Spin Wave Stiffness Parameter Research Articles

Influence of the Cooling Rate on the Heterogeneity of the Structure in the Rapid Quenched Fe – Based Alloys with the Si Addition

The paper presents the results of research from the analysis of primary magnetization curves for Fe based amorphous alloys. Structural defects in the form of pseudodislocation dipoles occur in amorphous alloys. Using the theory developed by H. Kronmuller called the approach to ferromagnetic saturation, it is possible to indirectly observe internal stresses occurring in the volume of amorphous alloys. The magnetic structure is sensitive to all kinds of inhomogeneities that become visible in the process of high-field magnetization. It has been shown that the cooling rate of the liquid alloy has a great influence on the migration of atoms during the solidification process. Longer time of alloy formation causes more atoms to occupy ordered positions, which results in a change in the distance between the magnetic atoms and a higher degree of structure relaxation. This is indicated by a significant difference in the value of the spin wave stiffness parameter Dspf. The structural differences of the alloys were also investigated using a magnetic balance. It has been shown that the cooling rate influences insignificant differences in the course of thermomagnetic curves and the Curie temperature.

Investigating optically excited terahertz standing spin waves using noncollinear magnetic bilayers

We investigate optically excited terahertz standing spin waves in noncollinear magnetic bilayers. Using femtosecond laser-pulse excitation, a spin current is generated in the first ferromagnetic (FM) layer, and flows through a conductive spacer layer to be injected into the second (transverse) FM layer, where it exerts a spin-transfer torque on the magnetization and excites higher-order standing spin waves. We show that the noncollinear magnetic bilayer is a convenient tool that allows easy excitation of terahertz spin waves, and can be used to investigate the dispersion and thereby the spin-wave stiffness parameter in the thin-film regime. This is experimentally demonstrated using wedge-shaped Co and CoB (absorption) layers. Furthermore, the damping of these terahertz spin waves is investigated, showing a strong increase of the damping with decreasing absorption layer thickness, much stronger than expected from interface spin pumping effects. Additionally, a previously unseen sudden decrease in the damping for the thinnest films is observed. A model for the additional damping contribution incorporating both these observations is proposed.

Influence of the Process for Obtaining Massive Amorphous Materials on the Defects of the Structure in the Alloy Fe61+xCo10-xY8W1B20 , where x = 0 or 1

As part of the work, a rapid cooled alloy was produced with the chemical composition Fe61+xCo10-xY8W1B20, where: x = 0 or 1 using two methods, suction in and injecting the molten alloy into the copper mold. The material was obtained in the form of 10 mm x 5 mm x 0.5 mm plates. Studies on the structure of the obtained alloys were carried out using X-ray diffraction. Obtained X-ray diffraction images are typical for amorphous materials. Using the vibration magnetometer, static magnetic hysteresis loops and primary magnetization curves were measured. The produced samples are characterized by a saturation value above 1T and a coercive field value below 200 A/m. Based on the theory of H. Kronm�ller, numerical analysis of the curves of the original magnetization was carried out. It has been found that the process of magnetizing the produced materials has defects of the amorphous structure in the form of pseudo-location dipoles and free volumes. In addition, the spin wave stiffness parameter was determined.

Microstructure and High-Field Magnetic Properties of Fe-Based Bulk Amorphous Alloys

In this paper the results of the structural and magnetic properties for bulk amorphous alloys Fe65Co10Y5B20 and Fe63Co10Y7B20 were presented. For the structural investigation was performed by X-ray diffractometry and Mossbauer spectroscopy. It was found that investigated samples were amorphous in the as-cast state. The magnetisation was measured within magnetic fields ranging from 0 to 2T using a vibrating sample magnetometer (VSM). On the basis of the hysteresis loops, the saturation magnetization value and the coercive field were determined.The investigation of the �magnetisation in the area close to ferromagnetic saturation� showed that the magnetisation process in strong magnetic fields is connected with point defects and Holstein-Primakoff paraprocess. Analysis of the high-field magnetization curves has facilitated the calculation of the spin wave stiffness parameter.

Proposed Bose–Einstein condensation of magnons in nanostructured films of Gd at low temperature and its manifestations in electrical resistivity and magnetoresistance

In this paper we report the observation of a proposed Bose–Einstein condensation (BEC) of magnons in a temperature range of around 15–20 K in nanostructured films of Gd with grain sizes that are much larger than the size range where superparamagentism is expected. The observation was carried out using magnetic as well as high precision resistivity and magnetoresistance (MR) measurements performed to low temperatures. We observe that the experimental observations depend crucially on one parameter, namely softening of the spin wave stiffness parameter D at BEC and the resistivity as well as MR can be related quantitatively to magnetic measurements through the temperature variation of the constant D in the vicinity of the transition. This paper establishes that the BEC reported before in nanocrystalline Gd can be extended to a somewhat larger size range.

