  Rare earth-transition metal ferrimagnets, featuring antiferromagnetically
coupled, inequivalent magnetic sublattices, have garnered increasing interest
in the burgeoning field of ferrimagnetic spintronics. However, controlling
their magnetism with low voltages,a key to reducing power consumption,remains
challenging, particularly due to the poorly understood mechanisms underlying
bulk perpendicular magnetic anisotropy (PMA). Here, we introduce a method
involving voltage-driven hydrogen insertion into interstitial sites between Tb
and Co atoms, selectively perturbing the atomic structure and enabling
ultra-low-voltage (1.2 V) switching of bulk PMA to in-plane directions in
ferrimagnetic films. Combining experimental and theoretical analysis, we find
that the anisotropy switching originates from the reorientation of Tb orbital
moments induced by the distortion of the crystal field. Initially aligned along
Tb-Co bonding directions, the easy magnetization axis undergoes reorientation
and switches by 90 deg,as substantiated by ab-initio calculations. Our study
not only establishes the groundwork for voltage-controlled magnetic anisotropy
in ferrimagnetic films, facilitating the development of
electrically-programmable ferrimagnetic spintronics, but also elucidates the
atomic origins of PMA in amorphous ferrimagnets, shedding light on a
long-standing question in this field.
