Magnetic Field Dependent Measurements Research Articles

Enhanced magnetization in multiferroic BiFe03 through structural distortion and particle size reduction

BiFeO3 is widely known multiferroic compound with high ferroelectric polarization but low magnetization values due to antiferromagnetic ordering. We used allied strategy of particle size reduction and structural modulation to improve the magnetization. Calcium doped Bismuth Ferrite nanoparticles were synthesized by sol-gel route. The Rietveld refinement of X-Ray Diffraction patterns indicates single phase formation with distorted rhombohedral (space group R3c) structure. The average crystallite size varies from 72 nm to 27 nm. The Unit cell volume decreases with increase in Ca concentration. The structural distortion is confirmed by Raman spectra through line width broadening and shifting of phonon frequencies. X-ray photoelectron spectroscopy revealed Fe+3 oxidation state and slight shifting of Bi 4f peak indicating the substitution of Ca2+ at Bi-site. The field dependent magnetization measurements show remnant magnetization up to 0.265 emu/gm and coercivity up to 543 Oe in Ca substituted samples.

Enhanced ferromagnetism by ion irradiation for substitutionally cobalt doped ZnO films

3 at.% Co doped sol-gel derived films were irradiated with 800 keV Ar ion beam. The studied films were un-irradiated and irradiated with fluences 5 × 1014, 2.5 × 1015 and 1016 ions/cm2. The films were characterized structurally (X-ray diffraction [XRD]), morphologically (Field Induced Scanning Electron Microscopy), optically (UV–visible, Raman spectroscopy and Photoluminescence [PL] measurements), magnetically (field and temperature dependent magnetization measurements) and electronically via local structure study (X-ray Absorption spectroscopy [XAS]). XAS data were interpreted in X-ray absorption near edge spectroscopy (XANEX) and extended X-ray absorption fine structure spectroscopy (EXAFS) measurements. XRD patterns show single-phase wurtzite structure of the films and indicate presence of defects. Raman spectra confirm single-phase structure of the films and indicate slight structural degradation in highest fluence irradiated film. The high substitutional incorporation of Co2+ at Zn2+ site was demonstrated from UV–visible spectra by d-d transition. The analysis of XANES and EXAFS spectra strongly reconfirms further the proper substitutional incorporation of Co2+ at Zn2+ site. All films are intrinsically ferromagnetic. The substantial enhancement of ferromagnetism (2.5 fold) has been observed upon irradiation (highest fluence) and it has been interpreted in terms of defects as evidenced from PL measurement. Hence the effect of Ar ion beam irradiation is highly enhanced ferromagnetism and proper Co substituted ZnO film.

Signatures of induced superconductivity in AlOx-capped topological heterostructures

In order to access exotic Dirac and Majorana states in (Bi,Sb)-based topological insulators (TIs), the physical surface of those crystals should not be exposed to air. 2–3 nm of in situ deposited Al on top of pristine TI thin films immediately oxidizes after taking the sample to ambient conditions. The native AlOx provides a favorable hard capping, which preserves the topological surface states during ex situ device fabrication. Here, we present a process on how to construct superconductor – topological insulator – superconductor (S-TI-S) junctions from in situ capped thin films comprised of 15 nm Sb2Te3 on top of 6 nm Bi2Te3. The thicknesses of the Sb2Te3 and the Bi2Te3 layer allow us to precisely tune the Fermi level of the upper surface of the Sb2Te3 layer. The challenge is to provide a transparent interface between Sb2Te3 and the superconductive Nb, while assuring an AlOx-capped weak link in between two closely separated Nb electrodes. Low temperature experiments on our junctions provide evidence for charge transport mediated by coherent Andreev states. Magnetic field dependent measurements yielded Fraunhofer-like patterns, whose periodicities are in good agreement with the effective areas of the respective junctions. Transmission electron micrographs of the narrowest junction confirm a crystalline and capped weak link. Our results provide the first reported signatures of induced superconductivity in S-TI-S junctions, which are capped by native AlOx. The presented process allows for accessing S-TI hybrid devices via magnetic flux, while assuring in situ conserved weak links. This makes as-prepared junctions a promising platform for proposed flux-controllable Majorana devices.

