Mössbauer and PAC studies of nanocrystalline Fe

The possible presence of a large magnetic field due to spin polarization of a Cd nucleus (decay product of 111In) at an Al substitutional site in AlN is investigated with perturbed angular correlation (PAC) spectroscopy. The PAC spectra of 111In/111Cd in AlN show two probe environments: a weak quadrupole interaction (quadrupole interaction constant, \(\nu _{\rm Q}^{\,\,\,\rm lattice} = 30\) MHz) due to 111In probes at a defect free Al substitutional site and an unknown large interaction (\(\nu _{\rm Q}^{\,\,\,\rm complex} = 300\) MHz) tentatively attributed to a nearest neighbour pair between 111In and a nitrogen vacancy (VN) aligned along the c-axis. Surprisingly, in density functional theory (DFT) calculations, such a large electric field gradient (EFG) could not be reproduced. However, an inclusion of spin polarization in the calculations indicates a strong magnetic field at ~50 % of the 111In/111Cd site. An attempt to verify the presence of the strong magnetic field and to explain the origin of the strong interaction is made. Orientation measurements show, the large interaction is not characterised by a magnetic interaction and is predominantly due to the EFG. However, in the presence of an external magnetic field, the strong interaction probe environment becomes more uniform and the EFG increases by ~10 %. This definitely hints towards some sort of magnetic interaction at the strong interaction probe site.

PAC study of martensitic transformations in RCu compounds

Perturbed angular correlation studies of 181Ta hyperfine interactions in Hf–Ni and Zr–Ni compounds

The perturbed angular correlation (PAC) method has been employed to study the hyperfine interaction of the 245-keV state in $^{111}\mathrm{Cd}$, following $^{111}\mathrm{In}$ implantation into the fcc metals Ag, Al, Cu, Ni, Pd, Au, and Pt and subsequent oxidation. Broad quadrupole frequency distributions around a peak frequency 〈${\ensuremath{\omega}}_{1}$〉=160 MHz were found in all systems and attributed to oxygen trapping at $^{111}\mathrm{In}$. The fraction of oxidized $^{111}\mathrm{In}$ increases with the oxidation temperature in such a way that about 50% of the implanted In ions have trapped oxygen at 60% of the melting temperature of the host. An interpretation of these frequency distributions has been attempted on the basis of PAC measurements for $^{111}\mathrm{In}$ implanted into the metal oxides AgO, ${\mathrm{Ag}}_{2}$O, and ${\mathrm{In}}_{2}$${\mathrm{O}}_{3}$. In these systems, well-defined electric field gradients have been found which can be related to the respective crystallographic structures. In addition, the magnetic hyperfine interaction of $^{111}\mathrm{In}$ in antiferromagnetic NiO proves substitutional implantation.

The study involves the investigation of basic magnetic properties of ultrafine rare-earth iron garnets in the size range 1.0-35 nm, prepared by a citrate precursor method. The crystallites of 10-35 nm size are monolithic ferrimagnetic particles which have saturation magnetization and magnetic interactions (as shown by the Curie temperature) comparable with those of bulk crystalline garnets. The small reduction in saturation magnetization in the 10-20 nm crystallites is attributed to spin canting and non-collinearity at the surface of the crystallites, and a model based on surface spin canting is proposed for the spin structure of these crystallites. The magnetization and magnetic interactions of smaller crystallites of 1.0-1.5 nm which exist as X-ray amorphous magnetic clusters are different from those in the bulk crystalline garnets. Reduction in the rare-earth sublattice contribution due to a weakening of the R3+-O2-Fe3+ super-exchange interaction in the disordered state accounts for the difference in saturation magnetization in these magnetic clusters, whereas the enhanced Curie temperatures are attributed to an increase in the number of effective magnetic interactions in the disordered state. A model based on bulk spin noncollinearity is proposed for the spin structure of the magnetic clusters consisting of 1.0-1.5 nm crystallites.

