Large Magnetic Change Research Articles

Effect of Cu addition on the physical and mechanical properties of a Ni–Mn–In–Co polycrystalline shape memory alloy

The martensite transformation (MT), magnetocaloric effect (MCE), and mechanical properties of the polycrystalline Ni46Mn36In14Co4 (Co4) alloys with doping of 4 at.% Cu were examined. A large magnetization change (ΔM, 92 emu/g), large magnetic entropy change (ΔSM, 37.3 J kg−1K−1), high refrigerant capacity (RC, 261 J kg−1), relatively high sensitivity to magnetic fields (2.8 K/T), and two-way magnetic-field-induced MT were attained at approximately room temperature. Moreover, the compressive fracture strength and strain of the alloy reached 335 MPa and 7.4 %, respectively. Finally, the mechanism of the excellent combination properties of the Ni42Mn36In14Co4Cu4 (Co4Cu4) alloy was discussed. This study can aid in the development of high-performance magnetic refrigeration and intelligent actuator materials.

In-Situ Measurement of Magnetoelectric Coupling and Strain Transfer in Multiferroic Nanocomposites of CoFe2O4 and Hf0.5Zr0.5O2 with Residual Porosity

With increasing applications for voltage-controlled magnetism, the need to more fully understand magnetoelectric coupling and strain transfer in nanostructured multiferroic composites has also increased. Here, multiferroic nanocomposites were synthesized using block copolymer templating to create mesoporous cobalt ferrite (CFO), followed by partly filling the pores with ferroelectric zirconium-substituted hafnia (HZO) using atomic layer deposition (ALD) to produce a porous multiferroic composite with enhanced mechanical flexibility. Upon electrical poling of the nanocomposite, we observed large changes in the magnetization. These changes partly relaxed upon removing the electric field, suggesting a strain-mediated mechanism. Both the anisotropic strain transfer from HZO to CFO and the strain relaxation after the field was removed were confirmed using high-resolution X-ray diffraction measurements collected during in-situ poling. The in-situ observation of both anisotropic strain transfer and large magnetization changes allows us to directly characterize the strong multiferroic coupling that can occur in flexible, nanostructured composites.

Photoinduced large magnetic change at room temperature and radical-quenched spin glass in a cyanide-bridged MnII-FeIII compound.

By the coordination assembly of a redox photoactive functional motif and a cyanide-bridged moiety, a cyanide-bridged MnII-FeIII compound with large photoinduced magnetic change at room-temperature due to photoinduced electron transfer was obtanied. This compound also shows unprecedented radical-quenched spin glass in molecule based magnets.

A new method to enhance the magnetocaloric effect in (Sc,Ti)Fe2 via magnetic phase separation

Magnetic refrigeration based on the magnetocaloric effect (MCE) is a novel refrigeration technology that will replace traditional vapor-compression refrigeration in the future. Improvement in the performance of MCE materials is crucial for the development of magnetic refrigeration technology. This study presents a new method that enhances the MCE performance of (Sc,Ti)Fe2 via magnetic phase separation. The maximum magnetic entropy change induced by the coexistence of an in-plane ferromagnetic phase (FMab) and a canting antiferromagnetic phase (CAFM) in Sc0.3Ti0.7Fe2 is twice that found in other (Sc,Ti)Fe2 compounds. Variable-temperature neutron diffraction experiments directly reveal that the large magnetic entropy change in Sc0.3Ti0.7Fe2 is dominated by the transformation from a highly ordered FMab state to a CAFM state with a lower magnetic order. The magnetic phase separation is a direct transition from a higher-ordered state with a larger lattice to a lower-ordered state with a smaller lattice that induces a large magnetic order change and lattice contraction. The combination of the metamagnetic transition and negative thermal expansion leads to enhanced MCE. This study suggests the possibility that magnetic phase separation can be an effective approach to achieving and controlling a large MCE in magnetic materials.

