Spin pumping from permalloy into uncompensated antiferromagnetic Co doped zinc oxide

Abstract

Heterostructures of Co-doped ZnO and Permalloy were investigated for their static and dynamic magnetic interaction. The highly Co-doped ZnO is paramagentic at room temperature and becomes an uncompensated antiferromagnet at low temperatures, showing a narrowly opened hysteresis and a vertical exchange bias shift even in the absence of any ferromagnetic layer. At low temperatures in combination with Permalloy an exchange bias is found causing a horizontal as well as vertical shift of the hysteresis of the heterostructure together with an increase in coercive field. Furthermore, an increase in the Gilbert damping parameter at room temperature was found by multifrequency FMR evidencing spin pumping. Temperature dependent FMR shows a maximum in magnetic damping close to the magnetic phase transition. These measurements also evidence the exchange bias interaction of Permalloy and long-range ordered Co-O-Co structures in ZnO, that are barely detectable by SQUID due to the shorter probing times in FMR.

Direct Imaging of the ac Component of Pumped Spin Polarization with Element Specificity

“…This leads to a Gilbert damping parameter of α Py = (5.5 ± 0.3) × 10 −3 for Py and an increased Gilbert damping parameter of (8.0 ± 0.3) × 10 −3 for the heterostructure. This has already been discussed before as experimental evidence that angular momentum transfer via spin pumping is possible in Co:ZnO/Py heterostructures [19]. The direct experimental evidence of the presence of a pumped spin polarization in the nonferromagnet is however lacking based on Fig.…”

“…1 is indeed a sign of spin pumping. The presence of spin pumping in a highly insulating paramagnetic oxide itself has already been indirectly reported [19] and led to two important observations. The pumping inferred from the FMR linewidth increases with increasing Co concentration.…”

“…The system of choice for the present study is a heterostructure consisting of Py in contact with Zn 0.5 Co 0.5 O (50% Co:ZnO), a system where the presence of spin pumping has been indirectly evidenced before via increased Gilbert damping in FMR [19]. Under the experimental conditions of the STXM FMR, i.e., at ambient temperature, 50% Co:ZnO is known to be weakly paramagnetic [19] and highly insulating. In addition the L 3 edge of Co is in close vicinity in terms of necessary photon energy of the Fe and Ni L 3 edges so that both Py and the Co:ZnO can be conveniently studied with the identical STXM setup.…”

“…Single films of Py, 50% Co:ZnO, and heterostructures thereof are fabricated on c-plane sapphire substrates as well as SiN membranes on highly insulating Si substrates by using reactive magnetron sputtering at a process pressure of 4 × 10 −3 mbar in the same ultrahigh vacuum chamber with a base pressure of 2 × 10 −9 mbar. For the 50% Co:ZnO layer, a metallic composite target is used, an Ar:O 2 ratio of 10:1 standard cubic centimeters per minute (sccm) is used as a process gas, and the substrate temperature is kept at 294 • C, which are optimized growth conditions for 50% Co:ZnO [19,20]. Py with a typical thickness of 20 nm is fabricated at room temperature using 10 sccm of only Ar as a process gas.…”

“…FMR precharacterization of the chosen materials for the spin pumping heterostructure is done in a home-built broadband FMR setup, as reported before [19]. For that, an unstructured 50% Co:ZnO/Py heterostructure and a single Py film grown on c-sapphire substrates are measured using a microwave short adapted after Ref.…”