Showing papers by "Robert J. Birgeneau published in 1971"

Critical Magnetic Scattering in K 2 Ni F 4

Abstract: A detailed study of the static and dynamic critical behavior of the planar antiferromagnet ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$ is reported. The dynamic experiments reveal several unusual features. Magnons over most of the Brillouin zone are observed to persist well above ${T}_{N}$. The magnons ultimately become unobservable because of lifetime effects rather than any appreciable renormalization of the real part of the energy. The spin-wave gap remains sharp right up to the phase transition and, in fact, it seems to renormalize like the sublattice magnetization at all temperatures. The transverse excitations for $q\ensuremath{ e}0$ in general exhibit no explicit recognition of the phase transition and even at $q=0$ there is no apparent variation through ${T}_{N}$ of the integrated intensity. The function ${\mathcal{S}}^{\ensuremath{\parallel}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}},\ensuremath{\omega})$, which manifests the expected critical behavior, is found to be very narrow in $q$, $\ensuremath{\omega}$ space. For $Tl{T}_{N}$ there is no measurable inelasticity of the critical fluctuations at $q=0$. For $Tg{T}_{N}$, ${\mathcal{S}}^{\ensuremath{\parallel}}(0,\ensuremath{\omega})$ has a finite width which goes to zero as $T\ensuremath{\rightarrow}{T}_{N}^{+}$ thus exhibiting the critical slowing down first predicted by Van Hove. Quasielastic measurements of ${\ensuremath{\chi}}^{\ensuremath{\parallel}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$ have been carried out for $T\ensuremath{\ge}{T}_{N}$. These experiments yield the critical exponents $\ensuremath{\eta}=0.4\ifmmode\pm\else\textpm\fi{}0.1$ measured at ${T}_{N}$ and $\ensuremath{ u}=0.57\ifmmode\pm\else\textpm\fi{}0.05$, $\ensuremath{\gamma}=1.0\ifmmode\pm\else\textpm\fi{}0.1$ for $Tg{T}_{N}$. The value for $\ensuremath{\eta}$ is similar to that for the two-dimensional Ising model, $\ensuremath{\eta}=0.25$, but $\ensuremath{ u}$, $\ensuremath{\gamma}$ differ considerably from the Ising-model values of 1, $\frac{7}{4}$. This latter result is not in agreement with current theory.

Neutron Crystal‐Field Spectroscopy in Rare‐Earth Metallic Compounds

Abstract: The technique of inelastic neutron scattering has been applied to the problem of crystal fields in rare‐earth metallic compounds. It is shown that in compounds where the Stark energies are somewhat greater than exchange energies, the complete energy‐level diagram can be readily obtained from the inelastic neutron spectra and hence the corresponding crystal‐field parameters may be deduced. In this paper we illustrate this by a series of measurements which we have performed on the praseodymium monopnictides, PrBi, PrSb, PrAs, PrP, and monochalcogenides PrTe, PrSe, PrS. It is found that the spectra can be reproduced in detail using ordinary crystal‐field theory together with a simple approximation to the instrumental resolution function. In addition, all of the crystal‐field levels in all seven compounds studied can be quantitatively accounted for using a nearest‐neighbor point charge model with an effective charge of −2. This is in spite of marked variations in covalency, carrier concentration and, indeed, in the widths of the crystal‐field transitions themselves over the series.

Neutron-Scattering Study of TmSb: A Model Crystal-Field-Only Metallic Paramagnet

Abstract: Inelastic-neutron-scattering studies have been carried out on the paramagnetic metallic compound TmSb. The ${\mathrm{Tm}}^{3+}$ ground-state multiplet $^{3}H_{6}$ is split by the octahedral crystal field into six levels ${\ensuremath{\Gamma}}_{1}$, ${\ensuremath{\Gamma}}_{4}$, ${\ensuremath{\Gamma}}_{5}^{(2)}$, ${\ensuremath{\Gamma}}_{2}$, ${\ensuremath{\Gamma}}_{5}^{(1)}$, ${\ensuremath{\Gamma}}_{3}$. Transitions between these levels have been observed and the parameters ${A}_{4}〈{r}^{4}〉=(6.86\ifmmode\pm\else\textpm\fi{}0.10)$ meV, ${A}_{6}〈{r}^{6}〉=(0.44\ifmmode\pm\else\textpm\fi{}0.04)$ meV have been deduced. The fourth-order term is consistent with that expected on the basis of a simple nearest-neighbor point-charge model, whereas the sixth-order term is an order of magnitude too large; this is in marked contrast with previous results in PrSb and other praseodymium pnictides and chalcogenides where the effective-point-charge model is quantitatively correct for both terms. It is found that the actual spectra can be reproduced in detail using ordinary crystal-field theory together with a simple approximation to the instrumental-resolution function. Furthermore, no appreciable exchange broadening of the transitions is observed at low temperatures, thus confirming TmSb as a model crystal-field-only metallic paramagnet.

Interactions between cerium ions in rare earth ethyl sulphates

Abstract: EPR measurements have been made of Ce nearest and second nearest neighbour pairs in europium, yttrium and lutecium ethyl sulphates. The information obtained from these measurements has been used, in conjunction with previous data on pure cerium ethyl sulphate, to obtain the elements of the interaction tensors. This sheds new light upon the interaction mechanisms. It is shown that the large nondipolar interaction cannot be due to exchange or electric quadrupole-quadrupole interactions alone, but is due mainly to virtual phonon exchange, with interaction parameters more than an order of magnitude larger than those hitherto calculated using a simple Debye model for the phonon spectrum and orbit- lattice coupling parameters estimated from the static crystal field.

Magnetic Neutron Scattering from a Nearly Ideal One‐Dimensional Antiferromagnet

Abstract: We have carried out a detailed investigation of the linear chain antiferromagnet, (CD3)4N MnCl3 using elastic and inelastic neutron scattering techniques. The MnCl3 chains in this compound are found to exhibit purely one‐dimensional paramagnetic behavior down to at least 1.1°K, with no evidence for the onset of three‐dimensional order. The interactions between Mn2+ ions along the chain are such that a molecular field theory would predict an ordering at 76°K. The quasielastic measurements, which yield ∫dωS(Q,ω)=1/N〈S(−Q,0)S(Q,0)〉 show planes of scattering intensity perpendicular to the chain axis. From the thermal evolution of the planar scattering, the temperature dependence of the correlation length has been obtained and is compared with the theory for the classical Heisenberg linear chain antiferromagnet with which it shows very good agreement. The inelastic measurements show that at 4.4°K for q and ω≠0, S(q, ω) may be described by long‐lived spin waves which accurately follow the dispersion relation ℏω...