Elias Palacios

Cryogenic Magnetocaloric Effect in a Ferromagnetic Molecular Dimer

Over the last few years, great interest has emerged in the synthesis and magnetothermal studies of molecular clusters based on paramagnetic ions, often referred to as molecular nanomagnets, in view of their potential application as lowtemperature magnetic refrigerants. What makes them promising is that their cryogenic magnetocaloric effect (MCE) can be considerably larger than that of any other magnetic refrigerant, for example, lanthanide alloys and magnetic nanoparticles. The MCE is the change of magnetic entropy (DSm) and related adiabatic temperature (DTad) in response to the change of applied magnetic field, and it can be exploited for cooling applications via a field-removal process called adiabatic demagnetization. Although the MCE is intrinsic to any magnetic material, in only a few cases are the changes sufficiently large to make them suitable for applications. The ideal molecular refrigerant comprises the following key characteristics: 1) a large spin ground state S, since the magnetic entropy amounts to R ln(2S+1); 2) a negligible magnetic anisotropy, which permits easy polarization of the net molecular spins in magnetic fields of weak or moderate strength; 3) the presence of low-lying excited spin states, which enhances the field dependence of the MCE owing to the increased number of populated spin states; 4) dominant ferromagnetic exchange, favoring a large S and hence a large field dependence of the MCE; 5) a relatively low molecular mass (or a large metal/ligand mass ratio), since the nonmagnetic ligands contribute passively to the MCE. Although this last point is crucial for obtaining an enhanced effect, it has beenmostly ignored to date. Molecular cluster compounds tend to have a very low magnetic density because of the large complex structural frameworks required to encase the multinuclear magnetic core. Herein we propose a drastically different approach by focusing on the simple and well-known ferromagnetic molecular dimer gadolinium acetate tetrahydrate, [{Gd(OAc)3(H2O)2}2]·4H2O (1). [4a,b] The structure of 1 (Figure 1) com-

A dense metal-organic framework for enhanced magnetic refrigeration

X iv :1 21 2. 28 77 v1 [ co nd -m at .m tr lsc i] 1 2 D ec 2 01 2 Magnetic cryocooling with Gd centers in a light and compact framework G. Lorusso, J. W. Sharples, E. Palacios, O. Roubeau, E. K. Brechin, R. Sessoli, A. Rossin, F. Tuna, E. J. L. McInnes, D. Collison, and M. Evangelisti a) Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC − Universidad de Zaragoza, Departamento de F́ısica de la Materia Condensada, 50009 Zaragoza, Spain School of Chemistry and Photon Science Institute, The University of Manchester, M13-9PL Manchester, United Kingdom School of Chemistry, The University of Edinburgh, EH9-3JJ Edinburgh, United Kingdom Department of Chemistry and INSTM, Università degli Studi di Firenze, 50019 Sesto Fiorentino, Italy Istituto di Chimica dei Composti Organometallici (ICCOM), CNR, 50019 Sesto Fiorentino, ItalyThe three-dimensional metal-organic framework Gd(HCOO)3 is characterized by a relatively compact crystal lattice of weakly interacting Gd(3+) spin centers interconnected via lightweight formate ligands, overall providing a remarkably large magnetic:non-magnetic elemental weight ratio. The resulting magnetocaloric effect per unit volume is decidedly superior in Gd(HCOO)3 than in the best known magnetic refrigerant materials for liquid-helium temperatures and low-moderate applied fields.

Entropy determinations and magnetocaloric parameters in systems with first-order transitions: Study of MnAs

We present a study of the giant magnetocaloric effect in MnAs produced by a magnetostructural first-order phase transition. Results deduced from magnetization, M, and heat capacity, Cp,B(T), are compared and discussed. Some spurious effects are explained, and especially a spike in the isothermal entropy change, ΔST, occurring at TC when obtained via the Maxwell relation (∂S/∂B)T=(∂M/∂T)B. Alternative determination methods are given to circumvent this problem. The spike is explained as an artifact due to the incorrect application of the Maxwell relation to path dependent thermodynamic functions that are not state functions. The added wrong contribution to ΔST has been calculated using calorimetric data, giving a good agreement with the result from the magnetization measurements.

Spontaneous Magnetization in Ni-Al and Ni-Fe Layered Double Hydroxides

Layered double hydroxides containing paramagnetic Ni (II) and diamagnetic/paramagnetic Al (III)/Fe (III) ions have been prepared and characterized. Ni 2Al(OH) 6(NO 3). nH 2O ( 1), Ni 2Fe(OH) 6(NO 3). nH 2O ( 2), Ni 2Fe(OH) 6(C 6H 8O 4) 0.5. nH 2O ( 3), and Ni 2Fe(OH) 6(C 10H 16O 4) 0.5. nH 2O ( 4) were prepared by coprecipitation at controlled pH as polycrystalline materials with the typical brucite-like structure, with alternating layers of hydroxide and the corresponding anions, which determine the interlayer separation. Magnetic studies show the appearance of spontaneous magnetization between 2 and 15 K for these compounds. Interestingly, the onset temperature for spontaneous magnetization follows a direct relationship with interlayer separation, since this is the only magnetic difference between compounds 2, 3, and 4. Magnetic and calorimetric data indicate that long-range magnetic ordering is not occurring in any of these materials, but rather a freezing of the magnetic system in 3D due to the magnetic disorder and competing intra- and interlayer interactions. Thus, these hydrotalcite-like magnetic materials can be regarded as spin glasses.