The magnetic properties of two natural magnesian hedenbergite samples with slightly different Fe contents (hereafter denoted HED1 and HED2) were studied by transmission 57Fe Mössbauer spectroscopy within the temperature range 4.2-35 K and in a longitudinal, external field of 60 kOe at 4.2 K. The magnetic zero-field Mössbauer spectra (MS) were adequately refined using a superposition of two model-independent hyperfine-field distributions, one for the dominant Fe2+ component and one for the weak Fe3+ contribution, the positions of the eight absorptions and their intensities for each composing elemental subspectrum being determined by diagonalization of the hyperfine-interaction Hamiltonian. The maximum-probability saturation hyperfine fields for Fe2+ were found to be 180 kOe and 185 kOe for HED1 and HED2, respectively, while a value of ~545 kOe was obtained for Fe3+. For both hedenbergite samples, the Fe2+ asymmetry parameter h of the electric field gradient (EFG) is quite high, namely 0.7-0.8 regardless of temperature. The orientation of the ferrous magnetic hyperfine field in the EFG principal-axes frame is ~(85°, 38°) and is not affected by the temperature of the absorber. The applied-field Mössbauer spectra show that the applied field does not disrupt the magnetic structure, and consequently that magnetic anisotropy is quite strong. The spectra were satisfactorily described by a two-parameter distribution model, taking into account distributions for the magnitude and the orientation of the hyperfine field with respect to the external field. The obtained hyperfine parameters are in excellent agreement with the results from the zerofield spectra. The temperature dependence of the hyperfine field seems to indicate that the magnetic structure in hedenbergite can be approximated by a two-dimensional rectangular Ising model and that the inter-chain and intra-chain magnetic exchange interactions are of similar magnitude.

The magnetic phase transitions of pseudo-binary rare earth (R) Laves phasesR1−xYxCo2 have been investigated by perturbed angular correlation (PAC)measurements of the magnetic hyperfine fields at the probe nucleus111Cd forR = Tb, Sm and Ho at variousY concentrations x and for R = Gd, Dy, Er, Nd and Pr at the concentrationx = 0.3. First-order transitionswere observed in Tb1−xYxCo2 and Sm1−xYxCo2 for x≥0.3,in Ho1−xYxCo2 forx≤0.4 andin R0.7Y0.3Co2 forR = Dy, Ho, Er, Ndand Pr. For Gd0.7Y0.3Co2, the temperature dependence of the average magnetic hyperfine fieldis compatible with a second-order transition. The discontinuity of themagnetic hyperfine interaction at the first-order transitions of heavyR1−xYxCo2, which mainly reflects the jump of 3d magnetization of the Co subsystem atTC, was found to increase monotonically with decreasing order temperature. TheTC dependence of the normalized magnetic frequency with T0 = 203(5) K for the boundary temperature between first- and second-order transitionscan be explained by the temperature dependence of the coefficient of theM4 term of the free energy in the Wohlfarth–Rhodes–Shimizu theory of itinerant electronmetamagnetism.

GdMnIn is reported to crystallize in the hexagonal MgNi2-type structure presenting a spin-glass behavior with no magnetic order attributed to the triangular spin frustration of magnetic ions. In the present work, FC-ZFC magnetization, specific heat and AC susceptibility measurements along with the local magnetic exchange measured by hyperfine interactions at In sites are used to investigate the magnetic behavior in GdMnIn compound. The ZFC-FC magnetization curves exhibit an inflection which was ascribed to the antiferromagnetic transition at TN= 145 K. These curves also give an indication of thermomagnetic irreversibility at 118 K, which along with the absence of inflection in specific heat results might be associated to spin-glass behavior. Results of AC susceptibility and magnetic hyperfine field measured using 111In(111Cd) probe nuclei carried out by perturbed angular correlations (PAC) technique did not show evidence of spin-glass behavior. The thermomagnetic irreversibility in FC-ZFC curves along with results of hyerfine interactions suggest the presence of magneto-crystalline anisotropy effects and a weak long-range coupling in GdMnIn.

