Determination of Paramagnetic Layer Localization Depth in LaF3-Based Gd3+ Delta-Doped Nanoparticles Using 3He NMR

Various model systems of core LaF3 and core–shell–shell LaF3–LaF3:Gd-LaF3 (delta-doped) nanoparticles were synthesized and studied by nuclear magnetic resonance (NMR) of adsorbed 3He in the 1.5–7 K temperature range and by 19F NMR in the 1.5–70 K temperature range. The NMR technique, employing T1 measurements of 3He adsorbed on a surface covered with various amounts of solid nitrogen, was applied to the synthesized nanoparticles to determine the depth of impurity localization, validate the approach, and control the synthesis of delta-doped magnetic nanoparticles. The paramagnetic impurities were introduced into the nanoparticles via a three-step synthesis by coating pill-shaped diamagnetic LaF3 particles (26 × 40 nm2 in size) with a thin (∼0.3 nm) LaF3:Gd3+ layer and subsequently coating them with a layer of diamagnetic LaF3 with a thickness of 0.5–1.0 nm (core–shell–shell structure). All experiments were performed in the 15–18 MHz NMR frequency range for both probe nuclei. The observed systematic deviation between the outer shell thicknesses determined from 3He NMR measurements and the expected values reflects unavoidable imperfections in the synthesis steps. Notably, the sensitivity of adsorbed 3He NMR to such deviations makes it a powerful tool for monitoring and controlling the synthesis of core–shell–shell nanoparticles. In sharp contrast to 19F NMR, which shows no sensitivity to shell thicknesses or even the Gd3+ concentration, the adsorbed 3He NMR technique is highly sensitive and therefore better-suited for characterizing such structures. The applied approach was verified under different conditions and can therefore be used for characterizing various magnetic nanoparticles and controlling their synthesis.