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The effect of competing crystal-field anisotropies on magnetic order has been investigated in a series of Ho/Er superlattices using a combination of x-ray and neutron scattering techniques. For temperatures in the interval TN(Er)≤T≤TN(Ho) the Ho basal-plane order propagates coherently through the paramagnetic Er over a typical length scale of 1000 A. At low temperatures the Ho moments retain their bulk-like helical configuration, whereas the magnetic structure in the Er blocks has both basal-plane and c-axis components. Below TN(Er), the coherence length of the basal-plane order decreases on cooling, while the longitudinal component of the Er moments fails to order across the Ho block. It is argued that these results require an extension of current models of indirect exchange in superlattices to explicitly include the superlattice band structure.

In pyrite-structure antiferromagnetic NiS{sub 2}, the magnetic susceptibility {chi}({ital T}) shows downward curvature above the Neel transition ({ital T}{sub {ital N}1}=39.2 K), suggesting that magnetic correlations persist to {ital T}=400 K. However, neutron-scattering measurements indicate that the bulk magnetic behavior above {ital T}{sub {ital N}1} is conventional. We present evidence that the unusual temperature dependence of {chi}({ital T}) arises from a surface contribution. At {ital T}{sub {ital N}2}=29.75 K a transition occurs to a weakly ferromagnetic state, accompanied by a small structural distortion. We show that this distortion is not tetragonal as suggested previously, but is most likely rhombohedral. Below {ital T}{sub {ital N}2}, {chi}({ital T}) is anisotropic; our data suggest that in this range {chi}({ital T}) is dominated by next-nearest-neighbor exchange interactions.

We report a neutron scattering study of the spin correlations in the model 2D, [ital S]=1/2, square-lattice Heisenberg antiferromagnetic Sr[sub 2]CuO[sub 2]Cl[sub 2]. The spin correlation lengths obtained agree quantitatively with values deduced from Monte Carlo simulations over a wide range of temperature. The combined data, which cover the length scale from 1 to 200 lattice constants, are predicted accurately with no adjustable parameters by renormalized classical theory for the quantum nonlinear sigma model.

We describe measurements of the magnetic critical fluctuations of holmium by x-ray scattering techniques. The x-ray results are compared to those obtained in neutron scattering experiments performed on the same sample.

We report a detailed synchrotron X-ray scattering study of the magnetic correlations in two samples of Mn0.75Zn0.25F2 as a function of temperature and applied field. The critical behavior of this system is believed to be isomorphic with that of the three-dimensional random field Ising model (RFIM). On cooling in an external magnetic field (FC), the first sample exhibits a transition to long range order (LRO) in the near-surface region, at a field dependent temperature,T N (H). In contrast, bulk neutron scattering studies show a long lived metastable domain state forming below a metastability boundary,T M (H). The transition temperatureT M (H), lies below the metastability boundary,T M (H). The temperature difference,T M (H)−T M (H), increases with increasing field and agrees closely with the value deduced from an extrapolation from aboveT M (H) of earlier, equilibrium neutron scattering results on the spin-spin correlation length. On cooling, the order parameter exponent is found to be large, β=0.30±0.05. We speculate that there is an imbalance in the random fields in the neighborhood of linear surface defects (scratches) in this sample, and that the consequent net staggered field initiates a regular random Ising transition. The second sample was cut from the first and underwent a more extensive polishing process, resulting in a smoother surface with a small density of visible defects. Interestingly, it does not attain a LRO state on cooling, but rather it forms a domain state consistent with that observed by neutron diffraction. Both samples may be prepared in a LRO state, either by cooling in zero field and subsequently applying a field (ZFC) or, at high fields, by heating from the XY phase (FHXY). We have studied the evolution of the metastable LRO state in each sample on warming. We find universal behavior in both samples at all fields studied. Specifically there is a powerlaw-like decay of the order parameter with exponent β=0.20±0.05, and a rounded transition region which may be described by a Gaussian distribution of transition temperatures. The width of this distribution scales asH2. A scaling plot of all the warming data as a function of the scaling variable (T−T C (H)/H2 is constructed. We label this non-equilibrium pseudo-critical behavior, “trompe l'oeil critical behaviour.” Phenomenologically, these results enable us to explain many previous, apparently contradictory, results in the literature.

