Reversible Magnetic Change Research Articles

Wide induction range analysis of DC magnetic properties and magnetization processes of Fe-based soft magnetic composites

The paper presents systematic analyses of a wide maximum induction range (0.001 T–1.4 T) DC magnetic properties of iron-based soft magnetic composite (SMC) materials by applying different methods to understand the specific features of magnetization reversal of this increasingly popular class of soft magnetic materials, including a hitherto less explored area of the Rayleigh region of very low magnetic fields, which has growing application potential in new, low-energy consuming electronic devices or ultra-low magnetic field shielding. The total permeability, coercive field and remanent magnetic induction were analysed in connection with the reciprocity factor and inner demagnetization factor and their relations to interparticle magnetic interaction, domain wall movability and predominance of individual magnetization processes. Their dependence on the magnetic induction and the properties of each sample were revealed and confirmed also by energy loss separation. The Rayleigh region analytical expressions for the coercive field, remanent magnetic induction, the ratio of irreversible to reversible magnetization changes and their percentages within the magnetizing cycle were used for SMCs for the first time.

Field-induced deformation of ferromagnetic soft nanocomposites

Uniaxial ferromagnetic Ni nanorods were prepared by the anodic aluminum oxide (AAO) template method. Reversible magnetization changes, measured perpendicular to the texture axis, were analyzed in terms of the Stoner–Wohlfarth model (SW). Using empirical model parameters, a quantitative and consistent description of the orientation- and field-dependent magnetic torque per particle was achieved. The model was extended (eSW) to take into account the local rotation of the magnetic nanorods in a soft-elastic matrix. The nanorods were characterized regarding their size, using transmission electron microscopy (TEM), their magnetic moment and colloidal volume fraction, determined from static field-dependent optical transmission (SFOT) measurements, and their rotational shape factor, obtained from oscillating field-dependent optical transmission (OFOT). The eSW-model was used in the simulation of simple bending and torsion of thin composite filaments. These simulations were compared with experimental results with the focus on the effect of finite magnetic anisotropy and local elastic rotation on the field-induced deformation of soft nanocomposites. The high sensitivity of thin filaments enabled the investigation of torque-induced deformation at nanorod volume density as low as 10−4 at which particle-particle interactions were negligible. In addition, reprogramming of the magnetic texture by magnetization reversal and the resulting modification in the deformation pattern was investigated.

Ferromagnetic Liquid Droplet on a Superhydrophobic Surface for the Transduction of Mechanical Energy to Electricity Based on Electromagnetic Induction.

Ferromagnetic liquids undergo reversible magnetization changes upon varying external magnetic field levels. The movement of ferromagnetic liquid droplets across a coil under an external magnetic field holds promise as an energy transducer from mechanical force to electricity; however, it suffers from an adhesive issue between the ferromagnetic liquid and the solid pedestal. We introduce a superhydrophobic support that uses antiwetting surfaces to remarkably reduce adhesion during the movement of ferromagnetic liquid droplets. Maxwell numerical simulation was utilized to analyze the working mechanism and improve further electrical outputs. By controlling the droplet size, the strength of the magnetic bottom and the tilting speed of the test condition, we generated a ferromagnetic liquid droplet-based superhydrophobic magnetoelectric energy transducer (FLD-SMET) that can convert vibrational energy to electricity. When a 100 μL ferromagnetic liquid droplet was used for FLD-SMET under a 13 mT magnetic field, an electrical voltage response of 280 μV and electrical current response of ∼7.5 μA were generated using a shaking machine with a tilting speed of 9.5°/s. We thus show that such a device can serve as a self-powered light buoy floating on a water surface. Our study presents an applied concept for the design of droplet-based energy harvesters to convert surrounding vibrational energy to electricity.

Reversible and magnetically unassisted voltage-driven switching of magnetization in FeRh/PMN-PT

Reversible control of magnetization by electric fields without assistance from a subsidiary magnetic field or electric current could help reduce the power consumption in spintronic devices. When increasing temperature above room temperature, FeRh displays an uncommon antiferromagnetic to ferromagnetic phase transition linked to a unit cell volume expansion. Thus, using the strain exerted by an adjacent piezoelectric layer, the relative amount of antiferromagnetic and ferromagnetic regions can be tuned by an electric field applied to the piezoelectric material. Indeed, large variations in the saturation magnetization have been observed when straining FeRh films grown on suitable piezoelectric substrates. In view of its applications, the variations in the remanent magnetization rather than those of the saturation magnetization are the most relevant. Here, we show that in the absence of any bias external magnetic field, permanent and reversible magnetization changes as high as 34% can be induced by an electric field, which remain after this has been zeroed. Bulk and local magnetoelectric characterization reveals that the fundamental reason for the large magnetoelectric response observed at remanence is the expansion (rather than the nucleation) of ferromagnetic nanoregions.

