Voltage-tunable ferromagnetism in semimagnetic quantum dots with few particles: magnetic polarons and electrical capacitance

TL;DR

This paper explores how voltage can tune ferromagnetism in semimagnetic quantum dots with few particles, revealing magnetic polaron behavior, shell effects, and potential magnetic state transitions relevant for quantum device applications.

Contribution

It demonstrates how quantum dot geometry and particle number influence magnetic polarons, their anisotropy, and ferromagnetic stability, introducing new insights into voltage-controlled magnetism at the nanoscale.

Findings

Magnetic polarons exhibit strong anisotropy due to valence band spin splitting.

Shell structure causes oscillations in binding energy and magnetization.

Capacitance spectra reveal formation and characteristics of polaron states.

Abstract

Magnetic semiconductor quantum dots with a few carriers represent an interesting model system where ferromagnetic interactions can be tuned by voltage. By designing the geometry of a doped quantum dot, one can tailor the anisotropic quantum states of magnetic polarons. The strong anisotropy of magnetic polaron states in disk-like quantum dots with holes comes from the spin splitting in the valence band. The binding energy and spontaneous magnetization of quantum dots oscillate with the number of particles and reflect the shell structure. Due to the Coulomb interaction, the maximum binding energy and spin polarization of magnetic polarons occur in the regime of Hund's rule when the total spin of holes in a quantum dot is maximum. With increasing number of particles in a quantum dot and for certain orbital configurations, the ferromagnetic state becomes especially stable or may have broken symmetry. In quantum dots with a strong ferromagnetic interaction, the ground state can undergo a transition from a magnetic to a nonmagnetic state with increasing temperature or decreasing exchange interaction. The characteristic temperature and fluctuations of magnetic polarons depend on the binding energy and degeneracy of the shell. The capacitance spectra of magnetic quantum dots with few particles reveal the formation of polaron states.