  Perpendicular magnetic anisotropy (PMA) of magnets is paramount for
electrically controlled spintronics due to their intrinsic potentials for
higher memory density, scalability, thermal stability and endurance, surpassing
an in-plane magnetic anisotropy (IMA). Nickel film is a long-lived fundamental
element ferromagnet, yet its electrical transport behavior associated with
magnetism has not been comprehensively studied, hindering corresponding
spintronic applications exploiting nickel-based compounds. Here, we
systematically investigate the highly versatile magnetism and corresponding
transport behavior of nickel films. As the thickness reduces within the general
thickness regime of a magnet layer for a memory device, the hardness of nickel
films' ferromagnetic loop of anomalous Hall effect increases and then
decreases, reflecting the magnetic transitions from IMA to PMA and back to IMA.
Additionally, the square ferromagnetic loop changes from a hard to a soft one
at rising temperatures, indicating a shift from PMA to IMA. Furthermore, we
observe a butterfly magnetoresistance resulting from the anisotropic
magnetoresistance effect, which evolves in conjunction with the thickness and
temperature-dependent magnetic transformations as a complementary support. Our
findings unveil the rich magnetic dynamics and most importantly settle down the
most useful guiding information for current-driven spintronic applications
based on nickel film: The hysteresis loop is squarest for the ~8 nm-thick
nickel film, of highest hardness with Rxyr/Rxys~1 and minimum Hs-Hc, up to 125
K; otherwise, extra care should be taken for a different thickness or at a
higher temperature.