Large reversible magnetocaloric effect and magnetoresistance by improving crystallographic compatibility condition in Ni(Co)-Mn-Ti all- d -metal Heusler alloys

Recently, all-$d$-metal Ni(Co)-Mn-Ti Heusler systems have become the research hotspot due to their magnetoresponsive properties associated with a tunable first-order magnetostructural transformation (MST) and excellent mechanical stability for potential applications. However, the presence of large thermal hysteresis acts as an obstacle to the cyclic operation of this novel material. In this present paper, we investigate a large reversible magnetocaloric effect (MCE) and magnetoresistance (MR) in ${\mathrm{Ni}}_{37\ensuremath{-}x}{\mathrm{Co}}_{13+x}{\mathrm{Mn}}_{34.5}{\mathrm{Ti}}_{15.5}$ all-$d$-metal Heusler alloys that undergo a first-order MST accompanied by a large magnetization change between ferromagnetic austenite and antiferromagnetic martensite phases. Tuning the small at.% of Co doping in ${\mathrm{Ni}}_{37\ensuremath{-}x}{\mathrm{Co}}_{13+x}{\mathrm{Mn}}_{34.5}{\mathrm{Ti}}_{15.5}$ systems, we achieved an optimum composition with $x$ = 1 where low thermal hysteresis of $\ensuremath{\sim}4.7$ K, a narrow transformation interval of $\ensuremath{\sim}11.2$ K, and improved sensitivity of transformation temperature $\ensuremath{\sim}2.8$ K/T is observed. In addition, the origin of small hysteresis is studied based on geometric compatibility between cubic austenite and monoclinic martensite phases, calculated from the powder x-ray diffraction data. These optimized parameters lead to a reversible magnetic field-induced inverse martensitic transition under the field cycling, yielding a large reversible magnetic entropy change ($\mathrm{\ensuremath{\Delta}}{\mathrm{S}}_{M}$) of $\ensuremath{\sim}17.78$ J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 277 K in a field change of 5 T. Moreover, a large reversible magnetoresistance (MR) of $\ensuremath{\sim}14%$ out of 32.6% is also obtained under 5-T magnetic field for $x$ = 1 sample in all-$d$-metal Heusler alloys. These reversible magnetoresponsive properties are comparable to other Ni-Mn-based Heusler alloys and have not been reported so far in the all-$d$-metal Heusler system. Therefore our present work demonstrates a pathway to design new cyclically stable multifunctional materials in all-$d$-metal Heusler systems for solid-state cooling devices and magnetic recording applications.

Simultaneously realized large low-temperature magnetocaloric effect and good mechanical properties in Ni36Co13Mn35Ti16 alloy

In this work, we present the observation of large low-temperature magnetocaloric effect and good mechanical properties in the Ni36Co13Mn35Ti16 alloy. The phase transition behaviors, magnetocaloric effect, and mechanical properties for the Ni36Co13Mn35Ti16 non-textured polycrystalline alloy were systematically investigated. Under the magnetic field changes of 5 T, a typical meta-magnetic behavior with a large magnetization change of about 110 A m2 kg−1 between the austenite and martensite can be observed, the maximum magnetic entropy changes (ΔSm) of the Ni36Co13Mn35Ti16 alloy is ∼16.1 J kg−1 K−1 at 148 K. Furthermore, the maximum compressive stress and strain are 894 MPa and 8.0%, respectively. The scanning electron microscope and first-principles calculations were used to analyze the fracture mechanism and the bonding interaction. In the combination of the large low-temperature magnetocaloric effect and good mechanical properties, the Ni36Co13Mn35Ti16 alloy has a good prospect for low-temperature magnetic refrigeration applications.

Observation of magnetic domain evolution in constrained epitaxial Ni–Mn–Ga thin films on MgO(0 0 1) substrate

• The evolutions of the magnetic domain of a typical epitaxial Ni–Mn–Ga thin film were investigated through wide-field magneto-optical Kerr-microscopy. • The abrupt magnetization changes in the hysteresis loops can be observed in the Type-Y area with the applied magentic field parallel to the twin interfaces. • The abrupt magnetization changes in the hysteresis loops should be attributed to the magnetic domain evolution instead of the magnetically induced variant reorientation. Epitaxial Ni–Mn–Ga thin films have promising application potential in micro-electro-mechanical sensing and actuation systems. To date, large abrupt magnetization changes have been observed in some epitaxial Ni–Mn–Ga thin films, but their origin - either from magnetically induced martensite variant reorientation (MIR) or magnetic domain evolution - has been discussed controversially. In the present work, we investigated the evolutions of the magnetic domain and microstructure of a typical epitaxial Ni–Mn–Ga thin film through wide-field magneto-optical Kerr-microscopy. It is demonstrated that the abrupt magnetization changes in the hysteresis loops should be attributed to the magnetic domain evolution instead of the MIR.

