Nuclear Resonant Forward Scattering Research Articles

Electronic and magnetic phase diagram of superconductors, SmFeAsO1−xFx

A crystallographic and magnetic phase diagram of SmFeAsO1−xFx is determined as a function of x in terms of temperature based on electrical transport and magnetization, synchrotron powder x-ray diffraction, 57Fe Mössbauer spectra (MS), and 149Sm nuclear resonant forward scattering (NRFS) measurements. MS revealed that the magnetic moments of Fe were aligned antiferromagnetically at ∼144 K (TN(Fe)). The magnetic moment of Fe (MFe) is estimated to be 0.34 μB/Fe at 4.2 K for undoped SmFeAsO; MFe is quenched in superconducting F-doped SmFeAsO. 149Sm NRFS spectra revealed that the magnetic moments of Sm start to order antiferromagnetically at 5.6 K (undoped) and 4.4 K (TN(Sm)) (x=0.069). Results clearly indicate that the antiferromagnetic (AF) Sm sublattice coexists with the superconducting phase in SmFeAsO1−xFx below TN(Sm), while the AF Fe sublattice does not coexist with the superconducting phase.

Coexistence of superconductivity and antiferromagnetic ordering in the layered superconductor SmFePO

Superconductivity and magnetic transition in SmFePO were examined by electric, magnetic, $^{57}\text{F}\text{e}$ M\"ossbauer, and $^{149}\text{S}\text{m}$ nuclear resonant forward-scattering (NRFS) measurements. Zero resistivity and negative magnetic susceptibility were observed in an undoped SmFePO, indicating superconducting transition temperature $({T}_{c})$ of $\ensuremath{\sim}3\text{ }\text{K}$. ${T}_{c}$ in undoped SmFePO appears in a narrow region of lattice constants with an optimum conductivity. Furthermore, in relation to magnetic susceptibility, NRFS spectra verify that magnetic moments of ${\text{Sm}}^{3+}$ ions order antiferromagnetically below magnetic transition $\ensuremath{\sim}5\text{ }\text{K}$, accompanying a discontinuous drop in electrical resistivity, whereas $^{57}\text{F}\text{e}$ M\"ossbauer spectra show that magnetic moments of Fe are quenched. These results indicate the coexistence of superconductivity and magnetic ordering in a crystallographic unit cell of SmFePO.

Pressure-induced electron spin transition in the paramagnetic phase of the GdFe3(BO3)4 Heisenberg magnet

The HS → LS spin crossover effect (high-spin → low-spin transition) induced by high pressure in the range 45–53 GPa is observed in trivalent Fe3+ ions in the paramagnetic phase of a Gd57Fe3(BO3)4 gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K β high-resolution x-ray emission spectroscopy (XES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.

57Fe Nuclear Resonant Forward Scattering of FeS under Pressure and with Magnetic Fields Using Synchrotron Radiation

⁵⁷Fe Nuclear Resonant Forward Scattering of FeS under Pressure and with Magnetic Fields Using Synchrotron Radiation

Dynamic structural disorder of the FeO2 moiety in oxymyoglobin studied by nuclear resonant forward scattering of synchrotron radiation.

Oxy- as well as deoxymyoglobin exhibit a pronounced temperature dependence of the quadrupole splitting of the heme iron as detected by conventional Mössbauer spectroscopy. With nuclear resonant forward scattering (NFS) of synchrotron radiation, which can be viewed as Mössbauer spectroscopy in the time domain, it is shown that this spectroscopic behavior, although it is phenomenologically similar in the two cases, is based on completely different physical mechanisms. It is demonstrated that stochastic fluctuations of the iron electric field gradient in MbO(2), which are due to the dynamic structural disorder of the FeO(2) moiety, are the reason for the temperature-dependent alterations of the coherent quantum beat pattern in the NFS spectra of MbO(2), in contrast to deoxyMb where transitions between orbital states of iron take place. This subtle spectroscopic difference cannot be inferred from conventional Mössbauer spectroscopy.

