MFM studies of magnetic domain patterns in bulk barium ferrite (BaFe12O19) single crystals

Magnetic domain patterns in bulk barium ferrite (BaFe₁₂O₁₉; BaM) single crystals on the basal plane and the prism plane were measured and studied by magnetic force microscopy (MFM). The surface domain pattern is in the form of flowers or star on the basal plane and long elongated spikes or stripe domains on the prism plane. The change in domain structure with applied field (H_app) and the thickness (T) dependence on domain width (δ) was observed. The domain width decreased from 32 to 9 μm for the crystals of 800-100 μm thicknesses, respectively.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

Magnetic entropy change in GdCo13−xSix intermetallic compounds

The magnetic entropy change in GdCo_13−xSi_x (x=3.8, 4, 4.1, and 4.2) intermetallic compounds has been investigated by means of magnetic measurements in the vicinity of their Curie temperature. It was found that the magnetic ordering temperatures decrease from 60 K at x=3.8 to 28 K for x=4.2. The magnetic entropy change is calculated from isothermal magnetization versus magnetic field at various temperatures using the Maxwell relation. As a result, the maximum magnetic entropy changes of the investigated compounds, at their Curie temperatures, decrease from 11.5 J/kg K for x=4.2 to 6.86 J/kg K for x=3.8 in a field change of 0-3 T, whereas it decreases from 5.13 J/kg K for x=4.2 to 2.60 J/kg K for x=3.8 in a field change of 0-1 T. Moreover, the maximum value of the magnetic entropy change obtained at a higher field for GdCo_13−xSi_x with x=4 (23.75 J/kg K at 5 T) is comparable to that of various types of compounds with a cubic NaZn₁₃-type structure. Finally, the maximum of the magnetic entropy change is found to decrease with increasing Si content.     

Magnetic spectra of Fe-based nanocrystalline wires with axial or transverse domains

The circular permeability μ′=μ′−jμ″ of two Fe-based soft magnetic wires with axial and transverse domains, has been determined from the measurements of impedance Z=R+jX as functions of frequency (f=10-10⁵ Hz) and AC current amplitude (I=0.1-100 mA). From the magnetic spectra of μ′−f and μ″−f for a few circular fields (H_φ=0.4, 1.2, 4, 12, 40 A/m), we found that the sample with axial domain structure exhibits a relaxational feature, while for the one with transverse domain resonance-like spectra were observed when the circular field H_φ≥4 A/m. These results have been discussed in terms of domain structure and circular magnetization processes.     

Structural,magnetic and magnetocaloric properties of AMn1−xGaxO3 compounds with 0≤x≤0.2

Structural, magnetic and magnetocaloric properties of manganites series with the AMn_1−xGa_xO₃ (A=La_0.75Ca_0.08Sr_0.17 and x=0, 0.05, 0.1 and 0.2) composition have been investigated to shed light on Ga-doping influence. Solid-state reaction method was used for preparation. From XRD study, all samples are found single phase and crystallize in the orthorhombic structure with the Pnma space group. The variation of the magnetization M vs. temperature T, under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. The experimental results indicate that T_C decreases from 336 to 135 K with increasing Ga substitution. Magnetocaloric effect (MCE) was estimated, in terms of isothermal magnetic entropy change (−ΔS_M), using the M(T, μ₀H) data and employing the thermodynamic Maxwell equation. The maximum entropy change and Relative Cooling Power (RCP) show non-monotonic behaviors with increasing the concentration of Gallium. In fact, the maximum value of ΔS_Mmaxof AMn_1−xGa_xO₃ for x=0.00 and 0.2 samples is found to be, respectively, 2.87 and 1.17 J/kg/K under an applied magnetic field change of 2 T. For the same applied magnetic field (μ₀H=2 T), the RCP values are found to vary between 97.58 and 89 J/kg.     

Magnetic entropy change in perovskite manganites La0.6Pr0.1Pb0.3MnO3 with double metal-insulator peaks

Following the double metal-insulator peaks found in series of perovskite manganites La_0.7−xPr_xPb_0.3MnO₃ (x=0, 0.05, 0.1), the magnetic entropy change of La_0.6Pr_0.1Pb_0.3MnO₃ was carefully investigated as a representative. The maximum magnetic entropy change (ΔS_H=−1.7 J/kg K at 300 K) and the expanded refrigerant capacity (about 123.8 J/kg) had been obtained under 10 kOe magnetic field variation, though the double peak of maximum magnetic entropy change had not occurred since the comparative faint magnetic signal from the Pr ions inhomogeneity existed in the octahedral frame submerged in the strong magnetic signal originated from the dominating octahedral frame both in the double exchange mechanism, but the width at half maximum in the magnetic entropy change comparatively broadened.     

