Abstract

The polycrystalline manganite Pr_0.55Ca_xSr_0.45-xMnO₃ (x = 0.00, 0.05, 0.1, and 0.2) has been prepared by the solid state reaction technique to investigate its crystal structure, magnetic, and magnetocaloric properties. Critical behaviour around PM-FM phase transition has also been analyzed through various methods including modified Arrott plots (MAP), and critical isotherm analysis of Pr_0.55Ca_xSr_0.45-xMnO₃ (x = 0.00, 0.05, 0.1, and 0.2). XRD analysis reveals that all the samples are found to be crystallized in the orthorhombic system with Pnma space group and lattice parameters a, b and c as well as the cell volume are found to decrease with increasing Ca (x) content. Microstructure is observed with the field emission scanning electron microscopy (FESEM) photograph and elemental compositions are determined by energy dispersive X-ray diffractometer (EDX). Temperature and field dependent magnetization measurement disclose that all the samples undergo second-order FM to PM phase transition but the Curie temperature (T_C) value decreases from 290 K to 245 K with the increase in Ca (x) content from 0.00 to 0.20. The magnetocaloric effect (MCE) in terms of maximum entropy change, (-ΔS_m)_max and relative cooling power (RCP) was calculated from isothermal magnetization measurements around T_C, using Maxwell’s thermodynamic relations. Both (-ΔS_m)_max and RCP increases with increasing Ca content suggests the suitability of this compound as a potential solid state refrigerent. Contribution of itinerant electron in the entropy change is found from modulating MCE with Landau theory of phase transition.

Abstract

CoPt nanoparticles (NPs) were synthesized by sol-gel route from metal chloride precursors. NPs annealed at 800 ◦C under hydrogen atmosphere which reveals the formation of bimetallic L10 face centered tetragonal (FCT) CoPt alloy NPs confirmed by XRD study and selected area diffraction (SAED) pattern. Here the magnetic properties were investigated from room temperature down to 77 K by vibrating sample magnetometer (VSM). Magnetic measurement revealed that NPs possess ferromagnetic property with a high coercivity 143.24 kAm􀀀 1 and remanence because of high anisotropic behavior. However, low temperature magnetization shows an anomalous antiferromagnetic nature of CoPt NPs due to spin canting effect. The band structure and electronic density of states (DOS) calculation indicate the metallic nature of the L10 FCT CoPt NPs. The magnetic moment
arises due to electronic spin polarization which is found to be 2.38 μB, calculated using spin density of states (SDOS).

Abstract

This paper reports on the structural and magnetic properties of ball-milled powder sample of (Fe_1−xMn_x)₇₅P₁₅C₁₀ (x = 0, 0.05, 0.1, 0.2 & 0.3) mixed metallic amorphous ribbons.XRD patterns of the as-made powder samples demonstrate a structural phase transition from amorphous to the nanocrystalline structure having a tetragonal (P) type structure.FESEM micrographs show that the particles form nanocrystalline structure presumably due to the stress relaxation upon ball milling the amorphous ribbons. The observed phase transformation and the changed magnetic properties e.g., significant enhancement in the coercive field leading to magnetic hardness are attributed to the controlled milling and also Mn substitution in this mixed metallic alloy system, which is the novelty of this research work.

Abstract

Conventional solid-state reaction technique was used to prepare the studied samples La_0.55Ca_xSr_0.45-xMnO₃ (x = 0.0, 0.1, 0.2). Their structural and magnetic properties have been investigated to find the effect of calcium doping on the D.C. and A.C. magnetic properties of the compounds. Room temperature XRD investigations confirm rhombohedral structure with space group R-3c for all the samples. Lattice parameter is found to decrease with increase in Ca(x) content. Microstructure was examined from the images taken by Scanning Electron Microscopy (SEM) and chemical compositions were determined by an energy dispersive X-ray diffractometer (EDX). The zero-field cooled (ZFC) and field cooled (FC) DC magnetization measurements reveal paramagnetic (PM) to ferromagnetic (FM) transition in them upon cooling through their Curie temperature (T_C). But the FM state coexists with the residual canted anti-ferromagnetic state rather than a pure FM state in the samples. The T_C values are dependent on Ca(x) content of the samples. Moreover, XRD analysis and EDX data reveals the presence of Mn deficiency in the studied samples. Presence of minor secondary Mn₃O₄ phase creates this deficiency resulting a significant decrease in T_C and strongly influences the magnetic properties. The values of A.C. permeability (μʹ) also depend on Ca(x) content as well as grain size that can be explained by Globus equation. However, μʹ values of the samples remain almost constant over a wide range of frequency which indicates the compositional stability of the perovskites. The magnetic modulus demonstrates the dynamics of their A.C. permeability by screening out the stray/any other effects in them. The Nyquist plots of magnetic modulus of the samples show the type of their magnetic relaxation.

