Zero-field-cooled Curves Research Articles

Comparing magnetic characteristics of CoSe3xFe2–4xO4 (x ≤ 0.10) spinel ferrite nanoparticles synthesized via Sol-Gel and hydrothermal approaches

In this study, Se substituted Co spinel ferrite nanoparticles (CoSe3xFe2–4xO4 (x ≤ 0.10), CoSe SFNPs) have been synthesized via sol-gel (SG) and hydrothermal (HT) routes and their structural, magnetic and morphological features have been compared. The effect of both the Se substitution and the method of synthesis on the structural, morphological and magnetic features of the host CoFe2O4 have been investigated. The structure and morphology of the SFNPs were examined using XRD, EDX, SEM, TEM and HR-TEM methods. XRD analysis confirmed the single-phase formation of CoSe SFNPs for both HT and SG routes. A TEM study for the HT process demonstrated the development of spherical nanoparticles with uniform size, whereas the SG procedure produced NPs with an indeterminate shape. By examining the hysteresis loops at 300 and 20 K and analysing the temperature-dependent changes in magnetization (M), the magnetic characteristics of CoSe (x ≤ 0.10) SFNPs prepared by the SG and HT methods were investigated. CoSe SFNPs possess magnetically soft and hard natures at 300 and 20 K, respectively. Under each synthesis condition, at 20 K, the SQR values are above 0.5, which signifies well-ordered magnetic domains at this temperature. Across all the materials, as the temperature decreases, zero field cooling (ZFC) branches exhibit a non-linear decline in magnetization which becomes nearly stable in the field cooling (FC) branches. The deviation of the ZFC curve with respect to FC one is more pronounced in the case of SFNPs prepared by the HT process. Our findings demonstrate that by employing the desired synthesis method, Se dopants can be applied to achieve particular magnetic characteristics of CoFe2O4 NPs.

Exploration of Griffiths phase, spin glass behaviour and memory effect in a frustrated Al2MnCoO7 pyrochlore compound

Al2MnCoO7 pyrochlore compound has been synthesized following the multistep solid-state ceramics route. The compound exhibits a monoclinic structure, which was confirmed through the X-ray diffraction pattern analysis. The magnetization measurements in both zero-field-cooled (ZFC) and field-cooled (FC) modes indicate that the compound undergoes a phase transition from paramagnetic (PM) to antiferromagnetic (AFM) state at the Néel transition temperature TN ∼31 K. Notably, below TN, there is a substantial discrepancy between the ZFC and FC curves, which can be ascribed to the existence of competing ferromagnetic (FM) and AFM interactions, resulting in a glassy behaviour. Additionally, the compound demonstrates a Griffiths phase above TN, signifying the presence of ferromagnetic clusters within the PM region of the material. The spin-glass behaviour is confirmed by the shift of maxima towards higher temperatures with increasing frequency in the ac susceptibility measurement. The memory effect also confirms the spin-glass nature of the compound. The Arrott's plots confirmed that the magnetic phase transition in the material to be a second-order type. Additionally, the material exhibited a maximum magnetic entropy change (−ΔS) value of 1.08 J/kg.K and a relative cooling power of 36.60 J/kg under an applied magnetic field of 5 T.

Temperature dependent magnetic and electrical transport properties of lanthanum and samarium substituted nanocrystalline nickel ferrite and their hyperthermia applications

