Single Crystal Absorbers Research Articles

M\xf6ssbauer measurements of GaFeO3 single crystal multiferroic

Mössbauer measurements on single crystal absorbers at room and at low temperatures were performed. The results are fully consistent with previously published reports by other groups. Spectra of single crystals were simultaneously analyzed including magnetic dipole and electric quadrupole interactions. The analysis shows that there is a small component of magnetic moments perpendicular to the magnetic easy axis. Mössbauer data seem not agree with commonly accepted ferrimagnetic structure of GaFeO3.

Determination of the electric field gradient and the magnetic field in M\xf6ssbauer spectroscopy

Problem of unambiguous determination of all components of the electric field gradient and hyperfine magnetic field in case of mixed hyperfine interaction was solved for the first time. Concept of a new method to determine orientation of hyperfine fields in the absorber Cartesian frame for Mössbauer spectroscopy for nuclear transitions between levels with spin 3/2 and 1/2 is presented. The method can be applied for single-crystal absorbers with well separated absorption lines in their spectra. Explicit formulas derived from velocity moments are presented. The new method allows full separation of the electric quadrupolar and magnetic dipolar hyperfine interactions by using unpolarized radiation.

A micro-Mössbauer study of chromites included in diamond and other mantle-related rocks

Oxygen fugacity (\( f_{{{\text{O}}_{ 2} }} \)) is a fundamental but little known intensive variable in mantle processes. It influences the P/T position of a mantle solidus and the composition of mantle-derived melts and fluids and constrains mantle-core equilibria and a number of geophysical properties of the mantle. An important source of information on oxidation states is the ferric–ferrous iron ratio in mantle spinels. Since the magnetite component is low in mantle spinels, normal analytical errors translate into considerable \( f_{{{\text{O}}_{ 2} }} \) uncertainties. In this study, we analyzed the Fe3+–Fetot ratio of chromites present as inclusions in diamond and other mantle-related occurrences by point-source Mossbauer spectroscopy using single-crystal absorbers as well as conventional Mossbauer spectroscopy using powder absorbers. The studied spinels have been previously analyzed by single-crystal X-ray diffraction and electron microprobe. The ferric–ferrous ratios found are normally similar to the different techniques apart from some samples where a large number of grains have been used for the analyses (powder absorbers). The general agreement between the different techniques allows us to conclude that the studied chromites are stoichiometric. However, conventional Mossbauer spectroscopy on powder absorbers should be conducted with great care, since the method requires a relatively large amount of sample material. Spinel frequently occurs as small grains, and the large number of crystals required may possess different degrees of oxidation/alteration and, consequently, different ferric–ferrous ratio leading to possible errors in the interpretation of the results.

Surface chemistry and electronics of semiconductor–nanosystem junctions I: metal-nanoemitter-based solar cells

Photoelectrochemically prepared nanotopographies on semiconductors are used for realization of nanoemitter solar devices that are active in the photovoltaic and the photoelectrocatalytic mode. The development of solar devices by a nonlinear electrochemical process and combined chemical/electrochemical metal deposition is described. Based on this low-temperature scalable approach, first efficiencies of 7.3% in the photovoltaic mode are reported for the photoelectrochemical solar cell n-Si/SiO2/Pt/I3 −–I−. With p-Si/Pt nanocomposite structures, light-induced H2 evolution is achieved. The surface chemistry and morphology is analyzed by photoelectron spectroscopy (PES), Fourier transform infrared spectroscopy, and high-resolution scanning electron microscopy. The operational principle of Pt-based nanoemitter solar devices that use silicon single crystal absorbers is analyzed by Mott–Schottky plots, chronoamperometric profiles, and PES. Related to simultaneous oxide formation during Pt deposition, evidence for the formation of a metal–oxide–semiconductor junction is obtained that explains the observed electronic behavior.

Neutron detection by superconducting tunnel junctions on a Li2B4O7 single-crystal absorber

We succeeded in detecting neutrons using superconducting tunnel junctions (STJs) fabricated on a single crystal of Li2B4O7. Neutrons are captured in the crystal by the nuclear reactions 6Li+n→T+alpha+4.78MeV and 10B+n→7Li+alpha+2.3MeV, which excite a large number of phonons in the substrate. The phonons propagate in the absorber and are measured by the STJs. We selected a single crystal of Li2B4O7 as the absorber material because of properties such as the large neutron cross-section of 6Li and 10B, low γ-ray sensitivity, short particle range in the substrate, and fast phonon velocity. Series-connected or multiple STJs on the crystal would enable two-dimensional neutron imaging with high detection efficiency, low γ-ray background, and a high spatial resolution of a few microns. In this paper, we demonstrate neutron detection by STJs and report the basic characteristics of the fabricated STJs, including their X-ray response and neutron detection. The correlation in pulse heights between two junctions located ∼1.3mm apart clearly indicates the possibility of neutron imaging.