Abstract

This paper reports on the experimental findings of ac conductivity as determined from the measurement of conductance by using the Waynekerr 6500B impedance analyzer for the stoichiometric and non-stoichiometric manganese doped cobalt ferrite nanoparticles. In the dispersion of ac conductivity over the frequency band 100Hz – 1MHz, two distinct conductive bands 100Hz – 1 kHz and 1 kHz – 1MHz for stoichiometric compositions, and 100Hz – 3 kHz and 3 kHz – 1MHz for non-stoichiometric compositions are observed. On the other hand, well-resolved peaks are observed in the dispersion of ac conductivity over the frequency band 1 – 120MHz that corresponds to the hopping resonance for both the compositions. The ac conductivity is found to be tunable by the Mn content and selectable by the frequency between cutoff frequencies for both the compositions. A comparatively higher magnitude of ac conductivity is observed for the representative sample of non-stoichiometric composition and a lower magnitude for the stoichiometric compositions. The presence of tunability of ac conductivity in the studied samples is, therefore, a concern on which basis, the Joule’s heat and Joule’s heat per unit time have been determined for the studied samples by using the usual formula and also found to be tunable by the manganese content. This tunability of the generated Joule’s heat may make these possible to use in the temperature switching/sensing, and/or thermoelectric/thermomagnetic devices.

Abstract

This paper reports on the complex impedance in stoichiometric and non-stoichiometric manganese doped cobalt nanoferrites. The decreasing trend in their impedance with the frequency shows an increasing trend in their conductivity, which is their normal behavior. The stoichiometric composition shows only the short-range mobility of charge carriers wherein n-type (Fe2+/Fe3+) charge carriers play a dominant role and the nonstoichiometric composition demonstrates both the longrange and short-range mobility of charge carriers in their respective Nyquist plot. The p-type (Co2+/Co3+) charge carriers are found to play a predominant role in the conduction mechanism of the non-stoichiometric composition. Thus, the complex impedance marks a single metallic band in the stoichiometric composition and a double metallic band in the non-stoichiometric composition. This double metallic band may make this sample suitable to be used in the switching as well as actuator devices. Conversely, the higher conductivity in the single metallic band is expected to generate heat by the eddy current loss for the stoichiometric composition, which may make it suitable to be used in hyperthermia and medical science-related research.

Abstract

The investigated samples of manganese doped cobalt nanoferrites were prepared by the conventional solid-state reaction route using a planetary ball milling technique. The XRD patterns confirm their crystallinity and single-phase formation of spinel structure. The average particle size is obtained in the nanoscale range from their field emission scanning micrographs. Their spectroscopic analysis was performed from the measurement of dielectric constant and permeability for microstructural correlation. In the spectra of the modulus, the appeared peaks at a particular frequency set the boundary between long-range and short-range mobility of charge carriers in the material. The enumerated dielectric relaxation time constant is found to be a maximum at a characteristic temperature, which signifies the ferromagnetic-to-spin-glass state transition. The activation energy in the long-range mobility wing is found to be greater than that at the short-range mobility wing. The observed increasing trend in the real part of the magnetic modulus with the concentration level may be attributed to the dominance of dipolar orientations across the grain boundaries whereas it’s decreasing trend is correlated with the decreasing trend in the crystallite/grain size. The temperature response of relaxation time constant, determined from the well-resolved peaks of the dispersion of imaginary part of magnetic modulus, exhibits the transformation of ferromagnetic phase to the glass transition at a particular temperature in the low-temperature regime due to ceasing of wall motion and spatially randomized spins by the freezing effects.

Abstract

The polycrystalline perovskite La_0.7Ca_0.2Pb_0.1CoO₃ has been prepared by the conventional solid-state reaction technique, and then the structural, magnetic, magnetocaloric, and dielectric properties of the sample have been investigated. The monoclinic structure with space group I2/a of the sample is confirmed by X-ray diffraction investigation at room temperature. The microstructure was examined by scanning electron microscopy (SEM), and chemical composition was determined using an energy dispersive X-ray diffractometer attached to the SEM. Magnetic measurement reveals that the sample undergoes ferromagnetic to paramagnetic transition with increasing temperature and behaves as a soft magnetic material. Field cooled and zero-field cooled dc magnetization curves at low field and low temperature show divergence, indicating the coexistence of antiferromagnetic and ferro-magnetic clusters in the sample. Magnetic inhomogeneity of the sample has also been clearly confirmed by the divergence between the Curie temperature, T_c, and the paramagnetic Curie temperature, θ. The sample also shows a magnetocaloric effect at a very low field (H = 0.01 T). The high negative value of the real part of complex permittivity, ε′, reveals metallic behavior of the sample at low frequency around room temperature. However, the frequency dependent ac conductivity (σ) exhibits three distinct bands, namely, the metallic, insulating, and anomalous band, around room temperature. Temperature dependent ε′ reveals that the sample undergoes insulator to metallic transition above −20 °C.

