Uniformly sized microspheres interacting via long-range magnetic dipolar forces are used to study diffusion-limited cluster aggregation in a plane. The results show that it is possible to scale the temporal evolution of the cluster size distribution and that there is a crossover in fractal dimension from D =1.52~0.05 to D =1.16+'0.05 in the limit of weak and strong dipolar coupling. External magnetic fields are shown to produce pronounced chaining with D approaching 1. The results compare favorably with computer simulations of aggregation of the same type of particles.

We report an x-ray-scattering study of the incommensurate modulations in pure chromium. X-ray magnetic scattering from the spin-density wave (SDW) is observed. No resonant enhancement of the signal is obtained near the Cr K edge. On cooling through the spin-flip transition, the magnetic signal falls to zero, consistent with the polarization dependence of nonresonant magnetic scattering and the known polarization of the SDW. Charge scattering is observed at the second harmonic due to the associated charge-density wave (CDW). The intensity of the second harmonic is unchanged on cooling through the spin-tlip transition. A survey of possible second-harmonic satellites reveals that a single Q state exists in the near-surface region.The amplitude of the lattice distortion is estimated to be {A2/a)=1.5+0.2X10, consistent with published reports. A fourth harmonic is also observed, suggesting that the CDW is not perfectly sinusoidal. The intensity of the fourth harmonic is 0.05% of the second and we find (A4/a) =2.8+0.2X10 '. The temperature dependence of the charge harmonics is found to obey mean-field scaling.

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Large-scale production of conical carbon nanostructures is possible through pyrolysis of hydrocarbons in a plasma torch process. The resulting carbon cones occur in five distinctly different forms, and disc-shaped particles are produced as well. The structure and properties of these carbon cones and discs have been relatively little explored until now. Here we characterize the structure of these particles using transmission electron microscopy, synchrotron x-ray and electron diffraction. The carbon nanocones are found to exhibit several interesting structural features; instead of having a uniform cross-section, the walls consist of a relatively thin inner graphite-like layer with a non-crystalline envelope, where the amount of the latter can be modified significantly by annealing. The cones appear with a well-defined faceting along the cone edge, demonstrating strict long-range atomic ordering; they also present occasional examples of symmetry breaking, such as two apexes appearing in the same carbon nanocone.

Experiments on a system of two magnetic holes (nonmagnetic microspheres in ferrofluid) subject to a rotating magnetic field show various types of behavior depending on the driving frequency. For two spheres (holes) mechanically bound together the stable rotation mode at low frequencies is replaced above a critical frequency by a mode with alternating rotation directions. This is described by simple nonlinear equations, and simulations show good quantitative agreement with the experiments. For two free spheres not bound together one observes a transition to a mode in which the spheres undergo both angular and radial motion around the center of mass of the system. Depending on the frequency and the anisotropy of the rotating magnetic field the motion passes through a sequence of states with sphere rotation mode-locked to the driving frequency at diR'erent ratios in good qualitative agreement with simulations. Adding a constant magnetic field normal to the plane causes the minimum sphere separation to increase from contact to a finite value. At high frequencies tlie spheres are influenced by an effective magnetic potential, giving rise to a static particle separation proportional to the normal component of the field.

We report the results of resonant x-ray magnetic scattering experiments on bulk and thin-film single crystals of holmium. The scattering at the principal magnetic reflection has been characterized as a function of the temperature in the spiral phases near and below their respective Neel temperatures. The integrated intensity of the principal magnetic peak in both samples shows power-law behavior versus reduced temperature with nearly equal exponents. The exponents for the scattering at the resonant second and third harmonics in the bulk sample are not simple integer multiples of the first, and motivate the consideration of simple scaling corrections to mean-field theory. We also present and compare the results of high-resolution measurements of the temperature dependence of the magnetic wave vectors, caxis lattice constants, and correlation lengths of the magnetic scattering of the two samples in their spiral phases. Although the qualitative behavior is similar, systematic differences are found, including uniformly larger magnetic wave vectors and the suppression of the 1/6 phase in the film. The spiral magnetic structure of the film forms a domain state at all temperatures in the ordered phase. The magnetic correlation lengths of both samples are greatest near the Neel temperature, where that of the film appears to exceed the translational correlation lengths of the lattice. As the temperature decreases, the magnetic correlation lengths also decrease. These results are discussed in terms of the strain present in the samples. all the rare earths) have remained controversial. Assuming that the magnetic ordering transition is second order, various experimental determinations of the critical exponent of the order parameter, P, and of the exponent of the specific heat, a, have been found to disagree. ' On the basis of dilatometry measurements, it has been sug-

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