ABSTRACT
A change in the Kondo lattice behavior of bulk YbAl3 has been observed when the alloy is shaped into nanoparticles (≈12 nm). Measurements of the electrical resistivity show inhibited coherence effects and deviation from the standard Fermi liquid behavior (T 2-dependence). These results are interpreted as being due to the effect of the disruption of the periodicity of the array of Kondo ions provoked by the size reduction process. Additionally, the ensemble of randomly placed nanoparticles also triggers an extra source of electronic scattering at very low temperatures (≈15 K) due to quantum interference effects.
ABSTRACT
The structural state and static and dynamic magnetic properties of TbCu2 nanoparticles are reported to be produced by mechanical milling under inert atmosphere. The randomly dispersed nanoparticles as detected by TEM retain the bulk symmetry with an orthorhombic Imma lattice and Tb and Cu in the 4e and 8h positions, respectively. Rietveld refinements confirm that the milling produces a controlled reduction of particle sizes reaching ≃6 nm and an increase of the microstrain up to ≃0.6%. The electrical resistivity indicates a metallic behavior and the presence of a magnetic contribution to the electronic scattering which decreases with milling times. The dc-susceptibility shows a reduction of the Néel transition (from 49 K to 43 K) and a progressive increase of a peak (from 9 K to 15 K) in the zero-field-cooled magnetization with size reduction. The exchange anisotropy is very weak (a bias field of ≃30 Oe) and is due to the presence of a disordered (thin) shell coupled to the antiferromagnetic core. The dynamic susceptibility evidences a critical slowing down in the spin-disordered state for the lowest temperature peak associated with a spin glass-like freezing with a tendency of zv and ß exponents to increase when the size becomes 6 nm (zv ≃ 6.6 and ß ≃ 0.85). A Rietveld analysis of the neutron diffraction patterns 1.8 ≤ T ≤ 60 K, including the magnetic structure determination, reveals that there is a reduction of the expected moment (≃80%), which must be connected to the presence of the disordered particle shell. The core magnetic structure retains the bulk antiferromagnetic arrangement. The overall interpretation is based on a superantiferromagnetic behavior which at low temperatures coexists with a canting of surface moments and a mismatch of the antiferromagnetic sublattices of the nanoparticles. We propose a novel magnetic phase diagram where changes are provoked by a combination of the decrease of size and the increase of microstrain.
ABSTRACT
The magnetic properties of nanometric TbAI2 alloys have been investigated. The Curie temperature (T(c)) of these nanometric alloys is strongly size dependent and decreases from 103 K for the bulk alloy down to 98 K for the 14 nm alloy, as the particle volume is reduced. This reduction of T(c) has been explained by a finite-size scaling law of type [T(c)(D) -T(c)(infinity)]/T(c)(infinity) = -(D/D0)-(1/vp), with v = 0.7 and D0 = 2.2a (a, the lattice parameter), in agreement with the three-dimensional Heisenberg model. The size dependence of the coercivity has also been established. An increase of the coercivity from 0.08 kOe (bulk) to 1 kOe for 10 h milled alloy, indicates the crossover from multidomain to single domain behavior around 85 nm, as expected from the estimate of the critical size of monodomain particles. The field dependence of the magnetization indicates a faster thermal reduction of the magnetization of the nanosized alloys (17% in 300 h milled alloy with mean particle size of 14 nm) related to the bulk (3%), in the temperature range between 5 K and 30 K. The results can be explained as a direct consequence of the competing effects of the surface and the purely finite-size effects, in an ensemble of nanometric particles suffering interactions.
