Temperature-dependent Magnetization Curves Research Articles

Magnetic properties of core/shell-structured CoFe2/CoFe2O4 composite nano-powders synthesized via oxidation reaction

The CoFe2/CoFe2O4 composite is a new type of exchange-spring magnet that has attracted much attention in recent years. To resolve the unstable chemical properties for CoFe2/CoFe2O4 composite nanoparticles with CoFe2 shells prepared by reducing CoFe2O4, we synthesized core/shell-structured CoFe2/CoFe2O4 composite nanoparticles with CoFe2O4 shells by oxidizing CoFe2 nanoparticles. The composition and micro-structure of the composite were characterized using an X-ray diffractometer, a transmission electron microscope and a high-resolution transmission electron microscope. Magnetic measurements, including magnetic hysteresis loops, Henkel plots, and temperature-dependent magnetization curves, were performed using a vibrating sample magnetometer. The results showed that the nanoparticles were composed of CoFe2 cores and CoFe2O4 shells, and a regular non-monotonous variation of properties was found. When the mass percentage of CoFe2O4 was 30%, the average crystallite size of CoFe2 reached its maximum value, a minimum coercivity was observed, and the exchange coupling between CoFe2 and CoFe2O4 was strengthened. This non-monotonous change in properties is attributed to the lattice vacancies and spin-glass-like state occurring when the mass percentage of CoFe2O4 is very small and dominant hard magnetic behavior exists in the larger CoFe2O4 phase.

Room temperature magnetism in zinc nano ferrite synthesized by a novel oxalate-ceramic method

Zinc nano-ferrite has been synthesized using a novel oxalate-ceramic method and its magnetic properties are reported in this paper. The hysteresis loop recorded at 300 K exhibits ferrimagnetic behavior. The Neel temperature was found to be 557 K. The AC susceptibility curve also indicates ferrimagnetic behavior. The temperature dependent magnetization curves, obtained in the temperature range of 4–300 K, show spin glass behavior. Using this method of synthesis large quantity of ferrite can be synthesized therefore this method can become useful technique for industrial scale production.

Magnetic interactions in BiFe₀.₅Mn₀.₅O₃ films and BiFeO₃/BiMnO₃ superlattices.

The clear understanding of exchange interactions between magnetic ions in substituted BiFeO3 is the prerequisite for the comprehensive studies on magnetic properties. BiFe0.5Mn0.5O3 films and BiFeO3/BiMnO3 superlattices have been fabricated by pulsed laser deposition on (001) SrTiO3 substrates. Using piezoresponse force microscopy (PFM), the ferroelectricity at room temperature has been inferred from the observation of PFM hysteresis loops and electrical writing of ferroelectric domains for both samples. Spin glass behavior has been observed in both samples by temperature dependent magnetization curves and decay of thermo-remnant magnetization with time. The magnetic ordering has been studied by X-ray magnetic circular dichroism measurements, and Fe-O-Mn interaction has been confirmed to be antiferromagnetic (AF). The observed spin glass in BiFe0.5Mn0.5O3 films has been attributed to cluster spin glass due to Mn-rich ferromagnetic (FM) clusters in AF matrix, while spin glass in BiFeO3/BiMnO3 superlattices is due to competition between AF Fe-O-Fe, AF Fe-O-Mn and FM Mn-O-Mn interactions in the well ordered square lattice with two Fe ions in BiFeO3 layer and two Mn ions in BiMnO3 layer at interfaces.

The multiferroic properties of BiFe0.5Mn0.5O3 and BiFeO3/BiMnO3 superlattice films

BiFe0.5Mn0.5O3 (BFMO) and BiFeO3/BiMnO3 superlattice (BFO/BMO) films were epitaxially grown on (111) SrTiO3 substrates by pulsed laser deposition. The ferroelectricity of both BFMO and BFO/BMO has been confirmed by piezoresponse force microscopy. Weak ferromagnetism was observed for both samples. The magnetization of BFO/BMO is smaller than that of BFMO, confirming the antiferromagnetic Fe3+-O2−-Mn3+ interaction. Two anomalies at low temperature, T1 = 235 K and T2 = 42 K for BFMO and T1 = 198 K and T2 = 77 K for BFO/BMO, were observed in temperature dependent magnetization (M-T) curves. The peaks at T1 in zero field cooled (ZFC) M-T curves for both samples can be understood by the onset of Fe3+-O2−-Mn3+ interaction. The peak at T2 for BFMO can be understood by the blocking of ferromagnetic Mn-rich clusters due to the inhomogeneous distribution of Mn. The peak at T2 for BFO/BMO observed in both field cooled and ZFC M-T curves has been ascribed to the enhanced single-ion magnetocrystalline anisotropy due to the highly distorted oxygen octahedra in the ordered structure of Fe3+ and Mn3+ ions along (111) orientation.