Magnetic behavior of magneto-rheological foam under uniaxial compression strain

This study reports the development of a magneto-rheological foam, which consists in a porous matrix filled by ferromagnetic particles. The porous matrix of such a composite being easily deformable, large magnetic properties changes are expected. The measurements of the magnetic properties of such a magneto-rheological foam submitted to a compressive strain are reported. Main aspect of the magnetic properties is the low field magnetic permeability as the function of the compression and filling factor. Then, larger field magnetization measurement allowed to investigate the saturation field as a function of the filling factor. Because of the large amount of pores in the material, the magnetic relative permeability, µ r, is quite small (µ r ∼ 1). However, these materials can be easily deformed over a large range of strain providing important relative variation of the magnetic properties under mechanical solicitation. The composite magnetic permeability is increasing under compression for all the considered filling factors. A model is then developed to understand the variation of the permeability with the strain. Hence, from a simple concept consisting of taking advantage of high deformation of foams, the present study demonstrates the interest of such a highly compressible while cheap composite for obtaining a large magneto-rheological effect.

Tuning the magnetic phase diagram of Ni-Mn-Ga by Cr and Co substitution

Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties.

Lumped Element Model for Thermomagnetic Generators Based on Magnetic SMA Films.

This paper presents a lumped element model (LEM) to describe the coupled dynamic properties of thermomagnetic generators (TMGs) based on magnetic shape memory alloy (MSMA) films. The TMG generators make use of the concept of resonant self-actuation of a freely movable cantilever, caused by a large abrupt temperature-dependent change of magnetization and rapid heat transfer inherent to the MSMA films. The LEM is validated for the case of a Ni-Mn-Ga film with Curie temperature TC of 375 K. For a heat source temperature of 443 K, the maximum power generated is 3.1 µW corresponding to a power density with respect to the active material’s volume of 80 mW/cm3. Corresponding LEM simulations allow for a detailed study of the time-resolved temperature change of the MSMA film, the change of magnetic field at the position of the film and of the corresponding film magnetization. Resonant self-actuation is observed at 114 Hz, while rapid temperature changes of about 10 K occur within 1 ms during mechanical contact between heat source and Ni-Mn-Ga film. The LEM is used to estimate the effect of decreasing TC on the lower limit of heat source temperature in order to predict possible routes towards waste heat recovery near room temperature.

Photoinduced magnetic phase transition and remarkable enhancement of magnetization for a photochromic single-molecule magnet

A photochromic single-molecule magnet with large magnetization change (+20.9%) at room temperature and paramagnetic to ferromagnetic transition was successfully achieved.

Mastering a 1.2 K hysteresis for martensitic para-ferromagnetic partial transformation in Ni-Mn(Cu)-Ga magnetocaloric material via binder jet 3D printing

Magnetocaloric (MC) materials have gained traction in the research and industry communities for their prospects in solid state magnetic refrigeration. Important to the commercialization of MC materials are: (1) establishment of a fabrication method that can combine high surface area for heat transfer and geometric freedom for designing an efficient heat exchanger which has low pressure drop for the coolant and (2) advancement of low cost alloys with appropriate MC properties. In this regard, additive manufacturing may provide the geometric freedom necessary for adapting designs to solid state cooling, and the Ni-Mn(Cu)-Ga Heusler ferromagnetic shape memory alloys (FSMAs), exhibiting a martensitic para-ferromagnetic transformation at Tms=304 K, can provide a low-cost MC material, very promising for magnetic cooling. In this study, a Ni49.5Mn19.1Cu6.6Ga24.8 (at.%) alloy is additively manufactured using powder bed binder jet 3D printing with subsequent sintering. This printed and sintered material enabled a large change of magnetization during partial transformation cycles with the smallest temperature hysteresis recorded for FSMAs, equal to about 1.2 K, regardless of the value of magnetic field applied. Under 2 T and at 304 K it exhibits an adiabatic temperature change (ΔTad) of 2 K and a stable cycling behavior of ΔTad = │1.7│ K for 100 cycles. The maximum of magnetic field-induced entropy change |ΔSm, 2T| ≈ 12.0 J/kg·K was estimated at 304 K. These results demonstrate the viability of powder bed binder jet 3D printing as an effective fabrication method for functional magnetocalorics, as well as the outstanding MC characteristics of a low-cost Ni-Mn(Cu)-Ga Heusler-type FSMA.

Bending Strain-Tailored Magnetic and Electronic Transport Properties of Reactively Sputtered γ'-Fe4N/Muscovite Epitaxial Heterostructures toward Flexible Spintronics.

