The following discussion would focus on current annealing induced magnetic coercivity enhancement, but the independent variable would be voltage, not current.
The current through the sample would reduce due to resistance increased during interface oxidation and became hard to show in graph, so the independent variable was showed in voltage.
Fig. 27. Normalized magnetic hysteresis loops of 5 nm Fe/2 nm ZnO structure was measured by longitudinal MOKE after applying different voltages. The coercivity gradually increased from 48 Oe after 5.5V and saturated at 180 Oe after 10 V applying. [33]
Fig. 27 showed normalized magnetic hysteresis loops of 5 nm Fe/2 nm ZnO structure measured by longitudinal MOKE after applying 0~12 voltages. The bias voltage was increased from 0 to 12V by each step of 0.5V. Hysteresis loops was measured after applying bias voltage 30 second. There was no any variety of hysteresis loops during the bias voltage increased to 5.5V. After 6V, the coercivity was increased from 48 Oe and saturated at 180 Oe with 9V. The shape of hysteresis loop became tilted but the ratio of remanence to saturation always maintained 100%. Bias Voltage higher then 9V didn’t make any change of hysteresis loop.
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Fig. 28. (a) Magnetic coercivity, (b) current, (c) resistance, and (d) sample temperature was graphed as a function of bias voltage. [33]
Fig. 28 showed more information about what happened when the magnetic coercivity (Hc) enhanced. The Hc,current, resistance, and sample temperature was graphed as a function of bias voltage. The sample temperature was measured by k-type thermocouple contact with the junction area when the voltage was applied. As voltage was bellowed 5V, the Hc kept like a constant and the current increased stably with raised voltage, so this indicated that the Fe/ZnO heterostructure was ohmic conducting behavior. After the voltage was increased over 5V, irreversible changes of magnetism and conducting property were observed. As the voltage increased, the sample current and Hc increased first and then got to maximum. The maximum temperature heated by current at Fe/ZnO junction was about 500K, but the lower limit of temperature to enhance coercivity was about 400K which could be observed by compare the temperature and coercivity function. When voltage exceeded 6V, the temperature was higher than 400K, and the coercivity began to increase. As voltage transcended 7V, the increasing of resistance due to the Fe/ZnO junction altered resulted in the decreasing of current and heating temperature, but the coercivity still increased because the
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temperature was higher than 400K. After the voltage was over 9V, the temperature reduced below 400K and stopped the increasing of coercivity.
Fig. 29. The magnetic coercivity exhibited as a function of (a) bias voltage and (b) current density in variety thickness of Fe/ZnO structure. In (b), the arrow showed the sequence of data about Hc with the variation of current density. [33]
Fig. 29 summed up the enhancement of coercivity in various Fe/ZnO structures. For each ZnO thickness, similar enhancement of coercivity always happened when the applied current density was large enough. In Fig. 29(a), the coercivity enhancement needed the voltage larger then 6-9 V in 2 nm ZnO system, but in 3 nm ZnO system, there needed the voltage larger then 12-14 V. it appeared that thicker the ZnO was,
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Fig. 30. The depth analyzing XPS of Fe measured at the indicated depth near the interface of Fe/ZnO.
Two samples were deposited with same parameters but different thickness. The right part of graphs were before annealing and lift part were after annealing. The peaks of Fe, Fe+2, and Fe+3 were shown by the arrows. [33]
In order to find out the evidence to prove the contention of current annealing inducing interface oxidation, the depth analyzing XPS of Fe was measured at the indicated depth near the interface of Fe/ZnO. Fig. 30 showed the XPS spectra of 2.5 and 10 nm Fe/ZnO structure with the depths analysis. There had two junctions deposited on one sample simultaneously. One was annealed by current, and another one was just checked by MOKE. So we could compare the XPS spectra from each sample. Many previous
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papers studied the characteristic XPS peak positions of Fe (707 eV), Fe+2 (709.9 eV), and Fe+3 (711.4 eV). [11-12] The peaks of Fe, Fe+2, and Fe+3 were shown by the arrows in Fig. 30. In Fig. 30(b), for 2.5 nm Fe system as deposited, the top of Fe layer was mostly pure metallic Fe, but the Fe near the interface of Fe/ZnO had been oxidized. The peak of Fe and Fe+2 simultaneously exist at depth. In Fig. 30(a), after the current annealing sufficient to induce an obvious Hc enhancement, a peak at 710.5 eV between Fe+2 and Fe+3 gradually became the main peak in XPS spectra when the depth is close to the Fe/ZnO interface. That meant there was Fe2O3 forming at the interface[13-15]. In the case of 10 nm Fe, the pure Fe peak was the main peak with a shoulder of Fe+2 in as-deposited sample shown in Fig. 30(d).
After annealing, the peak of Fe+2 gradually became the main peak the XPS spectrum with increasing depth shown in Fig. 30(a). Comparing two sample after annealing, the thicker Fe film tended to form Fe+2 due to the shortage of oxygen because of the limited ZnO thickness, so only FeO was formed in 10 nm Fe/ZnO, rather than the Fe3O4. Fe2O3
and Fe3O4 could be ferromagnetic, and FeO could be antiferromagnetic. That was the reason the coercivity would be larger after junction annealed.