No abstract

Over the last few years, great interest has emerged in the synthesis and magnetothermal studies of polymetallic molecular clusters based on paramagnetic ions, often referred to as molecular nanomagnets, in view of their potential application as lowtemperature magnetic refrigerants. [1,2] What makes them promising is that their cryogenic magnetocaloric effect (MCE) can be considerably larger than that of any other magnetic refrigerant, e.g. lanthanide alloys and magnetic nanoparticles.[3] The MCE is the change of magnetic entropy (∆S m ) and related adiabatic temperature (∆T ad ) following 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] (i) a large spin ground state S, since the magnetic entropy amounts to Rln(2S+1); (ii) a negligible magnetic anisotropy, which permits easy polarization of the net molecular spins in magnetic fields of weak or moderate strength; (iii) the presence of low-lying excited spin states, which enhances the field dependence of the MCE due to the increased number of populated spin states; (iv) dominant ferromagnetic exchange, [3(c)] favouring a large S and hence a large field dependence of the MCE; (v) a relatively low molecular mass (or a large metal:ligand mass ratio) since the non-magnetic ligands contribute passively to the MCE. Although this last point is crucial for obtaining an enhanced effect, it has been mostly 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 multi-metallic core.In this communication we propose a drastically different approach by focusing on the simple and well-known ferromagnetic molecular dimer gadolinium acetate tetrahydrate, [4] [{Gd(OAc) 3 (H 2 O) 2 } 2 ]•4H 2 O (1). The structure of 1 is depicted in Figure 1 and comprises a dimer of Gd 3+ ions bridged through two of the six carboxylate groups which bond in a η 2 :η 1 :µ 2 -fashion. The remaining acetates are chelating with the nine-coordinate [capped square anti-prismatic] geometry of the metal centres being completed by the presence of two terminally bound H 2 O molecules. These partake in intra-molecular H-bonding to the neighbouring chelating acetate ligands, and are responsible for both the direct inter-molecular H-bonds in the a-b plane and the inter-plane Hbonds mediated by the lattice H 2 O molecules ( Fig. S1 and Table S1). Our theoretical and experimental investigations (see Supporting Information for details) of the magnetothermal properties of 1 down to millikelvin temperatures reveal a truly enormous MCE. In addition to magnetization and heat capacity experiments, which we employ to indirectly estimate the MCE, we make use of a homemade experimental set-up th...

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.

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.

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.

The temperature variation of the ͑100͒ and ͑010͒ neutron diffraction peak intensities, related only to the Nd magnetic moments, have been measured on a NdFeO 3 single crystal, at temperatures down to 70 mK. The ͑100͒ peak becomes noticeable below 25 K while the ͑010͒ peak only gives an appreciable contribution below 1 K. Above T N2 Ϸ1 K the ͑100͒ peak intensity is accounted for by the electronic magnetic moments polarized by the Nd-Fe exchange field. Near T N2 a change of slope is observed in the temperature dependence of the ͑100͒ reflection intensity, demonstrating the crossover from the above polarization of Nd under the Nd-Fe exchange to proper long-range ordering due to Nd-Nd interaction. Below ϳ0.4 K another mechanism, polarization of Nd nuclear moments by hyperfine field, contributes to the intensity of the ͑100͒ and ͑010͒ peaks. A simple mean-field model explains consistently the observed temperature dependence of the diffraction intensities as well as earlier specific-heat data. The main feature of this model is allowance for Van Vleck susceptibility, which appears to play an important role in the overall polarization of Nd. The values of the hyperfine field at the Nd nuclei H hf ϭ1.0Ϯ0.15 MOe and of the Nd electronic magnetic moment Nd ϭ0.9 B are deduced, the ratio H hf / Nd being the same as in other Nd compounds.

The magnetic ordering structure of GdPO 4 is determined at T = 60 mK by the diffraction of hot neutrons with wavelength λ = 0.4696Å. It corresponds to a noncollinear antiferromagnetic arrangement of the Gd moments with propagation vector k = (1/2,0,1/2). This arrangement is found to minimize the dipole-dipole interaction and the crystal-field anisotropy energy, the magnetic superexchange being much smaller. The intensity of the magnetic reflections decreases with increasing temperature and vanishes at T ≈ 0.8 K, in agreement with the magnetic ordering temperature T N = 0.77 K, as reported in previous works based on heat capacity and magnetic susceptibility measurements. The magnetocaloric parameters have been determined from heat capacity data at constant applied fields up to 7 T, as well as from isothermal magnetization data. The magnetocaloric effect, for a field change B = 0 − 7 T, reaches − S T = 375.8 mJ/cm 3 K −1 at T = 2.1 K, largely exceeding the maximum values reported to date for Gd-based magnetic refrigerants.

Magnetocaloric properties of a Ni 50 Mn 36 Co 1 Sn 13 ferromagnetic shape memory alloy have been studied experimentally in the vicinity of a first-order magnetostructural phase-transition low-temperature paramagnetic martensite↔ high-temperature ferromagnetic austenite. The magnetic entropy change ⌬S m calculated from the magnetization M͑T͒ data measured upon cooling is higher than that estimated from M͑T͒ measured upon heating. Contrary to ⌬S m , the adiabatic temperature change ⌬T ad measured upon cooling is significantly smaller than that measured upon heating. The apparent discrepancy between ⌬S m and ⌬T ad ͑larger ⌬S m , smaller ⌬T ad upon cooling, and smaller ⌬S m , larger ⌬T ad upon heating͒ is caused by the hysteretical behavior of this magnetostructural transition, a feature common for all the alloys in the family of Ni 50 Mn 25+x Z 25−x ͑Z =In,Sn,Sb͒ ferromagnetic shape memory Heusler compounds. The hysteresis causes the magnetocaloric parameters to depend strongly on the temperature and field history of the experimental processes.