Strong electron-phonon coupling in superconducting bismuth nanoparticles

We report on the nanosized effect on superconducting properties of Bi nanoparticles (NPs) that extracted from the temperature and applied magnetic field dependent magnetization measurements of bismuth NPs, and revealed an enhanced TC and stronger coupling strength because of increased density of state and softened phonons ωln. A strong electron-phonon coupling (λep = 1.639) to low-lying phonons (ωln = 58 K) is found to be the leading mechanism behind the observed superconductivity with an enhanced TC ∼ 8.22 K and a strong superconducting coupling strength of α = 2.32(1) of 15 nm Bi NPs. Our results are well described by the Allen and Dynes formula within the framework of the McMillan formalism and are in agreement with the Ginzburg-Landau theory calculations.

The role of adding GdCrO3 in multiferroic CoCr2O4 nanoparticles

The role of adding GdCrO3 at various content in multiferroic CoCr2O4 spinel nanoparticles has been inclusively studied. The host (CoCr2O4) and secondary (GdCrO3) structures are crystallized in cubic and orthorhombic phases, respectively. The average crystallite size of host CoCr2O4 structure decreases with GdCrO3 content that it suppresses the formation of further CoCr2O4 crystals. The magnetic and magnetocaloric properties of CoCr2O4/(GdCrO3)x nanoparticles have been studied by performing temperature and magnetic field dependent magnetization measurements. Both saturation (~ 16× times) and remnant (~ 6× times) magnetizations are significantly enhanced by GdCrO3 contents compared to the CoCr2O4. The paramagnetic to ferrimagnetic phase transition temperature decreases from 96 to 92 K with increasing GdCrO3 content in the structure. Around the transition temperature, the magnetic entropy changes of samples were determined via the initial M-H curves and thermo-dynamical relations. The maximum magnetic entropy change is almost 3× enhanced by adding GdCrO3 into the CoCr2O4 structure compared to the pure CoCr2O4 structure. The relatively large magnetic entropy change could be obtained by combining two CoCr2O4 and GdCrO3 multiferroic materials. The results of samples are important for usage of them in a potential magnetic refrigeration system.

A Hydride Route to Ternary Alkali Metal Borides: A Case Study of Lithium Nickel Borides.

Ternary lithium nickel borides LiNi3 B1.8 and Li2.8 Ni16 B8 have been synthesized by using reactive LiH as a precursor. This synthetic route allows better mixing of the precursor powders, thus facilitating rapid preparation of the alkali-metal-containing ternary borides. This method is suitable for "fast screening" of multicomponent systems comprised of elements with drastically different reactivities. The crystal structures of the compounds LiNi3 B1.8 and Li2.8 Ni16 B8 have been re-investigated by a combination of single-crystal X-ray/synchrotron powder diffraction, solid-state 7 Li and 11 B NMR spectroscopies, and scanning transmission electron microscopy. This has allowed the determination of fine structural details, including the split position of Ni sites and the ordering of B vacancies. Field-dependent and temperature-dependent magnetization measurements are consistent with spin-glass behavior for both samples.

Magnetic Characteristics of Ni-Filled Luminescent Porous Silicon.

The aim of the presented work is to combine luminescent porous silicon (PSi) with a ferromagnetic metal (Ni) to modify on the one hand the photoluminescence by the presence of metal deposits and on the other hand to influence the optical properties by an external magnetic field. The optical properties are investigated especially with respect to the wavelength-shift of the photoluminescence due to the metal filling. With increasing metal deposits within PSi the photoluminescence peak is blue-shifted and furthermore an increase of the intensity is observed. Photoluminescence spectra of bare PSi show a maximum around 620 nm whereas in the case of Ni filled samples the peak is blue-shifted to around 580 nm for a deposition time of 15 min. Field dependent magnetic measurements performed with an applied field parallel and perpendicular to the surface, respectively, show a magnetic anisotropy which is in agreement with a thin film. This film-like behavior is caused by the interconnected Ni structures due to the branched porous silicon morphology. The coercivity increases with increasing metal deposition from about 150 Oe to about 450 Oe and also the magnetic anisotropy is enhanced with the growth of metal deposits. Within this work the influence of the magnetic metal filling on the optical properties and the magnetic characterization of the nanocomposites are discussed. The presented systems give not only rise to optoelectronics applications but also to magneto optical integrated devices.

