Anisotropic magnetocaloric effect in nanostructured magnetic clusters

Ferromagnetic Archimedean polyhedra {Fe24M18} (M = Fe, Ni, and Mn) with tunable electron configurations

Three symmetric nanocages {Fe24M18} that mimic the Archimedean polyhedra, namely pseudo-rhombicuboctahedron, were synthesized. Their electron configurations depend highly on the changes of metal ions and the deprotonation of auxiliary ligands.

A triple-triangle cluster derived from a simple tridentate ligand

The heptanuclear ferric cluster, [FeIII7(O)6(HL)5(H2L)(H2O)8](BF4)4 (H2L = 2,6-bis(1,5-diphenyl-1H-pyrazol-3-yl)pyridine), was synthesized by the reaction of the planar tridentate ligand H2L with iron(ii) tetrafluoroborate hexahydrate. The cluster has a corner-shared triple-triangle core structure connected by three μ-O atoms, and it has a spin-frustrated ferrimagnetic spin ground state of S = 19/2.

Interplay of Spin and Spatial Anisotropy in Low-Dimensional Quantum Magnets with Spin 1/2

Quantum Heisenberg chain and square lattices are important paradigms of a low-dimensional magnetism. Their ground states are determined by the strength of quantum fluctuations. Correspondingly, the ground state of a rectangular lattice interpolates between the spin liquid and the ordered collinear Néel state with the partially reduced order parameter. The diversity of additional exchange interactions offers variety of quantum models derived from the aforementioned paradigms. Besides the spatial anisotropy of the exchange coupling, controlling the lattice dimensionality and ground-state properties, the spin anisotropy (intrinsic or induced by the magnetic field) represents another important effect disturbing a rotational symmetry of the spin system. The S = 1/2 easy-axis and easy-plane XXZ models on the square lattice even for extremely weak spin anisotropies undergo Heisenberg-Ising and Heisenberg-XY crossovers, respectively, acting as precursors to the onset of the finite-temperature phase transitions within the two-dimensional Ising universality class (for the easy axis anisotropy) and a topological Berezinskii–Kosterlitz–Thouless phase transition (for the easy-plane anisotropy). Experimental realizations of the S = 1/2 two-dimensional XXZ models in bulk quantum magnets appeared only recently. Partial solutions of the problems associated with their experimental identifications are discussed and some possibilities of future investigations in quantum magnets on the square and rectangular lattice are outlined.

Coordination-Cluster-Based Molecular Magnetic Refrigerants

Coordination polymers serving as molecular magnetic refrigerants have been attracting great interest. In particular, coordination cluster compounds that demonstrate their apparent advantages on cryogenic magnetic refrigerants have attracted more attention in the last five years. Herein, we mainly focus on depicting aspects of syntheses, structures, and magnetothermal properties of coordination clusters that serve as magnetic refrigerants on account of the magnetocaloric effect. The documented molecular magnetic refrigerants are classified into two primary categories according to the types of metal centers, namely, homo- and heterometallic clusters. Every section is further divided into several subgroups based on the metal nuclearity and their dimensionalities, including discrete molecular clusters and those with extended structures constructed from molecular clusters. The objective is to present a rough overview of recent progress in coordination-cluster-based molecular magnetic refrigerants and provide a tutorial for researchers who are interested in the field.

The effect of magnetic coupling on magneto-caloric behaviour in two 3D Gd(iii)–glycolate coordination polymers

Two 3D Gd(iii)-based coordination polymers, [Gd(glc)(Hglc)(H₂O)]_n·nH₂O (1) and [Gd(Hglc)₃]_n (2) are reported. The MCE measurements indicate the weaker magnetic coupling in 2 produce a higher MCE than 1, especially at moderate fields (−ΔS_m,max = 41.1 J kg⁻¹ K⁻¹ and 24.8 J kg⁻¹ K⁻¹ for 1 and 2 respectively at ΔH = 3 T).

Magnetocaloric effect and thermal conductivity of Gd(OH)3 and Gd2O(OH)4(H2O)2

The magnetocaloric effect and thermal conductivity of two gadolinium hydroxides, Gd(OH)₃ (1) and Gd₂O(OH)₄(H₂O)₂ (2), were investigated, respectively, revealing that 1 is the most promising candidate for an ultra-low temperature magnetic refrigeration material.

Remarkable LnIII3FeIII2 clusters with magnetocaloric effect and slow magnetic relaxation

Two 3d–4f clusters were reported: Gd₃Fe₂ possesses significant magnetic entropy change and slow magnetic relaxation is observed in Dy₃Fe₂.

Recipes for enhanced molecular cooling

Molecular nanomagnets are considered valid candidates for magnetic refrigeration at low temperatures. Designing these materials for enhanced cooling requires the control and optimization of the quantum properties at the molecular level, in particular: spin ground state, magnetic anisotropy, and presence of low-lying excited spin states. Herein, we present the theoretical framework together with a critical review of recent results, and perspectives for future developments.

The origin of the non-monotonic field dependence of the blocking temperature in magnetic nanoparticles

The dependence of the peak temperature (T(P)) of the zero-field-cooled (ZFC) magnetization curves on the field in a magnetic nanoparticle system was studied using a diluted magnetic fluid composed of FePt nanoparticles. We found that the peak temperature increases with increasing applied field below 3 kOe; it then decreases when the applied field is increased further. We attribute the non-monotonic field dependence of the peak temperature to the anisotropic energy barrier distribution of the particles and to the slow decrease of high-field magnetization above the blocking temperature. Numerical simulations, based on magnetic dynamics, agree well with our experimental results.