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.

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-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.

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.

Influence of the production method of Fe61Co10Y8W1B20 amorphous alloy on the resulting microstructure and hyperfine field distribution

Mössbauer spectroscopy enables the determination of the influences of quenching rate and production method on the microstructure and magnetic properties of resulting ferromagnetic amorphous materials. On the basis of analysis of the hyperfine field distributions obtained from Mössbauer spectra, it has been ascertained that, in the volume of three samples manufactured by different production methods, differences exist in the local environments of the Fe atoms; further, that these differences contribute to observed variations in the magnetic properties. 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.

Evolution of Magnetotransport Properties and Spin-Glass Behavior of the La0.7-xNdxPb0.3MnO3 System

The effect of neodymium substitution on the lanthanum site in La0.7-xNdxPb0.3MnO3 perovskite polycrystalline has been systematically studied. The substitution of neodymium for lanthanum causes structure change from rhombohedral to orthorhombic. Ferromagnetism is suppressed as neodymium content increases. The magnetic order changes from a ferromagnetic long-range order with x=0.0 to a spin-glass nature with x=0.7. Besides, the ferromagnetic-paramagnetic transition temperature values are lower for the neodymium phases than for the lanthanum ones. Therefore, the introduction of neodymium degrades symmetry of the structure, diminishes the spin-coupling exchange interaction, and results in changes of magnetic properties. This fact is in good agreement with increasing the B value in Bloch's T3/2 law and decreasing the spin-wave stiffness parameter D in quadratic dispersion relation.

Magnetic Characteristics of the System of Exchange Coupled Layers

The interlayer exchange coupling of ferromagnetic transition metal layers through non-magnetic spacer has been intensively studied over the last decade (see, e.g. [1-6]). In comparison, little work was devoted to the magnetic characteristics of such systems. An oscillatory interlayer exchange coupling observed in many multilayered systems can be understood as an interference effect of electron waves partially reflected at each interface with spin dependent reflection coefficients [7]. Therefore, one can expect the magnetic properties, which are related to the interlayer exchange coupling parameter, to oscillate as a function of the spacer thickness. However, relatively few results have been reported concerning experimental investigations of basic magnetic properties of exchange coupled layers. Oscillations of the magnetic anisotropy and the Kerr rotation angle in magnetic films covered by non-magnetic overlayers have been reported [8, 9]. It has been also experimentally found that the Curie temperature, ground state magnetic moment, and spin wave stiffness parameter are the oscillatory functions of the thickness of non-magnetic layer [10]. Moreover, in the recent paper the onset of long-range order in exchange coupled epitaxial monolayers is investigated by temperature dependent susceptibility and element resolved magnetization measurements [11].

Concurrent Nd and Mn spin excitations in Nd0.6Sr0.4MnO3

The spin dynamics of Nd0.6Sr0.4MnO3 have been determined along [qqq], [qq0], and [00q] propagation directions using inelastic neutron scattering. At T=13 K, the low-q Mn–Mn mode energies varied quadratically with wave vector q, but exhibited marked softening on approach to the zone boundaries when compared with a Heisenberg nearest-neighbor exchange calculation. At the larger q’s the modes also show evidence of increasing spin wave energy width. The temperature dependence of the spin wave stiffness parameter D was measured up to Tc=273 K and showed conventional behavior. The 0 K value of D=150 meV Å2 yields D(0)/kTc=6.3. No anomalous E=0 central mode intensity was observed on approach to Tc. At low T additional modes were observed and are ascribed to Nd–Mn exchange interactions. These modes have q=0 spin wave gap of 2.1 meV compared to 1.1 meV for the Mn–Mn excitations.

Long-wavelength spin-wave energies and linewidths of the amorphous Invar alloy Fe100-xBx.

