Significant Magnetic Entropy Research Articles

Modulating Magnetic Refrigeration through Structural Variation in CoII/III-GdIII Clusters.

Three heterometallic aggregates, [(CoII)2(GdIII)2(tBuPO3)2(O2CtBu)2(HO2CtBu)2(NO3)4]·NEt3 (1), [(CoII)2(CoIII)2(GdIII)3(μ3-OH)2(tBuPO3)2(O2CtBu)9(deaH)2(H2O)2] (2), and (CoIII)2(GdIII)5(μ2-OH)(μ3-OH)2(tBuPO3)2(O2CtBu)10(HO2CtBu)(deaH)2]·MeOH (3), were successfully isolated in reactions of [Co2(μ-OH2)(O2CtBu)4]·(HO2CtBu)4, Gd(NO3)3·6H2O, tBu-PO3H2, and diethanolamine (deaH3) by varying the stoichiometry of the reactants and/or changing the solvent. The structures of the final products were profoundly affected by these minor changes in stoichiometry or a change in solvent. The metal-oxo core of these complexes displays a hemicubane or a defective dicubane-like view. Bond valence sum calculations and bond lengths indicate the presence of CoII centers in compound 1, mixed valent Co centers (CoII/CoIII) in compound 2, and only CoIII centers in compound 3 as required for the charge balances and supported by the magnetic measurements. Magnetic studies reveal significant magnetic entropy changes for complexes 1-3 (-ΔSm values of 28.14, 25.06, and 29.19 J kg-1 K-1 for 3 K and 7 T, respectively). This study shows how magnetic refrigeration can be affected by anisotropy, magnetic interactions (ferro- or antiferromagnetic), the metal/ligand ratio, and the content of GdIII in the molecule.

Two di- and trinuclear Gd(III) clusters derived from monocarboxylates exhibiting significant magnetic entropy changes

Two di- and trinuclear Gd(III) clusters with the formulas {[Gd2(omba)6(phen)2]·2H2O} (1) and [Gd3(Hsa)5(sa)2(phen)3] (2) (Homba=o-methoxybenzoic acid, H2sa=salicylic acid and phen=1,10-phenanthroline) have been prepared through the hydrothermal reactions. Complex 1 takes a dinuclear structure and two Gd(III) are bridged by two syn-syn-μ2-η1:η1 and two syn-syn-μ2-η1:η2 carboxylates. While complex 2 is a trinuclear cluster linked by one syn-syn-μ2-η1:η2 and two syn-syn-μ2-η1:η1 carboxylates. Magnetic studies reveal the key impact of magnetic density on the magnetocaloric effect in Gd(III) systems. They exhibit relatively weak magnetic interactions and significant magnetic entropy changes with −ΔSmmax=17.1Jkg−1K−1 and 22.5Jkg−1K−1 (T=2K, ΔH=7T), respectively.

Proton-Conducting Magnetic Coordination Polymers.

Three isostructural lanthanide-based two- dimensional coordination polymers (CPs) {[Ln2(L)3(H2O)2]n⋅2n CH3OH)⋅2n H2O} (Ln=Gd(3+) (1), Tb(3+) (2), Dy(3+) (3); H2L=cyclobutane-1,1-dicarboxylic acid) were synthesized by using a low molecular weight dicarboxylate ligand and characterized. Single-crystal structure analysis showed that in complexes 1-3 lanthanide centers are connected by μ3-bridging cyclobutanedicarboxylate ligands along the c axis to form a rod-shaped infinite 1D coordination chain, which is further linked with nearby chains by μ4-connected cyclobutanedicarboxylate ligands to form 2D CPs in the bc plane. Viewing the packing of the complexes down the b axis reveals that the lattice methanol molecules are located in the interlayer space between the adjacent 2D layers and form H-bonds with lattice and coordinated water molecules to form 1D chains. Magnetic properties of complexes 1-3 were thoroughly investigated. Complex 1 exhibits dominant ferromagnetic interaction between two nearby gadolinium centers and also acts as a cryogenic magnetic refrigerant having a significant magnetic entropy change of -ΔSm=32.8 J kg(-1) K(-1) for ΔH=7 T at 4 K (calculated from isothermal magnetization data). Complex 3 shows slow relaxation of magnetization below 10 K. Impedance analysis revealed that the complexes show humidity-dependent proton conductivity (σ=1.5×10(-5) S cm(-1) for 1, σ=2.07×10(-4) S cm(-1) for 2, and σ=1.1×10(-3) S cm(-1) for 3) at elevated temperature (>75 °C). They retain the conductivity for up to 10 h at high temperature and high humidity. Furthermore, the proton conductivity results were correlated with the number of water molecules from the water-vapor adsorption measurements. Water-vapor adsorption studies showed hysteretic and two-step water vapor adsorption (182,000 μL g(-1) for 1, 184,000 μL g(-1) for 2, and 1,874,000 μL g(-1) for 3) in the experimental pressure range. Simulation of water-vapor adsorption by the Monte Carlo method (for 1) confirmed the high density of adsorbed water molecules, preferentially in the interlayer space between the 2D layers.

