Low Temperature Magnetic Refrigeration Research Articles

Magnetic properties and cryogenic magnetocaloric effect in α-Gd2(MoO4)3 compound

Magnetic refrigeration technology has received extensive attention due to its application prospects in various fields. The research of magnetocaloric materials is the basis for the development of magnetic refrigeration technology. In this study, the α-Gd2(MoO4)3 was prepared by the high temperature solid state reaction. By X-ray diffraction characterization, α-Gd2(MoO4)3 crystallizes in monoclinic structure (space group C12/c1). Furthermore, its magnetism and magnetocaloric effect are investigated. The magnetic ordering temperature of α-Gd2(MoO4)3 is less than 1.8 K and there is a second-order phase transition. The maximum magnetic entropy changes are 20.8 and 32.7 J kg−1 K−1 under the field changes of 0–2 T and 0–5 T, respectively. Besides, the refrigeration capacity and relative cooling power are 142.0 and 190 J kg−1 under field change of 0–5 T. These properties indicate that α-Gd2(MoO4)3 is an excellent candidate for low temperature magnetic refrigeration.

Metamagnetic phase transition induced large magnetocaloric effect in a Dy0.5Ho0.5MnO3 single crystal.

Magnetic refrigeration based on the magnetocaloric effect is gaining interest in orthogonal or hexagonal rare-earth manganite. However, a more comprehensive understanding of the underlying mechanism is still required. We grew a high-quality single crystal of Dy0.5Ho0.5MnO3 using the optical floating zone method, since the parent crystals DyMnO3 and HoMnO3 have orthogonal and hexagonal structures, respectively. The magnetic and magnetocaloric properties and refrigeration mechanisms are thoroughly investigated. Doping modifies the magnetism according to the results obtained from the investigation of magnetic and dielectric properties and heat capacity. The spin reorientation transition shifts towards low temperature in comparison to HoMnO3. Near the Néel temperature of rare-earth sublattices (5 K), the highest changes in negative magnetic entropy under 0-70 kOe are 18 J kg-1 K-1 and 13 J kg-1 K-1 along the a- and c-axes, respectively. The low-temperature metamagnetic phase transition caused by the alterations in the magnetic symmetry of Ho3+ contributes to an increased magnetocaloric effect in comparison to the parent crystals, rendering it a promising choice for magnetic refrigeration applications. Dy0.5Ho0.5MnO3 exhibits a clear magnetocrystalline anisotropy with enhanced refrigeration capacity and negative magnetic entropy change along the a-axis. The adiabatic temperature change of Dy0.5Ho0.5MnO3 is 8.5 K, larger than that of HoMnO3, rendering it a promising choice for low-temperature magnetic refrigeration applications.

Large magnetocaloric effect in gadolinium-rich silicate NaGd9(SiO4)6O2

Magnetic cooling has aroused great interest in cryogenic research due to its high energy efficiency. Due to the large magnetic moment and completely quenched orbital momentum, Gd-based oxides have been promising candidates for cryogenic magnetic refrigerant. In this article, the magnetic properties and magnetocaloric effect of NaGd9(SiO4)6O2 (NGSO) single crystal are investigated. The maximum magnetic entropy change of 60 J·kg−1K−1 is observed with the magnetic field changing from 0–7 T at 2.6 K. The large magnetic entropy change can be attributed to high metal/ligand ratio and weak exchange interaction. Particularly, due to the large density of NGSO, the volumetric entropy change is identified to be 382.8 mJ·cm−3K−1, which surpasses most reported magnetic refrigerants in volume units. Besides, the excellent refrigerant capacity and adiabatic temperature change have been determined to be as high as 370.4 J·kg−1 and 7.4 K under the applied field change of 0–7 T and 0–2 T, respectively. These performances suggest that NGSO is a promising candidate for low-temperature magnetic refrigeration applications.

