Associate Professor, Cryogenic Research Center, The University of Tokyo
Group Leader, Tokura Spin Superstructure Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency (JST)
Anomalous Hall Effect and magnetic monopole in momentum space
In real space, magnetism always occurs due to a magnetic dipole, and a magnetic monopole has not been found to be naturally occurring. However, a magnetic monopole can appear in the momentum space of solids due to the anomalous Hall Effect in ferromagnetic metals. Here, we report experimental results together with first-principle calculations involving the ferromagnetic metal SrRuO3, which provide evidence for the existence of a magnetic monopole in momentum space.
Bulk crystals and high quality thin films of SrRuO3 are fabricated using flux-growth techniques and pulsed laser deposition, respectively. The anomalous Hall Effect, or the transverse conductivity, σxy changes with temperature or magnetization, changes sign and has a fairly large value of -60 Ω·1cm·1 at absolute zero (Fig.1). Theoretically, σxy is represented as an integral of the gauge field, which corresponds to the “effective magnetic field” that an electron feels and comes from the quantal phase, or the so-called Berry phase, of an electron Bloch wave function. The source, or sink, of the gauge field corresponds to the magnetic monopole in momentum space, and its position and distribution quantitatively govern the anomalous Hall Effect in this system (Fig.2).
We are convinced that the novel idea of “controlling the monopole in solids” will be a promising method for phase control in solids as well as be useful in future applications, such as colossal magnetoresistive or magneto-optical devices.
