Magnetic Transformation Research Articles

Structure and magnetic properties of a new hexaferrite (Ba,Pb)(Fe,Ti)9O15

A new hexaferrite with the composition (Ba,Pb)(Fe,Ti)9O15 was synthesized from lead(II) oxide flux, which lead to the incorporation of Pb2+ into the structure on the barium position. Like other ferrites, its structure can be described as the stacking of characteristic building blocks along [001]. Within the structure, the barium site is mixed occupied with lead. Octahedrally coordinated interstices are filled by iron and titanium in different ratios. Tetrahedra and trigonal bipyramids are solely occupied by iron. Wavelength dispersive X-ray spectroscopy provides the composition Ba0.69(1)Pb0.34(2)Fe7.28(7)Ti1.52(3)O15. Lastly, the compound shows ferrimagnetic behavior with a Curie temperature of TC = 309.3 °C and a strong indication for a magnetic phase transformation to ferromagnetic state at high magnetic field strengths and at temperatures below −123 °C.

Sintering time dependent structural and magnetic phase transformations in Pr doped BiFeO3 multiferroics

The structural stability and magnetic properties of Pr doped BiFeO3 (Bi0.8Pr0.2FeO3) multiferroic ceramics depends on the sintering time. As the sintering time is varied, the structural and magnetic phase transformations has been observed in the multiferroic ceramic. For the present investigation, the samples have been synthesized by conventional solid state reaction method and sintered for 1 h, 2 h and 3 h. The crystal structure of samples has been verified by the Rietveld refinement of XRD patterns. The sample sintered for 1 h is found to possess rhombohedral R3c as major phase. The dominating structural phase changes to orthorhombic Pbam for 3 h sintering. The remanent magnetization in the samples shows a significant increase with the sintering time as observed from M – H loops obtained by VSM. The enhancement in magnetization is suggested to be caused by the increase in canting angle while crossing over from polar rhombohedral R3c phase to antipolar orthorhombic Pbam symmetry.

Magnetic Difference Between Deep and Surface Soil Within an Agricultural Area in Southern China: Implications for Magnetic Mineral Transformation During Pedogenic Process Under Subtropical Climate

AbstractMineral magnetism is proved as a sensitive method of pedogenesis by numerous studies. However, formation pathways of magnetic minerals and subsequent magnetic response are quite different under different climate. In order to improve the understanding of magnetic assemblages in paddy soil layers in relation to soil formation conditions, a systematic magnetic investigation was performed for surface and its corresponding deep soil collected from 70 sampling sites within a paddy soil area under subtropical climate, south China. Rock magnetic results showed that superparamagnetic, single‐domain, and multiple domain maghematite and hematite were produced during ferrallitization pedogenic processes. Some fine superparamagnetic and single‐domain ferrimagnetic particles were dissolved during late gleyization of paddy soil formation with increased organic matter and reductive condition. As a result, magnetism of surface soil was weaker than that of deep soil despite basically consistent magnetic mineral composition.

Микросистемы с высоким коэффициентом преобразования слабого магнитного поля на основе магниторезистивных наноструктур

The magnetoresistive transducers of magnetic field have been claimed both for direct and indirect applications in various areas of industry, transport and special equipment. In the paper the special features of used terminology and dimensions in the main parameters of magnetic field magnetoresistive transformers have been revealed. The main results of experimental studies on the developed thin-film magnetoresistive nanostructures (TMN) with even and odd transfer characteristics have been presented. The TMN design on an anisotropic magnetoresistive effect has an odd transfer characteristic and conversion coefficient at the level of 8 mV/V/E. The TMN design based on a giant magnetoresistive effect has an even transfer characteristic and a conversion coefficient at the level of 27 mV/V/E. The results of promising design and technological solutions allowing reaching the values of conversion coefficient of TMN at the level exceeding 100 mV/V/E have been shown. The results of the study of the test spin-tunnel magnetoresistive nanostructure (spin-tunnel TMN) with giant magnetoresistive effect, exceeding 100% have been presented. The novelty of the obtained results has been reflected and the perspective of using highly sensitive TMN and creating the non-volatile MRAM on the basis of thin-film magnetoresistive structure with the spin-tunnel magnetoresistive effect have been determined.

