Temperature dependence of the coercivity in Nd60Fe30−xCoxAl10, x=0 or 10, and Pr58Fe24Al18 bulk amorphous ferromagnets

Bulk amorphous ferromagnet alloys of composition Nd₆₀Fe₃₀Al₁₀, Nd₆₀Fe₂₀Co₁₀Al₁₀ and Pr₅₈Fe₂₄Al₁₈ have been prepared by argon arc melting and quenching into a copper mould. General insight into the magnetic behaviour of the alloys was gained from measurement of the major hysteresis loop at room temperature, and from zero-field cooled and field-cooled magnetisation measurements in the range 10-400 K. Measurements of the coercivity were made from 10 to 400 K, and for all alloys, the coercivity is seen to increase steeply with decreasing temperature to a peak at a temperature in the range 25-50 K, before decreasing. For all alloys, the temperature dependence of the coercivity between 50 and 400 K is well explained by the strong pinning model of domain walls of Gaunt [Philos. Mag. B 48 (1983) 261]. Quantities deduced from the Gaunt model, along with other relevant magnetic parameters, are used to estimate values for the exchange and anisotropy constants.     

Structure,magnetic properties and magnetostriction of laves compounds Nd1−xPrx(Fe0.35Co0.55B0.1)2

The C15 Laves phases with composition Nd_1−xPr_x(Fe_0.35Co_0.55B_0.1)₂ (0?x?1) have been synthesized by arc melting and subsequent annealing. The Curie temperature T_c and the saturation magnetizations M_s at 5 and 295 K decrease with increasing Pr content. The linear anisotropic magnetostriction λ_a=λ_∥−λ_⊥ at room temperature for Nd_1−xPr_x(Fe_0.35Co_0.55B_0.1)₂ alloys with 0?x?0.4 initially reaches a negative minimum, then increases and changes its sign with increasing magnetic field H, and the λ_a for the alloys with x?0.6 is positive and increases as magnetic field H increases.     

Surface demagnetizing field as a source of uniaxial surface anisotropy in GdCoMo thin amorphous films

Surface magnetic anisotropy energy was studied for (Gd_0.26Co_0.74)_0.96Mo_0.04 and (Gd_0.29Co_0.71)_0.96Mo_0.04 thin amorphous films by means of microwave spectroscopy at the X-band within the temperature range 4–295 K. Excitations of surface spin waves were observed in the spin wave resonance spectra. The experiment was performed in a rotating external magnetic field. The angular dependence of the resonance field for the uniform mode (spin wave vector k=0) and the surface mode made it possible to determine the surface uniaxial anisotropy constant K_s and its temperature dependence. An inhomogeneity of the saturation magnetization M_s within a close-to-surface layer of thickness d can generate the surface anisotropy energy with anisotropy constant K_s given by the formula: K_s=4πM^b_s (M^b_s−M^surf_s)d, where the indexes b and surf correspond to the bulk and surface values, respectively. The temperature dependence of K_s calculated by means of the formula agrees qualitatively with temperature dependence of K_s found in the experiment.     

Anisotropy and FOMP in Tb3 (Fe28−xCox) V1.0 (x=0, 3 and 6) compounds

In this work, the structural and magnetic properties of Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) compounds have been investigated. The structural characterization of compounds by X-ray powder diffraction is an evidence for a monoclinic Nd₃(Fe, Ti)₂₉-type structure (A2/m space group). The refined lattice parameters a and b (but not c) and the unit cell volume V, obtained from the XRD data by the Rietveld method, are found to decrease with increasing Co concentration. The unit cell parameters behavior has been attributed to the smaller Co atoms and a preferential substitution of Fe by Co. The anisotropy field (H_a) as well as critical field (H_cr) was measured using the singular point detection (SPD) technique from 5 to 300 K in a pulsed magnetic field of up to 30 T. At T=5 K, a FOMP of type 2 was observed for all samples and persists at all temperatures up to 300 K. For sample x=0, H_cr=10.6 and 2.0 T at 5 and 300 K, respectively, is equal to that reported earlier. The occurrence of canting angles between the magnetic sublattices during the magnetization process instead of high-order anisotropy contributions (at room temperature are usually negligible) has been considered to explain the survival of the FOMP at room temperature. The anisotropy and critical fields behave differently for samples with x=0, 3 compared with x=6. The observed behavior has been related to the fact that the Co substitution for Fe takes place with a preferential entrance in the inequivalent crystallographic sites of the 3:29 structure. The contribution of the Tb-sublattice in the Tb₃(Fe, V)₂₉ compound with uniaxial anisotropy has been scaled from the anisotropy field measured on a Y₃(Fe, V)₂₉ single crystal with easy plane anisotropy.     