Structural, optical, temperature dependent magnetic and electrical properties have been investigated for the nanocrystalline NiFe1.97RE0.03O4 (RE = La3+ and Sm3+) compound. Without any signs of a secondary REFeO3 phase, the Rietveld refinement reveals a single-phase spinel structure possessing cubic symmetry for current samples. The decrease in lattice constant has been observed as a result of RE ions substitution. The TEM micrographs reveal nanosized particles with an average size of 35 ± 3 nm and 32 ± 3 nm for La3+ and Sm3+ substituted samples. EDX spectra of both samples show good compositional homogeneity. HRTEM micrographs of both samples show well resolved lattice fringes and their inter-planar spacing matches with that obtained from the Rietveld analysis of the XRD patterns. The concentration of Fe2+ and Fe3+ ions has been estimated from the XPS spectra analysis and it is found to be ∼ 22% and 78% for NiFe1.97La0.03O4 and, 20% and 80% for NiFe1.97Sm0.03O4 compound. The optical energy band gap for rare earth substituted samples is found to be more than that of pure nickel ferrite. The value of saturation magnetization (Ms) and magneto-crystalline anisotropy constant (K1) estimated from the “Law of Approach to Saturation” equation for rare earth substituted samples are found to be less than that of pure nickel ferrite. However, an enhanced value of coercivity has been observed with RE substitution. ZFC (zero field cooling) and FC (field cooling) DC magnetization curves measured at 100 Oe in the temperature range of 60–400 K reveal a combination of weak and intermediate forms of magnetic interaction between the particles for both samples. Temperature dependent analysis of saturation magnetization and coercivity employing modified Bloch’s law and Kneller’s relation supports the nanomagnetic behavior of both samples. Under an AC magnetic field of 14.92 kA/m and frequency 337 kHz, the SAR and ILP values have been found to be ∼ 331 W/g and 4.22 nHm2/kg for NiFe1.97La0.03O4 and, ∼ 241 W/g and 3.21 nHm2/kg for NiFe1.97Sm0.03O4. The dielectric relaxation data have been analyzed at various temperatures ranging from 40 to 300 0C over the frequency range 100 Hz-1 MHz using electrical impedance spectroscopy. The dielectric constant for rare earth substituted samples is found to be more and their AC conductivity is observed to be less than that of pure nickel ferrite. The analysis of temperature dependent AC conductivity employing Jonscher’s power law suggests that the charge carrier’s conduction mechanism of both samples follows the small polaron hopping mechanism below 200 0C, thereafter; it follows the correlated barrier hopping mechanism. The activation energy estimated by imaginary impedance spectra is found to be 0.411 eV and 0.398 eV for NiFe1.97La0.03O4 and NiFe1.97Sm0.03O4 which is more than that of pure nickel ferrite. The Cole-Cole plots at different temperatures suggest the presence of non-Debye-type relaxation. The modeling of Cole-Cole plots with equivalent circuits for both samples confirms the contribution of both grain and grain boundary in electrical conduction. The estimated stretching exponential factor by fitting the modified KWW (Kohlrausch-Williams-Watts) equation to the imaginary modulus curve reveals relatively more dipole-dipole interaction in NiFe1.97Sm0.03O4 than that of the La3+ substituted sample.

Observation of itinerant ferromagnetic behaviour in equiatomic medium entropy AlNiCo alloy

A medium entropy AlNiCo alloy has been synthesized by vacuum arc melting, which shows single phase B2 structure. Atom probe tomography analysis showed that the elements are homogeneously and randomly distributed in AlNiCo alloy without any clustering tendency. A detailed analysis of temperature and field dependent magnetization data suggests an itinerant ferromagnetic ground state of AlNiCo alloy. The magneto crystalline anisotropy is evaluated to be 1.03 × 105 ergs/cm3 at 5 K. A broad maximum at low temperature in zero-field-cooled (ZFC) magnetization, as well as irreversibility of ZFC and field-cooled (FC) curves indicated a glassy behavior. This observation is supported by the memory effects observed at low temperature. The observed anomalous magnetic behavior is discussed in terms of modifications in exchange interactions. To the best of our knowledge, this is the first report on microstructure and magnetic properties of an equiatomic ternary AlNiCo alloy.

