In order to achieved the purpose that using voltage to influence magnetic coercivity reversibly, the thickness of ZnO was raised to 320 nm. In the previous studies, ZnO had obvious electrostriction effect[4], so we imitated all the detail in that paper to deposit ZnO. The sample Au (2 nm) / Fe (3 nm) / ZnO (320 nm) / Au (40 nm) / Al2O3 (0001) was prepared. The increased thickness of bottom Au was for reduced the resistance.
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Fig. 31. (a) MOKE hysteresis loops with various bias voltages. (b) Summarized Hc values drawn as a function of bias voltage. The solid lines are guides for the eye.[34]
Fig. 31 showed the MOKE hysteresis loops measured at room temperature using a magnetic field along the in-plane direction with the applied bias voltag increased from 0 to 6 V. The magnetic coercivity (Hc) of the MOKE hysteresis loops decreased as the bias voltage increased, while the shape of hysteresis loop and the ratio of remanence were still invariant in Fig. 31(a). The Hc values drawn as a function of bias voltage from +6 V to -6 V in Fig. 31(b). The Hc was 112 ± 2 Oe without bias voltage. When the bias voltage increased from 0 to +3 V, the Hc decreased a little to 109 ± 2 Oe. While the bias voltage exceeded +3 V, the Hc intensely reduced and attaining a minimal Hc value of 93 ± 2 Oe at +6 V. There was the same variation about Hc reduced when the applied voltage was negative. So the Hc reduction was not related to the electric field direction.
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FIG. 32. (a) MOKE hysteresis loops were measured as the bias voltage was switched between 0 V and 6 V. (b) The value of Hc and switched bias voltage were shown as a function of time. [34]
Fig. 32 showed that the voltage-induced Hc reduction was reversible. The bias voltage was switched between 0 and 6V repeatedly and the MOKE hysteresis loops were recorded, as shown in Fig. 32(a). The hysteresis loop always deformed to same shape with 6V bias voltage applied. As shown in Fig. 32(b), the Hc switched between 112 ± 2Oe and 93 ± 2 Oe reversibly, which corresponded with the applied voltages between 0 and 6 V. Here we only used 6 V for the maximal voltage because that a large current generated by high voltage could heat the Fe/Zn junction and yielding further Fe-oxidation at the Fe/ZnO interface. This phenomenon would hinder the observation of voltage-induced Hc reduction, so the maximal voltage was set at 6 V.
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Fig. 33. MOKE hysteresis loops after different voltage annealing and under appropriate voltage to reduced coercivity. The hollow points represented that the loop was measured after annealing and without voltage. The solid points represented the loops was measured with appropriate voltage.
The previous data showed that the coercivity would be enhance due to the Fe/ZnO interface oxidation caused by enough current annealing. To investigate how voltage-induce reversible coercivity reducing would be after current voltage-induce interface oxidation, we annealed the sample by high voltage and measured hysteresis loops with suitable voltage. The Fe/ZnO junction was annealed under different voltage for 10min, and then measured the loop with appropriate voltage. The hollow points represented the loop measured after different voltage annealing, and the solid points represented the loops measured under appropriate voltage. Fig. 33 showed that even after annealing and interface oxidation, the voltage-induced coercivity enhancement was always effective.
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Fig. 34. The summarized Hc values were shown as a function of bias voltage for as-deposited sample and the samples after 8V, 10V, and 12V annealed to the Fe/ZnO junction for 10 min. The solid lines are guides for the eye. [34]
Fig. 34 showed the summarized Hc values as a function of bias voltage. After applying 10 V for 10 min, the Hc value irreversibly increased from 112 ± 2 eV to 147 ± 4Oe, which was measured at V = 0. Because the resistance of Fe/ZnO junction was increased after the 10 V annealing, the sample could bear a higher voltage of at least 8 V without further Fe oxidation. The Hc reduced by voltage was shown as a function of bias voltage.
After the 10 V annealing for 10 min, the Hc decreased from 147 ± 4 Oe to 112 ± 4 Oe as the bias voltage was increased from 0 V to 8 V. Certainly the Hc recovered to 147 ± 4 Oe after bias voltage was removed. After the 12 V annealing for 10 min, the Hc
increased again to 189 ± 4 Oe measured at V = 0, and the junction could bear at least 9 V. The Hc reversibly decreased from 189 ± 4 Oe to 147 ± 4 Oe as the bias voltage was increased from 0 V to 9 V. The above experimental observations could be summarized as following. (1) When the applying voltage was larger enough to anneal the junction, the Fe/ZnO interface oxidation would begin and lead to the irreversible Hc enhancement.