Abstract

Polycrystalline perovskite La_0.55Sr_0.45MnO₃ has been prepared by the conventional solid-state reaction route using planetary ball milling technique. The structural properties have been investigated using XRD and SEM data. Rhombohedral structure with space group R-3c of the sample is confirmed by room temperature XRD investigation. Microstructure and chemical composition were determined by Scanning Electron Microscopy (SEM), which includes an energy dispersive X-ray diffractometer (EDX). The study of the dielectric properties of the sample reveals that La_0.55Sr_0.45MnO₃ possesses a high dielectric constant at room temperature and at low frequencies. The real part of the dielectric constant ε’ is also observed to be strongly dependent on the temperature and the best properties are found in the sample at room temperature for which ε’ ≈10⁴ up to f = 4x10⁶ Hz. The dispersion of dielectric constants at low frequency is explained in the light of Maxwell–Wagner model. A pronounced increase in Capacitance C is observed from 273 K to 298 K and likely to be caused by the redistribution of ions due to the thermal agitation. The resistivity decreases with increasing temperature indicating that the sample has a semiconductor-like behavior. The constant value of μ’ over a wide range of frequency indicates the compositional stability and the range of the suitability of the perovskite. A significant decrease of ac permeability has occurred around 298 K indicating a magnetic phase transition from ferromagnetic to paramagnetic state. Low temperature magnetization measurement reveals paramagnetic to ferromagnetic (FM) transition upon cooling at T_C = 244 K. But the FM state coexists with the residual canted AFM (anti-ferromagnetic) state rather than a pure FM state. Presence of external field H favors the FM phase which expands at the expense of the AFM phase. Thus M−H measurement reveals high saturation magnetization.

Abstract

This paper reports the studies on the effect of A-site substitution by strontium on the structural properties and magnetic ordering in polycrystalline perovskite Sm_0.5Ca_0.5−xSr_xMnO₃ (x = 0.1, 0.2 and 0.3). The investigated samples are prepared by conventional solid-state reaction technique. XRD analysis at room temperature has confirmed orthorhombic structure of the sample with space group Pnma. The dependence of structural parameter, Curie temperature and coercivity on Sr doping content has been thoroughly investigated. It is observed that substitution of Sr²⁺ for Ca²⁺ increases lattice parameter, tolerance factor and the Curie temperature. However, the coercivity (H_c) decreases with increasing Sr content while the charge ordering process is weakened with increasing Sr content. Field cooled (FC) and zero-field cooled (ZFC) dc magnetizations measurements at low field and low temperature indicate that there is a spin-glass (SG) like state occurred. Temperature dependent ac susceptibility at different frequency indicates a spin-glass-like transition of the sample.

Abstract

This paper reports a series of investigations on the frequency and temperature dependent dielectric and magnetic properties of manganese doped cobalt ferrite nanoparticles. The investigated samples were prepared via solid state reaction using planetary ball milling. The nature in the variation of the relative dielectric constant of the investigated sample with Mn content (x) = 0.375 and 0.5 as a function of frequency demonstrates the normal behavior for the selected temperature at 77 K and 300 K. However, the relaxation single peaks are noted at 100 kHz for selected temperatures of 148 K, 165 K, and 236 K, which mark the ferrimagnetic-to-ferromagnetic phase transition. This transition may have originated from the dominance of anisotropy energy at a lower temperature. The values of relative dielectric constant for the samples are found to be higher than that at room temperature, which shows the relative dielectric constant to be strongly dependent on temperature. Their D-factor values are observed to be much lower in the low temperature region (below room temperature) and exhibit normal variation with an increase of frequency. The higher values of relative dielectric constant may make the samples suitable to be used in spaceborne applications. The enhanced saturation magnetization is observed due to calcination at elevated temperature (900°C). The effect of Mn content (x) is found to lessen the saturation magnetization due to the substitution of Fe³⁺ cation in the B site according to Neel’s two-sublattice model. The decreasing nature in the real part of permeability matches the spin-glass of the materials of the investigated system due to the tendency of their frozen spins in the lower temperature region. In addition, the decreasing trend in the Weiss constant with Mn content is also the signature of the spin-glass of the materials in the lower temperature region due to the decreased magnetic exchange interactions.

Abstract

This paper reports a series of investigations on the frequency and temperature dependent intrinsic electric properties of manganese doped cobalt ferrite nanoparticles. The investigated samples were prepared via solid state reaction route using the planetary ball milling technique. The XRD patterns confirm their crystallinity and single phase spinel structure. The Field Emission Electron Scanning Microscopic (FE-SEM) micrographs show the agglomerated particle with a nearly spherical shape. The estimated lattice constant is found to decrease but the crystallites size to increase with the Mn content. The ac resistivity is found to decrease exponentially with the increase of frequency due to the effect of multilayer capacitance. Conversely, the ac conductivity is observed to increase exponentially with the frequency due to increased hopping between metallic cationsFe²⁺and Fe³⁺. The temperature dependent ac resistivity exhibits the semiconducting behavior of the materials above room temperature. The variation in both the resistivity and conductivity is assumed to follow the Arrhenius equation at a selected frequency of 10 kHz. The estimated activation energy using the Arrhenius equation of ac conductivity in the ferro region is found to increase with the Mn content. This increasing trend in activation energy signifies ferrimagnetic-to-paramagnetic phase transition. Besides, the electric modulus shows the contributions of both the grain and grain boundaries to the conduction mechanism in the materials. This nature of temperature dependent resistivity is as usual for the majority of nanoferrite materials.

