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Magnetic interaction of Mn and Cr atoms on Ag(001) modified by graphene adsorption

We study the magnetic exchange interaction between Mn or Cr atoms adsorbed on Ag(001) surface by DFT as implemented in Quantum Espresso package. First we relax the structures in ferromagnetic spin configuration.Then, we calculate the total energies of various spin configurations in the obtained geometric configurations and obtain that the lowest energy spin configuration is antiferromagnetic for both kinds of adatoms. The same is obtained by fitting the Hamiltonian of Ising form to the DFT energies and obtaining the exchange parameters.We then turn to case of same system as before, but with graphene adsorbed on the adatoms. The structure is again relaxed in the ferromagnetic configuration and energies of various spin configurations are again calculated. The result is that the lowest energy spin configuration remains antiferromagnetic for Cr adatoms, while it changes for Mn adatoms.Since bonding with graphene broke symmetry between the adatoms, the system is relaxed in the minimum energy spin configurations and the energies are calculated again for sake of self-consistency, which is obtained already in first step. The relaxation however shows that the relation between the DFT energies of different spin configurations in same geometric configuration changes drastically with relaxation, as do the distances between the adatoms, which points to strong magnon-phonon coupling in the system. The Ising model may not be as appropriate in this case as in the case without graphene, but it follows the DFT results for the some part, and also shows that the exchange parameters change significantly upon change of geometry.

Magnetism ; surfaces ; 2D materials

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