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 effi-ciency 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.

Manipulation of topological spin textures in centrosymmetric rare-earth magnets

Topologically protected magnetic skyrmions are expected to be used in the next-generation spintronic devices.Realizing their nucleation and manipulation at room temperature is fundamental for future practical applications.Here,using in situ Lorentz transmission electron microscopy and micromagnetic simulation,we demonstrate that magnetic biskyrmions can spontaneously exist at room temperature in Nd_(1-x) Tb_(x)Co_(5)(x=0.3,0.5)alloys.The spontaneous biskyrmions are controllably obtained over a wide temperature range across room temperature by switching atomic chemical environment.Furthermore,the density of biskyrmions can be tuned by a small magnetic field.High-density biskyrmions are stimulated to form in the thinner region at room temperature by introducing an in-plane magnetic field component.These results provide valuable insights into the manipulation of topological states,which is of great significance to their practical applications.

Phase transition regulation and caloric effect

Solid state refrigeration based on caloric effect is regarded as a potential candidate for replacing vapor-compression refrigeration.Numerous methods have been proposed to optimize the refrigeration properties of caloric materials,of which single field tuning as a relatively simple way has been systemically studied.However,single field tuning with few tunable parameters usually obtains an excellent performance in one specific aspect at the cost of worsening the performance in other aspects,like attaining a large caloric effect with narrowing the transition temperature range and introducing hysteresis.Because of the shortcomings of the caloric effect driven by a single field,multifield tuning on multicaloric materials that have a coupling between different ferro-orders came into view.This review mainly focuses on recent studies that apply this method to improve the cooling performance of materials,consisting of enlarging caloric effects,reducing hysteresis losses,adjusting transition temperatures,and widening transition temperature spans,which indicate that further progress can be made in the application of this method.Furthermore,research on the sign of lattice and spin contributions to the magnetocaloric effect found new phonon evolution mechanisms,calling for more attention on multicaloric effects.Other progress including improving cyclability of FeRh alloys by introducing second phases and realizing a large reversible barocaloric effect by hybridizing carbon chains and inorganic groups is described in brief.

Large enhancement of magnetocaloric effect induced by dual regulation effects of hydrostatic pressure in Mn_(0.94)Fe_(0.06)NiGe compound

MM'X(M,M'=transition metals,X=carbon or boron group elements)compounds could exhibit large magnetocaloric effect due to the magnetostructural transition,and the composition regulation has been widely studied to realize the magnetostructural transition.Moreover,the magnetostructural transition is also sensitive to the pressure.Herein,the effect of hydrostatic pressure on magnetostructural transformation and magnetocaloric effect has been investigated in Mn_(0.94)Fe_(0.06)NiGe compound.Dual regulation effect of lowering structural transition temperature and strengthening ferromagnetic(FM)state of martensite is realized by applying hydrostatic pressure,which would greatly improve the magnetocaloric effect of Mn_(0.94)Fe_(0.06)NiGe compound.Moreover,the first-principles calculations have also been performed to discuss the origin of the regulation effect under hydrostatic pressure,and it indicates that the hydrostatic pressure can stabilize the hexagonal structure and decrease the structural transition temperature.The maximum isothermal entropy change increases by 109%from 4.3 J/(kg K)under 0 GPa to 9.0 J/(kg K)under 0.402 GPa for a magnetic field change of 0-3 T.This work proves that the hydrostatic pressure is an effective method to regulate the magnetostructural transition and enhance magnetocaloric effect in MM'X compounds.

Unipolar electric-field-controlled nonvolatile multistate magnetic memory in FeRh/(001)PMN-PT heterostructures over a broad temperature span

Multistate magnetic memory effect in heterostructures composed of FeRh thin films with antiferromagnetic(AFM)-ferromagnetic(FM)phase transition and(001)-oriented PMN-PT substrates has been investigated.Utilizing a unipolar electric field,the nonvolatile change in magnetization was nearly doubled compared with that obtained utilizing a conventional bipolar bias.Four stable nonvolatile magnetic states were obtained over a broad temperature span,from 320 to 390K,by adjusting the amplitude of the unipolar electric pulses,demonstrating the possibility of realizing a multistate nonvolatile magnetic memory in the FeRh/PMN-PT heterostructures.This work provides a new strategy for enhancing the magnetic response by utilizing unipolar electric fields and promotes the utilization of AFM-FM phase transition materials in multifunctional information storage and novel spintronic devices.