Spin-wave Stiffness Research Articles

Spin-wave dynamics in the helimagnet FeGe studied by small-angle neutron scattering

We have studied the spin-wave stiffness of the Dzyaloshinskii-Moriya helimagnet FeGe in a temperature range from 225~K up to $T_C \approx$~278.7~K by small-angle neutron scattering. The method we have used is based on [S. V. Grigoriev et al. Phys. Rev. B \textbf{92} 220415(R) (2015)] and was extended here for the application in polycrystalline samples. We confirm the validity of the anisotropic spin-wave dispersion for FeGe caused by the Dzyaloshinskii-Moriya interaction. We have shown that the spin-wave stiffness $A$ for FeGe helimagnet decreases with a temperature as $A(T) = 194(1-0.7(T/T_C)^{4.2})$ meV\AA$^2$. The finite value of the spin-wave stiffness $A = 58$ meV\AA$^2$ at $T_C$ classifies the order-disorder phase transition in FeGe as being the first order one.

Structural Relaxation in the Amorphous Alloys: FeMeMoCrNbB (where Me = Ni or Co)

In this paper, the results of structural and magnetic investigations are presented for the following amorphous alloys: FeMeMoCrNbB (where Me = Ni or Co). The structural investigations were performed using X-ray diffractometry. It was found that the investigated samples were amorphous in the as-cast state. The magnetisation was measured within magnetic fields ranging from 0 to 1 T using a vibrating sample magnetometer. Investigation of the “magnetisation in the area close to ferromagnetic saturation” showed that the magnetisation process in strong magnetic fields is connected with the rotation of magnetic moments in the vicinity of defects, which are the sources of short-range stresses. Analysis of the high-field magnetization curves facilitated the calculation of the spin-wave stiffness parameter.

The Effect of Isothermal Annealing on the Structural and Magnetic Properties of the Bulk Amorphous Alloy: FeCoTiYB

The aim of this paper was to conduct studies concerning the magnetization of bulk amorphous Fe61Co10Ti4Y5B20 alloy when subjected to strong magnetic felds in the area known as the approach to ferromagnetic saturation. The samples were produced using the suction-casting method. The amorphicity of the investigated alloy, in the as-quenched state and after annealing process in a temperature below the crystallization temperature was verified using X-ray diffractometry. The magnetization was measured using a vibrating sample magnetometer. On the basis of the obtained results, the type of structural defects having influence on magnetization in high magnetic fields were determined. Analysis of the high-field magnetization curves has facilitated the calculation of the spin wave stiffness parameter. From this parameter, the range and strength of the exchange interactions could be determined.

Low-temperature magnetization and magnetic exchange interactions in Fe40Ni40P14B6 bulk metallic glasses

One of the fundamental magnetic properties – the low-temperature magnetization as well as the exchange interactions derived by using a spin-wave excitation theory – has been well studied in glassy ribbons rather than bulk metallic glasses. The paper studies the temperature-dependent magnetization of a bulk Fe40Ni40P14B6 alloy, derives the range and fluctuation of exchange interactions, and compares these magnetic behaviors with those of well-studied glassy ribbons. The low-temperature magnetization of our bulk glass can be well expressed by a spin-wave excitation theory, which has been widely utilized to study the temperature-dependent magnetization of ferromagnetic glassy ribbons. Moreover, both the derived spin-wave stiffness coefficient and the derived range and fluctuation of exchange interactions in our bulk metallic glass are very similar to those in glassy ribbons, indicative of the similarity in magnetic exchange interaction between bulk metallic glasses and glassy ribbons.

Ferromagnetic and Spin-Wave Resonance on Heavy-Metal-Doped Permalloy Films: Temperature Effects

Broadband ferromagnetic resonance (FMR) spectroscopy is used to study the temperature $(T)$ and dopant-concentration dependence of the magnetodynamic properties of Permalloy $({\hbox{Py}} = {\hbox{Ni}}_{80}{\hbox{Fe}}_{20})$ and Py $_{100-x}$ M $_{x}$ films, where the dopant ${\hbox{M}} = {\hbox{Pt}}$ , Au, and Ag. The saturation magnetization $(M_{\text{S}})$ and Gilbert damping constant $(\alpha)$ are determined from the uniform FMR mode, while the spin wave stiffness $(D)$ is extracted using the first perpendicular standing spin-wave mode. The temperature dependence of $D$ is best described by a $T^{2}$ law, which suggests a noticeable effect of the itinerant character of the electrons. The spin wave stiffness is also estimated using Bloch's law and the two methods are compared. The results strongly imply that not only spin wave and Stoner excitations, but also other mechanisms contribute to the reduction of $M_{\mathrm{S}}$ . The damping increases with $T$ for all samples, but the enhancement is most pronounced for Py doped with 30 at.% Au.

