Abrupt Change In Magnetization Research Articles

Metamagnetic multiband Hall effect in Ising antiferromagnet ErGa2

Frustrated rare-earth-based intermetallics provide a promising platform for emergent magnetotransport properties through exchange coupling between conduction electrons and localized rare-earth magnetic moments. Metamagnetism, the abrupt change of magnetization under an external magnetic field, is a signature of first-order magnetic phase transitions; recently, metamagnetic transitions in frustrated rare earth intermetallics have attracted interest for their accompanying nontrivial spin structures (e.g., skyrmions) and associated nonlinear and topological Hall effects (THE). Here, we present metamagnetism-induced Hall anomalies in single-crystalline ErGa2, which recalls features arising from the THE but wherein the strong Ising-type anisotropy of Er moments prohibits noncoplanar spin structures. We show that the observed anomalies are neither due to anomalous Hall effect nor THE; instead, can be accounted for via 4f-5d interactions which produce a band-dependent mobility modulation. This leads to a pronounced multiband Hall response across the magnetization process-a metamagnetic multiband Hall effect that resembles a topological-Hall-like response but without nontrivial origins. The present findings may be of general relevance in itinerant metamagnetic systems regardless of coplanar/noncoplanar nature of spins and are important for the accurate identification of Hall signals due to emergent magnetic fields.

Giant magnetization jumps in multiscale-distortion dual-antiferromagnetic system

Large magnetization jumps (MJs) can realize an avalanched flip of the spin structure from a low spin state (antiferromagnetic) to a high spin state (ferromagnetic) and has potential applications in spin devices. Here, we report giant MJs in dual-antiferromagnetic hematite-ilmenite (Fe2O3)0.1(FeTiO3)0.9 (HI-9) solid solution. The obtained intensity of MJs (the ratio of an abrupt change in magnetization to saturation magnetization) increases to 53.3%, which is about twice as much as previously reported values in HI-9. These unusually large MJs are achieved by intentionally introducing multiscale distortions with high-stress compression deformation. Both experiments and Monte Carlo simulations demonstrate that the increase in MJs' intensity originates from the tunable atomic-scale and nano-scale distortions induced by crystal strain energy during the deformation process. Our findings provide an approach to modulate metamagnetic transitions and may inspire fresh ideas for creating high-performance antiferromagnetic materials.

Thermal, acoustic and magnetic noises emitted during martensitic transformation in single crystalline Ni45Co5Mn36.6In13.4 meta-magnetic shape memory alloy

Meta-magnetic shape memory alloys (MMSMAs) display an abrupt change in magnetization across the martensitic transformation. Simultaneous detection and statistical analysis of the emitted acoustic and magnetic noises, accompanied with martensitic transformations are important for better understanding of the magneto-structural coupling, and for utilization of well-characterized acoustic and magnetic signals for practical applications. In the present work, characteristics of thermal, acoustic and magnetic noises, detected during martensitic transformation in the Ni45Co5Mn36.6In13.4 MMSMA single crystals, were investigated for the first time. Statistical analysis of the thermal spikes, appeared during the calorimetric measurements at low cooling rates (typically below 1 K/min) showed that the energy distributions followed a power law with ε = 2.0 ± 0.3 exponent, which is in good agreement with the exponents of energy distribution of acoustic and magnetic emission events (ε = 1.7 ± 0.1 and ε = 1.7 ± 0.2, respectively). The above exponents were independent of the external magnetic field between B = 0 and 230 mT. Interestingly, the acoustic and magnetic exponents during cooling and heating were the same in contrast to the expected asymmetry based on the fact that the transformation demonstrates an isothermal character during cooling and an athermal one for heating. Upon interrupting cooling, and dwelling at a temperature above the martensite finish temperature, practically no acoustic and magnetic noises were detected, while upon continued cooling the noise appeared again, with the same critical exponents. This indicates that the main discontinuous events, leading to noise emissions, should be the same before and after dwelling as well as during cooling and heating. In magnetic noise measurements, under zero external field, the magnetic noise packets contained voltage peaks in the detector coils in both (positive and negative) directions. Under small external magnetic fields (between 20 and 230 mT), the activity of magnetic signals increased and the magnetic peaks, during both heating and cooling, appeared more and more unidirectional (i.e. peaks only in one direction were observed) which can be attributed to the increased magnetization of the austenite.

Structural and magnetic properties of Fe2TiO5 nanopowders prepared by ball-milling and post annealing

We investigate the structural and magnetic properties of Fe2TiO5 nanopowders prepared by ball-milling and post annealing. From the structural analysis, we observe the crystalline signature of the pseudobrookite phase. Moreover, by exploring distinct aspects of the magnetic behavior, we find fingerprints of the longitudinal spin freezing, with cluster spin glass transition taking place at 56 K, and of the presence of the Dzyaloshinskii-Moriya interaction in the system. Further, we observe antiferromagnetic features for the nanopowder at 300 K, and hysteresis loop with an abrupt change in magnetization characterizing the ferromagnetic behavior at 4 K. The results place the employed route as a simple, low-cost candidate to the production of nanopowders, especially Pseudobrookite.