Abstract

  This research demonstrated the development of graphene oxide (GO)-gold nanocrystals (AuNCs) composite-modified glassy carbon electrode (GCE) for the sensitive detection of dopamine (DA), uric acid (UA), and 4-aminophenol (4-AP). The GO was synthesized by modified Hummer's method, which was utilized to prepare GO-AuNCs composites by in situ synthesis method using sodium l(-) malate as a reducing agent. Morphological, X-ray diffraction, and spectral analysis revealed the homogeneous formation of AuNCs with high crystallinity and purity on the GO surface and the sizes of the NCs were decreased with increasing the concentrations of sodium l(-) malate. The optimal GCE/GO-AuNCs sensor exhibited excellent electrocatalytic activity towards the oxidation of DA, UA, and 4-AP. The sensor showed interference-free and selective detection of DA and UA with sensitivities of ca. 30.3 and 17.28 μA/cm²/μM, respectively, and detection limits of ca. 28 and 50 nM, respectively, with wider dynamic ranges, measured by differential pulse voltammetry (DPV) technique. Whereas, it displayed a sensitivity and detection limit of ca. 5.70 μA/cm²/mM and 0.017 nM, respectively, for the detection of 4-AP, using current density (J)-voltage (V) measurement method. The sensor also revealed excellent stability, reproducibility, and recoveries of DA, UA, and 4-AP in real samples.

Abstract

This paper reports the effect of Mn substitution in (Fe1−xMnx)75P15C10(0≤x≤0.3) amorphous alloys, which is synthesized by rapidly cooled melt spun technique. The magnetic properties of the fabricated ribbons have shown significant changes upon Mn substitution. M-H measurement at room temperature has shown a gradual transition from soft-to-hard magnetic phase with the increase of Mn concentration. The magnetoresistance is found to increase with Mn concentration at room temperature. The ribbons exhibit high effective ac permeability with low magnetic losses in the high frequency regime. The observed magnetic properties suggest that the antiferromagnetic interactions introduced by Mn atoms cause deviations from a pure ferromagnetic structure at low Mn concentrations.

Abstract

BiFeO₃ is a very promising material for future technical applications. The effects of the different size and valance state of Bi³⁺ and Ba²⁺ as A-site cations in BiFeO₃ (BFO) has been studied. The introduction of large-size Ba ions at Bi site resulted in significant lattice strain and a partial structural phase transformation from rhombohedral R3c to cubic Pm3¯mPm3¯m phase. Significant changes in the Raman spectrum, including variation in intensities and full-width at half-maxima, shifts in peak positions, and overlapping of Raman modes, were observed on incorporation of Ba into the lattice. The overlapping of various Raman modes for Ba-doped compositions confirmed the structural transformation. The decrease in intensity of A modes can be ascribed to replacement of Bi by Ba atoms, while the softening of E modes indicates presence of oxygen vacancies and suggests substantial changes in and destabilization of FeO₆octahedra. Destabilization of FeO₆ octahedra was confirmed by Fourier-transform infrared spectroscopy. Optical spectra showed the presence of charge transfer and doubly degenerate d–d transitions. A slight red-shift in the d–d transition was observed, attributed to variation in the crystal field strength. The bandgap increased with increasing Ba incorporation, which is related to weakening of dipolar moments due to the structural transformation from a noncentrosymmetric to centrosymmetric structure. Such better understanding of the mechanisms governing the optical response of BFO could enable researchers to engineer the bandgap and conductivity for enhanced photoferroelectric properties.

Abstract

This paper reports the significances of the negative real part of ac permeability and the transition of real part of ac permittivity. The paper also identifies the μ negative media and double negative media in the frequency range 3–120 MHz. The eddy current loss calculated from the measured complex permeability is found to diminish with increasing frequency which is an important factor to be considered for high frequency applications of these materials.

