Surfactant-assisted production of TbCu₂ nanoparticles

Abstract

The production of surfactant-assisted metallic nanoparticles of TbCu₂ has been achieved by the combination of high-energy ball milling in tungsten carbide containers and the use of oleic acid (C₁₈H₃₄O₂) and heptane (C₇H₁₆). The alloys were first produced in bulk pellets by arc melting and subsequently milled for only 2 and 5 h in oleic acid (15 and 30% mass weight). The powders consist of an ensemble of nanoparticles with a TbCu₂ lattice cell volume of ≈215 Å³, an average particle diameter between 9 and 12 nm and inhomogeneous lattice strain of 0.2–0.4%, as deduced from X-ray diffraction data. The nanometric sizes of the crystals with defined lattice planes are close to those obtained by transmission electron microscopy. Raman spectroscopy shows the existence of inelastic peaks between 1000 and 1650 cm⁻¹, a characteristic of C₁₈H₃₄O₂. The magnetisation shows a peak at the antiferromagnetic-paramagnetic transition with Néel temperatures around 48 K (below that of bulk alloy) and a distinctive metamagnetic transition at 5 K up to 40 K. The Curie-Weiss behaviour above the transition reveals effective Bohr magneton numbers (≈9.1–9.9 μ_B) which are close to what is expected for the free Tb³⁺ ion using Hund’s rules. The metamagnetic transition is slightly augmented with respect to the bulk value, reaching H = 24.5 kOe by the combined effect of the size reduction and the lattice strain increase and the increase of magnetic disorder. At low temperatures, there is irreversibility as a result of the existing magnetic disorder. The moment relaxation follows an Arrhenius model with uncompensated Tb moments, with activation energies between 295 and 326 K and pre-exponential factors between 10⁻¹¹ and 10⁻¹³ s. The results are interpreted as a consequence of the existence of a diamagnetic surfactant which drastically decreases the magnetic coupling between interparticle moments.