High Magnetic Entropy Change Research Articles

Near-room-temperature magnetic properties and magnetocaloric effect of polycrystalline and nano-scale manganites Pr(0.65-x)NdxSr0.35MnO3 (x ≤ 0.35)

Here we report investigations of structural, electrical and magnetic properties of bulk and nano-sized Pr0.65-xNdxSr0.35MnO3 compounds (x ≤ 0.35). Polycrystalline compounds were produced by solid – state reaction and nanocrystalline samples were obtained by sol–gel method. Analysis of x-ray diffraction patterns revealed a change in lattice structure from ‘Pbnm’ to ‘R3c’ symmetry, with diminishing cell volume upon increasing Nd ions substitution for all samples. Optical microscopy was used for bulk surface morphology and transmission electron microscopy was implemented for nano-sized samples. Iodometric titration showed oxygen deficiency for bulk compounds and oxygen excess for nano-sized particles. Measurements of resistivity of bulk samples revealed a single peak at temperatures associated with grain boundary conditions and with ferromagnetic/paramagnetic transition and negative magnetoresistivity. Critical magnetic behavior analysis disclosed that the polycrystalline samples are governed by a tricritical mean field and by 3D Heisenberg models while nanocrystalline samples are governed by a mean field model. Curie temperature Tc values remain in near room temperature range for all bulk compounds; they lower with increasing Nd ions substitution from 295 K for the parent bulk compound to 268 K for x = 0.35. Tc is lowered from 255 K for the parent nano-sized compound in small temperature intervals. All compounds exhibit second-order magnetic phase transitions and relatively high magnetic entropy change, with the highest value of 5.74 J/kgK for x = 0.25 bulk sample in μ˳∆H = 4 T. Strong magnetocaloric effect, stability and the possibility of fine-tuning Tc by Nd ions substitution make the investigated bulk polycrystalline compounds promising for application in magnetic refrigeration. Nano-sized samples possess lower magnetic entropy changes of maximum 2.4 J/kgK in μ˳∆H = 4 T for x = 0.15, but wider effective entropy change temperature (δTfwhm) and relative cooling power on par with other manganites, bringing them into the conversation as a viable option in cooling materials.

Rational design and synthesis of Cr1–x Te/Ag2Te composites for solid-state thermoelectromagnetic cooling near room temperature

Materials with both large magnetocaloric response and high thermoelectric performance are of vital importance for all-solid-state thermoelectromagnetic cooling. These two properties, however, hardly coexist in single phase materials except previously reported hexagonal Cr1−x Te half metal where a relatively high magnetic entropy change (−ΔS M) of ∼2.4 J⋅kg−1⋅K−1 @ 5 T and a moderate thermoelectric figure of merit (ZT) of ∼1.2 × 10−2@ 300 K are simultaneously recorded. Herein we aim to increase the thermoelectric performance of Cr1−x Te by compositing with semiconducting Ag2Te. It is discovered that the in-situ synthesis of Cr1−x Te/Ag2Te composites by reacting their constitute elements above melting temperatures is unsuccessful because of strong phase competition. Specifically, at elevated temperatures (T > 800 K), Cr1−x Te has a much lower deformation energy than Ag2Te and tends to become more Cr-deficient by capturing Te from Ag2Te. Therefore, Ag is insufficiently reacted and as a metal it deteriorates ZT. We then rationalize the synthesis of Cr1−x Te/Ag2Te composites by ex-situ mix of the pre-prepared Cr1−x Te and Ag2Te binary compounds followed by densification at a low sintering temperature of 573 K under a pressure of 3.5 GPa. We show that by compositing with 7 mol% Ag2Te, the Seebeck coefficient of Cr1−x Te is largely increased while the lattice thermal conductivity is considerably reduced, leading to 72% improvement of ZT. By comparison, −ΔS M is only slightly reduced by 10% in the composite. Our work demonstrates the potential of Cr1−x Te/Ag2Te composites for thermoelectromagnetic cooling.

