet technology, shape memory alloys, high frequency communications at low power loss, and other devices. In this purview, here we report synthesis and thermal properties of bulk amorphous alloys [{( Fe 0.5 Co 0.5)0.75 B 0.2 Si 0.05}96 Nb 4]100-x Cu x (x = 0, 1, 2, and 3). A copper mould casting of molten alloy was used to obtain a vitrified alloy in form of thin rods (1–2 mm diameter). Amorphous structure retains at a cooling rate as low as 100 K/s in argon atmosphere. Heat out-put measured in terms of a differential scanning calorimetric signal during heating and cooling cycles of these alloys demonstrate irreversibility in a compositional dependent melting point, which follows the glass transition temperature and successive crystallization. The irreversibility persists in this specific example of the bulk amorphous alloys even on sufficiently slow heating or cooling rates such as 0.33 K/s in argon. The copper inclusion tailors the melting point, the enthalpy of the fusion, and other thermodynamic parameters. Results are analyzed in corroboration to the magnetic properties.

An XMCD-PEEM study on magnetized Dy-doped Nd-Fe-B permanent magnets

We have succeeded in developing a method for photoemission electron microscopy (PEEM) on fully magnetized ferromagnetic bulk samples and have applied this technique to Dy-doped Nd-Fe-B permanent magnets. Remanence magnetization of the sample was approximately 1.2 T, and its dimension was 3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \times$</tex></formula> 3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\times$</tex></formula> 3 mm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{3}$</tex></formula> . By utilizing a yoke as an absorber of the stray magnetic field from the sample, we can obtain well-focused PEEM images of magnetized samples. We have observed not only chemical distributions to visualize Dy-rich and Dy-poor areas but also magnetic domains by x-ray magnetic circular dichroism. The formation of reversed magnetic domains is strongly suppressed at room temperature by Dy-doping. As the temperature of the Dy-doped sample is raised, starting from room temperature, the reversed magnetic domains first grow along the magnetization easy axis. Next, above approximately 80 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{ \circ}$</tex></formula> C, the shapes of reversed domains start to expand in the direction perpendicular to the easy axis. Above approximately 85 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}$</tex></formula> C, the reversed domains cover more than half of the field of view of 30 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu$</tex></formula> m. More importantly, the reversed magnetic domains tend to nucleate or extend in Dy-poor regions. We discuss the relationship between the chemical distribution and the magnetic domain structure.

Cluster Co-Existence and DC Field Effect on Re-entrant Spin Freezing Behavior in La0.85Ca0.15Mn0.95Fe0.05O Manganates

Linear and non-linear dynamic properties of a re-entrant ferromagnetic polycrystalline bulk La0.85Ca0.15Mn0.95Fe0.05O3 sample are studied using AC susceptibility. The Fe doped La0.85Ca0.15Mn0.95Fe0.05O3 undergoes a ferromagnetic transition at TC = 174 K and to spin glass (SG) at Tg∼100 K, and a decrease in the AC susceptibility as well as zero-field-cooled (ZFC) magnetization measured using very low DC magnetic fields. The dynamic behavior is investigated in the intermediate temperature range (Tg<T<TC). A prominent non-linear susceptibility is observed in this range, signifying the decomposition of the FM network into clusters by a random magnetic field which is expected to be generated by PM spins in the FM region. The results are supported by a study of the AC frequency and DC field effect on the non-linear susceptibility.