Spin Wave Stiffness Constant Research Articles

Growth, magnetic, transport and electronic properties of Co2TiSi Heusler alloy thin films

We have grown thin films of Co2TiSi (CTS) Heusler alloy on n-type Si (100) at different substrate temperatures by electron beam evaporator under a high vacuum. The structural, magnetic, and electric characterizations were carried out and found a strong correlation between the properties and the growth temperatures of the samples. The crystallinity and magnetic moment both were significantly improved with increasing growth temperatures from room temperature to 400 °C. The saturation magnetization of 1.85 µB at 10 K is close to the theoretical Slater–Pauling value. A large value of spin wave stiffness constant ( D = 7.2 meV- nm2) of CTS film manifests the robustness of magnetization. The temperature-dependent longitudinal resistivity demonstrated different scattering phenomenon like weak localization, electron-magnon, electron-election, and electron-phonon interactions. A decrease in residual resistivity (109–49 µΩ-cm) and an increase in residual resistivity ratio (1.2–1.39) indicate an enhancement of crystallinity and structural order of CTS films. By first principle study using DFT calculation we have analyzed the effect of atomic site disorder on electronic properties of both bulk and thin film (220) surface form of CTS Heusler alloy. It has been noticed that the ordered bulk structure shows perfect half-metallic nature but for thin film the spin polarization and moment both are deviated from identical value. The spin polarization and magnetic moment both are reduced a lot by introducing atomic site disorder in L21 ordered structure.

Study of structural, electronic, magnetic and vibrational properties of CoFeTiSi quaternary Heusler alloy

We have synthesized CoFeTiSi quaternary Heusler alloy and experimentally measured its structural and magnetic properties. Detailed ab-initio first principle calculations have been performed to investigate the half-metallic behavior of this quaternary Heusler alloy and the effect of disorder on its half-metallicity. The saturation magnetization and Curie temperature are obtained as 1.243 μB/f.u. and 583 K respectively. The spinwave-stiffness constant (D = 3.259 nm2-meV) is obtained from temperature dependence according to Bloch's law. Dynamical stability of the alloy is confirmed from the phonon dispersion curve of the alloy. The first principle calculations show that the alloy is 100% spin-polarized at the Fermi level and the spin-polarization is resistant to the uniform lattice distortion up to a large extent. The effect of anti-site disorder of atoms on the total magnetic moment and the electronic structure is also explored and the moment (and the half-metallicity) is found to be nearly independent (resistant) for the disorders due to Co–Fe and Ti–Si atomic swaps whereas it is affected in case of Co–Si and Fe–Ti atomic swaps. This study reveals that the alloy may be a viable candidate for spintronic applications at room temperature.

Investigation on structural, electronic and magnetic properties of Co2FeGe Heusler alloy: Experiment and theory

Experimental and computational studies are performed on Co2FeGe Heusler alloy. Significantly large saturation magnetization (Ms), Curie temperature (Tc) and spin wave stiffness constant (D) of 6.1 µB/f.u., 1073 K and 10.4 nm2-meV respectively were observed experimentally. The experimental values of Tc and D are reported first time and are among the highest reported values of the same in Heusler alloy domain. The Co2FeGe Heusler alloy strictly follows Slater-Pauling (SP) rule, however, the minor experimental deviation from its SP value is justified by doing full-potential density functional calculations, which gives more accurate result when electron–electron correlation parameter (U) is taken into account with conventional generalized gradient approximation (GGA) scheme. Effect of lattice strain and U on the total and atom-specific magnetic moments and spin-polarization is studied in detail from which we conclude that they have a key role in the deviation of the experimental results from the expected theoretical values. Dynamical stability of the alloy is studied from the phonon dispersion curve and the effect of U is found to be distinct on the phonon density of states. Role of different types of atomic site disorders on the saturation magnetization is also explored computationally and related to the experimental results. The computational results provide in-depth and detailed study followed by experimental validations.

