Room temperature organic magnets derived from sp3 functionalized graphene

Abstract

Materials based on metallic elements that have d orbitals and exhibit room temperature magnetism have been known for centuries and applied in a huge range of technologies. Development of room temperature carbon magnets containing exclusively sp orbitals is viewed as great challenge in chemistry, physics, spintronics and materials science. Here we describe a series of room temperature organic magnets prepared by a simple and controllable route based on the substitution of fluorine atoms in fluorographene with hydroxyl groups. Depending on the chemical composition (an F/OH ratio) and sp³ coverage, these new graphene derivatives show room temperature antiferromagnetic ordering, which has never been observed for any sp-based materials. Such 2D magnets undergo a transition to a ferromagnetic state at low temperatures, showing an extraordinarily high magnetic moment. The developed theoretical model addresses the origin of the room temperature magnetism in terms of sp²-conjugated diradical motifs embedded in an sp³ matrix and superexchange interactions via -OH functionalization.

Figure 1. Representative preparation of G(OH)F.

The scheme depicting the chemical procedure towards G(OH)F.

Figure 2. Physicochemical characterization of GF and G(OH)F.

(a) High-resolution C 1s spectrum of G(OH)F and GF precursor (inset). (b) Raman spectra of G(OH)F (black line) and GF (red line). (c) FT-IR spectra of G(OH)F (red line) and GF (blue line). (d) HRTEM image (scale bar, 100 nm) of a G(OH)F sheet with an inset (scale bar, 100 nm) demonstrating a single-sheet character of G(OH)F.

Figure 3. Chemical mapping of a G(OH)F sheet by STEM–HAADF.

(a) Distribution of fluorine atoms on the G(OH)F sheet (scale bar, 50 nm). (b) Distribution of oxygen atoms on the G(OH)F sheet (scale bar, 50 nm). (c) STEM/HAADF image of the G(OH)F sheet (scale bar, 10 nm). (d) Combined F/O chemical mapping of the G(OH)F sheet (scale bar, 6 nm). (e) Combined C/F/O chemical mapping of the G(OH)F sheet (scale bar, 6 nm).

Figure 4. Magnetization measurements.

(a) Temperature evolution of the mass magnetic susceptibility (χ_mass) of G(OH)F, measured under an external magnetic field (H_ext) of 10 kOe. The insets show the hysteresis loops of G(OH)F at 5 K, which indicate non-zero coercivity and a saturation magnetization of almost 1 emu g⁻¹. (b) Isothermal magnetization curves of G(OH)F at temperatures of 5–60 K. The insets show the profile of the hysteresis loops around the origin and the temperature dependence of coercivity (H_C). (c) Isothermal magnetization curves of G(OH)F, recorded from 100 to 400 K. The inset shows the profile of the isothermal magnetization curves at 350 and 400 K, demonstrating a dramatic decrease in the curve's gradient upon increasing temperature above 350 K; this implies a transition from an AFM state to a paramagnetic regime. (d) Temperature evolution of χ_mass of G(OH)F, measured under an external magnetic field of 10 kOe. The arrows show the reversibility of the χ_mass profile on warming the sample from 5 to 400 K and then cooling from 400 to 5 K. The inset depicts the behaviour of χ_mass between 300 and 400 K including its sudden drop above 370 K, which is indicative of a transition from an AFM state to the paramagnetic regime with a Néel transition temperature of about 372 K. Note: the paramagnetic signal from the non-interacting paramagnetic centres was subtracted from the χ_mass data.

Figure 5. Spin densities and densities of states.

(a,b) The m-xylylene-like motif (green) of G(OH)F embedded in an sp³ lattice with the corresponding FM (a) and AFM (b) phases, with up/down spin densities shown in yellow/blue. (c,d) sp³-embedded trimethylenemethane-like motif with the corresponding FM spin density. Brown/green, blue, red and pink balls represent carbon, fluorine, oxygen and hydrogen atoms, respectively. (e) DOS of the GS FM phase. (f) DOS of the AFM phase. For (e) and (f), orbital contributions of individual atoms (labelled in the inset legend) are presented in both spectra. Electronic states of carbon atoms with up/down magnetic moments are plotted by brown/black line. Electronic states with the same spin direction can hybridize with each other. The hybridization involves the occupied and unoccupied spin-up states (e) and the occupied and unoccupied spin-up or spin-down states (f). Such kind of coupling not involving a finite density of states at E_F is termed as the superexchange interaction. Both DOS spectra show a significant contribution of oxygen orbitals to the midgap states, which implies an important role of -OH group in the superexchange interactions.

Figure 6. Magnetic properties as a function of C₁₈(OH)_yF_x stoichiometry.

(a) The mean magnetization map indicates which C₁₈(OH)_yF_x (x=0–8, y=0–8) stoichiometries are likely to exist in ferromagnetic and non-magnetic ground states. Both –F and –OH groups are assumed to be randomly distributed across the sample (as suggested by STEM–HAADF elemental mapping, cf. Fig. 3) at zero temperature and the possibility of kinetically controlled –F/–OH migration is not considered. The white circles indicate experimentally studied samples: 1—C₁₈(OH)_1.8F_7.2 (ferromagnetic in the ground state), 2—C₁₈(OH)_1.5F₆ (ferromagnetic in the ground state), 3—C₁₈(OH)₂F₃ (diamagnetic in the ground state), 4—C₁₈(OH)_2.6F_4.7 (ferromagnetic in the ground state), 5—C₁₈(OH)_2.4F₇ (ferromagnetic in the ground state), 6—C₁₈F_2.5 (diamagnetic in the ground state), 7—C₁₈F₄ (ferromagnetic in the ground state) and 8—C₁₈F_6.3 (diamagnetic in the ground state). (b) The isothermal magnetization (M) curve of C₁₈(OH)_1.8F_7.2 (Sample No. 1) as a function of an external magnetic field (H), recorded at a temperature of 5 K. (c) M versus H curve of C₁₈(OH)_1.5F₆ (Sample No. 2), recorded at a temperature of 5 K. (d) M versus H curve of C₁₈(OH)₂F₃ (Sample No. 3), recorded at a temperature of 5 K. (e) M versus H curve of C₁₈(OH)_2.6F_4.7 (Sample No. 4), recorded at a temperature of 5 K. (f) M versus H curve of C₁₈(OH)_2.4F₇ (Sample No. 5), recorded at a temperature of 5 K. (g) M versus H curve of C₁₈F_2.5 (Sample No. 6), recorded at a temperature of 5 K. (h) M versus H curve of C₁₈F₄ (Sample No. 7), recorded at a temperature of 5 K. (i) M versus H curve of C₁₈F_6.3 (Sample No. 8), recorded at a temperature of 5 K. The insets in panel (b,c,e,f and h) show the behaviour of the respective hysteresis loops around the origin with the saturation magnetization (M_S) indicated.