First-Principles Prediction of Electronic, Magnetic, and Optical Properties of Co2MnAs Full-Heusler Half-Metallic Compound

Electronic, magnetic, and optical properties of Co2MnAs full-Heusler compound have been calculated using a first-principles approach with the full-potential linearized augmented plane-wave (FP-LAPW) method and generalized gradient approximation plus U (GGA + U). The results are compared with various properties of Co2MnZ (Z = Si, Ge, Al, Ga, Sn) full-Heusler compounds. The results of our calculations show that Co2MnAs is a half-metallic ferromagnetic compound with 100% spin polarization at the Fermi level. The total magnetic moment and half-metallic gap of Co2MnAs compound are found to be 6.00μB and 0.43 eV, respectively. It is also predicted that the spin-wave stiffness constant and Curie temperature of Co2MnAs compound are about 3.99 meV nm2 and 1109 K, respectively. The optical results show that the dominant behavior, at energy below 2 eV, is due to interactions of free electrons in the system. Interband optical transitions have been calculated based on the imaginary part of the dielectric function and analysis of critical points in the second energy derivative of the dielectric function. The results show that there is more than one plasmon energy for Co2MnAs compound, with the highest occurring at 25 eV. Also, the refractive index variations and optical reflectivity for radiation at normal incidence are calculated for Co2MnAs. Because of its high magnetic moment, high Curie temperature, and 100% spin polarization at the Fermi level as well as its optical properties, Co2MnAs is a good candidate for use in spintronic components and magnetooptical devices.

Coherence and stiffness of spin waves in diluted ferromagnets

We present results of a numerical analysis of magnon spectra in supercells simulating two-dimensional and bulk random diluted ferromagnets with long-ranged pair exchange interactions. We show that low-energy spectral regions for these strongly disordered systems contain a coherent component leading to interference phenomena manifested by a pronounced sensitivity of the lowest excitation energies to the adopted boundary conditions. The dependence of configuration averages of these excitation energies on the supercell size can be used for an efficient determination of the spin-wave stiffness D. The developed formalism is applied to the ferromagnetic Mn-doped GaAs semiconductor with optional incorporation of phosphorus; the obtained concentration trends of D are found in reasonable agreement with recent experiments. Moreover, a relation of the spin stiffness to the Curie temperature Tc has been studied for Mn-doped GaAs and GaN semiconductors. It is found that the ratio Tc/D exhibits qualitatively the same dependence on Mn concentration in both systems.

Comparative studies of magnetic and magnetocaloric properties in amorphous Gd0.67Y0.33 and Gd0.67Zr0.33 films

Abstract Amorphous Gd 0.67 × 0.3 (X=Y, Zr) films were grown by RF sputtering and their magnetic and magnetocaloric properties were studied. The magnetic properties were found to be improved in case of Gd 0.67 Y 0.33 with respect to Gd 0.67 Zr 0.33, in which the Curie temperature, magnetization and spin wave stiffness were higher for Gd 0.67 Y 0.33 , which we attribute to its larger Gd-Gd distance. The temperature dependence of magnetic entropy change (−ΔS M ) was calculated from the isothermal magnetization. Under a magnetic field change of 3 T, (−ΔS M ) were found to be 2.14 and 1.12 J/kg K, with corresponding relative cooling power (RCP) of 198.86 and 113.2 J/kg, for Gd 0.67 Y 0.33 and Gd 0.67 Zr 0.33 , respectively. The larger magnetocaloric properties for Gd 0.67 Y 0.33 are attributed to its higher magnetization.