Structural and magnetic properties of chemically synthesized dilute magnetic semiconducting (In0.9Fe0.1)2O3 nanoparticles

at.% Fe doped In2O3 dilute magnetic semi- conducting (DMS) nanoparticles are synthesized using low temperature 'pyrophoric reaction processes'. X-ray dif- fraction studies at room temperature have revealed single- phase cubic bixbyite structure of the sample. Nano size grain distribution has been observed by FESEM. The recorded temperature dependent magnetization curve exhibits concave nature in the low temperature range for both FC and ZFC conditions revealing insulating DMS like behavior. No phase transition has been noticed within the studied temperature range. Study of magnetic properties in detail confirms the weak ferromagnetic ordering along with paramagnetic behavior of the nanometric sample. From the temperature dependence of inverse magnetic susceptibility (χ −1 (T)) using Curie-Weiss plot, the effective Bohr mag- neton (μeff) has been estimated to be ~3.715 μB/Fe ion. Observed photoluminescence spectrum of Fe doped In2O3 nanostructures mainly results from the recombination of electrons occupying oxygen vacancies or Fe 3+ ions with photo-excited holes.

Griffiths phase, spin-phonon coupling, and exchange bias effect in double perovskite Pr2CoMnO6

The ceramic Pr2CoMnO6 of double perovskite structure is prepared by a solid-state reaction and the magnetic properties, phonon behaviors are studied in detail. Two ferromagnetic transitions at TC1 ∼ 172 K and TC2 ∼ 140 K are observed in the temperature-dependent magnetization curves, respectively. Furthermore, a detail analysis on the magnetic susceptibility reveals that a short-range ferromagnetic clustered state exists above TC1, which can be well described as the Griffiths phase with a well-defined Griffiths temperature TG ∼ 210 K. The presence of the B-site antisite defects is considered to contribute to the observed Griffiths singularity. Temperature-dependent Raman scattering experiment reveals an obvious softening of the phonon mode involving stretching vibrations of the (Co/Mn)O6 octahedra in FM temperature regions, indicating a close correlation between magnetism and lattice in Pr2CoMnO6. On the other hand, it is found that the phonon softening extends up to TG, which further confirms the preformation of the short-range ferromagnetic clusters up to TG. Moreover, the field-cooling magnetic hysteresis loop reveals that exchange bias phenomena is present, which is supposed to origin from the exchange coupling between Co/Mn ordered ferromagnetic phases with antiferromagnetic antiphase boundaries caused by the partially Co/Mn antisite disorders. These findings give a systematic understanding on the magnetic interaction in Pr2CoMnO6 which is closely related to the lattice and atomic distribution, and add special interest for application of this material.

Low temperature ferromagnetic ordering and dielectric properties of Bi1-xDyxFeO3 ceramics

Polycrystalline Bi1−xDyxFeO3 (x=0.03, 0.05, 0.07, 0.10, and 0.12) ceramics were synthesized by solid state reaction method. Rietveld refinement reveals that all the samples crystallize in the rhombohedral structure with non-centrosymmetric R3c space group. Substitution induced suppression of the spiral spin structure and Dy3+-Fe3+ interaction enhanced the room temperature magnetization. Temperature dependent magnetization curves suggest that the magnetic coupling between Dy3+-Fe3+ and Dy3+-Dy3+ ions enhanced at low temperatures range for x=0.07–0.12 samples. ESR study indicated enhanced magnetic properties due to change in magnetic behavior of Fe3+ ions. The change in the Fe-O-Fe bond angle corresponds to the increase in Neel transition temperature with increasing Dy concentration. The dielectric properties are improved with increasing Dy content in BiFeO3. The grain and grain boundary contributions decrease with increase in temperature for all samples, indicating a negative temperature coefficient of resistance.

