Observation of ferrotoroidic domains in a metal

Ferrotoroidic order has recently been established as a new form of spontaneous long-range order. Its inherent magnetoelectric properties have the potential to be utilized for spintronics based on magnetization-free systems. While spintronics applications have been mostly discussed for conducting systems, the ferroic properties of ferrotoroidic order have been mainly studied in insulating materials thus far. Here we report on the observation of ferrotoroidic order in a rare-earth tetraboride, NdB4, that shows metallic conductivity and no spontaneous magnetization. Using optical second harmonic generation (SHG), we spatially resolve the formation, distribution, and behavior of the ferrotoroidic domains across the ordering temperature. Due to the interference between the domains with the opposite ferrotoroidic polarity, SHG signals cancel out at the domain boundaries, which enables us to image the ferrotoroidic domain pattern. This study opens the way to explore unique functionalities of ferrotoroidic domains in metals, such as an electric-current-induced domain switching.