Bombardment-induced photon emission from GaAs as a function of target temperature

The results of experiments on bombardment-induced photon emission from a GaAs single crystal as a function of target temperature are described. The target temperature was varied in the range 200–500°C. The line emission from excited Ga atoms sputtered by argon ions of 8 keV energy was measured. The variation of the sputtering yield as a function of the target temperature does not affect the intensity of photon emission. One can assume that this effect indicates a two-particle mechanism of excitation that occurs at primary steps of the collision cascade inside the solid. The annealing of the surface layer as a factor that influences the sputtering process is also considered. The evolution of the simple cascade model and radiationless relaxation are proposed to explain the experimental data.     

Photon emission from sputtered atoms — The observation of apparent local thermodynamic equilibrium in the excitation

We develop, here, our previous LEED Kikuchi pattern theory in order to include the plamion excitation. Therefore, we assume that the intensity of electrons which have suffered plasmon excitations is given by a Poisson distribution. The theory is applied to calculate the contrast change of the aluminium (111) Kikuchi band due to the plasmon excitation, and the calculated values show a good agreement with our experiment. They confirm especially that the contrast due to the one plasmon excitation is larger than the contrast due to phonon excitation only.     

AC susceptibility and temperature modulation studies of gadolinium near the Curie point

Measurements of the ac susceptibility and temperature modulation studies of the ac and dc magnetic properties of polycrystalline gadolinium in the vicinity of the Curie point T_c are reported. Field-independent exchange-enhanced paramagnetism was observed above T_c. Below T_c the initial field-independent susceptibility was observed for applied fields < 8 A/m rms and exhibited a frequency dependence characteristic of a magnetic after-effect. Within about 3 K of T_c this susceptibility is strongly influenced by domain nucleation and a temperature dependent magnetic relaxation time was observed.     

Angular resonances in the light emission from atoms near a grating

A two step light emission process becomes possible when atoms are placed near a metallic grating. The atoms can excite surface plasmons in the grating. The surface plasmons can then emit light. This light is emitted at certain angles determined by momentum conservation in the plane of the grating. Thus, the angles depend on the spacing of the grating. Our measurements on light from nitrogen atoms at distances, d, ranging from 10 to 100 nm from a silver grating are in agreement with a recent theory of Aravind, Hood and Metiu. We also report the effect of a nonsinusoidal grating profile and the emission of light at angles which can be predicted from a straightforward extension of the theory.     

Conduction electrons in ferromagnetic semiconductors near the Curie point

Near the Curie point the conduction electron energy in a nondegenerate ferromagnetic semiconductor is determined by the short range magnetic ordering. This prohibits formation of the magnetic polaron and leads to an anomalous temperature shift of the conduction band bottom in the paramagnetic region.     

Photon emission as a random event

The statistical properties of the random events that correspond to the emission or the detection of a photon spontaneously emitted by a single atom are discussed. This statistics constitutes a full and nontrivial solution of the stochastic theory of random events. The results are most explicit for an atom with two relevant states. The formalism is also generalized to atomic models with nondegenerate states or with more than one driven transition.     

Low-frequency susceptibility of single-domain nickel particles from 2 K up to the Curie point

The magnetic susceptibility of small single-domain nickel particles chemisorbed on a silica carrier has been studied as a function of temperature in the range 1.7 < T < 700K. The mean particle size was obtained from the blocking temperature in good agreement with results deduced from magnetization measurements. A slightly sintered specimen shows the typical behaviour of bulk nickel. The remarkable thermostability of the small particles seems to be of considerable interest.     

Recoilless fraction studies of iron near the Curie temperature

The recoilless fractionf of⁵⁷Fe in iron is determined as a function of temperatureT in the vicinity of the Curie temperatureTc using a source of⁵⁷Co diffused in a foil of 100% enriched⁵⁶Fe. Our results show thatf does not undergo an abrupt change atTc, but that in the ferromagnetic phase |df/dT| is larger than in the non-ferromagnetic phase. This indicates that the ferromagnetic interaction increases the bond between the iron atoms, and thus contributes to the stability of the iron lattice belowTc.     

Heat diffusivity and heat conductivity of Ni near the Curie point

The heat diffusivity of Ni near the Curie point was measured by an a.c. oscillating method. The period of self-oscillation is related to the heat diffusivity. The principle and some technical details of the experimental method is given. The critical behavior of heat diffusivity near the Curie temperature is analysed. Using known specific heat data the heat conductivity for Ni can be calculated. A conclusion is made that the Wiedemann-Franz law is not valid near the ferromagnetic phase transition.     

The behaviour of physical quantities in thin films near the Curie point

The Valenta model of a thin ferromagnetic film in the critical region above the Curie point has been considered. Spatial and temperature dependence for spin correlation time and magnetic susceptibility has been obtained and discussed. The results have been generalized and expressions describing the behaviour of any physical quantity in more complicated models of a thin ferromagnetic film near the Curie temperature have been given.     

Dielectric losses in submicrometer barium titanate near the Curie temperature

Low-frequency dielectric losses (tanδ) in barium titanate ceramics with crystallite sizes of ~100 nm near the Curie temperature are studied. It is shown that the observed tanδ maximum is described by a low-frequency fluctuation mechanism of dielectric losses. An analysis of experimental data shows that the critical nucleus size in ultrafine material is smaller than the one in bulk barium titanate.     

Anomalous behaviour of GdNi5 near the Curie point

The a.c. permeability and the qualitative behaviour of the electrical resistivity of GdNi₅ have been investigated near the Curie point (T_c). The electrical resistivity shows no unusual properties near T_c but the a.c. permeability exhibits a sharp peak superimposed on the usual relatively slow decay in the transition region. The peak in the susceptibility occurs at 31.8 K in zero external field and may be suppressed by the application of a magnetic field.     

Oxidation of iron,cobalt and nickel at the Curie temperature

The large differences in the logarithmic oxidation kinetics reported as a change in activation energy with the oxidation rate decreasing from ferromagnetic to paramagnetic states are discussed for iron, cobalt and nickel. The observed anomalies at the Curie point are explained by a mechanism which takes into account both surface-state charge at the metal/oxide interface and space charge in the growing oxide.     

Mössbauer energy shift near the Curie temperature of Fe

A search for a possible magnetization dependence of the isomer shift in Fe yields negative results, both within 2 K of the Curie point, and in a wide region above and below the Curie point. These findings are based on both source and absorber experiments. They disagree with the observations of Preston near T_c, but are in accord with previous work over a wider temperature range.Research supported in part by the National Science Foundation, and the Research Corporation.     

The Curie temperature of dilute ferromagnetic alloys near the percolation threshold

Upper limit for the Curie temperature T_c(x) of the dilute Heisenberg ferromagnet near percolation threshold is derived. Qualitative arguments are put forward that this limit gives a correct dependence of T_c on the concentration of magnetic atoms x.