Magnetic Memory from Site Isolated Dy(III) on Silica Materials

Abstract

Achieving magnetic remanence at single isolated metal sites dispersed at the surface of a solid matrix has been envisioned as a key step toward information storage and processing in the smallest unit of matter. Here, we show that isolated Dy(III) sites distributed at the surface of silica nanoparticles, prepared with a simple and scalable two-step process, show magnetic remanence and display a hysteresis loop open at liquid ⁴He temperature, in contrast to the molecular precursor which does not display any magnetic memory. This singular behavior is achieved through the controlled grafting of a tailored Dy(III) siloxide complex on partially dehydroxylated silica nanoparticles followed by thermal annealing. This approach allows control of the density and the structure of isolated, "bare" Dy(III) sites bound to the silica surface. During the process, all organic fragments are removed, leaving the surface as the sole ligand, promoting magnetic remanence.

Figure 1

Immobilization strategies of single-molecule and single-atom magnets. (A) Immobilization of single-molecule magnets by grafting (a), through surface decoration with organic ligands (b), or by physisorption on the surface (c). (B) Immobilization on carbon nanotubes by supramolecular interaction (a) or encapsulation (b) (part of the nanotube wall is not represented for clarity). (C) Isolated atoms on flat metallic surface protected by an insulating decoupling layer. (D) Our strategy for immobilization of ions on metal oxide surfaces, based on a grafting step (a) and a thermolytic step (b).

Figure 2

(A) Grafting of 1 on SiO_2–700 to form 1/SiO₂ followed by a thermal annealing under high vacuum yielded Dy@SiO₂ (color code: carbon (gray), silicon (orange), oxygen (red), and dysprosium (blue)). (B) FTIR transmission spectra of SiO_2–700, 1/SiO₂, and Dy@SiO₂. Grafting of 1 (1.05 equiv/silanol or ca. 0.26 mmol g^–1) on SiO_2–700 leads to the disappearance of most isolated silanols while a broad band appears at 3650 cm^–1, consistent with the presence of unreacted silanols interacting with organic ligands. (C) Dy L_III-edge XANES of 1, 1/SiO₂, and Dy@SiO₂. (D) k³ weighted EXAFS fits in k-space (top) and R-space (bottom) of 1. The fit of 1 includes five O atoms in the first coordination sphere: three at 2.11 Å, and one each at 2.39 and 2.53 Å. (E) k³ weighted EXAFS fits in k-space (top) and R-space (bottom) of 1/SiO₂. Aside from a shortening of the Dy–O paths of the κ² neutral silanol at 2.26 and 2.40 Å compared to the molecular structure, the structure of 1/SiO₂ is similar to 1 (Figures S4 and S5, Table S3). (F) k³ weighted EXAFS fits in k-space (top) and R-space (bottom) of Dy@SiO₂. The best fit was obtained by including five O ligands, three with short (2.20 Å) and two with longer (2.42 Å) Dy–O distances (Figures S4 and S6, Table S3).

Figure 3

(A) Frequency dependence of the two components (χ_M′ and χ_M″) of the ac susceptibility for Dy@SiO₂ measured between 2 and 18 K in the absence of an external dc field. (B) Hysteresis loop for Dy@SiO₂ measured at 2 K and at a sweep rate of 16 Oe s^–1.