Exchange interactions and spin states in a V 15 magnetic molecular nanocluster
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SORDER, AND PHASE TRANSITION IN CONDENSED SYSTEMS
Exchange Interactions and Spin States in a V15 Magnetic Molecular Nanocluster V. V. Kostyuchenkoa and A. I. Popovb a Yaroslavl
Division, Institute of Physics and Technology, Russian Academy of Sciences, Yaroslavl, 150007 Russia b Moscow State Institute of Electronic Engineering, Moscow, 124498 Russia e-mail: [email protected] Received March 4, 2008
Abstract—Magnetization reversal in a V15 magnetic molecular cluster is studied theoretically and the results of calculations are compared with the available experimental data. Analytical and numerical methods (e.g., a modified Lanczos method) are used for calculating the energy spectrum of the cluster and for determining a set of exchange constants, which ensures good quantitative agreement between the results of theoretical calculations and experimental data on the behavior of magnetic susceptibility in the range of weak as well as strong fields. The fine structure of transitions from a low- to a high-spin state is predicted (in the range of ultrahigh fields, each of three transitions is a combination of two close transitions). PACS numbers: 75.50.Xx, 75.25.+z, 75.30.Et, 75.45.+j DOI: 10.1134/S1063776108100063
1. INTRODUCTION Magnetic molecular nanoclusters have become an object of intense experimental and theoretical investigations due to their unique physical properties and/or promising practical applications in high-density storage devices and quantum computers. Macroscopic quantum effects such as macroscopic quantum coherence and macroscopic quantum tunneling (see, for example, [1–3] and the literature cited therein) in such nanoclusters are being studied most actively. It should be noted that macroscopic quantum effects in magnetic molecular nanoclusters with a total integral spin have been studied most actively until recently. This is partly due to the synthesis of magnetic molecular nanoclusters like Mn12 and Fe8, which are convenient for investigations (see, for example, [3] and the literature cited therein). The steps on the hysteresis loop for molecular magnetic clusters with a large total integral spin (e.g., Mn12 and Fe8), which are associated with spin tunneling, can be explained on the basis of the multiplet approximation disregarding transitions between states with different total spins. However, analysis of clusters with a half-integral spin is essential for deeper understanding of the physics of magnetic nanoclusters. In accordance with the Kramers theorem, the properties of systems with a half-integral spin may differ qualitatively from the properties of systems with an integral spin. Polyoxovanadate V15 is an example of such a nanocluster. It is of considerable interest for studying molecular magnetism since it exhibits quite interesting effects like slow relaxation of magnetization at low temperatures and quantum coherence effects [4,
5]. This stimulated the interest of researchers in this molecular magnetic nanocluster. Experimental results on the temperature dependence of the magnetic susceptibili
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