Study on B2 structure epitaxial Fe/Co superlattice

The B2 structure Fe₅₀Co₅₀ alloy is very attractive material as a large spin conductance asymmetry. In this study, we have tried to fabricate epitaxial Fe/Co superlattice with B2 structure. In order to investigate the relationship between the film structure and the substrate temperature, the films were prepared at different substrate temperature. The film structure of Fe/Co was evaluated by reflection high energy electron diffraction (RHEED). The in-plane lattice spacing gradually decreased to that of a bulk Fe₅₀Co₅₀ as increase in the number of layers. The B2 structure ordered phase of Fe/Co superlattice was successfully confirmed by RHEED and X-ray diffraction (XRD).     

Synthetic Co50Pt50/Ru/CoFe hard–soft trilayer in spin valves

The influence of deposition power and seedlayer on the properties of hard magnet Co₅₀Pt₅₀ was studied. Co₅₀Pt₅₀(/Co₉₀Fe₁₀)/Ru/Co₉₀Fe₁₀ trilayer was used as pining/pinned layer in spin valves. The influences of different hard layer, soft layer and free layer on exchange bias, interlayer coupling, and magnetoresistance (MR) ratio were studied. Weak antiferromagnetic interlayer coupling was obtained by adjusting the thickness of hard and soft layers. MR of a spin valve with structure Cr2/CoFe0.5/CoPt4/CoFe0.5/Ru0.8/CoFe2.2/Cu2.05/CoFe2.6/Cu1.1/Ta1 reached 10.68% (unit in nm), which is comparable to those of IrMn-based synthetic spin valves. The increment of the coercivity of the free layer is mainly due to the static magnetic interaction between the hard layer and the free layer.     

双合成反铁磁结构及其对自旋阀巨磁电阻效应的影响

用直流磁控溅射方法制备了双合成反铁磁结构Co₉₀Fe₁₀(5 nm)/Ru(x nm)/Co₉₀Fe₁₀(3 nm)/Ru(y nm)/Co₉₀Fe₁₀(5 nm)(x=0.45,0.45,1.00; y=0.45,1.00,1.00)的系列样品,并对样品的性能及其作为钉扎层对自旋阀巨磁电阻(GMR)效应的影响进行了研究 关键词： 双合成反铁磁 自旋阀 巨磁电阻     

New perovskite related-compounds of Ru(V) in K2NiF4 type structure

The preparation of the new K₂NiF₄-type compounds, Sr₂Co_0.5Ru_0.5O₄, Sr₂Ni_0.5Ru_0.5O₄ and Sr₂Ga_0.5Ru_0.5O₄ by direct solid state synthesis and the subsequent characterization by powder X-ray diffraction measurements are described. From Rietveld refinements, we found that the compounds have a K₂NiF₄-type structure with the symmetry of space group I4/mmm. It was found that in these compounds, the substituted trivalent metal ions (M=Co, Ni, and Ga) and the Ru ions are disordered over the octahedral B sites of the K₂NiF₄ structure. The octahedral Ru-O environment is less elongated in the apical direction in these compounds than in the Sr₂RuO₄ compound. We also found that the octahedron in Sr₂Ga_0.5Ru_0.5O₄ is more symmetric than that of Sr₂Co_0.5Ru_0.5O₄ and Sr₂Ni_0.5Ru_0.5O₄. The Ru K-edge X-ray absorption measurements showed that Ru ions were in a pentavalent oxidation state.     

