Spin Wave Stiffness Constant Research Articles

Magnetization behavior and magnetoimpedance effect in melt-spun Fe 75− xCo xZr 8Nb 2B 15 ( x=5, 10, 15) alloys

Soft magnetic properties of melt-spuned (Fe–Co)–Zr alloy system can be achieved upon suitable partial nanocrystallization by proper thermal treatment. In this study, we have measured the spin-wave stiffness constant, magnetoimpedance and incremental permeability ratio (PR) of Fe 75− x Co x Zr 8Nb 2B 15 ( x=5, 10, 15) alloys to investigate the influence of the structural changes in the nanocrystallization process as well as the changes of the magnetic properties as a function of annealing temperatures. With increasing Co content, the spin wave stiffness constant and Curie temperature increase. The full-width at half-maximum in the longitudinal PR curves do not change very much for the samples annealed at various temperatures regardless of the presence of magnetic field during the heat treatment. The broadening MI ratio curves along with increment of frequency may arise from the circumferential permeability, which is somewhat different from the longitudinal permeability.

Variation of Magnetism and Structure on Mechanically Alloyed Co?C Powder

The variation of magnetic properties during mechanical alloying was studied in Co 100-x C x (x = 20, 50, 80) granular alloys. The structural evolution during mechanical alloying was complemented by X-ray diffractometry. With increasing processing time, X-ray diffraction peaks from Co and C phases became weaker and finally disappeared. The grain size was estimated to be about 20 nm from the broadening of the XRD peaks. The saturation of magnetization at a steady state value was another evidence for the formation of a saturated solid solution in the mechanically alloyed sample. During mechanical alloying, the exchange interaction among the Co atoms becomes weaker due to inter-diffusion of Co and C atoms. Both the Curie temperature, T C , and the spin wave stiffness constant, D, which were estimated from the temperature dependence of magnetization, decrease with increasing C content. This tendency indicates the isolation of Co clusters from the C matrix.

Magnetic properties of amorphousFe90−xMnxZr10(0<~x<~12)alloys

The magnetization as a function of field and temperature has been measured for a series of amorphous Fe 9 0 - x Mn x Zr 1 0 alloys with x=0-12 in the temperature range 4.2-300 K. All the samples of the present study show double transition (reentrant) behavior below room temperature. The high-temperature transition (T c ) decreases linearly at about 6 K/at. % of Mn, while the low-temperature transition (T s g ) increases at about 2.6 K/at. % of Mn. A detailed analysis of the temperature dependence of magnetization data reveals: (i) Spin-wave excitations at low-temperature, single-particle excitations and local-spin-density fluctuations (LSDF's) over a wide range of intermediate temperatures and enhanced fluctuations in the local magnetization for temperature close to T c contribute dominantly to the thermal demagnetization of spontaneous magnetizations; (ii) external applied magnetic field of strength ≥5 kOe suppresses the LSDF's; (iii) the spin-wave stiffness constant (D) decreases from 35.6′0.3 to 23.2′0.2 meV A 2 with increasing Mn concentration; and (iv) the D/T c ratio remains constant for all the compositions. A study of critical behavior of the magnetic order-disorder transition by various methods suggest that the critical exponents obtained below and above Curie temperature obey a scaling law [δ-1=γ/β and α+γ=2(1-β)] with a high degree of accuracy in the asymptotic critical region. The exponents are independent of composition and are in close agreement with the values those predicted for three-dimensional Heisenberg ferromagnets. The magnetic parameters such as high-field susceptibility, coercivity, local magnetic anisotropy, and spin-glass behavior, obtained from the low-temperature magnetization data, are consistent with the presence of a mixed magnetic state. The detailed analysis of high-field thermomagnetization data could be explained in terms of the nearest-neighbor Heisenberg model. Moreover, the temperature dependence of the magnetic behavior is discussed in terms of competing ferromagnetic and antiferromagnetic exchange interactions.

On the Role of Aluminum in Finemet

The role of aluminum with respect of its influence on some intrinsic magnetic properties of the nanocrystalline Finemet, such as saturation magnetization, Curie temperature, spin-wave stiffness constant and hyperfine magnetic fields was investigated. The strengthening effect of Al on the exchange interaction was observed for the alloys with small Al content (up to 3 at.%). Higher concentration of Al (5-7 at.%) led to considerably suppressed exchange interaction.

