Equilibrium susceptibilities of superparamagnets: longitudinal and transverse, quantum and classical

• Published in: Journal of physics. Condensed matter Vol. 21; no. 45; pp. 456006 - 456006 (15)
• Main Authors: García-Palacios, J L, Gong, J B, Luis, F
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 11.11.2009; Institute of Physics
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Diamagnetism, paramagnetism and superparamagnetism; Exact sciences and technology; Magnetic properties and materials; Physics; Kubo formula; Approximate method; Molecular clusters; Quantum statistical mechanics; Superparamagnetism; Magnetic susceptibility; Anisotropy; Temperature effects; Scaling laws; Paramagnetic materials; Modelling; Linear response theory; Classical theory; Quantum theory
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/21/45/456006

The equilibrium susceptibility of uniaxial paramagnets is studied in a unified framework which permits us to connect traditional results of the theory of quantum paramagnets, S = 1/2,1,3/2,..., with molecular magnetic clusters, S∼5,10,20 all the way up (S = 30,50,100,...,) to the theory of classical superparamagnets. This is done using standard tools of quantum statistical mechanics and linear-response theory (the Kubo correlator formalism). Several features of the temperature dependence of the susceptibility curves (crossovers, peaks, deviations from Curie law) are studied and their scalings with S identified and characterized. Both the longitudinal and transverse susceptibilities are discussed, as well as the response of the ensemble with anisotropy axes oriented at random. For the latter case a simple approximate formula is derived too, and its range of validity assessed, which could be used in the modelization of experiments.