On the theory of the polarization effects and its experimental test by angle-dependent Mössbauer spectroscopy with guanidinium nitroprusside (CN 3H 6) 2[Fe(CN) 5NO] (GNP) single crystals

The theory of the angle-dependent Mössbauer spectroscopy on nitroprusside-like single crystals has been complemented with expressions for the nuclear resonant cross-sections in case of the wave vector of the γ-radiation lying in the low symmetric non-mirror planes. They have been tested by rotation experiments in these planes using guanidinium nitroprusside (CN 3H 6) 2[Fe(CN) 5NO], (GNP) single crystal. As a result both the electric field gradient (EFG) and the mean square displacement (MSD) tensor parameters at the iron sites in GNP, have been determined at room temperature with high precision. The EFG asymmetry parameter is η=0.01(1) and the principal z-axes of the two EFG tensors are oriented at α=±8(1)° with respect to the c -axis in the mirror ac -plane. The corresponding orientation angle for the MSD tensor is ω=±11(4)° and lies also in the ac -plane. The crystal shows strong vibrational anisotropy with different Lamb–Mössbauer factors f LM ( i) for the principal crystal directions: f LM ( a) =0.118(3), f LM ( b) =0.174(2) and f LM ( c) =0.198(2). These values are in very good agreement with the results obtained by Mössbauer line broadening method and nuclear forward scattering of synchrotron radiation. In our analysis the saturation and polarization effects, which are very important for the case of single crystal absorbers with quadrupole split Mössbauer spectra and finite thickness, are taken into account. A total of 51 high quality spectra, recorded and calibrated with the sodium nitroprusside standard Na 2[Fe(CN) 5NO] · 2H 2O (SNP), have yielded for the quadrupole splitting the value Δ E Q=+1.854(1) mm/s and for the isomer shift relative to SNP the value IS=+0.006(1) mm/s (−0.251 mm/s relative to the α-Fe standard at room temperature). GNP has been confirmed as a very promising material for the investigation of long-lived metastable states observed in nitroprussides.

Electron-Phonon Coupling in Bolometers Used for Cryogenic Particle Detection

Superconducting transition-edge sensors have been used extensively in cryogenic particle detectors, either as thermometers for microcalorimetry or as bolometers for the detection of the prompt phonons resulting from a particle decay in a single crystal absorber. Bolometer action depends upon the energy coupling of the prompt phonons to the bolometer electrons. A study has been made of the electron-phonon coupling for a series of Au-Ti bolometers on a Si substrate and of the use of these bolometers for prompt phonon detection below 1 K. The electron-phonon coupling was found to be proportional to the normalized resistance (R/Rn) of the bolometer; R is the bolometer resistance and Rn is the normal resistance. When extrapolated to R/Rn = 1, this coupling was consistent with κ/VT3 = 3 × 109 W⋅m−3⋅K−4 where κ is the thermal conductance from the bolometer electrons to the Si phonons and V and T are the volume and transition temperature of the bolometer. The response of the bolometers to heat pulses generated by a thin film heater on the opposite face of a Si single crystal were similar to that generally seen above 1 K, apart from a delay time constant that varied from ∼0.5 to ∼1.3 µs as the transition temperature decreased from 600 to 200 mK. This delay time constant is attributed to the thermal equilibrium time of normal regions of the bolometer.

Series Array of Superconducting Tunnel Junctions as Phonon Sensor in a Dark Matter Detector

A design of a cryogenic detector for a WIMP dark matter search based on a single crystal absorber using Series Array of Superconducting Tunnel Junctions (SASTJs) as readout sensors has been presented. In order to investigate the behaviour of such a detector the response of two spatially separated Series Arrays of Superconducting Tunnel Junctions (SASTJs) to the non-equilibrium phonons generated in a single crystal of BaF2, by external α–particles has been measured. A strong position dependence in pulse height and pulse shape of the detector response has been observed. The fast time response of the SASTJs permits time resolved measurements of the spatially separated particle interactions in the crystal.

The design of a cryogenic dark matter detector based on the detection of the recoil direction of target nuclei

Abstract We discuss the design of a cryogenic detector for a WIMP dark matter search based on single crystal absorbers and using Series Arrays of Superconducting Tunnel Junctions (SASTJs). The distribution of recoil vectors of target nuclei from WIMP interactions are affected by the motion of the laboratory through the dark matter halo. The angular distribution of recoil directions is skewed due to the motion of the solar system around the galaxy and is modulated by the diurnal and annual rotation of the earth. We discuss the kinematics of the recoil events and how a directional signal might be identified in our cryogenic detectors using the fast response of SASTJs to the ballistic phonons arising in the absorber from WIMP interactions. We consider how the anisotropy of a dark matter recoil distribution can be used to place statistical limits on its component relative to the isotropic background signal. We also consider how the dark matter limit is altered if only the axis of the nuclear recoil, rather than the full recoil direction is available. We also briefly consider the effect of phonon focusing within single crystal absorbers. Focusing will modulate strongly the signal detected by the SASTJs, on the crystal surface, as the position of the interaction within the crystal varies. A comparison is made between the behaviour of phonons in strongly focusing crystals, such as Nb, Si and LiF, and their near isotropic propagation in BaF2.

