Old 2degs with new tricks: Antiferromagnetic order and magnetoelectricity of 2D charge carriers

Ulrich Zuelicke, Victoria University of Wellington

In magnetoelectric media, an electric field can induce a magnetization and a magnetic field can induce an electric polarization, while the system remains in thermal equilibrium. This effect requires that both space-inversion and time-reversal symmetry are broken. I will present a comprehensive theory for magnetoelectricity in magnetically ordered quasi-2D systems. Considering ferromagnetic (FM) zincblende and antiferromagnetic (AFM) diamond structures, quantitative expressions for the magnetoelectric responses due to electric and magnetic fields are obtained that reveal explicitly the inherent duality of these responses required by thermodynamics. The magnitude of magnetoelectric effects in quasi-2D systems is tunable, and typical values are sizable in quasi-2D hole systems where moderate electric fields can induce a magnetic moment of one Bohr magneton per charge carrier. For the microscopic understanding of magnetoelectric responses in these systems, AFM order plays a central role. We define a Néel operator t that describes AFM order, in the same way a magnetization mreflects FM order. While m is even under space inversion and odd under time reversal, t describes a toroidal moment that is odd under both symmetries. Thus m and t quantify complementary aspects of magnetic order in solids. In quasi-2D systems, FM order can be attributed to dipolar equilibrium currents that give rise to a magnetization. In the same way, AFM order arises from quadrupolar currents that generate the toroidal moment. The electric-field-induced magnetization can then be attributed to the electric manipulation of the quadrupolar currents. Our theory provides a broad framework for the manipulation of magnetic order by means of external fields.

PDF file of the talk available here