John E. Drumheller

Strong exchange narrowing in new two-dimensional paramagnetic systems☆

Abstract Single crystals of (CH3NH3)2 MnCl4 and (C2H5NH3)2 MnCl4 have a strong angular and temperature dependent ESR line width narrowed by exchange interaction. They are very probable two-dimensional paramagnets down to about 50°K.

Magnetic properties of the alkanediammonium copper halides

Abstract Powder magnetic susceptibility and magnetization data have been obtained for the remaining, unreported members of the quasi-two-dimensional series [NH 3 (CH 2 ) n NH 3 ] CuX 4 for n =2−10 and X=Cl and Br. Values for J (in-plane), J ′ (out-of-plane) and T c are tabulated for the entire series. An alternation between ferromagnetic and antiferromagnetic behavior for the low temperature phases in the bromine series was found. Several unusual low temperature phases which could not be classified were also seen.

Novel low‐dimensional spin 1/2 antiferromagnets: Two‐halide exchange pathways in A2CuBr4 salts

Magnetic susceptibility studies of several A2CuBr4 salts, where A is an organoammonium cation, are reported. These salts contain tetrahedrally distorted CuBr2−4 anions. Magnetic exchange pathways exist through interanionic Br⋅⋅⋅Br pathways. In this manner, various types of new antiferromagnetic spin 1/2 magnetic systems have been obtained. As examples, a one‐dimensional chain, a coupled pair of chains or ‘‘ladder,’’ a two‐dimensional square lattice, and a distorted ‘‘honeycomb’’ lattice are discussed.

Magnetic susceptibility of ((CH3)3NH)2Cu4Br10, chains of stacked linear tetramers

Abstract ((CH 3 ) 3 NH) 2 Cu 4 Br 10 consists of Cu 4 Br 10 linear tetramers which in turn are stacked to form chains. Powder magnetic susceptibility data have been fitted to a linear tetramer model using an end exchange constant J 1 / k =-180 K and a central exchange constant J 2 / k =-140 K.

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.

Phases of gallium nucleated by small particles

Several structural phases of gallium can be nucleated upon cooling of mixtures of liquid gallium and an excess of particulate material. One such structural phase, which we label the theta phase, was observed in gallium and fullerene mixtures and is efficiently nucleated only by particulate material of small particle size such as soot or the separation products of soot. The -phase is possibly a recently discovered phase of gallium.

Magnetic critical behavior and exponents of the quasi-two-dimensional ferromagnet [NH3(CH2)9NH3]CuBr4

The dc initial susceptibility, spontaneous magnetization, and critical isothermal magnetization measurements of powder [NH3(CH2)9NH3]CuBr4 (9DACuBr) are reported with emphasis on the anomalous behavior of the critical exponents. Because of the weak exchange interaction between the layers, it behaves as a nearly ideal two‐dimensional Heisenberg ferromagnet at finite temperature. The transition temperature Tc=11.35 K was determined by the measurements of spontaneous magnetization and initial susceptibility. The critical exponents β=0.28, γ=1.38 (T>Tc), and γ’=0.45 (T<Tc), as well as an empirical relation δ’=δ+cHd for the effective critical exponent with δ=0.65, c=0.185, and d=0.175, were found to fit the data.

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.

The temperature dependence of the symmetric exchange interaction by optical phonon modulation of the exchange integral

Abstract By assuming an anharmonic intermolecular potential for lattice displacement and an exponential form for the exchange energy, the exchange interaction is shown to be temperature dependent in the paramagnetic region. Bond strengths are taken from known tabulated results and overlap integrals are calculated with Slater-type orbitals so there are no adjustable parameters. Agreement with experimental results on K2CuCl4 · 2H2O and other layered compounds is shown.