A triangular [Mn3] cluster-based ferrimagnet with significant magnetic entropy change

A triangular [Mn3] cluster-based ferrimagnet [Na2Mn3(SO4)3(μ3-OH)2(μ2-OH2)2]n exhibits a significant magnetocaloric effect (−ΔSm = 14.4 J kg−1 K−1) at 13.5 K.

A family of Fe(3+) based double-stranded helicates showing a magnetocaloric effect, and Rhodamine B dye and DNA binding activities.

Herein, the synthesis, structural characterization, magnetic properties and guest binding activities of four Fe(3+) based double-stranded helicates namely; [Fe2(L)2](ClO4)(Cl)·4(CH3OH)·2(H2O) (), [Fe2(L)2](BF4)2·2(H2O) (), [Fe2(L)2](NO3)2·3(CH3OH)·2(H2O) (), and [Fe2(L)2](Cl)2·2(CH3OH)·4(H2O) () are reported. Complexes have been synthesized using the hydrazide-based ligand H2L (H2L = N'1,N'4-bis(2-hydroxybenzylidene)succinohydrazide) and the corresponding Fe(2+) salts. Each of the independent cationic complexes [Fe2(L)2](2+) shows double-stranded helicates from the self-assembly of the ligand and metal ions in a 2 : 2 ratio, where the individual Fe(3+) centre is lying on a C2-axis and the ligand strands wrap around it. In , ligand L adopts "pseudo-C" conformations and forms a double-stranded dinuclear helicate with a small cage in between them. Moreover, in , each of the independent cationic complexes [Fe2L2](2+) is inherently chiral and possesses P for right-hand and M for left-hand helicity and as a consequence is a racemic solid. Detailed magnetic studies of all the complexes reveal that the Fe(3+) centres are magnetically isolated and isotropic in nature. Estimation of the Magnetocaloric Effect (MCE) from magnetization data unveils a moderate MCE at a temperature of 3 K with magnetic entropy changes (-ΔSm) of 22.9, 27.7, 24.1, 26.5 J kg(-1) K(-1) at a magnetic field of 7 T for complexes , respectively. Also, the variation of the -ΔSm values was justified by considering the parameter of magnetization per unit mass. Stability of all the complexes in solution phase was confirmed by ESI-mass spectrometric analysis and liquid phase FT-IR spectroscopy. Further, the interaction of the complexes with Rhodamine B dye was examined by UV-vis and fluorescence spectroscopic study. The observed blue-shift in the fluorescence study and hyperchromicity and hypochromicity with the appearance of two isosbestic points in the UV-vis study ascertain the interactions of the dye with the complexes. A DNA binding study by absorption spectral titration suggests the weak external intercalation of complex within the nucleotide of calf thymus DNA. Computational study supports the isotropic nature of the metal centres and the consequent high spin multiplicity, which assists the complexes to show significant magnetic entropy changes.

Remarkable Ln(III)3Fe(III)2 clusters with magnetocaloric effect and slow magnetic relaxation.

Two remarkable pentanuclear 3d-4f clusters of Ln3Fe2 were synthesized and structurally characterized. Magnetic studies indicate that Gd3Fe2 possesses the significant magnetic entropy change (-ΔSm) of 31.7 J kg(-1) K(-1) and slow magnetic relaxation is observed in the anisotropic Dy-analogue.

