  Pinning-type magnets maintaining high coercivity, i.e. the ability to sustain
magnetization, at high temperature are at the core of thriving clean-energy
technologies. Among these, Sm2Co17-based magnets are excellent candidates owing
to their high-temperature stability. However, despite decades of efforts to
optimize the intragranular microstructure, the coercivity currently only
reaches 20~30% of the theoretical limits. Here, the roles of the grain-interior
nanostructure and the grain boundaries in controlling coercivity are
disentangled by an emerging magneto-electric approach. Through hydrogen
charging/discharging by applying voltages of only ~ 1 V, the coercivity is
reversibly tuned by an unprecedented value of ~ 1.3 T. In situ
magneto-structural measurements and atomic-scale tracking of hydrogen atoms
reveal that the segregation of hydrogen atoms at the grain boundaries, rather
than the change of the crystal structure, dominates the reversible and
substantial change of coercivity. Hydrogen lowers the local magnetocrystalline
anisotropy and facilitates the magnetization reversal starting from the grain
boundaries. Our study reveals the previously neglected critical role of grain
boundaries in the conventional magnetisation-switching paradigm, suggesting a
critical reconsideration of strategies to overcome the coercivity limits in
permanent magnets, via for instance atomic-scale grain boundary engineering.
