A full solid-state conceptual magnetocaloric refrigerator based on hybrid regeneration.

The environmental friendliness and high efficiency of magnetocaloric refrigeration make it a promising substitute for vapor compression refrigeration. However, the common use of heat transfer fluid in conventional passive magnetic regenerators (PMRs) and active magnetic regenerators (AMRs) makes only partial materials contribute to the regeneration process, which produces large regeneration loss and greatly limits the regeneration efficiency and refrigeration performance at high frequency. Herein, we propose a new conceptual hybrid magnetic regenerator (HMR) composed of multiple solid-state high thermal conductivity materials (HTCMs) and magnetocaloric materials (MCMs), in which both HTCM and MCM elements participate in the regeneration process. This novel working mode could greatly reduce regeneration losses caused by dead volume, pressure losses, and temperature nonuniformity in heat transfer substances to markedly improve regeneration efficiency at high working frequencies. Using the experimentally obtained adiabatic temperature change and magnetic work and with the help of finite element simulation, a maximum temperature of 26 K, a dramatically large cooling power of 8.3 kW/kg, and a maximum ideal exergy efficiency of 54.2% are achieved at the working frequency of 10 Hz for an ideal prototype device of a rotary HMR magnetocaloric refrigerator, which shows potential for achieving an integrative, advanced performance against current AMR/PMR systems.

Excellent thermomagnetic power generation for harvesting waste heat via a second-order ferromagnetic transition.

Thermomagnetic generation (TMG), a promising technology to convert low-grade waste heat to electricity, utilizes high performance TMG materials. However, the drawbacks of large hysteresis, poor mechanical properties and inadequate service life hinder the practical applications. For the first time, we evaluated the effect of different phase transitions on the TMG performance by systematically comparing the TMG performance of three typical Heusler alloys with similar composition but different phase transitions. Ni2Mn1.4In0.6 exhibits second-order magnetic transition (SOMT) from the ferromagnetic (FM) to paramagnetic (PM) state around TC = 316 K without thermal hysteresis. It presents highly comprehensive TMG performance, which is not only better than those of other two Heusler alloys with different phase transitions, but also better than those of most typical TMG materials. The maximum power density (1752.3 mW m-3), cost index (2.78 µW per €), and power generation index PGI (8.91 × 10-4) of Ni2Mn1.4In0.6 are 1-5, 1-4, and 1-7 orders of magnitude higher than those of most typical reported materials, respectively. In addition, Ni2Mn1.4In0.6 with SOMT also shows some advantages that first-order magnetic transition (FOMT) materials do not have, such as zero hysteresis and a long-term service life. In contrast to the short lifetime of a few minutes for the materials with FOMT, Ni2Mn1.4In0.6 with SOMT can serve for one month or even longer with excellent cycling stability. Consequently, we conclude that the SOMT Ni2Mn1.4In0.6 Heusler alloy with good TMG performance as well as zero hysteresis and long service life can be a better candidate than FOMT materials for practical applications of TMG.

High-performance thermomagnetic generator controlled by a magnetocaloric switch.

Low grade waste heat accounts for ~65% of total waste heat, but conventional waste heat recovery technology exhibits low conversion efficiency for low grade waste heat recovery. Hence, we designed a thermomagnetic generator for such applications. Unlike its usual role as the coil core or big magnetic yoke in previous works, here the magnetocaloric material acts as a switch that controls the magnetic circuit. This makes it not only have the advantage of flux reversal of the pretzel-like topology, but also present a simpler design, lower magnetic stray field, and higher performance by using less magnetocaloric material than preceding devices. The effects of key structural and system parameters were studied through a combination of experiments and finite element simulations. The optimized max power density PDmax produced by our device is significantly higher than those of other existing active thermomagnetic, thermo, and pyroelectric generators. Such high performance shows the effectiveness of our topology design of magnetic circuit with magnetocaloric switch.

Spontaneous Biskyrmion Lattice in a Centrosymmetric Rhombohedral Rare-Earth Magnet with Easy-Plane Anisotropy.

