Coalescence-driven magnetic order of the uncompensated antiferromagnetic Co doped ZnO

Abstract

The evolution of the structural and magnetic properties of Co doped ZnO has been investigated over an unprecedented concentration range above the coalescence limit. ZnO films with Co concentrations from 20% to 60% of the cationic lattice have been grown by reactive magnetron sputtering. The wurtzite crystal structure was maintained even for these high dopant concentrations. By measuring the x-ray absorption at the near edge and the linear and circular dichroism of the films at the Zn and Co K edge, it could be shown that Co substitutes predominantly for Zn in the lattice. No indications of metallic Co have been found in the samples. At low Co concentrations, the films are paramagnetic, but with increasing Co content, the films become antiferromagnetically ordered with increasing order temperature. Uncompensated spins, coupled to the antiferromagnetic dopant configurations, lead to a vertical exchange-bias-like effect, which increases with increasing Co concentration. In parallel, the single-ion anisotropy is gradually lost.

Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

“…Interestingly, there is no sign of metallic Co neither in the X‐ray absorption spectroscopy (XAS) (not shown) nor in the XMCD, indicating the magnetic signal arises from the oxides. In fact, simulations of the XMCD results indicate that the signal arises predominantly from Co 2+ ions in tetrahedral positions, as expected for wurtzite and zinc blende‐CoO (see Figure S3 in the Supporting Information) . Moreover, the XMCD signal at 300 K is considerable weaker than at low temperatures, where the magnetic moment (roughly proportional to the area of the XMCD spectra) decreases a factor 100 from 15 to 300 K (Figure , inset), in full agreement with the temperature dependence of M S , hence, corroborating the uncompensated spins as the origin of the ferromagnetic‐like behavior in the samples.…”

“…In fact, simulations of the XMCD results indicate that the signal arises predominantly from Co 2+ ions in tetrahedral positions, as expected for wurtzite and zinc blende-CoO (see Figure S3 in the Supporting Information). [96][97][98] Moreover, the XMCD signal at 300 K is considerable weaker than at low temperatures, where the magnetic moment (roughly proportional to the area of the XMCD spectra) decreases a factor 100 from 15 to 300 K (Figure 6, inset), in full agreement with the temperature dependence of M S , hence, corroborating the uncompensated spins as the origin of the ferromagnetic-like behavior in the samples. The uncompensated spins probably arise from surface effects and due to internal defects such as oxygen vacancies or stacking faults.…”

“…Interestingly, the results on wurtzite-CoO are also consistent with single crystalline wurtzite Zn 1−x Co x O films with large Co contents (x = 0.6), which show an antiferromagnetic order with uncompensated spins. [96,97]…”

Local structure and magnetism of Co3+ in wurtzite Co:ZnO

“…Obviously, the presence of Co 3+ or hole carriers in the samples does not lead to a more robust magnetization at higher temperatures compared to Co 2+ in ZnO, but on the contrary it lowers the order temperature for the "smaller" Co-O-Co...configurations, discussed in Refs. [25,26]. This indicates that the magnetic behavior of the samples is still governed by next-neighbor, i.e.…”

“…The occurrence of the hysteretic behavior together with the vertical shift has recently been explained for 60% Co:ZnO by uncompensated antiferromagnetic Co-O-Co-· · · -configurations of different size categories using a Stoner-Wohlfarth-like model [25] and investigated in dependence of the doping concentration in Ref. [26]. The presence of this vertical shift of the hystereses together with a finite slope at 5 T in Fig.…”