Size dependence of the Mössbauer effect in one-dimension

Employing a model of coupled harmonic oscillators we analyze the nature of recoilless emission in the Mössbauer effect for a finite sized 1D lattice. We demonstrate explicitly that under certain assumptions, recoilless probability first attains a maximum for a certain lattice size and then decreases with increasing lattice size. Further, we derive a scaling relation for this variation. Our treatment may have relevance to the recoilless probability in finite clusters such as nanocrystals and nanowires.     

Mössbauer study of CoFeRhO4

CoFeRhO₄ has been studied by Mössbauer spectroscopy and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant a₀=8.451±0.005 Å. The iron ions are in ferric states. The temperature dependence of the magnetic hyperfine field is analyzed by the Néel theory of ferrimagnetism. The intersublattice superexchange interaction is antiferromagnetic and strong with a strength of J_AB=−12.39k_B while the intrasublattice superexchange interactions are weak with strengths of J_AA=−4.96k_B and J_BB=6.20k_B. As the temperature increases toward the Néel temperature T_N, a systematic line broadening effect in the Mössbauer spectrum is observed and interpreted to originate from different temperature dependences of the magnetic hyperfine fields at various iron sites.     

Radiative detection of NMR using the Mössbauer effect

NMR has been observed in ⁵⁷Co dissolved in iron by measuring a change in the Mössbauer effect spectrum at 0.07 K.     

The Mössbauer effect in PbI2

Results are presented on the Mössbauer effect in mono- and poly-crystalline PbI₂ giving data for the isomer shifts and quadrupole couplings.     

Polarized radiation in Mössbauer spectroscopy

Mössbauer polarimetry is a spectroscopic technique sensitive to the orientations of hyperfine fields. The technique is particularly effective with monochromatic, polarized radiation: the measured spectra do not contain many additional transitions present when nonmonochromatic radiation is used. The paper reviews recent achievements in the construction of sources of polarized monochromatic radiation. Recently, filter techniques were adopted for achieving circularly and linearly polarized radiation from commercially available radioactive isotopes. A synchrotron source with nano-eV energy width, suitable for Mössbauer measurements was constructed. Applications are reviewed, in particular determination of the direction of the hyperfine magnetic field and the orientation of the electric field gradient. Special attention is paid to cases when the distributions of the hyperfine fields and mixed interactions result in poorly resolved spectra. Recent achievements in methodology are described. An explicit form of the intensity tensor is derived, which allows the transition probabilities to be calculated omitting the diagonalization of the Hamiltonian. The concept of the velocity moments is introduced. It is shown that some averages of the whole Mössbauer spectra relate to the averages of hyperfine fields and possess tensor properties.     

Tin(IV) derivatives of 2,6-pyridinedicarboxylate: A Sn Mössbauer spectroscopic investigation

A series of organotin(IV) derivatives of 2,6-pyridinedicarboxylate has been investigated by Mössbauer spectroscopy in order to elucidate aspects concerning bonding and structural features in the solid state. A geometrical pattern of five-fold coordination at the metal centre has been revealed for SnCl₃Bu and SnClBu₃ derivatives. Trans stereochemistry for the butyl and vinyl groups of SnCl₂Bu₂ and SnCl₂(Vin)₂ derivatives has also been identified by this method. The isomer shift for the divinyl derivative is concurrent to a 7-coordinate metal centre contrasting to that for the dibutyl one. Although there is a discrepancy in isomer shift between these compounds, both have seven-fold coordination at the Sn(IV) nucleus. The resulting data has given evidence that 2,6-pyridinedicarboxylate is acting as a tridentate ligand through pyridil and carbolxylate moiety to all derivatives except for SnClBu₃. For the latter, the coordination mode occurs via carboxylate groups. The overall data support distorted geometrical pattern to all complexes in solid state.     

A Mössbauer effect study of structural ordering in rapidly quenched Fe–Ga alloys

Samples of Fe_100−xGa_x (x=8.3, 17.9, 20.5 and 23.3) were prepared by rapid solidification from the melt using a single Cu roller. X-ray diffraction studies of all samples showed them to be single phase with the disordered BCC structure. No evidence of superlattice reflections from D0₃ ordering was observed for any of the samples. Room-temperature ⁵⁷Fe Mössbauer effect spectra indicated that all samples were ferromagnetically ordered. Spectra were fit to distributions of hyperfine fields. The x=8.3 sample showed a hyperfine field distribution that was single peaked and indicated a reasonably random distribution of local Fe environments. The x=17.9 and 20.5 samples showed hyperfine field distributions that were bimodal and indicated two distinct local Fe environments. The x=23.3 sample showed three distinct field components. It is suggested that the x=8.3, 17.9 and 20.5 alloys are primarily a disordered BCC phase. The x=8.3 alloy shows a small amount of short-range Ga–Ga pairing, while this short-range pairing is significantly greater in the x=17.9 and 20.5 alloys. The three field components in the x=23.3 alloy correspond well to the two sites associated with the D0₃ phase and a third component corresponding to a remaining L1₂ phase suggesting the presence of at least short-range D0₃ clustering in this alloy.     