Neutron scattering experiments have been performed to study the dynamic spin properties of La 1.98 Sr 0.02 CuO 4 , a crystal with just enough Sr to destroy three-dimensional long range magnetic order and induce spin-glass behavior at low temperatures. The q -integrated susceptibility \(\int \,{\rm d}^{2}q_{\rm 2D}\,{\rm Im}\,\chi _{\rm 2D}(q_{\rm 2D},\,\omega )\) follows the scaling function of ω/ T suggested by Keimer et al. Below 3 meV, however, clear deviation from the scaling function is observed at low temperatures. This probably originates from a gap in the spin-wave-like excitations due to the out-of-plane anisotropy.

Combined muon-spin-relaxation (${\mathrm{\ensuremath{\mu}}}^{+}$SR) and elastic magnetic neutron-scattering experiments on a large single crystal of ${\mathrm{La}}_{1.94}$${\mathrm{Sr}}_{0.06}$${\mathrm{CuO}}_{4}$ are reported. ${\mathrm{\ensuremath{\mu}}}^{+}$SR data taken without an external field indicate that the Cu spins are fluctuating slowly or frozen below ${\mathit{T}}_{\mathit{f}}^{\mathrm{\ensuremath{\mu}}}$\ensuremath{\approxeq}6.0 K. At 3.9 K, measurements done with an applied longitudinal field show that the muon depolarization is almost completely due to static moments. Neutron measurements of the magnetic peaks Q=(1,0,0) and (0,1,1) show that no three-dimensional long-range order is present down to 1.8 K. However, the quasielastic scattering intensity from the two-dimensional ridge Q=(1,k,0) at k=-0.12 increases sharply between 40 and 20 K. Below ${\mathit{T}}_{\mathit{f}}^{\mathit{N}}$=20 K the scattering intensity is roughly constant. The difference between these two freezing temperatures is explained in terms of the different range of fluctuation frequencies to which each of these probes is sensitive. The dependence of the freezing temperature on the frequency windows of different probes is similar to that seen in traditional spin-glass materials like CuMn.

The short-ranged correlations associated with magnetic ordering in the rare-earth antiferromagnet holmium have been characterized in high-resolution x-ray- and neutron-scattering studies. For temperatures within about 2 K above ${\mathit{T}}_{\mathit{N}}$, the short-ranged magnetic fluctuations exhibit two length scales, instead of the single one expected in an ideal system. Well above ${\mathit{T}}_{\mathit{N}}$, the shorter of the two length scales exhibits power-law behavior consistent with normal critical fluctuations; the line shape in momentum space is well described by a Lorentzian, and the measured critical exponents are \ensuremath{\nu}=0.54\ifmmode\pm\else\textpm\fi{}0.04 and \ensuremath{\gamma}=1.24\ifmmode\pm\else\textpm\fi{}0.15. The longer of the two length scales is well described by a squared-Lorentzian line shape, and exhibits a power-law temperature dependence. Both the shorter and the longer length-scale fluctuations are approximately spatially isotropic. We propose that the longer length-scale fluctuations are related to random strain fields which are localized at or near the sample surfae. These results are reminiscent of behavior observed at the cubic-to-tetragonal structural phase transitions of some perovskite materials.

The short-ranged correlations associated with magneitc ordering in the rare earth antiferromagnet holmium have been characterized in high-resolution x-ray and neutron scattering studies. We find that within about 2 K of ${\mathit{T}}_{\mathit{c}}$, the magnetic fluctuations exhibit two length scales, instead of one as expected in an ideal system. This result is reminiscent of behavior observed at the cubic-to-tegragonal structural phase transitions of the perovskites.

Only recently has it become clear that quantum fluctuations do not destroy long-range oreder in the ground state of the 2D S = 1 2 square-lattice Heisenberg antiferromagnet. We reporta neutron scattering study of the static structure factor of Su2CuO2Cl2, which is an optimal experimental realization of this model. Excellent agreement for the magnetic correlation length is found to exist between theory. Monte Carlo results and our experiment.

