Magnetic properties of β-MnO2 thin films grown by plasma-assisted molecular beam epitaxy

Single crystal beta-MnO2 films have been epitaxially grown on (001) LaAlO3 and (001) MgO substrates by using plasma-assisted molecular beam epitaxy. On the basis of reflective high energy electron diffraction, X-ray diffraction, Raman spectroscopy, and X-ray photoemission spectroscopy, the films are confirmed to be beta-MnO2 with slight oxygen vacancies resulting in coexistence of Mn3+ and Mn4+. These films are ferromagnetic at room temperature, which is reckoned to result from new forms of exchange interaction in MnO2 due to coexisting Mn3+ and Mn4+. The films on various substrates show different magnetic properties at 5 K attributable to different spin configurations between these films, which supports Yoshimori's theoretical prediction that an applied stress can vary the screw spin configuration. Two potential interaction mechanisms are proposed regarding the origins of the observed magnetic properties.