Structural, AC, and DC magnetic properties of Zn1−xCoxFe2O4

Structural, AC and DC magnetic properties of polycrystalline Zn_1−xCo_xFe₂O₄ (x=0.2, 0.4) samples sintered at various temperatures (1100-1300 °C), and various dwell times (0.2-15 h) have been investigated thoroughly. The bulk density of the Zn_0.60Co_0.40Fe₂O₄ samples increases as the sintering temperature (T_s) increases from 1100 to 1250 °C, and above 1250 °C the bulk density decreases slightly. The Zn_0.80Co_0.20Fe₂O₄ samples show similar behavior of changes to that of Zn_0.60Co_0.40Fe₂O₄ samples except that the bulk density is found to be highest at 1200 °C. The DC magnetization as a function of temperature curves show that the Zn_0.60Co_0.40Fe₂O₄ sample is ferrimagnetic at room temperature while the Zn_0.80Co_0.20Fe₂O₄ sample is paramagnetic at room temperature. The T_c of Zn_0.80Co_0.20Fe₂O₄ sample is found to be 170 K from DC magnetization measurement. Separate measurement (AC magnetization), initial permeability as a function of temperature shows that the T_c of the Zn_0.60Co_0.40Fe₂O₄ sample is 353 K. Slight variation of T_c is observed depending on sintering condition. The initial permeability for the Zn_0.60Co_0.40Fe₂O₄ composition sintered at 1250 °C is found to be maximum.     

Magnetic domain structure in Fe78.8−xCoxCu0.6Nb2.6Si9B9 nanocrystalline alloys studied by Lorentz microscopy

The magnetic domain structures of Fe_78.8−xCo_xCu_0.6Nb_2.6Si₉B₉ (x=0, 20, 40, 60) alloys are investigated by Lorentz microscopy coupled with the focused ion beam method. The specimen prepared using the FIB method is found to have a considerably more uniform thickness compared to that prepared using the ion-milling method. In Fe_38.8Co₄₀Cu_0.6Nb_2.6Si₉B₉ and Fe_18.8Co₆₀Cu_0.6Nb_2.6Si₉B₉ alloys, 180° domain walls extending in the direction of the induced magnetic anisotropy are observed. Analysis with Lorentz microscopy reveals that the width of the magnetic domains decreases with an increase in the cobalt content or the induced magnetic anisotropy K_u, that is, the domain width d is proportional to the induced magnetic anisotropy (K_u)^−1/4. On the other hand, in the in situ Lorentz microscopy observation as a function of temperature, magnetic ripple structures are found to appear in a localized area due to the fluctuation of magnetization vectors from 423 K. It is observed that the induced magnetic anisotropy caused by the applied magnetic field at 803 K is not suppressed by the magnetic ripple structures observed at 423–443 K.     

Beneficial effect of Co substitution on the structure and magnetic properties of mechanically alloyed (Pr,Tb)–Fe–(C,B) magnets

The effect of Co substitution on the structure and magnetic properties of mechanically alloyed Pr₁₄Tb₂Fe_76−xCo_xC₆B₂ and Pr₁₆Fe_76−xCo_xC₆B₂ (x=0–20$x=0–20$) alloys has been studied systematically. The main phase in the alloys is Pr₂Fe₁₄C-type carbide, coexisting with a small amount of α-Fe and rare-earth-rich phases. In addition to the increasing of the Curie temperature of the Pr₂Fe₁₄C-type phase, Co substitution can affect the magnetic properties by adjusting the α-Fe fraction of the alloys. The increase of both coercivity and remanence has been realized in a certain composition range. This increase may be attributed mainly to the enhancement of the effective anisotropy constant _Keff$K_{\mathrm{eff}}$ of the magnets due to the reduced α-Fe fraction with a small Co addition. The highest coercivity _iH_c of 20.3 kOe and the optimum energy product (BH)_max of 10.3 MG Oe have been obtained for the Pr₁₄Tb₂Fe_69.5Co_6.5C₆B₂ alloy.     

Controlled fabrication of nano-scale double barrier magnetic tunnel junctions using focused ion beam milling method

The controlled fabrication method for nano-scale double barrier magnetic tunnel junctions (DBMTJs) with the layer structure of Ta(5)/Cu(10)/Ni₇₉Fe₂₁(5)/Ir₂₂Mn₇₈(12)/Co₆₀Fe₂₀B₂₀(4)/Al(1)–oxide/Co₆₀Fe₂₀B₂₀(6)/Al(1)–oxide/Co₆₀Fe₂₀B₂₀(4)/Ir₂₂Mn₇₈(12)/Ni₇₉Fe₂₁(5)/Ta(5) (thickness unit: nm) was used. This method involved depositing thin multi-layer stacks by sputtering system, and depositing a Pt nano-pillar using a focused ion beam which acted both as a top contact and as an etching mask. The advantages of this process over the traditional process using e-beam and optical lithography in that it involve only few processing steps, e.g. it does not involve any lift-off steps. In order to evaluate the nanofabrication techniques, the DBMTJs with the dimensions of 200 nm×400 nm, 200 nm×200 nm nano-scale were prepared and their R–H, I–V characteristics were measured.     

