Flux trapping in the 9.3 GHz modulated microwave absorption spectra observed near 4 K from ceramic and powdered ceramic specimens of two separately prepared YBa2Cu3O7-δ samples has been used to separate the intergranular and intragranular contributions to the spectra. In the denser, glassy sample, a broad absorption with a peak near 400 Oe for forward sweeps was observed with appreciable intensity after the maximum flux was trapped. This spectrum is attributed to intergranular junctions, since its relative intensity was reduced on powdering and suspending in wax. In the less dense, more uniform sample, the latter spectrum was appreciably weaker in both ceramic and powder. Both types of junction appear to contribute to the narrow low-field absorption which was observed after zero field cooling in all the samples.

The original motivation for the work outlined in this paper started with an attempt to find a method for measuring the anisotropy energy as well as the interplane antiferromagnetic (AF) exchange in the common layered structures of n-ethylammonium CuCU (nEA) and n-ethanediammonium CuCU (nDA). Most of these order ferromagnetically within the plane of CuCU's but the weak ferromagnetic (F) exchange between planes yields a 3D AF compound below the ordering temperature. In F resonance the strong exchange between ions is not explicit in the free energy or Hamiltorian since in the ordered F state all spins are aligned. That is, the exchange energy has already been used to provide the magnetization vector M. AF resonance, on the other hand, is typically treated as a two sublattice system where all the spins pointing in one direction are considered to be one sublattice and all those in the other direc-* tion are from the other sublattice. In this case the strong exchange in a standard 3D AF is still the exchange between neighboring spins and, therefore, becomes the exchange between sublattices and as such becomes part of the explicit free energy of the system. The fact that all the spins on one sublat-

Low-temperature microwave absorption measurements at 9.3 GHz in ceramic $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ were made on the broad modulated signal observed after maximum flux had been trapped. Noise occurred on the extrema of this signal if the modulation frequency and amplitude were sufficiently high and the temperature was close to the lowest that could be attained when the limiting factor was the microwave heating of the sample. The source of the noise is proposed to be thermal instabilities, which produce small flux avalanches in the superconducting grains.

The general form for the magnetic resonance frequency \ensuremath{\omega} of anisotropic ferromagnets as derived from the free energy F by Smit and Beljers (\ensuremath{\omega}/\ensuremath{\gamma}${)}^{2}$=(M sin\ensuremath{\theta}${)}^{\mathrm{\ensuremath{-}}2}$(${\mathit{F}}_{\mathrm{\ensuremath{\theta}}\mathrm{\ensuremath{\theta}}}$${\mathit{F}}_{\mathrm{\ensuremath{\varphi}}\mathrm{\ensuremath{\varphi}}}$-${\mathit{F}}_{\mathrm{\ensuremath{\theta}}\mathrm{\ensuremath{\varphi}}}^{2}$), although numerically correct, is physically not convenient, because the origin of the different terms in F is obscured by an angular-dependent mixing. This mixing is avoided by using the relation (\ensuremath{\omega}/\ensuremath{\gamma}${)}^{2}$=1/${\mathit{M}}^{2}$ [${\mathit{F}}_{\mathrm{\ensuremath{\theta}}\mathrm{\ensuremath{\theta}}}$(${\mathit{F}}_{\mathrm{\ensuremath{\varphi}}\mathrm{\ensuremath{\varphi}}}$ /${\mathrm{sin}}^{2}$\ensuremath{\theta}+cos\ensuremath{\theta}/sin\ensuremath{\theta}${\mathit{F}}_{\mathrm{\ensuremath{\theta}}}$)-(${\mathit{F}}_{\mathrm{\ensuremath{\theta}}\mathrm{\ensuremath{\varphi}}}$/sin\ensuremath{\theta}-cos\ensuremath{\theta}/sin\ensuremath{\theta} ${\mathit{F}}_{\mathrm{\ensuremath{\varphi}}}$/sin\ensuremath{\theta}${)}^{2}$]. .sp Explicit expressions will show the symmetry of each of the terms in F for all magnitudes and directions of H. In addition, an alternate method which uses only rectangular coordinates and which can easily be generalized for multisublattice systems is described.

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.

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.

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.

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.

This paper reviews the status of point defect studies in HgCdTe and presents new data on the characterization of impurities in CdTe, on shallow and deep acceptors in arsenic-doped HgCdTe, and on the ability of electron paramagnetic resonance (EPR) to detect defects in HgCdTe. Point defects are important in controlling carrier concentrations, minority-carrier lifetimes, and noise in HgCdTe. Shallow impurities from dopants are fairly well understood and their effects follow what is expected from the periodic table. Intrinsic defects such as the mercury vacancy, which is believed to act as a shallow acceptor, are less well understood and suffer from a lack of characterization techniques that can identify individual defects. Deep-level defects are present with concentrations proportional to the shallow acceptor concentrations. These centers often control lifetime and noise. Many of these have been electrically characterized, but not identified. Theoretical work by several groups to calculate defect levels has begun. At present, the errors in this work are larger than the HgCdTe band gaps, but the calculated trends are important. Experimental data on identified deep levels are needed both for guiding material improvement and for improving the theoretical modeling.

The $c$ axis dielectric permittivity at 1 kHz was measured for a 71.4 at.% deuterated crystal of ${\mathrm{Rb}}_{0.52}$${({\mathrm{ND}}_{4})}_{0.48}$${\mathrm{D}}_{2}$${\mathrm{PO}}_{4}$ from 4 to 300 K. The permittivity follows, down to 150 K, a Curie-Weiss law with a Curie temperature of 0 K. Below this temperature the susceptibility rounds off to a broad maximum at 80 K, and below 50 K, starts decreasing rapidly. Between 25 and 40 K, the inverse susceptibility obeys a Curie-Weiss law which extrapolates to zero at 43 K. At 4 K, the relative permittivity flattens out to a value of 11.5. The results show general agreement with predictions of a Landau model giving a second-order transition to an antiferroelectric state at 43 K, but the rounding of the susceptibility peak over a very wide temperature range agrees better with predictions of a model which considers the asymmetry of the typical hydrogen bond caused by the crystal being only partly ammoniated. Permittivity results of Courtens and of Iida and Terauchi for undeuterated crystals with 35% and 60% ammonium, respectively, are also compared with predictions of this second model.