Stable magnets from single atoms: An important goal in molecular magnetism is to create a permanent magnet from a single atom. Metal atoms adsorbed on surfaces can develop strong magnetization in an applied field (paramagnetism). Donati et al. show that single holmium atoms adsorbed on a magnesium oxide film grown on a silver substrate show residual magnetism for temperatures up to 30 K and bistabilty that lasts for 1500 s at 10 K (see the Perspective by Khajetoorians and Heinrich). The atom avoids spin relaxation by a combination of quantum-state symmetry and by the oxide film preventing the spin from interacting with the underlying metal via tunneling. Science , this issue p. 318 ; see also p. 296

The development of chemical systems with switchable molecular spins could lead to the architecture of materials with controllable magnetic or spintronic properties. Here, we present conclusive evidence that the spin of an organometallic molecule coupled to a ferromagnetic substrate can be switched between magnetic off and on states by a chemical stimulus. This is achieved by nitric oxide (NO) functioning as an axial ligand of cobalt(II)tetraphenylporphyrin (CoTPP) ferromagnetically coupled to nickel thin-film (Ni(001)). On NO addition, the coordination sphere of Co²⁺ is modified and a NO–CoTPP nitrosyl complex is formed, which corresponds to an off state of the Co spin. Thermal dissociation of NO from the nitrosyl complex restores the on state of the Co spin. The NO-induced reversible off–on switching of surface-adsorbed molecular spins observed here is attributed to a spin trans effect.

Realization of long-range magnetic order in surface-supported two-dimensional systems has been challenging, mainly due to the competition between fundamental magnetic interactions as the short-range Kondo effect and spin-stabilizing magnetic exchange interactions. Spin-bearing molecules on conducting substrates represent a rich platform to investigate the interplay of these fundamental magnetic interactions. Here we demonstrate the direct observation of long-range ferrimagnetic order emerging in a two-dimensional supramolecular Kondo lattice. The lattice consists of paramagnetic hexadeca-fluorinated iron phthalocyanine (FeFPc) and manganese phthalocyanine (MnPc) molecules co-assembled into a checkerboard pattern on single-crystalline Au(111) substrates. Remarkably, the remanent magnetic moments are oriented in the out-of-plane direction with significant contribution from orbital moments. First-principles calculations reveal that the FeFPc-MnPc antiferromagnetic nearest-neighbour coupling is mediated by the Ruderman–Kittel–Kasuya–Yosida exchange interaction via the Au substrate electronic states. Our findings suggest the use of molecular frameworks to engineer novel low-dimensional magnetically ordered materials and their application in molecular quantum devices.

Paramagnetic transition-metal complexes assembled on surfaces are of great interest for potential applications in organic spintronics. The magnetochemical interactions of the spin of the metal centers with both ferromagnetic surfaces and optional axial ligands are yet to be understood. We use a combination of X-ray magnetic circular dichroism (XMCD) and quantum-chemical simulations based on density functional theory (DFT + U) to investigate these metal–organic interfaces with chemically tunable magnetization. The interplay between an optional axial ligand (NO, spin S = 1/2 or NH₃ , S = 0) and Ni and Co ferromagnetic surfaces affecting the spin of Co(II) tetraphenylporphyrin (d⁷, S = 1/2), Fe(II) tetraphenylporphyrin (d⁶, S = 1), Mn(II) tetraphenylporphyrin (d⁵, S = 5/2) and Mn(II) phthalocyanine (d⁵, S = 3/2) is studied. We find that the structural trans effect on the surface rules the molecular spin state, as well as the sign and strength of the exchange interaction with the substrate. We refer to this observation as the surface spin-trans effect.