Structure and cryogenic magnetocaloric effects of NaREO2 (RE = Er, Ho) compounds

Rare-earth (RE) based oxides have presented great application prospects in low-temperature magnetic refrigeration technology. Thus far, studies on magnetic and especially magnetocaloric properties of alkali-metal RE oxides are limited, largely due to the difficulty in synthesizing these oxides. In this work, NaREO2 (RE = Er, Ho) compounds with both the rhombohedral (R3‾m space group) and cubic (Fm3‾m space group) structures were prepared by a facile ball-milling-aided sintering approach. The cubic structural compounds exhibit a spherical microstructure composed of vertically-grown irregular hexahedra, which sharply contrast with a loose lamellar microstructure of the rhombohedral structural compound. The as-prepared NaHoO2 or NaErO2 compound exhibits an antiferromagnetic ordering of RE3+ moments at a temperature of 2.4 K or lower than 2 K respectively. An excellent comprehensive magnetic refrigeration performance at cryogenic temperatures in a low magnetic field change of 0–20 kOe was obtained in these compounds, including a complete reversibility of the magnetocaloric effect, a maximal magnetic-entropy change ΔSM at 2 K of 12 J/kg·K for the rhombohedral NaErO2 and a relative cooling power (RCP) of 79.7 J/kg for the cubic NaHoO2.

Synthesis, structure and properties of RNiBN (R = rare-earth elements)

We report the structural and physical properties of new nitridoborates RNiBN (R = Nd, Sm, Gd, Tb, Dy, Ho, and Er). These compounds crystallize in a tetragonal LaNiBN-type structure with two-dimensional Ni2B2 and RN layers. The physical properties measurements indicate that they are antiferromagnetic metals. For R = Gd and Ho, a successive magnetic transition was observed below N é el temperature, possibly associated with spin re-orientation due to Ruderman-Kittel-Kasuya-Yosida interaction. A giant magnetocaloric effect was measured in the R = Er and Ho compounds, which is related to a field-induced first-order metamagnetic transition from antiferromagnetic to ferromagnetic state. Our results indicate that RNiBN (R = Er and Ho) may be promising candidates for low temperature magnetic refrigeration

Tunning the magnetism and magnetocaloric effects of EuAl4 single crystals by Si and Cu doping

The effects of Si and Cu doping on crystal structures, magnetism, and the magnetocaloric effect of EuAl4 have been investigated. EuAl4, EuAl3.35Si0.65, and EuAl3.82Cu0.18 single crystals grown by the Al flux method are isotypic and crystallize in BaAl4-type structure with space group I4/mmm. Compared to the pristine compound EuAl4, the lattice parameter a keeps almost unchanged and c decreases obviously by Si doping, while a decreases and c increases upon Cu-substitution. These irregular changes in the lattice parameters may contribute to the electronegativity difference between Si and Cu. Cu and Si doping enhances the ferromagnetic interaction, and the direction of easy magnetization changes from the ab plane to the c axis upon the Cu-substitution. Multiple-step metamagnetic transitions are still observed in EuAl3.35Si0.65 as applying the magnetic field along the c axis, while EuAl3.82Cu0.18 shows ferromagnetic behavior along the ab plane and c axis. As a consequence, the low-field magnetocaloric effect is significantly enhanced. The maximum magnetic entropy change under a field change of 20 kOe increases from − 8.4 J/kg K for EuAl4 to − 12.9 J/kg K for EuAl3.35Si0.65 and − 15.9 J/kg K for EuAl3.82Cu0.18. The corresponding values of refrigerating capacity are 67, 140, and 147 J/kg, respectively. The remarkable magnetocaloric performances suggest that the Cu-doped EuAl4 compound is an excellent candidate material for low-temperature magnetic refrigeration.

Tunable magnetic and magnetocaloric properties of scandium doped Gd3Fe5-xScxO12 garnet compound

This paper presents the tunable magnetic and magnetocaloric properties of Scandium-doped gadolinium iron garnets, Gd3 Fe5-xScxO12 (x = 0.0 to 0.25) compounds prepared by a facile auto-combustion method. The sample analyzed has a dominant cubic crystal structure (space group: Ia3¯d)) with a small fraction of an orthorhombic secondary phase, as determined by Rietveld analysis of X-ray diffraction patterns. The structural, magnetic, Mössbauer spectroscopy and first-principles density functional theory (DFT) studies show a preferential substitution of Sc3+ at the octahedral site of Fe3+ions. The ferrimagnetic (FIM) transition is present in all samples, with the transition temperature decreasing from 560 K for × = 0.0 to 521 K for × = 0.25. The Sc3+dopedGd3Fe4.75Sc0.25O12exhibits an improved magnetocaloric effect (MCE) with a maximum magnetic entropy change (-ΔSMmax)3.82 J kg-1K−1, and a higher relative cooling power (RCP) value of 408 J kg−1, which is ∼ 7 % higher than the Gd3Fe5O12(380 J kg−1) sample. These findings suggest that by incorporating Sc3+, the magnetic and magnetocaloric properties of Gd3Fe5-xScxO12 can be tailored for potential applications in low-temperature magnetic refrigeration.