VSM and Mössbauer study of nanostructured hematite

In the present work, we have synthesized nanostructured hematite samples using chemical precipitation method. The crystal structure and the grain size of the samples were studied using XRD. The zero field cooled and field cooled magnetization curves of the samples were recorded in the temperature range from 300 to 10 K. The variations of Morin transition temperature and blocking temperature with the grain size of the samples were investigated. The hysterics curves of the samples were recorded and the samples showed a superparamagnetic nature at room temperature whereas, at 10 K the samples showed open hysteresis curves. The sample with smaller grain size showed higher value of coercivity compared to samples with larger grain size. Mössbauer spectra of the samples were recorded and the grain size dependence on Mössbauer parameters was investigated.     

The Mössbauer effect isomer shift of Fe impurities in CuZn alloys

The Mössbauer effect isomer shift has been measured for Fe in each room temperature phase of the CuZn alloy system. The isomer shift contribution in the α phase due to Zn electrons is deduced from pressure data.     

Fe-contacts on InAs(100) and InP(100) characterised by conversion electron Mössbauer spectroscopy

We have grown 4 nm thin films of ⁵⁷Fe on InAs(100) and InP(100) surfaces by use of MBE and studied the samples by ⁵⁷Fe conversion electron Mössbauer spectroscopy. In the case of InAs, the Mössbauer spectrum showed a sextet due to α-Fe and a further magnetically split component with slightly smaller hyperfine field, which is attributed to interface components. This result indicates that there is a relatively sharp interface between Fe and InAs. On the contrary, the spectrum of the InP sample showed a sextet with very broad lines and a smaller average hyperfine field. This suggests a strong chemical reaction between iron and the substrate, which results in the formation of a poorly crystalline phase.     

Localisation of iron ions in NH4,Na-Y zeolite: A Mössbauer effect study

Localisation of Fe ions in deep-bed treated NH₄,Na-Y zeolite at 830K, 940K, and 1050K, as well as after reduction by hydrogen, and dealumination with subsequent calcination is studied by⁵⁷Fe Mössbauer spectroscopy. Most of the incorporated iron is present in the form of octahedrally coordinated ferric species with two distinct sets of hyperfine parameters assigned to atoms inside the zeolite structure and in extraframework positions. Fe²⁺ ions with high coordination are also detected and their amount increases with the temperature of deep-bed treatment. Low-temperature Mössbauer effect measurements at 77K and 4.2K were employed to facilitate the identification of iron sites.     

Electric hyperfine alignment of Au nuclei in KAu(CN)2 observed by the Mössbauer effect

A large electric hyperfine alignment of ¹⁹⁷Au nuclei has been observed by Mössbauer absorption measurements of the 77.3 keV γ-transition of ¹⁹⁷Au in diamagnetic KAu(CN)₂. the measurements were extended down to 36 mK by employing a ³He/⁴He dilution refrigerator. The nuclear spin-lattice relaxation time was estimated from the time dependence of the asymmetry in Mössbauer spectra. It is shown that the spectra of oriented specimen can be used for determination of the recoilless factor of the absorber.     

Nucleogenic iron studied by emission Mössbauer spectroscopy in Co-metal

Results obtained by means of the emission Mössbauer spectroscopy in metallic cobalt are reported. The Mössbauer line of the 14.4-keV energy connecting the first excited state of the stable ⁵⁷Fe nucleus with its ground state was used. Radioactive ⁵⁷Co was used as the precursor of the above nuclear state. It was dissolved in the natural metallic cobalt with the concentration of about 40 at. ppm including nucleogenic iron generated during decay of the cobalt precursor. Mössbauer spectra were collected in the temperature range between room temperature (RT) and 1075 K with the sample kept under vacuum. A transition from the low temperature hexagonal phase to the face centered cubic high temperature phase at about 690 K has no influence on the iron magnetic hyperfine field arising due to the ferromagnetic ordering of the host. On the other hand, the electron charge density on the iron nucleus has some relatively narrow maximum in the vicinity of the transition temperature. There is some discontinuity in the recoilless fraction as well indicating that the high temperature cubic phase provides somewhat stronger bonds for the isolated iron impurity. The anharmonic behavior of the lattice vibrations could be seen in the cubic phase well above the transition point. No measurable electric quadrupole interaction was found in the hexagonal phase. The evolution of the magnetic hyperfine field with the temperature is reasonably described by the spin wave formalism provided strong magnon-magnon scattering is allowed for. On the other hand, charge density on the iron nucleus follows thermal expansion except some singularity in the vicinity of the transition point.     