On the correct estimation of the magnetic entropy change across the magneto-structural transition from the Maxwell relation: Study of MnCoGeBx alloy ribbons

An accurate calculation of the different magnetocaloric-related magnitudes derived from the temperature dependence of the magnetic entropy change in materials exhibiting first-order magnetocaloric effect is imperative to correctly estimate the true potential of a specific material for refrigeration purposes. In this contribution, we present a meticulous study of two different thermal procedures to measure the set of isothermal magnetization curves from which the total field induced magnetic entropy change, ΔST, is calculated using the adequate Maxwell relation. If the accurate determination of ΔST for any temperature is pursued the thermal and magnetic history of the materials must be taken into account, and then, the unidirectional measurement of reversible isothermal magnetization curves after a thermal cycle is required. The analysis was conducted on MnCoGeB0.01 alloy ribbons that show a giant ΔST at the coupled magneto-structural transition, from a ferromagnetic (TiNiSi-type) phase to a paramagnetic (NiIn2-type) one, owing to the concomitant abrupt magnetization change. We suggest that the conclusions reached can be applicable to any other system displaying magnetocaloric effect originated at a first-order phase transition.

High Frequency Thermal Energy Harvesting Using Magnetic Shape Memory Films

A new method for thermal energy harvesting at small temperature difference and high cycling frequency is presented that exploits the unique magnetic properties and actuation capability of magnetic shape memory alloy (MSMA) films. Polycrystalline films of the Ni50.4Co3.7Mn32.8In13.1 alloy are tailored, showing a large abrupt change of magnetization and low thermal hysteresis well above room temperature. Based on this material, a free‐standing film device is designed that exhibits thermomagnetically induced actuation between a heat source and sink with short heat transfer times. The cycling frequency of the device is tuned by mechanical frequency up‐conversion to over 200 Hz. An integrated pick‐up coil converts the thermally induced change of magnetization as well as the kinetic energy to electricity. For a temperature change of 10 K, the maximum peak power density is in the order of 5 mW cm‐3.

Stress-Induced Anisotropy in Electroplated FeNi Racetrack Fluxgate Cores

Magnetic anisotropy plays an important role in the behavior of a fluxgate core. It is desired to have an anisotropy orthogonal to the direction of the excitation field to move from one saturated state to the opposite saturated state without abrupt change of magnetization. In this paper, we present a method for electroplating a Permalloy film in the shape of a racetrack core with built-in anisotropy. The core is electroplated under bending and later released so that the resulting stress on the film generates the anisotropy. We show how the BH loop of the film changes as we increase the bending radius. Moreover, we demonstrate that electroplating under bending extends the frequency range before the inductance drops. Therefore, the fluxgates based on such cores can be used for higher frequencies (at the expense of lower sensitivity given by lower permeability).

Magnetic properties of single-phase MnBi grown from MnBi49 melt

The single-phase NiAs-type MnBi, embedded in Bi matrix, was grown from homogeneous MnBi49 melt at low temperatures to prevent the formation of Mn1.08Bi. An abrupt magnetization change was observed at ∼240 K. The origin of this change was ascribed to the movement of the Mn atoms between the regular sites and the interstitial sites in the MnBi lattices. The splitting of the x-ray photoelectron lines of MnBi indicates the presence of two binding states of Mn atoms, one of which was ascribed to interstitial Mn atoms. A large coercivity up to 1.79 T at 400 K was observed in the as-grown bulk isotropic MnBi alloys.

Microstructure and magnetic properties in melt-spun MnV0.02CoGe0.99 ribbons

The melt-spun MnV0.02CoGe0.99 ribbons were successfully prepared and the microstructure, magnetic and magnetocaloric properties of these ribbons were investigated. A single phase with the hexagonal Ni2In-type structure was confirmed by XRD methodology at room temperature. In these ribbons, a ferromagnetic phase transition (~252K) with the second-order nature was observed accompanied by an abrupt change of magnetization, which induces moderate values of magnetic entropy change and relative cooling power, 0.9J/kgK and 36J/kg, respectively, in the magnetic field change of ΔH=10kOe. The appreciable magnetocaloric effect with an excellent thermal/magnetic reversibility as well as more economical would make these ribbons materials suitable as a magnetic refrigerant.