Switchable Ferro-, Ferri-, and Antiferromagnetic States in a Piezo- and Hydrochromic Metal-Organic Framework.

The Mn(II) metal-organic framework with a viologen-based tetracarboxylate ligand exhibits reversible optical (color) and magnetic changes concomitant with stimuli-induced electron transfer from carboxylate to viologen. Compression causes a magnetic transformation from ferro- to ferrimagnetic, while water release/reuptake switches the magnetic behavior between ferro- and antiferromagnetic.

Magnetic Sponge Behavior via Electronic State Modulations.

A reversible magnetic change in response to external stimuli is a desired function of molecular magnetic materials. The magnetic change induced by a change in the intrinsic spin is significant because the magnetic change is inevitable and could become drastic. In this study, we demonstrate a reversible magnetic change closely associated with electronic state modulations, as well as structural modifications realized by solvation/desolvation cycles of a magnetic sponge. The compound was a D2A-type layered magnet, [{Ru2(O2CPh-2,3,5-Cl3)4}2(TCNQMe2)]·4DCM (1; 2,3,5-Cl3PhCO2- = 2,3,5-trichlorobenzoate; TCNQMe2 = 2,5-dimethyl-7,7,8,8-tetracyanoquinodimethane; DCM = dichloromethane), where [Ru2(O2CPh-2,3,5-Cl3)4] ([Ru2II,II]) is an electron donor (D) and TCNQMe2 is an electron acceptor (A). Compound 1 had a one-electron-transferred, charge-ordered state with a [{Ru2II,II}-TCNQMe2•--{Ru2II,III}+] (1e-I) formula. Strong intralayer antiferromagnetic couplings between [Ru2II,II] with S = 1 or [Ru2II,III]+ with S = 3/2 and TCNQMe2•- with S = 1/2, as well as ferromagnetic interlayer interactions, induced long-range ferrimagnetic ordering at Tc = 101 K. Interstitial DCM molecules were located between layers, and these were gradually eliminated under vacuum at 80 °C to form a solvent-free compound (1-dry) without loss of crystallinity. The electronic state of 1-dry thermally fluctuated and eventually provided a charge-disproportionate disordered state, with a [{Ru2}0.5+-TCNQMe21.5--{Ru2II,III}+] (1.5e-I) formula as the ground state. The Tc in 1-dry was 34 K because of the presence of diamagnetic TCNQMe22- in some parts of the framework. A large Tc variation with Δ Tc ≈ 70 K was switchable; switching was achieved by charge-state modulations accompanied by subtle structural modifications in solvation/desolvation treatments.

Free and forced Barkhausen noises in magnetic thin film based cross-junctions

Barkhausen noise, driven by thermal fluctuations in stationary magnetic field, and Barkhausen jumps, driven by sweeping magnetic field, are demonstrated to be effects of different orders of magnitude. The critical magnetic field for domain walls depinning, followed by avalanched and irreversible magnetization jumps, is determined. Magnetoresistive response of NiFe/M/NiFe (M = Au, Ta, Ag) trilayers to stationary and sweeping magnetic field is studied by means of anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) measurements. Thermal fluctuations result in local and reversible changes of magnetization of the layers in thin film magnetic junctions, while the sweeping magnetic field results in reversible and irreversible avalanched domain motion, dependently on the ratio between the values of sweeping magnetic field and domain wall depinning field. The correlation between AMR and PHE responses to Barkhausen jumps is studied. The value of this correlation is found to be dependent on the α angle between the directions of magnetic field and current path.

Reversible Single-Crystal-to-Single-Crystal Transformation and Magnetic Change of Nonporous Copper(II) Complexes by the Chemisorption/Desorption of HCl and H2O.

Vapor-responsive magnetic materials are highly promising for applications as chemical switches or sensors. Compared with porous materials, nonporous species benefit in overcoming the intrinsic conflict between magnetic exchange and porosity but usually suffer from the powdering of single crystals, which hinders the understanding of the structural nature of vapor response and magnetic switch. Single-crystal-to-single-crystal (SCSC) transformation of nonporous compounds through the desorption/absorption of gaseous HCl is unprecedented. Reported here is a discrete nonporous copper(II) complex, (H3O)[K(15-crown-5)2][CuCl4], that exhibits reversible SCSC transformation and magnetic change by the chemisorption/desorption of HCl and H2O. Significant changes in the coordination number (4 ↔ 3), space group (P1̅ ↔ P21/c), color (green ↔ red), and magnetic behavior (antiferromagnetic ↔ paramagnetic) were found during the SCSC transformation.