Integrated Current Balancing Transformer Based Input-Parallel Output-Parallel LLC Resonant Converter Modules

The input-parallel output-parallel (IPOP) connected converter system allows the use of a low-power converter module for high-power applications. However, mismatches in the IPOP circuit parameters cause uneven current sharing between the constituent modules, which makes the IPOP system unreliable for practical use. Different techniques have been proposed so far; however, they cause an increase in magnetic volume, cost, component count, and complexity of the IPOP system. In this article, a reliable and efficient magnetic integration technique is proposed for the current sharing requirements of the IPOP system based on LLC resonant converter modules. The proposed integrated magnetic current balancing transformer (ICBT) based IPOP LLC resonant converter modules ensures both the input current sharing and output current sharing between constituent LLC modules, under the open-loop condition quite effectively. The proposed ICBT is the resonant inductors of LLC resonant converter modules, coupled to perform the function of current balancing as well as generates enough leakage inductances for resonance operation, simultaneously. Thus, it can reduce the overall magnetic volume and improves power density of the IPOP LLC modules. Moreover, the current sharing performances of the proposed ICBT in both the steady-state and dynamic-state are analyzed based on the magnetic and electrical model. To validate the performance of the proposed ICBT based IPOP two LLC modules, a 2.6-kW hardware prototype was designed, fabricated, and tested successfully.

Features of the electrical resistivity as a function of temperature in GdxPb1−xMo6S8 (x = 0.5, 0.7, and 0.9) superconductors

For the first time, the temperature ρ(T) and magnetic field ρ(H) dependences of the electrical resistivity have been studied in compounds GdxPb1−xMo6S8 (x = 0.5, 0.7, 0.9), in the temperature range 2–300 K and magnetic fields up to 9 T. In the absence of a magnetic field, the compounds Gd0.9Pb0.1Mo6S8 and Gd0.7Pb0.3Mo6S8 exhibit features along ρ(T) (maximum and minimum for Gd0.9Pb0.1Mo6S8, and an inflection for Gd0.7Pb0.3Mo6S8), in some temperature ranges. The ρ(T) dependence of Gd0.5Pb0.5Mo6S8 is monotonic over the entire investigated temperature range. The presence of a small positive magnetoresistance (up to 2%) is established, which peaks at temperatures where there are features. It is shown that the dependence of the magnetization M(T) in the studied temperature range does not have features associated with long-range magnetic transformations. It is suggested that the features along the ρ(T) of Gd0.9Pb0.1Mo6S8 and Gd0.7Pb0.3Mo6S8 can be caused by band structure rearrangement, and the appearance of two charge carrier channels that are responsible for the semiconductor and metallic behavior of the temperature dependences of the electrical resistivity.

Gas-Dependent Reversible Structural and Magnetic Transformation between Two Ladder Compounds

We report reversible structural transformation that occurs in two ladder compounds: Cu2CO3(ClO4)2(NH3)6 (1) and Cu2CO3(ClO4)2(NH3)5(H2O) (2), when they are exposed to gaseous vapors. The ladder structures of both 1 and 2 consist of two Cu2+ ions and one CO32− ion. In 1, the Cu2+ ions are coordinated by three NH3 molecules on each side, while those in 2 are coordinated by three NH3 molecules on one side, and two NH3 molecules and one H2O molecule on the other side. We demonstrated reversible transformation of 1 and 2 via the exposure of 1 to H2O vapor and the exposure of 2 to NH3 vapor using a simple bench-scale method. The minor structural change observed led to a significant difference in physical properties, which we observed using several methods.