Structural and Magnetization Changes at High Temperature in Co50Mn30In20 Alloy

In this work we report on microstructural and magnetic characterization of Co50Mn30In20 alloy melt-spun ribbons in its as-cast state and after being annealed at 923 K during 5 h. Microstructure was analysed by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques, while magnetic measurements (hysteresis loop and magnetization) were performed in the temperature range 1.8-1000 K. XRD measurements confirm the presence of Heusler phase Co2MnIn in the annealed material with crystallites sizing around 35 nm. Ferromagnetic ordering temperatures in Co-based Heusler systems (Co2MnIn) are considerably higher than in the corresponding Ni2MnIn ones. In this case, T(c) = 520 K for the annealed sample is lower than the corresponding one (T(c) = 550 K) for the as-cast ribbon. At around 840 K, the very abrupt magnetization change for the annealed sample is higher than the one obtained for the as-cast ribbon. This temperature value is the same than the recrystallization phase transition temperature detected in the DSC measurement.

A Spatio-temporal Description of the Abrupt Changes in the Photospheric Magnetic and Lorentz-Force Vectors During the 15 February 2011 X2.2 Flare

The active region NOAA 11158 produced the first X-class flare of Solar Cycle 24, an X2.2 flare at 01:44 UT on 2011 February 15. Here we analyze SDO/HMI magnetograms covering a 12-hour interval centered at the time of this flare. We describe the spatial distributions of the photospheric magnetic changes associated with this flare, including the abrupt changes in the field vector, vertical electric current and Lorentz force vector. We also trace these parameters' temporal evolution. The abrupt magnetic changes were concentrated near the neutral line and in two neighboring sunspots. Near the neutral line, the field vectors became stronger and more horizontal during the flare and the shear increased. This was due to an increase in strength of the horizontal field components near the neutral line, most significant in the horizontal component parallel to the neutral line but the perpendicular component also increased in strength. The vertical component did not show a significant, permanent overall change at the neutral line. The increase in total flux at the neutral line was accompanied by a compensating flux decrease in the surrounding volume. In the two sunspots near the neutral line the azimuthal flux abruptly decreased during the flare but this change was permanent in only one of the spots. There was a large, abrupt, downward vertical Lorentz force change during the flare, consistent with results of past analyses and recent theoretical work. The horizontal Lorentz force acted in opposite directions along each side of neutral line, with the two sunspots at each end subject to abrupt torsional forces. The shearing forces were consistent with field contraction and decrease of shear near the neutral line, whereas the field itself became more sheared as a result of the flux collapsing towards the neutral line from the surrounding volume.

A Review on the Regulation of Magnetic Transitions and the Related Magnetocaloric Properties in Ni-Mn-Co-Sn Alloys

In Ni-Mn-X (X =In, Sn, Sb) ferromagnetic shape memory alloys, a ferromagnetic transition from paramagnetic to ferromagnetic austenite and a martensitic transformation from ferromagnetic austenite to weak magnetic martensite occur in some particular composition ranges, in which abundant physical properties have been observed by the abrupt change of magnetization and resistivity around their transition temperatures in these alloys. Therefore, tuning the martensitic transformation temperature (TM) and enlarging the working-temperature interval for Ni-Mn-X (X=In, Sn, Sb) alloys, are of great importance. In the present paper, we will focus on the effect of external factors, including pre-deformation, annealing, and high pressure annealing, on the magnetic transitions and the related magnetocaloric properties in Ni-Mn-Co-Sn ferromagnetic shape memory alloys. Our approaches and the main results in this particular field will be reviewed.

Griffiths phase-like behavior and spin-phonon coupling in double perovskite Tb2NiMnO6

The Griffiths phase-like features and the spin-phonon coupling effects observed in Tb2NiMnO6 are reported. The double perovskite compound crystallizes in monoclinic P21/n space group and exhibits a magnetic phase transition at Tc ∼ 111 K as an abrupt change in magnetization. A negative deviation from ideal Curie--Weiss law exhibited by 1/χ(T) curves and less-than-unity susceptibility exponents from the power-law analysis of inverse susceptibility are reminiscent of Griffiths phase-like features. Arrott plots derived from magnetization isotherms support the inhomogeneous nature of magnetism in this material. The observed effects originate from antiferromagnetic interactions that arise from inherent disorder in the system. Raman scattering experiments display no magnetic-order-induced phonon renormalization below Tc in Tb2NiMnO6, which is different from the results observed in other double perovskites and is correlated to the smaller size of the rare earth. The temperature evolution of full-width-at-half-maximum for the stretching mode at 645 cm−1 presents an anomaly that coincides with the magnetic transition temperature and signals a close connection between magnetism and lattice in this material.

Electrically controlled magnetization switching in a multiferroic heterostructure

A demonstration of magnetization reversal via the application of electric field across a multiferroic heterostructure, consisting of a FeCoV ribbon bonded to a lead magnesium niobate-lead titanate crystal, is presented. The magnetization switching occurs by an abrupt change in magnetization near ferromagnetic coercivity, coinciding with an electrical field-induced magnetic anisotropy field. Experiments reveal a converse magnetoelectric coupling of α=μ0(dM/dE)=1.6×10−7 s m−1 upon magnetization reversal in the strain-mediated heterostructure. The frequency dependence of magnetization switching is presented and explained within the framework of a relaxation model for the multiferroic heterostructure.