Atomic‐resolution Interfacial Microstructure and Thermo‐electro‐magnetic Energy Conversion Performance of Gd/Bi0.5Sb1.5Te3 Composites

Thermo‐electro‐magnetic materials with simultaneously large magnetocaloric (MC) and thermoelectric (TE) effects are the core part for designing TE/MC all‐solid‐state cooling devices. Compositing MC phase with TE material is an effective approach. However, the elemental diffusion and chemical reaction occurring at the two‐phase interfaces could significantly impair the cooling performance. Herein, Gd/Bi0.5Sb1.5Te3 (Gd/BST) composites were prepared by a low‐temperature high‐pressure spark plasma sintering method with an aim to control the extent of interfacial reaction. The reaction of Gd with the diffusive Te and the formation of GdTe nanocrystals were identified at the Gd/BST interfaces by the atomic‐resolution microscope. The formed antisite defects and enhanced {000 l} preferential orientation in BST are responsible for the increased carrier concentration and mobility, which leads to optimized electrical properties. The heterogeneous interface phases, along with antisite defects, favor the phonon scattering enhancement and lattice thermal conductivity suppression. The optimized composite sintered at 693 K exhibited a maximum ZT of 1.27 at 300 K. Furthermore, the well‐controlled interfacial reaction has a slight impact on the magnetic properties of Gd and a high magnetic entropy change is retained in the composites. This work provides a universal approach to fabricating thermo‐electro‐magnetic materials with excellent MC and TE properties.

Near-room-temperature magnetocaloric properties at 1.5 T coupled with relaxor dielectric properties in La2CoMnO6 nanoparticles

Achieving high magnetic entropy change at low magnetic fields near room temperature may pave the way to attaining room temperature magnetic refrigeration. We report herein on high magnetic entropy change at 1.5 T around 291 K in La2CoMnO6 manganite nanoparticles. The magnetic, magnetocaloric, and dielectric properties of La2CoMnO6 nanoparticles of sizes D = 30–40 nm are reported. Powder X-ray diffraction analysis and Raman spectra have confirmed the co-existence of a monoclinic ordered phase and a partially disordered phase. Magnetization measurements have showna long-range ferromagnetic to paramagnetic transition at a Curie temperature of 291.9 K. A maximum magnetocaloric entropy change of 0.88 J kg−1 K−1 has been achieved upon application of a 1.5 T external magnetic field. Dielectric measurements have shown relaxation behavior and a high dielectric constant. The detailed electrical conduction mechanism is explained in terms of various polaron hopping mechanisms. The mutual origin of magnetocaloric and dielectric relaxor behavior in La2CoMnO6, along with its high dielectric constant, confirms that there is strong coupling between the magnetic and dielectric properties of this material, making it suitable for multifunctional applications.

Mechanism for Improved Curie Temperature and Magnetic Entropy Change in Sm-Doped Fe88Zr8B4 Amorphous Alloys

In the present work, Fe88Zr8−xSmxB4 (x = 2, 4) amorphous alloys (AAs) were successfully synthesized into the shape of 40-micrometer-thick ribbons and their magnetic properties were measured. The Fe88Zr8−xSmxB4 (x = 2, 4) AAs exhibited a rather high maximum magnetic entropy change (−ΔSmpeak): ~3.53 J/(K × kg) near 317 K for x = 2 and ~3.79 J/(K × kg) near 348 K for x = 4 under 5 T. The effects of a Sm substitution for Zr on the Curie temperature (Tc) and −ΔSmpeak were studied and compared to those of Nd and Pr substitutions, for the purpose of revealing the mechanism involved in more detail.

Critical behavior and room temperature magnetocaloric effect of La-doped Pr0.7Sr0.3MnO3 compounds

