J.A.H. Coaquira

Structural and Mössbauer spectroscopic study of hexagonal Laves-phase Zr(FexCr1-x)2 alloys and their hydrides

Abstract Hexagonal (C14) Laves-phase Zr(FexCr1−x)2 alloys with x=0.3, 0.4, 0.5, 0.6 and 0.7 have been prepared by arc melting and the corresponding hydrides have been formed by hydrogen uptake at 1 atm pressure. X-ray diffractograms have been measured and analyzed with the Rietveld method. 57Fe Mossbauer spectra, measured at room temperature, could be fitted with two doublets with 3:1 area ratio, corresponding to the two Fe sites in the C14 structure. The quadrupole splitting (Δ) and isomer shift (δ) dependences on x are discussed in terms of volume and charge effects. The hydrides showed volume expansions of up to 22% without any crystal structure change. Both Mossbauer hyperfine parameters Δ and δ increased markedly in the hydrides as compared to the alloys. Tetrahedral hole radii have been calculated from measured crystalline parameters and their relationship to hydrogen site preferences is discussed.

Mössbauer and magnetic characterisation of carbon-coated small iron particles

Abstract Carbon-coated Fe particles have been produced by the Kratschmer–Huffmann carbon-arc discharge method. Soot, collarette and cathode samples have been separately characterised by Mossbauer spectroscopy and magnetisation measurements in the temperature range 4.2–300xa0K. From the analysis of the results the content of the different Fe phases and Fe-carbon solid solutions in our samples has been determined, as well as an estimation of particle sizes, which were found to be approximately 9 and 13xa0nm for the soot and collarette samples, respectively.

Structural, magnetic and hyperfine properties of Zr(Cr1−xFex)2 hydrides

Abstract Laves-phase compounds with the hexagonal C14-type structure Zr(Cr 1− x Fe x ) 2 ( x =0.3–0.7) were charged to maximum H capacity, which ranged from 3.6 to 2.5 H/f.u. for increasing x . The biggest cell volume expansion was 22%. Magnetic properties were investigated with magnetometry and Mossbauer spectroscopy. The hydrides exhibited spin-glass behavior, similar to their parent compounds, but with larger Fe moments and lower freezing temperatures. Electric hyperfine interactions were measured in the 78–350 K temperature range at the Fe sites and at the Zr site, using 57 Fe MS and 181 Ta TDPAC, respectively. Activation energies for H jump diffusion were determined by these two complementary techniques, yielding 72(4) and 246(30) meV respectively.

Mössbauer investigation of RCo3Sn compounds (R=Gd–Tm)

Abstract 119 Sn Mossbauer spectra of RCo 3 Sn (R=Gd–Tm) compounds have been measured in the temperature range 4.2–150 K. The T C values obtained from the hyperfine field ( B hf ) temperature dependence were found to vary linearly with the de Gennes factor, extrapolating to 30 K for J →0. B hf (measured at 4.2 K) was found to depend on the R element in a way that could be explained in terms of rare-earth spin and orbital contributions to the electron polarization, plus a contribution from the Co moments. Additionally, our results provide evidence against a recent claim that RCo 3 Sn compounds actually belong to the R 3 Co 8 Sn 4 structure.

Magnetic properties of Zr(Cr1 − xFex)2 alloys and their hydrides

Abstract The magnetic properties of MgZn2-type Zr(Cr1 −xFex)2 (0.3 ⩽ x ⩽ 0.7) and Zr(Cr1 −xFex)2Hy (y ∼ 3) alloys have been investigated. Arrott plots showed no evidence for ferromagnetic ordering. ZFC/FC magnetization and Mossbauer spectroscopic data indicated spin glass-like freezing. Saturation moments in the unhydrided alloys decreased with decreasing Fe concentration, vanishing for x ≈ 0.3. Hydrogen absorption made magnetic moments increase and freezing temperatures decrease.

