$\pi$-anisotropy: A nanocarbon route to hard magnetism

High coercivity magnets are an important resource for renewable energy, electric vehicles, and memory technologies. Most hard magnetic materials incorporate rare earths such as neodymium and samarium, but concerns about the environmental impact and supply stability of these materials are prompting research into alternatives. Here, we present a hybrid bilayer of cobalt and the nanocarbon molecule ${\text{C}}_{60}$ which exhibits significantly enhanced coercivity with minimal reduction in magnetization. We demonstrate how this anisotropy enhancing effect cannot be described by existing models of molecule-metal magnetic interfaces. We outline a form of anisotropy, arising from asymmetric magnetoelectric coupling in the metal-molecule interface. Because this phenomenon arises from $\pi \text{−}d$ hybrid orbitals, we propose calling this effect $\pi$-anisotropy. While the critical temperature of this effect is currently limited by the rotational degree of freedom of the chosen molecule, ${\text{C}}_{60}$, we describe how surface functionalization would allow for the design of room-temperature, carbon-based hard magnetic films.

Figure 1

(a) $MH$ curves for two indentical bilayer films of Ta(4 nm)/Co(3 nm), cooled to 5 K in a 2 T applied field. The red curve (stars) is an uncapped film while the black curve (circles) is capped with a 35-nm film of ${\text{C}}_{60}$. The increase in the maximum energy product with the addition of ${\text{C}}_{60}$ is $520\%$. The right-hand images show the expected orientation of the ${\text{C}}_{60}$ molecule on the Co surface before demagnetization (top) and after (bottom). (b) The energy product for a Co/${\text{C}}_{60}$ film as a function of temperature. There are two distinct regimes above and below the rotational transition of ${\text{C}}_{60}$ at 100 K. The red and blue fits are for the temperature-dependent pinning factor as described in the temperature-dependent Jiles-Atherton model, Eq. (1) [13].

Figure 2

(a) Minor loops for increasing values of $H_a$ for the first and second demagnetization sweeps. (b) Hysteron density plots for the first and second demagnetization of a Co/${\text{C}}_{60}$ film cooled to 5 K in a 2 T applied field. The distribution of reversal modes is significantly changed from the first to the second sweep.

Figure 3

(a) Representations of the Co/${\text{C}}_{60}$ stationary points during rotation as simulated via DFT. Three atoms on the hexagonal ring are marked for reference. The transition state (TS) shows the point in the HH-HP rotation where the surface energy is maximized. This is a first approximation to the energy barrier which must be overcome by the magnetoelectric torque at the interface. The ${\text{C}}_{60}$ will then reach the metastable HH state. (b) Hysteresis loop simulated using the mumax3 code. The surface pinning replicates the vertical domain wall nucleation predicted in the $\pi$-anisotropy model. In the second sweep, this surface anisotropy is reduced, resulting in the formation of in-plane domains. (c) Shows a color map for the slab simulated in (b). Red indicates spins pointing in the $+x$ direction and blue in the $-x$ direction.

Figure 4

(a) Top: hysteresis loops obtained from the first and second sweeps after cooling a Co/${\text{C}}_{60}$ bilayer in a 2 T applied field to 5 K. The exchange bias and asymmetry is completely destroyed after a single cycle. Middle: AMR (anisotropic magnetoresistance) recorded during the first sweep. Note that the magnetoresistance, $\mathrm{\Delta }\rho$, is zero at point B. Bottom: AMR recorded during second sweep. Note that the AMR now exhibits expected behavior for both forward and backward sweeps. (b) Comparison of hysteresis loops at 5 K for Co/${\text{C}}_{60}$ bilayer (black circles), a Co/${\text{C}}_{70}$ bilayer (red stars), and the same Co/${\text{C}}_{60}$ bilayer after removing the molecular film with a combination of acetone and UV (blue hollow circles). (c) Dependence of the maximum recorded coercivity at 5 K on the thickness of the Ta seed layer, showing the importance of seeding the correct structure in the Co thin film.