Neutron inelastic scattering experiments have been performed to investigate the long-wavelength spin dynamics of the amorphous isotropic ferromagnet ${\mathrm{Fe}}_{100\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{B}}_{\mathrm{x}}$ (for x=14 and 18). At both iron concentrations this system exhibits Invar behavior. The spin-wave energies are found to be well described by a quadratic dispersion relation E=D(T)${q}^{2}$+\ensuremath{\Delta}, where D(T) is the stiffness parameter and \ensuremath{\Delta} is a small (\ensuremath{\sim}0.05 meV) energy gap originating primarily from dipole interactions. The stiffness parameter D renormalizes with temperature as predicted by the two-magnon interaction theory of the Heisenberg ferromagnet, although the renormalization is more dramatic than expected for short-range interactions. The T=0 stiffness parameters obtained from the neutron scattering measurements are 131 and 122 meV A${\r{}}^{2}$ for two samples of nominal concentration x=14, and 165 meV A${\r{}}^{2}$ for x=18. These values are almost twice as large as those derived from low-temperature magnetization measurements. It has been argued that possible mechanisms that could explain this discrepancy are the existence of additional low-lying magnetic excitations, and/or anomalous magnon linewidths that would contribute to the rapid decrease of the magnetization. Our extensive analysis of the damping of the long-wavelength spin-wave excitations in ${\mathrm{Fe}}_{86}$${\mathrm{B}}_{14}$ has revealed that the temperature and wave-vector dependence of the spin-wave intrinsic linewidths are in good agreement with the predictions of the conventional two-magnon interaction theory of a Heisenberg ferromagnet. We therefore conclude that the discrepancy between the stiffness parameter derived from neutron scattering and magnetization measurements is likely a consequence of additional low-lying magnetic excitations, which contribute to the rather rapid renormalization of the spin-wave stiffness parameter with temperature.

Spin wave stiffness constants in some ferrimagnetics

The magnitude of the spin wave stiffness parameter. D and its dependence on temperature in YIG, Fe3O4 and Li-Zn ferrite has been examined and compared with theory. It is shown that values of D obtained using the exchange constants that satisfactorily explain the equilibrium magnetic properties of these systems yield results in agreement with experiment for YIG and Fe3O4 but are inappropriate for the Li-Zn ferrite. The temperature dependence of D in YIG and Fe3O4 is satisfactorily explained on the basis of the interaction of acoustic magnons with optic magnons.

Brillouin light scattering on cobalt-chromium films

Brillouin light scattering has been used to observe high frequency thermal magnetic excitations in ion-beam sputtered Co-Cr thin films. The measurements were performed on 82 at. % Co samples, 100 Å, 350 Å and 750 Å in thickness, sputtered on single crystal Si substrates. The Brillouin scattering signals from these Co-Cr films were quite strong. All spectra show scattering due to surface magnons propagating in the film plane. Scattering due to bulk spin waves was found for the two thicker films. From the measured magnon dispersion for the surface mode, Damon-Eshbach theory was used to calculate the magnetization, the perpendicular anisotropy field and the gyromagnetic ratio for the films. The spin wave stiffness parameter was determined from the bulk magnon dispersion. These magnetic parameters are in reasonable agreement with results from ferromagnetic resonance.

Brilliouin light scattering determination of the spin-wave stiffness parameter in lithium-zinc ferrite

The spin-wave stiffness parameter for Zn substituted Li-ferrite has been measured by the technique of Brillouin light scattering. A range of compositions from pure Li-ferrite through Li-Zn ferrite with 0.65 Zn at./formula unit was studied. The magnon frequency vs. field determinations from light scattering and separate magnetization data were used to determine the spin-wave stiffness parameter, D. The data give a D-value of 10.1 × 10-9Oe.cm2for pure lithium ferrite, approximately twice as large as for yttrium iron garnet. A rapid decrease occurs with Zn substitution and D is essentially zero for about 0.4 Zn at./formula unit. These are the first reported results of Brillouin scattering from a polycrystalline ferrite. A molecular field calculation of the spin-wave stiffness yields a much slower decrease in D with zinc than is found experimentally.

Ferromagnetic and spin glass behavior near the critical concentration in amorphous (FexNi100−x)75Gl25

The magnetic properties of amorphous (FexNi100−x)75P16B6Al3 have been investigated above the critical concentration (x = 17) for long range order via neutron scattering. For x = 30 the spin wave stiffness parameter D first increases with decreasing temperature, but then decreases at lower temperatures as the spin glass state is approached. Accompanied by the decrease in D is an increase in the spin wave linewidths.

Spin Waves in Amorphous Ferromagnets

The spin wave spectrum of dense random packing amorphous ferromagnets is discussed. The genetal formula of spin wave stiffness parameter which appears in the long wavelength dispersion relation is given. An explanation for the roton-like minimum observed in Mook's neutron diffraction measurement is also given within the quasicrystalline approximation.