Study of structural, electronic and magnetic properties of Ti doped Co2FeGe Heusler alloy: Co2Fe1−x Ti x Ge (x = 0, 0.5, and 0.75)

Tunability of structural, magnetic and electronic properties of Co2FeGe Heusler alloy is experimentally demonstrated by doping Ti in the Fe site (i.e. Co2Fe1−x Ti x Ge), followed by in-depth first principle calculations. Co2FeGe in its pure phase shows very high saturation magnetization, Curie temperature and spin-wave stiffness constant which were reported in our earlier work. With gradual increase in Ti doping concentration (x = 0.5 and 0.75), the experimental saturation magnetization is found to be decreased to 4.3 μ B/f.u. and 3.1 μ B/f.u. respectively as compared to the parent alloy (x = 0) having the saturation magnetization of 6.1 μ B/f.u. Variation of spinwave stiffness constant is also studied for different x and found to be decreasing from peak value of 10.4 nm2 meV (for x = 0) to the least value of 2.56 nm2 meV for x = 0.5. Justification of the experimental results is given with first principle calculations. Computational phase diagram of the alloys is found in terms of formation energy showing that the doping in Fe site (i.e. Co2Fe1−x Ti x Ge) is more stable rather than in Co site (i.e. Co2−x FeTi x Ge). The change in magnetic moment and half-metallicity with Ti doping concentration is better explained under GGA + U approach as compared to GGA approach signifying that the electron–electron correlation (U) has a distinct role to play in the alloys. Effect of variation of U for Ti atom is studied and optimized with reference to the experimental results. The dynamical stability of the Co2Fe1−x Ti x Ge alloy crystal structure is explained in terms of phonon dispersion relations and the effect of U on the phonon density of states is also explored. Close agreement between the experimental and theoretical results is observed.

Magnetic and Ferromagnetic Resonance Studies in Amorphous Co–Zr Films

The magnetic properties of amorphous Co[Formula: see text]Zrx films prepared by co-evaporation have been studied by means of vibrating sample magnetometry and ferromagnetic resonance. The temperature dependence of spontaneous magnetization [Formula: see text] in the low temperature range can be described using Bloch’s law. In addition, the spin wave stiffness constant, the distance between nearest magnetic atoms and the exchange interaction between cobalt and its nearest neighbors were calculated from the experimental results for [Formula: see text]. Spin-wave resonance modes were obtained and the change of the resonant field as a function of the mode number [Formula: see text] obeys the so-called [Formula: see text] law perpendicular to the film plane at 300[Formula: see text]K.

Effects of Cr substitution on the low temperature magnetization behavior in amorphous Fe68+xCr12-xSi8B12 ribbons

Amorphous Fe68+xCr12-xSi8B12 alloys (x=0, 2, 4, 6, 8 and 10) were prepared using single wheel melt spinning technique. Magnetic measurements were carried out in the temperature range from 5–300 K and at an external applied field of 30 kOe. The Curie temperature was found to increase by substituting Cr by Fe. The magnetic behavior at low temperature was analyzed using spin wave theory. The low temperature magnetization curve was found to obey Bloch's T3/2 law. Some parameters are extracted like the spin wave stiffness constant (D) and mean square range of exchange interaction for the different compositions. The values of spin waves constant (D) determined were found to increase with substitution the Cr by Fe. The values of the mean square range of exchange interaction were found to be characteristic of amorphous ferromagnets. The local random anisotropy constant KL was derived from the approach of saturation in the magnetization. KL increases as Cr concentration decreases.

Anisotropy and temperature dependence of the spin-wave stiffness in Nd2Fe14B : An inelastic neutron scattering investigation

The anisotropy and the temperature dependence of the spin-wave stiffness constants of the permanent magnet ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ are determined in the range 160--440 K using single-crystal inelastic neutron scattering technique. It is found that the deviation from isotropic spin-wave dispersion, between the $a$ and the $c$ axis of the tetragonal structure, is small and eventually the anisotropy decreases when the temperature increases. The softening of the stiffness constants can be described by a linear temperature variation with an average rate of about $\ensuremath{-}0.46$ meV ${\AA{}}^{2}/\mathrm{K}$. These results are compared with the most recent atomistic and multiscale calculations.

Exchange stiffness of ferromagnets

Spin-wave and exchange stiffness constants of 22 ferromagnetic compounds have been deduced from their spontaneous magnetization, $$M_{\mathrm{s}}$$ , by using an improved technique. The improvement consists in utilizing the entire $$M_{\mathrm{s}}(T)$$ curve, up to the Curie point, rather than just its low-temperature part, with $$T\ll T_{\mathrm{C}}$$ . For 17 of the 22 ferromagnets, literature data have been used, while 5 compounds have been studied anew, on single crystals grown for the purpose.

