Magnetite was detected in the particulate matter collected from diesel engine exhaust using a total exhaust dilution tunnel. This discovery is very important in determining the health effects of exposure to magnetite or its interaction with static magnetic fields or low-frequency electromagnetic fields. Magnetite is the best absorber of microwave radiation of any biological material in the 0.5–10 GHz frequency range through the process of electromagnetic resonance. This includes the frequencies that are normally used in the cellular telephone industry.

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From detailed magnetization data taken under various field-cooling conditions, the magnetic phase diagram of temperature versus composition is determined for disordered Ni-Mn of Mn concentration (x) near 25 at. %. With increasing x, the ferromagnetic Curie-point (T,) line descends and meets the ascending line for the spin-glass reentrance temperature (Tfg) at a multicritical point (MCP) located at x =23.9, T =102 K, from which the spin-glass freezing-temperature (Tg) line emerges and reaches a maximum at higher x. The reentrant spin-glass (SG) ordering is accompanied by a net ferromagnetic (FM) moment, thus describing a mixed ferro-spin-glass (FSG) state, which is separated from the normal SG state by a boundary line that extends essentially vertically down in temperature from the MCP. Moreover, it is shown that the SG ordering of the FSG state probably persists into the FM regime above Tfg, where Tfg (like Tg) remains defined operationally by the appearance of irreversible and time-dependent magnetic effects.

With decreasing Mn concentration near 25 at. %, disordered Ni-Mn alloys are seen to evolve from a characteristic spin-glass behavior, with the magnetic polarizability rapidly increasing, to a magnetic behavior that signifies the onset of ferromagnetic order and the appearance of a spin-glass-like phase at lower temperatures. Moreover, the latter phase is ‘‘mixed’’ in that its equilibrium state (in the fields applied during cooling) manifests a spontaneous ferromagnetic moment, while its magnetization reversals with field exhibit the unidirectional anisotropy of a spin glass. The unusual magnetic properties of the mixed phase in the zero-field-cooled state can be ascribed to a domain structure in which the domains have unidirectional anisotropies of random orientation.

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