  The Anomalous Hall Effect (AHE) manifests as a transverse voltage
proportional to magnetization in ferromagnetic materials under the application
of a charge current, being an indispensable tool for probing magnetism,
especially in nanoscale devices. However, the AHE primarily sensitizes to
out-of-plane magnetization, thereby hindering its capacity to discern the
in-plane magnetization, a characteristic prevalent in ferromagnetic films. Here
we challenge this conventional understanding by demonstrating the in-plane
magnetization-induced AHE in iron and nickel, two ubiquitous ferromagnets. This
observation of the in-plane AHE is remarkable as it contradicts existing
theories that forbid such phenomena in cubic crystal systems. We trace the
origin of this unanticipated phenomenon to a hitherto unconsidered octupole of
the anomalous Hall conductivity in the magnetization space, a mechanism we
propose could enable the detection of in-plane AHE in a wide range of
ferromagnetic materials. This work realizes the in-plane AHE in common
ferromagnets by exploiting the anomalous Hall conductivity octupole, revealing
a new physical origin of the AHE and promising to revolutionize the design of
magnetic devices and sensors.