Magnetic and magnetocaloric properties of R55Co30Al10Si5 (R = Ho, Dy) metallic glass ribbons

Metallic glass alloys are suitable systems for the development of magnetocaloric refrigerated materials due to the stable physical properties and the adjustable magnetic phase transition temperature. Here, the glass formation ability (GFA), magnetic properties and magnetocaloric effects of R55Co30Al10Si5 (R = Ho, Dy) amorphous ribbons prepared by the melt-spinning technique were investigated. The X-ray diffraction and DSC measurement indicated that the metallic glass alloys have a completely amorphous structure and excellent glass formation ability. The amorphous ribbons were demonstrated to undergo a second-order magnetic phase transition (ferromagnetic to paramagnetic) around Tc ∼ 18 K for Ho55Co30Al10Si5, ∼34 K for Dy55Co30Al10Si5, respectively. The maximum magnetic entropy change (-ΔSMmax) and the corresponding refrigerant capacity (RC) values are 12.7 J/kg K, 343 J/kg for Ho55Co30Al10Si5, 9.1 J/kg K, 334 J/kg for Dy55Co30Al10Si5 under a field change ΔH= 5 T. The large -ΔSMmax and high RC suggest that R55Co30Al10Si5 (R = Ho, Dy) amorphous ribbons have potential applications in low temperature magnetic refrigeration.

Giant low-field reversible magnetocaloric effect at liquid helium temperature of niobium and iron co-substituted EuTiO3 compounds

Giant magnetocaloric effect (MCE) materials in the liquid helium temperature region have attracted a lot of attention in the field of low-temperature magnetic refrigeration (MR). In this study, a series of niobium (Nb) and iron (Fe) co-substituted EuTiO3 perovskites with cubic structure (space group pm3m) was successfully fabricated, and their magnetic properties as well as cryogenic magnetocaloric effects were investigated in detail. As expected, the introduction of Nb and Fe can significantly modulate the magnetic phase transition and magnetocaloric effect of the EuTiO3 compounds. With increasing Fe concentration, two local minima corresponding to the AFM-FM magnetic phase transition near 5.0 K and FM-PM transition near 10 K with no hysteresis in the thermomagnetic curves are observed, which is attributed to an enhancement of FM coupling. At the same time, the gradually widened –ΔSM–T curves and the two peaks with a broad shoulder lead to considerable refrigeration capacity (RC). With the field change of ΔH = 2 T, the calculated values of −ΔSMmax for the EuTi0.9375–xNb0.0625FexO3 (x = 0.075, 0.1, 0.125, 0.15) compounds are 24.2, 17.6, 14.5 and 14.0 J/(kg·K), respectively. The corresponding RC values were calculated to be 144.6, 138.3, 151.2 and 159 J/kg, respectively. Especially, the values of −ΔSMmax for EuTi0.8625Nb0.0625Fe0.075O3 are 8.6 and 15.1 J/(kg·K) under low field changes of 0.5 and 1 T, respectively. The giant low-field reversible magnetocaloric effect makes them attractive candidates for magnetic refrigeration in the liquid helium temperature region.

Structural, magnetic, and magnetocaloric properties of chromium doped Gd3Fe5-xCrxO12 garnet compound

The detailed structural, magnetic, and cryogenic magneto-caloric properties of chromium-substituted gadolinium iron garnet (Gd3Fe5-xCrxO12) nanocrystalline powders were studied using the facile autocombustion method and a calcination temperature of 1100 °C. The X-ray diffraction pattern showed that all samples were single-phase with cubic Ia3d symmetry. The temperature and field-dependent magnetization data of Gd3Fe5-xCrxO12 samples revealed a ferrimagnetic ordering at low temperatures. Upon Cr3+ substitution, the Curie temperature reduced by 7% at x = 0.25 from 560 K for x = 0.00 sample. In a field up to 5 T, the maximum magnetic entropy change was observed as ΔSM ∼ 3.8 J K−1 kg−1 for x = 0.00 and −ΔSM ∼ 3.9 J K−1 kg−1 for x = 0.25 sample, while the maximum relative cooling power, RCP, value of 420 J kg−1 was measured for x = 0.25 sample, which is 10% larger than the x = 0.00 (RCP ∼ 380 J kg−1). Therefore, Cr3+ substituted Gd3Fe5-xCrxO12 samples exhibit promising magneto-caloric performance and have potential low-temperature magnetic refrigeration applications.