Features of Synthesis and Magnetic Characteristics of Yttrium Orthoferrite Produced by Gel Combustion

The effect of an organic fuel on the combustion mode, features of combustion, and the phase composition of the end product of the synthesis of yttrium orthoferrite was studied. It was shown that the production of YFeO3 requires a fuel that favors the formation of a single polymer metal-containing complex determining the gel structure and ensures the combustion in an intense self-sustaining mode. Yttrium orthoferrite was synthesized by gel combustion using a mixture of citric acid and ammonium nitrate as a fuel. The unit cell parameters of orthorhombic (space group Pnma) yttrium orthoferrite were found to be a = 5.594(2) A, b = 7.604(3) A, c = 5.282(1) A, and V = 224.7(3) A3. The temperature dependence of the specific magnetization of YFeO3 in the range 80–800 K was investigated. The temperature of the magnetic phase transformation magnetic order–magnetic disorder was determined to be TC ~ 620 K.

The influence of self-doping of stibnite ore with impurities on the preparation, heat capacity, magnetic and transport properties of tetrahedrite Cu12Sb4S13

Abstract Stibnite mineral (mainly Sb2S3) has been employed for the synthesis of tetrahedrite Cu12Sb4S13 bulk material by spark plasma sintering. High purity Cu12Sb4S13 can be quickly obtained by two sintering procedures at temperatures from the range of 420 °C to 440 °C for 1 h. Appropriate reduction of Cu content (Cu12+xSb4S13, x ⩽ –0.05) or CuS content (Cu12−ySb4S13−y, y = 0.1 or 0.3) was beneficial to fabricate Cu12Sb4S13. The secondary resintering improved the purity of Cu12Sb4S13 material. The first-order magnetic phase transformation with magnetic hysteresis effect was confirmed by the behavior of susceptibility, heat capacity and resistivity. The magnetization showed a linear increase with increasing field (up to 7 T) and non-saturation behavior was observed. The impurities in stibnite mineral Sb2S3 had a weak influence on the transformation temperature but affected the low-temperature magnetization value (~0.15, close to natural tetrahedrite). Similar transformation was observed by the analysis of heat capacity. The properties such as electrical resistivity, Seebeck coefficient and thermal conductivity were also measured for Cu11:9Sb4S13 and Cu11:9Sb4S12:9. The maximum figure of merit ZT of Cu11:9Sb4S12:9 was 0.22 at 367 K.

An Analog Magnetic Isolator for Space Power Applications

In power electronic applications, often a galvanic isolation barrier has to be passed. An example of this is the feedback control of isolated DC/DC converters where the secondary voltage has to be passed to the primary side where the control circuitry lies. For these applications, the use of optocouplers is well known in terrestrial applications. However, optocouplers tend to suffer degradation from the radiation damage induced by the space environment. For this reason, some space grade DC/DC converters use some form of magnetic feedback. In this paper, a magnetic analog isolator is presented. It will allow for passing an analog voltage through an isolation barrier by means of a magnetic transformer based circuit. It will be based on an LLC DC/DC converter so the gain between both sides of the isolator can be tuned. Design methodology will be presented so the circuit can be tailored for any foreseen application. In this paper, a simulation of its application in a Latching Current Limiter is presented. All the components will be discrete components which have a rad-hard version. A design example having a bandwidth around 20 kHz and a gain of 4.6 dB is shown in the Experimental Results sections to support the analysis.

On the temperature-dependent coercivities of anisotropic Nd-Fe-B magnet

We have studied the thermal stability of coercivity in anisotropic Nd-Fe-B magnet from 300 K to 500 K by combining the micromagnetic simulation with the numerical estimation of coercivity reduction caused by thermal activation. The numerically achieved coercivities agree well with the measured value of a conventional sintered magnet. The concave shape of Hc(T) which is different from linear shape of HA(T) in anisotropic Nd-Fe-B magnets, as well as the break of linearity in the fitting of Hc/Msvs. HA/Ms are found both originated from the temperature dependence of magnetization of the ferromagnetic grain boundary phase. This curving of Hc(T) can be eliminated via transformation of grain boundary magnetism into non-ferromagnetic. We have demonstrated this hypothesis experimentally in the hot-deformed magnet heavily infiltrated with Nd–Cu alloy. The parameters, α and Neff, achieved in the fitting of Kronmüller equation are found to be largely deviated from those achieved in micromagnetic method, which is due to the ignorance of temperature dependent grain boundary magnetization and the thermal activation effect in the fitting.

