PNNL

Abstract

Dilute magnetic semiconductors (DMS) which exhibit ferromagnetism (FM) at and above room temperature are a highly desirable class of materials for future spin- tronics devices. Zn1-xCoxO (Co:ZnO) is a heavily studied DMS material in this context. Although controversially discussed in the literature, there is a growing con- sensus, that phase-pure Co:ZnO is paramagnetic (PM)[1–3]. Altering the preparation conditions can easily lead to phase separation and consequently superparam- agnetism (SPM) [3]. Nonetheless there are recent experimental data claiming that FM can be switched on inCo:ZnO by controlling the carrier concentration [4]. On the other hand, no FM was found in structurally excellent Al-codoped Co:ZnO [5]. However, in the latter work the magnetic characterization was restricted to room temperature measurements. In parallel, theory has also revealed that defect-free, insulating Co:ZnO is not ferromagnetic [6, 7] whereas the role of n-type carriers remains under debate, ranging from ferromagnetic coupling [8], or oscil- latory behavior with Co-Co distance [9] to antiferromagnetic coupling [10]. It is rather common to manipulate the n-type carrier concentration of ZnO by Al-doping to yield high conductivity [4, 11]. On the other hand, it had been shown that Al-codoping of Co:ZnO may promote the onset of phase separation [11]. It is extremely difficult to detect such secondary Co-containing phases even with the most careful x-ray diffraction (XRD) analysis [11, 12] or depth-profiling photoelectron spectroscopy (DP-XPS) [13]. Such careful materials characterization is lacking in Ref. [4]. An alternative to extensive XRD or DP-XPS to look for potential phase separation in Co:ZnO is the combination of x-ray absorption near edge spectra (XANES), x-ray linear dichroism (XLD), and x-ray magnetic circular dichroism (XMCD). This suite of atom-specific x-ray spectroscopies nicely complements integral superconducting quantum interference device (SQUID) magnetometry. For example, combined XLD simulations and experiments at the Co K-edge have been used to verify the phase purity of Co:ZnO [2] and characteristic spectroscopic signatures with appropriate quality thresholds for PM and SPM have been identified recently in the XANES and XMCD at the Co K-edge of Co:ZnO [3]. Along the same line, a careful combination of XANES and extended x-ray absorption fine structure (EXAFS) was employed to study Co:ZnO films similar to those in [4] which found evidence for Co(0) secondary phases [14].