Transmission electron microscopy study of large field induced anisotropy (Co1−xFex)89Zr7B4 nanocomposite ribbons with dilute Fe-contents

Electron microscopy was employed to investigate the structure of magnetic field crystallized (Co_1−xFe_x)₈₉Zr₇B₄ alloys with only dilute Fe-contents (x=0, 0.025, 0.05, and 0.10). The x=0.025 and 0.05 alloys exhibit very large field induced anisotropies and multiple nanocrystalline phases (BCC, FCC, and HCP) surrounded by an intergranular amorphous phase. Correlation between the volume fraction crystallized and the measured value of H_K suggests that the large K_U values are associated with the crystalline phases that form. Multiple crystalline phases are present for the highest K_U alloys and so the presence of FCC and/or HCP-type nanocrystals may be responsible for these observations. High-resolution transmission electron microscopy (HRTEM) illustrates a number of microstructural features including (1) high densities of stacking faults in many of the FCC and, in particular, the HCP-type nanocrystals, (2) infrequent BCC/FCC orientation relationships, and (3) nanocrystals with disordered or long period stacking sequences of close-packed planes. High densities of planar faults are suggested as a potential source of K_U for the FCC and HCP-type nanocrystals, but the origin of the large values of K_U found in dilute Fe-containing, Co-rich “nanocomposite” alloys is an area where further work is needed.     

Partial crystallization in amorphous magnetic film induced by Ru layer interface

Partial crystallization in amorphous magnetic film is observed in this study. The film of Co₄₆Fe₄₆Zr₅B₃/Ru/Co₄₆Fe₄₆Zr₅B₃ (CFZB/Ru/CFZB) was prepared on glass substrate by DC magnetron sputtering. The CFZB underlayer (CFZB‐UL) and the CFZB overlayer (CFZB‐OL) were deposited under nominally same sputtering conditions, i.e. target, working gas, working pressure, input power, and deposition duration. The transmission electron microscopy (TEM) images with fast Fourier transform (FFT) patterns revealed that the CFZB‐UL was amorphous while the CFZB‐OL became unexpectedly partially crystallized. The result of X‐ray diffraction (XRD) spectra verified the TEM observation. The cause of the partial crystallization is attributed to the lower concentration of glass formers and the lattice matching between the overlayer and the Ru layer below.     

Research on La–Co-substituted strontium ferrite magnets for high intrinsic coercive force

M-type strontium ferrites with substitution of Sr²⁺ by rare-earth La³⁺, and a little amount of Fe³⁺ by Co²⁺ according to the formula Sr_1−xLa_xFe_12−xCo_xO₁₉, are prepared by the ceramic process. Effects of the substituted amount of La³⁺ and Co²⁺ on structure and magnetic properties of Sr_1−xLa_xFe_12−xCo_xO₁₉ compounds have systematically been investigated by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and B–H hysteresis curve measurements. In our results, the suitable amount of La³⁺–Co²⁺ substitution may remarkably increase saturation magnetization. Intrinsic coercive force (H_cj) of Sr ferrite magnets is evidently increased without significant decrease in residual flux density (B_r) by La³⁺–Co²⁺ substitution.     

