We have studied the magnetization-reversal process of a Nd 2 Fe 14 B=Fe 3 B nanocomposite using small-angle neutron scattering. Based on the computation of the autocorrelation function of the spin misalignment, we have estimated the characteristic size l C of spin inhomogeneities around the Nd 2 Fe 14 B nanoparticles. The quantity l C approaches a constant value of about 12.5 nm ($average Nd 2 Fe 14 B particle radius) at 14 T and takes on a maximum value of about 18.5 nm at the coercive field of À0.55 T. The field dependence of l C can be described by a model that takes into account the convolution relationship between the nuclear and the magnetic microstructure.

The structural and magnetic properties of a cobalt nanorod array have been studied by means of magnetic field dependent small-angle neutron scattering (SANS). Measurement of the unpolarized SANS cross section dAE/d of the saturated sample in the two scattering geometries where the applied magnetic field H is either perpendicular or parallel to the wavevector k i of the incoming neutron beam allows one to separate nuclear from magnetic SANS, without employing the usual sector-averaging procedure. The analysis of the SANS data in the saturated state provides structural parameters (rod radius and centre-tocentre distance) that are in good agreement with results from electron microscopy. Between saturation and the coercive field, a strong field dependence of dAE/d is observed (in both geometries), which cannot be explained using the conventional expression of the magnetic SANS cross section of magnetic nanoparticles in a homogeneous nonmagnetic matrix. The origin of the strong field dependence of dAE/d is believed to be related to intradomain spin misalignment, due to magnetocrystalline and magnetoelastic anisotropies and magnetostatic stray fields.

We report magnetic-field-dependent small-angle neutron scattering (SANS) experiments on a Nd2Fe14B/Fe3B nanocomposite. For the two scattering geometries where the applied magnetic field is either perpendicular or parallel to the incoming neutron beam, we have independently analyzed the field-dependent SANS data in terms of micromagnetic theory, taking into account demagnetizing-field effects. The approach in reciprocal space is supported by an analysis of the data in real space and provides consistent results for the exchange-stiffness parameter and the mean magnetic anisotropy-field radius.

We report on the results of grain-size and temperature-dependent magnetization, specific-heat, and neutron-scattering experiments on the heavy rare-earth metals terbium and holmium, with particular emphasis on the temperature regions where the helical antiferromagnetic phases exist. In contrast to Ho, we find that the helical structure in Tb is relative strongly affected by microstructural disorder, specifically, it can no longer be detected for the smallest studied grain size of D = 18 nm. Moreover, in coarse-grained Tb a helical structure persists even in the ferromagnetic regime, down to about T = 215 K, in agreement with angle-resolved photoelectron spectroscopy (ARPES) data, which reveal a nesting feature of the bulk Fermi surface at the L point of the Brillouin zone at T = 210 K. As samples for the ARPES measurements, we used 10-nm-thick single-crystalline Tb films that show a bulk electronic valance-band structure. Thus, our ARPES measurements are used to discuss temperature-induced effects observed in the coarse-grained samples.

The results are reported of magnetic field-dependent neutron diffraction experiments on polycrystalline inert-gas condensed holmium with a nanometre crystallite size (D = 33 nm). At T = 50 K, no evidence is found for the existence of helifan(3/2) or helifan(2) structures for the nanocrystalline sample, in contrast with results reported in the literature for the single crystal. Instead, when the applied field H is increased, the helix pattern transforms progressively, most likely into a fan structure. It is the component of H which acts on the basal-plane spins of a given nanocrystallite that drives the disappearance of the helix; for nanocrystalline Ho, this field is about 1.3 T, and it is related to a characteristic kink in the virgin magnetization curve. For a coarse-grained Ho sample, concomitant with the destruction of the helix phase, the emergence of an unusual angular anisotropy (streak pattern) and the appearance of novel spin structures are observed.