Exchange interactions and Curie temperatures of 3D- and 2D-ferromagnets

Effective exchange interactions in bulk ferromagnets as well as in magnetic overlayers on Cu(001) covered by a Cu-cap layer of varying thickness were determined from first principles by mapping of corresponding total energies onto the effective Heisenberg model in the framework of the adiabatic approximation and magnetic force theorem. The effective Heisenberg model is then used to determine spin-wave stiffness constants and Curie temperatures evaluated in the framework of the random-phase approximation. Calculations are in a fair agreement with available experimental data for bulk ferromagnets and reproduce an oscillatory Curie temperature of magnetic overlayers as a function of Cu-cap thickness in a qualitative agreement with recent experiments.

Random anisotropy model approach on ion beam sputtered Co 20Cu 80 granular alloy

The Co 20Cu 80 granular film has been elaborated using ion beam sputtering technique. The magnetic properties of the sample were studied in the temperature range 5–300 K at H⩽50 kOe. From the thermomagnetisation curve, which is found to obey to the Bloch law, we have extracted the spin wave stiffness constant D and the exchange constant A . The magnetic experimental results have been interpreted in the framework of random anisotropy model. We have determined the local anisotropy constant K L and the local correlation length of anisotropy axis R a, which is compared to the experimental grains size obtained by transmission electronic microscopy.

Magnetic study in amorphous Fe67Y33 alloy

An amorphous Fe67Y33 alloy has been prepared by the melt-spinning method and its magnetic properties have been studied. The thermomagnetization curve is found to obey the Bloch law; spin wave stiffness constant D and the exchange constant A were calculated from the experimental results. From an analysis of the approach to saturation magnetization, the local random anisotropy constant KL has been extracted. A and KL are found to decrease with increasing temperature. Experimental data show that the ferromagnetic correlation Rf increases with increasing temperature.

Oscillatory Curie temperature of 2D-ferromagnets

The effective exchange interactions of the magnetic overlayer Fe/Cu(0 0 1) covered by a Cu-cap layer of varying thickness were calculated in real space from first principles. The effective two-dimensional Heisenberg Hamiltonian was constructed and used to estimate spin-wave stiffness constants and overlayer Curie temperatures within the random-phase approximation. Oscillatory behavior of the overlayer Curie temperature, spin-wave stiffness, and magnetic moment as a function of the cap-layer thickness was found and explained.

Ab initiocalculations of exchange interactions, spin-wave stiffness constants, and Curie temperatures of Fe, Co, and Ni

We have calculated Heisenberg exchange parameters for bcc-Fe, fcc-Co, and fcc-Ni using the scalar-relativistic spin-polarized Green function technique within the tight-binding linear muffin-tin orbital method and by employing the magnetic force theorem to calculate total energy changes associated with a local rotation of magnetization directions. We have also determined spin-wave stiffness constants and found the dispersion curves for metals in question employing the Fourier transform of calculated Heisenberg exchange parameters. Curie temperatures were calculated both in the mean-field approximation and within the Green function random phase approximation. The latter results were found to be in a better agreement with available experimental data.

Analysis of the small-angle neutron scattering of nanocrystalline ferromagnets using a micromagnetics model

In ferromagnets with a nonuniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline (nc-) or cold-worked (cw-) polycrystalline materials, the static magnetic microstructure gives rise to strong elastic magnetic small-angle neutron scattering (SANS). The paper explores a method for analyzing field-dependent SANS data from such materials in terms of a model based on the theory of micromagnetics. Samples of cw Ni and of electrodeposited nc Ni and nc Co were characterized by x-ray scattering and magnetometry, and were investigated by SANS both with and without polarization of the neutron beam. The variation of the differential scattering cross section with the scattering vector and with the applied magnetic field is well described by the model. Also, experimental results for the exchange stiffness constant A and for the spin-wave stiffness constant D obtained from the analysis are found to agree with literature data obtained by inelastic neutron scattering on single-crystal specimens. The model supplies an ``anisotropy field scattering function'' that contains information on the magnitude of the magnetic anisotropy in the material, and on the characteristic length scales on which the anisotropy changes direction. The results suggest that the anisotropy may be strongly nonuniform in each crystallite, possibly due to twinning, and that some magnetic moments in the Ni samples are strongly pinned at defects.