A Mossbauer study of temperature-driven spin reorientations in Dy1-xHoxFeO3

Single crystals of the mixed rare-earth orthoferrite Dy1-xHoxFeO3 (x=0.25, 0.5, and 0.75) have been studied by Mossbauer spectroscopy in the 1.3-298 K temperature range. Several temperature-induced Fe3+ spin reorientation transitions have been observed. On cooling, a locally discontinuous reorientation from the a to the b orthorhombic axis took place near 40 K followed by a continuous reorientation from the b to the c axis. The onset temperature of the latter transition depends linearly on Ho concentration x. An x-T phase diagram of Dy1-xHoxFeO3 is proposed which extrapolates well to the results of previous studies of pure HoFeO3 and pure DyFeO3. The continuous Fe3+ spin reorientation in the bc plane showed the hyperfine interactions of the 57Fe nuclei to split into two branches. This branching is associated with different Fe unit cell positions. The temperature variation of the difference in magnetic hyperfine fields agrees well with a previous calculation of a similar effect in NMR. We also observed a new, very sharp Fe3+ spin reorientation from the c to the b orthorhombic axis upon cooling a Dy1-xHoxFeO3 single-crystal absorber between 3 K and 2.3 R. We suggest that this transition is driven by ordering of the rare-earth spins and a microscopic mechanism is proposed.

Mößbauer study of magnetic and structural phase transitions of a layered system

Mößauer measurements of hexane-diammonium-tetrachloro-ferrate over the temperature range 78–320 K for powdered and single-crystal absorbers show a transition to the antiferromagnetically ordered state at (105.0 ± 0.2) K. The center shift indicates an electronic ground state of 3d 64s 0.2. The quadrupole splitting implies that Fe(II) exists in a distorted octahedral field and that the ground state is a doublet, with negative electric field gradient. The recoilless fraction, the center shift and the quadrupole splitting reveal the existence of a structural phase change at (235 ± 5) K. The Debye temperature of the lower temperature phase is (166 ± 5) K aand (155 ± 3) K for the higher. Transition to the ordered state takes place between 105 and 110 K where an increase of the spin correlation time is found. A three-dimensional transition associated with a weak ferromagnetic interaction as a result of canting of the spins is observed at about 102 K. At 77 K the effective magnetic field strength was found to be (225 ± 3) kOe and to make an angle of 80° ± 3° with the principle axis of the electric field gradient.

A Mossbauer study of temperature-driven spin-reorientation transitions in TbFeO3

Reorientations of Fe3+ spins have been studied in TbFeO3 single-crystal absorbers by Mossbauer spectroscopy. At low temperature we observed two closely spaced changes of direction of the Fe3+ spins. Below 6.5 K the spins moved from the a towards the c orthorhombic axis with decreasing temperature. This movement was reversed at approximately 4.2 K and the movement back to the a axis was complete at 3.5 K. The maximum deviation from the a axis was about 55 degrees and somewhat dependent on crystal stress. These rotations are attributed to the temperature dependence of the Fe-Tb and Tb-Tb interactions.

A Mossbauer study of (NH4)2FeCl5.H2O

Mossbauer spectra of (NH4)2FeCl5.H2O have been measured over the temperature range 1.5-297 K for powder and various single-crystal absorbers. This antiferromagnet has shown complicated magnetic behaviour but, contrary to a previous suggestion, the spectra show that below the ordering temperature the Fe spins are aligned along the alpha axis. The antiferromagnetic spectra may be fitted with a small number of components coupled with relaxation broadening due to critical fluctuations. The data suggest two closely spaced ordering temperatures. Details of the hyperfine electrostatic interaction have been deduced from the paramagnetic spectra. No evidence was found for a reported structural phase change in the paramagnetic region.

On the Multipole Character of the X‐Ray Transitions in the Pre‐Edge Structure of Fe K Absorption Spectra. An Experimental Study

AbstractThe K absorption edge of pure Fe and the pre‐edge Fe K absorption of FeS2 (pyrite), Fe2O3 (hematite), and FeCO3 (siderite) are investigated with single crystal absorbers of these materials using linearly polarized synchrotron X‐radiation. From the angular dependence of the pre‐edge K absorption the existence of predominantly ligand‐field‐induced dipole transition in Fe2O3 and of quadrupole transitions in FeCO3 is concluded. The multipole character and the mechanism of the pre‐edge K absorption are discussed.

