Magnetic impurities in nanowires

Strong electron-correlation effects and in particular magnetism are among the most interesting and challenging topics in nanoscience. The unconventional properties of magnetic impurities in metal hosts reflect the competition between the tendency of electrons to delocalize, in order to minimize their kinetic energy, and the resulting local charge fluctuations, which increase the Coulomb-repulsion energy and which favor the formation of localized states. This competition depends so critically on the magnetic coupling between impurity and host electrons, that small magnetic energy gaps appear in the low-energy spectrum. Even the magnetic nature of the ground state is difficult to predict. Consequently, a remarkable low- temperature magnetic behavior is observed.

The purpose of this work is to investigate the properties of magnetic impurities in nanowires in the framework of Hohenberg-Kohn-Sham's (HKS) density-functional theory (DFT). One of the qualitatively most important outcomes of our calculations is that the optimal total spin Sz is not minimal but one above minimal. According to HKS-DFT this is a consequence of a reduction of the local atomic moment at the impurity, rather than the result of magnetic screening. In fact, as shown in the figure, the coupling between the impurity moment and the induced moments at the metal host is ferromagnetic-like. The results for the local density of electronic states at the impurity atoms confirm that the impurity moments are largely dominated by the d-electron contributions. Moreover, a 100% minority-spin polarization of the electronic states at the Fermi energy is observed. As illustrated in the Table, a more detailed analysis of the corresponding d wave functions reveals a remarkable correlation between the rotational symmetry and the degree of delocalization of the impurity states. These predictions should be detectable in STM experiments.

This work has been supported by the Deutsche Forschungsgemeinschaft through the Priority Program "Nanowires and Nanotubes" and by the DAAD-CONACyT exchange program PROALMEX.

[1] Electronic and magnetic properties of Co and Ni impurities in Cu wires: First-principles investigation of local moment formation in one dimension, Matthieu Saubanère, J. L. Ricardo-Chávez, and G. M. Pastor, Phys. Rev. B 82, 054436 (2010).

[2] First principles theoretical study of complex magnetic order in transition-metal nanowires, M. Saubanère, M. Tanveer, P. Ruiz-Díaz, and G. M. Pastor, Phys. Status Solidi B 247, 2610-2620 (2010).

Illustration of the charge-density distribution in the minority-spin orbitals ?k? close to the Fermi energy ?F of Cu5CoCu5.