Nuclear Resonant Inelastic and Forward Scattering of Synchrotron Radiation by 40K

We achieved excitation of the first excited state of 40K and confirmed both energy and lifetime. Furthermore, we observed nuclear resonant inelastic scattering by 40K in a powdered KCl sample at room temperature using a high-resolution monochromator. The time spectrum of the nuclear resonant forward scattering was measured at 50 K. Our observations of nuclear resonant inelastic and forward scattering by 40K make electronic and dynamic studies for potassium practical. The measurements of nuclear resonant scattering for the radioactive 40K nuclide will enable and lead to further studies of other radioactive nuclides.

Mössbauer spectroscopy in the time domain applied to the study of single‐crystalline guanidinium nitroprusside

Guanidinium nitroprusside GNP, (CN3H6)2[Fe(CN)5NO] has been investigated in single‐crystalline form by nuclear resonant forward scattering (NFS) using synchrotron radiation (Mossbauer spectroscopy in the time domain). This method provides a direct measure of effective absorber thickness and therefore also of the Lamb–Mossbauer factor fLM. GNP has the advantage that all [Fe(CN5)NO]2- anions are practically aligned within the crystal. For the two different crystal orientations, with the crystallographic a- and c-direction parallel to the synchrotron beam, respectively, we have obtained fLM(a)= 0.122(10) and fLM(c)= 0.206(10), i.e., GNP exhibits significant anisotropic vibrational behavior. The quantum beat pattern of the NFS spectra obtained for the two different crystal orientations is discussed on the basis of radiation characteristics of the polarized synchrotron beam and the multipole transitions of oriented 57Fe nuclei.

Mössbauer spectroscopy with synchrotron radiation: a new technique entering biological inorganic chemistry

Nuclear resonant forward scattering (NFS) of synchrotron radiation was employed as Mössbauer spectroscopy in the time domain only recently, while conventional Mössbauer spectroscopy in the energy domain is widely applied since its discovery in 1958. Experimental setup and theoretical background required for NFS are explained, and examples are given for detecting quadrupole splitting, isomer shift, thickness effect and magnetic hyperfine interaction. Nuclear inelastic scattering provides the possibility to detect molecular vibrations.

Relaxation as seen by nuclei: Comparison between conventional Mössbauer spectra and nuclear resonant scattering of synchrotron radiation

The mechanism of spin‐lattice relaxation has been investigated in the “picket‐fence” porphyrin [Fe(CH3COO)(TPpivP)]-, a high‐spin iron(II) complex with unusual large quadrupole splitting of 4.25 mm s-1, by conventional Mossbauer spectroscopy as well as by nuclear resonant forward scattering (NFS). Superparamagnetism with a blocking temperature of about 8 K is observable by both methods in the spectra of bacterioferritin from S. olivaceus. From these two examples general conclusions about the merits of both methods can be drawn.

Iron porphyrins reinvestigated by a new method: Mössbauer spectroscopy using synchrotron radiation

Nuclear resonant forward scattering (NFS) of synchrotron radiation re- presents Mossbauer spectroscopy in the time domain. This new technique com- plements the conventional nuclear resonance absorption, e.g. Mossbauer spectro- scopy in the energy domain, by supplying highly brilliant, polarized, collimated and timed radiation. In NFS the hyperfine interaction of coherently excited nuclei manifests itself as quantum beats, i.e. as modulation of the time-dependent inten- sity of the transmitted radiation, which is delayed with respect to the incoming synchrotron pulse. We have investigated 57Fe-enriched iron porphyrins to test NFS for first biophysical applications. NFS spectra of the diamagnetic porphyrin Fe02(SC6HF4)(TPpivP) and of the paramagnetic porphyrin (Fe(CH3COO)(TPpiVP))- were recorded at various temperatures, with and without reference scatterer, with and without applied field , Dynamic molecular properties, e.g. dynamic structural disorder or spin-lattice relaxation document as variation of the time-delayed count rate. Measured NFS spectra were analysed theoretically by programs which are basically the analogue in the time domain compared to the usual calculations in the energy domain.