Magnetic Cluster Excitations

Abstract

Magnetic clusters, i.e., assemblies of a finite number (between two or three and several hundred) of interacting spin centers which are magnetically decoupled from their environment, can be found in many materials ranging from inorganic compounds, magnetic molecules, artificial metal structures formed on surfaces to metalloproteins. The magnetic excitation spectra in them are determined by the nature of the spin centers, the nature of the magnetic interactions, and the particular arrangement of the mutual interaction paths between the spin centers. Small clusters of up to four magnetic ions are ideal model systems to examine the fundamental magnetic interactions which are usually dominated by Heisenberg exchange, but often complemented by anisotropic and/or higher-order interactions. In large magnetic clusters which may potentially deal with a dozen or more spin centers, the possibility of novel many-body quantum states and quantum phenomena are in focus. In this review the necessary theoretical concepts and experimental techniques to study the magnetic cluster excitations and the resulting characteristic magnetic properties are introduced, followed by examples of small clusters demonstrating the enormous amount of detailed physical information which can be retrieved. The current understanding of the excitations and their physical interpretation in the molecular nanomagnets which represent large magnetic clusters is then presented, with an own section devoted to the subclass of the single-molecule magnets which are distinguished by displaying quantum tunneling of the magnetization. Finally, some quantum many-body states are summarized which evolve in magnetic insulators characterized by built-in or field-induced magnetic clusters. The review concludes addressing future perspectives in the field of magnetic cluster excitations.

59 pages, 64 figures, to appear in Rev. Mod. Phys

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• Quantum Magnetism in Minerals
• Excitations in the ordered and paramagnetic states of honeycomb magnet Na2Co2TeO6
• Giant magnetocaloric effect, magnetization plateaux and jumps of the regular Ising polyhedra
• The even-odd effect in short antiferromagnetic Heisenberg chains
• Magnetic Exchange Interactions in the Molecular Nanomagnet Mn${}_{12}$
• Spin-Orbit Excitons in CoO
• Disentangling orbital and spin exchange interactions for Co${}^{2+}$ on a rocksalt lattice
• Local probe of fractional edge states of S=1 Heisenberg spin chains
• Combined use of translational and spin-rotational invariance for spin systems
• Disorder-induced spin-liquid-like behavior in kagome-lattice compounds
• Magnetization process and magnetocaloric effect in geometrically frustrated Ising antiferromagnet and spin ice models on a `Star of David' nanocluster
• Neutron Scattering Studies of the Breathing Pyrochlore Antiferromagnet LiGaCr${}_4$O${}_8$
• Probing molecular spin clusters by local measurements
• Influence of intermolecular interactions on magnetic observables
• Magnetization process and magnetocaloric effect of the spin-1/2 XXZ Heisenberg cuboctahedron
• Magnetocaloric properties of frustrated tetrahedra-based spin nanoclusters
• Unravelling the Spin Dynamics of Molecular Nanomagnets with Four-Dimensional Inelastic Neutron Scattering
• Magnetic Fluctuations and the Spin-Orbit Interaction in Mott Insulating CoO
• Magnetocaloric and electrocaloric properties of the Hubbard pair cluster
• Toroidal magnetic molecules stripped to their basics
• Orphan spins and bound in-gap states in the $S=\frac{5}{2}$ antiferromagnet CaFe${}_2$O${}_4$
• Magnetic spectra in the tridiminished-icosahedron \{Fe${}_9$\} nano-cluster by inelastic neutron scattering
• Phase diagram of the alternating-spin Heisenberg chain with extra isotropic three-body exchange interactions
• On the Extraction of Exchange Parameters in Transition-Metal Perovskites
• Control of entanglement transitions in quantum spin clusters
• Conventional and inverse magnetocaloric and electrocaloric effects of a mixed spin-(1/2, 1) Heisenberg dimer
• Magnetic and thermodynamic properties of the octanuclear nickel phosphonate-based cage
• Electric field tuning of magnetic states in single magnetic molecules
• Generalization of polarized spin excitations for asymmetric dimeric systems
• Experimental observation of magnetic dimers in diluted Yb:YAlO${}_3$
• Evolution of the thermodynamic properties and inelastic neutron scattering intensities for spin-1/2 antiferromagnetic quantum rings
• Spin-Coupling Topology in the Copper Hexamer Compounds A${}_2$Cu${}_3$O(SO${}_4$)${}_3$ (A=Na, K)
• Description of molecular nanomagnets by the multi-orbital Hubbard model with correlated hopping
• Effect of Sr doping on the magnetic exchange interactions in manganites
• Manipulation of spin cluster qubits by electric-field induced modulation of exchange coupling, $g$-factor, and axial anisotropy
• On the size of linear superpositions in molecular nanomagnets
• Geometrical Frustration and Cluster Spin Glass with Random Graphs
• Temperature Dependence of the Effective Interdimeric Exchange Interaction in a Weakly Coupled Antiferromagnetic Dimeric Copper Compound
• Non-Collinearity in Small Magnetic Cobalt-Benzene Molecules
• Alternating-spin S=3/2 and sigma=1/2 Heisenberg chain with three-body exchange interactions
• Antiferromagnetic molecular nanomagnets with odd-numbered coupled spins
• Magnetic Interactions between Nanoscale Domains in Correlated Liquids
• Symmetry-Induced Universal Momentum-Transfer Dependencies for Inelastic Neutron Scattering on Anisotropic Spin Clusters
• Magnetic excitations from an S=1/2 diamond-shaped tetramer compound Cu2PO4OH
• Partitioned Density Matrices and Entanglement Correlators
• Localized Singlets and Ferromagnetic Fluctuations in the Dilute Magnetic Topological Insulator Sn${}_{0.95}$Mn${}_{0.05}$Te
• Magnetization distribution in a spin ladder-shaped quantum nanomagnet
• The Linear and Non-linear Magnetic Response of a Tri-Uranium Single Molecule Magnet
• Propagation of defects in doped magnetic materials of different dimensionality
• Defect propagation in one-, two-, and three-dimensional compounds doped by magnetic atoms
• Magnetic excitations in the trimeric compounds A${}_3$Cu${}_3$(PO${}_4$)${}_4$ (A = Ca, Sr, Pb)
• Noncoplanar and chiral spin states on the way towards Néel ordering in fullerene Heisenberg models
• Ground-state magnetic properties of spin ladder-shaped quantum nanomagnet: Exact diagonalization study
• Thermal DMRG for highly frustrated quantum spin chains: a user perspective
• Heisenberg spin chains with additional isotropic three-site exchange interactions
• Effect of Single-ion Anisotropy on Magnetocaloric Properties of Frustrated Spin-$s$ Ising Nanoclusters
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• A systematic approach to determine the spectral characteristics of molecular magnets
• Quantum continuum fluctuations in glassy perovskite Ca(Co${}_{0.15}$Ru${}_{0.85}$)O${}_3$