Structural and magnetic properties of SmCo5.6Ti0.4 alloy

Three series of SmCo_5.6Ti_0.4 samples were prepared by quenching, melt spinning, and ball milling, respectively. Annealing at different temperatures was carried out for the three series. The influence of the processing routes on the structural and magnetic properties was systematically investigated for this alloy. The as-quenched bulk sample consisted of three phases with a rather coarse grain microstructure. Low intrinsic coercivity (_iH_c) of 0.12 T was obtained in this sample. While the as-spun ribbons and as-milled/annealed powders showed the CaCu₅-type phase (1:5) plus Th₂Zn₁₇-type phase (2:17), and the 1:5 phase plus TbCu₇-type phase (1:7), respectively, with nanograin microstructure. The _iH_c of as-spun ribbons and as-milled/annealed (700 °C for 2 h) powders was found to be 0.59 and 2.23 T, respectively. Coercivity mechanism of these as-spun ribbons is mainly of nucleation type. In the as-milled/annealed powders, the network of the nanograin boundaries is believed to provide strong pinning sites for the domain wall movement.     

Structure and magnetic properties of SmCoxTi0.4-1:7 ribbons

SmCo_xTi_0.4 (x=6.6, 7.1, 7.6, 8.1) ribbons have been prepared by melt spinning at a wheel speed of 42 m/s, followed by annealing at 750 °C for 2 h. Both as-spun and as-annealed ribbons possess the disordered TbCu7-type (1:7) phase even when the Sm/(Co,Ti) atomic ratio deviates from 1/7. The c/a ratio increases with increasing Co concentration x, but the unit cell volume decreases. The Curie temperatures show above 700 °C, increasing from 707 °C for x=6.6 to 782 °C for x=8.1. The saturation magnetizations increase almost linearly with increasing Co content. The observed magnetic hardening is believed to arise from the high magnetocrystalline anisotropy of the 1:7 phase and the fine nanograin structure. The intrinsic coercivity of 9797 Oe has been obtained in the melt-spun SmCo_7.1Ti_0.4 ribbons.     

Microstructures and coercivities of SmCox and Sm(Co,Cu)5 films prepared by magnetron sputtering

We have investigated the influence of composition and annealing conditions on the magnetic properties and microstructural features of SmCo_x films that were prepared by sputtering and subsequent annealing. A huge in-plane coercivity of 5.6 T was obtained from an optimally annealed Sm–Co film, which was attributed to the nanometer sized polycrystalline microstructure of the highly anisotropic SmCo₅ phase. Although a high density of planar defects were observed in the films that were annealed at high temperatures, they did not act as strong pinning sites for domain wall motion. The effect of Cu on [SmCo_4.5(9 nm)/Cu(xnm)]₁₀ multilayer thin films was also studied. An appropriate Cu content increased the coercivity.     

The structure and magnetic properties of Zn1−xNixFe2O4 ferrite nanoparticles prepared by sol–gel auto-combustion

Zn_1−xNi_xFe₂O₄ ferrite nanoparticles were prepared by sol–gel auto-combustion and then annealed at 700 °C for 4 h. The results of differential thermal analysis indicate that the thermal decomposition temperature is about 210 °C and Ni–Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. The microstructure and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscope, and Vibrating sample magnetometer. It is observed that all the spherical nanoparticles with an average grain size of about 35 nm are of pure spinel cubic structure. The crystal lattice constant declines gradually with increasing x from 0.8435 nm (x=0.20) to 0.8352 nm (x=1.00). Different from the composition of Zn_0.5Ni_0.5Fe₂O₄ for the bulk, the maximum M_s is found in the composition of Zn_0.3Ni_0.7Fe₂O₄ for nanoparticles. The H_c of samples is much larger than the bulk ferrites and increases with the enlarging x. The results of Zn_0.3Ni_0.7Fe₂O₄ annealed at different temperatures indicate that the maximum M_s (83.2 emu/g) appears in the sample annealed at 900 °C. The H_c of Zn_0.3Ni_0.7Fe₂O₄ firstly increases slightly as the grain size increases, and presents a maximum value of 115 Oe when the grains grow up to about 30 nm, and then declines rapidly with the grains further growing. The critical diameter (under the critical diameter, the grain is of single domain) of Zn_0.3Ni_0.7Fe₂O₄ nanoparticles is found to be about 30 nm.     