Electronic and Magnetic Properties of Pt Based Intermetalic LaPtAs and LaPt2As Compounds

We report the synthesis of polycrystalline LaPtAs and LaPt2As via solid state reactions and their temperature dependent magnetic and electrical transport properties. Crystal structures of both samples are suggested to be orthorhombic with a Pmmm space group. Temperature and field dependent magnetization measurements show that the splitting of field cooling and zero field cooling curves (initiation of irreversibility) take place at temperatures just below the Curie temperatures for both samples. This behavior is a characteristic of magnetic nanoparticles. Therefore, we propose that the magnetization arises from nanosized platinum clusters due to partial filling of the available interstitial sites. Temperature (T) dependent resistivity measurements of both alloys show metallic behavior at high temperatures and resistivity curve bends away from classical linear T dependence toward the temperature axis in the intermediate temperature range. This behavior is attributed to phonon induced interband s–d scattering. The main contribution to the resistivity at low temperatures is suggested to arise from the scattering of conduction electrons by spin-waves, in the presence of strong spin–orbit coupling due to Pt atoms. We discuss the role of spin–orbit effect on the electronic structure and the magnetic state of both samples based on various nano-scale structures.

Implications of including a magnetic ion (Cr3+ and Fe3+) at the vanadium site in a geometrically frustrated spinel MgV2O4: magnetic and catalytic properties.

The vanadium sublattice in a geometrically frustrated MgV2O4 is substituted partially with Cr3+ and Fe3+. With a successful Rietveld refinement of the powder X-ray diffraction patterns of MgVCrO4 and MgVFeO4, Cr and Fe occupy the octahedral sites of a spinel structure. Extensive field-dependent and temperature magnetic measurements on these samples reveal exciting results. MgV2O4 remains paramagnetic till 3 K, with a small divergence at around 20 K between ZFC and FC data. MgVCrO4 exhibits antiferromagnetic behavior with a Néel temperature of 13.6 K. MgVFeO4 shows spin-glass behavior resulting from the frustration with a glass transition temperature of 194 K. This sample shows a typical ferromagnetic behavior, with a coercivity of 194.5 Oe within an applied field of ±2 kOe. All three systems have been found to have a frustration index in the range of 1-25, and the effective magnetic moment decreases in the order MgVFeO4 > MgVCrO4 > MgV2O4. While the inclusion of chromium does not alter the bandgap of MgV2O4, iron substitution increases the bandgap. The partial replacement of V3+ with Cr3+ and Fe3+ increases the catalytic ability of the system in terms of the oxidative degradation of methylene blue dye. The catalytic efficiency follows the order MgVFeO4 > MgVCrO4 > MgV2O4, matching well with the trend noticed in the porosity, surface area, and redox ability of Fe3+, Cr3+ and V3+ in these samples. The degradation pathway has been followed by analyzing the intermediates from these experiments by mass spectrometry, and a plausible mechanism is proposed.

Effect of doping in Ce1−TM O2 (TM = Mn, Cr, Co and Fe) nanoparticles obtained by hydrothermal method

The structural and magnetic properties of TM-doped CeO2 nanoparticles obtained by hydrothermal method are reported. X-ray diffraction (XRD), X-ray fluorescence (XRF), transmission electronic microscopy (TEM), UV/vis absorption and temperature, field-dependent magnetic measurements are used to study the samples. The Rietveld refinement analysis of XRD patterns show that nanoparticles have a cubic structure with average sizes ranging from 8 to 12 nm. TEM images reveal that nanoparticles have a spherical-like shape and a small particles size distribution. The UV/vis absorption data allow us to state that the band gap remain unaffected by the doping. The magnetic measurements indicate that the TM-doped samples exhibit a magnetic saturation at T = 5 K and paramagnetic behavior at T = 300 K. Besides, the number of paramagnetic ions, n, was evaluated for fitting the χ(T) curves to Curie-Weiss law. The obtained n values show to be TM- and concentration-dependent and smaller than the nominal concentration for all samples.