Evidence of Hysteresis Free Ferromagnetic Nature and Significant Magnetocaloric Parameters in FeNi Binary Alloy

In this study, our aim is to investigate the structural, magnetic, and magnetocaloric properties of the FeNi binary alloy. The FeNi alloy with Fe65Ni35 composition was prepared by ball milling followed by the annealing and quenching processes. A Rietveld refinement analysis of structural results reveals that this system has coexisting cubic structural phases with a dominant face-centered cubic phase (Fm-3m;γ-FeNi). Magnetization results of this compound indicate the presence of ferromagnetic ordering and the magnetic transition observed around 100 K. Moreover, an Arrott plot study provides information about the order of phase transition, which is found in the second-order near the ordering temperature, whereas first-order nature is also noted in the low-temperature region. The significant magnetocaloric parameters, i.e., magnetic entropy change (ΔSM~0.495 J/kg-K) and relative cooling power (88 J/kg), are noted over a wide temperature range. The power law dependency of magnetic entropy change with the applied field is also investigated. Due to their significant magnetocaloric performance over a wide temperature range, these multiphase alloys may be a good candidate for room-temperature to low-temperature magnetic refrigeration.

Tunable magnetic and magnetocaloric properties by thermal annealing in ErCo2 atomized particles

Processing magnetocaloric materials into magnetic refrigerants with appropriate shapes is essential for the development of magnetic refrigeration systems. In this context, the impact of processing on the physical properties of the materials is one of the important issues. Here, we investigate the crystallographic, magnetic, and magnetocaloric properties of gas-atomized particles of the intermetallic compound ErCo2, a giant magnetocaloric material for low-temperature applications. The results demonstrate that the physical properties of ErCo2 are significantly changed by atomization and subsequent thermal annealing. In the as-atomized particles, the magnetic transition temperature increases from 34 to 56 K, and the phase transition changes from first order to second order. The thermal annealing shifts the transition temperature back to the original one and restores the first-order phase transition characteristic. The changes in magnetic properties are closely related to those in crystallographic properties, suggesting the importance of the magneto-structural coupling. The magnetic entropy change −ΔSM of the particles can be tuned in size, shape, and peak temperature depending on the annealing conditions. The peak value of −ΔSM varies in the range of 9–33 J kg−1 K−1 for a field change of 0–5 T. All the ErCo2 atomized particles have magnetocaloric properties comparable or superior to other promising candidates for low-temperature magnetic refrigerants.

Structural and cryogenic magnetic properties of the RE2MoO6 (RE = Er and Ho) compounds

The rare earth (RE) based magnetic materials with large low temperature magnetocaloric effect (MCE) have attracted much research interest due to their potential application for the environment-friendly magnetic refrigeration (MR) technology. Herein, a systematic investigation on the crystal structure, micro morphology together with the low temperature magnetic properties and MCE performances have been conducted on the RE2MoO6 (RE = Er and Ho) compounds. Both compounds crystallize in monoclinic structure and exhibit large reversible low temperature MCE. The maximum magnetic entropy changes (−ΔSMmax) at magnetic field change (ΔH) of 0–5 T reach 15.3 and 15.8 J kg−1 K−1 for Er2MoO6 and Ho2MoO6, respectively. The corresponding relative cooling power/refrigeration capacity (RCP/RC) values are 197.4/149.7 and 286.0/222.3 J/kg. These obtained MCE parameters for present RE2MoO6 compounds are close to those of reported RE based MCE materials, which make them of potential for low temperature MR application.

Nonlinear elasticity and strain-tunable magnetocalorics of antiferromagnetic monolayer MnPS[formula omitted

The nonlinear elasticity and strain-tunable magnetocaloric effect (MCE) of antiferromagnetic monolayer MnPS3 are demonstrated by multiscale modeling and simulations. A continuum constitutive model expanding the elastic strain energy density by a Taylor series is formulated to describe the nonlinear and large-deformation elasticity of monolayer MnPS3. Fourteen independent elastic constants of the model are determined using the ab initio results. The nonlinear behavior is found to initiate at about 7% strain and nearly isotropic before the rupture. MCE (adiabatic temperature change ΔT and isothermal entropy change −ΔS) of monolayer MnPS3 is found anisotropic due to the anisotropy in the temperature derivative of magnetic susceptibility, i.e. stronger in-plane MCE. An in-plane biaxial strain could improve MCE by a factor of 1.5–2. The maximum −ΔS and ΔT occur at around 10 K and are evaluated as 2.8 μJ m−2 K−1 and 2.2 K, respectively, suggesting monolayer MnPS3 as a potential candidate for low-temperature (¡ 20 K) magnetic refrigeration.

