Po Chun Chang

Magnetism modulation of Fe/ZnO heterostructure by interface oxidation

In this study, the magnetic coercivity (Hc) of Fe/ZnO heterostructure was significantly enhanced by 2–3 times after applying a suitable current. This Hc enhancement originates from the Fe-oxidation at the Fe/ZnO interface induced by direct current heating. Depth-profiling X-ray photoemission spectroscopy analysis confirmed the formation of FeO, Fe3O4, and Fe2O3 close to the interface region, depending on the Fe thickness and annealing process. This study demonstrates that direct current heating can moderately change the local interface oxidation and modulate the magnetic properties. These results clearly reveal the correlation between magnetism and interface properties in the Fe/ZnO heterostructure and provide valuable information for future applications.

Voltage-induced reversible changes in the magnetic coercivity of Fe/ZnO heterostructures

In this study, the magnetic coercivity (Hc) of Fe/ZnO heterostructure monotonically decreased as voltage was applied. The reversibility of this effect was demonstrated by cyclically changing the bias voltage from 0 to 6–9 V; the Hc decreased 15%–20%. The Hc value exhibited the same variation whether the applied voltage was positive or negative. As thick Fe-oxide gradually formed at the interface by using direct current heating, the Hc increased and the Fe/ZnO heterostructure demonstrated a similar voltage-induced reduction of Hc.

Voltage induced reversible and irreversible change of magnetic coercivity in Co/ZnO heterostructure

In this study, the application of bias voltage to 4–8 nm Co/275 nm ZnO heterostructures changed the magnetic behavior reversibly or irreversibly, depending on the different voltage-induced mechanisms. The magnetic coercivity (Hc) monotonically decreased 20% when the small voltages of 0–8 V were applied. The Hc reduction was symmetric with the voltage polarity, and the reversibility was demonstrated by cyclically switching the bias voltage between 0 and 7 V. While a large voltage up to 40 V was applied to the Co/ZnO junction, the current heating effect became considerable and the Co-oxide was formed, as confirmed by depth-profiling X-ray photoemission spectroscopy analysis. The presence of Co-oxide in the Co films induced the irreversible reduction of the Kerr signal and Hc at room temperature. The considerable Hc enhancement at 130 K also indicates the exchange bias coupling effect from the antiferromagnetic Co-oxide.

Voltage-induced Interface Reconstruction and Electrical Instability of the Ferromagnet-Semiconductor Device

Using x-ray magnetic spectroscopy with in-situ electrical characterizations, we investigated the effects of external voltage on the spin-electronic and transport properties at the interface of a Fe/ZnO device. Layer-, element-, and spin-resolved information of the device was obtained by cross-tuning of the x-ray mode and photon energy, when voltage was applied. At the early stage of the operation, the device exhibited a low-resistance state featuring robust Fe-O bonds. However, the Fe-O bonds were broken with increasing voltage. Breaking of the Fe-O bonds caused the formation of oxygen vacancies and resulted in a high-resistance state. Such interface reconstruction was coupled to a charge-transfer effect via Fe-O hybridization, which suppressed/enhanced the magnetization/coercivity of Fe electronically. Nevertheless, the interface became stabilized with the metallic phase if the device was continuously polarized. During this stage, the spin-polarization of Fe was enhanced whereas the coercivity was lowered by voltage, but changes of both characteristics were reversible. This stage is desirable for spintronic device applications, owing to a different voltage-induced electronic transition compared to the first stage. The study enabled a straightforward detection of the spin-electronic state at the ferromagnet-semiconductor interface in relation to the transport and reversal properties during operation process of the device.

Hydrogen-mediated magnetic domain formation and domain wall motion in Co 30 Pd 70 alloy films

In this study, the microscopic origin of the hydrogen effect on magnetic materials was explored through the characterization of time-dependent magnetic domain evolution. We prepared 25-nm Co30Pd70 alloy films with canted magnetic moment on SiO2/Si(001) substrates. From macroscopic Kerr hysteresis loops, considerable hydrogen-induced reduction of magnetic coercivity by a factor of 1/5 in a longitudinal direction and enhancement of magnetic remanence to saturation ratio from 60% to 100% were observed. The magnetic reversal behavior of the Co30Pd70 alloy films gradually transformed from nucleation- to domain-wall-motion dominance when H2 pressure was increased from a vacuum of 1 × 10−5 mbar to 0.8 bar. Domain size also increased considerably with H2 pressure. When H2 pressure was above 0.4 bar, the domain wall (DW) motion was clear to observe and the DW velocity was approximately 10−6–10−5 m/s. Greater hydrogen content in the Co30Pd70 alloy films promoted DW motion that was closer to the behavior of a thermally activated model. The hydrogen effects on magnetism were observed to be reversible and could have valuable future application in spintronic devices for hydrogen sensing.

Voltage-induced reversible and irreversible changes in the magnetic coercivity of Fe, Co/ZnO heterostructures

This study investigates the magnetic coercivity of heterostructures composed of a transition metal (Fe and Co) and a semiconductor (ZnO). An enhancement in the magnetic coercivity is observed which is due to direct current heating-induced oxidation at the interface. Depth-profiling X-ray photoemission spectroscopy observations confirm oxide formation at the interface due to annealing. It is shown that magnetic coercivity of heterostructures monotonically decreased as a relatively small voltage is applied and its reversibility is demonstrated by cyclically changing the bias voltage.

Sensitive hydrogenation effect on magnetic behavior of CoPd alloy thin films

Pd has long been used as a high efficient catalyst for hydrogen dissociation and absorption. Pd thin films of few nm thickness are widely used in metal-hydride systems. In the Pd-alloys, the hydroge-nation of Pd may easily bring in proximity effect upon the underneath functional materials. This effect is important from fundamental point of view and also applicable in future techniques. Besides of hydrogen catalyst, in spintronicss point of view, Pd is also a key element for memory storage. The strain and magnetostriction effect in Co, Fe-Pd alloy films dominate the magnetoelastic anisot-ropy and yield the perpendicular magnetization, which can significantly increase the memory density. Thus Pd is a multifunctional element for the devices or sensors applicable in both spintron-ics and energy applications. In our experiment, Co-Pd alloy thin films of different alloy compositions were prepared for the investigation of reversible hydrogenation effect on the magnetic and optical properties. As shown in Fig. 1, after hydrogenation the magnetic coercivity (Hc) was and the squareness of hysteresis loops (i.e. remanence/saturation) were significantly modified. The hydrogenation effects varies with the alloy composition, from Co-rich to Pd-rich. The detailed comparison was discussed in this report.