Magnetic viscosity-coercivity relation for Fe1−xCox fine particles

The variation of the applied field results in a subsequent change of magnetization with time. There is a relationship between the coercivity (H_c), as the equilibrium characteristic of the system, and its magnetic stability (1/S), as a parameter characterizing the time dependence. 1/S as a function of H_c has been measured and studied for different Fe_1−xCo_x samples. We synthesized several samples with different values of x by applying various magnetic fields during the grains’ growth, and observed a linear relationship between 1/S and H_c.     

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.     

Magnetic and electrical properties of amorphous Ge1−xCrx thin films grown by low temperature vapor deposition

Amorphous Ge_1−xCr_x thin films are deposited on (1 0 0)Si by using a thermal evaporator. Amorphous phase is obtained when Cr concentration is lower than 30.7 at%. The electrical resistivities are 1.89×10⁻³–0.96×10² Ω cm at 300 K, and decrease with Cr concentration. The Ge_1−xCr_x thin films are p-type. The hole concentrations are 5×10¹⁶–7×10²¹ cm⁻³ at 300 K, and increase with Cr concentration. Magnetizations are 7.60–1.57 emu/cm³ at 5 K in the applied field of 2 T. The magnetizations decrease with Cr concentration and temperature. Magnetization characteristics show that the Ge_1−xCr_x thin films are paramagnetic.     

Magnetic behavior of MnAs precipitates in Ga1−xMnxAs diluted magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x≈4.7–5.5%) were grown on (1 0 0) GaAs substrates by molecular beam epitaxy. These p-type (Ga,Mn)As films were revealed to have a ferromagnetic structure and ferromagnetism is observed up to a Curie temperature of 318 K, which is ascribed to the presence of MnAs secondary magnetic phases within the film. It is highly likely that the phase segregation occurs due to the high Mn cell temperature around 890–920 °C, as it is well established that GaMnAs is unstable at such a high temperature. The MnAs precipitate in the samples with x≈4.7–5.5% has a Curie temperature T_c≈318 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.     

Soft magnetic properties and giant magneto-impedance effect of Fe73.5−xCrxSi13.5B9Nb3Au1 (x=1–5) alloys

In this paper, the effect of microstructural and surface morphological developments on the soft magnetic properties and giant magneto-impedance (GMI) effect of Fe_73.5−xCr_xSi_13.5B₉Nb₃Au₁ (x=1, 2, 3, 4, 5) alloys was investigated. It was found that the Cr addition causes slight decrease in the mean grain size of α-Fe(Si) grains. AFM results indicated a large variation of surface morphology of density and size of protrusions along the ribbon plane due to structural changes caused by thermal treatments with increasing Cr content. Ultrasoft magnetic properties such as the increase of magnetic permeability and the decrease of coercivity were observed in the samples annealed at 540 °C for 30 min. Accordingly, the GMI effect was also observed in the annealed samples.     

Structural and magnetic properties of Co1−xZnxFe2O4 nanoparticles by co-precipitation method

Co_1−xZn_xFe₂O₄ nanoparticles were prepared by co-precipitation method with x varying from 0 to 1.0. The powder samples were characterized by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Fourier transform infrared spectroscopy (FTIR). The average crystallite sizes of the particles were determined from XRD. X-ray analysis showed that the samples were cubic spinel. The average crystallite size (D_aveXR) of the particles precipitated was found to vary from 6.92 to 12.02 nm decreasing with the increase in zinc substitution. The lattice constant (a_o) increased with the increase in zinc substitution. The specific saturation magnetization (M_S) of the particles was measured at room temperature. The magnetic parameters such as M_S, H_c, and M_r were found to decrease with the increase in zinc substitution. FTIR spectra of the Co_1−xZn_xFe₂O₄ with x varying from 0 to 1.0 in the range 400–4000 cm⁻¹ were reported. The spinel structure and the crystalline water adsorption of Co_1−xZn_xFe₂O₄ nanoparticles were studied by using FTIR.     