Among Cu/sup 2+/ compounds, cupric oxide (CuO) has unusual magnetic properties, some of which are shared by the superconducting copper oxides. In order to study these properties further, neutron scattering experiments on a CuO single crystal have been performed. Two magnetic phase transitions at T/sub N//sub 1/ = 231 K and T/sub N//sub 2/ = 212.5 K have been observed, with the magnetic structure below T/sub N//sub 2/ antiferromagnetic and the structure between T/sub N//sub 1/ and T/sub N//sub 2/ helical in nature. The dynamics of the magnetism was also studied. As in La/sub 2/CuO/sub 4/, the dispersion of the low-temperature spin-wave excitations is unusually steep (dE/dk = 250 +- 75 meV A/sup -1/), indicating strong magnetic interactions. Inelastic magnetic scattering has also been measured above T/sub N//sub 1/. At temperatures slightly above the 231-K phase transition, dynamic critical slowing has been observed.

We review the results of recent neutron-scattering studies of the spin fluctuations in samples of La 1.89 Sr 0.11 CuO 4 which are ∼80% superconducting with T c = 10 K. The structure factor, S(Q), reflects three-dimensional modulated spin correlations with an in-plane correlation length of order 18 ± 6 A. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasi-elastic (|ΔE| < 0.5 meV) at low temperatures. No significant differences are observed in the normal and superconducting states.

We report a neutron scattering study of the spin fluctuations in samples of ${\mathrm{La}}_{1.89}$${\mathrm{Sr}}_{0.11}$Cu${\mathrm{O}}_{4}$ which are \ensuremath{\sim}80% superconducting with ${T}_{c}=10$ K. The structure factor $S(\mathrm{Q})$ reflects three-dimensional modulated spin correlations with an in-plane correlation length of order 18\ifmmode\pm\else\textpm\fi{}6 \AA{}. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasielastic ($|\ensuremath{\Delta}E|l0.5$ meV) at low temperatures. No significant differences are observed in the normal and superconducting states.

Neutron-scattering studies provide direct evidence for a field-driven magnetic transition which originates from the canting of spins out of the Cu${\mathrm{O}}_{2}$ planes: At the transition between antiferroand ferromagnetic ordering of the canted component of the spins in the layers, the (100) Bragg peak vanishes while the (201) peak appears. Detailed measurements and analysis suggest that the phase transition can be described by the mean-field theory of Thio et al. only close to the N\'eel temperature.

The soft optical phonons associated with the tetragonal-to-orthorhombic structural phase transition in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ have been investigated using inelastic-neutron-scattering techniques. Single crystals with Sr concentration of x = 0, 0.08, and 0.14 were studied. Both branches of the doubly degenerate X-point phonon which go soft at the tetragonal-to-orthorhombic transition have been observed in the orthorhombic phase. In all three samples one branch hardens, while the other remains relatively soft with behavior indicative of a further incipient structural phase transition. We identify the hard and soft branches of the orthorhombic phase as zone center and zone-boundary phonons, respectively. The zone-center mode has A/sub g/ symmetry and is, consequently, Raman active. The relation of these measurements to other structural properties and to the superconductivity in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ is discussed.

We review the results of recent neutron scattering studies of the spin fluctuations in samples of La1.89Sr0.11CuO4 which are ~ 80% superconducting with Tc = 10 K. The structure factor, S(Q), reflects three dimensional modulated spin correlations with an in-plane correlation length of order 18 ± 6 A. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasielastic (|ΔE| < 0.5 meV) at low temperatures. No significant differences are observed in the normal and superconducting states.

The results of recent neutron scattering studies on La 2−x Sr x CuO 4 are reviewed. For × = 0 the material is an antiferromagnetic insulator, and the spins in the CuO 2 sheets are well-described by the 2D S = 1/2 square lattice Heisenberg model. With increasing × the Neel state is destroyed and is replaced at low temperatures, at least for samples which are nonmetallic or have low superconducting T c 's, by a spin glass. The instantaneous correlations as well as those of the glass are incommensurate with the crystal lattice. The spin-spin correlation length in the doped CuO 2 sheets equals the average separation between the holes which ultimately carry the supercurrent.