In this work, we present the critical and the magnetocaloric properties of Pr0.7-xLaxSr0.3MnO3 compounds around room temperature. The samples exhibit a single phase with an orthorhombic structure. However, the lattice parameters increase with an increase in La content (x). The Curie temperature (TC) of the ferromagnetic (FM)-paramagnetic (PM) phase transition of compounds increases from 256 K for x = 0–319 K for x = 0.3. We observed a large magnetocaloric effect with high maximum magnetic entropy change (|ΔSmax|) values occurring around room temperature, corresponding to |ΔSmax| = 6.65 and 5.21 J/kgK under a magnetic field change (ΔH) of 50 kOe for x = 0.2 and 0.3, respectively. We also pointed out that the change in |ΔSmax| obeys an exponential function of ΔH (|ΔSmax| ∼ ΔHn), where the value of field exponent (n) is approximately 0.52. Furthermore, the critical behavior around TC for compounds with x = 0.2 and 0.3 has been investigated. The results indicate that these compounds undergo a second-order phase transition with the critical parameters (β = 0.311, γ = 1.137, and δ = 4.656 at TC = 298.4 K for x = 0.2; and β = 0.319, γ = 1.098, and δ = 4.442 at TC = 318.7 K for x = 0.3) that are quite close to those expected for the 3D-Ising model. This suggests the existence of the short-range magnetic interactions in these compounds.

Analysis of magnetocaloric effect in parallel Ni-Mn-Ga Heusler alloy nanowires

In this article, a magnetocaloric effect of electrodeposited Ni-Mn-Ga - based Heusler nanowires is presented using the indirect approach. The structural characterization revealed that the prepared nanowires exhibit a martensitic transformation capability above room temperature (at 350 K). The martensitic transformation results in a relatively high magnetic entropy change of> 2.5 J.kg−1. K−1. Moreover, the magnetocaloric effect analysis reveals a maximum magnetic entropy change at a moderate magnetic field of only 2 T. The presented cheap fabrication method, together with the strong magnetocaloric response of the nanowires at moderate magnetic fields, opens up new possibilities in the field of nanosized Heusler alloys operating as refrigerants, actuators, or sensors

Magnetocaloric effect of the Fe87M8B5 (M = Zr, Ce) amorphous alloys

The magnetic and magnetocaloric properties of the amorphous Fe87Ce8B5 ribbon were investigated in comparison with the Fe87Zr8B5 amorphous alloy (AA). It was found that the Fe87Ce8B5 AA shows a rather high maximum magnetic entropy change (−ΔSmpeak, ∼ 3.65 J/(kg × K) under 5 T and 1.47 J/(kg × K) under 1.5 T) near its Curie temperature (Tc, ∼ 283 K), which is much higher than the − ΔSmpeak of the Fe87Zr8B5 AA near 306 K. The mechanism for the enhanced − ΔSmpeak and decreased Tc by the Ce substitution for Zr in the Fe87M8B5 AAs were investigated. The results are helpful for achieving higher − ΔSmpeak of the iron-based AAs at the temperatures slightly lower than the room temperature, which is conducive to construct a table-like magnetic entropy change (−ΔSm) curve with higher average − ΔSm within the working temperature range of a domestic air conditioner.

Formation and magnetocaloric properties of the amorphous Fe88La7-xCexB5 (x = 0, 1, 3, 5, 7) ribbons

The formability and magnetocaloric response of the amorphous Fe88La7-xCexB5 (x = 0, 1, 3, 5, 7) alloys were studied in this work. When the alloys were prepared into ribbons with a thickness of ∼40 μm, only the Fe88La7-xCexB5 (x = 3, 5, 7) ribbons are fully amorphous and the best glass former is Fe88La2Ce5B5. The amorphous ribbons show rather high magnetic entropy change (−ΔSm) peak, while the Curie temperature of these ribbons decreases with the Ce addition. Based on these results, several amorphous hybrids with flattened −ΔSm profiles were designed. Among these, the hybrid composed of 45% (wt.%) Fe88Ce7B5, 3% (wt.%) Fe88La2Ce5B5 and 52% (wt.%) Fe86Ce7B5Co2 amorphous ribbons is considered to be a better candidate for the working materials of a domestic magnetic air conditioner.

