Albert Narath

X‐Ray Diffraction and Nuclear Quadrupole Resonance Studies of Chromium Trichloride

The infinite‐layer compound CrCl3 is found to undergo a first‐order phase transformation near 240°K. The crystal structures of the high‐ and low‐temperature phases have been elucidated by detailed x‐ray diffraction and 35Cl nuclear quadrupole resonance studies. The previously reported structure for CrCl3 is shown to be incorrect. The present single‐crystal diffraction results at 298° and 225°K give monoclinic (C2/m: a0=5.959 A, b0=10.321, c0=6.114 A, β=108.49°) and rhombohedral (R3: a0=5.942 A, c0=17.333 A) structures, respectively. The 35Cl quadrupole resonance frequencies, measured in polycrystalline samples, have been compared with results of field‐gradient calculations based on the ionic point‐charge model. The ionic contribution accounts for a large fraction of the total gradient making it impossible to evaluate the covalency parameters of the Cr–Cl bonds from the measured quadrupole coupling constants.

Nuclear Magnetic Resonance in Palladium‐Silver Alloys

The 109Ag NMR has been studied in a series of Pd1−xAgx alloys by spin‐echo techniques in external fields near 55 kOe and at temperatures of 1°–4°K. Knight shifts (K), spin‐lattice (T1), and spin‐spin (T2) relaxation times were determined for 12 compositions in the range 0.025≤x≤1. The shifts are negative for x≤0.35. From the observed lineshape in the x = 0.025 alloy (K = −1.41%) it is estimated that K = −1.46% in the limit of infinite dilution. After adjustment for an estimated direct contact shift of +0.34%, this value yields a hyperfine field of −140(±10) kOe/μB of palladium d‐spin magnetization, which presumably arises from spin polarization of the s conduction electrons by the palladium host. This process is found to be ineffective as a spin‐lattice relaxation mechanism. The observed relaxation times T1T increase gradually from a value of 9.6(±0.5) sec·°K for pure silver to a broad maximum of 26(±3) sec·°K near x = 0.3 and remain nearly constant at lower concentrations [T1T = 22(±4) sec·°K for x = 0.0...

Effect of Hydrostatic Pressure on the Metamagnetic Transitions in FeCl2·2H2O, CoCl2·2H2O, FeCl2, and FeBr2

The pressure derivatives of the two interchain exchange constants in FeCl2·2H2O and CoCl2·2H2O, and of the interlayer exchange constant in FeCl2 and FeBr2 have been obtained from measurements of the respective metamagnetic transition fields in the temperature range 1°–4°K. Hydrostatic pressure to 4 kbar were generated by isobaric freezing of 4He. The antiferromagnetic exchange constants in these compounds exhibit large positive pressure derivatives which range from 2% to 9% per kbar. The magnitudes of the average interlayer exchange energies in the hexagonal layer compounds FeCl2 and FeBr2 appear to be strongly influenced by crystal imperfections such as stacking faults. In FeCl2 a polymorphic transformation (presumably between CdCl2 and CdI2 structure types) is observed near 2 kbar, which is accompanied by a 12% increase in the transition field.The pressure derivatives of the two interchain exchange constants in FeCl2·2H2O and CoCl2·2H2O, and of the interlayer exchange constant in FeCl2 and FeBr2 have been obtained from measurements of the respective metamagnetic transition fields in the temperature range 1°–4°K. Hydrostatic pressure to 4 kbar were generated by isobaric freezing of 4He. The antiferromagnetic exchange constants in these compounds exhibit large positive pressure derivatives which range from 2% to 9% per kbar. The magnitudes of the average interlayer exchange energies in the hexagonal layer compounds FeCl2 and FeBr2 appear to be strongly influenced by crystal imperfections such as stacking faults. In FeCl2 a polymorphic transformation (presumably between CdCl2 and CdI2 structure types) is observed near 2 kbar, which is accompanied by a 12% increase in the transition field.

Anomaly in the NMR shifts of P and As in CeP and CeAs

Abstract The NMR frequency shifts for 31 P and 75 As in the compounds CeP and CeAs have been measured between the Neel point and 200°K. Between 10 and 60°K the ratio of resonance shift to bulk magnetic susceptibility, expected to be independent of temperature, was found to increase with increasing temperature by approximately a factor of two. Possible causes are temperature dependent or non-linear hyperfine couplings.

