Absence of ferromagnetic-transport signatures in epitaxial paramagnetic and superparamagneticZn0.95Co0.05Ofilms

Abstract

Paramagnetic (PM) and superparamagnetic (SPM) ${\text{Zn}}_{0.95}{\text{Co}}_{0.05}\text{O}$ epitaxial films display similar temperature and magnetic field dependent anisotropic magnetoresistance (MR) effects. The high structural quality of the PM films is confirmed by x-ray linear dichroism. A classical two-band model describes these MR effects well and reveals the same intrinsic origin of the transport signatures in PM and SPM ${\text{Zn}}_{0.95}{\text{Co}}_{0.05}\text{O}$ films. The temperature-dependent resistivity of the respective films arises from a Mott variable-range hopping process. The absence of the anomalous Hall effect in the SPM film provides another evidence for lacking contributions from the SPM phase to the magnetotransport properties. Moreover, above the blocking temperature of SPM ${\text{Zn}}_{0.95}{\text{Co}}_{0.05}\text{O}$ films, the $M(H)$ curve can be described by a Langevin function, indicating the presence of approximately 2 nm large magnetic nanoparticles. Therefore, only the contribution of PM ${\text{Co}}^{2+}$ ions in ${\text{Zn}}_{0.95}{\text{Co}}_{0.05}\text{O}$ films to the transport behavior can be found, thus demonstrating that 2 nm large magnetic Co nanoparticles does not interact with the carriers.