G. K. Shenoy

The effect of a crystalline electric field on the magnetic transition temperatures of rare-earth rhodium borides

Abstract A simple molecular field model is presented for the prediction of magnetic transition temperatures of rare-earth (RE) compounds when crystalline electric field (CEF) splittings are significant. The model is applied to the RERh4B4 (RE=Gd−Tm) series, using what is known about the crystal field in these materials.

EXAFS study of iron monomers and dimers isolated in solid argon

Abstract We report a study of local structure of iron monomers and dimers isolated in a rare gas Ar matrix using extended X-ray absorption fine structure measurements. The results have been compared with Mossbauer measurements on such systems. The interatomic distance found for Fe2 in these measurements is 1.87 ± 0.13 A.

Structural relationships in rare earth-transition metal hydrides

Several structural types of form AB/sub 2/, AB/sub 3/, A/sub 2/B/sub 7/, and AB/sub 5/ (A = rare earth, B = transition metal) are known to be closely related, with local environments which are very similar among the different compounds. Based on these relationships, we suggest that hydride phases A/sub l/B/sub m/H/sub n/ should be related according to the relationships n(AB/sub 3/) = 1/3n(AB/sub 5/) + 2/3n(AB/sub 2/) and n(A/sub 2/B/sub 7/) = n(AB/sub 5/) + n(AB/sub 2/) where the n(AB/sub m/) are observed hydrogen phase concentrations. It is shown that the phases observed in pressure-composition isotherms for AB/sub 3/ and A/sub 2/B/sub 7/ systems can be understood from this viewpoint. A discussion is given of the maximum hydrogen concentration possible in the compounds having these structure types.

57Fe Mössbauer studies on Si-substituted Er2Fe17

Abstract The compound Er 2 Fe 17 crystallizes in the hexagonal Th 2 Ni 17 structure, and is ordered magnetically with a Curie temperature T c of 305 K. We have investigated the effect of substituting small amounts of Si for Fe on the magnetic behavior of this compound by low temperature 57 Fe Mossbauer spectroscopy and magnetization measurements. Fe can be replaced by Si to form Er 2 Fe 17−x Si x alloys with x up to 3.0 without changing the crystal structure, though a small amount of a second phase appears in X = 3.0 sample. The substitution of Si causes a decrease in the average Fe magnetic moment. The lattice parameters decrease upon Si substitution, and T c increase from 305 K for x = 0 to 498 K for x = 3.0. 57 Fe Mossbauer spectroscopy measurements indicates no preferential subtitution of Si among the four crystallographically different Fe sites.

Effects of in situ vacuum annealing on the surface and luminescent properties of ZnS nanowires

We have monitored the changes that occur in the x-ray-excited optical luminescence, absorption, and photoemission spectra as a function of vacuum annealing time and temperature for ZnS nanowires. All measurements were done in situ. Initial heating causes desorption of surface oxides and a concurrent reduction in the intensity of all the luminescence peaks, which we attribute to the creation of surface states that quench the luminescence. Extended annealing causes diffusion of Au from the particle used to nucleate the wire growth, which results in an increase in intensity of its associated luminescent band at 520nm. Changes were also observed in the ZnL- and SK-edge x-ray absorption spectra, which are consistent with this interpretation.

The coordination environment of Eu(III) ions in hydrated A and Y zeolites as determined by luminescence lifetime and EXAFS measurements

Luminescence lifetimes and EXAFS show that the average coordination of Eu3+ in the large‐pore zeolite Y is identical to that in aqueous solution, while the coordination number in the small‐pore zeolite A is lower, probably because of steric constraints.

Magnetic Behavior of the FeSiO3–MgSiO3 Orthopyroxene System From NGR in 57Fe

The magnetic behavior of (FexMg1−x)SiO3 with 0.269≤x≤1.0 was investigated between 300° and 1.7°K using NGR of 57Fe. For x≳0.76 magnetic ordering was observed at the following temperatures: x=1.0, TC=38°K; x=0.866, TC=18°K; x=0.758, TC=11°K. Ferrous ions occupy two different lattice sites (M1 and M2) of low symmetry. The resonance spectra consist of two superimposed hyperfine patterns, which reflect different electronic ground states for Fe2+ at the two sites. The M2 site is characterized by a smaller magnetic hyperfine field and a more negative isomer shift compared to the M1 site. Typical values for FeSiO3 at 4.2°K are 113 kOe, 1.29 mm/sec, 45 MHz at M2 and 290 kOe, 1.34 mm/sec, 69 MHz at M1. Above TC two quadrupolar doublets are observed. The quadrupole coupling of the M1 site shows a strong temperature dependence, while that of the M2 site remains fairly constant. The magnetically more dilute samples (x≲0.4) did not show ordering in the temperature region above 1.7°K, but produced paramagnetic relaxati...

Spin glass behavior in the new amorphous alloys M2SnTe4 (M=Cr, Mn, Fe)

Abstract The amorphous alloys M 2 SnTe 4 (M=Cr, Mn, Fe) are prepared by a new method involving the oxidation of main group polyanions (Zintl anions) by transition metal cations in solution at or below room temperature. The M 2 SnTe 4 materials undergo a transition to a spin glass state at 12 K ⩽ T f ⩽ 20 K and were characterized by dc magnetization, 57 Fe and 119 Sn Mossbauer and x-ray diffraction measurements as well as the behavior of the remanent magnetization.

Magnetic properties of DyFe2H2 from 57Fe, 161Dy Mössbauer effect and magnetization measurements

A single phase stable hydride with nominal composition DyFe2H2 is shown to have the cubic C15 Laves structure with a 5% larger lattice constant. 57Fe Mossbauer spectra show two sites with nearly equal population and with different isomer shifts and hyperfine fields. Based on the average hyperfine field at 4.2 K, an Fe moment of approximately 1.6 μB is estimated. From the 161Dy data at 4.2 K, which show a single site, a nearly free‐ion moment of 9.8 μB is determined. Relaxation effects in the 161Dy spectra above 77 K, combined with a sharp decrease in the moment with increasing temperature, imply a weakening of the Dy‐Fe magnetic coupling induced by the presence of the hydrogen.

Surface segregation of iron by hydrogenation of FeTi: An 57Fe mössbauer conversion electron study

Abstract Mossbauer spectroscopy of 57 Fe conversion electrons was used to study FeTi subjected to 51 hydrogen absorption-desorption cycles. This technique probes both the surface and part of the bulk, and it can also distinguish between magnetic and non-magnetic iron in the lattice. Our measurements suggest the presence of a magnetic overlayer on FeTi made up of iron clusters and the absence of magnetic moments on the iron contained in the bulk. This results shows that the magnetic moment observed in bulk magnetization studies in fact originates from this segregated iron layer on the surface of FeTi.