A155Gd Mossbauer study of a Gd2Fe14B single crystal

Mossbauer spectra with 155Gd were taken of a Gd2Fe14B single-crystal absorber. These spectra, which ave a more pronounced structure than those of powder absorber, lead to considerably improved precision in the hyperfine interaction parameter values and in estimates for the second-order crystal field parameters. A comparison between spectra taken in zero field and in a field 8 T shows the magnetic hyperfine field at Gd nuclei to be positive at both lattice sites.

Determination of electric-field—gradient orientation and asymmetry with Mössbauer absorption of linearly polarizedγradiation

Zory's method of electric-field---gradient orientation determination by using the area ratio of a two-line spectrum in the M\"ossbauer absorption, by $^{57}\mathrm{Fe}$, using unpolarized $\ensuremath{\gamma}$ radiation in a thin single-crystal absorber is extended to that of using linearly polarized $\ensuremath{\gamma}$ radiation. The method is applied to Fe${\mathrm{S}}_{2}$ pyrite and to marcasite, and some definite predictions on how the area ratio should vary with the directions of incidence as well as that of the plane of polarization are obtained.

Magnetic behaviour of NpAs 2 from Mössbauer spectroscopy

The Mössbauer hyperfine spectra of the 60 keV resonance of 237Np in powder and single crystal absorbers of NpAs 2 were measured between 4.2 and 60 K. Below 18 K a simple magnetic plus quadrupole pattern is seen in accordance with a ferromagnetic spin structure in tetragonal NpAs 2. The isomer shift favors the 4+ charge state, the hyperfine field of 288 T implies a moment of 1.5μ B at the Np ion. The large reduction compared to the free ion values points towards a strong mixing of the electronic ground state by crystalline field interactions. Above 18 K the spectrum changes into a complex hyperfine pattern indicating a sinusoidally modulated spin structure. Near 54 K a transition into the paramagnetic state is observed. Both magnetic transitions (18 and 54 K) exhibit a feature typical for a first-order character.

Magnetic ordering in the compounds RbFeCl 3·2H 2O and CsFeCl 3·2H 2O

Mössbauer experiments have been performed in the magnetically ordered phase of RbFeCl 3·2H 2O and CsFeCl 3·2H 2O. Magnetic ordering is observed in both compounds, which are reported to show low-dimensional interactions. The observed temperature dependence of the hyperfine field of RbFeCl 3·2H 2O fits well to the S′ = 1 2 rectangular Ising model, with exchange constants | J 1/ k| = 50±7 K and | J 2/ k| = 0.40±0.25 K, and T N = 12.08±0.06 K. The low-temperature orientation of the hyperfine field of RbFeCl 3·2H 2O has been determined by measurements on a single-crystal absorber in an applied field. The hyperfine field is oriented parallel to the a axis, which deviates slightly from the earlier reported magnetic moment direction at 16° to the a axis. This deviation is attributed to the dipolar contribution in the hyperfine field. The low value of the saturation hyperfine field (-7.5T for RbFeCl 3·2H 2O) and the relative importance of the dipolar contribution are discussed in relation to the crystal field effects of a tetragonally distorted octahedron.

Crystal field effects in the compounds RbFeCl 3·2H 2O and CsFeCl 3·2H 2O

Mössbauer experiments on the compounds RbFeCl 3·2H 2O and CsFeCl 3·2H 2O have been performed in the temperature region of 4.2 to 300 K. The temperature dependence of the center shift and the quadrupole splitting is reported. A qualitative analysis of these data indicates an orbital-doublet ground state for the octahedrally coordinated ferrous ion. This is confirmed by the results from experiments in an applied magnetic field. A quantitative analysis of the quadrupole splitting data, using the crystal field model, results in the assignment of a singlet 685±50 cm −1 above the orbital-doublet. The rhombic distortion will lift the degeneracy of the orbital ground state by a splitting of no more than 30 cm −. Single-crystal absorbers of RbFeCl 3·2H 2O were measured along the crystal axes, as well as in several directions within the bc plane, in order to determine the principal axes of the electric field gradient tensor (EFG). Additional experiments were performed in the paramagnetic region, by the application of an external magnetic field, and in the magnetically ordered region. Analysis of the spectra results in the assignment of the Cl IFeCl I axis as the maing axis of the EFG tensor.

Electric field gradient and mean-square displacement of the iron sites in FeS2(pyrite)

The electric-field-gradient tensor and the mean-square-displacement tensor (MSD) are considered for the four equivelent, but differently oriented, Fe sites in pyrite. Most of the parameters are obtained through symmetry considerations and a simple relation between the area ratio of the M\ossbauer absorption peaks and the orientation of a single-crystal absorber is derived, which can be used to determine the MSD anisotropy parameter.