Martensitic transformation and magnetocaloric properties of NiCoMnSn magnetic shape memory alloys

We report on the magnetic, magnetocaloric properties, and hysteresis of Ni45Co5Mn38Sn12 and Ni45Co5Mn38.5Sn11.5 Heusler polycrystalline alloys under several magnetic field values. Both alloys have a moderate magnetization difference (≈70 emu/gr) between austenite and martensite phase with a moderate thermal hysteresis (≈8 K) around the martensitic transition. Magnetic field dependent measurements reveal that the application of a magnetic field stabilizes the austenite phase, which has higher magnetization. Both alloys have substantial inverse magnetic entropy changes and adiabatic temperature changes. In addition, we investigate the accessibility of the calculated entropy change that is quantified by means of the minimum magnetic field required to start and to complete the magnetostructural phase transition.

Colossal dielectric, relaxor ferroelectric, diamagnetic and weak ferromagnetic properties of NdCrO3 perovskite nanoparticles

The NdCrO3 perovskite nanoparticles have been successfully synthesized using sol–gel auto-combustion process. Structural and morphological characterizations of the material have been performed at room temperature using XRD and SEM, respectively. Frequency- and temperature-dependent dielectric measurements have been performed in the range of 1 Hz–10 MHz and RT − 300 °C, respectively. The dielectric constant is found to be in the order of e′ > 103 at room temperature, indicating the colossal dielectric constant behaviour. Ferroelectric phase transition temperature has increased with an increase in frequency, suggesting that the material is a relaxor ferroelectric nature. Temperature- and field-dependent magnetization measurements have been carried out in the temperature range (5–400 K) and field up to 5 T. M–T curve exhibited a diamagnetic-like behaviour in low temperature and low fields. Two magnetic transitions were found at TN1 (223 K) and TN2 (33 K) which correspond to para to weak ferromagnetic transition and Nd3+ spin ordering, respectively. Hysteresis loop noticed weak ferromagnetism.

Effect of barium doping on the microstructure, dielectric and magnetic properties of GdMnO3 multiferroic ceramics

Undoped GdMnO3 (GMO) and Gd0.95Ba0.05MnO3 (GBMO) multiferroic ceramics were prepared by solid state reaction method, and the effects of Ba doping on the structure, morphology, dielectric and magnetic properties of GMO system were investigated. The XRD results show that the two samples yield single phase, and Ba doping induce structure distortion. The SEM images show that Ba doping can inhibit the grain growth. The XPS analysis indicates that more Mn4+ and oxygen vacancies are involved in the Ba doped GMO ceramics. Raman measurements show that Ba substitutes Gd into the lattice of GMO and induces lattice disorder. The dielectric measurements reveal that the Ba doped GMO exhibits the giant dielectric properties, but the dielectric loss of Ba doped samples is higher than that of the undoped samples. Temperature and magnetic field dependent magnetization measurements reveal that the low temperature magnetic behaviors of GMO system can be altered by Ba doping. The paramagnetic–antiferromagnetic transition temperature and magnetization of GMO system are decreased after Ba doping. In addition, Ba doping can also influence the long-range order of Gd moments at about 8 K. The possible reasons for the above results are discussed.

Magnetocaloric materials as switchable high contrast ratio MRI labels.