Magnetic skyrmion and its derivatives have demonstrated fascinating topological behaviors with potential applications in future spintronic devices. Despite the recent progress, the spontaneous skyrmion lattice and successive topological transition in the magnets with easy-plane magnetic anisotropy are still elusive especially at room temperature. Here, in a centrosymmetric rhombohedral Nd2Co17 magnet with easy-plane magnetic anisotropy, spontaneous biskyrmions are observed over a wide temperature range across room temperature, and then evolve into enclosed in-plane domains with nanometric size due to the enhancement of the planar magnetic anisotropy. The spontaneous generation of the biskyrmion lattice and its evolution along different crystal orientations demonstrate the crucial role of intrinsic bi-anisotropy and demagnetization effects. This discovery provides a fundamental insight into the nature of topological magnetic textures in easy-plane anisotropy materials and suggests an arena to explore the topological states in rare-earth magnets as well as their applications in spintronics.

Spontaneous Topological States and Their Mutual Transformations in a Rare-Earth Ferrimagnet.

Nontrivial chiral spin textures with nanometric sizes and novel characteristics (e.g., magnetic skyrmions) are promising for encoding information bits in future energy-efficient and high-density spintronic devices. Because of antiferromagnetic exchange coupling, skyrmions in ferrimagnetic materials exhibit many advantages in terms of size and efficient manipulation, which allow them to overcome the limitations of ferromagnetic skyrmions. Despite recent progress, ferrimagnetic skyrmions have been observed only in few films in the presence of external fields, while those in ferrimagnetic bulks remain elusive. This study reports on spontaneously generated zero-field ground-state magnetic skyrmions and their subsequent transformation into traditional magnetic bubbles via intermediate states of (bi-)target bubbles during a magnetic anisotropy change in the rare-earth ferrimagnetic crystal DyFe11 Ti. Spontaneous reversible topological transformation driven by a temperature-induced spin reorientation transition is directly distinguished using Lorentz transmission electron microscopy. The spontaneous generation of magnetic skyrmions and successive topological transformations in ferrimagnetic DyFe11 Ti are expected to advance the design of topological spin textures with versatile properties and potential applications in rare-earth magnets.

Regulation of Magnetocaloric Effect in Ni40Co10Mn40Sn10 Alloys by Using a Homemade Uniaxial Strain Pressure Cell.

In this study, a homemade uniaxial strain pressure cell was designed to be directly used in the standard magnetometers whereby the magnetic properties of samples subjected to a uniaxial strain and magnetic field were characterized. Its feasibility has been demonstrated by the uniaxial strain control of the phase transition and magnetocaloric effect in Ni40Co10Mn40Sn10 (NCMS) alloys. With the assistance of a uniaxial strain of ~0.5%, the cooling temperature span of NCMS alloys is broadened by 2 K, and the refrigeration capacity under a 3 T magnetic field change increases from 246 to 277 J/kg. This research provides not only direct experimental assistance for the tuning of phase transition by the uniaxial strain but also possibilities for studying the coupled caloric effect in first-order phase transition materials under a combined uniaxial strain and magnetic field by the thermodynamic analysis.

Enhanced Performance of ΔTad upon Frequent Alternating Magnetic Fields in FeRh Alloys by Introducing Second Phases.

The cyclability and frequency dependence of the adiabatic temperature change (ΔTad) under an alternating magnetic field (AMF) are significantly important from the viewpoint of refrigeration application. Our studies demonstrated, by direct measurements, that the cyclability and low-magnetic-field performance of ΔTad in FeRh alloys can be largely enhanced by introducing second phases. The ΔTad under a 1.8 T, 0.13 Hz AMF is reduced by 14%, which is much better than that (40-50%) of monophase FeRh previously reported. More importantly, the introduction of second phases enables the antiferromagnetic-ferromagnetic phase transition to be driven by a lower magnetic field. Thus, ΔTad is significantly enhanced under a 0.62 T, 1 Hz AMF, and its value is 70% larger than that of monophase FeRh previously reported. Although frequency dependence of ΔTad occurs, the specific cooling power largely increases by 11 times from 0.17 to 1.9 W/g, as the frequency increases from 1 to 18.4 Hz under an AMF of 0.62 T. Our analysis of the phase transition dynamics based on magnetic relaxation measurements indicates that the activation energy barrier is lowered owing to the existence of second phases in FeRh alloys, which should be responsible for the reduction of the driving field. This work provides an effective way to enhance the cyclability and low-magnetic-field performance of ΔTad under an AMF in FeRh alloys by introducing second phases.

Spontaneous Topological Magnetic Transitions in NdCo5 Rare-Earth Magnets.