Magnetic interactions and spin-wave stiffness constant of In-substituted yttrium iron garnets

Abstract Indium-substituted yttrium iron garnet samples, Y3Fe5−xInxO12 (x = 0–0.7, step 0.1), were prepared using a citrate solgel method. Synchrotron X-ray diffraction (SXRD) measurements combined with the Rietveld refinement technique were used to investigate the crystallization, structure parameters and lattice distortion in the samples. Magnetization and Curie temperature of the samples were measured with the SQUID and VSM equipments. The non-magnetic indium ions were found to reside at the octahedral sites, leading to an increase of the total magnetization and a decrease in the Curie temperature. Molecular field coefficients Naa, Ndd and Nad were determined by fitting the experimental thermomagnetization curves in the framework of a two-sublattice ferrimagnetic model. The stiffness constant of the spin waves, D, in these samples was calculated based on the exchange interaction constants, Jij, and its temperature dependence was derived for the sublattice magetizations. The calculated results for D(T) are in good agreement with the experimentally measured data and are significantly large around room temperature.

Superspinglass behavior of maghemite nanoparticles dispersed in mesoporous silica

Abstract γ-Fe2O3 nanoparticles were dispersed in a mesoporous silica (SBA15) matrix using the co-precipitation method. While structural properties were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, micro-Raman and transmission electron microscopy, magnetic studies were performed with DC and AC magnetometry. The γ-Fe2O3 nanoparticles have the regular cubic spinel structure. A strong reduction in the saturation magnetization (∼30 emu/gFe, 300 K) was measured and can be mainly attributed to the large fraction (60%) of surface spin disorder. This phenomenon was studied using the modified Kneller’s law and a modified Bloch spin wave model with a surface disorder term. Values of spin wave stiffness constant D of 391 meVA2 and exchange integral Jex equal to 1.12 meV were obtained, suggesting the presence of a block spinel ferrimagnetic state. The Vogel-Fulcher relation was applied to study the nanoparticles magnetic interactions. The values of zv = 5.3 (5), Tf = 184 K and τ 0 = 10 - 12 s suggest a superspinglass behavior. Thermoremanent experiments have also shown that magnetic relaxation occurs even below Tf (180 K) with an exponential decay with scale parameters in the range from 0.56 to 0.6, in agreement with the slowing down law. These facts indicate that the superspinglass effect is mainly caused by the strong surface spin disorder and frustrated collective state related to an interacting system with a broad particle size distribution.

Ni thickness influence on magnetic properties (Co/Ni/Co/Pt) multilayers with perpendicular magnetic anisotropy

We present a study of perpendicular magnetic anisotropy in [Co/Ni(t)/Co/Pt]×8 multilayers for use as free layers in magnetic tunnel junctions (MTJ) and spin valves. The thickness t of the Ni sub-layer was varied and the resulting magnetic properties were investigated. As determined from magnetic force microscopy and magnetometry measurements, all multilayers exhibited a perpendicular magnetic anisotropy with an increase of saturation magnetization with thickness t. From the temperature dependence of the magnetization, well described by a Bloch law, we find that the spin-wave stiffness constant of the [Co/Ni(t)/Co/Pt]×8 multilayers is larger compared to (Co/Ni) multilayers. These multilayers could be the basis for spintronic devices where the reduction of total Pt content could help to reduce the damping constant while keeping the magnetic anisotropy energy relatively high. These are conflicting requirements needed for high performance magnetic memory devices.

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.

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.

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.

First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors

Employing first-principles electronic-structure calculations in conjunction with the frozen-magnon method, we calculate exchange interactions, spin-wave dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin gapless semiconductor (SGS) compounds ${\text{Mn}}_{2}\text{CoAl}, {\text{Ti}}_{2}\text{MnAl}, {\text{Cr}}_{2}\text{ZnSi}, {\text{Ti}}_{2}\text{CoSi}$, and ${\text{Ti}}_{2}\text{VAs}$. We find that their magnetic behavior is similar to the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice exchange interactions play an essential role in the formation of the magnetic ground state and in determining the Curie temperature ${T}_{\mathrm{c}}$. All compounds, except ${\text{Ti}}_{2}\text{CoSi}$, possess a ferrimagnetic ground state. Due to the finite energy gap in one spin channel, the exchange interactions decay sharply with the distance, and hence magnetism of these SGSs can be described considering only nearest- and next-nearest-neighbor exchange interactions. The calculated spin-wave dispersion curves are typical for ferrimagnets and ferromagnets. The spin-wave stiffness constants turn out to be larger than those of the elementary $3d$ ferromagnets. Calculated exchange parameters are used as input to determine the temperature dependence of the magnetization and ${T}_{\mathrm{c}}$ of the SGSs. We find that the ${T}_{\mathrm{c}}$ of all compounds is much above the room temperature. The calculated magnetization curve for ${\text{Mn}}_{2}\text{CoAl}$ as well as the Curie temperature are in very good agreement with available experimental data. This study is expected to pave the way for a deeper understanding of the magnetic properties of the inverse-Heusler-based SGSs and enhance the interest in these materials for application in spintronic and magnetoelectronic devices.