Abstract

Polycrystalline Bi_1−xBa_xFeO₃ (0 ≤ x ≤ 0.30) samples have been prepared by two stage solid state reaction method. The surface texture of the prepared material exhibited a uniform grain distribution with minor empty space suggesting that the density of the pelletized samples is high and grain size is found to decrease with Ba substitution. An important observation is the stabilization of a dielectric anomaly near the magnetic transition temperature in temperature dependent dielectric studies which is ascribed to the presence of magnetoelectric (ME) coupling. This ME coupling effect has been demonstrated by measuring the effect of varying magnetic fields on the dielectric constant. The dielectric constant increased with increasing applied magnetic field giving a coupling coefficient (ε_r(H) − ε_r(0))/ε_r(0) of 1% at H = 5 kOe at room temperature which demonstrate the importance of these material from application point of view. The dielectric constant is high at low frequencies and is found to decrease at moderate frequencies indicating the Maxwell–Wagner contribution to the dielectric response. The increase in Ba content and temperature resulted in an increase in bulk resistance and exhibits a typical negative temperature coefficient of resistance behavior. A temperature dependent and non-Debye type of relaxation process is revealed by impedance spectroscopic studies. The conductivity spectra revealed the presence of hopping mechanism in the electrical transport process and the activation energies lying in the range from 0.45 to 0.70 eV indicate that the conduction of oxygen vacancies through hopping mechanism.

Abstract

This work reports the synthesis and studies of semi-soft ferrimagnetic CoFe₂O₄ nanoparticles using sol–gel method. The X-ray diffraction patterns confirm the formation of cubic spinel CoFe₂O₄
nanoparticles. The average crystallite size was found from XRD data is about 30 nm. The high resolution transmission electron microscopy analysis shows that nanoparticles are highly crystalline. The magnetic properties reveal that the particles are ferrimagnetically ordered soft magnetic materials with coercive field of 620 Oe and saturation magnetization Ms = 60 emug⁻¹. The higher value of saturation magnetization is due to the ordered single-domains magnetic nanoparticles and lower magnitude of coercivity is attributed to the decrease of interparticle interactions and magneto-elastic anisotropy. The value of remanence ratio R < 0.5 is responsible for magnetostatic interactions of the particles. The nanoparticles possess low values of dielectric constant which decreased with increasing frequency. The low dielectric constant makes the nanoparticles as a promising candidate for high frequency magnetic devices.

Abstract

This paper reports on the magnetocaloric effect exploited from the manganese doped cobalt ferrite nanoparticles using indirect method. The optimum values of magnetic entropy change, adiabatic temperature change, refrigeration capacity, and maximum specific cooling energy are found to be -0.37J kg -1K -1 , 65.01K, 24.05 J kg -1and 192.95J kg -1 under the magnetic field difference 0.2 T between 0.2 – 0.4 T below 200 K temperature range. As such the investigated materials are expected to be suitable for the applications in low temperature physics and also promising for space born applications.

Abstract

The electrical and magnetic properties of polycrystalline as-sintered and annealed Co_0.3Mn_0.2Zn_0.5Fe₂O₄prepared by using conventional electroceramic processing steps in the laboratory are presented. The microstructures are examined via scanning electron micrographs and energy dispersive X-ray (EDX)analysis. The ac electrical behavior represented via complex plane plots in the frequency range 160–211 MHz reveals inductive contribution via resonance originated within the material system. The ac permeability is presented in the frequency range 50–300 MHz affirmed resonance behavior. The inductive response in the complex planes (impedance Z and modulus M) reflecting resonance behavior due to magnetic softness is obvious around 140 MHz whereas in the permeability dispersion negative domain is observed at about150 MHz attributing to the spin rotation.

Abstract

Abstract

(Fe_100-xV_x)₇₅P₁₅C₁₀ (x=0, 5, 10 and 15) semi-amorphous alloys were prepared by the melt spinning technique and their structural, thermal and magnetic properties were investigated. The structure of the alloys was studied by X-ray diffraction (XRD) for annealing temperatures of 400 ºC to 650 ºC within a 30 minute annealing time. The XRD patterns show that the samples contain a BCC structure for temperatures between 400 ºC to 450 ºC and a hexagonal structure for the temperatures between 500 ºC to 650 ºC. The lattice parameter of the BCC structure changes from 2.854 to 2.870 Å with the temperature. The lattice parameters a and c for the hexagonal structure are (5.011- 5.045) Å and (13.676-13.822) Å, respectively. The grain size is found to vary from 10 to 60 nm. The recrystallization temperatures of the amorphous phase volume and weight changes were measured by differential thermal analysis (DTA) and thermogravimetry (TG), respectively. The saturation magnetization gradually decreases with the increase of the substitution of Fe by V at room temperature (RT).

Abstract

A melt-spun Fe₇₁(NdPr)₄Co₅Cu_0.5Nb₁B_18.5 amorphous alloy has been studied for their exchange spring behavior in the nanocrystalline state and hence its potential use as an exchange spring magnet. Steep recoil curves of the samples annealed at 870 and 970 K show the typical spring exchange characteristic with smaller loop area. Temperature dependence of (BH)_max and the remanence ratio M_r/M_s have been investigated over a wide range of temperature from 5 up to 380 K using a superconducting quantum interference device (SQUID). It is observed that the temperature dependence of these parameters decrease with increasing temperature indicating gradual weakening of exchange coupling energy between the magnetic nanograins with the rise of temperature. A change in the shape of the hysteresis loop is observed below 100 K indicating a possible spin reorientation in the hard magnetic phase.