Particle-like magnetic textures with nanometric sizes, such as skyrmions, are potentially suitable for designing high-efficiency information bits in future spintronics devices. In general, the Dzyaloshinskii-Moriya interactions and dipolar interactions are the dominant factors for generating nonlinear spin configurations. However, to stabilize the topological skyrmions, an external magnetic field is usually required. In this study, the spontaneous emergence of skyrmions is directly observed, together with the unique successive topological domain evolution during the spin reorientation transition in a neodymium-cobalt (NdCo5 ) rare-earth magnet. On decreasing the temperature, nanometric skyrmion lattices evolve into enclosed in-plane domains (EIPDs) similar to mini bar-magnets with size below 120 nm. The internal magnetization rotates with magnetic anisotropy, demonstrating the ability to manipulate the mini bar-magnets. The nanoscale EIPD lattices remain robust over the wide temperature range of 241-167 K, indicating the possibility of high-density in-plane magnetic information storage. The generation of spontaneous magnetic skyrmions and the successive domain transformation in the traditional NdCo5 rare-earth magnet may prompt application exploration for topological magnetic spin textures with novel physical mechanisms in versatile magnets.

Large Low-Field Magnetoresistance (LFMR) Effect in Free-Standing La0.7Sr0.3MnO3 Films.

The realization of a large low-field magnetoresistance (LFMR) effect in free-standing magnetic oxide films is a crucial goal toward promoting the development of flexible, low power consumption, and nonvolatile memory devices for information storage. La0.7Sr0.3MnO3 (LSMO) is an ideal material for spintronic devices due to its excellent magnetic and electronic properties. However, it is difficult to achieve both a large LFMR effect and high flexibility in LSMO films due to the lack of research on LFMR-related mechanisms and the strict LSMO growth conditions, which require rigid substrates. Here, we induced a large LFMR effect in an LSMO/mica heterostructure by utilizing a disorder-related spin-polarized tunneling effect and developed a simple transfer method to obtain free-standing LSMO films for the first time. Electrical and magnetic characterizations of these free-standing LSMO films revealed that all of the principal properties of LSMO were sustained under compressive and tensile conditions. Notably, the magnetoresistance of the processed LSMO film reached up to 16% under an ultrasmall magnetic field (0.1 T), which is 80 times that of a traditional LSMO film. As a demonstration, a stable nonvolatile multivalue storage function in flexible LSMO films was successfully achieved. Our work may pave the way for future wearable resistive memory device applications.

Electric field control of magnetism through modulating phase separation in (011)-Nd0.5Sr0.5MnO3/PMN-PT heterostructures.

Large and non-volatile electric field control of magnetization is promising to develop memory devices with reduced energy consumption. Herein, we report the electric field control of magnetization with a non-volatile memory effect in an intermediate band Nd0.5Sr0.5MnO3 film grown on a (011)-cut 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystal. Applying an electric field across the ferroelectric PMN-PT increases the magnetization of the Nd0.5Sr0.5MnO3 film along both in-plane [100] and [011[combining macron]] directions. Moreover, the magnetization does not recover to its original state after withdrawal of the electric field at temperatures below 70 K, demonstrating a non-volatile memory effect. Detailed investigation showed that (011)-PMN-PT exhibits an anisotropic in-plane strain due to an electric field-induced rhombohedral to orthorhombic phase transition. This electric field-induced anisotropic strain can dynamically transfer to Nd0.5Sr0.5MnO3 film and modulate the magnetization of the Nd0.5Sr0.5MnO3 film through adjusting its phase balance between ferromagnetic (FM) and charge-orbital ordered antiferromagnetic (COO AFM) phases. The non-volatile memory effect can be ascribed to the competition of thermal energy and energy barriers between the FM and COO AFM phases at low temperatures. This work broadens the knowledge of electric field control of magnetism in the intermediate band-manganite ferromagnetic/ferroelectric multiferroic heterostructures, and may also pave a way for the control of antiferromagnetism and to design antiferromagnet-based memories.

In situ TEM study on diversified martensitic transition behaviour in Ni50Mn35In15 alloys.

