Gerald V. Rubenacker

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.

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.

Magnetic susceptibility of [NH3(CH2)nNH3]CuCl4xBr4(1−x) with n=5 and 7

Abstract Powder magnetic susceptibility measurements were obtained for [NH 3 (CH 2 ) x NH 3 ]CuCl 4 x Br (4(1- x ) with various chloride and bromide concentrations ( n = 5 or 7, and x between 0 and 1). Although the pure bromide compounds, x = 0, are three dimensional ferromagnets, while the pure chloride compounds, x = 1, are three dimensional antiferromagnets, no spin glass behavior was observed. Temperature versus composition phase diagrams showing possible tetracitical points with mixed phases were constructed for both series of compounds and are discussed.

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.

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.

Physical processing effects on polycrystalline YBa 2 Cu 3 O x

The effect of heating YBa 2 Cu 3 O x in vacuum to 600 °C has been studied using photoelectron spectroscopy and diamagnetic susceptibility measurements. Evidence of two chemically distinct copper and barium species is found in single phase samples at room temperature cleaned by gentle heating at 450 °C. Such annealing also increases the volume diamagnetic susceptibility of the samples which suggests that the preferred stoichiometry of growth does not lead to an optimum superconducting phase. Samples cleaned by vacuum scraping or ion bombardment reveal more amorphous XPS structure and are less indicative of bulk properties.

Low magnetic field superconducting phase diagram of the high-Tc YBa2Cu3O9-δ

Abstract The low magnetic field phase diagram of the high-Tc superconductor YBa2Cu3O9-δ with δ∼2.1 has been determined in fields to 5000 0e. The lower critical field is about 300 0e near T=0 and less than 100 0e at liquid nitrogen temperature. The mixed phase shows significant relaxation and field cooling effects.

Magnetic study of acetamidinium tetrachlorocuprate II

Magnetic susceptibility measurements of the tetrahedrally coordinated system [(CH3C(NH2)2]2CuCl4 show it to be a weak pseudo‐one‐dimensional antiferromagnet with the dominant exchange, J2/kB =−2.15 K, directed along chains of magnetic Cu++ ions. A weaker intraplanar exchange, J1/kB =0.66 K, indicates ferromagnetic ordering between chains. Susceptibility data reveal an unexpected double peak at 2.5 and 1.8 K, suggesting the interesting possibility of two phase transitions corresponding to antiferromagnetic ordering of different sublattice pairs. Consistent with crystallographic data, these sublattice pairs are formed by two crystallographically distinct distorted tetrahedral CuCl4 groups linked into chains by intermediate acetamidinium ions. This interpretation is consistent with critical electron paramagnetic resonance linewidth broadening of two separate resonances at the two reported temperatures.