Critical Behavior and Magnetocaloric Effect in Co2CrAl Heusler Alloy

The analysis of structural, electrical transport, critical magnetic behavior, and magnetocaloric properties of Co2CrAl Heusler alloys has been conducted here. Co2CrAl full Heusler alloy has been prepared by the conventional arc melting method in Ar atmosphere and post‐annealing in a vacuum‐sealed quartz tube. X‐ray diffraction (XRD) pattern reveals the crystallization of the alloy to the Fmm space group with L21 crystal symmetry. The correspondence between the magnetic and electrical transport properties of the Co2CrAl alloy has been observed. This alloy exhibits paramagnetic to ferromagnetic transition (Curie temperature) just above room temperature at around 340 K. Metallic to semiconducting transition occurs near the magnetic Curie temperature. Critical magnetic behavior has been analyzed by employing the Arrott plot, Kouvel–Fisher methods, and critical isotherm analysis. These conclude that the exchange interaction in Co2CrAl alloy follows between mean‐field theory having spin–spin long‐range interaction and the 3D Heisenberg model. The magnetocaloric effect has been explored from the isothermal magnetization data by employing Maxwell's equations. The highest magnetic entropy change is found to be ≈0.99 J (kg K)−1 for a magnetic field change of 30 kOe. A large working temperature window of 57 K is the advantage of the present alloy.

Magnetic refrigeration material operating at a full temperature range required for hydrogen liquefaction

Magnetic refrigeration (MR) is a key technique for hydrogen liquefaction. Although the MR has ideally higher performance than the conventional gas compression technique around the hydrogen liquefaction temperature, the lack of MR materials with high magnetic entropy change in a wide temperature range required for the hydrogen liquefaction is a bottle-neck for practical applications of MR cooling systems. Here, we show a series of materials with a giant magnetocaloric effect (MCE) in magnetic entropy change (-∆Sm > 0.2 J cm−3K−1) in the Er(Ho)Co2-based compounds, suitable for operation in the full temperature range required for hydrogen liquefaction (20-77 K). We also demonstrate that the giant MCE becomes reversible, enabling sustainable use of the MR materials, by eliminating the magneto-structural phase transition that leads to deterioration of the MCE. This discovery can lead to the application of Er(Ho)Co2-based alloys for the hydrogen liquefaction using MR cooling technology for the future green fuel society.

Magnetostriction and heat-capacity study on the metamagnetic phase transition of Dy2In1-xAlx alloys

In Dy2In1-xAlx (x = 0, 0.2, 0.3 and 0.4) alloys, the hexagonal structure is gradually transformed into orthorhombic one with x = 0.3 as the critical composition. In Dy2In0.7Al0.3 alloy, the highest magnetic entropy change is accompanied with highest saturation magnetostriction ∼995 ppm under 7 T near phase transition temperature, indicating the typical strong magnetoelastic coupling. From heat-capacity (Cp) measurement, a sharp peak is observed on the Cp - T curves for Dy2In1-xAlx (x = 0, 0.2 and 0.3) alloys. Meanwhile, more Al substitution for In leads into the reduced density of state near Fermi level of each alloy judged from Cp/T - T2 curves. The maximized ΔTad for a field of 7 T is slightly increased from 6.5 K in Dy2In to 7 K in Dy2In0.7Al0.3.

High magnetic entropy change of Pr1−xDyxNi2 compounds with second-order magnetic phase transition

High magnetic entropy change of Pr1−xDyxNi2 compounds with second-order magnetic phase transition

Defect Engineering: Electron-Exchange Integral Manipulation to Generate a Large Magnetocaloric Effect in Ni41Mn43Co6Sn10 Alloys.

A promising magnetocaloric effect has been obtained in Ni-(Co)-Mn-X (X = Sn, In, Sb)-based Heusler alloys, but the low isothermal magnetic entropy change ΔSM restricts the further promotion of such materials. Defect engineering is a useful method to modulate magnetic performance and shows great potential in improving the magnetocaloric effect. In this work, dense Ni vacancies are introduced in Ni41Mn43Co6Sn10 alloys by employing high-energy electron irradiation to adjust the magnetic properties. These vacancies bring about intense lattice distortion to change the distance between adjacent magnetic atoms, leading to a significant enhancement of the average magnetic moment. As a result, the saturation magnetization of ferromagnetic austenite is accordingly improved to generate a high isothermal magnetic entropy change ΔSM of 20.0 J/(kg K) at a very low magnetic field of ∼2 T.

