  The control of domains in ferroic devices lies at the heart of their
potential for technological applications. Multiferroic materials offer another
level of complexity as domains can be either or both of a ferroelectric and
magnetic nature. Here we report the discovery of a novel magnetic state in the
orthoferrite TbFeO3 using neutron diffraction under an applied magnetic field.
This state has a very long incommensurate period ranging from 340 Angstrom at
3K to 2700 Angstrom at the lowest temperatures and exhibits an anomalously
large number of higher-order harmonics, allowing us to identify it with the
periodic array of sharp domain walls of Tb spins separated by many lattice
constants. The Tb domain walls interact by exchanging spin waves propagating
through the Fe magnetic sublattice. The resulting Yukawa-like force, familiar
from particle physics, has a finite range that determines the period of the
incommensurate state.