Mössbauer study of antiferromagnetic CuFeO2

We have studied the magnetic spin structure of antiferromagnetic CuFeO₂ by X-ray diffraction (XRD) and Mössbauer spectroscopy. Its crystal structure determined by XRD analysis was a rhombohedral structure (space group R-3m) and lattice constants a₀ and c₀ were 3.0333 and 17.1595 Å, respectively. In spite of 4-Fe sublattices in a delafossite CuFeO₂, its Mössbauer spectra were analyzed with 1-set (6-Lorentzian lines) below 10 K due to the collinear-commensurate spin structure, but the spectra were fitted with 4-sextet above 10 K due to the incommensurate spin structure. This phenomenon was attributed to the spin–lattice relaxation effect. Magnetic Néel temperature was also determined at 18 K, which corresponded to the high-spin Fe³⁺ valance state. On the other hand, CuFe_0.98Al_0.02O₂ powder with a noncollinear spin structure was fitted with 4-sextet at 4.2 K.     

Magnetic and Mössbauer studies of Ni substituted Li-Zn ferrite

Li-Zn ferrites substituted with Ni having the compositional formula Li_0.4−0.5xZn_0.2Ni_xFe_2.4−0.5xO₄ where x=0.02?x?0.1 in steps of 0.02 were fabricated by the citrate precursor method. This method has been employed to get nanosized particles and good magnetic properties. The spinel phase structure of the prepared ferrites was confirmed by XRD analysis. The effect of Ni concentration on magnetic properties such as saturation magnetization and Curie temperature were investigated. A good knowledge of these magnetic properties is desirable from application point of view. The values observed are large and both quantities were found to decrease with substitution. The saturation magnetizations were found to vary from 78 to 94 emu/gm while the Curie temperature which limits the operating temperature of the system ranges between 563 and 584 °C. Mössbauer data were also recorded at room temperature and the hyperfine parameters like isomer shift, quadrupole splitting and internal magnetic field estimated. The results obtained and mechanisms involved are discussed.     

Nucleogenic iron charge states in CoO studied by Mössbauer spectroscopy

Single crystal of CoO was doped by about 40-at. ppm of ⁵⁷Co. Mössbauer spectra were measured vs. sample orientation, temperature and under reduced oxygen pressure, i.e. for a low cobalt vacancy concentration. Distinctly different iron charge states coexist till about 866 K. A mixed-valent state is observed above this temperature. It evolves gradually towards the Fe²⁺ state above 1255 K due to the decreasing mobility of holes, as compared to the mobility of the electrons. The ratio of the mobility of holes to the mobility of electrons increases with the increasing vacancy concentration, i.e. with the increasing oxygen pressure. Defects order into clusters below 866 K leaving behind almost perfect structure. A recoilless fraction exhibits some anharmonicity above 923 K.     

A Mössbauer study of antiferroelectric transition in Fe: NH4H2PO4

Mössbauer studies in ⁵⁷Fe³⁺: NH₄H₂PO₄ (ADP) were conducted over a range of temperatures covering the antiferroelectric transition at 148 K, using both powder and single crystal absorbers. No anomaly was observed in the temperature dependence of chemical isomershift and normalised resonance intensity at the transition point T_c. However, a sudden increase in the line width around T_c was observed, consistent with the expected increase in electric field gradient created by the onset of antiferroelectric ordering. These experiments clearly show that the impurity Fe atoms do not play any role in the soft mode responsible for the spontaneous polarization in ADP.     

The Mössbauer rotor experiment and the general theory of relativity

In the recent paper Yarman et al. (2015), the authors claim that our general relativistic analysis in Corda (2015), with the additional effect due to clock synchronization, cannot explain the extra energy shift in the Mössbauer rotor experiment. In their opinion, the extra energy shift due to the clock synchronization is of order 10⁻¹³ and cannot be detected by the detectors of γ$\gamma$-quanta which are completely insensitive to such a very low order of energy shifts. In addition, they claim to have shown that the extra energy shift can be explained in the framework of the so-called YARK gravitational theory. They indeed claim that such a theory should replace the general theory of relativity (GTR) as the correct theory of gravity.     

Emission Mössbauer spectroscopy in the Co-O system under high oxygen pressure

Results obtained by means of the emission Mössbauer spectroscopy in CoO and Co₃O₄ systems kept under relatively high oxygen pressure (air close to the normal pressure) are reported. Broadening of the 14.4-keV ⁵⁷Fe Mössbauer line due to the diffusion of iron atoms in CoO has been investigated in the temperature range between 1275 and 1450 K. The excess of the linewidth caused by diffusive motions obeys the Arrhenius law with the activation energy of 1.86(8) eV. Mixed valence oxide Co₃O₄ was examined vs. temperature. Measurements were performed from the room temperature till 1073 K. Tetrahedrally coordinated iron impurities in the Co₃O₄ host lattice are in the high spin trivalent state despite that parent Co is divalent. The anharmonic contribution to the lattice vibrations at high temperatures was observed and analyzed for both CoO and Co₃O₄.