To develop switchable and tunable labels with high contrast ratio for MRI using magnetocaloric materials that have sharp first-order magnetic phase transitions at physiological temperatures and typical MRI magnetic field strengths. A prototypical magnetocaloric material iron-rhodium (FeRh) was prepared by melt mixing, high-temperature annealing, and ice-water quenching. Temperature- and magnetic field-dependent magnetization measurements of wire-cut FeRh samples were performed on a vibrating sample magnetometer. Temperature-dependent MRI of FeRh samples was performed on a 4.7T MRI. Temperature-dependent MRI clearly demonstrated image contrast changes due to the sharp magnetic state transition of the FeRh samples in the MRI magnetic field (4.7T) and at a physiologically relevant temperature (~37°C). A magnetocaloric material, FeRh, was demonstrated to act as a high contrast ratio switchable MRI contrast agent due to its sharp first-order magnetic phase transition in the DC magnetic field of MRI and at physiologically relevant temperatures. A wide range of magnetocaloric materials are available that can be tuned by materials science techniques to optimize their response under MRI-appropriate conditions and be controllably switched in situ with temperature, magnetic field, or a combination of both.

Magnetic field dependent thermal conductivity measurements of magnetic nanofluids by 3ω method

Abstract This study aims to investigate the thermal conductivity of the Fe3O4-water magnetic nanofluid under the influence of external magnetic field. The Fe3O4-water magnetic nanofluid sample with 4.8% vol. concentration was purchased. Then, it is diluted to four different vol. concentrations of 1, 2, 3, 4%. The external magnetic field in the range of 0–400 G was applied in parallel and perpendicular directions to the temperature gradient generated by the thermal conductivity measurement probe. Before thermal conductivity measurements, the external magnetic field generated in the air gap between permanent magnets has been analyzed both numerically and experimentally. Then a non-uniformity parameter was defined to relate magnetic field in measurement region to chain aggregation of the nanoparticles. Thermal conductivity measurements were conducted with 3ω method. The results showed that the thermal conductivity enhances with the increasing of the magnetic field strength up to a specific point, then the enhancement decreases. It is also observed that this specific point varies according to the particle concentration. Unlike the most of available study in the literature, in this study the lower thermal conductivity enhancements were obtained. The maximum enhancements with the highest concentration were 10% and 5.6% in parallel and perpendicular orientations, respectively.

Synthesis, structure, photoluminescent, optical and magnetic properties of a novel thulium p-hydroxybenzenesulfonate complex

Abstract A novel thulium p-hydroxybenzenesulfonate complex [Tm(C6H5O4S)2(H2O)6](C6H5O4S)·3H2O (1) was solvothermally synthesized and characterized by elemental analyses, photoluminescence, solid-state UV/vis diffuse reflectance, magnetic measurements and single-crystal X-ray diffraction. Complex 1 features an ionic structure with the thulium ion possessing a square antiprismatic geometry. Complex 1 crystallized in the monoclinic system with space group P21. Photoluminescent measurements with solid-state sample demonstrate that the anti-Stokes emission bands in the red/NIR spectral region 710 and 812 nm are observed from the Tm3+ 4f intrashell transitions from the 3F2,3 excited states to the 3H6 and 1G4 excited state to the 3H5 state, respectively. Solid-state UV/vis diffuse reflectance spectra of complex 1 show the existence of a wide optical band gap of 3.56 eV. Variable-temperature magnetic susceptibility and field dependence magnetization measurements are also studied and the magnetic susceptibility obeys the Curie-Weiss law (χ m =c/(T−θ)) with the value C being of 8.6 K and a negative Weiss constant θ being of −0.2 K.

Magnetoelectric coupling in the gapless NH4CuCl3

NH4CuCl3, a well-studied antiferromagnet best known for its gapless magnetization and unusual magnetization plateaus, is shown here to exhibit magnetoelectric behavior below the antiferromagnetic transition temperature of 1.3 K. This is based on magnetic field and temperature dependent capacitance measurements made at millikelvin temperatures, along with the appearance of spontaneous polarization and electric-field reversible pyroelectric current. The possession of both long-range magnetic and dielectric order, combined with magnetoelectric coupling, renders this material a rather unique Cu-based multiferroic and serves as a unique template for the design of such materials.

