Lanthanide metal-organic network featuring strong perpendicular magnetic anisotropy

Abstract

The coordination of lanthanides atoms in two-dimensional surface-confined metal-organic networks is a promising path to achieve an ordered array of single atom magnets. These networks are highly versatile with plenty of combinations of molecular linkers and metallic atoms. Notably, with an appropriate choice of molecules and lanthanide atoms it should be feasible to tailor the orientation and intensity of the magnetic anisotropy. However, up to now only tilted and almost in-plane easy axis of magnetizations were reported in lanthanide-based architectures. Here we introduce an Er-directed two-dimensional metallosupramolecular network on Cu(111) featuring strong out-of-plane magnetic anisotropy. Our results will contribute to pave avenues for the use of lanthanides in potential applications in nanomagnetism and spintronics.

Fig. 1. Structure of Er-TDA network on Cu(111). (a) High resolution STM image with an atomistic model superimposed. Scanning parameters: V_b = 500 mV and I_t = 500 pA, T = 4 K, scale bar: 2.0 nm. (b) Side and top views of the DFT-optimized network. C, H, O, Er and Cu atoms are represented by black, white, red, green and orange balls, respectively.

Fig. 2. Electronic properties of Er-TDA metal–organic network on Cu(111). (a) STS spectra taken at Er center (purple), TDA molecule (red), and Cu substrate (green). The dashed lines indicated the energies were the dI/dV maps were taken. The inset presents a STM image showing the positions were the STS where taken. Scanning parameters: V_b = 500 mV and I_t = 500 pA, T = 4 K, scale bar: 3.0 nm. STS parameters: tip stabilization at V_b = 3 V and I_t = 700 pA; lock-in modulation of 20 mV. (b) Constant-current dI/dV map taken at 1.73 V with a tip functionalized with CO, showing the LUMO resonance. Scale bar: 2.0 nm. (c) Reference STM image for (b). Scanning parameters: V_b = 1.73 V and I_t = 200 pA, T = 4 K, scale bar: 2.0 nm. (d) Constant-current dI/dV map taken at 2.67 V with a tip functionalized with CO, showing the Er resonance. Scale bar: 2.0 nm. (e) Reference STM image for (d). Scanning parameters: V_b = 2.67 V and I_t = 200 pA, T = 4 K, scale bar: 2.0 nm.

Fig. 3. Characterization of the magnetic properties of the Er-TDA network on Cu(111). (a and b) STM images of the Er-TDA network on Cu(111). Scanning parameters: (a) V_b = 500 mV, I_t = 300 pA, T = 90 K, scale bar: 10 nm; (b) V_b = 500 mV, I_t = 300 pA, T = 90 K, scale bar: 2 nm. (c) Magnetization curves constructed by measuring the XMCD intensity at the most intense peak of the M₅-edge at NI (red) and GI (blue). Dark (light) dots represent the descending (ascending) branches. The inset shows field dependent measurements of the XMCD intensity for some selected fields (T = 1.7 K). See the ESI for more details. (d) XAS spectra acquired with positive (μ₊, green dots) and negative (μ₋, orange dots) circularly polarized light and XMCD (μ₋–μ₊) taken at the Er M_4,5-edges at NI (red dots) and GI (blue dots) (B = 6 T, T = 1.7 K). (e) XAS spectra acquired with vertical (μ_V, brown dots) and horizontal (μ_H, purple dots) linearly polarized light and XLD (μ_V–μ_H) taken at the Er M₅-edge at GI at 0.05 T (black dots) and 6 T (pink dots) (T = 1.7 K). Multiplet calculations are presented in (d and e) together with the experimental data with brighter lines.