Magnetic properties of YFe12−xMox compounds and magnetocaloric effect of YFe9.5Mo2.5

The characterization and magnetic properties of YFe_12−xMo_x (x=2.0, 2.5 and 3.0) with the ThMn₁₂-type structure, and the magnetocaloric effect of YFe_9.5Mo_2.5 were investigated. A directional growth was observed in YFe₁₀Mo₂ alloy. A broad peak in the zero-field-cooling (ZFC) magnetization curve of the YFe_12−xMo_x compounds is ascribed to the existence of ferromagnetic clusters with different site moments and scattered orientations of the moments. The broad range of the peak is reduced with increasing Mo content. A weak peak is observed near 190 K in the ZFC curve of YFe₉Mo₃, which is associated with the 8i sites being mostly occupied by Mo atoms. YFe_9.5Mo_2.5 has a magnetic entropy change of −1.09 J/kg K for a field change of 5 T at 277 K.     

Structural and magnetic properties of Mg(In2−xMnx)O4 system

We synthesized the Mn-doped Mg(In_2−xMn_x)O₄ oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn³⁺ ions to the larger In³⁺ ions. The undoped MgIn₂O₄ oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10⁻³ emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO₂, Mn₂O₃ oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.     

Structure and magnetic phase diagram of mixed rare-earth Nd1−xTbxFe10.5Mo1.5 compounds

The structural and magnetic properties of Nd_1−xTb_xFe_10.5Mo_1.5 (x=0$x=0$, 0.2, 0.4, 0.6, 0.8, 1.0) compounds have been investigated by means of X-ray diffraction and magnetic measurements. All the investigated compounds crystallize in the tetragonal ThMn₁₂-type structure with I4/mmm space group. The lattice parameters a, c and the unit-cell volume V decrease with increasing x. The Curie temperatures T_C are almost independent x. There exists a unique spin-reorientation transition for the end compositions of Nd_1−xTb_xFe_10.5Mo_1.5 compounds with x=0$x=0$ and x=1$x=1$, while two spin-reorientation transitions are observed for x=0.2–0.8$x=0.2–0.8$. The room-temperature magnetocrystalline anisotropy of Nd_1−xTb_xFe_10.5Mo_1.5 compounds changes from uniaxial to planar with increasing x content. Based on magnetic measurements, a magnetic phase diagram of Nd_1−xTb_xFe_10.5Mo_1.5 compounds is constructed. By minimizing the magnetocrystalline anisotropy energy, a theoretical magnetic phase diagram for the Nd_1−xTb_xFe_10.5Mo_1.5 system is derived, showing a reasonable agreement with the observations.     

Analysis of Raman scattering of Ga1−xMnxAs dilute magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x=1.4-3.0%) grown on (1 0 0) GaAs substrates by molecular beam epitaxy were characterized using Raman spectroscopy. As Mn is introduced into GaAs, a marked increase in disorder in the material occurs, as indicated by the growth of the disorder-allowed transverse-optical Raman line. Another important result is that as the Mn concentration in Ga_1−xMn_xAs increases further beyond ca. 2%, Raman-active coupled-plasmon-longitudinal-optical phonon modes arise, which signals the increasing presence of holes, and thus provides a useful tool for determining their concentration. Using the depletion-layer approach from the Raman spectroscopy data, we determined the carrier concentration for samples with x=2.2% and 3.0% was to be 7.2×10¹⁹ and 8.3×10²⁰ cm⁻³, respectively.     

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.     

Enhancement of initial permeability due to Mn substitution in polycrystalline Ni0.50−xMnxZn0.50Fe2O4

The structural and magnetic properties of Mn substituted Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.00, 0.10 and 0.20) sintered at various temperatures have been investigated thoroughly. The lattice parameter, average grain size and initial permeability increase with Mn substitution. Both bulk density and initial permeability increase with increasing sintering temperature from 1250 to 1300 °C and above 1300 °C they decrease. The Ni_0.30Mn_0.20Zn_0.50Fe₂O₄ sintered at 1300 °C shows the highest relative quality factor and highest initial permeability among the studied samples. The initial permeability strongly depends on average grain size and intragranular porosity. From the magnetization as a function of applied magnetic field, M(H), it is clear that at room temperature all samples are in ferrimagnetic state. The number of Bohr magneton, n(μ_B), and Neel temperature, T_N, decrease with increasing Mn substitution. It is found that Mn substitution in Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.20) decreases the Neel temperature by 25% but increases the initial permeability by 76%. Possible explanation for the observed characteristics of microstructure, initial permeability, DC magnetization and Neel temperature of the studied samples are discussed.     