Non-equilibrium magnetic response in concentrated spin-glass Au0.89Fe0.11 alloy

We report a detailed study of dc magnetization and ac susceptibility performed on the zero field cooled (ZFC) and field cooled (FC) state of polycrystalline Au0.89Fe0.11 alloy. The temperature variation of ZFC and FC dc magnetization at low fields shows a distinct peak around Tf = 33 K, which indicates the cooperative freezing of the finite size spin clusters. A weak thermomagnetic irreversibility between ZFC and FC magnetization appears at a temperature Tir, which is slightly below Tf. Further down the temperature at Tsh<Tir, the ZFC and FC magnetization curves exhibit strong thermomagnetic irreversibility. The ac susceptibility recorded after cooling in presence of field shows significant dispersion below Tf, and also, the FC state of Au0.89Fe0.11 alloy exhibits a pronounced memory effect in the dc magnetization which clearly underline that the FC state is not an equilibrium state. In contrast to the general perception obtained through the mean-field theories of thermodynamic phase transition in spin-glass envisaging the FC state to be an equilibrium state, the present experimental results clearly indicate that the energy landscape of the FC state of Au0.89Fe0.11 alloy is a nontrivial one.

Effects of Pr3+ ion doping on magnetic features of Ni–Co nanospinel ferrites synthesized via sonochemical approach

The praseodymium ion (Pr3+) substituted Ni0.5Co0.5Fe2O4 nanospinel ferrites, (Co0.5Ni0.5PrxFe2-xO4 (Pr → CoNi) (x ≤ 0.10) NSFs), were synthesized via sonochemical route. XRD confirmed the single cubic structure of all samples with the crystallite size within the range of 9–11 nm.The morphology of (Pr → CoNi) (x ≤ 0.10) NSFs) particles was analyzed by SEM and TEM and showed the spherical shape. Mossbauer study showed that the larger Pr3+ ions prefer the B site. According to the magnetic study, the superparamagnetic character increased with increase of Pr3+ions. The magnetic characteristics are elucidated through both hysteresis curves and temperature (T) dependence of magnetization (M). Magnetization (M) vs external field (H) hysteresis curves were performed at both 300 (RT) and 10 K. It is revealed that Pr → CoNi NSFs exhibit superparamagnetic (SPM) nature performed at RT. Saturation magnetization (Ms), magnetic moment (nB), squareness ratio (SQR), remanence (Mr), and coercivity (Hc) were determined. The Hc of Pr → CoNi (x ≤ 0.10) NSFs was found to be larger than that of the host material. On the other hand, at both temperatures, the magnetization data demonstrates the crucial impact of Pr3+ ions which diminish the Ms, Mr and nB of the host material. The magnetic parameters for Pr → CoNi (x ≤ 0.10) NSFs barely fluctuate and depend on the cation distribution. At 10 K, the SQR of all samples is calculated to be ∼ 0.5 which reflects the uniaxial symmetry and a single-domain structure formation. For Pr → CoNi (x ≤ 0.10) NSFs, much lower SQR values are attained at room temperature (RT), indicating a decrease in the exchange interaction, and suggesting a multi-domain wall structure. For Pr → CoNi (x ≤ 0.10) NSFs, the M values through field-cooling (FC) curves exhibit a slight decrease with increasing T. Their zero field-cooling (ZFC) curves display a nonmonotonic decrease with the decreasing T and result in a low magnetization at low Ts. ZFC and FC curves of Pr → CoNi (x ≤ 0.10) NSFs indicate a large irreversibility.

