Neutron scattering study of the magnetic microstructure of nanocrystalline gadolinium

Abstract

We report grain-size-dependent results on nanocrystalline bulk Gd obtained by magnetic small-angle neutron scattering (SANS) and magnetometry. This approach allows one to study systematically how the magnetic microstructure of this rare-earth metal is affected by defects in the atomic microstructure, which are largely present in nanocrystalline materials, predominantly in the form of grain boundaries. The neutron scattering data reveal two types of angular anisotropies in the magnetic-field-dependent scattering cross section that are typically not seen in the coarse-grained polycrystal. In particular, a cloverleaf-shaped anisotropy and an elongation of the scattering pattern in the direction of the applied magnetic field have been detected. While the first result, which is an exceptional finding even in the nanocrystalline state, can be attributed to pronounced spin disorder in the vicinity of the Gd grain boundaries, the second anisotropy is related to spin misalignment due to the random magnetocrystalline anisotropy within the individual crystallites. Furthermore, we have calculated the correlation function of the spin misalignment from the radially averaged data, which gives access to the characteristic length scales on which the magnetization is perturbed by crystal defects. The results of this real-space analysis independently support the findings from magnetometry and field-dependent SANS. Wide-angle x-ray diffraction data indicate that stacking faults may limit the range of spin-misalignment fluctuations due to random anisotropy in this material.

Magnetic SANS study of a sintered Nd–Fe–B magnet: Estimation of defect size

“…The parameter l C has been determined in the usual way as the value of r for which C(r) has decayed to exp(À1) of its value C(r = 0) at the origin, where the latter is obtained by extrapolation of the C(r) data; this extrapolation, from C(r min ) to C(r = 0), has been done using different functional dependencies (linear and quadratic), and the results for l C agree within 2-3 nm (for further details, see Refs. [23,24]). Fig.…”

Experimental and modeling studies on phase stability of nanocrystalline magnetic Sm2Co7

“…The increase of magnetic properties of nanocrystalline magnets is mainly attributed to the ultrafine nanograin microstructure [31][32][33]. On one hand, it has been found that the coercivity of nanocrystalline alloy increases first and then decreases with the decrease of grain size on the nanoscale [34].…”

Magnetic disorder in TbAl2nanoparticles

“…There is a linear behavior which can be connected to a reduction of the Tb 3+ magnetic moment can be explained by the existence of a spin disorder layer at the surface of the particles. In this sense, and as was also applied in other nanoparticle systems [30,31,32], the width of this surface layer can be estimated from the plot of M S vs the inverse of the mean diameter of the particles (1/D), assuming a core-shell structure in which the non-magnetic shell has a constant thickness (t). The behaviour of this relationship is well accounted by the expression M S = M bulk (1 − 6t/D) [30], being t = 0.8 nm in the case of the TbAl 2 alloys.…”