  Recent advances in the study of materials with topological electronic band
structures have revealed magnetic materials exhibiting giant anomalous Hall
effects (AHE). The giant AHE has not only attracted the research interest in
its mechanism but also opened up the possibility of practical application in
magnetic sensors. In this article, we describe simulation-based investigations
of AHE magnetic sensors for the applications to read head sensors (readers) of
hard disk drives. With the shrinking of magnetic recording patterns, the reader
technology, which currently uses multilayer-based tunnel magnetoresistance
(TMR) devices, is associated with fundamental challenges, such as insufficient
spatial resolution and signal-to-noise ratio (SNR) in sensors with dimensions
below 20 nm. The structure of an AHE-based device composed of a single
ferromagnetic material is advantageous for magnetic sensors with nanoscale
dimensions. We found that AHE readers using topological ferromagnets with giant
AHE, such as Co2MnGa, can achieve a higher SNR than current TMR readers. The
higher SNR originates from the large output signal of the giant AHE as well as
from the reduced thermal magnetic noise, which is the dominant noise in TMR
readers. We highlight a major challenge in the development of AHE readers: the
reduction in the output signal due to the shunting of the bias current and the
leakage of the Hall voltage through the soft magnetic shields surrounding the
AHE reader. We propose reader structures that overcome this challenge. Finally,
we discuss the scope for future research to realize AHE readers.