(2) If the applying voltage was not enough to heat up the sample, interface oxidation would not happen and the reversible Hc reducing was observed within the small voltage region.
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have Kerr microscopy recently, so we could observe any change of magnetic domain after sample absorbed hydrogen. In this research, we prepared Co30Pd70 25 nm on SiO2/Si(001). The SiO2/Si(001) substrate was chosen not the Al2O3 because that the light passed through Al2O3 and reflected from the bottom would interfere the measurement of Kerr microscopy.
Fig. 35. (a) Longitudinal and (b) polar MOKE hysteresis loops was measured under vacuum and 0.2-0.8 bar H2 pressures. The dashed red curve in Fig.35(a) was measured after the recovery to vacuum and was almost same as the hysteresis loop of the initial vacuum condition (solid black curve).[35]
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The hysteresis loops of longitudinal direction and polar direction are shown in Fig. 35.
The polar MOKE hysteresis loop was not saturated because the limit of our polar vacuum coil. In vacuum, the longitudinal hysteresis loop was tilted, and the coercivity was 700 Oe. In polar direction, the coercivity was 450 Oe and the hysteresis loop was tilted also, but the real coercivity should be larger than that. Under 0.2 bar H2, the coercivity reduced to 140 Oe in longitudinal direction and to 200 Oe in polar direction.
The shape of hysteresis loops was become square and the remanence was increased to 100%. When the pressure of H2 increased from 0.2 bar to 0.8 bar, the remanence was still 100%, but the coercivity increased slightly. The coercivity increased from 140 Oe to 194 Oe in longitudinal and from 200 Oe to 435 Oe in polar direction. The dashed red curve in Fig. 35(a) was measured after the recovery to vacuum and was almost same as the hysteresis loop of the initial vacuum condition.
Fig. 36. The magnetic coercivity andremanence changed in longitudinal and polar direction under vacuum and different H2 pressure. [35]
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Fig. 37. Longitudinal Kerr images were taken with magnetic field near the coercivity under (a) a vacuum, (b) 0.2 bar H2, and (c) 0.8 bar H2. All Kerr images were taken at same area. The image size was 450 × 450 μm2. [35]
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Those domain images were taken as magnetic hysteresis loops showed in Fig. 37 measuring, and the defect with ‘K’ shape at lower right corner Implied the measure was at same area. The image of Co30Pd70 in vacuum during reversal in (a) were shown.
There was no observable domain contrast in the Kerr image due to the very small nucleation motion dominated the magnetization reversal. The nucleation was too small and reversed randomly so only the gray scale became darker and darker with the increasing of magnetic field. When the pressure of H2 was 0.2 bar, there was a lot of needle-like domain structure were seen at 0.91 Hc of magnetic field. The number and size of domains were increased when the magnetic field increased to 0.98 Hc. As the field increased to 1.01 Hc, the divided domain expanded until contact with neighbor domain, and the domain motion leaded the reversal. For the sample exposed to 0.8 bar H2, as showed in Fig. 37(c), the appeared domain was much larger and like diamond shaped with a magnetic field of 0.91 Hc, instant of needlelike that under 0.2 bar H2. As the magnetic field was increased to 0.97 and 1.0 Hc, there was less new nucleation domain appeared, and the reversal motion was replaced by expanding and merging of domain compared with the case that under 0.2 bar H2. From Fig. 37(a)-(c), we could make a conclusion that the hydrogen absorbed into Co30Pd70 made the number of domain decreased, the size of domain increased and the domain wall motion leaded the reversal gradually with the increased of H2 pressure. From vacuum to 0.2 bar H2, the observable domain formed and replaced the small domain, but the reversal was still dominated by nucleation motion until the magnetic field larger than 1.01 Hc. From 0.2 bar H2 to 0.8 bar H2, the size of domain became larger and the number of domain almost none increased with larger magnetic field, and the reversal leaded by the domain wall motion once the domain appeared.