Superconductivity and magnetism in a novel La0.85Ga0.15FeAsO compound

We report magnetization measurements on samples of La0.85Ga0.15FeAsO in the wide temperature range of 10 K-400 K, up to the field of 90 kOe having been prepared by solid-state reaction. This novel compound crystallizes with the tetragonal ZrCuSiAs-type structure. The crystal structure was determined by X-ray powder diffraction (P4/nmm: a = 4.032 Å, and c = 8.741 Å).Temperature dependence of magnetization for this compound is measured with zero-field cooling (ZFC) and field cooling (FC) with an application of H = 50 Oe. The large bifurcation between the ZFC and FC curves has been observed starting at around 370 K. At lower temperatures below Tscset−on∼37 K, superconductivity and magnetism coexist. Intermediate temperature range (37 K< T < 300 K), the sample is in a mixed state of the ferromagnetic clusters with paramagnetic moments, described well by the Modified Curie Weiss expression. At higher temperatures above 360 K, it is a pure paramagnetic, described perfectly by the simple Curie-Weiss expression.The natures of the superconducting flux-pinning centers are revealed based on analysis of the pinning force, Fpin at T = 10 K. The normalized pinning force, Fp/Fp, max versus H indicates the filamentary character owing to the pinning parameters, p = 0.5, q = 2. These results point out that, in the superconductivity, the dominance of grain boundary with single-vortex pinning (μ =1/7) mechanismThe Curie temperature, Tcurie was estimated using the modified Arrot plots. The critical exponents, β, ϒ, and δ were estimated, and their values are in close agreement with those predicted for a mean-field-like behavior at a magnetic phase transition.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO 3

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO ₃

Sol-gel synthesized double perovskite Gd2FeCrO6 nanoparticles: Structural, magnetic and optical properties

Lead-free double perovskites are overtaking single perovskites as solar harvesting materials due to their superior stability, excellent catalytic efficiency and minimal toxicity. In this investigation, we have synthesized double perovskite Gd2FeCrO6 (GFCO) nanoparticles for the first time via a facile sol-gel technique to investigate their structural, magnetic and optical properties. The double perovskite GFCO crystallized in monoclinic structure with P21/n space group. The Fe/Cr-O bond length was calculated as ~ 1.95 Å from the Raman spectrum which was consistent with the value, ~ 1.99 Å obtained from X-ray diffraction analysis. The average size of the nanoparticles was determined to be ~ 70 nm by both field emission scanning electron microscopy and transmission electron microscopy. The existence of mixed valence states of Fe and Cr was confirmed by X-ray photoelectron spectroscopy. The zero field cooled (ZFC) and field cooled (FC) curves largely diverged below 20 K. A downturn was observed in the ZFC curve at 15 K which corresponds to an antiferromagnetic, Néel transition. The narrow magnetic hysteresis loop recorded at 5 K was nearly saturated and demonstrated an asymmetric shift along the magnetic field axis indicating the concurrence of ferromagnetic and antiferromagnetic domains in GFCO nanoparticles. The UV–visible and photoluminescence spectroscopic analyses unveiled the semiconducting nature of nanostructured GFCO with an optical band gap of 2.0 eV. The as-synthesized thermally stable lead-free GFCO semiconductor might be a potential perovskite material to be employed in photocatalytic and related solar energy applications due to its ability to absorb the visible spectrum of the solar light.

Observation of large exchange bias field in Mn rich NiMnAl Heusler alloy thin film

In the present work exchange bias in Mn rich Ni-Mn-Al film has been studied. The film has been deposited by cosputtering from the targets of Ni, Mn and Al. It exhibits an antiferromagnetic B2 phase at room temperature. At low temperature, magnetization versus temperature (M-T) measurement shows a bifurcation among ZFC and FC curves. Hysteresis (M-H) loops measurements reveal the occurrence of exchange bias phenomenon at low temperature. Large HEB of 2.2 kOe for 20 kOe cooling field has been observed. There occurs a field induced ordering at temperature ~50 K reflected as coinciding ZFC (zero-field cooled) and FC (field-cooled) curves at large magnetic field. AC susceptibility measurement indicates the occurrence of cluster glass behaviour at low temperature. The exchange constant obtained from the best fit to the experimental data was found to be Ji ≈ −2 meV. The negative interface exchange constant indicates AFM coupling occurs among the coexisting phases. Exchange bias phenomenon may be associated to the coexistence of antiferromagnetic and ferromagnetic like ordering at low temperature. Cooling field dependence of HEB and coercive field HC indicate that an increase in anisotropy and growth of ferromagnetic clusters occurs at high magnetic field.