The strain modulation on the magnetic and electronic transport properties of the ferromagnetic films is one of the hot topics due to the practical applications in flexible and wearable spintronic devices. However, the large strain-induced saturation magnetization and resistance change is not easy to achieve because most of the ferromagnetic films deposited on flexible substrates are polycrystalline or amorphous. Here, the flexible epitaxial γ'-Fe4N/mica films are fabricated by facing-target reactive sputtering. At a tensile strain with a radius of curvature (ROC) of 3 mm, the saturation magnetization (Ms) of the γ'-Fe4N/mica film is tailored significantly with a maximal variation of 210%. Meanwhile, the magnetic anisotropy was broadly tunable at different strains, where the out-of-plane Mr/Ms at a tensile strain of ROC = 2 mm is six times larger than that at the unbent state. Besides, the strain-tailored longitudinal resistance Rxx and anomalous Hall resistivity ρxy appear where the drop of Rxx (ρxy) reaches 5% (22%) at a tensile strain of ROC = 3 mm. The shift of the nitrogen position in the γ'-Fe4N unit cell at different bending strains plays a key role in the strain-tailored magnetic and electronic transport properties. The flexible epitaxial γ'-Fe4N films have the potential applications in magneto- and electromechanical wearable spintronic devices.

Design of Reversible Low-Field Magnetocaloric Effect at Room Temperature in Hexagonal MnMX Ferromagnets

Giant magnetocaloric effect is widely achieved in hexagonal MnMX-based (M = Co or Ni, X = Si or Ge) ferromagnets at their first-order magnetostructural transition. However, the thermal hysteresis and the low sensitivity of the magnetostructural transition to the magnetic field inevitably lead to a sizeable irreversibility of the low-field magnetocaloric effect. In this work, we show an alternative way to realize a reversible low-field magnetocaloric effect in MnMX-based alloys by taking advantage of the second-order phase transition. With introducing Cu into Co in MnCoGe alloy, the martensitic transition is stabilized at high temperature, while the Curie temperature of the orthorhombic phase is reduced to room temperature. As a result, a second-order magnetic transition with negligible thermal hysteresis and a large magnetization change can be observed, enabling a large reversible magnetocaloric effect. By both calorimetric and direct measurements, a reversible adiabatic temperature change of about 1 K is obtained under a field change of 0-1 T at 304 K, which is larger than that obtained in a first-order magnetostructural transition. To get a better insight into the origin of these experimental results, first-principles calculations are carried out to characterize the chemical bonds and the magnetic exchange interaction. Our work provides a new understanding of the MnCoGe alloy and indicates a feasible route to improve the reversibility of the low-field magnetocaloric effect in the MnMX system.

Low-pressure-induced giant barocaloric effect in an all-d-metal Heusler Ni35.5Co14.5Mn35Ti15 magnetic shape memory alloy

The clean and energy-efficient solid-state refrigeration based on magnetostructural phase transformation is a promising alternative technology for vapor-compression refrigeration. Herein, using a unique quasi-direct calorimetric method in a high hydrostatic pressure condition, we have studied the barocaloric effect for a Ni35.5Co14.5Mn35Ti15 all-d-metal Heusler alloy that undergoes a martensitic transformation accompanied by a large magnetization change. The martensitic transformation is found to be sensitive to either applied pressure or magnetic field with the large transformation temperature driving rates of 5.8 K kbar−1 and 2.0 K T−1. Such a sensitive response to external stimuli originates from the large lattice/volume- and magnetization-discontinuity on the phase transformation. The Ni35.5Co14.5Mn35Ti15 alloy exhibits a large barocaloric effect with an isothermal entropy change of −24.2 J kg−1 K−1and an adiabatic temperature change of 4.2 K, concomitant with a relatively low pressure change of 1 kbar, appearing to be the largest values among those of reported magnetic shape memory alloys. Moreover, by virtue of the strong magnetostructural coupling in the Ni35.5Co14.5Mn35Ti15 alloy, we propose a strategy of applying/releasing pressure and magnetic field in a proper sequence to eliminate the hysteresis and effectively enhance the reproducibility of the barocaloric effect.

Prediction of giant magnetocaloric effect in Ni40Co10Mn36Al14 Heusler alloys: An insight from ab initio and Monte Carlo calculations

In this work, based on limited experimental magnetocaloric data for Ni–Co–Mn–Al Heusler alloys, we present a theoretical study to predict a composition with higher magnetocaloric properties. By analogy with Ni–Co–Mn–(In, Sn) alloys exhibiting a large magnetization change across the structural transformation, we suppose that the addition of 10 at. % Co in Ni–Mn–Al would yield a similar trend. Our approach is based on the combination of ab initio calculations and Monte Carlo simulations within the framework of the Potts–Blume–Emery–Griffiths model. It follows from ab initio calculations that Co addition modifies the exchange interactions and enhances the ferromagnetism in austenite, while for martensite, the ferromagnetism is substantially suppressed due to the strongest antiferromagnetic Mn–Mn interactions. Thermo-magnetization curves and magnetocaloric properties under magnetic fields of 0.5 and 2 T are simulated by the Monte Carlo method assuming the ab initio exchange-interaction parameters. A large change in magnetization of approximately 100 A m2kg−1, leading to a giant magnetocaloric effect (ΔTad≈−7 K) across the martensite–austenite transformation, is predicted.