Structure and magnetic properties of (Nd,Dy)16(Fe,Co)76−xTixB8 powders prepared by mechanical alloying

Nanocrystalline (Nd,Dy)₁₆(Fe,Co)_76−xTi_xB₈ magnets were prepared by mechanical alloying and respective heat treatment at 973–1073 K/30–60 min. An addition of 0.5 at % of Ti results in an increase of coercivity from 796 to 1115 kA m⁻¹. Partial substitution of Nd by Dy results in an additional increase of coercivity up to 1234 kA m⁻¹. Mössbauer investigations shows that for x?1 the (Nd,Dy)₁₆(Fe,Co)_76−xTi_xB₈ powders are single phase. For higher Ti contents (x>1) the mechanically alloyed powders heat treated at 973 K are no more single phase, and the coercivity decreases due to the presence of an amorphous phase. A heat treatment at a higher temperature (1073 K) for longer time (1 h) results in the full recrystallisation of powders. The mean hyperfine field of the Nd₂Fe₁₄B phase decreases for titanium contents of 0?x?1, and remains constant for x>1. This indicates that the Ti content in the Nd₂Fe₁₄B phase reaches its maximum value.     

Structural,electrical transport,and magnetic properties of Ni1−xZnxFe2O4

Structural, electrical, and magnetic properties of Ni_1−xZn_xFe₂O₄ (x=0.2, 0.4) samples sintered at various temperatures have been investigated thoroughly. The bulk density of the Ni_0.8Zn_0.2Fe₂O₄ samples increases as the sintering temperature (T_s) increases from 1200 to 1300 °C and above 1300 °C the bulk density decreases slightly. The Ni_0.6Zn_0.4Fe₂O₄ samples show similar behavior of changes to that of Ni_0.8Zn_0.2Fe₂O₄ samples, except that the bulk density is found to be the highest at 1350 °C. The DC electrical resistivity, ρ(T)$\rho (T)$, decreases as the temperature increases indicating that the samples have semiconductor-like behavior. As the Zn content increases, the Curie temperature (T_c), resistivity, and the activation energy decrease while the magnetization, initial permeability, and the relative quality factor (Q) increases. A Hopkinson peak is obtained near T_c in the real part of the initial permeability vs. temperature curves. The ferrite with higher permeability has a relatively lower resonance frequency. The initial permeability and magnetization of the samples has been found to correlate with density, average grain sizes. Possible explanation for the observed structural, magnetic, and changes of resistivity behavior with various Zn content are discussed.     

Spin-glass-like phase behaviour in CeNi2−xCuxSn2

We report the results of our investigation in CeNi_2−xCu_xSn₂ (x=0, 0.4, 1.0, 1.6 and 2.0), a new pseudoternary series with CaBe₂Ge₂-type tetragonal structure. Substitution of Cu for Ni leads to a linear increase in the constants a, c and the unit cell volume v. As probed by the low temperature dependence of ac susceptibility χ_ac(T), the T_f temperature, which corresponds to the freezing temperature of the spin-glass clusters, is annihilated above 2.0 K significantly for the samples with x≥1.6. This observation proves conclusively that the Ni-rich samples in the series CeNi_2−xCu_xSn₂ have the advantage of forming the spin-glass-like state.     

Magnetic properties of nanocrystalline Co1−xZnxFe2O4 prepared by forced hydrolysis method

Nanocrystalline zinc-substituted cobalt ferrite powders, Co_1−xZn_xFe₂O₄ (x=0, 0.2, 0.4), were for the first time prepared by forced hydrolysis method. Magnetic and structural properties in these specimens were investigated. The average crystallite size is about 3.0 nm. When the zinc substitution increases from x=0 to x=0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g and the coercive field decreases from 1.22 to 0.71 T. All samples are superparamagnetic at room temperature and ferrimagnetic at temperatures below the blocking temperature. The high value of the saturation magnetization and the very thin thickness of the disorder surface layer of all samples suggests that this forced hydrolysis method is suitable not only for preparing two metal element systems but also for three or more ones.