A study on the spectral,microstructural, and magnetic properties of Eu–Nd double-substituted Ba0.5Sr0.5Fe12O19 hexaferrites synthesized by an ultrasonic-assisted approach

In this study, an examination on the spectral, microstructural, and magnetic characteristics of Eu–Nd double-substituted Ba_0.5Sr_0.5Fe₁₂O₁₉ hexaferrites (Ba_0.5Sr_0.5Nd_xEu_xFe_12−2xO₁₉ (x = 0.00–0.05) HFs) fabricated by an ultrasonic-assisted approach has been presented. An UZ SONOPULS HD 2070 ultrasonic homogenizer with frequency of 20 kHz and power of 70 W was used. The chemical bonding, structure and the morphology of the products were evaluated by Fourier-Transform Infrared (FT-IR) Spectroscopy, XRD (X-ray diffraction), scanning and transmission electron microscopy and techniques. The textural properties of the prepared nanomaterials were examined by using the Brunauer-Emmett-Teller (BET) method. The magnetic properties were studied using a vibrating sample magnetometer (VSM) at room temperature (RT) and low temperature 10 K. The magnitudes of various magnetic parameters including M_s (saturation magnetization), M_r (remanence) and H_c (coercivity) were estimated and evaluated. The M-H loops revealed the hard ferrimagnetic nature for all products at both temperatures. The M_s and M_r values showed a decreasing tendency with increasing degree of Eu³⁺ and Nd³⁺ substitutions whereas H_c values displayed an increasing trend. At RT, M_s, M_r and H_c values lie in the ranges of 63.0–68.8 emu·g⁻¹, 24.6–39.2 emu·g⁻¹ and 2252.4–2782.1 Oe, respectively. At 10 K, the values of M_s, M_r and H_c lie between 87.5–97.1 emu·g⁻¹, 33.5–40.1 emu·g⁻¹ and 2060.6–2417.2 Oe, respectively. The observed magnetic properties make the prepared products promising candidates to be applied in the recording media.     

Inducing anisotropy in bulk Nd-Fe-Co-Al-B nanocrystalline alloys by quenching in magnetic field

We have observed magnetic anisotropy in bulk Nd_55−xCo_xFe₃₀Al₁₀B₅ (x=10, 15 and 20) alloys prepared by copper mold suction casting method with a presence of external magnetic field (quenching field) μ₀H=0.25 T. By changing direction of the measuring field from perpendicular to parallel one in comparison with that of the quenching field, coercive force of the alloys slightly decreases while remanent magnetization and squareness of hysteresis loop increase more clearly. It is also found that the higher Co-concentration in the alloys the larger magnetic anisotropy is induced. The structure analyses manifest nanocrystalline particles embedded in residual amorphous matrix of the alloys. The size of the particles is in range of 10-30 nm and their crystalline phases consist of Nd₂(Fe,Co)₁₄B, Nd₃Co, Nd₃Al, NdAl₂ and Nd.     

Study on magnetic properties of nanocrystalline La-, Nd-, or Gd-substituted Ni–Mn ferrite at low temperatures

The effects of rare-earth ions with different radii and magnetic moments on the magnetic properties of Ni–Mn ferrite are investigated. X-ray diffraction pattern has shown the presence of cubic structure of spinel ferrite for all samples. The values of M_s and H_c are decreasing with increasing of testing temperatures for all samples. The H_c value of Ni_0.7Mn_0.3La_0.1Fe_1.9O₄ reaches 1082 Oe at 2 K. Mössbauer spectra tested at 273 K indicate the presence of superparamagnetism for samples calcined at 873 K.     

