Spin-reorientation transitions and domain structure in TbFe<Subscript>11−<Emphasis Type="Italic">x</Emphasis></Subscript> Co<Subscript>x</Subscript>Ti single crystals

The magnetic structure of single-crystal TbFe_11?xCo_xTi compounds has been studied over a broad temperature range and in strong magnetic fields (up to 14 T). Measurements of magnetization and magnetostriction and a study of the domain structure revealed that spin-reorientation transitions (SRTs) in TbFe_11?xCo_xTi single crystals depend substantially on the cobalt concentration. It was established that the SRT temperatures and threshold magnetic fields are governed by the interplay between the magnetic anisotropies of the 3d and terbium sublattices. It is shown that, in these compounds, the low-temperature phase with planar anisotropy is separated in temperature from the high-temperature phase with uniaxial anisotropy by an intermediate metastable phase containing domains of the uniaxial or planar phase.     

A magnetic, magnetostrictive and Mössbauer study of Tb0.3Dy0.7−xPrx(Fe0.9Al0.1)1.95 alloys

The effect of Pr substitution for Dy on the magnetization, magnetostriction, anisotropy and spin reorientation of a series of Tb_0.3Dy_0.7−xPr_x(Fe_0.9Al_0.1)_1.95 alloys (x=0, 0.1, 0.20, 0.25, 0.30, 0.35) at room temperature has been investigated. It was found that the magnetization and magnetostriction of the homogenized Tb_0.3Dy_0.7−xPr_x(Fe_0.9Al_0.1)_1.95 alloys decreases drastically with increasing x and the magnetostrictive effect disappears for x>0.2, but the spontaneous magnetostriction λ₁₁₁ increases approximately linearly with increasing x. Moreover, the magnetostriction exhibits slightly bigger value at x=0.1 than the free alloys and is saturated more easily with the magnetic field H, showing that a small amount of Pr substitution is beneficial to a decrease in the magnetocrystalline anisotropy. The analysis of the Mössbauer spectra indicated that the easy magnetization direction in the {1 1 0} plane deviates slightly from the main axis of symmetry with Pr concentration x, namely spin reorientation. Comparing with the Al substitution, the effect of Pr substitution for Dy on the spin reorientation is smaller.     

Magnetostriction and anisotropy of CoxFe3xO4 (x≤0.1) single crystals

Using the strain gauge technique, the magnetostriction constants λ_[100] and λ_[111] have been measured on single crystals of Co_xFe_3-xO₄ with compositions 0.0005≤x≤0.1 in the temperature range between 100 and 300 K and using magnetic fields up to 23 kOe. The presence of Co²⁺-ions gives rise to a very large negative value of λ_[100] and a fairly large positive value of λ_[111]. A linear relationship is found between the magnetostriction constants and the cobalt concentration.Some conclusions about the anisotropic beheviour of the samples can be made, based on the magnetostriction measurements. The change of the direction of easy magnetization from [111] to [100] in the temperature range just above the Verwey transition, is illustrated and discussed. It turned out to be possible to determine an effective anisotropy constant K'₁ through a curve fitting procedure. It is found that 1n K'₁ varies linearly with T.     

Magnetotransport and magnetoelastic effects in Co-doped La0.7Sr0.3MnO3 nanocrystalline perovskites

La_0.7Sr_0.3Mn_1−xCo_xO₃ (x=0, 0.05, 0.1) nanoparticles, prepared by sol-gel method, were studied by means of X-ray diffraction, transmission electron microscopy, resistivity, magnetoresistance, thermal expansion and magnetostriction measurements. Results show that partial substitution of Mn by Co leads to a reduction in lattice parameters, enhancement of resistivity and room temperature magnetoresistance MR, decrease of metal-insulator transition temperature T_MI and T_C, an increase in thermal expansion coefficient, volume magnetostriction and anisotropic magnetostriction. The latter increases about one order of magnitude with 10% Co substitution. In comparison with Mn ions, the Co ions possess higher anisotropy energy, larger magnetostriction effect, smaller ionic size and spin state transitions with increase in temperature and magnetic field; this suggests that Co substitution leads to double-exchange interaction weakening, resulting in suppression of ferromagnetic long-range order and metallic state and increase of magnetic anisotropy. Furthermore, our samples have a relatively lower T_MI and T_C, higher resistivity and MR, compared with the reported values for similar compounds with larger particle sizes. This is attributed to the nanometric grain size and spin-polarized tunneling between neighboring grains.     