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.

Evaluating the properties of the intermetallic compound HoN for magnetic refrigerator application: Combined DFT and Monte Carlo simulation

Evaluating the properties of the magnetocaloric material regenerators is a crucial step in order to exploit them subsequently as the working substances in magnetic cooling devices. So, the main objective of this latter is to evaluate the electronic, magnetic properties as well as the magnetocaloric effect of HoN compound. To do so, a combination of DFT study and Monte Carlo simulations has been performed on HoN compound which undergoes a second-order ferromagnetic-paramagnetic phase transition. For this material, the temperature-dependent total magnetization, heat capacity and magnetic susceptibility have been calculated and are revealed that the curie temperature is acceptable concurrence with the experimental value. Furthermore, it was showed that under an applied magnetic field of 5T, the magnetocaloric effect (MCE) in regard to the maximum value of magnetic entropy change ΔSmagmax around its Curie temperature, agree well with the experimental one. Besides, the relative cooling power (RCP) values are found to be 28.5, 162.2 and 227 J. Kg−1 under different magnetic fields of 1,3 and 5T. All findings which are presented in the present work indicate that the large values of RCP and ΔSmagmax found in HoN are interesting for low-temperature magnetic refrigeration with excellent performance.

Study of magnetic and magnetocaloric effect of REMnO3(RE=Dy,Eu)manganites

We have systematically investigated the magnetic and magnetocaloric effect (MCE) of REMnO3 (RE = Dy, Eu) manganites sintered by solid-state reaction method. The Neel temperature (TN), the maximum magnetic entropy change (-ΔSmmax) and the relative cooling power (RCP) of the sample were obtained by measuring the magnetization and heat capacity. Under the magnetic field change of 50 kOe, large -ΔSmmax of 8.93J/kg K and 9.31J/kg K are achieved around the TN of 14 K and 50 K for DyMnO3 and EuMnO3, respectively. It is proved by the slope of the Arrott plot and the universal phenomenological curve that the sample undergoes second order phase transition. The critical behavior of antiferromagnetic to paramagnetic phase transition is studied by modifying Arrott plot, which shows that the spin order of the sample decreases at the phase transition temperature and tends to change into shortrange order. The large magnetic entropy change calculated by Landau theory and experimental data indicates that the sample has a large magnetocaloric effect and has potential application in low temperature magnetic refrigeration materials.

First principles study of magnetic transition of strain induced monolayer NbSi<sub>2</sub>N<sub>4</sub>

The effective control of two-dimensional material magnetism is a frontier research field. In this work, the influences of in-plane biaxial tension strain on the electronic structure, magnetic properties, and Curie temperature of monolayer NbSi<sub>2</sub>N<sub>4</sub> are investigated by first-principles calculations based on density functional theory and Monte Carlo simulations in the frame of the Heisenberg model. We demonstrate that the monolayer NbSi<sub>2</sub>N<sub>4</sub> has favorable dynamic and thermal stability through the phonon spectral calculations and ab initio molecular dynamics simulations. It is found that the intrinsic monolayer NbSi<sub>2</sub>N<sub>4</sub> is a non-magnetic metal, which can be transformed into a ferromagnetic metal by 1.5% tensile strain. The electronic structure analysis of monolayer NbSi<sub>2</sub>N<sub>4</sub> shows that the ferromagnetism induced by tensile strain is caused by traveling electrons. There is a half-full band at the monolayer NbSi<sub>2</sub>N<sub>4</sub> Fermi level, which is mainly contributed by the dz<sup>2</sup> orbital of the Nb atom. When there is no additional strain, the band is spin-degenerate. Tensile strain can make this band more localized, which leads to Stoner instability, resulting in the ferromagnetic ordering of monolayer NbSi<sub>2</sub>N<sub>4</sub> traveling electrons. The stability of the ferromagnetic coupling is enhanced with the increase of the strain degree. The calculation results of the magnetic anisotropy energy show that the strain can make the direction of the easy magnetization axis of the monolayer NbSi<sub>2</sub>N<sub>4</sub> reverse from the vertical direction to the in-plane, and then back to the vertical direction. Furthermore, the strain can significantly increase the Curie temperature of monolayer NbSi<sub>2</sub>N<sub>4</sub>. The Curie temperature of monolayer NbSi<sub>2</sub>N<sub>4</sub> is 18 K at 2% strain and 87.5 K at 6% strain, which is 386% higher than that at 2% strain. Strain engineering can effectively control the magnetic ground state and Curie temperature of single-layer NbSi<sub>2</sub>N<sub>4</sub>. The research results are expected to promote the development of <i>MA</i><sub>2</sub><i>Z</i><sub>4</sub> materials in the field of mechanical sensing device design and low-temperature magnetic refrigeration.