Magnetic transformation of Mn from anti-ferromagnetism to ferromagnetism in FeCoNiZMnx (Z = Si, Al, Sn, Ge) high entropy alloys

We design high entropy alloys (HEAs) with different induction elements (Si/Al/Sn). In order to keep the crystal structure invariant and to investigate how the increment in saturation magnetization (Ms) is caused only by the change of electron spin state, each set of HEAs contains a different amount of Mn. Synergistic effects among induction elements that induce the magnetic transformation of Mn from anti-ferromagnetism to ferromagnetism are found. Ms of added Mn reduces when a particular induction element (Si0.4/Al0.4/Sn0.4) exists, while a larger increment of Ms appears when two induction elements coexist, Si0.4Al0.4 (25.79 emu/g) and Sn0.4Al0.4 (15.43 emu/g). This is reflected in the microcosmic magnetic structure for the emergence of closed domains due to large demagnetization energy, which is confirmed by the Lorentz transmission electron microscope (LTEM) data. The calculated magnetic moments and the exchange integral constants from density functional theory based on the Exact Muffin-Tin Orbits formalism reveal that the magnetic state and the strength of ferromagnetic and anti-ferromagnetic coupling determine the variation of Ms in different chemical environments. The difference in energy levels of coexisting multiple induction elements also leads to a larger increment of Ms, Si0.4Al0.4Sn0.4 (29.78 emu/g), and Si0.4Al0.4Ge0.4Sn0.4 (31.00 emu/g).

Restoration of ion beam irradiation induced metastable magnetic states and lattice structures of FeRh thin films by heat treatments at elevated temperatures

The restoration process of irradiation-induced metastable magnetic states of FeRh thin films during the heat treatments at 373–1073 K was investigated by measuring the magnetization-magnetic field (M−H) curves and the magnetization-temperature (M−T) curves for the FeRh thin films irradiated with 50 keV Ar ions to various fluences. The recovery of lattice structures accompanying the magnetic restoration was also examined by an x-ray diffraction (XRD) measurement. For the ferromagnetic state induced by low fluence irradiation, the magnetic restoration to the thermal equilibrium antiferromagnetic state occurs during the heat treatment. Whereas for the paramagnetic state induced by high fluence irradiation, it once changes to the ferromagnetic state, and then, is restored to the antiferromagnetic (i.e., thermal equilibrium) state during the heat treatment. The above magnetic transformation occurs by the heat treatments at higher temperatures for the higher fluence irradiation. Moreover, the magnetic transition temperature from the antiferromagnetic state, which has been thermally restored from the irradiation induced ferromagnetic or paramagnetic state, to the ferromagnetic state decreases with increasing the irradiation fluence. This result implies that the antiferromagnetic state with larger amount of lattice defects is more unstable against the thermal vibration of lattice atoms.

Advanced ferrogels with high magnetic response and wear resistance using carbon nanotubes

