The neutron scattering cross section below the Neel temperature in RbMnF3 has been studied with the aid of neutron spin polarisation analysis. In addition to the spin-wave scattering, a small central component was observed and found to be longitudinal in character. This longitudinal scattering is quasi-elastic, with an intensity that decreases with increasing wavevector and with decreasing temperature below TN.

The phase transitions at the R point in KMnF 3 doped with 1% and 10% of KMgF3, with 1% of NaMnF3, and with 1% of KCaF3, have been studied with x-ray scattering techniques. The Mg impurities lower T c while the others raise T c , by amounts which are roughly consistent with the differences in the atomic radii. In both cases it is found that the fluctuation-driven first-order transition becomes continuous when the concentration of impurity exceeds a critical amount. PACS numbers: 63.75,+z, 64.70.Dv The effect of impurities on phase transitions in general, and on structural phase transitions in particular, is still not fully understood. KMnF3 has a structural phase transition at 186 K which has three components of the order parameter and so might be expected to have a continuous phase transition described by the n= s d=3 Heisenberg model. Experimentally, however, it is found that the transition is not continuous, and this is believed to arise because when the critical fluctuations are so anisotropic they drive the transition to be of first order. 1 ' 2 We have studied the effect of impurities on this transition by the introduction of substitutional ions which either raise or lower T c . In both cases the results show that the impurities tend to change the character of the transition, and we suggest that above a small critical concentration the transition is continuous. We conclude that random-bond defects alter the order of fluctuationdriven first-order phase transitions.Single crystals of KMnF3 containing 1% and 10% of KMgF3 were grown by the Crystal Growth Facility at Oxford University and two similar crystals containing 1% of NaMnF3 and 1% of KCaF3 were grown at Le Mans University by Dr. J. Nouet. All the crystals were cut with a face perpendicular to a [100] crystallographic axis, and the surfaces polished and then etched with use of orthophosphoric acid to remove any damage. Each crystal was mounted with the [Oil] axis vertical in a closed-cycle cryostat so that the temperature could be controlled to ±0.02 K. The cryostat was then positioned in the extended face geometry on an x-ray diffractometer which could be operated either as a twoaxis diffractometer or as a high-resolution three-axis diffractometer, as used and described for work on KMnF 3 (Ref. 3) and RbCaF 3 . 4 " 6 The crystals were found to be of excellent crystallographic quality, with mosaic spreads of =0.02°.Detailed measurements were made of the variation with temperature of the (400) Bragg reflection in order to investigate the temperature dependence of the lattice parameter, and to study the structure of domains below the transition temperature. In nominally pure KMnF3 (Ref. 3) and RbCaF 3 (Refs. 4 and 5), it was found that well below the transition temperature the domain structure varied as the x-ray beam was scanned across the surface of the crystal. In the crystals containing KMgF3 all possible domains were observed with roughly equal intensities as the beam position was varied. We conclude 3 t/t § 2 »» rar -O ro -1

The wavevector dependence of the critical fluctuations at the cubic-to-tetragonal structural phase transition in KMnF3 doped with 1% and 10% of KMgF3 has been studied using X-ray scattering techniques. It is found that the introduction of impurities lowers Tc and that the fluctuation-driven first-order transition becomes continuous when the impurity concentration exceeds a certain threshold. At temperatures far above Tc, the temperature dependence of the X-ray critical scattering cross section measured in the samples KMnF3, KMn0.99Mg0.01F3 and KMn0.9Mg0.1F3 is described by the same critical exponents nu and gamma . Contrary to expectation, the magnitude of the anomalous narrow component observed in the critical scattering in pure KMnF3 close to Tc is significantly reduced by the introduction of the impurities.

We have investigated the etching characteristics of GaAs and AI,,,Ga,,,As using SiCI, as t h e etching gas. The effect of damage caused on the etched surface has been characterized by employing Schottky diode performance, low-temperature integrated band-gap photoluminescence, Raman scattering and high-resolution x-ray photoelectron spectroscopy, and, for the first time, the usefulness of employing glancing x-ray reflectivity for the identification of surface damage is explored. Using high-resolution fabrication techniques, a novel procedure lor constructing a sidewall Schottky diode has been developed to allow the amount of induced sidewall damage to be quantified. In addition, the deteriorative effect of the surface damage layer is observed to remain unchanged with etch time

The frequency dependence of the critical fluctuations at the antiferrodistortive phase transition exhibited by the disordered crystals KMnn 99Mgn and KMno ,Mg" IF3 has been studied using neutron scattering techniques. In pure KMnF, the soft RaS mode is overdamped for temperatures below -(Tc + 40) K. A similar behaviour is observed in the disordered crystals, but the damping constant is increased by the presence of the impurities. The amplitude of the central peak observed in the spectral response for pure KMnF, is not significantly increased by the addition of the impurities. This is consistent with measurements of the anomalous component observed close to T, in complimentary x-ray scattering studies, which showed that this component was reduced by the addition of impurities. The results further indicate that although the temperature dependence of the anomalous scattering is similar in both the x-ray and neutron scattering experiments, the neutron central peak persists over the temperature range T, to (Tc + 20) K while the x-ray anomaly was only observed for a few degrees above T,. Thus, while the anomalous scattering measured using the two techniques is related to the impurity concentration in the disordered crystals, the two features must be to some extent distinct.