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Superparamagnetic fluctuation of magnetic nanoparticles is studied. We examine the validity of the simplified approach using the difference between zero-field-cooled magnetization and field-cooled one. The results show that it is valuable for estimating the blocking temperature distribution of polydispersive particles even in magnetic fields, if both surface effects and inter-particle interactions are negligible. Index Terms--Magnetic hysteresis, magnetic materials, magnetization reversal.

Magnetization curves were measured for a frozen iron-nitride magnetic fluid with uniform particles. The results are compared with calculated curves for the single domain particles with randomly oriented uniaxial easy axes. It is clarified that the magnetization curve for isolated particles shows appreciable deviation from the superparamagnetic superposition in a wide range of temperature above the blocking temperature. This deviation can be attributed to the anisotropy energy on each particle. Index Terms - Anisotropy energy, Fine particles, Magnetization curves, Superparamagnetism

The phase diagram is studied for an magnetic fluid with uniform iron-nitride particles. The magnetic properties were measured at various temperatures in weak magnetic fields. The results show a temperature-induced transition. Weak magnetic fields stabilize the state observed at the higher temperatures. Index Terms--Magnetic fluid, magnetic property, phase diagram.

To clarify the universal features of spin glasses, we carefully studied slow dynamics in a geometrically frustrated magnet ZnFe2O4 with slight disorders, regarded as an "unconventional" Heisenberg spin glass, using time-resolved neutron diffractometry and magnetometry. The results indicate that "aging" can be attributed not to growth of the short-range order detected by a diffuse scattering but to aging of a hidden aperiodic correlation, as expected from theories for spin glasses. Concerning aging, peculiar behavior was found; the decay of thermoremanent magnetization is extremely accelerated if the sample is heated/cooled briefly midway through the isothermal slow relaxation. Conversely, magnetization surprisingly increases despite the absence of a magnetic field when the temperature returns after the brief heating/cooling. The behavior can be explained as a destabilization of the aged spin configuration due to the thermal perturbations and subsequent spontaneous restoration of the original spin configuration after the destabilization. Whereas such destabilization and restoration do not occur during freezing into numerous metastable states in a fixed energy landscape, these are possible in an energy landscape with a temperature-sensitive funnel-like structure. These features, consistent with the ghost domain scenario of the droplet picture, are the same as for conventional Heisenberg spin glasses such as dilute magnetic alloys and dilute magnetic semiconductors. In other words, they are universal features in Heisenberg spin glasses including unconventional ones. [ABSTRACT FROM AUTHOR]