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Fig. 38. Time-dependent longitudinal Kerr images taken under (a) vacuum, (b) 0.2 bar H2, and (c) 0.8 bar H2. The three series of Kerr images were taken over the same area at 1, 2, 3, and 6 s after the application of the magnetic field which was 0.9 Hc. The image size was 450×450 mm2. [35]
To analyze the hydrogen-mediated magnetic domain wall motion, the time-dependent magnetic domain reversal processes were monitored with the different magnetic fields and under different H2 gas pressures. Fig. 38(a) showed the series of Kerr images were taken under vacuum with an applied magnetic field of 0.9 Hc. No observable magnetic domain contrast came out at 1-6 s after the applied magnetic field, no matter how large the magnetic field was applied or the length of time we waited after the field was applied.
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In Fig. 38(b), after exposed to 0.2 bar H2, a lot of needle-shape reversal nucleation appeared at 1 second after the applied magnetic field of 0.9 Hc,. Over the following 2-6 s, the nucleation number was nearly unchanged, which meant the density of nucleation was no increased, and the domain extended by domain wall motion. In Fig.
38(c), under exposed to 0.8 bar H2, reversed nucleation started to expand by domain wall motion at 1 s after applied magnetic field of 0.9 Hc. Compared with the condition of 0.2 bar H2 in Fig. 38(b), the sample exposed to 0.8 bar H2 had less nucleation density and large domain size, but domain wall motion dominated the magnetization reversal at the waiting under 0.2 and 0.8 bar H2.
Fig. 39. Time-dependent magnetization reversal curves were recorded under (a) vacuum, (b) 0.2 bar H2, and (c) 0.8 bar H2 after applied various reverse magnetic fields. The Kerr intensity was obtained from the sum of pixel from the Kerr images showed in Fig. 38. [35]
In order to get more information about the reversal, we made the Time-dependent magnetization reversal curves were recorded under different H2 pressure after applied various reverse magnetic fields. Before the magnetization reversal, the sample was saturated by a large positive field, and retained at the remanence state, then various negative magnetic field was applied to start the domain evolution. Fig. 39. showed the time-dependent magnetization reversal curves were recorded under (a) vacuum, (b) 0.2 bar H2, and (c) 0.8 bar H2 after applied various reverse magnetic fields. As previously proposed by Bruno et al., time-dependent magnetization reversal can be described by a kinetic model as follows: [17, 18]
𝑀(𝑡) = 2𝑀 ⋅ 𝑒𝑥𝑝(− 𝑡 𝜏⁄ ) − 𝑀 (1)
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the magnetic field, then slowly decreased with a nearly constant slope, and that was almost same under variant magnetic field. The magnetization reversal curves in Fig.
39(a) was totally different behavior from what described by Eq. (1). The reason might be shown in Fig. 39(a) and 39(a), where the magnetization reversal under vacuum was dominated by submicrometer scale nucleation without observable domain wall motion.
The magnetization reversed immediately by nucleation motion after (< 1 s) the magnetic field was applied, and that was accounted for the intense jump in the first second of data shown in Fig. 39(a). When the Co30Pd70 alloy film was exposed to 0.2-0.8 bar H2, comparing to the nucleation motion under vacuum, domain wall motion was observed and dominated the magnetization reversal, so Eq. (1) can successfully fit the data curves in Fig. 39(b) and (c).
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Fig. 40. Series of time constant τ under various magnetic fields and different H2 pressures were calculate from the curve which was fitted in Fig. 39 by Eq. (1). ln(τ) was linear with applied magnetic field H according to Eq. (4). The dots represented the experimental data and the solid lines were the results of linear fitting. [35]
The time constant τ depended on the activation energy EA for the magnetic flipping of each microscopic element compared with the thermal energy 𝑘𝐵𝑇.[17, 18]
= 𝜏
0⋅ exp (− 𝐸𝐴⁄𝑘𝐵𝑇) (2)The activation energy EA was linear with the magnetic field H which was applied in the magnetization reversal process, as shown in Eq. (3). [17, 18]
𝐸𝐴 = 𝑉 ⋅ 𝑀(𝐻 + 𝐻𝑝) (3)
The V was the Barkhausen volume which was reversed within a single activation motion in the magnetization. Hp was the field required for magnetization reversal without any other activation processes. We could observe that the time constant τ had the relation with the applied magnetic field H from the combination of equations (2) and (3). ln(τ) is linear with H. [17,18]]
𝑙𝑛(𝜏) = − (𝑉⋅𝑀
𝑘𝐵⋅𝑇) ⋅ 𝐻 + 𝑙𝑛(𝜏0) −𝑉⋅𝑀⋅𝐻𝑝
𝑘𝐵⋅𝑇
(4)
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Fig. 41. Time-dependent polar Kerr images were taken under (a) vacuum, (b) 0.2 bar H2, (c) 0.6 bar H2, and (d) 0.8 bar H2. The four series of Kerr images were taken over the same area at 2, 3, 4, and 6 s after the application of the magnetic field which was 0.92 Hc. The image size was 450×450 mm2. [35]
Because the 25-nm Co30Pd70 alloy film revealed polar magnetization reversal behavior, which was shown in Fig. 41 that the sample was coexistence of longitudinal and polar, so we also needed to investigate the domain wall reversal motion in polar moment. Fig.