The influence of sintering conditions on the phase purity of bulk EuTiO3 and Eu0.5Ba0.5TiO3 ceramics

EuTiO₃ and Eu_0.5Ba_0.5TiO₃ ceramics were synthesized using mechanochemical activation of oxide precursors and then calcined. The uniaxially as well as isostatically pressed samples were sintered in different kinds of reducing atmospheres, namely Ar?+?(7–10)%H₂, respectively, 99.99%H₂ in the case of pressureless sintering or in vacuum (enriched by CO vapors) in the case of pressure-assisted spark plasma sintering (SPS). The samples prepared by SPS contained the pyrochlore phase as the second phase. In contrast with SPS, pressureless sintered samples were phase pure, although thermodynamics calculations showed that CO atmosphere in SPS is more reducing than pure hydrogen. This is explained by short sintering times in SPS that do not allow establishment of the thermodynamic equilibrium. The proper choice of sintering temperature, time, and atmosphere enabled preparation of dense and phase pure samples of Eu _x Ba_{1? x} TiO₃ ceramics suitable for the evaluation of “true” physical properties (e.g., infrared reflectivity), or for experimental confirmation of specific functionalities proposed from theory.     

Magnetostriction and magnetic anisotropy of (Sm,Ce)Fe2 compounds

The structural, magnetic and magnetostrictive properties of Sm_1−xCe_xFe₂ (0≤x≤1) Laves compounds have been investigated. The magnetostriction coefficient λ₁₁₁ and the anisotropy of the Sm_1−xCe_xFe₂ compounds decrease with increasing Ce content. The decreasing of anisotropy results in an enhancement of the anisotropic magnetostriction at low magnetic fields for the compounds with a small amount of Ce substituted for Sm.     

Magnetostriction of TbxHo0.75−xPr0.25(Fe0.9B0.1)2 (x=0–0.3) compounds

The Tb_xHo_0.75−xPr_0.25(Fe_0.9B_0.1)₂ (x=0, 0.1, 0.15, 0.2, 0.25, and 0.3) compounds are found to stabilize in a cubic Laves phase structure. The lattice parameter, magnetostriction (at 10 kOe), and Curie temperature are found to increase with increasing Tb content. The compound with x=0.15 exhibits a possible anisotropy compensation between the Tb and (Ho/Pr) sublattices. The easy magnetization direction rotates towards the 〈1 1 1〉 from the 〈1 0 0〉 direction, with increasing Tb content. The splitting of the (4 4 0) peak accompanied by the spontaneous magnetostriction-induced rhombohedral distortion is observed for compounds with x?0.15 and the spontaneous magnetostriction (λ_1 1 1) is found to increase with Tb content.     

Experiments concerning the origin of stress anneal induced magnetic anisotropy in metallic glass ribbons

The stress anneal induced anisotropy has been studied in metallic glass ribbons of the compositions (Co_1-xFe_x)₇₅Si₁₅B ₁₀, 0≤x≤0.12. The induced anisotropies are found to consist of two contributions, an irreversable one and a recoverable one. From their thermal variations and from their stress dependence the anisotropy constants have tentatively been related to the constituents of the saturation magnetostriction coefficients. The kinetics reveal for the recoverable anisotropy a continuous spectrum of activation energies in the range 1.55–1.8 eV.     

Detailed study of ultra-soft magnetic properties of Fe74Cu0.8Nb2.7Si15.5B7

The magnetic properties of nanocrystalline Fe₇₄Cu_0.8Nb_2.7Si_15.5B₇ alloy, which were rapidly solidified and then annealed at various temperatures between 475 and 650°C for different holding time, have been studied. Grain size, silicon content and the lattice parameter of α-Fe(Si) nanograins at the annealing temperatures were determined. Curie temperature of the amorphous phase was determined from the temperature dependence of permeability. For higher annealing temperatures and times, some Si diffused out of the α-Fe(Si) phase and formed an ordered DO₃ phase of Fe₃Si. This changed the overall magnetostriction and average anisotropy of the matrix, which deteriorated the magnetic softness of the material at higher annealing temperatures. Ultra-soft magnetic properties were achieved by averaging the random anisotropy via exchange interaction. Hysteresis loops for samples in as-cast and annealed conditions have also been studied.     

