William Duffy

Magnetic Susceptibilities of Crystalline Stable Free Radicals in the 77–293°K Temperature Range

The powder susceptibilities of crystalline stable free radical samples of picryl‐n‐amino carbazyl, Wursters blue, DPPH and 1,3‐bisdiphenylene‐2‐phenyl allyl have been measured as a function of temperature from 77–293°K. All exhibit negative paramagnetic Curie points with values in the range —51° to —6°K. For Wursters blue, the Neel temperature is found to be 186°K.

ACOUSTIC QUALITY FACTOR OF ALUMINUM ALLOYS FROM 50 MK TO 300 K

Measurements of the acoustic quality factor (reciprocal internal friction) of commercial aluminum alloys 2090, 5056, and 6061 in a torsional vibrational mode at a frequency of 1 kHz in the temperature range from 50 mK to 300 K are reported. The temperature dependence of the mode frequency, which is a measure of the shear modulus of the material, is also given. The quality factor (Q) of all alloys exhibited the highest values at the lowest measurement temperature. A heat‐treated sample of the 5056 alloy gave the highest Q of 6.5×107 at 53 mK. Maximum Q values of 8.9×106 in 2090 and 6.5×106 in 6061 were observed. Internal friction mechanisms which may explain the measured Q values are discussed. The results provide support for the planned construction and operation of very sensitive 50‐mK aluminum‐alloy bar gravity‐wave detectors.

Antiferromagnetic Linear Chains in the Crystalline Free Radical BDPA

The static magnetic susceptibility from 1.6 to 300°K and the heat capacity in fields of 0 and 5 kG from 0.9 to 10.2°K have been measured on polycrystalline samples of the complex of 1,3‐bisdiphenylene‐2‐phenylallyl with benzene, commonly abbreviated BDPA. The data are shown to correspond to antiferro‐magnetic Heisenberg linear chains characterized by a nearest‐neighbor exchange integral of −4.6°K. Weak interchain coupling results in a paramagnetic—antiferromagnetic phase transition at 1.695°K in zero field, as evidenced by the sharp heat capacity anomaly at that temperature. Resolution of the heat capacity into a lattice T3 contribution and a magnetic contribution yields a Debye temperature of 52.5°K and a magnetic entropy per mole of 101% of the expected Rln2.

Paramagnetism of Carbazyl and Hydrazyl Free Radicals

Static magnetic‐susceptibility measurements in the temperature range 1.4°—300°K are reported on powder samples of the stable aromatic free radicals N‐picryl−9‐aminocarbazyl (PAC), 2,2‐diphenyl−1‐picrylhydrazyl in the 106°C mp [DPPH(I)] and 137°C mp [DPPH(II)] crystal modifications, 2‐phenyl−2‐p‐nitrophenyl−1‐picrylhydrazyl (DHNO2·) and 2,2‐bis(p‐nitrophenyl)−1‐picrylhydrazyl [D(NO2)2·]. Radical concentrations in the range from 0.81 to 0.99 and Weiss constants in the range −36° to −5.2°K are determined. The measurements are interpreted assuming that the magnetic behavior is due to alternating antiferromagnetic exchange‐coupled linear chains. Exchange integrals are inferred from fitting theoretical curves to the susceptibility maxima. Evidence is presented for the presence of random dilution by diamagnetic sites in PAC.

Acoustic quality factor of molybdenum and tungsten at low temperatures

The amplitude‐independent acoustical mechanical quality factors Q of 99.95% pure polycrystalline molybdenum and tungsten have been measured as a function of temperature and heat treatment from 50 mK to 300 K. Data were obtained from 1 kHz torsional modes of resonators designed to minimize extraneous loss effects. Thermal annealing at temperatures above the recrystallization temperatures yielded strongly enhanced Q. Maximum Q values of 4.38×107 in Mo and 1.23×107 in W were observed in annealed material at the lowest measurement temperatures. Internal friction peaks and backgrounds are discussed. The temperature dependence of the resonator frequency is reported. Mo and W are compared with Al‐5056 as suitable materials for fabrication of cryogenic resonant‐bar gravity‐wave antennas.