41 showed the polar MOKE images recorded from 2 s to 6 s after applied reverse magnetic field with 0.92 Hc under different H2 pressure. No observable magnetic domain structure for the Co30Pd70 alloy film existed under vacuum, which was same as
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the results of longitudinal measurement shown in Figs. 37 and 38. The gray scale of the polar Kerr images dark as the magnetic field applied without any observable contrasting structures, and slowly darker and darker with time. The reversal curve with different magnetic field also could not be fit by Eq. (3) because the reverse magnetic field only triggered magnetization switching and couldn’t spread nucleation sites or domain.
Under 0.2 bar H2, as showed in Fig. 41(a), many small needle-like domains appeared after 2 s at left-hand side and then spread toward right-hand side. In Fig. 41(b), the nucleation under 0.4 bar H2 was considerably less than those under 0.2 bar H2 but much larger. Domain wall motion and domain merging also became faster and more clear.
As the H2 gas pressure increased from 0.4 to 0.8 bar, the number of nucleation decreased and the domain wall motion was slightly faster than that at 0.4 bar. All the situations in polar motion were almost same as longitudinal.
Fig. 42. Series of time constant τ under various polar magnetic fields and different H2 pressures were calculate from the curve which was fitted in Fig. 41 by Eq. (1). ln(τ) was linear with applied magnetic field H according to Eq. (4). The dots represented the experimental data and the solid lines were the results of linear fitting. [35]
By fitting the time-dependent magnetization reversal data with Eq. (1), The values of time constant τ could be deduced. Fig. 42 showed the ln(τ) was linear with applied magnetic field H according to Eq. (4), and from the slope of the fitting lines, the Barkhausen volume V can be calculated with the parameters of T = 300 K and M ≈ 0.7±0.1μ𝐵/atom.
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Fig. 43. Barkhausen volume was respectively calculated from the fitting in Figs. 40 and 42 for the polar (open circle) and longitudinal (open square) magnetization reversal. [35]
Fig. 43 showed the Barkhausen volume V for the longitudinal and polar magnetization reversal, which were calculated from the fitting curves in Figs. 40 and 42. Because the thickness of Co30Pd70 alloy film was 25 nm, we could calculate the activation area in each magnetization reversal event by dividing V with 25 nm.
In the polar Kerr measurement, when H2 pressure was increased from 0.2 to 0.8 bar, V decreased from 6800 ± 500 to 3000 ± 500 nm3. Identically, the activation area A decreased from 272 ± 20 to 120 ± 20 nm2. In the longitudinal Kerr measurement, V decreased from 4600 ± 500 to 3000 ± 500 nm3 when the H2 pressure increased from 0.2 to 0.8 bar. Identically, the activation area A decreased from 184 ± 20 to 120 ± 20 nm2. The deviation between polar and longitudinal measurements might be due to the reorientation of moment tilted angle. Nevertheless, the polar and longitudinal measurements showed same tendency that the V was decreased with H2 pressure.
From Eq. (3), we could know that the activation energy EA in a single magnetization reversal event was proportional to V. Therefore, the hydrogen absorption in Co30Pd70
thin film induced the decrease of V suggested that the hydrogen content reduced the activation energy of single magnetization reversal event and promoted domain wall motion.On the other side, without enough H2 gas in the environment, large activation energy was needed to reverse a single magnetization. This phenomenon indicated that
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reversing a single domain became harder in air or vacuum, so the domain wall motion was almost unobservable and the nucleation motion would dominate magnetization reversal.