Evaluation of magnetic force distribution on a pair of superconducting bulk magnets

We study the construction of superconducting permanent magnets by RE123 bulk materials and the investigation of these industrial applications such as a magnetic separation. A bulk magnet can generate strong magnetic fields exceeding 2 T, which is the limit of ordinary iron-cored electromagnets, in a compact device with a low running cost. A magnetic field distribution of the bulk magnet is a cone shape, and it contributes to an increase of magnetic force which is proportional to the product of a magnetic field and its gradient. It is important to evaluate magnetic force when the application of the bulk magnet is discussed. In this paper, two Gd123 bulk materials of 65 mm in diameter were magnetized using a pair of superconducting bulk magnet system and three-axis components of magnetic flux density (B_x,⋅B_y, and B_z) in an open space between the magnetic poles were scanned with pitch of 2 mm in each direction. From these measured data, the axial and radial components of magnetic force factor, B_z⋅dB_z/dz and B_r⋅dB_r/dr, were calculated. At 10 mm gap, the B_z⋅dB_z/dz value reached 180.6 T²/m for a field of 2.33 T, which is comparable to B_z = 6.76 T for a common 10 T–100 mm∅ superconducting magnet.     

Density of states effective mass of n-type CuInSe2 from the temperature dependence of Hall coefficient in the activation regime

The Hall coefficient R_H of n-type CuInSe₂ single crystals is measured between 10 and 300 K in pulsed magnetic field up to 35 T. The threshold field B_th, above which the magnetic freezeout starts to occur, varies linearly with temperature. From the analysis of the temperature dependence of electron concentration in the activation regime above 100 K at different field values, it is established that the density of states effective mass is independent of the magnetic field B and the activation energy E_D, above around 6 T, varies as B^1/3. Similar B^1/3 dependence of the magnetoresistance in the high magnetic field regime, reported earlier in the same material, suggests that theoretical work that could explain this coincidence is needed.     

Effect of thermal process on magnetic anisotropy in FeCoB soft underlayer

Relationship between magnetic anisotropy field H_k and thermal processes during the preparation has been studied for FeCoB thin films. The FeCoB films deposited on the glass substrates by facing targets sputtering successfully showed strong magnetic anisotropy when the substrate was heated at the substrate temperature T_s above 100 °C. Additionally, the lattice spacing of FeCo(1 1 0) in the perpendicular direction was found to decrease depending on the substrate temperature T_s. Among various temperature histories, the heating processes with a phase of increasing T_s revealed the further improvement of H_k. Meanwhile, high H_k in the films disappears after the post-deposition annealing at the temperature above 400 °C.     

Magnetocaloric properties in the Cr-doped La0.7Sr0.3MnO3 manganites

Single-phase polycrystalline samples of La_0.7Sr_0.3Mn_1-xCr_xO₃ with nominal composition of x=0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5-400 K and magnetic field range 0-8 T. It was found that the Curie temperature T_C decreases with increasing x and the maximum magnetic entropy change (−ΔS_M) for x=0.20 is ∼1.203 and ∼2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K.     

Effect of a high magnetic field on the morphology and magnetic properties of the MnBi compound during the Mn1.08Bi-MnBi phase transformation process

Effect of a 10 T high magnetic field on the morphology and magnetic properties of the MnBi compounds during the Mn_1.08Bi-MnBi phase transformation has been investigated. Results indicate that the field has split the MnBi crystal along the (0 0 1)-crystal plane during the Mn_1.08Bi-MnBi phase transformation process and the split MnBi crystals align and aggregate along the magnetic field direction. Along with the change of the MnBi phase morphology, the magnetic property changes greatly. Indeed, with the alignment and aggregation of the spit MnBi phases, the saturation magnetization M_s and the magnetic susceptibility χ increase, and the coercive field H_c and the remnant magnetization M_r decrease. This implies that a high magnetic field may have caused the magnetic property of the MnBi phase to transform towards soft magnetism. Above results may be attributed to the enhancement of the magnetization and the Mn_1.08Bi-MnBi phase transformation in a high magnetic field.     