Room temperature ferromagnetism in Zn-doped NiO nanoparticles: An experimental and DFT+U approach

Abstract We report the room temperature ferromagnetism in Zn-doped NiO nanoparticles of Ni1−x ZnxO (x = 0.00, 0.01, 0.03, 0.05 and 0.1) prepared by the sol-gel method. Zn dopant has significant influence on structural, optical, and magnetic properties of NiO crystal when there is a size reduction and lattice distortions. The crystalline nature of the samples were characterized by the room temperature X-ray Diffraction and we found that the range of crystallite size is about 7 ± 0.01 to 9 ± 0.01 nm for 3–10% of Zn doping concentration. The lattice parameter and the effective strain increases with the increase of doping concentration. Local structure and defects are probed through Raman characterization which reveals that there are no secondary or impurity peaks on the Zn-doped NiO samples. Photoluminescence spectra at room temperature shows two major emission peaks at wavelength 357 nm and 393 nm for both pure and Zn-doped NiO powder respectively. There also exists a blue shift for wavelength 357 nm emission band with the increase of Zn doping concentration. The band gap energy calculated from the PL spectra is ≈3.46 eV for pure NiO and it increases with the doping upto 3% then it decreases with further doping. To understand the effect of doping on magnetic properties, we measured the temperature dependent M-H loops and M-T curve for zero field cooled (ZFC) and field cooled (FC) condition in the temperature range 25–350 K using Vibrating Sample Magnetometer. We observed that the Ni1−x ZnxO nanoparticles are ferromagnetic in nature at room temperature with highest magnetic moment of ≈1.9 emu/g when the doping of Zn is 3% and it decreases with further increase of doping concentration. M-T data bifurcation peak was observed around 300 K and a broad peak in ZFC curve below the bifurcation. This broad peak is attributed to the thermal relaxation of uncompensated spins. We have also investigated the first-principle DFT+U calculation to study the electronic properties of the materials. The results show that the Zn-doped NiO with oxygen vacancies are ferromagnetic in nature whereas without defects the compound is non-magnetic, which agrees quite well with the experimental results. The nanoparticle with ferro-magnetic properties have potential application in the field of nanomagnetism and also its optical properties could be useful for optical device applications.

Sample Dependence of Magnetism in the Next-Generation Cathode Material LiNi0.8Mn0.1Co0.1O2.

We present a structural and magnetic study of two batches of polycrystalline LiNi0.8Mn0.1Co0.1O2 (commonly known as Li NMC 811), a Ni-rich Li ion battery cathode material, using elemental analysis, X-ray and neutron diffraction, magnetometry, and polarized neutron scattering measurements. We find that the samples, labeled S1 and S2, have the composition Li1-xNi0.9+x-yMnyCo0.1O2, with x = 0.025(2), y = 0.120(2) for S1 and x = 0.002(2), y = 0.094(2) for S2, corresponding to different concentrations of magnetic ions and excess Ni2+ in the Li+ layers. Both samples show a peak in the zero-field-cooled (ZFC) dc susceptibility at 8.0(2) K, but the temperature at which the ZFC and FC (field-cooled) curves deviate is substantially different: 64(2) K for S1 and 122(2) K for S2. The ac susceptibility measurements show that the transition for S1 shifts with frequency whereas no such shift is observed for S2 within the resolution of our measurements. Our results demonstrate the sample dependence of magnetic properties in Li NMC 811, consistent with previous reports on the parent material LiNiO2. We further establish that a combination of experimental techniques is necessary to accurately determine the chemical composition of next-generation battery materials with multiple cations.

