We use the zero-temperature random-field Ising model to study hysteretic behavior at first-order phase transitions. Sweeping the external field through zero, the model exhibits hysteresis, the return-point memory effect, and avalanche fluctuations.There is a critical value of disorder at which a jump in the magnetization (corresponding to an infinite avalanche) first occurs. We study the universal behavior at this critical point using mean-field theory, and also present preliminary results of numerical simulations in three dimensions.

No abstract

This publication includes all data on superconductive materials intercepted through March 1975. Dala on the bulk elements have been critically evaluated. and values on alloys. compounrl~. anti nth"T forms have been selected and condensed to indicate the probable value and spread of values observed. Prov"n non-superconductors have been noted. Conflict in data values has been noted. All data have been ,keyed to the literature in one or more of the tables. Special subdivisions are presented for $uperconduc· t~ve materials with organic constituents and for those based on semiconductive materials. The proper-tIes. ~res~nted are superconductive critical temperature, critical magnetic fields, material state and com· posItIOn mclu~ing crystal-structure type where noted, a key to thin-film forms, and the presence of thermodynamlc data (generally the electronic specific heat, 'Y, and Debye 6). High-magnetic·field superconductors are noted with listing 01' He,. H c ' He" and He, plus the temperature of observation T.~5

In a random solid solution, if the two atoms have appreciably different sizes, the nearest-neighbor distances and to a lesser extent the higher neighbor distances will be of three kinds, γAA, γAB, and γBB. The effect produces modulations in the diffuse intensity similar to those produced by short-range order. The size effect is important when the difference in scattering power is large, the difference in size is large, and the short-range order is small. The size effect is illustrated by a single crystal pattern of Cu3Au and a powder pattern of Ni3Au2. An asymmetry in the wings about a fundamental reflection is a result of the size effect.

The intermetallic compound, BiMn, has been studied by neutron diffraction at temperatures from 4.2°K to 733°K (460°C). In the temperature range 340-360°C to 445°C a disordered arrangement of Mn atoms on regular and interstitial lattice sites fits the data well if the Mn atoms are assumed to be in the paramagnetic state. This model conflicts with the suggestion made by Guillaud of an antiferromagnetic state in this temperature range. The temperature hysteresis associated with both magnetization and the large cell distortions are qualitatively explained by the disordering and recovery at the transformation temperatures. The effective moment per Mn atom below the transformation agrees with the magnetic measurements of Heikes within experimental error. Measurements of spin direction made at temperatures below 84°K show that only partial rotation occurs in zero magnetic field.