Low-temperature specific heat in hydrogenated and Mn-dopedLa(Fe,Si)13

It is now well established that the paramagnetic-to-ferromagnetic transition in the magnetocaloric $\mathrm{La}{(\mathrm{FeSi})}_{13}$ is a cooperative effect involving spin, charge, and lattice degrees of freedom. However, the influence of this correlated behavior on the ferromagnetic state is as yet little studied. Here we measure the specific heat at low temperatures in a systematic set of $\mathrm{LaF}{\mathrm{e}}_{x}\mathrm{M}{\mathrm{n}}_{y}\mathrm{S}{\mathrm{i}}_{z}$ samples, with and without hydrogen, to extract the Sommerfeld coefficient, the Debye temperature, and the spin-wave stiffness. Substantial and systematic changes in magnitude of the Sommerfeld coefficient are observed with Mn substitution and introduction of hydrogen, showing that over and above the changes to the density of states at the Fermi energy there are significant enhanced $d$-band electronic interactions at play. The Sommerfeld coefficient is found to be $90--210\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$, unusually high compared to that expected from band-structure calculations. The Debye temperature determined from the specific heat measurement is insensitive to Mn and Si doping but increases when hydrogen is introduced into the system. The Sommerfeld coefficient is reduced in magnetic field for all compositions that have a measurable spin-wave contribution. These results move our understanding of the cooperative effects forward in this important and interesting class of materials significantly and provide a basis for future theoretical development.

Nanocrystalline alloys: synthesis and characterization of non-stoichiometric Co2FeAl nanocrystals

Non-stoichiometric Co2FeAl nanoparticles are formed by the in-solution thermal decomposition of the corresponding metal acetylacetonate complexes in the presence of capping ligands followed by reduction of the obtained material under an H2-containing atmosphere. Transmission electron microscopy indicates that sub-100 nm nanoparticles are obtained, with reasonable size control. Magnetic measurements indicate that the saturation magnetization, Bloch behavior, magnetization reversal, spin-wave stiffness, and exchange stiffness are all comparable to those observed for bulk and thin-film Co2FeAl, indicating that these nanomaterials are promising for use in nanoparticle-based spintronic devices.

Structure and Soft Magnetic Properties of the Amorphous Alloys: Fe61Co10Ti3-xY6+xB20 (x = 0, 1)

This paper presents studies relating to the structure, soft magnetic properties and thermal stability of the following bulk amorphous alloys: Fe61Co10Ti3-xY6+xB20 (x = 0, 1). On the basis of the performed X-ray diffraction studies and Mössbauer spectroscopy, it was found that investigated samples were amorphous in the as-cast state. The DSC curve obtained for Fe61Co10Ti2Y7B20 alloy exhibited one exothermic peak, while for the Fe61Co10Ti3Y6B20 sample, two peaks were distinguishable. The change in the chemical structure of the investigated alloys has a major effect on their soft magnetic properties; especially on coercivity and saturation magnetization. On the basis of the magnetization curves analysis, the spin wave stiffness parameter Dsp were determined for the investigated alloys.

Spin–orbit stiffness of the spin‐polarized electron gas

In a spin-polarized electron gas, Coulomb interaction couples the spin and motion degrees of freedom to build propagating spin waves. The spin wave stiffness Ssw quantifies the energy cost to trigger such excitation by perturbing the kinetic energy of the electron gas (i.e. putting it in motion). Here we introduce the concept of spin–orbit stiffness, Sso, as the energy necessary to excite a spin wave with a spin polarization induced by spin–orbit coupling. This quantity governs the Coulombic enhancement of the spin–orbit field acting of the spin wave. First-principles calculations and electronic Raman scattering experiments carried out on a model spin-polarized electron gas, embedded in a CdMnTe quantum well, demonstrate that Sso = Ssw. Through optical gating of the structure, we demonstrate the reproducible tuning of Sso by a factor of 3, highlighting the great potential of spin–orbit control of spin waves in view of spintronics applications. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Spin wave and percolation studies in epitaxial La2/3Sr1/3MnO3 thin films grown by pulsed laser deposition

We investigate the magnetic and transport properties of high quality La2/3Sr1/3MnO3 thin films grown by pulsed laser deposition. X-ray diffraction shows that the deposited films are epitaxial with the expected pseudo-cubic structure. Using the spin wave theory, the temperature dependence of magnetization was satisfactory modeled at low temperature, in which several fundamental magnetic parameters were obtained (spin wave stiffness, exchange constants, Fermi wave-vector, Mn–Mn interatomic distance). The transport properties were studied via the temperature dependence of electrical resistivity [ρ(T)], which shows a peak at Curie temperature due to metal to insulator transition. The percolation theory was used to simulate ρ(T) in both the ferromagnetic and paramagnetic phases. Reasonable agreement with the experimental data is reported.