Ni-Mn-In magnetic shape-memory alloys are attractive materials due to their important functional properties relating to the martensitic transition. Understanding the complex martensitic magnetism and the transition process is of crucial importance not only from a fundamental but also from a technological point of view. Here, we demonstrate the dynamic magnetic domains and microstructures during the martensitic transition in the bulk and melt-spun ribbons of Ni50Mn35In15via in situ Lorentz transmission electron microscopy. The significant evolutionary differences in correlation with the temperature dependence of magnetization are identified between the bulk and ribbons. For a bulk alloy with L21 crystal structure at room temperature, the complete martensite with 7 M modulation in the paramagnetic state and the successive stripe magnetic domains in ferromagnetic martensite develop with a further decrease in the temperature. The stripe domains evolve into biskyrmion-like spin configurations when a perpendicular magnetic field is applied. In contrast, the partial austenitic phase always coexists with the martensitic phase in the ribbons even far below the martensitic transition temperatures and the martensitic phase presents a dominant twinning stack morphology with 5 M modulation and various magnetic domains. During the subsequent reheating-cooling cycles, the thermal hysteresis behavior and the transition reversibility in the bulk and ribbons are represented via the microstructural evolution.

Anisotropic nonvolatile magnetization controlled by electric field in amorphous SmCo thin films grown on (011)-cut PMN-PT substrates.

The tunable, nonvolatile electrical modulation of magnetization at room temperature is firstly demonstrated in a magnetically hard amorphous SmCo film grown on a (011)-cut 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) substrate. Uniaxial in-plane anisotropy with hard and easy axes lying in the [100] and [01-1] directions, respectively, occurs. Bipolar electric field, E, across the thickness direction enhances the remnant magnetization, Mr, along the hard axis, while suppresses the Mr along the easy axis, and the maximal regulation is about -5.8% and +2.2%, respectively. Detailed analysis indicates that the induced effective uniaxial magnetic anisotropy field, which arises from the magnetostrictive properties of the amorphous SmCo thin film and the anisotropic strain from the PMN-PT substrate, is mainly responsible for the anisotropic tunability. The variation of the directional pair ordering of the SmCo film, which is caused by the anisotropic strain due to the electric field, also contributes to the tunability. More importantly, nonvolatile modulation and a stable two-state memory effect are demonstrated for the bipolar case, and in situ X-ray diffraction and X-ray diffraction reciprocal space mapping reveal that these phenomena originate from the electric-field-induced rhombohedral-orthorhombic phase transformation in the PMN-PT substrate. Moreover, by unipolarizing the ferroelectric substrate, a nonvolatile modulation is also observed. The anisotropic nonvolatile control of magnetization in SmCo amorphous films opens a new avenue for developing multifunctional information storage and novel spintronics devices based on hard magnetic materials.

Enhanced Field Modulation Sensitivity and Anomalous Polarity-Dependency Emerged in Spatial-Confined Manganite Strips.

An anomalous polarity-dependent electrostatic field modulation effect, facilitated by spatial confinement, is found in an oxide-based field-effect prototype device with a spatial-confined Pr0.7(Ca0.6Sr0.4)0.3MnO3 channel. It is revealed that the dominant field modulation mode under a small bias field varies from a polarity-independent strain-mediated one to a nonvolatile polarity-dependent one with enhanced modulation sensitivity as the channel width narrows down to several micrometers. Specially, in the structure confined to length scales similar to that of the phase domains, the field modulation exhibits a greatly increased modulation amplitude around the transition temperature and an anomalous bias-polarity dependence that is diametrically opposite to the normal one observed in regular polarization field-effect. Further simulations show that a large in-plane polarization field is unexpectedly induced by a small out-of-plane bias field of 4 kV/cm in the narrow strip (up to 790 kV/cm for the 3 µm strip). Such large in-plane polarization field, facilitated and enhanced by size reduction, drives phase transitions in the narrow channel film, leading to the reconfiguration of percolation channel and nonvolatile modulation of transport properties. Accordingly, the accompanied polarity relationship between the induced in-plane polarization field and the applied vertical bias field well explains the observed anomalous polarity-dependence of the modulation. Our studies reveal a new acting channel in the nanoscale control of lateral configurations of electronic phase separation and macroscopic behaviors by a small vertical electric bias field in spatial-confined field-effect structures. This distinct acting mechanism offers new possibilities for designing low-power all-oxide-based electronic devices and exploiting new types of multifunctionality to other strongly correlated materials where electronic phase competition exists.