Room-temperature magnetocaloric effect in Fe-substituted CoCr2O4 spinels

In this study, structural, magnetic and magnetocaloric properties of Co(Cr1−xFex)2O4 (x = 0.0, 0.25, 0.50, 0.75 and 1.0) cubic spinel compounds produced by solgel method were investigated. The cubic crystal structure of CoCr2O4 has not disrupted by Fe substitution, but the lattice parameter increases with increasing Fe content due to bigger ionic radii of Fe3+ ions than Cr3+ ions. The magnetic transition temperature of CoCr2O4 (96 K) is increased to 312 K when 50% Fe is substituted with Cr in CoCr2O4 structure. The maximum magnetic entropy change values are − 0.88 J/kgK, − 1.27 J/kgK and − 1.39 J/kgK under 7 T magnetic field for x = 0.0, 0.25 and 0.5 in Co(Cr1−xFex)2O4, respectively. The relative cooling power values are enhanced × 5 when 50% Fe3+ ions are substituted with Cr3+ ions in CoCr2O4. Therefore, the CoFeCrO4 sample might be a potential material for magnetic refrigerant applications due to its considerable high magnetization values, having room-temperature magnetic transition temperature, highest magnetic entropy change and relative cooling power compared to other samples.

Research progress of critical behaviors and magnetocaloric effects of perovskite manganites

Hole-doped perovskite-type manganites have received intensive attention due to their intriguing physical phenomena such as giant magnetocaloric effect and magnetic-phase transitions. However, the mechanism of internal ferromagnetic interaction still needs to be further explored due to the complex natures of competing double-exchange (DE) and super-exchange (SE) interaction, Jahn-Teller (JT) polaron localization, charge ordering, and phase separation scenarios. Critical exponent analysis near magnetic phase transition is a powerful tool to investigate the details of the ferromagnetic interactions and has been used frequently in various magnetocaloric materials. In this article, the critical behavior analyses of perovskite manganites in recent years are comprehensively reviewed. A large number of studies have shown that even in single-phase materials with uniform structure and composition, the critical behavior can be affected by multiple factors such as grain boundary density and the degree of disorder, making them difficult to fully describe the intrinsic ferromagnetism. In this review, firstly, the critical behaviors of typical manganites with different bandwidths in single crystal and polycrystalline are discussed. In a double-exchange dominated system such as La-Sr-Mn-O, short-range 3D-Heisenberg model is basically in good accordance with optimally-doped single crystal sample. However, it would be replaced by long-range mean-field critical behavior in polycrystalline sample when the correlation length exceeds the crystallite size. In a typical intermediate bandwidth system such as La-Ca-Mn-O exhibiting a complex phase diagram described by competing SE/DE interactions, JT polaron localization/delocalization, and Griffith phase disorder, the critical exponent can vary from 3D-Heisenberg model to tricritical mean-field model, for the crossover from first to second order phase transition. Secondly, the studies of elements doping and different fabrication methods indicate that the critical behavior of manganites can be effectively modulated, and vary between different theoretical models including even nonuniversal exponent for highly disordered magnetic system. In the following part, the influence of magnetic field on the critical behavior and field induced crossover phenomena of La-Ca-Mn-O system near tricritical point is analyzed and discussed in detail. Furthermore, the magnetocaloric effects of materials near the tricritical point collected in many studies are listed and compared with each other. Excellent magnetocaloric properties with high magnetic entropy change and relative cooling power in plenty of researches indicate that ideal magnetocaloric material would be very likely to be found in the materials near the tricritical point, which lay at the borderline between first-order and second-order phase transition. Consequently, it is suggested that perovskite manganites are still quite promising in the potential magnetic refrigeration applications, and need to be further developed.

Lanthanide clusters of phenanthroline containing a pyridine-pyrazole based ligand: magnetism and cell imaging.