Tuning the giant Magnetocaloric Effect and refrigerant capacity in Gd<sub>1–x</sub>Y<sub>x</sub>CrO<sub>3</sub> (0.0 ≤ x ≤ 0.9) perovskites nanoparticles

<abstract> <p>Different compounds of rare-earth orthochromites Gd<sub>1–x</sub>Y<sub>x</sub>CrO<sub>3</sub> (where x is 0.0–0.9) powder nanoparticles, were synthesized by the auto-combustion method followed by annealing at 700 ℃. All the compounds showed single-phase and crystallized into a distorted orthorhombic structure with the space group (Pbnm). The average particle size for all the samples were in the range 53–110 nm. The detailed and systematic magnetic measurements and analysis showed that all the samples up to x = 0.9 have large magnetization and large values of the change in the magnetic entropy. The magnitude of the change in the magnetic entropy (at 4.5 K and for all the values of the change in the applied magnetic field between 1 and 9 T) is found to increase with increasing x reaching a maximum value at x = 0.3 then it decreases as we increase the yttrium concentration. The nanoparticle compounds with low yttrium concentrations showed a giant change in the magnetic entropy and a giant relative cooling power. Based on the slopes of Arrott plots curves the order parameter of the magnetic transition has been estimated and found to be second order. The giant change in the magnetic entropy and the relative cooling power were tuned in the rages (-45.6 to -8.7 J/kg·K at a change in the applied magnetic field of 9 T; and 136–746 J/kg), around the helium liquefaction temperature. The magnitude of the change in the magnetic entropy is significantly larger for large range of temperatures, up to the nitrogen liquefaction temperature. The giant change in the magnetic entropy and the giant relative cooling power at low temperatures (in the range about 4 to 20 K.) make these samples candidate materials for the low temperature magnetic refrigerant applications, based on the magnetocaloric effect.</p> </abstract>

Synthesis, structure and magnetocaloric properties of a new two-dimensional gadolinium(III) coordination polymer based on azobenzene-2,2',3,3'-tetracarboxylic acid.

A new Gd3+ coordination polymer (CP), namely, poly[diaqua[μ4-1'-carboxy-3,3'-(diazene-1,2-diyl)dibenzene-1,2,2'-tricarboxylato]gadolinium(III)], [Gd(C16H7N2O8)(H2O)2]n, (I), has been synthesized hydrothermally from Gd(NO3)3·6H2O and azobenzene-2,2',3,3'-tetracarboxylic acid (H4abtc). The target solid has been characterized by single-crystal and powder X-ray diffraction, elemental analysis, IR spectroscopy and susceptibility measurements. CP (I) crystallizes in the monoclinic space group C2/c. The structure features a 4-connected topology in which Gd3+ ions are connected by carboxylate groups into a linear chain along the monoclinic symmetry direction. Adjacent one-dimensional aggregates are bridged by Habtc3- ligands to form a two-dimensional CP in the (10-1) plane. A very short hydrogen bond [O...O = 2.4393 (4) Å] links neighbouring layers into a three-dimensional network. A magnetic study revealed antiferromagnetic Gd...Gd coupling within the chain direction. CP (I) displays a significant magnetocaloric effect (MCE), with a maximum -ΔSm of 27.26 J kg-1 K-1 for ΔH = 7 T at 3.0 K. As the MCE in (I) exceeds that of the commercial magnetic refrigerant GGG (Gd3Ga5O12, -ΔSm = 24 J kg-1 K-1, ΔH = 30 kG), CP (I) can be regarded as a potential cryogenic material for low-temperature magnetic refrigeration.