Stuart L. Hutton

Microwave spectroscopy and magnetization measurements of flux trapping and hysteresis in YBa2Cu3O7−δ

Microwave spectroscopic measurements at 9.2 GHz on a superconducting sample of YBa2Cu3O7−δ have established the occurrence of flux trapping and hysteresis associated with a low‐field signal. The position of the signal peak is related to the field at which an abrupt change in the slope of the magnetization curve is observed. The data are consistent with a superconducting glass model.

Magnetic susceptibility and magnetic resonance in the ordered state of single‐crystal [NH3(CH2)2HN3]CuBr4

We report here the single‐crystal magnetic susceptibility data and the magnetic resonance behavior in the ordered state of (NH3(CH2)2NH3)CuBr4, which is a strongly coupled, pseudo‐1D, Heisenberg antiferromagnet with strong in‐plane ferromagnetic coupling. The Neel temperature is 58 K and appears to be sample‐dependent. The observed resonances occur near g=2 with small anisotropies and suggest hidden spin canting with a second magnetic phase transition into a different ordered state at 20 K.

Temperature dependence of the zero‐field splitting of Ni2+ in ZnSiF6 ⋅ 6H2O and ZnSiF6 ⋅ 6D2O at low temperatures

EPR measurements at 9.25 GHz were made on single crystals of Ni2+ in ZnSiF6 ⋅ 6H2O and ZnSiF6 ⋅ 6D2O at small temperature intervals from 4 K to above 100 K. Least‐squares fits of the zero‐field splitting data below 70 K for the two crystals to a theoretical model of Shrivastava, based on the interaction in the long wavelength approximation, yielded values for the Debye temperature θD of 130 K for ZnSiF6 ⋅ 6H2O and 99 K for ZnSiF6 ⋅ 6D2O. The value of θD for the hydrated salt is roughly 30% larger than the more direct estimate of Vasyukov et al. from measurements of the stress coefficients. The deviations from the theoretical curves for both crystals are appreciable above 75 K.

Magnetic properties of the copper trimer chains 4‐picolinium2 Cu3Cl8 and 4‐picolinium2 Cu3Br8

(4‐pic)2Cu3Cl8 and (4‐pic)2Cu3Br8 contain planar, bibridged copper halide trimers. The trimers are then stacked with longer semicoordinate Cu–halide–Cu bonds between adjacent trimers. The magnetic susceptibilities show that trimers are in a spin‐1/2 ground state at low temperatures with coupling constants J/k=30 and −100 K, respectively, determined by fitting the data to a new high‐temperature expansion with only exchange constants and external field as expansion parameters. We show that even though the spin‐1/2 trimers are antiferromagnetically exchange coupled along the chain, they order ferromagnetically at low temperature providing examples of pseudo‐one‐dimensional, S=1/2, Heisenberg ferromagnets. Electron paramagnetic resonance measurements at the X band were also performed and confirm the susceptibility measurements.

Temperature dependence of the EPR zero‐field splitting of Mn2+ in ZnSiF6⋅6H2O

The temperature dependence of the EPR spectrum of Mn2+ in ZnSiF6⋅6H2O was measured at 9.2 GHz between 15 and 297 K. Both the g and A tensors were found to be isotropic with values close to 2.000 and −90×10−4 cm−1, respectively. A slight decrease in the magnitude of A with increasing temperature was observed. The zero‐field splitting parameters D and a–F were found to vary from −130.5×10−4 and 9.9×10−4 cm−1, respectively, at 15 K to −170.2×10−4 and 7.8×10−4 cm−1, respectively, at 297 K. The data for D below 100 K were fitted to the orbit–lattice interaction in the long‐wavelength approximation with a Debye temperature of 138 K. An estimate of the low‐temperature ratio of D for Ni2+ and Mn2+ in ZnSiF6⋅6H2O based on the long‐wavelength fits was in good agreement with the measured ratio of 10 at 4 K.

Measurement of Debye temperature from the zero‐field splitting of Ni2+ in ZnSiF6⋅6H2O

The variation of the zero‐field splitting parameter D for Ni2+ in ZnSiF6⋅6H2O between 4 and 250 K has been used to calculate the temperature dependence of the Debye temperature θD(T) in ZnSiF6⋅6H2O. θD(T) was determined to be 143 K at a temperature of 30 K and 127 K at 250 K. There was a pronounced minimum of 105 K in θD(T) near 60 K. The behavior of θD(T) is qualitatively similar to that found for alkali halides from specific heat data.

Phase transitions in ZnTiF6•6H2O observed by the electron paramagnetic resonance of Mn2+

The temperature dependence and hysteresis of the trigonal–monoclinic phase transition occurring near 180 K in ZnTiF6⋅6H2O were studied as functions of Mn concentration, and spin–Hamiltonian parameters were determined in both the trigonal and monoclinic phases. Changes in the 9 GHz electron paramagnetic resonance (EPR) spectrum between 30 and 10 K indicated a first order phase transition on cooling to a more disordered structure. Additional lines showing ligand hyperfine structure, with an even number of components, are attributed to paramagnetic impurities occupying Ti4+ sites with single F− vacancies.

Temperature dependence of the g value of Ni2+ in ZnSiF6⋅6H2O

The measured increase in g∥ between 4 and 100 K for Ni2+ in ZnSiF6⋅H2O has been explained as a consequence of the dynamical phonon coupling between the spins and the lattice. The Debye temperature of θ=130 K previously determined from the zero‐field splitting has been found to agree with that obtained from the temperature dependence of the g value. The zero‐point vibrational correction to the g value has been deduced.

Electron paramagnetic resonance of Mn2+ in FeSiF6⋅6H2O and FeSiF6⋅6D2O above 100 K

In electron paramagnetic resonance measurements on Mn2+ in single crystals of FeSiF6⋅6H2O and FeSiF6⋅6D2O at 9.3 GHz, changes in the resolution of the outer sets of hyperfine lines were used to observe the trigonal to monoclinic structural phase transition, which occurred over the range 223–227 K in both crystals. The magnitude of the axial spin‐Hamiltonian parameter D for FeSiF6⋅6D2O was (273±1)×10−4 cm−1 at 300 K and decreased slightly with decreasing temperature in the trigonal phase. In the monoclinic phase ‖D‖ was (263±1)×10−4 cm−1 at 220 K and increased slightly with decreasing temperature. Similar values were obtained for FeSiF6⋅6H2O. An abrupt linewidth increase on warming through the transition is postulated to result from a decrease in the effective Mn2+–Fe2+ exchange interaction due to SiF2−6 reorientation. A gradual decrease in linewidth with temperature at higher temperatures is ascribed to a reduction of the second moment of the Mn2+–Fe2+ dipolar interaction.

On the unusual signals observed in a ceramic Y-Ba-Cu-O sample

Abstract Shrivastava [Solid State Comm. 68, 259 (1988)] has interpreted the sudden appearance of noise on a broad modulated microwave absorption signal observed on cooling a particular ceramic Y-Ba-Cu-O sample below 10 K as evidence for a flux-lattice transition, attributing the noise itself to random sets of periodic signals. However, it has been shown that the noise in question, which occurs only under conditions of large modulation amplitude (∼ 10 Oe), is an artifact associated with helium gas-flow cooling systems. The manifestation of temperature cycling in such systems is also considered.