Low temperature heat capacity and magnetic susceptibility of the DPPH–benzene complex

An experimental study of the heat capacity in fields of 0 and 50 kOe from 0.55 to 20 K and the powder magnetic susceptibility from 0.4 K to room temperature of the DPPH–benzene complex is reported. Magnetic and lattice contributions to the heat capacity are well resolved. The susceptibility and magnetic heat capacity are compared with pair, chain, and quadratic net Heisenberg model predictions, none of which give satisfactory agreement with experiment. Calculations based on McConnell’s spin density Hamiltonian indicate the likelihood that dominant intermolecular exchange coupling is of comparable magnitude to six near neighbors, in layers separated from adjacent layers by benzene solvent molecules.An experimental study of the heat capacity in fields of 0 and 50 kOe from 0.55 to 20 K and the powder magnetic susceptibility from 0.4 K to room temperature of the DPPH–benzene complex is reported. Magnetic and lattice contributions to the heat capacity are well resolved. The susceptibility and magnetic heat capacity are compared with pair, chain, and quadratic net Heisenberg model predictions, none of which give satisfactory agreement with experiment. Calculations based on McConnell’s spin density Hamiltonian indicate the likelihood that dominant intermolecular exchange coupling is of comparable magnitude to six near neighbors, in layers separated from adjacent layers by benzene solvent molecules.

Acoustic quality factor of copper, brass and beryllium copper from 50 mK to 300 K

Abstract The amplitude-independent background mechanical quality factors (reciprocal internal frictions) of oxygen-free electronic (OFE) copper (C 10100), free-turning brass (C36000) and beryllium copper (C17200) were measured in the temperature range 50 mk – 300 K. The data were obtained from measurements on 1 kHz torsional modes of mechanical resonators configurated to minimize extraneous internal friction effects. The resonant frequency dependence on temperatures was also obtained, giving the temperature dependence of the dynamic shear modulus. Strongly enhanced quality factors were obtained from appropriate heat treatment.

Electron Magnetic Resonance Spectrum and Magnetic Susceptibility of K4MnCl6 from 1.4° to 300°K

The electron magnetic resonance spectrum and magnetic susceptibility of polycrystalline K4MnCl6 have been investigated over the temperature range from 1.4° to 300°K. The susceptibility follows a Curie–Weiss behavior above 95°K with a Weiss constant θ of −7.8°K. Between 13° and 80°K, a Curie–Weiss behavior is still observed but with a θ value of −6.0°K, different than that observed in the higher temperature range. Coinciding with the susceptibility change is a resonance linewidth change from 23 G above 90°K to approximately 175 G below 70°K. The origin of these changes is not known. No evidence for a transition to a magnetically ordered state has been observed, even down to 1.4°K. Comparison of experimental and theoretical resonance linewidths suggest that the dominant superexchange interaction involves next‐nearest‐neighbor Mn2+ ions. This interaction is conveyed by two intervening Cl− ions. The exchange integral for the next‐nearest‐neighbor interaction is estimated to be 0.22°K. Results suggest that there is approximately a 0.8% probability for transfer of an electron from a Cl− ion to a Mn2+ ion.

ACOUSTIC QUALITY FACTOR OF NIOBIUM AND VANADIUM AT LOW TEMPERATURES

The amplitude‐independent acoustical mechanical quality factors Q of polycrystalline commercially prepared samples of 99.8% pure niobium and 99.75% pure vanadium have been measured as a function of temperature and heat treatment from 50 mK to 300 K. Data were obtained from 1 kHz torsional modes of resonators designed to minimize extraneous loss effects. Thermal annealing at temperatures above the recrystallization temperatures yielded strongly enhanced Q. Maximum Q values of 4.24×107 in Nb and 3.91×107 in V were observed in annealed material at the lowest measurement temperatures. Internal friction peaks are interpreted in terms of mechanisms identified from previous studies of predeformed and hydrogen‐charged materials. Intergrain thermal diffusion is shown to account for a significant portion of the background acoustic loss from 60 to 300 K. Relative shear moduli, obtained from the square of the resonator frequency, are given from 50 mK to 300 K. High‐resolution frequency measurements at the lowest temp...

Acoustic quality factor of aluminium and selected aluminium alloys from 50 mK to 300 K

Abstract The amplitude-independent quality factors ( Q s) of polycrystalline aluminium of 99.999 wt% purity, and the commercial aluminium alloys 2024, 6061 and 7075 have been measured in the temperature range 50 mK–300 K. Data were obtained at about 1 kHz from a torsional mode of machined monolithic mechanical resonators. The as-machined alloys exhibited maximum Q s of 1.5 to 9.5 million. The annealed alloys exhibited maximum Q s from 4.3 to 7.4 million. After several weeks at room temperature, substantially higher Q s were observed in all of the annealed materials, as compared to values obtained within a few days subsequent to the thermal treatment. Nevertheless, the maximum Q s were appreciably lower than those which have been measured in aluminium–magnesium alloys. Three loss peaks were observed in the aluminium, and a single peak in two of the alloys, even after annealing. The temperature dependence of the shear moduli was also obtained. This study is part of the continuing effort in this laboratory to evaluate the acoustic loss of high- Q materials at low temperatures, of particular interest for the construction of sensitive detectors of gravitational radiation.