C. E. Violet

Mössbauer Line Positions and Hyperfine Interactions in α Iron

The 57Fe Mossbauer spectrum of NBS standard reference material 1541 α‐iron foil has been determined at 298±2 and 4.2±0.1°K, using a palladium source at 298±2°K. Line positions at 298 and 4.3°K were determined for the calibration of Mossbauer spectrometers. The magnetic splitting, in mm/sec, of the 57Fe ground state and first excited state are g0=(3.9098±0.0008), g1=(2.2342±0.0008) at 298°K and g0=(4.0117±0.0010), g1=(2.2931±0.0010) at 4.3°K. The center shift is − (0.1748±0.0008) mm/sec at 298°K and − (0.0556±0.0012) mm/sec at 4.3°K; the temperature shift is − (0.1192±0.0014) mm/sec. The hyperfine magnetic field is − (330.4±0.3) kOe at 298°K and − (339.0±0.3) kOe at 4.3°K. The 57Fe magnetic moment ratio is μ1/μ0=−1.7145±0.0005. The magnetic moment of the first excited state is μ1=−0.1547±0.0001 nm. The hyperfine magnetic field ratio is H(298°K)/H(4.3°K)=0.9746±0.0001. The electric quadrupole effect in terms of e, the shift of the ±32 levels, is 4e=+0.0023±0.0015 mm/sec at 298°K and 4e=+0.0088±0.0025 mm/sec...

Magnetic behavior of Ho1Ba2(Cu1−xFex)3O7−y

We have carried out dc magnetization (x=0 and 0.05) and 57Fe Mossbauer spectroscopy (x=0.05) measurements of Ho1Ba2(Cu1−x Fex)3O7−y , between room temperature (RT) and 4.2 K. Based on x‐ray diffraction results all samples are single phase, and either orthorhombic (x=0) or tegragonal (x=0.05). Magnetization measurements determined the onset Tc to be either 90 K (orthorhombic) or 48 K (tetragonal). Mossbauer spectra between RT and 20 K are nearly identical, each consisting of three superposed quadrupole doublets. As the temperature is lowered below about 10 K the Mossbauer spectra exhibit paramagnetic relaxation. The relaxation time increases by about two orders of magnitude as the temperature decreases from 10 to 4.2 K. No magnetic ordering of the Fe moments was observed.

Metallurgical explanation of ‘‘double magnetic transitions’’ and allied phenomena in AuFe alloys

Evidence for two magnetic ordering temperatures in AuFe alloys (0.1<XFe<0.28) is given by ac and static susceptibility measurements and Mossbauer experiments. These data have given rise to a variety of qualitative to semiquantitative theories based upon fluctuations in the random distribution of Fe atoms. Our explanation of the two ordering temperatures is based upon the precipitation of metastable Fe‐enriched regions which arise via a decomposition and which magnetically couple to the spin‐glass matrix. The high temperature transition denotes the onset of ferromagnetic order within the ‘‘precipitate’’ whose field is then coupled to the paramagnetic matrix. When the temperature of the system is decreased below the spin‐glass ordering temperature, the apparent high temperature ferromagnetic character is destroyed, attesting to the dominance of the random spin‐glass structure and the strength of the coupling between the two phases. Hence, it is the temperature dependence of the coupling between distinguisha...

Anisotropic production of laser‐plasma soft x rays and fast electrons from thin plastic targets

In separate experiments incident pulses from two different Nd‐glass lasers and a CO2 laser were fired at planar targets of thin plastic, and the angular distributions of the soft x rays and fast electrons were measured. The focused power densities of the first Nd‐glass laser and the CO2 laser were 2×1012 W/mm2, of the second Nd‐glass laser, 2×1013 W/mm2. The electron efficiency of the CO2 laser plasmas is strongly peaked antiparallel to the incident pulse, the backward‐to‐forward ratio being 100. The electron efficiency of both Nd‐glass lasers is less than 150% of their x‐ray efficiency. The x‐ray efficiencies of the Nd‐glass laser plasmas are at least an order of magnitude greater than that of the CO2 laser plasmas. The x‐ray efficiency of the first Nd‐glass laser was isotropic. The x‐ray distribution for both the second Nd‐glass laser and the CO2 laser plasmas had the backward‐to‐forward ratio of 4.