Annihilation of percolative correlation signals in sulfur doped highly oriented pyrolytic graphite with hexagonal Moiré superlattices

We report a novel study on the magnetic properties of exfoliated HOPG lamellae exhibiting hexagonal Moiré superlattices with periodicity of ~40 nm, ~26 nm, ~23 nm and ~7.7 nm corresponding to rotational angles θrot of ~0.35°, ~0.55o, ~0.62° and ~1.8o. SQUID magnetometry revealed presence of percolative ferromagnetic contributions arising from topologically disordered areas and absence of superconductive transitions. Field-dependent zero field cooled (ZFC) and field cooled (FC) measurements of the magnetization evidenced presence of magnetic irreversibilities, arising from percolative ferromagnetism. Similar magnetic features could be detected in other exfoliated lamellae and in natural graphite. Experiments involving doping of the exfoliated lamellae with sulfur, evidenced further an unusual demagnetization effect; ZFC and FC magnetic curves of doped samples revealed a partial annihilation of the ferromagnetic signals as a consequence of interactions between the sulfur species and the topologically disordered regions. Raman spectroscopy highlighted the appearance of unknown broad bands in the regions of ~500 cm−1, 1200 cm−1 and 1400 cm−1 resulting from sulfur-defect interactions.

On the detection of surface spin freezing in iron oxide nanoparticles and its long-term evolution under ambient oxidation

Exchange bias (EB) effects linked to surface spin freezing (SSF) are commonly found in iron oxide nanoparticles, while signatures of SSF in low-field temperature-dependent magnetization curves have been much less frequently reported. Here, we present magnetic properties of dense assemblies of similar-sized (∼8 nm diameter) particles synthesized by a magnetite (sample S1) and a maghemite (sample S2) method, and the influence of long-term (4 year) sample aging under ambient conditions on these properties. The size of the EB field of the different sample (fresh or aged) states is found to correlate with (a) whether a low-temperature hump feature signaling the SSF transition is detected in out-of-phase ac susceptibility or zero-field-cooled (ZFC) dc magnetization recorded at low field and with (b) the prominence of irreversibility between FC and ZFC curves recorded at high field. Sample S1 displays a lower magnetization than S2, and it is in S1 where the largest SSF effects are found. These effects are significantly weakened by aging but remain larger than the SSF effects in S2, where the influence of aging is considerably smaller. A non-saturating component due to spin disorder in S1 also weakens with aging, accompanied by, we infer, an increase in the superspin and the radius of the ordered nanoparticle cores. X-ray diffraction and Mössbauer spectroscopy provide indication of maghemite-like stoichiometry in both aged samples as well as thicker disordered particle shells in aged-S1 relative to aged-S2 (crystallographically-disordered and spin-disordered according to diffraction and Mössbauer, respectively). The pronounced diminution in SSF effects with aging in S1 is attributed to a (long-term) transition, caused by ambient oxidation, from magnetite-like to maghemite-like stoichiometry, and a concomitant softening of the spin-disordered shell anisotropy. We assess the impact of this anisotropy on the nature of the blocking of the nanoparticle superspins.

Determination of the Magnetic Structure and Properties of the FeS Compound by using Mössbauer Spectroscopy

This study investigates the magnetic hyperfine structure and the magnetic properties of FeS by using an X-ray diffractometer (XRD), a vibrating sample magnetometer (VSM), and Mossbauer spectroscopy. The XRD FeS pattern showed NiAs hexagonal structure with space groups P-62c, P63/mmc, and P63/mc. The hysteresis loop with the maximum applied field of 15 kOe was measured at room temperature by using VSM, and the saturation magnetization (Ms) and the coercivity (Hc) values were observed to saturate. The temperature dependence of the zero-field-cooled (ZFC) and the field-cooled (FC) curves were examined at temperatures from 4.2 to 295 K at 100 Oe. Below 280 K, the ZFC curve displayed antiferromagnetic behavior; it was shown that the α-transition temperature (Tα) was shown to begins at a temperature high than 280 K in the FeS compound. The Mossbauer spectra of FeS were taken at various temperatures ranging from 4.2 to 295 K, and at each of them, the spectrum was analyzed using two sets of sextets containing A-sites, B-sites, and C-sites. The charge state was determined to be Fe2+ and Fe3+ based on the isomer shift, and the Curie temperature (TC) was found to be 630 K.

