Light-induced changes in the coercivity ΔHc are reported for cobalt ferrite nanoparticles. These changes arise from charge transfer initiated by optical absorption near 2 eV. An insulator-to-metal transition at 170 K is an upper limit for significant ΔHc. The larger ΔHc for smaller (17 nm) particles was correlated with a distortion about the Co2+ ions at B sites, which increases the absorption probability.

The metastable Ni 3 C phase has been produced by mechanically alloying Ni and C. Ni 3 C particles of diameter 10 nm are produced after 90 h of mechanical alloying with no evidence of crystalline Ni in x ray or electron diffraction. Linear muffin-tin orbital band-structure calculations show that Ni 3 C is not expected to be ferromagnetic due to strong Ni-C hybridization in the ordered alloy; however, the introduction of even small amounts of disorder produces locally Ni-rich regions that can sustain magnetism. Mechanically alloyed Ni 3 C is ferromagnetic, with a room-temperature coercivity of 70 Oe and a magnetization of 0.8 emu/g at 5.5 T, although the hysteresis loop is not saturated. The theoretical prediction that interacting locally nickel-rich regions may be responsible for ferromagnetic behavior is supported by the observation of magnetically glassy behavior at low magnetic fields.

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Spin-echo nuclear magnetic resonance (NMR) experiments have been carried out at 4.2 K and room temperature on carbon-coated nanoscale (average diameter ≈20 nm) face-centered-cubic (fcc) Co particles prepared by the Kratschmer carbon arc process for 0⩽H⩽9.0 kOe. Information concerning the magnetic structure and paramagnetic relaxation behavior of the nanoscale particle system has been obtained.