Effect of Mn on the magnetic properties of Fe3B/Nd2Fe14B nanocomposites

The magnetic properties of Nd_4.5Fe_77−xMn_xB_18.5 (x=0, 1 and 2) nanocomposites prepared by the crystallization of amorphous precursors were investigated. Addition of Mn is found to decrease the crystallization temperature of the amorphous ribbons. The intrinsic coercivity _iH_c and maximum energy product (BH)_max increase from 2.6 kOe and 9.1 MGOe for x=0 to 3.1 kOe and 10.3 MGOe for x=1, respectively, and the remanence ratio M_r/M_s increases from 0.70 to 0.72. The effect of Mn on Curie temperature T_C and the thermal stability of M_r and _iH_c were also studied. ⁵⁷Fe Mössbauer spectra have been recorded for x=0, 1 and 2 ribbons at room temperature and site preference of the Mn atoms in Fe₃B and Nd₂Fe₁₄B phases is discussed using the Mössbauer spectroscopy.     

Study of the field dependence of the magnetocaloric effect in Nd1.25Fe11Ti: A multiphase magnetic system

The field dependence of magnetic entropy change ΔS_M(T,H) has been studied in the crystalline sample Nd_1.25Fe₁₁Ti, a multiphase system constituted by three phases: Fe₁₇Nd₂, Fe₇Nd and Fe₁₁TiNd. The magnetic entropy change has been calculated from the numerical derivative of magnetization curves M(T,H) with respect to temperature and subsequent integration in field. To determine the field dependence of the experimental ΔS_M, a local exponent n(T,H) can be calculated from the logarithmic derivative of the magnetic entropy change vs. field. In contrast with the results for single phase materials, where n at the Curie temperature T_C is field independent, it is shown that for a multiphase system n evolves with field both at the Curie temperature of the system and the Curie temperatures of the constituent phases. This is in agreement with numerical simulations using the Arrott-Noakes equation of state.     

Thermomagnetic transitions and coercivity mechanism in bulk composite Nd60Fe30Al10 alloys

The thermomagnetic behaviour (within the temperature range 553-300 K) for the bulk composite Nd₆₀Fe₃₀Al₁₀ alloy is described in terms of a transition from paramagnetic to superferromagnetic state at T=553 K, followed by a ferromagnetic ordering for T<473 K. For the superferromagnetic regime, the alloy thermomagnetic response was associated to a homogeneous distribution of magnetic clusters with mean magnetic moment and size of 1072 μ_B and 2.5 nm, respectively. For T<473 K, a pinning model of domain walls described properly the alloy coercivity dependence with temperature, from which the domain wall width and the magnetic anisotropy constant were estimated as being of ≈8 nm and ≈10⁵ J/m³, typical values of hard magnetic phases. Results are supported by microstructural and magnetic domain observations.     

Nd1.9M0.1Fe12Co2B,M = Ti or Hf as a material for permanent magnets

Magnetic properties (saturation magnetizations, anisotropy fields and Curie temperatures) of the alloys of the composition Nd_1.9M_0.1Fe₁₂Co₂B, with M = Ti and Hf are presented. All of the compounds crystallize in the tetragonal Nd₂Fe₁₄B structure. With substitution of Nd by Ti or Hf, the saturation moment and Curie temperature decrease, but the anisotropy fields, H_A, are found to increase significantly. Theoretical energy products are reduced from 65 to 63 MGOe when Nd is replaced by Ti or Hf to the extent of x = 0.1.     

Micromagnetic studies for bit patterned media above 2 Tbit/in.