Magnetic and Structural Properties of MnBi1-xTixAlloys

<TEX>$MnBi_{1-x}Ti_x$</TEX> (x = 0, 0.4, 0.7, 1) alloys were prepared by arc-melting, followed by heat treatment. X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) were used to measure and investigate the phase structure and magnetic properties. The temperature dependent magnetization curves indicate that the phase transitions between LTP and HTP MnBi occur with heating or cooling in <TEX>$MnBi_{1-x}Ti_x$</TEX> (<TEX>$x{\leq}0.7$</TEX>) samples. However, MnTi samples are in <TEX>$Mn_2Ti$</TEX> single-phase, with very low magnetic properties. Furthermore, the coercivity exhibits a positive temperature coefficient. The results show that the optimal content of Ti for the coercivity of <TEX>$MnBi_{1-x}Ti_x$</TEX> alloy is x = 0.4. For MnBi sample, the coercivity reaches a maximum value of 1.13 T at 550 K. However, the remanence and energy product show apparent decrease with the addition of Ti in <TEX>$MnBi_{1-x}Ti_x$</TEX> alloys.

Anomalous magnetic behavior in nanocomposite materials of reduced graphene oxide-Ni/NiFe2O4

Magnetic Reduced Graphene Oxide-Nickel/NiFe2O4 (RGO-Ni/NF) nanocomposite has been synthesized by one pot solvothermal method. Respective phase formations and their purities in the composite are confirmed by High Resolution Transmission Electron Microscope and X Ray Diffraction, respectively. For the RGO-Ni/NF composite material finite-size effects lead to the anomalous magnetic behavior, which is corroborated in temperature and field dependent magnetization curves. Here, we are reporting the behavior of higher magnetization values for Zero Field Cooled condition to that of Field Cooled for the RGO-Ni/NF nanocomposite. Also, the observed negative and positive moments in Hysteresis loops at relatively smaller applied fields (100 Oe and 200 Oe) are explained on the basis of surface spin disorder.

Magnetocaloric effect and critical behavior in Pr0.5Sr0.5MnO3: an analysis of the validity of the Maxwell relation and the nature of the phase transitions

The Maxwell relation, the Clausius–Clapeyron equation, and a non–iterative method to obtain the critical exponents have been used to characterize the magnetocaloric effect (MCE) and the nature of the phase transitions in Pr0.5Sr0.5MnO3, which undergoes a second-order paramagnetic to ferromagnetic (PM-FM) transition at , and a first-order ferromagnetic to antiferromagnetic (FM-AFM) transition at . We find that around the second-order PM-FM transition, the MCE (as represented by the magnetic entropy change, ΔSM) can be precisely determined from magnetization measurements using the Maxwell relation. However, around the first-order FM-AFM transition, values of ΔSM calculated with the Maxwell relation deviate significantly from those calculated by the Clausius–Clapeyron equation at the magnetic field and temperature ranges where a conversion between the AFM and FM phases occurs. A detailed analysis of the critical exponents of the second-order PM-FM transition allows us to correlate the short-range type magnetic interactions with the MCE. Using the Arrott–Noakes equation of state with the appropriate values of the critical exponents, the field- and temperature-dependent magnetization curves, and hence the curves, have been simulated and compared with experimental data. A good agreement between the experimental and simulated data has been found in the vicinity of the Curie temperature TC, but a noticeable discrepancy is present for . This discrepancy arises mainly from the coexistence of AFM and FM phases and the presence of ferromagnetic clusters in the AFM matrix.

Structural and magnetic properties of BiFe1−xCrxO3 synthesized samples

Chromium doping effects on the structure and the magnetic properties of bismuth ferrite BiFe1−xCrxO3 (x = 0–0.3) (BFCxO) polycrystalline samples are examined. The Perovskite-type oxide samples are synthesized by the conventional solid state reaction at a high pressure of 7 GPa and a temperature of 1273 K. The X-ray powder diffraction patterns at room temperature show that all the samples with x = 0.0–0.3 are described by the rhombohedral structure. In the meantime, it is revealed that the doping of Cr can induce noticeable lattice distortions in the doping samples, and the largest distortion is observed in the case x = 0. 1. The magnetic hysteresis loops measured at room temperature exhibit week ferromagnetic behaviors of the samples and the magnetization is found to increase with the increase in Cr concentration. The temperature-dependent magnetization curves indicate antiferromagnetic features in samples. Moreover, Cr-doping tends to reduce the ordering temperature.

Entanglement of lock-in transition and exchange bias in [formula omitted

We report here the structural, magnetic and exchange bias in a well characterized single phase sample of Co(Cr0.9Co0.1)2O4. A pronounced signature of thermal hysteresis in temperature dependent magnetic susceptibility curves is obtained across the lock-in transition TL≃10K. Concomitantly, exchange bias is observed only below the lock-in transition (TL≃10K). This indicates a possible coupling of exchange bias to the lock-in transition in this system.

