Many dense magnetic nanoparticle systems exhibit slow dynamics which is qualitatively indistinguishable from that observed in atomic spin glasses and its origin is attributed to dipole interactions among particle moments (or superspins). However, even in dilute nanoparticle systems where the dipole interactions are vanishingly small, slow dynamics is observed and is attributed solely to a broad distribution of relaxation times which in turn comes from that of the anisotropy energy barriers. To clarify characteristic differences between the two types of slow dynamics, we study a simple model of a non-interacting nanoparticle system (a superparamagnet) analytically as well as ferritin (a superparamagnet) and a dense Fe-N nanoparticle system (a superspin glass) experimentally. It is found that superparamagnets in fact show aging (a waiting time dependence) of the thermoremanent-magnetization as well as various memory effects. We also find some dynamical phenomena peculiar only to superspin glasses such as the flatness of the field-cooled magnetization below the critical temperature and memory effects in the zero-field-cooled magnetization. These dynamical phenomena are qualitatively reproduced by the random energy model, and are well interpreted by the so-called droplet theory in the field of the spin-glass study.

Equilibrium susceptibility x eq of frozen iron nitride magnetic fluids is estimated as the convergent value of the relaxation curves for various initial states. In the lower temperature range in which the field cooled susceptibility x FC shows a plateau, x eq of the dense sample is nearly the same as x FC , while x eq of the diluted sample increases with decreasing temperature. These indicate that blocked moments are observed for the isolated particles owing to a finite measurement time and that the magnetic moments of the interacting particles freeze cooperatively as seen in a spin glass. [S0031-9007 (97)04918-1] PACS numbers: 75.50.Lk, 75.50.MmGlassy behavior due to random anisotropy and dipolar interactions has been intensively investigated in ferromagnetic fine particle systems [1][2][3]. However, there is little agreement as to the existence of the spin glasslike phase. One reason is that most properties for the cooperative freezing of spin glass are similar to those for the blocking of isolated particles in appearance. For example, a plateau for the temperature dependence of the field cooled susceptibility has been observed for both systems. Recently, typical spin glass dynamics has been observed in a concentrated system of g-Fe 2 O 3 fine particles by Jonsson et al., where the relaxation depends on the time spent at constant temperatures before applying the magnetic field [4]. However, the existence of this dynamics does not by itself imply that there is a thermodynamic spin glass phase. To distinguish a thermodynamic phase from a nonequilibrium state, the equilibrium properties, in addition to critical phenomena, should be examined. In the spin glass phase, it is known that the equilibrium susceptibility x eq is almost independent of the temperature, and that the nonlinear susceptibility x 2 diverges at the transition. On the other hand, x eq of the blocked magnetic moments is predicted not to be constant and to be superparamagnetic. In this Letter, x eq and x 2 are estimated and the results for isolated particles and interacting particles are compared with the superparamagnetic model and spin glass.The samples are iron nitride e-Fe 3 N magnetic fluids with kerosene as the carrier liquid [5]. Band structure calculation [6] has shown that e-Fe 3 N is a ferromagnetic substance with magnetic moment per Fe of about 1.9m B and that the easy axis is along the c axis of the hexagonal structure. Electron microscopy shows that the particles have isotropic shapes and uniform size, as shown in Fig. 1. There is a single domain within each particle for this size. Therefore, we can take the magnetization of each particle as a rigid dipole moment m [the mean value is ͗m͑T ͒͘]. The small angle x-ray scattering profiles show no particle agglomeration. Measurements were made on samples cooled in a zero field to 150 K below the freezing point for kerosene. These samples, therefore, are randomly oriented single-domain particles embedded in solidified kerosene. Sample d1 is an as-prepared magnetic fluid and the other samples ...

Targeted hyperthermia treatment using magnetic nanoparticles is a promising cancer therapy. However, the mechanisms of heat dissipation in the large alternating magnetic field used during such treatment have not been clarified. In this study, we numerically compared the magnetic loss in rotatable nanoparticles in aqueous media with that of non-rotatable nanoparticles anchored to localised structures. In the former, the relaxation loss in superparamagnetic nanoparticles has a secondary maximum because of slow rotation of the magnetic easy axis of each nanoparticle in the large field in addition to the known primary maximum caused by rapid Néel relaxation. Irradiation of rotatable ferromagnetic nanoparticles with a high-frequency axial field generates structures oriented in a longitudinal or planar direction irrespective of the free energy. Consequently, these dissipative structures significantly affect the conditions for maximum hysteresis loss. These findings shed new light on the design of targeted magnetic hyperthermia treatments.

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The magnetic properties and the crystal structure of MgV2O4 and Mg(V0.85Al0.15)2O4 have been studied. Both compounds are the normal cubic spinels with highly frustrated magnetic lattice. Around T2=65 K, MgV2O4 has magnetic orders accompanied with the cubic-tetragonal transition. Below T2, the susceptibility shows complex behavior. In Mg(V0.85Al0.15)2O4, the spin-glasslike state appears. The V51-Knight shift of MgV2O4 has an anomalous temperature dependence, which is not simply related by that of the susceptibility.