Thermal demagnetization due to spin waves in nanocrystalline Ni

An elaborate analysis of the decline of magnetization with increasing temperature, observed in nanocrystalline (nc-) Ni samples with average crystallite size, d, varying from d=10nm to 40nm, permits us to (i) completely rule out a possible Stoner single-particle contribution to the thermal demagnetization, M(T), at external magnetic fields 10kOe≤H≤70kOe and temperatures T≤350K, (ii) demonstrate that spin-wave (SW) excitations alone account for the observed MH(T) and (iii) the thermal renormalization of spin-wave stiffness is primarily due to the magnon–magnon interactions. For fields H≥20kOe, the spin-wave stiffness at 0K,D(T=0,H), is found to decrease with field as D(T=0,H)∼H1/2 for all the nanocrystalline samples. Extrapolation of the D(T=0,H)−H1/2 straight line to H=0 yields D0≡D(T=0,H=0) for a sample of given d. D0 varies with d as d4/3. This power law behavior of D0 asserts that the crystallite size is the relevant length scale for spin waves. Strong departures from the spin-wave behavior are observed at low temperatures T≤T†(H) in nc-Ni with d=10nm. Such departures, marked by the T4/3 power law behavior of [MH(T)]2, are a manifestation of damped spin waves, which act as non-propagating spin fluctuations and get suppressed by magnetic field in accordance with the H1/2 power law.

Spin waves in full-polarized state of Dzyaloshinskii-Moriya helimagnets: Small-angle neutron scattering study

We develop the technique to study the spin-wave dynamics of the full-polarized state of the Dzyaloshinskii-Moriya helimagnets by polarized small-angle neutron scattering. We have experimentally proven that the spin-waves dispersion in this state has the anisotropic form. We show that the neutron scattering image displays a circle with a certain radius which is centered at the momentum transfer corresponding to the helix wave vector in helimagnetic phase ${k}_{s}$, which is oriented along the applied magnetic field $\mathbf{H}$. The radius of this circle is directly related to the spin-wave stiffness of this system. This scattering depends on the neutron polarization showing the one-handed nature of the spin waves in Dzyaloshinskii-Moriya helimagnets in the full-polarized phase. We show that the spin-wave stiffness $A$ for MnSi helimagnet decreased twice as the temperature increases from zero to the critical temperature ${T}_{c}$.

Effect of growth temperature on the electronic transport and anomalous Hall effect response in co-sputtered Co2FeSi thin films

Co-sputtered Co2FeSi thin films are studied by varying the growth temperature (Ts) as a control parameter in terms of the appreciable change in the disorder. The effect of Ts on structural, magnetic, electrical, and magneto-transport properties was investigated. As Ts is increased from room temperature to 400 °C, an improvement in the crystallinity and atomic ordering are observed. These are found to be correlated with the associated reduction in residual resistivity (ρxx0) from 410 to 88 μΩ cm, an increment in residual resistivity ratio (r) from 0.8 to 1.23, and an increase in saturation magnetization from 1074 to 1196 emu/cc. The spin wave stiffness constant in these films is found to increase with Ts, with a reasonably high value of 358 meVÅ2 at the optimum value of Ts of 400 °C. Further, the obtained high carrier concentration and mobility values (at 10 K) of ∼30 e−s/f.u. and ∼0.11 cm2 V−1 s−1 for the films deposited at Ts = 400 °C shows the presence of compensated Fermi surface. The transport properties are investigated qualitatively from the scaling of anomalous Hall resistivity ρxys(T) with the longitudinal resistivity ρxx(T) data, employing the extrinsic (skew- and side-jump scatterings) and intrinsic scattering contributions. The variation in the intrinsic scattering contributions observed via the variation in linear dependence of ρxys on ρxx2 with the change in Ts is found to be associated with the improvement in the crystallinity of these films.

