Properties of Zn$_{1-x}$Co$_x$O films produced by pulsed laser deposition with fast particle separation

The study is concerned with the structural, optical, magnetic, and transport properties of Zn$_{1-x}$Co$_x$O ($x$ = 0.05–0.45) films produced on Al$_2$O$_3$ (0001) substrates at a temperature of $T_s$ = 500$^\circ$C by pulsed laser deposition with fast particle separation. The film thickness is $d$ = 60–300 nm. It is found that the Zn$_{1-x}$Co$_x$O ternary alloy retains its wurtzite-type crystal structure up to $x$ = 0.35, if the films are produced at low buffer-oxygen pressures ($\sim$10$^{-6}$ Torr). It is established that, in these conditions, the electron concentration is higher than 10$^{20}$ cm$^{-3}$ because of the high density of oxygen donor vacancies. In this case, the films start to exhibit ferromagnetism in the magnetization and the anomalous Hall effect at temperatures above 100 K. The sign of the anomalous Hall effect is found to be positive and opposite to the sign of the normal Hall effect, as occurs in Co metal layers. This is indicative of the cluster nature of ferromagnetism of the Zn$_{1-x}$Co$_x$O films. For thin Zn$_{1-x}$Co$_x$O layers ($d$ = 60 nm, $x$ = 0.2) in a transverse magnetic field, profound hysteresis of the magnetoresistance is observed, which is indicative of the out-of-plain easy axis of magnetization of the films. The magnetic anisotropy is attributed to the structuring of the layers (elongation of magnetic clusters along the growth axis of the films). The structuring can lead to noticeable strengthening of the layer ferromagnetism.