Low-temperature specific heat of La 0.78Pb 0.22MnO 3−δ manganite

The specific heat of the perovskite manganite La 0.78Pb 0.22MnO 3−δ was measured from 0.5 to 25 K. The results yield values of the electronic density of states, the Debye temperature, and the spin-wave stiffness constant of the compound. The magnetic contribution was found to be consistent with the existence of a gap in the spin-wave spectrum, and a phonon term in T 5 was needed to fit the data.

Oscillatory curie temperature of two-dimensional ferromagnets.

The effective exchange interactions of magnetic overlayers Fe/Cu(001) and Co/Cu(001) covered by a Cu-cap layer of varying thickness were calculated in real space from first principles. The effective two-dimensional Heisenberg Hamiltonian was constructed and used to estimate magnon dispersion laws, spin-wave stiffness constants, and overlayer Curie temperatures within the mean-field and random-phase approximations. Overlayer Curie temperature oscillates as a function of the cap-layer thickness in a qualitative agreement with a recent experiment.

Measuring the exchange-stiffness constant of nanocrystalline solids by elastic small-angle neutron scattering

In ferromagnets with a non-uniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline or cold-worked polycrystalline materials, the static magnetic microstructure gives rise to elastic magnetic small-angle neutron scattering (SANS). The paper explores a method for determining the exchange-stiffness constant A by analysis of the dependence of the elastic SANS cross-section on the applied magnetic field. Experimental results for A and for the spin-wave stiffness constant D in cold-worked or nanocrystalline Ni and Co are found to agree with literature data obtained by inelastic neutron scattering on single-crystal specimens.

Brillouin light scattering from spin waves in Co 100− xCr x alloy films

Collective spin waves in Co 100− x Cr x alloy films ( x=0, 9.2, 15.7 and 35.2 in at%) of 300±5 Å in thickness have been studied by means of Brillouin light scattering (BLS) at room temperature in magnetic fields of up to 4.0 kOe. Except for the Co 64.8Cr 35.2 film, we observed scattering from spin waves and determined the spin wave stiffness constant D B. We obtained a value of D B=(3.47±0.35)×10 −9 Oe cm 2 (436±44 meV Å 2) for pure Co. Although this value is in good agreement with recent BLS values, it is rather smaller than the value of 510 meV Å 2 obtained from neutron scattering. For the alloys, the D B value at x=15.7 is strongly reduced to ∼1/4 of the value at x=9.2, while the saturation magnetization decreases almost linearly as the Cr content increases. We discuss this D B behavior in terms of the Cr segregation which takes a place for Cr contents above x∼12.

A correlation between the spin wave stiffness constant and the cation size disorder in the perovskite manganites

Abstract We investigated the temperature dependence of magnetization in the ferromagnetic phase of La0.67−2xNd2xCa0.33−xSrxMnO3 (x=0, 0.1, 0.02 and 0.33) manganites in terms of the spin wave theory. A relation between the spin wave stiffness constant (D) and the cation size mismatch was obtained. By means of this relation we could reproduce the values of D determined by different techniques reported in the literature for some samples. These findings may have important implications on the spin dynamics and the metallic electrical resistivity at low temperatures.

Low temperature properties of nanocrystalline Fe 73.5Cu 1Nb 3Si 13.5B 9 ribbons

Many investigations on nanocrystalline samples were performed between room temperature and elevated temperatures but there is still a lack of low-temperature studies especially that of coercivity and magnetostriction. In this work we present measurements of the low-temperature behavior of magnetic properties like coercivity, magnetization and magnetostriction of Fe 73.5Cu 1Nb 3Si 13.5B 9 samples at different stages of crystallization and also in comparison to the amorphous state. The temperature dependence of the magnetostriction was measured by the small-angle magnetization rotation method which exhibits a high sensitivity. The magnetization data of the samples were used for analyzing the temperature dependence of magnetostriction in the low-temperature range in order to obtain information about the physical mechanism of the magnetostriction for such a kind of samples. From the temperature dependence of the magnetization, which follows the Bloch T 3/2 law, the spin wave stiffness constant can be estimated. The temperature dependence of coercivity H c below room temperature is also discussed.