Magnetic and magnetoelastic properties of Zn-doped cobalt-ferrites—CoFe2−xZnxO4 (x=0, 0.1, 0.2, and 0.3)

Cobalt-ferrite (CoFe₂O₄) based materials are suitable candidates for magnetomechanical sensor applications owing to a strong sensitivity of their magnetostriction to an applied magnetic field. Zn-doped cobalt-ferrites, with nominal compositions CoFe_2−xZn_xO₄ (x=0–0.3), were synthesized by auto-combustion technique using Co- , Fe- , and Zn-nitrate as precursors. X-ray spectra analysis and Transmission electron microscopy studies revealed that the as-prepared powders were comprised of nano-crystalline (∼25–30 nm) cubic-spinel phase with irregularly-shaped grains morphology along with minor impurity phases. Calcination (800 °C for 3 h) of the precursor followed by sintering (1300 °C for 12 h) resulted in a single phase cubic-spinel structure with average grain size ∼2–4 μm, as revealed from scanning electron micrographs. The magnitude of coercive field decreases from ∼540 Oe for x=0 to 105 Oe for x=0.30. Saturation magnetization initially increases and peaks to ∼87 emu/g for x=0.2 and then decreases. The peak value of magnetostriction monotonically decreases with increasing Zn content in the range 0.0–0.3; however the piezomagnetic coefficient (dλ/dH) reaches a maximum value of 105×10⁻⁹ Oe⁻¹ for x=0.1. The observed variation in piezomagnetic coefficient in the Zn substituted cobalt ferrite is related to the reduced anisotropy of the system. The Zn-doped cobalt-ferrite (x=0.1) having high strain derivative could be a potential material for stress sensor application.     

Structure, magnetic and magnetostrictive properties of as-deposited Fe-Ga thin films

Polycrystalline Fe_100−xGa_x (19?x?23) films were grown on Si(1 0 0) substrates at different partial pressures of sputtering gas ranging from 3 to 7 μbar. Microstructural, magnetic and magnetostrictive properties were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS) and magneto-optic Kerr effect (MOKE) magnetometry respectively. X-ray diffraction showed that all films have the body-centered cubic (bcc) Fe-Ga phase with the 〈1 1 0〉 direction out of the film plane. Magnetic characterization of the films showed that the films prepared at 3 μbar had weak uniaxial anisotropy whereas films grown at Ar pressures in the range 4-7 μbar were magnetically isotropic. The effective saturation magnetostriction constants (λ_eff) of the films were measured using the Villari effect. It was found that effective saturation magnetostriction constants were almost constant over the Ga composition range achieved by varying the sputtering pressure. The measured effective magnetostriction constants fit closely to the calculated saturation magnetostriction constants of 〈1 1 0〉 textured polycrystalline films with the 〈1 1 0〉 directions slightly canted with respect to the normal to the sample surface. It was found that a high pressure of the sputtering gas effected the magnetic softness of the films. The saturation field increased and remanence ratio decreased with increase in pressure.     

Magnetic anisotropy of amorphous (Fe1−x Co x )78Si10B12 alloys

In order to better understand the problem of magnetic anisotropy in amorphous alloys produced by a rapidly quenching technique, in-plane magnetic anisotropy of amorphous (Fe_1−x Co _x (₇₈Si₁₀B₁₂ alloys was measured by means of a torque magnetometer using a disk specimen made from the amorphous alloy ribbon. The amorphous ribbons were prepared by a single roller type quenching apparatus. It was found that the anisotropy had mostly twofold symmetry in all the alloy cases, and that the concentrationx dependence of the anisotropy constant behaved differently from that of the magnetostriction. Moreover, the anisotropy did not disappear by subsequent annealing at high temperatures where the internal stress relief and the crystallization were completed.     

A survey on the effect of vanadium content on the magnetoelastic properties of YFe12−xVx alloys

Experimental results on the thermal expansion and magnetostriction of YFe_12−xV_x (1.5≤x≤3.5) alloys are reported. The results show that the anisotropic magnetostriction (Δλ) at a finite field (1.5 T) increases with increasing vanadium content in the range of x<2. But for x>2, a decrease in the magnetic anisotropy with increasing vanadium content causes a decrease in the saturation values of Δλ. In addition, the thermal expansion coefficient becomes a minimum for x≈2. Experimental curves exhibit that the forced volume magnetostriction (ΔV/V) is positive and increases linearly with the applied field at high fields. But in the low field region (≤0.5 T), a minimum appears in the isothermal curves of ΔV/V around the saturation field. The results are explained by considering the influence of vanadium content on the magnetization anisotropy of YFe_12−xV_x compounds.     

Cubic harmonic analysis of magnetic anisotropy measurements on single crystal HoxTb1−xFe2 laves phase compounds

Room temperature magnetic anisotropy measurements have been made on a series of low anisotropy Ho_xTb_1?xFe₂ compounds, with 0.826 < x < 0.908, using torque magnetometry techniques. An analysis of the data using the conventional expansion of anisotropy energy in terms of the direction cosines proved inadequate because of the presence of higher order contributions to the anisotropy. When these contributions were included in the expansion, non unique anisotropy constants were obtained which were not consistent with the predictions of the single ion model. An analysis of the data using an expansion of the anisotropy in terms of an orthonormal set of functions, namely the cubic harmonics developed by Mueller and Priestly, proved more successful. The results showed that the two lowest order anisotropy constants, κ₄ and κ₆, both varied linearly with holmium concentrations as predicted by the single ion model; however, κ₈, the next higher order term, was not consistent with the predicted behavior. The origin of this contribution is believed to be related to the magnetostriction.     