Neutron Diffraction Determination of the Intermediate‐Field Magnetic Structure of CoCl2·2H2O

Recent magnetization measurements have shown that antiferromagnetic CoCl2·2H2O undergoes two metamagnetic transitions when a magnetic field is applied along the monoclinic b axis. At 4.0°K and 32 kOe there is an abrupt increase in magnetization to 1 μB per Co2+ ion followed at 46 kOe by a second increase to 3 μB per Co2+ ion, which is believed to correspond to the ferromagnetic state. Neutron diffraction data have been obtained with fields of up to 46 kOe applied along the b axis of a single crystal mounted within a split‐coil superconducting magnet. At the first transition, magnetic reflections characteristic of the antiferromagnetic phase disappear, and a number of new reflections, based upon a unit cell in which the a axis is tripled, are observed. These are in good agreement with a simple collinear ferrimagnetic model recently proposed in which the moments are parallel within (100) planes, which in turn are ordered in the sequence −++−++ in the a direction. The observed moment is about 3.3 μB per Co2+...

Magnetic Susceptibility of Dilute Pd(Rh) and Pd(Ag) Alloys

High‐resolution measurements of magnetic susceptibility χ of dilute alloys of Pd1−aRha (a = 0.001, 0.075, 0.015, 0.03, and 0.07) and Pd1−aAga (a = 0.001, 0.005, 0.012, 0.023, and 0.025) are presented for temperatures from 4.2° to 300°K and for fields up to 55 kG. The general features show a broad maximum of χ(4.2°K) between 0.03 Rh and 0.07 Rh for Pd(Rh), then a rapid decrease in χ(4.2°K) to the value of χ for pure Pd, and a further decrease of χ with increasing Ag content. Small effects of Fe impurities are noted and briefly discussed. The very rapid variation of χ versus Rh content is consistent with earlier measurements. The maximum of χ(T), observed at about 80°K for pure Pd, moves toward 0°K as Rh content increases to 0.03, but the maximum in χ(T) is almost independent of increasing Ag content. The measured susceptibility and electronic specific heat are used to estimate the Stoner exchange enhancement in Pd and Pd0.95Rh0.05 and the results are in reasonable agreement with independent high‐field resu...

Nuclear Magnetic Resonance Studies of Magnetic Impurities in Metals

The application of nuclear magnetic resonance techniques to the study of magnetic impurity states in metals is reviewed. Two types of experimental information are considered. The first concerns resonance shifts and spin‐lattice relaxation rates of isolated impurities, while the second concerns the effects of impurity‐impurity interactions on these quantities. The theory of exchange‐enhanced hyperfine interactions in metals is discussed for both elemental metals as well as impurities in metals. It is shown that the available low‐temperature impurity data, which span a large range of 0°K susceptibilities, can be accounted for by treating the intra‐atomic Coulomb and exchange interactions in the Hartree‐Fock approximation. This even applies to two alloys, Au(V) and Mo(Co), which have previously been treated as Kondo systems. The present results therefore appear to remove the distinction between magnetic and nonmagnetic regimes of the magnetic impurity problem.

The impurity susceptibility of dilute AuV alloys: 51V NMR studies☆

Abstract Measurements of the 51V NMR shift in Au V are reported for the temperature range 4–540°K. The results, which can be separated into a temperature-dependent spin contribution and a temperature-independent orbital contribution, are in general agreement with the bulk susceptibility measurements of Van Dam and Gubbens.

Low Frequency Superregenerative Oscillator Design for the Detection of Broad‐Line Nuclear Magnetic Resonances

Two superregenerative nuclear magnetic resonance detectors are described which have proved useful for the detection of broad resonances. These oscillators are capable of sensitive operation at radio frequencies as low as 2.5 Mc and quench frequencies ranging from about 50 kc near 3 Mc to 120 kc at 10 Mc. In the one case for which a comparison was made, the sensitivity of the superregenerative oscillator design was found to be comparable to that of a crossed‐coil spectrometer operating under optimum conditions.

A phase‐coherent, extended‐frequency‐range, high‐power rf transmitter system for pulsed nuclear magnetic resonance

A high‐power transmitter for pulsed nuclear magnetic resonance experiments in the frequency range 2–200 MHz is described. Frequencies below 40 MHz are provided by a multistage, 15 kW (peak rms power), variable‐tuned, rf amplifier; the power output can be increased to 100 kW by means of a single‐stage booster. Frequencies between 40 and 200 MHz are provided by a three‐stage 20 kW frequency multiplier which can be tuned to either triple or quintuple the output of the low‐frequency amplifier. Extension of the high‐power multiplication scheme to higher frequencies is discussed. Finally, a versatile spectrometer system is described which illustrates the practical utilization of the transmitter.