Polyacrylamide hydrogel with nickel nanoparticles (ferrogel) have received increasing attention because of their viability in magnetic biosensors, magnetic field sensitive transformers, field-assisted drug deliverers, strain sensors and wearable electronics etc. In traditional ferrogels, wear can destruct durability and magnetic sensitivity, which might make them inept for further functioning. In this work, carbon nanotubes (CNTs) were incorporated into polyacrylamide hydrogel embedded with nickel nanoparticles to obtain high magnetic sensitivity, strength and wear resistance. The incorporation of CNTs, offered an increase in saturation value of magnetic moment form 0.76 emu/g to 1.41 emu/g, i.e. an 85% increase was demonstrated, owing to the maximum grain refinement (28 nm) and spin transport-based magnetization in CNTs. The enriched magnetism of CNT added ferrogel is also linked to the increment in domain sizes (70–120 nm), domain wall width (40–80 nm) and magnetostatic energy (∼0.136 erg/cm2), calculated by using magnetic force microscopic data. Along with this, the wear rate is also demonstrated to have dropped ∼40% with the adding up CNTs into the ferrogel, due to the lubricity effect of CNTs as well as the synergistic effect created between Ni nano-particles and CNTs. Thus, CNT added ferrogel shows higher magnetism and limited wear damage and can be an advanced polymer composite for magnetic sensors and drug delivery applications.

Influence of Interfacial Stress on Microstructural Evolution in NiAl Alloys

A phase-field model for the phase transition between austenite and martensite and twinning between two martensitic variants is presented from our previous theory [1] with the main focus on the influence of interfacial stress that is consistent with the sharp interface limit. Each variant-variant transformation can be represented by only one order parameter. Thus, it allows us to get the analytical solution of interface energy and width. Coupled phase-field and elasticity equations are solved for cubic-to-tetragonal phase transformation in NiAl shape memory alloy. The effects of interfacial stress are studied for martensite-martensite interfaces in detail, which was absent in [1]. Additionally, stress and temperature-induced growth of the martensitic phase inside austenite and twining are simulated. Some of the nontrivial experimentally observed microstructures reproduced in the simulations [1] are analyzed in detail. It includes tip splitting and bending, and twins crossing. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

Phase Transformations in Iron Oxide under the Action of Microwave Radiation

It has been shown that microwave radiation induces structural and, as a consequence, magnetic phase transformations in iron oxide α-Fe2O3 (hematite). After microwave irradiation of fine-dispersed partially amorphized Fe2O3 particles for 10 min in wet air, the percentage of crystalline hematite drops by 40% but the total amount of crystalline components increases owing to the formation of a new ferromagnetic modification, α-Fe2O3 maghemite. The initial antiferromagnetic and final ferromagnetic Fe2O3 samples are powders consisting of spherical particles with nearly equisized (40–60 nm) domains of X-ray coherent scattering.

Large refrigeration capacity in a Ni48Co1Mn37In14 polycrystalline alloy with low thermal hysteresis

For Ni–Mn–In based alloys, simultaneously achieving large magnetic field-induced transformation temperature shift and low thermal hysteresis is of vital significance to obtain significant magnetocaloric properties. In this work, we employ low content Co doping to tune the magnetostructural transformation parameters and demonstrate large inverse magnetocaloric effect in a Ni48Co1Mn37In14 alloy. It is shown that 1% Co doping confers a strong magnetic field dependence of transformation temperature, i.e., 7.2 KT−1, giving rise to a wide refrigeration temperature window of 34 K under the magnetic field change of 5 T. Together with relatively low thermal hysteresis, i.e., ~8 K, giant effective refrigeration capacity up to 284 Jkg−1 is achieved under the magnetic field change of 5T. The present work is expected to offer some instructions for the design of Ni–Mn-based alloys with excellent magnetocaloric properties.

Influence of cation distribution on magnetic response of polycrystalline Co1−xNixFe2O4 (0 ≤ x ≤ 1) ferrites