Synthesis of α-Fe2O3 nanobelts and nanoflakes by thermal oxidation and study to their magnetic properties

α-Fe₂O₃ nanobelts and nanoflakes have been successfully synthesized by oxidation of iron-coated ITO glass in air. The X-ray diffraction, Raman spectrum and scanning electron microscopy are carried out to characterize the nanobelts and nanoflakes. The formation mechanism has been presented. Significantly, the magnetic investigations show that the magnetic properties are strongly shape-dependent. The magnetization measurements of belt-like and flake-like α-Fe₂O₃ in perpendicular exhibit ferromagnetic feature with the coercivity (H_c) and saturation magnetization (M_s) of 334.5 Oe and 1.35 emu/g, 239.5 Oe and 0.12 emu/g, respectively. For the parallel, belt-like and flake-like α-Fe₂O₃ also exhibit ferromagnetic feature with the H_c and M_s of 205.5 Oe and 1.44 emu/g, 159.6 Oe and 0.15 emu/g, respectively.     

Synthesis of goethite from Fe(OH)2 precipitates: Influence of Fe(II) concentration and stirring speed

Magnetic methods are efficient tools in soil and environmental science. But in such natural environments, several magnetic minerals are generally present. So, synthetic standard samples are necessary for calibration of laboratory techniques. The aim of this study was to synthesise goethite free of magnetic impurities (concentration <∼1 μg kg⁻¹) with different crystal sizes. Goethite was prepared by oxidation of aqueous suspensions of Fe(OH)₂ precipitates. Final products were characterised by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy and magnetic methods. Goethite could be obtained in the absence of any trace of strong magnetic minerals using FeSO₄·7H₂O and NaOH as reactants with the following experimental conditions: temperature=45 °C, [FeSO₄·7H₂O]=0.50 mol L⁻¹, [NaOH]=0.20 mol L⁻¹, stirring speed=760 rpm. The Fe(II) concentration and the stirring speed were varied. It proved possible to modify the size of the goethite crystals by varying the Fe(II) concentration and the stirring speed, but important changes of these parameters induced the formation of other phases, lepidocrocite when the oxidation reaction was drastically accelerated and Fe₃O₄ when the reaction was slowed down. In any case, for weak magnetic fields, a low-coercivity magnetic mineral saturating at weak magnetic fields was observed. It may correspond to traces of δ-FeOOH or to domains structurally similar to δ-FeOOH inside the multidomainic crystals of δ-FeOOH.     

Effects of the Hf content on the microstructure and magnetic properties of Co-Hf-Ta thin films

Effects of the Hf content in Co-Hf-Ta thin films on the microstructure and magnetic properties were investigated in this study. It was found that appropriate Hf addition can effectively refine the Co grain size. Co grain sizes sharply decreased from 50 nm down to 2.3 nm with increasing the Hf content from 1.02 at.% to 2.81 at.%, leading to the reduced magneto-crystalline anisotropy. The Co-Hf-Ta thin films with small Co grains reveal low anisotropy field, low coercivity, and high resistivity. By optimizing the Hf content, the film with Hf concentration of 2.81 at.% exhibits excellent soft magnetic properties: high saturation magnetization (4πM_S ∼ 13.6 kG), and low coercivity (H_C ∼ 0.6 Oe). The effective permeability of the film reaches 800 and remains constant up to 1 GHz.     

Structural and magnetic study of the Ti-doped barium hexaferrite ceramic samples: Theoretical and experimental results

We present a theoretical and experimental study of the structural and magnetic properties of M-type Ti⁴⁺-doped barium hexaferrite BaFe_{(12−(4/3)x)}Ti_xO₁₉ with x=0, 0.1, 0.2, 0.3, 0.5, 0.7 and 1.3. The XRD patterns and magnetic measurements show appreciable variations in the values of the saturation magnetization and the magnetic anisotropy field, H_an, with increasing Ti⁴⁺ content. We did not observe significant changes in the Lotgering factor along the (0 0 l) direction and in the texture coefficient, C_ex, which was estimated from the torque curves. The magnetic properties of these materials are explained by the combined effect of the coherent rotation of the magnetic domains and the replacements of Fe³⁺ by Ti⁴⁺ ions in the octahedral and tetrahedral sites. The influence of the Ti⁴⁺ content on the samples was studied theoretically by using a statistical phenomenological model. The main purpose of the model is to make preliminary predictions of the distribution of any dopant cation in the Fe³⁺ sites. As a result, we are able to analyze both structural and magnetic features of M-type barium hexaferrite.     