Spin-wave stiffness parameter in ferrimagnetic systems: Nanoparticulate powders of (Mg,Zn)Fe2O4 mixed ferrites

We have evaluated the spin-wave stiffness parameter in nanoparticulate powders of Mg1−xZnxFe2O4 (0.0≤x≤0.6) mixed ferrites from magnetization data obtained at two different ranges of temperature: 5−300 K and 300−750 K. At the lower temperature range the T-dependence of the saturation magnetization, Ms, data could be fitted to the Bloch's law with T3/2. The spin-wave stiffness parameters D were determined from the coefficient of T3/2; being ∼132 and ∼86 meVÅ for x = 0.0 and 0.6, respectively, with the corresponding exchange constant JAB of ∼1.10 and ∼0.72 meV, respectively. The values of D determined from the experimental Curie temperature Tc were ∼212 and ∼163 meVÅ2 for x = 0.0 and 0.6, respectively, with the corresponding exchange constant JAB of ∼1.77 and ∼1.30 meV. The difference in both JAB and D values obtained from the coefficient of T3/2 and from Tc may be attributed to the fact that the magnetic measurements were performed at a different range of temperatures. The results are discussed in terms of the cation distribution among A- and B-sites of occupation on these spinel ferrites.

Metallic behavior induced by slight N doping in Sr2FeMoO6double perovskite compound

The N-doped compounds with double perovskite structure were synthesized by ammonolysis method and are characterized by means of X-ray diffraction, magnetic and electrical resistivity measurements. The effect of dopant on the crystal structure, magnetic properties and electrical behavior was studied. Rietveld analysis shows that for N-doped samples the unit-cell parameters, a and c , first decrease, then increase with ammonolyzed time. Thermal magnetization shows that the spontaneous magnetization for all samples follows Bloch’s law in low temperature range. The spin wave stiffness constant, Dsw, increases with ammonolyzed time from 0 to 8 h, followed by a sharp decrease with ammonolyzed time from 8 to 10 h. Isothermal magnetization for N-doped samples demonstrates that the saturation magnetic moments at different temperatures first increase, then decrease with N doping. The resistivity increases with ammonolyzed time from 3 to 10 h. For slight N-doped samples the electrical resistivity demonstrates metallic behavior and electrical transport behavior for all samples shows the disordered characteristic. The degree of B-site cationic order in N-doped samples is lower than that in N-undoped sample.

Hydrogen enhanced magnetization and exchange interaction in amorphous (FeCo)0.70Ge0.30-H films

(FeCo)xGe1–x-H and (FeCo)xGe1–x films with high FeCo concentration were prepared by magnetron sputtering, and hydrogen enhanced magnetization and exchange interaction were found by static and dynamic magnetization measurements. The static magnetization measurements demonstrate that the saturation magnetization of (FeCo)0.70Ge0.30-H films is high up to 567 emu/cm3 at room temperature, which is about 170% higher than that of (FeCo)0.70Ge0.30 films. The dynamic magnetization measurements indicate that the exchange interaction in term of spin-wave stiffness constant D is 176.2 meV·Å2 in (FeCo)0.70Ge0.30-H films, which is about 156% larger than that in (FeCo)0.70Ge0.30 films. It is proposed that the hydrogen enhanced exchange interaction is mediated by both holes and H1s electrons in (FeCo)xGe1–x-H films. This may open an alternative way to design new spintronic materials by hydrogen enhancing magnetization and exchange interaction.

Structural Defects In The FeCoYB Amorphous Alloys

Abstract The aim of this work was to determine the nature of the structural defects that have a major influence on the magnetisation process within the investigated alloys. The structure of the alloys in the as-quenched state was investigated by means of X-ray diffractometry. It was confirmed that the samples were amorphous. The magnetisation was measured within magnetic fields ranging from 0 to 2T using a vibrating sample magnetometer (VSM). The investigation of the ‘magnetisation in the area close to ferromagnetic saturation’ showed that, for this class of alloys, the magnetisation process in strong magnetic fields is connected with the following two influences: 1) Firstly, the rotation of the magnetic moments in the vicinity of the defects, which are the sources of the short-range stresses, and, 2) The dumping of the thermally-induced spin waves by the magnetic field. In the case of the Fe63Co10Y7B20 alloy, the magnetisation process is connected with both point and linear defects, whereas for the Fe64Co10Y6B20 alloy, only with linear defects. This suggests that the size of the defects, determining the character of the magnetisation in the vicinity of ferromagnetic saturation, depends on the atomic packing density. On the basis of analysis of the magnetisation curves, the spin wave stiffness parameter (Dsp) was calculated.