The present paper reports the interconnection between cation distribution and physical properties of Co1−xNixFe2O4 (0 ≤ x ≤ 1) samples to tune them for suitable applications at room temperature. A set of five Co1−xNixFe2O4 (CNFO) samples with different nickel concentration (x) were prepared by using citrate-based ‘sol-gel’ method. The x-ray diffraction (XRD), Raman spectroscopic and scanning electron microscopic techniques were employed to characterize the samples structurally and morphologically. The Rietveld refinement of diffraction data confirms the monophasic spinel cubic structure with space group. The cationic arrangement between the tetrahedral (A-) and octahedral (B-)sites of (AB2O4-type) CNFO samples was estimated from the diffraction intensities of XRD and the spectral peaks of Raman data. Based on the cation arrangement, the estimated magnetic moment values, by adopting Neel’s sublattice model, were in correlation with each other and found to reduce with the rise in nickel content. A systematic change in the lattice, spectral and magnetic parameters with an increase in nickel content in CNFO, clearly ensures the transformation from hard to soft magnetic behavior of the samples. The marked magnetic transformation was elucidated in the framework of cation distribution. The tuning of magnetic parameters and cation distribution in ferrites with an appropriate element substitution might be a viable approach to make them for desired high-frequency magnetic applications.

Creating, probing and confirming tetragonality in bulk FeNi alloys

Effects derived from the simultaneous application of passive, uniform saturating magnetic field and tensile stress during long-time annealing utilizing a unique and simple processing tool, the MultiDriver furnace, are reported. In particular, the evolution of the crystalline structure and the magnetic domain configuration of severely deformed and subsequently annealed equiatomic FeNi alloys were revealed by high-energy synchrotron X-Ray diffraction and Magneto-Optic Kerr Microscopy. These alloys are known to undergo a first-order magnetic phase transformation to a chemically ordered tetragonal (L10) structure in astronomical timeframes. MultiDriver-annealed specimens are found to retain a tetragonal crystallographic state and texture that is similar to that of the precursor deformed state, with a smaller unit cell volume and a larger c/a ratio relative to those of control samples that were annealed under the same conditions but without magnetic field and stress drivers. Magnetic domain images echo these observations: while as-cast FeNi alloys exhibit domain patterns consistent with the presence of a stress-modified cubic magnetocrystalline anisotropy, large areas of domains with uniaxial anisotropy are present in MultiDriver-annealed samples. While no chemical order was demonstrated in these samples, a strategy is proposed for achieving the ordered state by employing a passive magnetic gradient in the MultiDriver furnace to enhance diffusion.

The elastic, magnetic, and electrical properties of Sr1–уYуCoO3–x cobaltites in the composition range 0.2 ≤ y ≤ 0.3

Anion-deficient layered cobaltites Sr0.75Ln0.25CoO3–x (Ln is a lanthanide) have attracted the special attention of the scientists who study the nature of phase transformations in perovskite-like cobaltites, the anomalous behavior of the temperature magnetization of which is still the subject of scientific discussion. The purpose of this work is to investigate the regularity of changes in the elastic, magnetic, and electrical properties of layered cobaltites Sr1–уYуCoO3–x in the composition range 0.2 ≤ y ≤ 0.3 over a wide temperature range. The studied polycrystalline samples were obtained by the known ceramic technology in the air. Electron microscopic studies were performed on a LEO 1455 PV scanning electron microscope. The temperature dependence of the Young’s modulus was studied by the method of resonance vibrations in the frequency range 1000–6000 Hz and in the temperature range 100–450 K. X-ray phase analysis was performed on a DRON-3M diffractometer under Cu-Kα radiation. Magnetic measurements were performed using a physical property measurement system (Cryogenic Ltd.) in the temperature range 5–325 K. As a result of the studies, it was found that in the temperature range 25–300 K, Sr1–уYуCoO3–x solid solutions (0.2 ≤ y ≤ 0.3) are characterized by the semiconductor-like conductivity. No significant magnetoresistive effect was observed in this temperature range for the studied compositions. It was shown that the Sr1–уYуCoO3–x solid solution (у = 0.25) exhibits two magnetic phase transformations: low-temperature near 220 K and high-temperature at 350 K. The nearby compositions of the concentration range 0.2 ≤ y ≤ 0.3 exhibit magnetic phase transformations at temperatures above room temperature. No low-temperature phase transitions were detected in them. It has been established that magnetic phase transformations are accompanied by structural transitions at corresponding temperatures.