  The coexistence of superconductivity and ferromagnetism is a long-standing
issue in superconductivity due to the antagonistic nature of these two ordered
states. Experimentally identifying and characterizing novel heterointerface
superconductors that coexist with magnetism presents significant challenges.
Here, we report the experimental observation of two-dimensional long-range
ferromagnetic order in the KTaO$_3$ heterointerface superconductor, showing the
coexistence of superconductivity and ferromagnetism. Remarkably, our direct
current superconducting quantum interference device measurements reveal an
in-plane magnetization hysteresis loop persisting above room temperature.
Moreover, the first-principles calculations and X-ray magnetic circular
dichroism measurements provide decisive insights into the origin of the
observed robust ferromagnetism, attributing it to oxygen vacancies that
localize electrons in nearby Ta 5$d$ states. Our findings not only suggest
KTaO$_3$ heterointerfaces as time-reversal symmetry breaking superconductors,
but also inject fresh momentum into the exploration of the intricate interplay
between superconductivity and magnetism, enhanced by the strong spin-orbit
coupling inherent to the heavy Ta in 5$d$ orbitals of KTaO$_3$
heterointerfaces.
