Unconventional Magnetic Domain Structure in the Ferromagnetic Phase of MnP Single Crystals

We have studied ferromagnetic (FM) structures in the FM phase of MnP single crystals by low-temperature Lorentz transmission electron microscopy and small-angle electron diffraction analysis. In Lorentz Fresnel micrographs, striped FM domain structures were observed at an external magnetic field less than 10 Oe in specimens with the $ab$-plane in their plane. From real- and reciprocal-space analyses, it was clearly identified that striped FM domains oriented to the $c$-axis appear with Bloch-type domain walls in the $b$-direction and order regularly along the $a$-axis with a constant separation less than 100 nm. Moreover, the magnetic chirality reverses in alternate FM domain walls. These specific spin configuration of striped FM domains will affect the magnetic phase transition from the FM phase to the proper screw spiral phase at low temperature or to the FAN phase in magnetic fields in MnP.