Magnetic cluster excitations

Abstract

Magnetic clusters, i.e., assemblies of a finite number (between two or three\nand several hundred) of interacting spin centers which are magnetically\ndecoupled from their environment, can be found in many materials ranging from\ninorganic compounds, magnetic molecules, artificial metal structures formed on\nsurfaces to metalloproteins. The magnetic excitation spectra in them are\ndetermined by the nature of the spin centers, the nature of the magnetic\ninteractions, and the particular arrangement of the mutual interaction paths\nbetween the spin centers. Small clusters of up to four magnetic ions are ideal\nmodel systems to examine the fundamental magnetic interactions which are\nusually dominated by Heisenberg exchange, but often complemented by anisotropic\nand/or higher-order interactions. In large magnetic clusters which may\npotentially deal with a dozen or more spin centers, the possibility of novel\nmany-body quantum states and quantum phenomena are in focus. In this review the\nnecessary theoretical concepts and experimental techniques to study the\nmagnetic cluster excitations and the resulting characteristic magnetic\nproperties are introduced, followed by examples of small clusters demonstrating\nthe enormous amount of detailed physical information which can be retrieved.\nThe current understanding of the excitations and their physical interpretation\nin the molecular nanomagnets which represent large magnetic clusters is then\npresented, with an own section devoted to the subclass of the single-molecule\nmagnets which are distinguished by displaying quantum tunneling of the\nmagnetization. Finally, some quantum many-body states are summarized which\nevolve in magnetic insulators characterized by built-in or field-induced\nmagnetic clusters. The review concludes addressing future perspectives in the\nfield of magnetic cluster excitations.\n