Investigation of structural and magnetic properties of Nd0.7Ba0.3Mn1-xTixO3 (x = 0.05, 0.15 and 0.25) manganites synthesized through a single-step process

The polycrystalline Nd0.7Ba0.3Mn1-xTixO3 (x = 0.05, 0.15 and 0.25) perovskite manganites have been synthesized using combustion synthesis method followed by single-step calcination process. Rietveld analysis of the X-ray powder diffraction patterns reveals that the samples crystallize in single phase body-centered orthorhombic crystal structure with space group Imma. The lattice parameters and unit cell volume increase exponentially with doping of Ti4+ ions for Nd0.7Ba0.3Mn1-xTixO3 (x = 0.05, 0.15 and 0.25). The temperature dependence of magnetization measurements reveal that all samples undergo transition from paramagnetic (PM) to ferromagnetic (FM) phase on cooling below room temperature. The values of Curie temperature (TC), Curie-Weiss temperature (TCW) and experimental effective paramagnetic moment (μeffexp) decrease exponentially on the doping of Ti4+ ions. The field dependent magnetization measurements reveal coexistence of FM and antiferromagnetic (AFM) states in the low-temperature region. The experimentally observed saturation moment (μsatexp) of all the samples is lower than the theoretically calculated values and decreases exponentially with doping of Ti4+, due to AFM component in the FM ordering. The negative slope in the low field region of the M2 vs. H/M Arrott-plots for all the samples reveals that Nd0.7Ba0.3Mn1-xTixO3 manganites exhibit a first-order magnetic phase transition. The critical exponents β, γ and δ were estimated for Nd0.7Ba0.3Mn1-xTixO3 perovskites.

DySc2N@C80 Single-Molecule Magnetic Metallofullerene Encapsulated in a Single-Walled Carbon Nanotube.

DySc2N@C80 is an endohedral metallofullerene showing single-molecule magnet (SMM) behavior. In this work, we encapsulated DySc2N@C80-SMMs into the internal one-dimensional nanospace of single-walled carbon nanotubes (SWCNTs). Using transmission electron microscopy, "peapod" structures were clearly observed. From magnetic field dependent measurements, DySc2N@C80 showed stepwise hysteresis characteristic of SMMs even inside the SWCNTs, and the coercivity increased from 0.5 to 4 kOe. In addition, it showed slow relaxation of the magnetization without an applied external magnetic field. This system is the first example where SMMs have been encapsulated in SWCNTs, and this system could be used in future molecular-spintronics devices.

Structural, magnetic, magnetocaloric and critical properties of La0.9Ba0.1MnO3 manganite

Polycrystalline La0.9Ba0.1MnO3 (LBMO) manganite samples are synthesized by usual solid state reaction technique. X-ray powder diffraction at room temperature indicates that our samples crystallize in the rhombohedral structure with an R3¯c space group. Magnetocaloric and critical behavior of the sample is probed through temperature and magnetic field dependent magnetization measurements. The compound shows paramagnetic-ferromagnetic (PM-FM) transition at critical temperature (TC) of 280 K and the nature of this transition is of second order. The deviation in the inverse of susceptibility versus temperature plot from Curie–Weiss law near TC unveiled the presence of Griffiths phase (GP) at TG = 292 K with susceptibility exponent λ = 0.3. The presence of Griffiths phase (GP) proves the existence of ferromagnetic clusters in the paramagnetic domain. Having second order phase transition sample shows a maximum magnetic entropy change (ΔSMmax) of 0.665 J/kg K for the magnetic field change of 4 Tesla. The sample also exhibits high relative cooling power (RCP) of 28.83 J/kg against this field. The critical behavior of second order phase transition in this sample is analyzed using Modified Arrott plots (MAP), Kouvel Fisher (KF) and critical isotherm analysis method. The estimated critical exponents of the sample are β ≈ 0.3, γ ≈ 1.32 and δ ≈ 5.1. These values of the critical exponents are also verified by scaling equations of state. Further the change in magnetic entropy (ΔSM) versus applied field obeys the power law ΔSM = (μoH) n with exponent value n = 0.55 at TC = 280 K.