Influence of Magnetic Field Intensity on the Temperature Dependence of Magnetization of Ni2.08Mn0.96Ga0.96 Alloy

Results of investigation of the temperature dependence of magnetization of Ni2.08Mn0.96Ga0.96 alloy in the magnetic fields of various intensities are reported. An abrupt change in magnetization at transformation of low temperature phase to the high temperature one is observed. Magnetization increases during the phase transition in the magnetic field having intensity below 500 kA/m and decreases at higher intensities. The explanation is based on zigzag configuration of domains in twinned structure. In the Curie temperature region the ferromagnetic ? paramagnetic phase transition occurs sharply at low field strength, while at higher field strength the transition is smooth. It is concluded that the increase in flatness of the curve σ = f (T) and the increase of ferromagnetic state destruction temperature with increase of the intensity of the magnetic field is indicative of the main role of Mn in magnetization of the alloy.

Low-temperature annealing effects on (Ga,Mn)As/Zn-GaAs superlattice structures grown on GaAs(0 0 1) substrates

(Ga,Mn)As/Zn-doped GaAs superlattices (SLs) have been prepared by molecular beam epitaxy (MBE), and the effect of low-temperature thermal annealing on their structural, electrical, and magnetic properties has been investigated. The SL structures studied in this work consists of 5-periods of 20-nm (Ga,Mn)As/5-nm GaAs. In order to introduce extra holes in (Ga,Mn)As layers, the GaAs spacer layers were doped with Zn acceptor atoms. Low-temperature thermal annealing was also performed to increase the hole concentration by reducing lattice defects like Mn interstitials and As antisite defects in (Ga,Mn)As layers. As-grown SL sample showed a ferromagnetic transition temperature T C of ∼50 K, and the presence of noticeable abrupt change in magnetization around ∼35 K in the M– T curve which is possibly due to the rotation of in-plane uniaxial anisotropy with changing temperature. Low-temperature annealing led to an increase in T C up to 65 K accompanied with a decrease in the resistivity.

Magnetostructural phase transition and magnetocaloric effect in off-stoichiometric Mn1.9−xNixGe alloys

Phase transitions and magnetic entropy changes are studied in Mn1.9−xNixGe (x=0.85, 0.855) alloys. In these off-stoichiometric alloys, the crystallographic transition temperature decreases remarkably due to the deficiency of transition-metal atoms, and, consequently, a magnetostructural transition from the antiferromagnetic TiNiSi-type structure to the ferromagnetic Ni2In-type structure is observed. Owing to the abrupt change in magnetization, large magnetic entropy changes are obtained. The effect of transition-metal vacancies on the phase transition temperature is discussed.

Martensitic transformation and magnetocaloric effect in Ni50Mn35In15 Heusler alloy

Polycrystalline Ni50Mn35In15 alloy was fabricated by arc-melting, and its martensitic transformation and magnetocaloric effect were systematically investigated with magnetic property measurement. The experimental results indicated that, with decrease of temperature, the second-order magnetic transition and the martensitic transition with characteristic of the first-order structural transition happened successively near room temperature, leading to an abrupt change of magnetization. At the same time, exchange bias was found by means of hysteresis loop measurement at low temperature, which clearly suggested coexistence of ferromagnetic and anti-ferromagnetic phases at low temperatures. In addition, a large magnetic entropy change during martensitic transition was calculated from Maxwell equations. Its value can reach 22.3 J/kg K at 309K in applied field of 5T.

Effect of Uniaxial Strain on Verwey Transition in Magnetite

We present a study on magnetization in a single crystal of magnetite under uniaxial strain applied along the [1 1 0] direction using a SQUID magnetometer. An abrupt change in magnetization was observed at the metal–insulator transition temperature T V (Verwey transition temperature). We observed that T V increased with increasing uniaxial compression. The pressure coefficient d T V /d P was obtained to be 1.6 ±0.1 K/kbar. The result obtained is different from that obtained under hydrostatic pressure, which showed that T V decreases with increasing pressure.

Large magnetic entropy changes in NdFe12B6 compound

Magnetic entropy changes of the NdFe12B6 compound, which was crystallized from amorphous as-quenched ribbons, were investigated. Differential thermal analysis shows that it is stable below 940K. An abrupt change of magnetization was observed around its Curie temperature of 218K. The maximum values of magnetic entropy change of this compound are 8.4 and 5.4J∕kgK for the applied fields of 10 and 7kOe, respectively. These results suggest that this ternary compound can be considered as a good candidate for magnetic refrigeration.