Rapid synthesis and room temperature ferromagnetism of Ni doped ZnO DMS nanoflakes

NixZn1−xO (x=0.0, 0.05, 0.1, and 0.2) nanoflakes were successfully synthesized by microwave-assisted combustion method. The structural and morphological characterizations of the as synthesized samples were done by using XRD and SEM. Optical and magnetic properties of NixZn1−xO samples were analyzed by a UV–vis spectrophotometer and VSM magnetometer. XRD patterns indicate the hexagonal wurtzite structure of all samples without any impurity phases. SEM micrographs indicated that all samples have grains mainly in the form of nanoflakes. From UV–vis spectra of the samples, it is observed that pure ZnO sample has an energy band gap of 3.13eV and it is decreased by Ni doping. Magnetic property investigations have revealed that all samples have room-temperature ferromagnetic properties. The saturation magnetizations of the samples were increased by Ni doping. The difference between temperature dependent magnetization curves (the difference between field cooled-FC and zero field cooled-FC magnetization curves) also indicated the ferromagnetic behaviors of the samples at all measuring temperatures with a Curie temperature above the room temperature.

Investigation of site preference of Zn doped Ba3Co2−xZnxFe24O41 by Mössbauer spectroscopy

The polycrystalline Ba3Co2−xZnxFe24O41 (x = 0.0, 0.5, 1.0) samples were prepared by using solid-state-reaction method. The crystal structures and magnetic properties of samples were investigated with x-ray diffractometer, vibrating sample magnetometer, and Mössbauer spectroscopy. The crystal structure of Ba3Co2−xZnxFe24O41 (x = 0.0, 0.5, 1.0) samples was determined to be a hexagonal structure with P63/mmc space group at 295 K, and the saturation magnetization (Ms) of Ba3Co2−xZnxFe24O41 (x = 0.0, 0.5, 1.0) samples were found to be Ms = 50.9, 53.1, 55.0 emu/g, respectively. From the temperature dependence of magnetization curves under 100 Oe between 4.2 and 740 K, we were able to observe the spin transition, and both spin transition temperature (Ts) and Curie temperature (TC) decrease with increasing Zn concentration. Mössbauer spectra of all samples were obtained and analyzed at various temperatures ranging from 4.2 to 295 K. With ten-sextets for Fe sites corresponding to the Z-type hexagonal crystallographic sites, all spectra below TC were fitted by least-square method. In addition, from the site occupation numbers of Fe, calculated from the relative areas fitted to the Mössbauer spectra, we find that Zn ions preferentially occupy the tetrahedral sublattices of down sites.

MOCVD selective growth of orthorhombic or hexagonal YMnO3 phase on Si(100) substrate

Multiferroic YMnO3 (YMO) thin films were grown by pulsed injection metal organic chemical vapor deposition on (1 0 0)-oriented silicon substrate. The growth conditions were optimized in order to obtain either orthorhombic (o-YMO) or hexagonal (h-YMO) YMO phases. The most significant deposition parameters in the selective growth of these phases are the substrate temperature and the Y/Mn ratio in the precursor solution. Polycrystalline films with a preferential growth direction are obtained for h-YMO whereas grains with random orientation are observed for o-YMO as shown by X-ray diffraction and transmission electron microscopy. The h-YMO (300 nm thick) and o-YMO (450 nm thick) films present a columnar growth with column diameter comprised between 15 and 35 nm. The Raman spectrum of o-YMO film is reported and compared to the o-YMO bulk spectrum. The temperature-dependent magnetization curves display a λ-shape for both o- and h-YMO phases, indicating a spin-glass state.

The effects of Co-Ti co-doping on the magnetic, electrical, and magnetodielectric behaviors of M-type barium hexaferrites

Magnetic, electrical and magnetodielectric properties have been studied in Co-Ti co-doped M-type hexaferrite BaCoxTixFe12-2xO19 (x = 0 ∼ 4). With the incorporation of Co-Ti, both their ferromagnetic magnetization and coercivity have been greatly changed. The temperature dependent magnetization curve shows a apparent hump at around 420 K, likely in association with more complicated cycloidal spin ordering, which is closely related to ferroelectric polarization. Interestingly, a significantly enhancement in resistivity (∼3 orders in magnitude) can be obtained in co-doped samples (x &amp;gt; 2), which is beneficial for magnetoelectric properties. The magnetoelectric effect were examined by dielectric tunibility under external magnetic field, which shows apparent tunability up to ∼−3% for sample with x = 2 at 1T magnetic field, further supporting it is a room temperature single phase mutliferroic material.