Saturation and forced volume magnetostriction of Fe-rich FeZr amorphous alloys

AC susceptibility, saturation and forced volume magnetostriction were studied on iron-rich zirconium amorphous alloys a-Fe_100-xZr_x (8 at% ≤ x ≤ 12 at%). The experimental results are discussed in relation to the re-entrant spin glass state. It is found that the transverse magnetostriction accompanies a remarkable magnetic aftereffect in the spin glass phase, and that the forced volume magnetostriction shows an apparent peak at the ferromagnet to spin glass transition where the ac susceptibility has a cusp.     

A study on magnetoelastic properties of Tb3 (Fe28−xCox) V1.0 (x=0, 3, 6) compounds

In this work, The magnetoelastic properties of polycrystalline samples of Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) intermetallic compounds are investigated by means of linear thermal expansion and magnetostriction measurements in the temperature range of 77–515 K under applied magnetic fields up to 1.5 T. The linear thermal expansion increases with the Co content. The well-defined anomalies observed in the linear thermal expansion coefficients for Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) compounds are associated with the magnetic ordering temperature for x=0 and spin reorientation temperatures for x=3, 6. Below transition temperatures, the value of the longitudinal magnetostriction (λ_Pa) at 1.6 T increases with Co content.     

Structural, magnetic properties and magnetostriction studies of Sm1−xNdxFe1.55 alloys

Structural, magnetic properties and magnetostriction studies of Sm_1−xNd_xFe_1.55 (0≤x≤0.56) alloys have been performed. X-ray diffraction analysis confirms the presence of single cubic Laves phase in Sm_1-xNd_xFe_1.55 alloys with 0≤x≤0.48. The lattice parameter of alloys increases linearly with increase in Nd content while the Curie temperature behaves in the opposite way. The alloy x=0.08 exhibits a giant magnetostriction value (λ_∥-λ_⊥) of −2187 ppm at a magnetic field of 12 kOe due to the anisotropy compensation between Sm³⁺ and Nd³⁺ ions.     

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.     

Magnetic properties of metastable Gd-Cr alloys

We report on the magnetization, magnetocaloric effect, magnetic ordering temperatures, saturation magnetic moments and anisotropy of sputter-deposited Gd_xCr_1−x alloys with Gd atomic concentrations, x, ranging from 0.13 to 0.52. The complex magnetic nature of the Gd-Cr films was revealed from the M×H isotherms, which do not show saturation even at an applied field of 70 kOe and a temperature of 2 K and do not exhibit a linear behavior at higher temperatures. For some of the samples, the isotherms were used to determine the isothermal entropy variation as a function of temperature, for a change of 50 kOe in the applied magnetic field. The saturation magnetic moment varies with x and follows the dilution law, implying that the Cr atoms do not contribute to the total moment of the Gd-Cr alloys. Both static magnetization and dynamic susceptibility measurements reveal the existence of a magnetic glassy behavior in the alloys, which occurs below a freezing temperature. The existence of anisotropy at low temperatures for all samples was revealed by their M×H hysteresis loops from which the in-plane coercive fields, H_c, were determined. A monotonical increase in H_c with increasing Gd concentration was observed.     

High magnetostriction at low fields of (Tb1−xDyx)0.2Pr0.8(Fe0.4Co0.6)1.88C0.05 intermetallic compounds

The structure and magnetostriction of the (Tb_1−xDy_x)_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 intermetallic compounds (0≤x≤1) were studied by X-ray diffraction and magnetic measurements. The formation of an approximate single Laves phase with a MgCu2-type cubic structure was observed in this series of compounds. It was found that the Curie temperature and the saturation magnetization of the compounds would decrease with increase in the Dy content up to x=1. The magnetostriction λ_a (λ_a=λ_‖-λ_⊥) gently rises when x≤0.6, and follows with a precipitous fall when x exceeds 0.6, with the highest value of λ_a being reached in the compounds with x=0.6. The magnetostriction of all the samples was observed to approach their own saturation in the magnetic fields higher than 4 kOe. This indicates that the addition of a small amount of Dy could effectively improve the low-field magnetostriction of the Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 compounds, which could become a kind of promising magnetostrictive material.