Bit patterned media (BPM) recording is a candidate for extremely high density magnetic recording. A micromagnetic model is built up to analyze the phase diagram of the correct-write-in condition in BPM above 2 Tb/in.² fabricated by lithography or ion irradiation methods. The target of the study is to acquire the relationship between the recording performance and the magnetic properties of the media. The medium includes the polycrystalline grains and grain boundary. In BPM fabricated by lithography with FCT structure, two phase diagrams of the correct-write-in condition are found for the anisotropy angular distribution Δθ, the ratio of tetragonal anisotropy K₂₂ to uniaxial anisotropy K₁ and the uniaxial anisotropy distribution ΔK₁. In BPM fabricated by ion irradiation methods, two phase diagrams of the correct-write-in condition are analyzed for the ratio of saturation magnetization M′_s/M_s, anisotropy field H′_k/H_k and the exchange field H′_ex/H_ex in the ion irradiated region and the bit islands.     

Magnetic Properties of Nanoparticles Useful for SQUID Relaxometry in Biomedical Applications

We use dynamic susceptometry measurements to extract semiempirical temperature-dependent, 255-400 K, magnetic parameters that determine the behavior of single-core nanoparticles useful for SQUID relaxometry in biomedical applications. Volume susceptibility measurements were made in 5 K degree steps at nine frequencies in the 0.1-1000 Hz range, with a 0.2 mT amplitude probe field. The saturation magnetization (M_s) and anisotropy energy density (K) derived from the fitting of theoretical susceptibility to the measurements both increase with decreasing temperature; good agreement between the parameter values derived separately from the real and imaginary components is obtained. Characterization of the Néel relaxation time indicates that the conventional prefactor, 0.1 ns, is an upper limit, strongly correlated with the anisotropy energy density. This prefactor decreases substantially for lower temperatures as K increases. We find, using the values of the parameters determined from the real part of the susceptibility measurements at 300 K, that SQUID relaxometry measurements of relaxation and excitation curves on the same sample are well described.     

Soft magnetic properties of FeCo films with Co underlayer

Bilayered Fe₆₅Co₃₅ (=FeCo)/Co films were prepared by facing targets sputtering with 4πM_s∼24 kg. The soft magnetic properties of FeCo films were induced by a Co underlayer. H_c decreased rapidly when the Co underlayer was 2 nm or more. The films showed well-defined in-plane uniaxial anisotropy with the typical values of H_ce=10 Oe and H_ch=3 Oe, respectively. High frequency characteristics of the films show the films can work at 0.8 GHz with real permeability as high as 250.     

Prediction of a domain-drag effect in uniaxial,non-compensated,ferromagnetic metals

An electric current is predicted to exert a dragging force on ferromagnetic domain walls. The effect arises from the non-uniform current distribution, recently predicted and observed at 4·2 K in the neighborhood of a wall in metals (Co, Ni) with large Hall angle. When the average current density j? exceeds a certain “coercive” value j?_c, the domains are set into motion in the same direction as the charge carriers. When j?j?_c, the wall speed approaches the drift speed of the carriers. Ohm's law fails when the walls move. The limits of very small and of very large Hall angle are both considered, using the single-wall model of Williams, Shockley and Kittel, and also the stratified-medium model of Herring. We assume B = M_s inside each domain; this is appropriate if the sample thickness t in the direction of the saturation magnetization M_s, and the wall spacing a, obey t > a. Materials with large anisotropy field H_A (h.c.p. Co, Gd) are necessary if the field H of the current is not to remove all walls normal to the current. For similar reasons, the sample should be in the shape of a flat ribbon (or rod) normal to M_s, of thickness (or diameter) t obeying t < dH_A/H_c, where H_c is the coercive field and d the wall length in the direction normal to the current and to M_s.     

Current induced domain wall motion in nanostripes with perpendicular magnetic anisotropy

We report micromagnetic modeling results of current induced domain wall (DW) motion in magnetic devices with perpendicular magnetic anisotropy by solving the Landau-Lifschitz-Gilbert equation including adiabatic and non-adiabatic terms. A nanostripe model system with dimensions of 500 nm (L)×25 nm (W)×5 nm (H) was selected for calculating the DW motion and its width, as a function of various parameters such as non-adiabatic contribution, anisotropy constant (K_u), saturation magnetization (M_s), and temperature (T). The DW velocity was found to increase when the values of K_u and T were increased and the M_s value decreased. In addition, a reduction of the domain wall width could be achieved by increasing K_u and lowering M_s values regardless of the non-adiabatic constant value.     

