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Inelastic neutron measurements of the high temperature lattice excitations in NaI show that in thermal equilibrium at 555 K an intrinsic mode, localized in three dimensions, occurs at a single frequency near the center of the spectral phonon gap, polarized along [111]. At higher temperatures the intrinsic localized mode gains intensity. Higher energy inelastic neutron and x-ray scattering measurements on a room temperature NaI crystal indicate that the creation energy of the ground state of the intrinsic localized mode is 299 meV.

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Specific-heat and magnetic-susceptibility measurements are reported for the polycrystalline spinel compounds GeNi 2 O 4 and GeCo 2 O 4 in magnetic fields up to 14 T and 0.5 K Յ T Յ 400 K. Both compounds have first-order antiferromagnetic transitions. There are two sharp closely spaced magnetic-ordering anomalies for GeNi 2 O 4 at Néel temperatures T N1 ͑0͒ = 12.080 K and T N2 ͑0͒ = 11.433 K in zero magnetic field. There is also a broad anomaly in the specific heat centered at ϳ5 K, which is present for all fields. Spin waves with an average gap of 10.9 K are associated with this anomaly, which is confirmed by neutron-scattering measurements. An unusual feature of the antiferromagnetism for GeNi 2 O 4 is the simultaneous presence of both gapped and ungapped spin waves in the Néel state, inferred from the specific-heat data. GeCo 2 O 4 has a single anomaly at T N ͑0͒ = 20.617 K in zero magnetic field. Spin waves with an average gap of 38.7 K are derived from fitting the low-temperature specific heat and are also observed by neutron scattering. For both compounds ϳ50% of the derived magnetic entropy is below the ordering temperatures, and the total magnetic entropies are only ϳ60% of that predicted for the Ni 2+ and Co 2+ single-ion ground-state configurations. The missing entropy is not linked to magnetic disorder in the ground state or hidden ordering below 0.5 K. It is postulated that the missing entropy is accounted for by the presence of substantial magnetic correlations well above the Néel temperatures. Fitting the GeNi 2 O 4 susceptibilities to the Curie-Weiss law yields parameters that are consistent with those found for Ni 2+ ions in a crystal-electric-field environment including octahedral and trigonal components. The application of the Curie-Weiss law to the GeCo 2 O 4 susceptibilities is not valid because of low-lying crystal-electric-field states.

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We have measured the spin-wave spectrum of iron by neutron inelastic scattering using the Low-Resolution Medium-Energy Chopper Spectrometer at the Intense Pulsed Neutron Source (IPNS) of the Argonne National Laboratory. Interest focuses on the magnetic excitations at high energy transfers where the spin-wave dispersion relation has not previously been determined. In measurements performed at 10K using a 23-gram single crystal of pure iron, we observed magnetic scattering around the (110) reciprocal lattice point with spin-wave energies from 40 to 160 meV. The spin-waves over the entire range of energy are found to be consistent with an isotropic spin-wave dispersion relation. With the present experimental sensitivity we were unable to observe any band structure effects such as Stoner excitations or optical magnons in this range of wave vector and energy.

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