Toward On‐and‐Off Magnetism: Reversible Electrochemistry to Control Magnetic Phase Transitions in Spinel Ferrites

The magnetoelectric effect, i.e., electric‐field control of magnetism in artificial heterostructures is usually limited to surface/interface atoms of the magnetic materials. In order to attain electrical control of magnetism in bulk ferromagnets, this study proposes to extend the definition of magnetoelectric phenomena to include reversible, chemistry‐controlled magnetization switching. A large and reversible change in the room temperature magnetization in strong ferromagnets is reported, with electrochemistry‐driven Li‐ion exchange; carefully chosen spinel ferrites demonstrate a reversible magnetization variation up to 50% for CuFe2O4 and 70% for ZnFe2O4. In case of CuFe2O4, the magnetization variation is predominantly associated with the preferential reduction of Cu2+ to Cu+ ions, and, hence, abides a nearly one‐to‐one relationship with the amount of injected Li‐ions. In addition, the reduction of Cu2+ also annihilates the Fe3+OCu2+ magnetic interaction, resulting in a marked decrease in the Neél temperature of CuFe2O4. In contrast, the electrical tuning of superexchange interactions is found to play the decisive role in ZnFe2O4, where the simple electrochemical reduction model of magnetic cations can only explain a nominal fraction of the total magnetization variation, and indeed an electrochemically controlled reversible change in transition temperature is found necessary to account for the large magnetization variation observed.

Intercalation‐Driven Reversible Control of Magnetism in Bulk Ferromagnets

An extension in magnetoelectric effects is proposed to include reversible chemistry-controlled magnetization variations. This ion-intercalation-driven magnetic control can be fully reversible and pertinent to bulk material volumes. The concept is demonstrated for ferromagnetic iron oxide where the intercalated lithium ions cause valence change and partial redistribution of Fe(3+) cations yielding a large and fully reversible change in magnetization at room temperature.

Temperature induced reversible structural and magnetic changes in a crystal of tetrachlorosemiquinone anion radical

The anion radicals with nonaromatic π-systems, such as semiquinones, are interesting in the design of functional materials. Magnetic properties of solvates of alkali salts of tetrachlorosemiquinone (chloranil) radical anion are tuned by crystal engineering using different solvents (2-butanone and acetonitrile) and/or cations (potassium and ammonium) in crystals. The structural and magnetic characteristics of two salts were studied by variable-temperature single crystal X-ray diffraction and magnetic susceptibility measurements. DFT and CAS-MP2 calculations were used to correlate magnetic and structural properties of radical anions. The solid-state structure of potassium tetrachlorosemiquinone 2-butanone solvate, influenced by temperature, switches between a paramagnetic-like monomeric radical form at high temperature (200 K) and diamagnetic dimeric one at low temperature (100 K). The reversible transition is controlled by solvent and cation types which affect intrastack distance and magnetic properties.

Electric field manipulation of magnetization at room temperature in multiferroic CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures

Multiferroic heterostructures were fabricated by growing ferrimagnetic CoFe2O4 films on ferroelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 substrates using pulsed laser deposition. Upon applying an electric field, the in-plane magnetization of the heterostructures increases and the out-of-plane magnetization decreases. Sharp and reversible changes in magnetization under electric field were also observed for the poled sample. The relative change in magnetization-electric field hysteresis loops were obtained for both the in-plane and out-of-plane magnetizations. Analysis of the results suggests that the electric field induced change in magnetic anisotropy via strain plays an important role in the magnetoelectric coupling in the heterostructures.

The thermal and magnetic pre-treatments dependence of the magneto-mechanical behavior of Ni-Mn-Ga single crystal

The effects of thermomagnetic pretreatments on the magnetism and mechanical properties of Ni50.3Mn28.7Ga21 single crystal were studied. Two samples A and B were heated to a temperature above their Curie temperature （TC） and then cooled down to room temperature via different cooling processes. An external magnetic field was applied during cooling of sample A from T>TC to room temperature. For sample B, the same magnetic field was applied at T below its TC but higher than its austenite to martensitic transformation temperature during cooling. The reversible strain and change in magnetization （ΔM） measured at room temperature for sample A are much smaller than those for sample B. It was supposed that with the appearance of the magnetic field during the transformation from austenite to martensitic phase, the field tends to induce a favorable single martensitic domain in sample B, leading to the greater reversible strain and ΔM. However, preferential magnetic domains may develop in sample A by applying the same magnetic field during the transition from paramagnetism to ferromagnetism, leading to the different alignments of martensitic domains from those in sample B. This might be the reason for the smaller reversible strain and ΔM in sample A.