Structural, magnetic, and magnetocaloric studies of La0.67Ba0.22Sr0.11Mn1−x Co x O3 manganites

In this work, we report the effect of Co doping on the structural and magnetic properties of La0.67Ba0.22Sr0.11Mn1−x Co x O3 (x = 0.0, 0.1, 0.2, 0.3 and 1) nanopowder samples synthesized by sol–gel method. All the samples crystallized in the orthorhombic system with Pnma space group, and the unit cell volume decreased significantly for x = 0.1, and then increased with increasing Co concentration up to x = 0.3. The stoichiometry and grain morphology of all samples were investigated by energy dispersive X-ray analysis and scanning electron microscopy. The magnetization measurements indicated that all samples exhibited a paramagnetic–ferromagnetic transition, and the transition temperature decreased with the increasing Co concentration. The magnetocaloric measurements on this series gave a significant magnetic entropy change (−ΔS M) near Curie temperature. The undoped sample gave a maximum magnetic entropy change of 2.75 J/kg K in a field change of 5 T, with a relative cooling power (RCP) of 290 J/kg. Intermediate level of substitution of Co for Mn (x = 0.2) was found to result in a decrease of the maximum magnetic entropy change, and to induce significant broadening of the peak, resulting in a relatively high RCP value of 270 J/kg. The observed maximum entropy change was found to be slightly lower than that calculated using the Landau model, and more significant deviations from the calculated values were observed at low temperatures. Our results on the magnetocaloric properties suggest that the parent compound La0.67Ba0.22Sr0.11MnO3 is attractive as a possible material for magnetic refrigeration around ambient temperature, whereas Co-doped samples are more efficient at lower temperatures.

Room temperature magnetocaloric effect and refrigerant capacitance in La0.7Sr0.3MnO3 nanotube arrays

High aspect ratio La0.7Sr0.3MnO3 nanotube (NT) arrays have been synthesized using nitrates based sol-gel precursor by nanoporous anodized aluminum oxide template assisted method. Their phase purity and microstructures were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. Magnetocaloric effect (MCE) of as prepared NTs was investigated by means of field dependence magnetization measurements. Significant magnetic entropy change, −△SM = 1.6 J/kg K, and the refrigerant capacitance, RC = 69 J/kg, were achieved near the transition temperature at 315 K for 5 T. For comparison, a bulk sample was also prepared using the same precursor solution which gives a value of −△SM = 4.2 J/kg K and a RC = 165 J/kg. Though the bulk sample exhibits higher △SM value, the NTs present an expanded temperature dependence of −△SM curves that spread over a broad temperature range and assured to be appropriate for active magnetic refrigeration. The diminutive MCE observed in manganite NTs is explained by the increased influence of surface sites of nanograins which affect the structural phase transition occurred by external magnetic field due to the coupling between magnetism and the lattice in manganese perovskites. Our report paves the way for further investigation in 1D manganite nanostructured materials towards applications in such magnetic refrigeration technology or even on hyperthermia/drug delivery.

Magnetic susceptibility and heat capacity measurements of single crystal TbMnO3

Measurements of the magnetic susceptibility χ and heat capacity C on single crystals of the multiferroic TbMnO3 are presented. A non-magnetic isostructural compound, LaGaO3, was used to isolate the magnetic component of the heat capacity. An anisotropic magnetic susceptibility, deviations from Curie-Weiss behaviour and a significant magnetic entropy above the antiferromagnetic ordering temperature TN1 = 41 K are attributed to a combination of crystal-field effects and short-range order between the Mn moments. Heat capacity in a magnetic field applied along the a axis confirms the saturation of Tb3+ moments in 90 kOe. A hyperfine contribution from the Tb and Mn nuclear moments that may be convolved with a contribution from low-lying Tb crystal-field levels leads to a low-temperature rise in C(T)/T.

The multiple core–shell structure in Cu24Ln6 cluster with magnetocaloric effect and slow magnetization relaxation

Triple core-shell structures in the Cu24Gd6 and Cu24Dy6 clusters were first observed and structurally characterized. The magnetic investigations reveal that Cu24Gd6 possesses the significant magnetic entropy change (-ΔSm) of 21.2 J kg(-1) K(-1) for ΔH = 7 T, and a slow magnetization relaxation is observed in Cu24Dy6.