Magnetic properties of Co-ferrite-doped hydroxyapatite nanoparticles having a core/shell structure

The magnetic properties of Co-ferrite-doped hydroxyapatite (HAP) nanoparticles of composition Ca_10−3xFe_2xCo_x(PO₄)₆(OH)₂ (where x=0, 0.1, 0.2, 0.3, 0.4 and 0.5% mole) are studied. Transmission electron microscope micrograms show that the 90 nm size nanoparticles annealed at 1250 °C have a core/shell structure. Their electron diffraction patterns show that the shell is composed of the hydroxyapatite and the core is composed of the Co-ferrite, CoFe₂O₄. Electron spin resonance measurements indicate that the Co²⁺ ions are being substituted into the Ca(1) sites in HAP lattice. X-ray diffraction studies show the formation of impurity phases as higher amounts of the Fe³⁺/Co²⁺ ions which are substituted into the HAP host matrix. The presence of two sextets (one for the A-site Fe³⁺ and the other for the B-site Fe³⁺) in the Mössbauer spectrum for all the doped samples clearly indicates that the CoFe₂O₄.cores are in the ferromagnetic state. Evidence of the impurity phases is seen in the appearance of doublet patterns in the Mössbauer spectrums for the heavier-doped (x=0.4 and 0.5) specimens. The decrease in the saturation magnetizations and other magnetic properties of the nanoparticles at the higher doping levels is consistent with some of the Fe³⁺ and Co²⁺ which being used to form the CoO and Fe₂O₃ impurity phase seen in the XRD patterns.     

Effects of Mn substitution of Co and Fe in spinel CoFe2O4 thin films

Effects of Mn substitution for Co and Fe on the structural and magnetic properties of inverse-spinel CoFe₂O₄ have been investigated. Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ thin films were prepared by a sol–gel method. The observed increase of the lattice constant of Mn_xCo_1−xFe₂O₄ indicates that Mn²⁺ ions substitute the octahedral Co²⁺ sites. Conversion electron Mössbauer spectroscopy data indicate that a fraction of octahedral Co²⁺ ions exchange sites with tetrahedral Fe³⁺ ions through Mn doping. Vibrating-sample magnetometry data exhibit a large increase of saturation magnetization for both Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ films compared to that of the CoFe₂O₄ film. Such enhancement of magnetization can be explained in terms of a breaking of ferrimagnetic order induced by the Co²⁺ migration.     

Magnetoresistive properties of Zn1−xCoxFe2O4 ferrites

The magnetic and magnetoresistive properties of spinel-type Zn_1−xCo_xFe₂O₄ (x=0, 0.2 and 0.4) ferrites are extensively investigated in this study. A large negative magnetoresistance (MR) effect is observed in Zn_1−xCo_xFe₂O₄ ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature, and show a spin-(cluster) glass transition at low temperatures, depending on the chemical compositions. The MR curves as a function of magnetic fields, MR(H), are parabolic at all temperatures for paramagnetic polycrystalline ZnFe₂O₄. The MR for ZnFe₂O₄ at 110 K in the presence of 9 T applied magnetic field is 30%. On the other hand, MR(H) are linear for x=0.2 and 0.4 ferrimagnetic Zn_1−xCo_xFe₂O₄ samples up to 9 T. The MR effect is independent of the sintering temperatures, and can be explained with the help of the spin-dependent scattering and the Yafet–Kittel angle of Zn_1−xCo_xFe₂O₄ mixed ferrites.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

Structure and magnetic properties of magnetostrictive compounds Tb0.36Dy0.64(Fe0.85Co0.15)2−xBx (0⩽x⩽0.15)

Crystal structure and magnetic properties of magnetostrictive compounds Tb_0.36Dy_0.64(Fe_0.85Co_0.15)_2−xB_x (0?x?0.15) have been investigated at room temperature. The matrix of these compounds keeps a cubic MgCu₂-type structure. Lattice parameter a of the Laves phase decreases to reach a minimum at x=0.10, then increases with increasing boron content. Through analyzing the Mössbauer spectra, the easy magnetization direction (EMD) for all samples is confirmed to lie along 〈111〉 direction at room temperature, suggesting the presence of the giant magnetostriction. The mean hyperfine field H_hf and the deduced iron moment μ_Fe increase with increasing boron content, resulting in the enhancement of both Curie temperature T_C and spin reorientation temperature T_r. Although the addition of B enlarges the magnetocrystalline anisotropy constant K₁, the composition dependence of the ratio λ/K₁ for Tb_0.36Dy_0.64(Fe_0.85Co_0.15)_2−xB_x, however, reaches a maximum value at x=0. 05 under high magnetic fields.     

