Quenching and temperature dependence of perpendicular magnetic anisotropy of Pt/Co multilayers

Magnetic metallic multilayers separated by nonmagnetic metal films are of great importance in magnetoelectronics and spintronics, due to their capacity of giving rise to giant magneto-resistance as well as the electric field control of ferromagnetism. Co/Pt multilayers are one of the typical platforms that own perpendicular magnetic anisotropy which can be tuned in various ways. Since previous investigations focus on the anomalous Hall(transverse) resistivity which characterizes the magnetization of the multilayers, much less attention has been paid to the longitudinal resistivity. In this work, we find that the longitudinal resistivity also gives rich phenomena that need further theoretical treatment. We have grown two Co/Pt multilayer structures that have different spacings between neighboring ferromagnetic layers. The one with smaller spacing shows a superparamagnetic behavior in its Hall resistivity even at a temperature as low as 1.5 K, but the longitudinal resistivity shows a well established hysteresis. The other sample shows square hysteresis in the Hall resistivity at all available temperatures up to 300 K, while the longitudinal resistivity gives no significant signals because they are mostly engulfed in the noises. The corresponding temperature dependence of the coercive field are also different. While the former gives an approximately exponential function of the temperature T, the latter can be divided to two zones, each of which can be characterized by a lnT^s dependence, where s is not necessarily an integer. Such distinct features may be deeply related to the microstructures as well as the magnon scattering, which require further investigations.