  The mass-production of fuel-cell vehicles and the eventual transition to the
hydrogen economy will require safe, inexpensive and reliable sensors capable of
simultaneously detecting low concentrations of leaking hydrogen and measuring
broad ranges of hydrogen concentration in storage and energy generating
systems. Although several competing sensor technologies can potentially be used
in this role, just a few of them have thus far demonstrated a combination of
all desirable characteristics. This group of devices also includes
magneto-electronic sensors that can detect the presence of hydrogen gas in a
range of hydrogen concentrations from zero to 100% at atmospheric pressure with
the response time approaching the industry standard of one second. The hydrogen
gas sensing mechanism underpinning the operation of magneto-electronic sensors
is based on the physical processes of ferromagnetic resonance, magneto-optical
Kerr effect and anomalous Hall effect that enable one to measure
hydrogen-induced changes in the magnetic properties of structures combining Pd
with one or several ferromagnetic metals such as Co, Fe or Ni. In this chapter,
we overview the physical foundations of emergent ferromagnetic Pd-alloy-based
magneto-electronic hydrogen sensors and compare their characteristics with
those of high-performing multilayer thin film-based counterparts that have
already demonstrated a potential to find commercial applications.