Size-dependent phase transformation and its correlation with magnetic properties of multiferroic 0.8BiFeO3– 0.2CaTiO3 nanoparticles

Structural and magnetic properties of 0.8BiFeO3–0.2CaTiO3 (BF-20CT) nanoparticles were investigated systematically as a function of heat treatment at 500 °C, 650 °C, 800 °C and 950 °C of a sol-gel derived powder. The structural investigation by the Rietveld refinement revealed the phase transformation from monoclinic (Cc) to rhombohedral (R3c) structure. The lattice parameters, bond angles, bond distance, and unit cell volume significantly changed as temperature increases from 500 to 950 °C. TEM investigation has confirmed the size, morphology, and microstructure of the BF-20CT nanoparticles. Enhanced magnetization with the finite coercive field was observed as phase changed from monoclinic (Cc) to rhombohedral (R3c). The splitting in zero-field cooling (ZFC) and field cooling (FC) magnetization curves with a sharp peak in the ZFC curves revealing a spin-glass type behavior.

Structural characterization, dielectric, magnetic and optical properties of double perovskite Bi2FeMnO6 ceramics

Double perovskite Bi2FeMnO6 (BFMO) oxides are promising candidate for single-phase multiferroic system. However, up to date only a limited works are carried out to synthesize the BFMO ceramics, and most of them have the issues of impure phases. In this work, we report on the structural, dielectric, magnetic, and optical properties of single-phase BFMO ceramics synthesized by solid-state reaction method. The BFMO ceramics crystallized in a rhombohedral structure with R3c space group, and their grains exhibited polyhedron morphology with average size of 15.0 µm. X-ray energy dispersive spectroscopy revealed the ceramic grains with atomic ratio of Bi:Fe:Mn equal to 2.0:1.16:0.90. XPS spectra of Fe 2p and Mn 2p core levels confirm the mixed chemical valence states of Fe (Fe2+ and Fe3+) and Mn (Mn3+ and Mn4+) in the BFMO ceramics. O 1s XPS spectra reveal the oxygen existing in the forms of lattice oxygen and oxygen vacancies while bismuth is present as Bi3+. Dielectric measurements demonstrate that the BFMO ceramics have much high dielectric constant (in the order of 104) in the frequency below 102 Hz but decrease fast with increasing the frequency. The same phenomenon appeared for the dielectric loss. A dielectric relaxation behavior with activation energy of 0.34 eV was observed in the BFMO ceramics, which was due to the thermal motion of the oxygen vacancies. The BFMO ceramics exhibit antiferromagnetic nature but a weak ferromagnetic behavior at 2 K. Sharp cusp peaks were observed in the zero-field-cooling (ZFC) curves and bifurcations between the field-cooling (FC) and ZFC curves were observed. Both of them shifted to low temperature as increasing the external applied magnetic field, indicating a spin-glass like behavior in the system. UV–Vis absorption spectra demonstrated a broad absorption peak in the wavelength range of 300–800 nm and a direct optical band gap of ~1.23 eV for the BFMO ceramics. Such narrow optical band gap as well as the high dielectric constant at room temperature enable the BFMO ceramics to be promising candidate used for solar cells, sensors, bolometer and optoelectronic devices.

Synthesis and magnetic properties of Ba2Fe14O22 Y-type hexaferrite

Using the solid state reaction method, Y-type hexaferrite Ba2Fe2+2Fe3+12O22 (Fe2Y) polycrystalline ceramics were fabricated by calcining and sintering in a nitrogen atmosphere, and their magnetic properties were systematically investigated. The kink occurs at around 130 K in the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves is independent of measuring frequencies, which is probably derived by the temperature-induced spin reorientation transitions of Fe2Y phase. While the kink at 50 K in the ZFC magnetization and the irreversibility between ZFC and FC curves were observed and ascribed to the spin-glass-like transition in the low temperature region. Both ac susceptibility and thermoremanent magnetization (TRM) measurements confirmed this assumption.

