Exchange-Driven Colective Behavior in a 3D Array of Nanoparticles

다국어 초록 (Multilingual Abstract)

A Monte Carlo simulation is performed in a cubic lattice of interacting identical Stoner-Wohlfarth nanoparticles. The model system is a randomly-anisotropic Heisenberg spin system with a small anisotropy-to-exchange ratio D=J = 3:5. The dc susceptibility, Χdc(T), shows a Curie-Weiss-like transition at a temperature TC/J 1.5, followed by alow-temperature glassy behavior manifested by cusps in both the zero-field-cooled and the field-cooled curves. The ac susceptibility, Χac(T,ω), at various frequencies, ω, shows that with decreasing temperature, a non-Arrhenius dispersive peak occurs at Tb(ω), succeeded by another dispersionless peak at Tg/J 1.20 in the in-phase part, Χ'(T,ω), of Χac(T,ω) while the out-of-phase part, Χ''(T,ω), shows only one peak. A dynamic scaling analysis shows that the system exhibits a critical slowing-down at Tg with a quite small exponent zυ 1.65. However, no universal collapse is seen for the fully-scaled data of Χ''(T,ω). These observed behaviors are interpreted under the droplet-like hypothesis that the formation and development of exchange-induced correlated clusters drive ensembles of nanoparticles undergoing a transition from a paramagnetic order to a short-range order (SRO) at TC, followed by a transition at Tg to the magnetic state in which a magnetic glassy order and a magnetic quasi-long-rangeorder (QLRO) coexist. In addition, our simulation shows that the onset of the latter transition, which is peculiarly manifested by the dispersionless peak, occurs only for those ensembles possessing the anisotropy strength in the region 1.0 ≤ D/J ≤ 5.0. When this range is exceeded, this onset totally suppressed. The reason that the QLRO is prohibited in ensembles of strong random anisotropy may account for this phenomenon.

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