Tuning martensite transformation behavior and magneto-caloric effect in Ni44Mn36In14Co6 alloy through doping the fifth element Cu

The martensite transformation (MT) behavior and magneto-caloric effect (MCE) in polycrystalline Ni44Mn36In14Co6 alloy were tuned by doping the fifth element, 2 at.% Cu. The process of heat-induced MT was observed by in-situ scanning electron microscopy and the magnetic-field-induced reversible MT was confirmed by magnetic properties test near room temperature. A large magnetization change (ΔM, 111emu/g), relatively high sensitivity to magnetic fields (3.8 K/T), a large MCE (33.7J kg−1 K−1) and the two-way magnetic-field-induced transformation were attained around the MT. The combination properties in the Ni42Mn36In14Co6Cu2 alloy are instructive for the development of high-performance magnetic refrigeration and intelligent actuator materials.

Negative Thermal Expansion in the Plateau State of a Magnetically Frustrated Spinel.

We report on negative thermal expansion (NTE) in the high-field, half-magnetization plateau phase of the frustrated magnetic insulator CdCr_{2}O_{4}. Using dilatometry, we precisely map the phase diagram at fields of up to 30T and identify a strong NTE associated with the collinear half-magnetization plateau for B>27 T. The resulting phase diagram is compared with a microscopic theory for spin-lattice coupling, and the origin of the NTE is identified as a large negative change in magnetization with temperature, coming from a nearly localized band of spin excitations in the plateau phase. These results provide useful guidelines for the discovery of new NTE materials.

Hydration of gadolinium oxide ( GdOx ) and its effect on voltage-induced Co oxidation in a Pt/Co/GdOx/Au heterostructure

Magneto-ionic control of magnetism has garnered great interest in recent years due to the large magnetic changes that can be induced using a relatively small voltage. One model structure for this is $\mathrm{Pt}/\mathrm{Co}/\mathrm{Gd}{\mathrm{O}}_{x}/\mathrm{Au}$, where Co is the magnetic layer and $\mathrm{Gd}{\mathrm{O}}_{x}$ is the ionic conductor, with the magnetic properties dependent on the oxidation state of Co. While this structure is commonly used, there is limited understanding of the effect of $\mathrm{Gd}{\mathrm{O}}_{x}$ properties on voltage-induced magnetic changes. In this work, we show that hydration of $\mathrm{G}{\mathrm{d}}_{2}{\mathrm{O}}_{3}$ to form $\mathrm{Gd}{(\mathrm{OH})}_{3}$ is crucial for voltage-induced Co oxidation in a $\mathrm{Pt}/\mathrm{Co}/\mathrm{Gd}{\mathrm{O}}_{x}/\mathrm{Au}$ device. By examining the rate of Co oxidation in nonhydrated and hydrated devices, we conclude that ${\mathrm{H}}_{2}\mathrm{O}$ in the $\mathrm{Gd}{\mathrm{O}}_{x}$ layer acts as an oxidant during the voltage-induced Co oxidation process. Co oxidation through this interfacial reaction process is confirmed by in situ x-ray absorption spectroscopy.

First-principles study of Ni-Co-Mn-Sn alloys with regular and inverse Heusler structure

Abstract The influence of partial Mn and Co content on magnetic properties and martensitic transformation in full Heusler compounds Ni-Co-Mn-Sn with the regular and inverse structure is studied theoretically by first-principles calculations within density functional theory. The optimized lattice parameters, magnetic moments and bulk moduli are calculated. It is shown that the substitution of Sn for Mn atom in the regular structure and of Sn for Ni atom in the inverse structure leads to an appearance of stable martensitic phase. For both considered systems with excess Ni and Mn content, the magnetic reference state of austenite and martensite is found to be the ferrimagnetic one except for Ni 1.75 Co 0.25 Mn 1.75 Sn 0.25 . For the latter, martensitic and austenitic phases are ordered ferri- and ferromagnetically that results in a large change in magnetization.