K. Dwight

Low temperature crystallographic and magnetic study of LaCoO3

Abstract The structural and magnetic properties of LaCoO3 between 300 and 4·2°K are studied. Previous investigators have usually reported the existence of a local maximum in the susceptibility at 90°K in this material, but a recent single crystal study by Naiman and coworkers indicated no such phenomenon. We find that the magnetic properties in chemically pure materials are strongly dependent on the method of preparation. Thus we have been able to obtain samples exhibiting a susceptibility maximum, and then remove it by additional regrinding and renring. Samples both with and without a susceptibility maximum were studied using neutron diffraction. Our results indicate that both sample types have R 3c symmetry at room temperature and below. In addition, neither type exhibits any long-range bulk magnetic order at 4·2°K. This result is in accord with that of Koehler and Wollan. Both sample types were studied with a vibrating-coil magnetometer. An anomalously rapid increase of magnetic moment with decreasing temperature was observed below 40°K in a field of 10 kOe. The presence of a small ferromagnetic component was established by the presence of hysteresis at 4·2°K, a trace of this hysteresis remaining at 77δK. The magnetic moment, as measured in 200 Oe, was found to decrease gradually with increasing temperature above 4·2°K. The absence of any abrupt drop indicates the remanent moment to be due to isolated regions of a magnetic phase dispersed in a non-magnetic matrix. We conclude that variations in the size and number of these regions are the cause of the discrepancies between observed magnetic properties of different samples of LaCoO3, that they are virtually unavoidable because of near-degeneracy with the bulk diamagnetic phase, and that their presence precludes definitive measurement of the low-temperature susceptibility of LaCoO3.

Effect of Pressure on the Magnetic Properties of Ca1−xSrxMnO3

In order to distinguish among the effects of interatomic distance, B–O–B angle and chemical substitution in perovskites, a study has been made of the variation of Neel points of several samples in the system Ca1−xSrxMnO3 as functions of x and pressure. It is found that throughout the region 0 0 throughout this region, which indicates just the opposite dependence on cell size. This apparent contradication is compatible with Goodenoughs hypothesis regarding the effect of covalence of the nonmagnetic A‐site cation on the magnetic interaction. In addition, it is found that application of hydrostatic pressure reduces the weak ferromagnetic moment. This is most marked at intermediate values of x, where a 25% decrease in moment can be obtained upon application of a hydrostatic pressure of 8 kbar. Increasing x at atmospheric pressure also decreases the weak ferromagnetic moment. These findings indicate that orthor...

High‐Pressure RbFeCl3‐A Transparent Ferrimagnet

The structure of RbFeCl3 at atmospheric pressure contains hexagonal close‐packed RbCl3 layers with Fe filling all the chlorine octahedra, which form continuous face‐shared chains. RbFeCl3 transforms at 700°C and 20–35 kbar to the RbNiF3 structure, which has two cubic‐stacked for every hexagonal‐stacked RbCl3 layer to give face‐shared pairs of octahedra joined by octahedra sharing only corners. The high‐pressure phase is retained under ambient conditions and is ferrimagnetic with Tc = 109°K. Above 225°K, the susceptibility obeys the Curie‐Weiss law with CM = 3.89 emu°K/mole (μeff = 5.58 μB) and θp = −134°K. The powder samples do not saturate at 4.2°K in fields up to 17.2 kOe. The saturation magnetization calculated from a 1/H vs M plot is 1.1 μB (25.4 emu/g), which approaches the theoretical value of 1.3 μB predicted for ferromagnetic pairs of iron‐filled octahedra coupled antiferromagnetically to a third octahedra, as is found in RbNiF3 and CsFeF3. Optical transmission data on the tan powdered samples pre...

MAGNETIC PROPERTIES OF EUROPIUM: PRESSURE AND IMPURITY EFFECTS.

A study has been made of the magnetic properties of europium in the vicinity of the transition temperature Tt as functions of hydrostatic pressure. A marked drop in the moment, consistent with a first‐order transition, is observed at atmospheric pressure, but the sharpness of this transition decreases at higher pressures. The variation of Tt with pressure is nonlinear above 4 kbar, with ΔTt/ΔP=0.45°±0.03°K/kbar in the limit P→0. This value corresponds, via the Clausius‐Clapeyron relation, to twice the volume change previously estimated on the basis of direct thermal‐expansion measurements. In addition, it is found that dθ/dP<0, where θ is the paramagnetic Curie temperature. Below Tt, the magnetic susceptibility of europium is known to rise anomalously. Although our sample was nominally ``high‐purity europium, the presence of a ferromagnetic component is observed at 4.2°K. In addition, the temperature variation of the susceptibility is shown to be consistent with a presence of a small amount of some ferr...

Pressure Variation of the Curie Temperature and Spontaneous Magnetization in Fe2P and Fe2P0.9As0.1

The transition‐metal pnictides (M1−yMy′)P1−xAsx exhibit structural relationships and magnetic properties that indicate the presence of filled valence bands, empty conduction bands, and partially filled 3d bands active in metal‐metal bonding. In many cases they support spontaneous magnetism, thereby offering the opportunity to study itinerant‐electron magnetism as a function of 3d bandwidth and occupancy. In particular, the hexagonal system Fe2P1−xAsx is ferromagnetic, but for xu2009<u2009u20090.33 its spontaneous moment at Tu2009=u20090K is reduced from the μou2009=u20093.0μB/molecule predicted for itinerant, spin‐only ferromagnetism. We investigated the pressure dependence of Tc and μo to 11 kbar. In Fe2P, the relation between ΔTc [° C] and P [kbar] is: Pu2009=u2009−0.252(ΔTc)u2009−0.0012(ΔTc)2. In Fe2P0.9As0.1, it is: Pu2009=u2009−0.71(ΔTc)u2009−0.0017(ΔTc)2. Pressure did not change significantly the value of μ at 58K, but it promoted a remarkably exchange‐enhanced susceptibility above Tc. We interpret these results to mean that the reduced moment in Fe2...