Large magnetocaloric effect in La0.75Ca0.25–xNaxMnO3 (0 ≤ x ≤ 0.10) manganites

We have studied the effect of a partial substitution of Ca2+ by Na+ on the structural, magnetic and magnetocaloric properties of the mixed valence perovskites $${\text{La}}_{{{0}{\text{.75}}}} {\text{Ca}}_{{{0}{\text{.25 - }}x}} {\text{Na}}_{x} {\text{MnO}}_{{3}}$$ (0 ≤ x ≤ 0.10), prepared by the flux method. X-ray diffraction studies revealed that our samples crystallize in the orthorhombic structure with a Pbnm space group. Rietveld analysis showed that MnO6 octahedron has a little distortion and that θMn–O–Mn bond angles increase with increasing Na+ content. The field cooled (FC) and zero field cooled (ZFC) magnetizations indicated that our samples have a paramagnetic-ferromagnetic phase transition, when decreasing temperature. A small deviation was observed between FC and ZFC curves. This deviation can be explained by the competition between the ferromagnetic and antiferromagnetic interactions, leading to a glassy behavior. The Curie temperature, TC, increased from 268 K for x = 0 to 278 K for x = 0.10. The magnetizations isotherms of our compounds, with a second-order phase transition, were investigated. For the x = 0.10 sample, the maximum of the magnetic entropy change was found to be 3.15 and 6.44 J kg−1 K−1, under an applied magnetic field of 2 and 5 T, respectively. Moreover, it has important relative cooling power values of about 134.82 and 317.79 J kg−1, under the same applied magnetic field. In addition, rescaled entropy data collapsed into the same curve, which indicates the universal behavior for the magnetocaloric effect (MCE) in our compounds. These materials could be potential candidates as working substances in magnetic refrigeration.

Preparation of Ba2FeNbO6 double perovskite ceramics from molten-salt synthesized powders and their dielectric and magnetic properties

Double perovskite Ba2FeNbO6 (BFN) ceramics were prepared from the molten-salt synthesized BFN powders. The BFN powders have a cubic structure with $$ Fm\bar{3}m $$ space group, confirmed by X-ray diffraction patterns. Their average particle sizes increased with the annealing temperature and dwell time whereas decreased with the molten-salt content. X-ray photoemission spectra revealed both Fe2+ and Fe3+ ions present in the BFN powders and oxygen in the forms of lattice oxygen and oxygen vacancies, while niobium was present as Nb5+. The BFN ceramics exhibited significant frequency-dependent dielectric behavior, and two kinds of dielectric relaxor processes were observed. Their activation energies were 0.16 eV and 0.47 eV, respectively. The electron hopping process (between Fe2+ and Fe3+ ions) with the activation energy of 0.16 eV contributes the low-temperature dielectric relaxation process, and an activation energy of 0.47 eV associated with the high-temperature dielectric relaxation is originally from the barrier energy of the thermal motion of the oxygen vacancies. The dielectric relaxation observed at high-temperature region can be explained by Maxwell–Wagner interfacial polarization in the BFN ceramics. M–H plots of the BFN ceramics at different temperatures revealed their antiferromagnetic nature and a weak ferromagnetic behavior at 2 K. The remanent magnetization (Mr) of the BFN ceramics was measured to be 1.75 emu/g at 2 K and coercive field (Hc) as 1.26 kOe. Magnetization splitting was observed at ~ 290 K between the temperature-dependent magnetizations of the BFN ceramics, which were measured under zero field-cooled (ZFC) and field-cooled modes with an external field of 1 kOe. In the ZFC curve, a maximum peak was observed at 14 K, corresponding to the Neel temperature (TN). The TN of the present BFN ceramics shifts toward low temperature as compared with those prepared by conventional solid-state reaction methods. Such a reduction of the Neel temperature is ascribed to the size effect.