Reversible light-induced magnetization change in an azobenzene-attached pyridylbenzimidazole complex of iron(ii) at room temperature

Reversible photomagnetic effects were demonstrated at room temperature by the use of a new iron(II) complex t100c0-[Fe()3](BF4)(2).3H2O ( =phenyl(2-pyridin-2-yl-3H-benzoimidazol-5-yl)diazene), based on the strategy of the so-called "ligand-driven light-induced spin change" (LD-LISC) effect. The complex showed thermal spin crossover in both solid and solution states, showing a transition temperature T1/2=279(16) K in acetone. trans-cis Photoisomerization of azobenzene moieties using different energy lights afforded reversible photomagnetic effects, which were directly determined using the Evans 1H NMR method.

Identifying reversible and irreversible magnetization changes in prototype patterned media using first- and second-order reversal curves

Arrays of nanomagnets have important potential applications as future generation ultrahigh-density patterned magnetic recording media, in which each nanomagnet constitutes a single bit. We introduce a powerful technique to identify and quantify reversible and irreversible magnetization changes, a key challenge in characterizing these systems. The experimental protocol consists of measuring a few families of second-order reversal curves along selected profiles in the first-order-reversal-curve diagram, which then can be decomposed into truly irreversible switching events and reversible magnetization changes. The viability of the method is demonstrated for arrays of sub-100-nm Fe nanomagnets, which exhibit complex magnetization reversal processes.

Photoinduced Magnetism and Random Magnetic Anisotropy in Organic-Based Magnetic SemiconductorV(TCNE)xFilms, forx∼2

The V(TCNE){x}, x approximately 2 is an organic-based amorphous ferrimagnet, whose magnetic behavior is significantly affected in the low field regime by the random magnetic anisotropy. It was determined that this material has thermally reversible persistent change in both magnetization and conductivity driven by the optical excitation. Here, we report results of a ferrimagnetic resonance study of the photoinduced magnetism in V(TCNE){x} film. Upon optical excitation (lambda approximately 457.9 nm), the ferrimagnetic resonance spectra display substantial changes in their linewidths and line shifts, which reflect a substantial increase in the random magnetic anistropy. The results reflect the role of magnetic anisotropy in disordered magnets and suggest a novel mechanism of photoinduced magnetism in V(TCNE){x} induced by the increased structural disorder in the system.

A Dynamic Porous Magnet Exhibiting Reversible Guest‐Induced Magnetic Behavior Modulation

A flexible 3D porous magnet [KCo(7)(OH)(3)(ip)(6)(H(2)O)(4)]center dot 12H(2)O based on unprecedented trigonal-prismatic heptanuclear Co(II) clusters (see figure) is reported to undergo a reversible single-crystal-to-single-crystal transformation on the desorption/adsorption of guest molecules, accompanied by reversible magnetic property changes. Ibis might be applied for the sensing of coordinative, paramagnetic, small molecules such as 02 and NO.

Temperature dependence of reversible field-induced strain in Ni–Mn–Ga single crystal

Abstract Temperature dependence of reversible magnetic shape memory effect and magnetization changes were studied in Ni 49.7 Mn 29.1 Ga 21.2 . The twinning stress, equivalent magnetic stress, switching field and maximum strain all increase with decreasing temperature. A 5–6% reversible strain is observed in the temperature range of 307–263 K and it sharply decreases to 1–2% at temperatures below 263 K. The main features of the observed behavior are explained using a simple energy-controlled model. Predictions of the model agree well with the experiment.

Novel Co complex with high transformation temperature of valence tautomerism

The preparation and characterization of [Co II-HS(dpa)(3,5-DBSQ) 2] are presented first time. From the magnetic curve, the phase transition temperatures, T c is 380 K. To our knowledge, the T c value is the highest among the species exhibiting thermal valence tautomerism. After illumination with 532 nm light at 5 K, the magnetization values increase from μ eff=1.73 to 2.0 μ B, showing valence tautomerism. The metastable state will recover to the original state with the increase of temperature. The alternate illumination with 532 and 830 nm light at 5 K can induce a reversible change in magnetization. The quick response to light means that this compound can be used for the development of optical–magnetic switch.

Exchange torque and spin transfer between spin polarized current and ferromagnetic layers

Magnetization reversal triggered by spin injection is measured in electrodeposited Co/Cu/Co pillars (diameter about 60 nm). Two protocols are used. (i) switching of magnetization after a current pulse is monitored as a function of applied field. The maximum offset from the switching field at which irreversible switching occurs is a measure of the strength of the effect; and (ii) irreversible and reversible magnetization changes are observed while the current is ramped at fixed applied field. (i) and (ii) show that irreversible transitions occur only from antiparallel to parallel magnetic configurations and for electrons flow from the polarizer to the analyzer.