Fig. 50. (a) Longitudinal and (b) transverse MOKE hysteresis loops of a 3-nm Pd/2-nm Fe/2-nm Pd/3-nm Fe thin film on MgO(001) were measured with the different azimuthal angle φ from 0˚ to 180˚ by a step of 5˚. Split minor loops were observed in the region of φ = 135˚ ± 30˚. [36]
After we understood the uniaxial magnetic anisotropy energy in the Fe/MgO(001), we explored magnetic interlayer coupling in the Fe/Pd/Fe structure. Fig. 50. (a) Longitudinal and (b) transverse MOKE hysteresis loops of a 3-nm Pd/2-nm Fe/2-nm Pd/3-nm Fe thin film on MgO(001) were measured with the different azimuthal angle φ from 0˚ to 180˚ by a step of 5˚.
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φ was going to 100˚, we observed strange hysteresis loop, which was included a square loop at center with two rotational symmetric loops. When φ was increased from 100˚
to 135˚, the two minor loops seemed to move toward the center loop. At φ = 135˚, the Kerr intensity of the hysteresis loop decreased and the loop was almost unobservable compared with others.
Form the MOKE measurement, we could imagine that there was multi-symmetric magnetic behavior on Pd/Fe/Pd/Fe/MgO system. For example, in transverse MOKE, the hysteresis loop at 45˚ and 135˚ indicated the presence of a four-fold symmetry.
However, in longitudinal MOKE, the magnetic behavior at 0˚ ~ 90˚ and 90˚ ~180˚
indicated the presence of a two-fold symmetry of magnetic anisotropy energy. Thus, a simple model about the magnetic easy axes was proposed to explain this complex magnetic behavior. In this double Fe layer system, the bottom-Fe layer was affected by the miscut of the substrate, so an uniaxial magnetic behavior was shown which was similar in Fig. 49. The top-Fe layer was far away from the substrate, so the feature of Fe(001) crystalline dominated and the easy axes became four-fold symmetry.
Considering the two layer with different symmetry of easy axes, we could simulate the complex MOKE hysteresis loops shown in Fig. 51. and Fig. 52.
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Fig. 51. Illustration of the magnetization reversal mechanism at different angular φ = 0˚ (a), 45˚ (b) and 90˚ (c) were displayed. The longitudinal and transverse MOKE hysteresis loops were shown in the left and right panels. The numbers denoted the different magnetic states and respectively compared with the proposed steps of magnetic moment-switching. In the middle illustrations, red and blue represented the magnetism of the top and bottom Fe layer. The arrows indicated magnetic moment, and the dashed lines indicated the 4- and 2-fold magnetic easy axes. The gray arrow indicated the direction of the external magnetic field. [36]
Figs. 51 and 52 showed several L- and T-MOKE loops at φ = 0˚, 45˚, 90˚ ,110˚, 120˚, and 135˚ with the detailed magnetization reversal processes. The longitudinal and transverse MOKE hysteresis loops were shown in the left and right panels. The numbers denoted the different magnetic states and respectively compared with the proposed steps of magnetic moment-switching. In the middle illustrations, red and blue represented the magnetism of the top and bottom Fe layer. The arrows indicated magnetic moment, and the dashed lines indicated the 4- and 2-fold magnetic easy axes.
The gray arrow indicated the direction of the external magnetic field; longer the gray arrow, larger was the magnetic field applied. For the bottom Fe layer, the magnetic easy
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we would use the MOKE data at 110˚, 125˚ and 135˚ to verify the model we built here.
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Fig. 52. Illustration of the magnetization reversal mechanism at different angular φ = 0˚ (a), 45˚ (b) and 90˚ (c) were displayed. The longitudinal and transverse MOKE hysteresis loops were shown in the left and right panels. The numbers denoted the different magnetic states and respectively compared with the proposed steps of magnetic moment-switching. In the middle illustrations, red and blue represented the magnetism of the top and bottom Fe layer. The arrows indicated magnetic moment, and the dashed lines indicated the 4- and 2-fold magnetic easy axes. The gray arrow indicated the direction of the external magnetic field; and the longer the arrow, the larger magnetic field was applied. [36]
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vector of magnetization moment at longitudinal MOKE was reduced, and at transverse MOKE was increased. In State 3, the magnetic field changed to a negative direction. A small negative field, which could not break the coupling but enough to reversal the magnetization of Fe, switched the bound Fe layers from 45˚ to 225˚. Thus, the component vector of magnetization moment at longitudinal MOKE was just reduced a little, but the component vector at transverse MOKE was switched to the negative side intensely. In State 4, the negative magnetic field was increased to a larger magnitude and twisted the magnetization of top-Fe layer to 315˚. This four steps of magnetization process could be confirmed from the variation of Kerr signal measured in L- and T-MOKE curves. When the magnetic field was rotated to 125˚, it was hard to switch the magnetization from 45˚ to 225˚ by a small negative magnetic field. Thus, the situation of State-2-to-3 in 110˚ would not happen in 125˚. So the loop in L-MOKE became a double loop (Fig. 52b). As shown in Fig. 52(c), when the magnetic field was rotated to 135˚ and perpendicular to the easy axis of the bottom-Fe layer, the bottom-Fe layer would become no preference for either of the two easy axes. When the magnetic field was small and unable to decouple the two Fe layers, the equal distribution of the bottom-Fe layer in both easy directions pinned the top-Fe layer along the positive and negative transverse directions. Thus, we got a T-MOKE loop with a center loop smaller than 110˚-130˚. All of those were the detail of magnetization switching in our Fe/Pd/Fe trilayers system. We could know that the two loop MOKE was due to the pinning of uniaxial easy axis, rather than the antiferromagnetic coupling of two Fe layers.
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Fig. 53. Longitudinal MOKE hysteresis loops of 3-nm Pd/2-nm Fe/3-nm Pd/4-nm Fe was measured under different H2 gas pressure. The magnetic field was perpendicular to the easy axis of bottom Fe layer but parallel to the one easy axis of top Fe layer. The double minor loops were due to the coupling of two Fe layers. The dashed lines indicated the center of the minor loops. [36]
To investigate the effect of hydrogenation on long-range coupling in Fe/Pd/Fe, we focused on the double hysteresis loop at longitudinal direction of φ approximately 130̊.
Fig. 53 showed the double hysteresis loops of 3-nm Pd/2-nm Fe/3-nm Pd/4-nm Fe were measured under different H2 gas pressures. The magnetic field was perpendicular to the easy axis of bottom Fe layer but parallel to the one easy axis of top Fe layer. The dashed lines indicate the center of the minor loops.
The hysteresis loops with double minor loop measured in air and vacuum confirmed
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Fig. 54. Reversible 90˚ rotation of the magnetic moment in the top Fe layer during the absorption and desorption of hydrogen was possible if we applied a suitable magnetic field. [36]
At the final, we tried to control the magnetization of the top Fe layer like a censer by the absorption and desorption of hydrogen. If a magnetic field Ha about 6 Oe was applied to the sample, the absorption and desorption of hydrogen would shift the minor loop to left or right side, thus causing a reversible 90˚ rotation of the top Fe layer. This effect could be applied for magnetoresistance, such as that in a GMR device, for hydrogen sensing.
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