Competing magnetoresistance contributions in sputtered FePt thin films

Fe-Pt thin films were deposited by rf sputtering on an MgO substrate heated at different temperatures to induce the formation of the perpendicular Fe-Pt L1₀ phase with a different grain morphology on the nanometer scale. All films are characterized by a mazelike pattern of FePt nanograins with interconnected bases. MFM images and magnetization curves indicate that all samples have a strong perpendicular magnetic anisotropy arising from (0 0 1) growth. The temperature behaviour of the electrical resistance indicates that a percolating path exists for conduction electrons in the mazelike pattern. The magnetoresistance was measured as a function of magnetic field (applied longitudinally) and temperature in the ranges −70 kOe<H<+70 kOe and 4 K<T<150 K, respectively. All samples display a complex behaviour of the electrical resistance as a function of applied field. The role of the different magnetoresistance effects (both intrinsic and extrinsic) measured in these FePt thin films is elucidated.     

Magnetization of epitaxial MnGe alloys on Ge(1 1 1) substrates

The structural and magnetic properties of an Mn rich solid phase epitaxy Mn_xGe_1−x alloy grown on a clean 2 × 8-Ge(1 1 1) surface, with a Curie temperature of about 300 K are investigated. Magneto-optical  Kerr effect infers the existence of in-plane easy magnetization direction. We describe the epitaxial registry condition, the room temperature—zero field magnetic structure observed by magnetic force microscopy and the magneto-optical properties. The observations are consistent with the formation of epitaxial Mn₅Ge₃ alloy, with a modulated magnetic structure characterized by asymmetric 180° Bloch walls consisting of a vortex-like configuration of the local magnetization.     

Stress-MI and domain studies in Co-based nanocrystalline ribbons

A systematic investigation of the influence of different types of annealing on the magnetoimpedance (MI) effect in melt-spun (Co_1−xFe_x)₈₉Zr₇B₄ [x=0, 0.025, 0.05] and (Co_0.88Fe_0.12)_78.4Nb_2.6Si₉B₉Al ribbons has been carried out in the frequency range 500 kHz-13 MHz and under dc magnetic fields (H_dc) up to 80 Oe. In the stress annealed ribbons, the strain-induced transverse anisotropy is seen to result in large MI. Magnetic domains were investigated in the ribbons through magnetic force microscopy.     

The effect of magnetic field heat treatment on magnetostriction of Terfenol-D 2-2 composites

Based on the analysis of the magnetostriction for Terfenol-D composites, Terfenol-D 2-2 magnetostrictive composites have been prepared with laminations perpendicular to [1 1 2] axes. Then one of the samples was annealed in the vacuum at 423 K for 15 min at the magnetic field of 240 kA/m, which is along the direction of laminations and vertical to the [1 1 2] axes of the specimen. The static magnetostriction λ and dynamic magnetostrictive coefficient d₃₃ of samples were measured under the compressive stress of 0, 2, 4, 6 and 8 MPa. Effects of the compressive stress and the magnetic field heat treatment on the magnetostriction λ have been investigated. It is found that the magnetostriction of 2-2 composites can be improved under the compressive stress when the magnetic field is larger than 20 kA/m. The magnetostriction of 2-2 composites with the magnetic field heat treatment increases under compressive stress, and it can reach 1390×10⁻⁶ at the magnetic field of 200 kA/m and under the compressive stress of 4 MPa, much larger than the value of 860×10⁻⁶ without the magnetic field heat treatment. The highest magnetostriction of the 2-2 composite with the magnetic field heat treatment can reach 1530×10⁻⁶. The dynamic magnetostrictive coefficient d₃₃ of 2-2 composites with the magnetic field heat treatment have been improved, compared with that without magnetic field heat treatment. The maximum value of d₃₃ of the sample with magnetic field heat treatment is 71% larger than that without magnetic field heat treatment.     

Exchange bias in bulk Ni-Mn-In-based Heusler alloys

The crystal structure and exchange bias of the bulk Heusler alloys Ni₅₀Mn_50−xIn_x with 14.5?x?15.2 have been investigated using X-ray diffraction and magnetization measurements, respectively. Magnetic measurements were performed with SQUID magnetometry after samples were zero-field cooled and field cooled (FC) in positive magnetic fields up to H=50 kOe, from a temperature T=380 K. Three temperatures of the phase transitions, T₁<T_M<T_C, and a shift of the FC (50 kOe) magnetic hysteresis loops up to 120 Oe at 5 K have been detected for all samples. The exchange bias field (H_E) was almost constant for intermediate In concentrations 14.8<x<15.2, and sharply decreased to about 20 Oe on the borders of this concentration interval (x?14.5; 15.2?x). The changes of H_E have been related to changes in the ratio of T₁ to T_M: the overlapping of transitions at T₁ and T_M (for x=14.8, and 15.2) results in a decrease in H_E.