In this contribution, we report the synthesis, characterization and luminescence-magnetic properties of Ln-clusters (Ln = Gd3+, Eu3+ and Tb3+) using a new pyridine-pyrazole functionalized ligand fitted with a chromophoric phenanthroline backbone. The unorthodox N-rich ligand forms isostructural trinuclear lanthanide complexes with a topology that closely resembles two interdigitating hairpins. The clusters crystallize in chiral space groups and also exhibit chirality for bulk samples, which were further confirmed using solid state CD spectra. Magnetic studies on the complexes reveal their interesting features while the Gd cluster shows a significant cryogenic magnetic cooling behaviour with a moderately high magnetic entropy change of -23.42 J kg-1 K-1 at 7 T and 2 K. On the other hand, Eu and Tb complexes exhibit interesting fluorescence properties. The compounds were subsequently used as fluorescent probes for the imaging of human breast adenocarcinoma (MCF7) cells. Live cell confocal microscopy images show that the complexes penetrate beyond the usual cytoplasm region and can be useful in imaging the nucleus region of MCF7 cells.

Effect of Si substitution on the Structural, Magnetic and Magnetocaloric Properties of Ni–Mn–In Heusler alloys

The effect of partial substitution of manganese by silicon in Ni47Mn40−xSixIn13 (x = 1, 2, 3) on the structural (martensitic), magnetic transitions and associated magnetocaloric properties around the martensitic transition of these alloys, was investigated. The alloys exhibited the single austenite structure at the room temperature. The lattice contraction was observed with an increase in Si content, thereby inducing the FM and AFM interactions between manganese atoms, which further influence the magnetic and martensitic transitions. The increase in Si concentration was observed with the decrease in valence electron to atom ratio (e/a ratio) and thus influencing the martensitic transitions. The transition temperatures such as martensitic transition (TM) and the Curie transition (TC) of the austenite phase decrease with increasing Si-content. With the increase in Si content from x = 1 to 3, the TM was found to decrease from 259 to 169 K. A high magnetic entropy change (∆S)M of 12.3, 3.1 and 30.6 J kg−1 K−1 was observed for the x = 1, 2, and 3 alloys at their respective martensitic transitions for a magnetic field of 2 T.

Research Trend of Metal-Organic Frameworks for Magnetic Refrigeration Materials Application

The magnetocaloric effect (MCE), which is the reversible temperature change of magnetic materials due to an applied magnetic field, occurs largely in the vicinity of the magnetic phase transition temperature. This phenomenon can be used to induce magnetic refrigeration, a viable, energy-efficient solid-state cooling technology. Recently, Metal-organic frameworks (MOFs), due to their structural diversity of tunable crystalline pore structure and chemical functionality, have been studied as good candidates for magnetic refrigeration materials in the cryogenic region. In cryogenic cooling applications, MCE using MOF can have great potential, and is even considered comparable to conventional lanthanum alloys and magnetic nanoparticles. Owing to the presence of large internal pores, however, MOF also exhibits the drawback of low magnetic density. To overcome this problem, therefore, recent reports in literature that achieve high magnetic entropy change using a dense structure formation and ligand tuning are introduced.

Table-like magnetocaloric effect and enhanced refrigerant capacity of HPS La(Fe,Si)13-based composites by Ce–Co grain boundary diffusion

The structure and magnetic properties of LaFe11.6Si1.4/Ce40Co60 bulk composites prepared by hot pressing sintering (HPS), followed by high-temperature diffusion annealing, were investigated. During the HPS process, the La(Fe,Si)13 alloy powder particles combined with Ce40Co60 binder and a porous structure with high mechanical strength were obtained. The addition of Ce40Co60 can also promote the peritectic reaction. Annealing has important effects on the magnetocaloric properties due to the diffusion of Ce and Co into the La(Fe,Si)13 phase during annealing. HPS samples were annealed at 1373 K for 24 h, and a table-like magnetocaloric effect, exhibiting a constant high magnetic entropy change (− ∆SM) of ~ 4.0 J/(kg K) in a commercially useful temperature region of 28 K (236–264 K), was obtained for a field change of 2 T. The values of full width at half maximum of (− ∆SM)–T plots (ΔTFWHM) and enhanced refrigeration capacity for the composites are 55 K and 173 J/kg, respectively, for a field change of 2 T. The composites exhibit high compressive strength of up to 312 MPa. The present results indicate that LaFe11.6Si1.4/Ce40Co60 bulk composites can meet the requirement of near room temperature magnetic refrigeration based on the Ericsson cycle.