The temperature and concentration dependence of the hyperfine field in AuFe alloys in the concentration range of 15 to 28 at%Fe was measured by perturbed angular correlation (PAC) spectroscopy. The temperature dependence of the hyperfine field shows a slope change at T/TC = 0.25 for concentration below 20 at%Fe, while for higher concentration a continuous behaviour is seen.

Using the Monte Carlo method, we studied magnetic properties of the models of Re-Tb and Re-Gd amorphous alloys as well as pure amorphous Tb and Gd. The magnetic phase diagrams for the models of amorphous Tb and Gd were constructed. We determined the values of D/J0 (for Tb) and J1/|J2| (for Gd) at which the transition into spin-glass-like state takes place. The local magnetic structure was studied by the spin correlation functions and the angular spin correlation functions. The difference in magnetic structures in amorphous alloys with the random anisotropy and with fluctuations of exchange interaction was revealed.

To understand more fully the magnetic structure of GdCo2 we have investigated the field and temperature dependence of the Co hyperfine field in this cubic‐Laves phase ferrimagnet. The hyperfine field at Co nuclei is 61.3 kOe at 1.4 K and the sign of this field is positive with respect to the sample magnetization. Below 200 K the Co resonance is basically a single line. Above this temperature, near which a pronounced inflection appears in the saturation magnetization vs. temperature curve, the Co spectrum becomes complex. Above 260 K it is composed of two closely spaced but resolved lines. The temperature dependence of the sublattice magnetizations are analyzed in the light of the bulk magnetization behavior and this hyperfine field data. The significance of the spectral structure and the temperature dependence of the Co sublattice magnetization are explored within the context of existing models of the magnetic structure.

In the binary Ni98.5Al1.5 alloy we observed well resolved satellite hyperfine fields at111Cd nuclear probes. The hyperfine shifts derivated are −4.0(3); −8.5(3) and 14.0(3)% relative to the pure nickel value. From the relative intensity of the four components as well as from the quadrupole interaction data we identify the satellites as impurity-probe configurations containing zero, one, two or three Al impurities in the third neighbour shell only. The third neighbour contribution to the hyperfine field at Cd in nickel equals ΔH3= +4.5kG/μB-atom, allowing an estimate ΔH1≃ −17.8 kG/μB-atom for the first shell contribution, confirmed by a similar experiment on Ni98.5Pd1.5.

We report the complex magnetic phase diagram and electronic structure of Cr_{2}(Te_{1-x}W_{x})O_{6} systems. While compounds with different x values possess the same crystal structure, they display different magnetic structures below and above x_{c}=0.7, where both the transition temperature T_{N} and sublattice magnetization (M_{s}) reach a minimum. Unlike many known cases where magnetic interactions are controlled either by injection of charge carriers or by structural distortion induced via chemical doping, in the present case it is achieved by tuning the orbital hybridization between Cr 3d and O 2p orbitals through W 5d states. The result is supported by abinitio electronic structure calculations. Through this concept, we introduce a new approach to tune magnetic and electronic properties via chemical doping.

The microscopic magnetic local structure of Botallackite-type layer structured compounds Cu2(OD)3X (X = NO3 − and HCOO−) exhibiting a nonequilateral planar triangular magnetic lattice was determined by the solid-state high resolution deuterium NMR of deuterated hydroxy groups in the high temperature region above 190 K. The magnetic interactions in a copper ion layer were probed by the paramagnetic NMR shifts of the two chemically distinct hydroxy groups. Isotropic NMR shift of each hydroxy group showed different temperature dependence, suggesting non-uniform magnetic interaction. The magnetic interaction in the copper layer could be decomposed into a sum of 1D-Heisenberg ferro- and antiferromagnetic chains in the high temperature region. Two distinct copper chains with ferro- and antiferromagnetic exchange interactions J= ± 19 ± 11 and − 21 ± 3 K were found for x = NO3− from the temperature dependence of the two distinct NMR signals, while J = ± 13 ± 7 and − 13 ± 5 K for x = HCOO−. The derived values of J almost reproduced the temperature behavior of the magnetic susceptibility χiA T vs. T.

Alumina surfaces and interfaces under non-ultrahigh vacuum conditions