  Conversion of thermal energy into magnonic spin currents and/or effective
electric polarization promises new device functionalities. A versatile approach
is presented here for generating and controlling open circuit magnonic spin
currents and an effective multiferroicity at a uniform temperature with the aid
of spatially inhomogeneous, external, static electric fields. This field
applied to a ferromagnetic insulator with a Dzyaloshinskii-Moriya type coupling
changes locally the magnon dispersion and modifies the density of thermally
excited magnons in a region of the scale of the field inhomogeneity. The
resulting gradient in the magnon density can be viewed as a gradient in the
effective magnon temperature. This effective thermal gradient together with
local magnon dispersion result in an open-circuit, electric field controlled
magnonic spin current. In fact, for a moderate variation in the external
electric field the predicted magnonic spin current is on the scale of the spin
(Seebeck) current generated by a comparable external temperature gradient.
Analytical methods supported by full-fledge numerics confirm that both, a
finite temperature and an inhomogeneous electric field are necessary for this
emergent non-equilibrium phenomena. The proposal can be integrated in magnonic
and multiferroic circuits, for instance to convert heat into electrically
controlled pure spin current using for example nanopatterning, without the need
to generate large thermal gradients on the nanoscale.