Magnetic behavior of Y‐Ba‐Cu‐O superconductors prepared under different oxygen anneals

In our study of three YBa2 Cu3 oxides prepared with different oxygen anneals, we observed that the temperature derivative of the magnetization or susceptibility χ(T) of these high‐Tc superconductors has a maximum at approximately the transition temperature, Tc . This temperature as estimated by the peak value of (∂χ/∂T)H , ranged between 55 and 65 K. The most oxygen‐poor sample showed the lowest Tc and the most oxygen‐rich sample showed the highest Tc . In the subcritical region, a novel irreversibility is observed in the field‐cooled warm‐up and cool‐down behaviors.

ac susceptibility of thermally annealed and neutron irradiated Cu-Ni alloys

The effect of thermal annealing and high flux neutron irradiation on the ac susceptibility of Cu‐Ni alloys of composition 40, 50, and 60 at. % Ni have been studied. In this compositional region the short‐range order and cluster changes introduced by the annealing and irradiation can greatly effect the magnetic state of the Cu‐Ni system. Neutron diffraction studies on the Cu‐Ni system have indicated the possibility of a miscibility gap forming in this system. The ac susceptibility of samples quenched from high temperature (900 °C), annealed at low temperature (350 °C), or irradiated at the LLNL, RTNS‐II 14‐MeV neutron source were measured as a function of temperature. The quenched 60 at. % Ni alloy exhibited a ferromagnetic transition near 195 °K that remained unchanged under neutron irradiation. The low‐temperature annealed sample gave a transition occurring over a wider temperature range. The quenched 50 at. % Ni alloy exhibits a ferromagnetic transition at a lower temperature with a χ(T) curve that is p...

Determining Polar Angles from Mössbauer Intensity Ratios

Polar angles cannot be determined in general from intensity ratios of Mossbauer hyperfine lines. However, conditions can be found that specify the polar angle as either perpendicular or parallel to the Mossbauer photon.

Temperature hysteresis and relaxation effects in amorphous Fe‐Ni‐Zr alloys (abstract)

Amorphous Fe‐Ni‐Zr alloys in the ‘‘as quenched’’ and irradiated state have been studied with 57Fe Mossbauer spectroscopy and ac and dc susceptibility. Samples were irradiated by 14‐MeV neutrons at room temperature to fluences of ≊1017 ns/cm2 corresponding to a dpa of 3×10−4 at the Lawrence Livermore National Laboratory RTNS‐II facility. Two samples (Fe89.7Ni0.3Zr10 and Fe70Ni20Zr10) ‘‘as quenched’’ and irradiated, have been studied between room temperature and ≊4 K. ac susceptibility shows reentrant magnetic behavior for both alloys. Temperature hysteresis (no applied fields) is observed initially in the Mossbauer spectra for both alloys, ‘‘as quenched’’ and irradiated. It decreases and virtually disappears after a few thermal cycles between 20 K and room temperature. Relaxation effects are present in the magnetic hyperfine spectra of both alloys, ‘‘as quenched’’ and irradiated. The usually assumed proportionality between the magnetic hyperfine splitting and the magnetization is probably not valid for the...

151Eu Mössbauer study of the high‐Tc superconductor Bi2Ca0.5Eu0.5Sr2Cu2Ox (abstract)

Isomer shift measurements show the Eu to be trivalent and nonmagnetic with no apparent anomaly between measurements made above and below the transition temperature (Tc=87 K). The value of the isomer shift at room temperature (0.69±0.02 mm/s) falls within the values observed by previous researchers on other types of Eu‐based high‐Tc superconductors. A discussion on the isomer shift and f factor as a function of temperature will be reported and compared with previous results found in Eu‐based high‐Tc superconductors.1–3