Hydrogen absorption effects in the Zr(Fe0.5Cr0.5)2 compound

Zr(Fe0.5Cr0.5)2 Laves phase samples have been made to absorb different hydrogen amounts up to 3.3 H/f.u. XRD showed all ZrFeCrHy samples to have the C14 structure and, for 0 < y < 2.6, to be composed of a H-poor α-phase and a H-rich β-phase having different c/a ratios. Both phases could be distinguished in the Mössbauer spectra. The β-phase fraction was determined both from X-ray and Mössbauer analysis, with excellent agreement. β-phase lattice constants and hyperfine parameters vs. y exhibited a small jump at y ≈ 1.75, suggesting a phase transition. Mössbauer spectra of ZrFeCrH3.3 in the 78–350 K range revealed that, for Fe at the 6h site, quadrupole splitting and linewidth increased for decreasing temperature. These effects were attributed to H atomic jump diffusion, and an activation energy of ≈70 meV was estimated.

Phase Monitoring during Nd(Fe,M)12 (M=Mo or Ti) Compound Nitrogenation by Chemical Reaction with Sodium Azide (NaN3)

NdFe 10.75 MO 1.25 and NdFe 11 Ti alloys have been successfully nitrogenated by heating a mixture of powdered alloys with sodium azide (NaN 3 ) at temperatures between 330 and 450 °C. Among the characterization techniques used to evaluate the final product, Mossbauer spectroscopy was invaluable for quantitative determination of 1-12 and α-Fe phases, after and before nitrogenation. The α-Fe phase increased indicated that a competition between the interstitially modified compound formation (alloy + N) and alloy dissociation occurred. This effect was more pronounced in Ti alloys nitrogenated at 450 °C.

Magnetic properties and hyperfine field at Sn site in GdCo3Sn

Abstract The magnetic properties of the novel compound GdCo 3 Sn have been investigated with susceptibility, magnetization and 119 Sn Mossbauer spectroscopy measurements in the 4.2–300xa0K range. The sample was prepared in an arc furnace and annealed for 30 days. XRD data showed the presence of Gd 2 Co 17 and GdCoSn in addition to the compound of interest. Magnetization curves showed ferromagnetic behavior. The Mossbauer spectra consisted of two magnetic subspectra, one corresponding to each Sn-containing compound. Magnetic ordering temperatures of both GdCo 3 Sn ( T C =133±2xa0K) and GdCoSn ( T N =27±2xa0K) have been determined from the hyperfine field thermal dependence. The corresponding hyperfine fields at 4.2xa0K were 16.0 and 11.5xa0T, respectively. The GdCo 3 Sn saturation moment was estimated as 6.3xa0μ B /f.u.

Spin-glass behavior of Zr(FexCr1−x)2 compounds

Abstract The magnetic properties of C14 Laves-phase Zr(FexCr1−x)2 (0.7⩽x⩽0.85) alloys have been investigated. Arrott plots showed evidence of ferromagnetic ordering for alloys with x>0.7 but not for x=0.7. Zero field cooling/field cooling magnetization and Mossbauer spectroscopy data indicated spin-glass-like freezing for the sample with x=0.7 and a reentrant spin-glass behavior for samples with x>0.7. Dynamical behavior of the spin-glass sample (x=0.7) was studied by means of AC susceptibility. Two critical lines described by T f (H)−T(0) ∼H 2/δ were obtained with δ=4.4 and 2.5, respectively.

Hyperfine Interactions in CeT2Ge2 (T = Mn, Co) Heavy Fermions Compounds Measured by TDPAC

Time Differential Perturbed γ–γ Angular Correlation (TDPAC) technique was used to measure the magnetic hyperfine field at both Ge and Ce sites in CeMn2Ge2 and CeCo2Ge2 intermetallic compounds. The 111In (111Cd) probe nuclei was used to investigate the hyperfine interaction at Ge sites, while the 140La (140Ce) nuclei was used to measure the magnetic hyperfine field at Ce site. The present measurements cover the temperature ranges from 10–460 K for CeMn2Ge2 and 9–295 K for CeCo2Ge2, respectively. The result for 111Cd probe showed two distinct electric quadrupole frequencies above magnetic transition temperatures, in both compounds and a combined interaction in the magnetic region. The temperature dependence of the magnetic hyperfine field at 111Cd at Mn site for the CeMn2Ge2 compound showed a transition from ferromagnetic to antiferromagnetic phase around 320 K and from antiferromagnetic to paramagnetic phase at 420 K. While a small magnetic field was measured on 111Cd at Co site, no magnetic field on 140Ce site was observed in CeCo2Ge2.