Modulating the magnetic properties by structural modification in a family of Co-Ln (Ln = Gd, Dy) molecular aggregates.

Two types of heterometallic aggregates of the general formula [(Co(II))3(Co(III))2Ln3(μ3-OH)5(O2C(t)Bu)12(L)2]·2H2O (Ln = Gd(III) (), Dy(III) ()) and [(Co(III))3Ln3(μ3-OH)4(O2C(t)Bu)6(L)3](NO3)2·2CH3CN·2H2O (Ln = Gd(III) (), Dy(III) ()) were successfully isolated in reactions with [Co2(μ-OH2)(O2C(t)Bu)4]·(HO2C(t)Bu)4, Ln(NO3)3 and n-N-butyldiethanolamine (H2L) under ambient conditions by a change in the stoichiometry of the reactants from 1 : 1 : 1 to 1 : 1 : 2 in order. Bond Valence Sum (BVS) calculations and bond lengths indicate the presence of mixed valent Co (Co(II), Co(III)) centres in compounds and and only Co(III) centres in and as required for the charge balances and supported by the magnetic measurements. Isostructural crab shaped complexes and feature distorted cubane cores that edge share to each other whereas the metallic core of or displays hemicubane like arrangement of metal centres and oxygen atoms. Overall structural symmetry was found to enhance on moving from the former to the latter series of complexes. Magnetic studies reveal significant magnetic entropy changes for complexes and (-ΔSm = 21.57 and 19.39 J kg(-1) K(-1)) and single molecular magnetic behaviour for and .

Synthesis, crystal structure and study of magnetocaloric effect and single molecular magnetic behaviour in discrete lanthanide complexes.

The synthesis, crystal structure and magnetic properties of four polynuclear lanthanide coordination complexes having molecular formulae, [Gd3(2)(1)L(H2O)8(Cl)](Cl)4·10H2O (1), [Dy3L(2)(1)(H2O)9](Cl)5·6H2O (2) [Gd6L(2)(2)(HCO2)4(μ3-OH)4(DMF)6(H2O)2](Cl)2·4H2O (3) and [Dy6L(2)(2)(HCO2)4(μ3-OH)4(DMF)6(H2O)2](Cl)2·4H2O (4) (where H2L(1) = bis[(2-pyridyl)methylene]pyridine-2,6-dicarbohydrazide and H4L(2) = bis[2-hydroxy-benzylidene]pyridine-2,6-dicarbohydrazide) are reported. Structural investigation by X-ray crystallography reveals similar structural features for complexes 1 and 2 and they exhibit butterfly like shapes of the molecules. Non-covalent interactions between the molecules create double helical arrangements for both molecules. Complexes 3 and 4 are isostructural and the core structures feature four distorted hemi-cubanes connected by vertex sharing. Magnetic studies unveil significant magnetic entropy changes for complexes 1, 3 and slow relaxation of magnetization for both dysprosium analogues 2 and 4.

Magnetic refrigeration and slow magnetic relaxation in tetranuclear lanthanide cages (Ln = Gd, Dy) with in situ ligand transformation

Three tetranuclear lanthanide cages featuring either hemicubane (1 and 2) or distorted hemicubane (3) like cores are reported. Magnetic studies reveal significant magnetic entropy changes for complex 1 and a slow relaxation of magnetisation for 3.

Enhancement of the magnetocaloric effect in composites: Experimental validation

Recent calculations have shown that the refrigerant capacity (RC) of magnetic refrigerants can be enhanced using multiphase materials or composites, which expand the temperature range over which a significant magnetic entropy change can be obtained. This work is a systematic experimental validation of the improvement of RC (RCI) using layered composites comprised of two Fe88−2yCoyNiyZr7B4Cu1 amorphous alloy constituents, with y=8.25 and y=11 compositions. RCI has a nonmonotonic dependence on the applied magnetic field H and the fraction x of the two constituent phases. In contrast to common assumptions, the composite has a smaller RCI than its constituent phases for small values of H and x, and there are critical values of each for which RCI is maximized. This work demonstrates the outstanding agreement between the experimental results and the continuous curves predicted by numerical calculations, indicating that this approach can be used to design magnetic refrigerant materials with enhanced magnetocaloric response for moderate magnetic fields.