Abstract

Among the perovskite manganites, a series of La_1-xCa_xMnO₃ has the largest magneto-caloric effect (MCE) (|ΔS_m|_max=3.2-6.7 J/kg K at ΔH=13.5 kOe), but the Curie temperatures, T_C, are quite low (165-270 K). The system of LaSrMnO₃ has quite high T_C but its MCE is not so large. The manganites La_0.7(Ca_1-xSr_x)_0.3MnO₃ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25) have been prepared by solid state reaction technique with an expectation of large MCE at room temperature region. The samples are of single phase with orthorhombic structure. The lattice parameters as well as the volume of unit cell are continuously increased with the increase of x due to large Sr²⁺ ions substituted for smaller Ca²⁺ ions. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic measurements at low field and low temperatures indicate that there is a spin-glass like (or cluster glass) state occurred. The Curie temperature T_C increases continuously from 258 K (for x=0) to 293 K (for x=0.25). A large MCE of 5 J/kg K has been observed around 293 K at the magnetic field change ΔH=13.5 kOe for the sample x=0.25. The studied samples can be considered as giant magneto-caloric materials, which is an excellent candidate for magnetic refrigeration at room temperature region.

Abstract

Recoil hysteresis and temperature dependence of coercivity have been measured for the sample of composition Nd₄Fe_73.5Co₃Hf_0.5Ga_0.5B_18.5 prepared by rapid quenching method with a wheel speed of 25 m/s and after subsequent annealing in an evacuated quartz tube under a pressure of 10^–5 mbar at different temperatures and times. In the optimum and over‐annealed condition, recoil hysteresis loops have been measured by the application of progressively increasing negative field and successive withdrawal to the remanence and found to have steep recoil loop and small recoil area, which is characteristic to exchange‐biased system. Temperature dependence of coercivity in the range of 5 K to 380 K has been measured for the sample of composition Nd₄Fe_73.5Co₃Hf_0.5Ga_0.5B_18.5. Variation in the shape of the hysteresis loop has been observed in this temperature range. In the low temperature region i.e. below 150 K hysteresis loops are constricted due to spin reorientation of Nd₂Fe₁₄B at 135 K while above this temperature the hysteresis loops are characterized by convex M (H) curve similar to single‐phase permanent magnet. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract

Among perovskite manganites, a series La_1−xCa_xMnO₃ has the largest magnetocaloric effect (MCE) (|ΔS_m|_max=3.2–6.7 J/kg K in ΔH=1.35 T) but their Curie temperatures, T_C, are quite low (165–270 K). A system La_1−xPb_xMnO₃ has quite high T_C (235–360 K) but exhibits not so large MCE (0.65–1.53 J/kg K). The manganites La_0.7Ca_0.3−xPb_xMnO₃ (x=0.05, 0.10, 0.15, and 0.20) have been fabricated by solid-state reaction technique with the expectation that they could establish large MCE at room temperature region. The prepared samples are of single phase with orthorhombic structure. Lattice parameters as well as volume of unit cell are continuously increased with increasing x due to large Pb²⁺ ions substituted for smaller Ca²⁺ ions. There is spin-glass (or cluster glass)-like state occurring in studied samples. T_Cincreases continuously from 270 K (for x=0.05) to 338 K (for x=0.20). Large MCE has been obtained in all samples and |ΔS_m|_max reached the highest value of 3.72 J/kg K (with magnetic field variation ΔH=1.35 T) for the sample x=0.05 and the studied samples could be considered as giant magnetocaloric materials working for magnetic refrigeration. The magnetoresistance of samples exhibited colossal value.

Abstract

Soft magnetic properties of Co-based amorphous alloy of the composition Co₆₇Fe₄Ni₂Si₁₅B₁₂ have been investigated by isothermal heat treatment up to the conventional crystallization temperature. In the as-cast condition the Curie temperature of the sample is 272 °C and saturation magnetization is 74 emu/g. Magnetic properties undergo variation depending on the heat treatment temperature. For the heat treatment temperatures of around 420 and 490 °C, superior soft magnetic properties are obtained. For both the temperatures initial permeability, μ′ reaches value up to ten times the value of permeability in the as-received samples. Annealing effect on giant magneto-impedance has been observed for the current-driving frequencies of 4.5 and 6 MHz. Field dependence of magneto-impedance shows hysteresis at low field, which is related to the changes in the magnetization process of the sample.