Quantum signatures of a molecular nanomagnet in direct magnetocaloric measurements.

Geometric spin frustration in low-dimensional materials, such as the two-dimensional kagome or triangular antiferromagnetic nets, can significantly enhance the change of the magnetic entropy and adiabatic temperature following a change in the applied magnetic field, that is, the magnetocaloric effect. In principle, an equivalent outcome should also be observable in certain high-symmetry zero-dimensional, that is, molecular, structures with frustrated topologies. Here we report experimental realization of this in a heptametallic gadolinium molecule. Adiabatic demagnetization experiments reach ~200 mK, the first sub-Kelvin cooling with any molecular nanomagnet, and reveal isentropes (the constant entropy paths followed in the temperature-field plane) with a rich structure. The latter is shown to be a direct manifestation of the trigonal antiferromagnetic net structure, allowing study of frustration-enhanced magnetocaloric effects in a finite system.

Two-Step Thermal Spin Transition and LIESST Relaxation of the Polymeric Spin-Crossover Compounds Fe(X-py)2[Ag(CN)2]2 (X=H, 3-methyl, 4-methyl, 3,4-dimethyl, 3-Cl)†

In the series of polymeric spin-crossover compounds Fe(X-py)(2)[Ag(CN)(2))](2) (py=pyridine, X=H, 3-Cl, 3-methyl, 4-methyl, 3,4-dimethyl), magnetic and calorimetric measurements have revealed that the conversion from the high-spin (HS) to the low-spin (LS) state occurs by two-step transitions for three out of five members of the family (X=H, 4-methyl, and X=3,4-dimethyl). The two other compounds (X=3-Cl and 3-methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin-crossover behaviour of the compound undergoing two-step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X=3-methyl undergo light-induced excited spin state trapping (LIESST) at low temperatures. Isothermal HS-to-LS relaxation curves at different temperatures support the low-cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX=H and X=3,4-dimethyl the behaviour is well described by a double exponential function in accordance with the two-step thermal spin transition. The thermodynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the experimental data.

A single-crystal neutron diffraction study of NH4MnF3

Neutron diffraction measurements have been performed on a single crystal of the ammonium perovskite NH4MnF3 at 220 K, 80 K and 20 K. This perovskite is known to undergo a transition from cubic (Pm3m, Z=1) to orthorhombic (Pnma, Z=4) structure at Tc=182 K and an antiferromagnetic transition at TN=75 K. In the cubic phase, the rotational structure factor method has been used to obtain the parameters of the interaction potential of the NH4+ molecule with its crystal environment. In the orthorhombic phase, the refinement shows that a single orientation of the NH4+ molecule in the crystal is populated. This gives rise to a fully ordered sublattice of NH4+ groups related to the distortion of the perovskite and the appearance of two types of hydrogen bond. In the magnetic phase, an antiferromagnetic Gy structure is deduced with a magnetic moment of 3.6(2) mu B.

Direct Measurement of the Magnetocaloric Effect in Gd5Si2Ge1.9Ga0.1

Doping with some metals (Fe, Cu, Ga, Mn, Al) reduces or removes the strong thermal hysteresis in the giant magnetocaloric alloys Gd5(SixGe1−x)4. We present heat capacity and direct measurements of the isothermal entropy change, ΔST, and adiabatic temperature increment, ΔTS, of Gd5Si2Ge1.9Ga0.1. TC = 293.6 ± 0.2 K is quite higher than in the non-substituted alloys and similar to the values in the Si rich compounds (i.e. x > 0.5). The results indicate that even this small addition of Ga makes the transition of second-order type, as a usual magnetic transition in the orthorhombic phase. The magnetocaloric parameters are lower than in the non-substituted compound and comparable to those for pure Gd.

Magnetic moment ordering of Nd3+ and Fe3+ in NdFe03 at low temperature

Abstract Magnetic ordering of Nd 3+ and Fe 3+ ions in orthoferrite, NdFe0 3 , was investigated by neutron diffraction. Neutron diffraction patterns of polycrystalline NdFe0 3 at 70 mK, 1. I K and 4 K were obtained at neutron wavelength λ = 2.38 A. C-type ordering for Nd 3+ magnetic moments was confirmed. A deviation from the pure G-type configuration for Fe 3+ magnetic moments is observed at these temperatures.

Neutron diffraction and magnetization studies of ErFe/sub 10.5/Mo/sub 1.5/D/sub x/ (x=0, 0.9)

A powder neutron diffraction and magnetic measurements have been performed on the ErFe/sub 10.5/Mo/sub 1.5/D/sub x/ compounds (x=0, 0.9). The results allow to locate the deuterium atoms in the structure and to determine the magnetic structures of these compounds. >