Surface-induced magnetism of the solids with impurities and vacancies

TL;DR

This study uses quantum-mechanical calculations to explore how surfaces and impurities induce magnetic states in solids, revealing surface-specific magnetic phenomena and potential mechanisms for room-temperature ferromagnetism in certain oxides.

Contribution

It demonstrates that surface impurities and vacancies can induce magnetic triplet states, differing from bulk behavior, and suggests a new mechanism for ferromagnetism in oxide thin films.

Findings

Surface impurities near solids' surfaces favor magnetic triplet ground states.

Surface defect pairs exhibit ferromagnetic interactions at nanometer distances.

Metastable magnetic states suggest possible room-temperature ferromagnetism mechanisms.

Abstract

Using the quantum-mechanical approach combined with the image charge method we calculated the lowest energy levels of the impurities and neutral vacancies with two electrons or holes located in the vicinity of flat surface of different solids. We obtained that the magnetic triplet state is the ground state of the impurities and neutral vacancies in the vicinity of surface, while the nonmagnetic singlet is the ground state in the bulk for e.g. He atom, Li+, Be++, etc. ions. The energy difference between the lowest triplet and singlet states strongly depends on the electron (hole) effective mass, dielectric permittivity of the solid and the distance from the surface. Pair interaction of the identical surface defects (two doubly charged impurities or vacancies with two electrons or holes) reveals the ferromagnetic spin state with the maximal exchange energy at the definite distance between the defects (~5-25 nm). We obtained that the nonmagnetic singlet state is the lowest one for a molecule with two electrons formed by a pair of identical surface impurities (like surface hydrogen), while its next state with deep enough negative energy minimum is the magnetic triplet. The metastable magnetic triplet state appeared for such molecule at the surface indicates the possibility of metastable orto-states of the hydrogen-like molecules, while they are absent in the bulk of material. We hope that obtained results could provide an alternative mechanism of the room temperature ferromagnetism observed in TiO2, HfO2, and In2O3 thin films with contribution of the oxygen vacancies.