Abstract

An amorphous alloy of composition (NdPr)₄Fe₇₁Co₅Cu_0.5Nb₁B_18.5 prepared by a melt spinning technique in an argon atmosphere has been studied to observe exchange‐spring behavior in the nanocrystalline state. Hysteresis loop measurements at room temperature revealed that the highest value of the coercivity, H_c, of 4.08 kOe has been obtained for the sample annealed at 913 K for 10 min. At this annealing temperature (BH)_max and M_r/M_s have been found to be 11.06 MGOe and 0.716, respectively. Steep recoil curves of the samples annealed at 873 and 973 K show typical characteristics of the exchange‐spring mechanism having smaller loop area. The temperature dependences of H_c, M_r/M_s and (BH)_max in the range of 5 to 380 K generally decrease with the increase of temperature. From the temperature dependence of the hysteresis loop parameters, it has been found that at 5 K the values of H_c and (BH)_max are 6.89 kOe and 13.72 MGOe respectively for the sample annealed at 873 K for 10 min and 6.69 kOe and 13.74 MGOe respectively for the sample annealed at 923 K for 10 min. A change in the shape of the hysteresis loops was observed at temperatures below 100 K, related to the spin reorientation in the hard magnetic phase. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract

Tb-substituted (Nd,Tb)₂Fe₁₄B/Fe₃B nanocomposite ribbons have been synthesized by melt spinning of Nd₃Tb₁Fe₇₆Cu_0.5Nb₁B_18.5 alloys. Tb substitution has significantly enhanced the value of coercivity and Curie temperature. Highest value of coercivity has been obtained as 4.76 kOe for the sample annealed at 953 K for 10 min. Curie temperature of Tb substituted sample, Nd₃Tb₁Fe₇₆Cu_0.5Nb₁B_18.5 is 549 K while Curie temperature of the sample without Tb, Nd₄Fe₇₆Cu_0.5Nb₁B_18.5 is 535 K. Recoil hysteresis loops measured along the major demagnetization curve are steep having small recoil loop area. Temperature dependence of coercivity, remanent ratio and maximum energy product have been measured for the sample annealed at 893 K and 923 K for 10 min. At 5 K, coercivity and maximum energy product of the sample annealed at 893 K for 10 min are 5.2 kOe and 11.5 MGOe respectively and the sample annealed at 923 K for 10 min are 6 kOe and 13.1 MGOe respectively.

Abstract

Soft magnetic properties of Co-based amorphous alloy of the composition Co₆₇Fe₄Ni₂Si₁₅B₁₂ have been investigated by isothermal heat treatment up to the conventional crystallization temperature. In the as-cast condition the Curie temperature of the sample is 272 °C and saturation magnetization is 74 emu/g. Magnetic properties undergo variation depending on the heat treatment temperature. For the heat treatment temperatures of around 420 and 490 °C, superior soft magnetic properties are obtained. For both the temperatures initial permeability, μ′ reaches value up to ten times the value of permeability in the as-received samples. Annealing effect on giant magneto-impedance has been observed for the current-driving frequencies of 4.5 and 6 MHz. Field dependence of magneto-impedance shows hysteresis at low field, which is related to the changes in the magnetization process of the sample.

Abstract

Co-based amorphous alloy exhibits superior soft magnetic properties after appropriate annealing treatment below the crystallization temperature by devitrification of the amorphous phase. The initiation of crystallization temperature has been found as 518 °C from differential thermal analysis for the heating rate of 10 °C/min. Ultra-soft magnetic properties manifested by enhanced initial permeability, μ′ of the order of 5.1 × 10⁴ and reduction of relative loss factor, tan δ/μ′ of the order of 9 × 10^− 6 has been achieved for the annealing temperature of 500 °C for 30 min. This has been achieved because at this stage local anisotropy is averaged out by exchange coupling between nanograins embedded in the residual amorphous matrix. Since enhanced magneto-impedance is a typical characteristic of Co-based amorphous alloy, field dependence of magneto-impedance has been measured for as-cast and annealed samples at current driving frequency of 4.5 MHz. Field dependence of magneto-impedance shows hysteresis at low field, which is related to the changes in the magnetization process of the sample.

Abstract

In this study, we have investigated Cr- and Co-substituted Fe₃B/Nd₂Fe₁₄B-based nanocomposite alloy. Samples have been prepared by using melt-spin technique in an Ar atmosphere. Crystallization temperatures have been identified by differential thermal analysis. Samples have been annealed in an evacuated quartz tube using a pressure of around 10⁻⁵ mbar following differential thermal analysis. Structural analysis has been carried out by X-ray diffraction. Coercivity, remanent ratio and maximum energy product have been found to depend strongly on the annealing temperature. Highest value of coercivity of 4.6 kOe has been obtained for the sample annealed at 923 K for 20 min. Temperature dependence of hysteresis loop below 150 K is affected by the spin reorientation temperature of Nd₂Fe₁₄B at 135 K. In the temperature range of 250–380 K temperature coefficient of coercivity is −0.21%/°C, which is much lower. Recoil hysteresis loops have been measured by the application of progressively increasing negative field and successive withdrawal to the remanence and found to have small recoil area and high recoil permeability, which is characteristic of exchange-biased system.