Very high field magnetization and AC susceptibility of native horse spleen ferritin

The magnetization of native horse spleen ferritin protein is measured in pulsed magnetic fields to 55 T at T=1.52 K. The magnetization rises smoothly with negative curvature due to uncompensated Fe³⁺ spins and with a large high field slope due to the underlying antiferromagnetic ferritin core. Even at highest fields the magnetic moment is only ∼4% of the saturation moment of the full complement of Fe³⁺ in the ferritin molecule. The AC magnetic susceptibility, χ_AC(T,f), responding to the uncompensated spins, reaches a maximum near the superparamagnetic blocking temperature with the temperature of the maximum, T_M, varying with excitation frequency, T_M⁻¹ α log f for 10?f?10⁴ Hz.     

Magnetic properties of CdSb doped with Ni

Magnetic properties of the group II–V semiconductor CdSb single crystals doped with Ni (2 at%) are investigated. Deviation of the zero-field-cooled susceptibility, χ_ZFC, from the field-cooled susceptibility is observed below 300 K, along with a broad maximum of χ_ZFC (T) at T_b in fields below the anisotropy field B_K∼4 kG. T_b(B) obeys the law [T_b(B)/T_b(0)]^1/2=1–B/B_K with T_b(0)∼100 K. The magnetization exhibits saturation above ∼20–30 kG, a weak temperature dependence and anisotropy of the saturation value M_s. The coercive field is much smaller then B_K and displays anisotropy inverted with respect to that of M_s. Such magnetic behavior is expected for spheroidal Ni-rich Ni_1−xSb_x nanoparticles with high aspect ratio, broad distribution of the sizes and with orientations of the major axis distributed around a preferred direction.     

Temperature dependence of magnetic anisotropy in nanoparticles of CoxFe(3−x)O4

In this paper we present a study of the magnetic anisotropy constant of nanocrystalline magnetic particles of Co_xFe_(3−x)O₄, with x ranging from 0.05 to 1.6, synthesized by a combustion reaction. The magnetic anisotropy constants were obtained by fitting the high-field part of the major hysteresis loops with the law of approach equation down to temperatures of 4 K and up to fields of 60 kOe. The anisotropy constant depends strongly on both temperature and cobalt content x, exhibiting a nonmonotic dome-shaped dependence on x with a maximum at x=1.0. We found that fits at lower temperatures, i.e., 4 and 72 K, give values of K₁ that are approximately one order of magnitude higher than those at higher temperatures, i.e., 272 and 340 K. For example, K₁ for specimens with x=0.8 and 1.0 are 4.21×10⁷ and 4.22×10⁷ ergs/cm³ at 4 K, and 7.64×10⁶ and 7.51×10⁶ ergs/cm³ at 340 K, respectively. Thus, our determination of temperature-dependence of the anisotropy constant represents an improvement over existing works.     

Ferromagnetism and spin reorientation in Sm12Fe14Al5

The single crystal of the new ternary compound Sm₁₂Fe₁₄Al₅ was grown and its crystallographic and magnetic properties were investigated. Sm₁₂Fe₁₄Al₅ has a hexagonal structure of the space group p-3m1 and shows ferromagnetism with a Curie temperature of 245 K. The easy direction of magnetization is parallel to the c-axis at temperatures between 245 and 85 K; however, it changes to the c-plane below 85 K through a first-order-like phase transition. No saturation is observed in the magnetization curve even under the applied field of 55 kOe at 5 K. Sm₁₂Fe₁₄Al₅ seems to have a large coercive field at very low temperatures. The anisotropy field was estimated at 5 and 120 K and the saturation magnetization of low temperature phase is explained assuming a ferromagnetic coupling between Fe and Sm sublattices.