MAGNETIC PROPERTIES OF MnO2 SHRIMPS-LIKE NANOSTRUCTURES SYNTHESIZED BY HYDROTHERMAL ROUTE

In this paper, we report the field-dependent magnetization (M–H) and temperature-dependent magnetization (M–T) properties of hexagonal MnO 2 shrimps-like nanostructures which have been successfully synthesized by hydrothermal route using KMnO 4 and HNO 3 as precursors. The magnetic properties were characterized by vibrating sample magnetometer (VSM). Field-dependent magnetization (M–H) measured at 300 K exhibit paramagnetic nature of MnO 2 nanostructures, while an antiferromagnetic component is observed for the as-prepared product at 55 K. The Néel temperature (TN) is calculated as 89 K by plotting temperature-dependent magnetization (M–T) curve for the as-prepared MnO 2 nanostructures. X-ray photoelectron spectroscopy (XPS) is employed to calculate the binding energies of Mn (2p3/2, 2p1/2) and O (1s) observed in the spectrum.

The preparation and antiferromagnetic properties of epitaxial rocksalt-type CoN films

Abstract We have grown epitaxial rocksalt-type CoN films on SrTiO3 and sapphire substrates by ablating a cobalt target in activated nitrogen atmosphere. The structural and magnetic properties have been investigated. X-ray diffraction profiles show CoN films grow along the [1 0 0] and [1 1 1] direction on SrTiO3(1 0 0) and α-Al2O3(0 0 0 1), respectively. High-resolution transmission electron microscopy measurement has been performed on CoN/SrTiO3 system, which shows a good epitaxial growth. Temperature-dependent magnetization curves reveal that the as-grown films have antiferromagnetic properties with TN of ∼310 K.

Mössbauer studies of Y-type hexaferrite with aluminum doping

The crystal structures of polycrystalline Ba2Co1.5Mg0.5(Fe1−xAlx)12O22 (x = 0.00, 0.01, 0.02) samples were determined to be rhombohedral with a space group of R−3m. From the temperature-dependent magnetization curves under 100 Oe, all samples showed helimagnet-to-ferrimagnet transitions. Based on the applied-field-dependent magnetization measurements up to 10 kOe at 295 K, the saturation magnetization (Ms) and coercivity (Hc) of Ba2Co1.5Mg0.5(Fe1−xAlx)12O22 (x = 0.00, 0.01, 0.02) samples were found to be Ms = 28.0, 24.7, 23.4 emu/g and Hc = 255.4, 222.0, 196.2 Oe, respectively. Also, from the Mossbauer spectra of 295 K, the isomer shift values of all samples indicate charge states of Fe3+. We expect that the decrease in the Ms with increasing Al ion doping is due to the fact that Al ions preferentially occupy the 3bV I, 18hV I and 3aV I octahedral sublattices, which are up-spin sites.

Structural, optical and magnetic properties of Zn 1−x Co x O prepared by the sol–gel route

Effects of Co doping on the structural, optical and magnetic properties of ZnO samples prepared by the sol–gel method are reported. The X-ray diffraction, X-ray photoelectron spectroscopy and UV–visible spectroscopy confirmed the substitution of Co ions on Zn sites without changing the wurtzite structure. No segregated secondary phases or Co rich clusters were detected. Optical absorption spectra of the samples exhibit a blue shift in the absorption band edge with increasing dopant concentration. The photoluminescence measurements show a blue shift in UV emission peak with the increase in Co concentration and a slight shift in the green emission band at around 509 nm which gets suppressed for higher sintering temperature. The field dependence of magnetization observed at room temperature exhibits clear ferromagnetic behavior. Efforts have been made to fit the experimental M – H data using the magnetic polarons model (BMP) which involves localized carriers and magnetic cations. The calculated concentration of the BMPs is found to be below the typical percolation threshold in ZnO. Thus BMP model alone is not sufficient to explain the room temperature ferromagnetic behavior in ZnO. To know the exact magnetic ordering in the system, we also attempted to fit temperature dependent magnetization curves with the Curie–Weiss Law which shows antiferromagnetic ordering in all samples.