Optimization of the refrigerant capacity in multiphase magnetocaloric materials

The refrigerant capacity (RC) of magnetocaloric materials can be enhanced using multiphase materials or composites, which expand the temperature range over which a significant magnetic entropy change can be obtained. Numerical simulations show that by controlling the parameters of the composite (the fraction of the different phases and their Curie temperatures) improvements of RC of ∼83% are possible. The maximum applied field plays a crucial, nonmonotonic, role in the optimization. As a proof of concept, it is shown that the combination of two Fe88−2xCoxNixZr7B4Cu1 alloys produces an enhancement in RC of ∼37%, making it ∼92% larger than that of Gd5Si2Ge1.9Fe0.1.

Long-range and short-range magnetic order in NaVGe 2O 6

The metaloxide pyroxene compound NaVGe 2O 6 containing isolated edge-sharing VO 6 ( S=1) chains undergoes a transition into a long-range antiferromagnetic state at T N=16 K. The broad maximum in the temperature dependence of magnetic susceptibility at T M=26 K indicates the low-dimensional character of magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Neel temperature.

Renormalisation of the Néel temperature and magnetocaloric effect in Tb 2Agln

The influence of an applied magnetic field on the antiferromagnetic transition temperature of the localised metallic antiferromagnet Tb 2AgIn has been studied. Magnetisation measurements show that as the applied field strength increases the transition temperature is shifted to lower temperatures with a B 2 dependence as expected on the grounds of symmetry. On the basis of measurements made in static fields of up to 5.5 T, it was estimated that a field of 7.7 T would be sufficient to suppress antiferromagnetic order at T=0 K. This prediction has been confirmed using pulsed high field magnetisation and neutron diffraction measurements in an applied field. The magnetic isotherms have a complex field dependence producing an entropy change Δ S which becomes negative above the Néel temperature ∼58 K. The temperature at which the entropy change passes through zero provides a more precise value for the Néel temperature than can be obtained from Arrott plots. Significant magnetic entropy persists above 58 K and specific heat measurements show that the R ln(13) expected for Tb 3+ is only recovered at ∼2.5 T N.

Long-range and short-range magnetic order in new compound NaVGe2O6

New metal oxide pyroxene compound NaVGe2O6 containing isolated edge-sharing VO6 (S=1) chains undergoes transition into a long-range antiferromagnetic state at TN=16 K. The broad maximum in the temperature dependence of magnetic susceptibility at TM=26 K indicates the low-dimensional character of the magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Neel temperature.

Investigation of the magnetic properties of the Gd 1− xEr x alloy system in the x < 0.62 composition range

The magnetic properties of the GdEr system (with erbium concentrations ranging from 0 to 62 at%) were investigated at temperatures between 4.2 and 350 K under external magnetic fields up to 1 T. The results show that, similar to other rare-earth systems, simple ferromagnetism prevails for concentrations with a de Gennes factor G higher than about 11.5 (≈ 30 at % Er) while complex magnetic structures are present for higher Er concentrations ( G ≤ 11.5. The magnetic transition temperature decreases with increasing Er content, showing good agreement with phenomenological expectations. In the paramagnetic region, the Curie constant of the alloy corresponds to the linear combination of the constituent elements' contribution, thus proving that both Gd and Er maintain their individual magnetic moments. From magnetization data below the transition, the change in magnetic entropy for a 1 T field was calculated and this showed distinct differences between ferromagnetic-paramagnetic transitions and antiferromagnetic transitions. The Landau theory of second-order phase transitions can be well applied for evaluating the thermodynamic transition temperature and the specific heat anomaly in the ferromagnetic alloys but it fails for the more complex structures if the applied field is not far higher than the critical field. The magnetic entropy change displays well defined peaks at the transition temperature for both ferromagnetic and antiferromagnetic alloys. There is an indication that the spin-reorientation transition occurring in alloys with higher Er concentrations is also coupled with significant magnetic entropy changes.