Small-angle neutron scattering by the magnetic microstructure of nanocrystalline ferromagnets near saturation

The paper presents a theoretical analysis of elastic magnetic small-angle neutron scattering (SANS) due to the nonuniform magnetic microstructure in nanocrystalline ferromagnets. The reaction of the magnetization to the magnetocrystalline and magnetoelastic anisotropy fields is derived using the theory of micromagnetics. In the limit where the scattering volume is a single magnetic domain, and the magnetization is nearly aligned with the direction of the magnetic field, closed form solutions are given for the differential scattering cross-section as a function of the scattering vector and of the magnetic field. These expressions involve an anisotropy field scattering function, that depends only on the Fourier components of the anisotropy field microstructure, not on the applied field, and a micromagnetic response function for SANS, that can be computed from tabulated values of the materials parameters saturation magnetization and exchange stiffness constant or spin wave stiffness constant. Based on these results, it is suggested that the anisotropy field scattering function SH can be extracted from experimental SANS data. A sum rule for SH suggests measurement of the volumetric mean square anisotropy field. When magnetocrystalline anisotropy is dominant, then a mean grain size or the grain size distribution may be determined by analysis of SH.

MAGNETIC PROPERTIES AND MAGNETOIMPEDANCE EFFECT IN TM70Cr5Si10Bl5 (TM = Fe, Co, Ni) ALLOYS

Amorphous TM70Cr5Sil0B15 (TM = Fe, Co, Ni) alloys were prepared by rapid quenching technique. Saturation magnetization of the amorphous ribbons was investigated by using SQUID and vibrating sample magnetometer from 5 to 800 K. For these two ferromagnetic alloys, the saturation magnetization up to 0.4 Tc can be described by the Bloch relation, Ms(T) = Ms(0)[1 - BT3/2 - CT5/2]. The spin wave stiffness constants and the range of exchange interaction were analyzed from the magnetization behaviour. And the temperature dependence of the magnetoimpedance effect was investigated in conjecture with their magnetic properties from 10 to 300 K.

Low temperature magnetostriction of amorphous Fe85B15 ribbons

In this work we present measurements of the low temperature behavior of the magnetostriction of amorphous Fe 85 B 15 samples that were produced by planar flow casting. For these low temperature measurements we developed a new system in order to characterize amorphous or nanocrystalline ribbons in a temperature range between 2 K and 300 K. With the system the low temperature dependence of magnetostriction can be measured by the SAMR-(Small Angle Magnetization Rotation) method which exhibits a high sensitivity. Additionally magnetic measurements like hysteresis loop, initial susceptibility and pinning field were performed with the same equipment simultaneously on the same piece of ribbon. The magnetostriction and the magnetization data of the Fe 85 B 15 sample were used for analyzing the temperature dependence of magnetostriction in the low temperature range. From scaling laws between the magnetostriction and the saturation magnetization information about the physical mechanism of magnetostriction can be obtained. The temperature dependence of coercivity H c will also be analyzed by various models. From the temperature dependence of the magnetization, which follows the Bloch T 3/2 law, the spin wave stiffness constant can be estimated.

Structure, crystallization and magnetic behaviour of Fe-RE-Cu-Si-B alloys (RE = Pr, Nd, Gd)

Nb was replaced by RE atoms (Gd. Pr. Nd) in classic FINEMET and the influence of this substitution on the structure, crystallization and magnetic behaviour was investigated. Thermomagnetic measurements, TEM and XRD observations have shown that Gd and Pr are suitable for grain growth suppressing during crystallization and take part in formation of nanocrystalline structure. The crystallization kinetics of the Nd containing sample is entirely different. Coercivity reflects the structural changes of the sample during annealing. Low-temperature magnetization measurements enable the study of annealing behaviour of the magnetic moment μ Fe+RE and of the spin-wave stiffness constant D SP . The results suggest that the light rare-earth Pr and Nd atoms couple ferromagnetically, while the heavy rare-earth Gd atoms couple ferrimagnetically to Fe atoms. The influence of RE atoms on Curie temperature was also observed.