Annealing Effect on the Structural and Magnetic Properties of Granular Cu80Fe10Ni10 Ribbons

We report on the effect of annealing process on the structural and magnetic properties of granular Cu80Fe10Ni10 ribbons prepared by melt spinning technique. Energy-filtered transmission electron microscopy measurements show that the precipitates (Fe10Ni10) are localized at the grain boundaries of the Cu matrix for the non-annealed samples. After an annealing treatment at 600 °C, a strong diffusion of the precipitates within the Cu matrix is observed. The temperature dependence of magnetization has been analyzed using the Bloch law. Spin wave stiffness constant D and the exchange constant A were calculated from the experimental results. Furthermore, we show that the random magnetic anisotropy (RMA) model is applicable to describe the experimental results. Other fundamental parameters have also been calculated using the approach to saturation magnetization. Ab initio calculations based on Korringa-Kohn-Rostocker (KKR) method are carried out in order to characterize both electronic and magnetic properties.

Magnetization dynamics in La0.67Ca0.33MnO3 epitaxial films probed with resonant and non-resonant microwave absorption

Temperature (T) dependent microwave absorption measurements are performed on La0.67Ca0.33MnO3 (LCMO) epitaxial thin films of thickness 100 and 200 nm in an electron paramagnetic resonance spectrometer operating in X-band. The resonant absorption peak is monitored for out-of-plane (H⊥) and in-plane (H∥) dc magnetic field (H) as the system goes through magnetic ordering. These data suggest a resilient transformation to the ferromagnetic (FM) phase in the vicinity of the Curie temperature (TC), indicative of a phase separation, which is dominant in the thinner film. The saturation magnetization is calculated from SQUID magnetometry on the same film. A pronounced zero-field absorption is seen in H∥ geometry displaying anomalous growth in 100 nm film at T < TC. This feature is correlated with the magneto-conductivity of the manganite which is colossal in the vicinity of TC in the well-ordered film of thickness 200 nm. Signature of standing spin wave modes is seen in H⊥ measurements which are analyzed to calculate the spin wave stiffness constant D(T) in the limit of zero temperature. The same is also inferred from the decay of equilibrium magnetization in the framework of Bloch law. These studies reveal that a bulk like LCMO is obtained in the fully relaxed thicker films.

Temperature dependent coercivity and magnetization of light rare-earth Nd doped permalloy thin films

Abstract The temperature dependence of the magnetic properties of Ni80Fe20 thin films with dilute Nd dopants has been studied. The effect of Nd dopants on the hysteresis loops of Nd-doped Ni80Fe20 thin films is small at room temperature but large at low temperature. The measurements show an interesting temperature dependence of the coercivity with different concentrations of Nd impurities, which might be related to the film structure. The temperature dependence of the saturation magnetization for Nd-doped Ni80Fe20 thin films is fitted by a theoretical expression of spin-wave excitation plus Stoner-type-like contribution, in which the spin-wave excitation plays a main role at low temperature. The spin-wave stiffness constant D evaluated from the fitting shows a strong doping concentration dependence.

Room temperature ferromagnetism down to 10 nanometer Ni–Fe–Mo alloy films

Magnetic behavior of a few pulsed laser deposited soft ferromagnetic thin films of Ni–Fe–Mo alloys of different thickness on sapphire single crystals is interpreted on the basis of their structural characteristics. Highly textured thin films have high void density due to island-like growth. X-ray reflectivity (XRR) of the thin films indicate that instead of a uniform density there are effectively three layers with density gradient across the thickness, which is further supported by atomic force microscopy and cross-sectional scanning electron microscopy. Rutherford backscattering spectroscopy and energy dispersive spectrum measurements reveal that the composition in the films is not too far from that of the bulk target with a trend of enhanced Fe yield in the films. The structural disorder strongly affected the magnetic property of the films resulting in much higher values of the Curie temperature TC and coercive field HC than those of the bulk targets. Bifurcations of low-field zero-field-cooled and field-cooled magnetization reflect the disorder-induced anisotropy in the thin films. The spin wave stiffness constants D are higher than their bulk counterparts which are supportive of the enhanced Fe yield in the films. The saturation magnetization, M calculated from measurements in field transverse to the films strongly supports the thickness found from XRR. Finally, even the 10nm thin films have sizable M and HC and TC >300K, making them good candidates for magnetic applications. Overall, the magnetic behavior and the structural characteristics have reasonably complemented each other.