Coupling of single molecule magnets to ferromagnetic metals

Conventional magnets rely on the collective behaviour of the unpaired electron spins of millions of atoms. Single molecule magnets (SMM), on the other hand, may contain just one spin centre enclosed by an organic cage, which is the case for the bis-phthalocyaninato terbium complex (TbPc₂) [1]. Because they are so small and uniform, single molecule magnets are ideal candidates for magnetic data storage and quantum computing applications [2]. Yet, despite intense research efforts, their usefulness to fabricate molecular memories is severely limited by two factors. Firstly, SMM display remanent magnetisation only at very low temperature [1]. Secondly, interfacing SMM with solid state systems (electrodes, metal surfaces, etc.) has proven far from trivial [3].

We thus examined the interaction of SMM with ferromagnetic metal films, which is a prerequisite to embed such molecules into hybrid metal-organic architectures to make, for example, a molecular spin valve device [2]. We focused on the TbPc₂ complex owing to its compact structure, which constitutes an advantage for the establishment of an exchange-coupling path between the magnetic core of the molecule (the Tb atom) and the underlying magnetic film (see Figure 1). Ni films were deposited on Cu(100) and Ag(100) single crystals providing substrates with either out-of-plane or in-plane easy magnetisation, a parameter that can be controlled through epitaxial strain without changing the chemical composition of the interface.

Figure 1. TbPc2 single molecule magnets coupled to ferromagnetic Ni films with (a) out-of-plane and (b) in-plane easy magnetisation axis. The plots show element-resolved out-of-plane magnetisation loops of Ni (blue) and Tb (red) obtained by X-ray magnetic circular dichroism measurements. The arrows indicate the relative orientation of the molecule and substrate magnetic moments.

By performing element-resolved magnetisation measurements using X-ray magnetic circular dichroism (XMCD) at beamline ID08, we proved that the magnetic moment of TbPc₂ deposited on Ni is coupled antiparallel, that is, antiferromagnetic, to that of the Ni substrate. If the easy magnetisation axis of TbPc₂, which is out-of-plane, coincides with that of the Ni film, the magnetic moment of the molecule is effectively stabilised by the interaction with the substrate, resulting in a square magnetisation hysteresis curve with nearly saturated magnetic remanence at zero applied field (Figure 1a). Finite magnetic remanence persists up to 100 K, a temperature that is two orders of magnitude higher than that for isolated TbPc₂ [1]. Depending on the strength of the applied magnetic field, we observe that both antiparallel and parallel magnetic configurations can be reached, as the Zeeman interaction compensates and eventually overcomes the exchange coupling between Tb and Ni. Moreover, if the easy magnetisation axis of the Ni film is orthogonal to that of TbPc₂, we observe pronounced frustration effects as the molecule magnetisation cannot align with the substrate at equilibrium, and exhibits zero remanence at zero field (Figure 1b).

The detailed mechanism mediating the coupling between Tb and Ni atoms remains to be clarified. Given that Tb and Ni are physically separated by a Pc ligand, our data point towards an indirect exchange mechanism mediated by electrons hopping back and forth between the Pc macrocycle and Tb on one side and Ni on the other (Tb-Pc-Ni superexchange). We find that the strength of the molecule-substrate coupling can be tuned by electron or hole doping of the interface, which is expected to change the occupation of the Pc electron orbitals [4]. This behaviour also shows how the interface chemistry and magnetic response are intimately related in such systems.

In summary, these results demonstrate that SMM behave as coupled but separate magnetic units from an underlying ferromagnetic substrate. The enhanced thermal stability of the TbPc₂ magnetic moment and the possibility to orient it parallel or antiparallel to a macroscopic ferromagnetic layer make this complex very interesting for applications in metal-organic magnetoelectronic devices. Further experiments taking advantage of the element-resolving power of X-rays may address the coupling of different families of SMM to magnetic layers, and extend this research to more complex multilayer structures.

Principal publication and authors
A. Lodi Rizzini (a), C. Krull (a), T. Balashov (a), J.J. Kavich (a), A. Mugarza (a), P.S. Miedema (b), P.K. Thakur (c), V. Sessi (c), S. Klyatskaya (d), M. Ruben (d,e), S. Stepanow (f), and P. Gambardella (a,g,h), Phys. Rev. Lett. 107, 177205 (2011).

References
[1] N. Ishikawa, Polyhedron 26, 2147 (2007).
[2] L. Bogani and W. Wernsdorfer, Nat. Mater. 7, 179 (2008).
[3] M. Mannini et al., Nat. Mater. 8, 194 (2009); H. Wende, Nat. Mater. 8, 165 (2009). 
[4] A. Mugarza, C. Krull, R. Robles, S. Stepanow, G. Ceballos, and P. Gambardella, Nature Comm. 2, 490 (2011).

Top image: Scheme of a TbPc2 single molecule magnet deposited on a Ni film, with relative orientation of the magnetic moments (arrows).