Abstract

Temperature dependence of real part initial permeability (μ′) of Fe_75.5Cu_0.6Nb_2.4Si₁₃B_8.5 has been measured with different crystalline state of the material. Enhanced value of initial permeability by two orders of magnitude and very low value of relative loss factor of the order of 10⁻⁵ has been observed with the variation of annealing temperature and time, which shows that Fe-based alloys of this composition with lower Cu and Nb content also bears ultra-soft magnetic properties like conventional FINEMET. The results have been correlated with the volume fraction of amorphous and crystalline phases obtained by differential scanning calorimetry. Temperature dependence of initial permeability curve for the annealing temperature, T_a < 475 °C, is characterized by the single amorphous phase. The curve is characterized by the two-phase structure for 475 °C < T_a < 625 °C. The curve is characterized by presence of iron boride phase for T_a > 625 °C.

Abstract

Among perovskite manganites, a series La_1-x Ca _x MnO₃ has the largest magnetocaloric effect (MCE) (|ΔS _m|_max=3.2-6.7 J/kg K in ΔH=1.35 T) but their Curie temperatures, T _C, are quite low (165-270 K). A system La_1-x Pb _x MnO₃ has quite high T _C(235-360 K) but exhibits not so large MCE (0.65-1.53 J/kg K). The manganites La_0.7Ca_0.3-x Pb _x MnO₃ (x=0.05, 0.10, 0.15, and 0.20) have been fabricated by solid-state reaction technique with the expectation that they could establish large MCE at room temperature region. The prepared samples are of single phase with orthorhombic structure. Lattice parameters as well as volume of unit cell are continuously increased with increasing x due to large Pb²⁺ ions substituted for smaller Ca²⁺ ions. There is spin-glass (or cluster glass)-like state occurring in studied samples. T _C increases continuously from 270 K (for x=0.05) to 338 K (for x=0.20). Large MCE has been obtained in all samples and vertical bar ΔS _m vertical bar_max reached the highest value of 3.72 J/kg K (with magnetic field variation ΔH=1.35 T) for the sample x=0.05 and the studied samples could be considered as giant magnetocaloric materials working for magnetic refrigeration. The magnetoresistance of samples exhibited colossal value.

Abstract

Magnetic softening and its correlation with the giant magneto‐impedance effect have been investigated for Co₆₈Fe₄Ni₁Si₁₅B₁₂ subjected to different annealing temperatures. In the as‐cast condition the Curie temperature is 230 °C and the saturation magnetization is 72.3 emu/g. A very high value of initial permeability of around 59 × 10³ and a low value of relative loss factor of the order of 10^–6 have been obtained for samples annealed at 420 °C for 1 h. The sensitivity of the field dependence of the giant magneto‐impedance ratio (GMIR) for this annealing temperature is higher than that for as‐cast and other annealed samples. From X‐ray diffraction and differential scanning calorimetry results it has been found that for samples annealed at 420 °C for 1 h the ribbon is in a fully amorphous state. The extraordinary soft magnetic properties at this annealing temperature are mainly due to the structural relaxation, which determines the profile of the field dependence of the GMIR. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract

The thermoelectric power S of (Fe_1—xMn_x)₇₅P₁₅C₁₀ ferromagnetic metallic amorphous alloys with x = 0, 0.05, 0.20, 0.30 has been measured in the temperature range 30 K < T < 350 K. For all compounds, the sign of S is negative, suggesting that the transport properties are dominated by electrons. A humplike structure at low temperatures which is superimposed to a linear rise and seemingly correlated to magnetic order suggests in particular magnon drag added on diffusion thermopower as the main contributions to S(T).

Abstract

Magnetizations of the binary alloy system Ni_1−xMo_x [x(wt%) = 4, 6, 8, 10] single crystals have been measured at 4.2, 77 and 300 K, respectively, using the Vibrating Sample Magnetometer (VSM) and the Superconducting Quantum Interference Device (SQUID). The determination of magnetocrystalline anisotropy constants have been done by the integral technique using the M versus H curves in the three principal crystallographic directions [1 1 1], [1 1 0] and [1 0 0]. The effect of alloying Nickel with Molybdenum on magnetization is found to be not linear with increasing Mo concentration and the non-linearity increases more rapidly at higher Mo concentration. The ferromagnetic ordering vanishes around 8 at% Mo. The results of magnetocrystalline anisotropy constants agree with the unpublished previous data of J.E. Scott (Ph.D. Thesis, University of Sheffield, UK, 1969) determined by magnetic torque measurements on this alloy system.

Abstract

The influence of dipolar interaction in a frozen ferrofluid has been experimentally studied. The ferrofluid consisted of particles of γ- Fe2O3 with mean diameter 70 Å. Four samples with volume concentration of magnetic particles ranging from 0.03% to 17% have been investigated. The magnetic relaxation of the most concentrated particle system shows typical spin glass dynamics at low temperature, e.g., the relaxation depends on the time spent at constant temperature before applying the magnetic field—the system ages. The most diluted sample shows isolated particle dynamics and no aging.