Structural, magnetic and electrical transport properties of Co doping in LaKFeMoO6 double perovskite

The structural, magnetic and transport properties of La_1+xK_1−xFe_1−yCo_yMoO₆ (0.0≤x≤0.1 and 0.1≤y≤0.2) series are studied. At room temperature, the crystal structure is a monoclinic system with space group P2₁/n. The antisite defect lowers with Co doping in LaKFe_1−yCo_yMoO₆ series. However, it increases with the substitution of K by La. Magnetizations increase with the increase in Co content (x=0) and with the La substitution for K, respectively. All compounds demonstrate semiconducting behavior. Their electrical resistivities increase with Co content for LaKFe_1−yCo_yMoO₆ and also increase with La for La_1+xK_1−xFe_1−yCo_yMoO₆. For the LaKFe_1−yCo_yMoO₆ the electrical transport behavior can be described by Mott variable range hopping model in the studied temperature range, whereas for the La_1+xK_1−xFe_1−yCo_yMoO₆ (x≠0 and y≠0) the electrical transport behavior follows the Mott and ES variable range hopping model in high and low temperature ranges, respectively. Each sample exhibits a large magnetoresistance effect.     

Exchange coupled magnetic nanocomposites of Sm(Co1−xFex)5 / Fe3O4 with core/shell structure

Magnetic nanocomposites of Sm(Co_1−xFe_x)₅/Fe₃O₄ (x≈0.1) with the core/shell type structure were successfully fabricated using a two-step polyol process, where as-prepared SmCo_5(1−x) nanoparticles were used as seeds for the ferrite coating. The core/shell composites are quite stable in air and show a typical hysteric behavior of single component, yielding an enhanced coercivity of 2.2 kOe with a saturated magnetization of 130 emu/g at 5 T. The magnetization data clearly reveal the presence of effective exchange coupling between the hard-magnetic Sm(Co_1−xFe_x)₅ core and soft-magnetic Fe₃O₄ shell, suggestive of a single-phase structure rather than a distinctive two-phase one.     

Characterization of different magnetite–cobalt nanoparticles in hydrocarbon-based magnetic fluids by means of static and dynamic magnetization measurements

Magnetic nanoparticles with different compositions (Co_xFe_3−xO₄, 0?x?0.1) were synthesized from metal salts using a coprecipitation technique to produce magnetic fluids following a peptization technique. The liquid carrier was the hydrocarbon Isopar M and the surfactant was oleic acid. The colloidal-sized ferrimagnetic nanoparticles produced were found to be superparamagnetic. Measurements of the complex magnetic susceptibility were carried out to evaluate the resonant frequency f_res, the anisotropy constant K, and anisotropy field H_A. f_res was found to be a linear function of the cobalt content of the magnetic nanoparticles over the range of cobalt content studied.     

Mössbauer study of Fe0.92−xCoxP0.08 nanowire arrays

Arrays of Fe_0.92−xCo_xP_0.08 (0.22≤x≤0.78) ternary alloy nanowires were fabricated in anodic aluminium oxide templates by electrochemical deposition. The broadened peaks in transmission Mössbauer spectra and the halo in selected area electron diffraction patterns indicate that the structure of Fe_0.92−xCo_xP_0.08 nanowires is amorphous. However, the short-range order of Fe_0.92−xCo_xP_0.08 nanowires has a bcc structure with a [110]-preferred orientation that is parallel to the nanowires. The magnetic texture results in the magnetic moment direction of the Fe atoms being along the nanowires. The short-range order around the Fe atoms reaches a minimum at x=0.45. With increasing Co content, the average hyperfine field decreases, while the isomer shift and quadrupole splitting remain almost constant, which result from the variation of 3d and 4s electron volume density at the Fe sites.