Magnetic Anisotropy of Ultrathin Fe Films on GaAs(110)

In this section the magnetic properties of low-temperature (LT) grown Fe/GaAs{110}

interfaces are investigated by in situ MOKE measurements. The samples were prepared as described in section 2.3.

In Figure 1.8 of section 1.6 it has already been discussed that for ultrathin Fe films on GaAs{110} a spin reorientation transition takes place. At a film thickness of around 10 monolayers (ML) the Fe film has the magnetic easy axis (EA) in <110> direction. For Fe film thicknesses below 4 ML the EA shifts into the <001> direction [46, 47, 52]. Figure 7.3 shows an in situ longitudinal Kerr measurement of a 10 ML thick Fe film on the GaAs(1�1�0) surface. At a rotation angle of the in-plane magnetic field and the plane of incidence of the laser beam of 𝛼𝛼=𝜗𝜗= 90° a sharp square-shaped hysteresis curve is detected indicating the EA along the < 1�10 > direction. Furthermore, if the plane of indicence and the magnetic field are rotated by 180° with respect to the sample to 𝛼𝛼= 𝜗𝜗= 270° (or the sample itself is rotated as actually done in the experiment), essentially

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the same signal is detected. In both cases the Kerr rotation amounts to ~12 mdeg and the sense of the hysteresis curve is counterclockwise. Exactly this behavior is expected for an in-plane EA in < 1�10 > direction.

Figure 7.3: In situ longitudinal MOKE measurement for a 10 ML thick Fe film grown on p-type GaAs(1�1�0). The Kerr rotation is plotted against the applied magnetic field. Data was taken with an in-plane magnetic field (𝛽𝛽= 90°) and at an angle of incidence of 𝜃𝜃= 15°

and a laser wavelength of 𝜆𝜆 ≈632.8 nm.

In contrast, samples with Fe films in the thickness range of 2—3 ML show a completely different behavior as has been shown for the first time in the diploma thesis of Iffländer [52]. The left panel of Figure 7.4(a) shows the Kerr rotation of a longitudinal (in-plane) MOKE measurement of a 2.5 ML thick Fe film on a GaAs(1�1�0) surface with 𝛼𝛼=𝜗𝜗= 0°. The sharp square-shaped hysteresis curve with coercive fields of less than 1 mT indicates that the EA is parallel to the [001] direction (which corresponds to the rotation angles 𝛼𝛼=𝜗𝜗= 0°). The sense of the obtained hysteresis curve is clockwise as indicated by the arrows. If now the in-plane magnetic field and the plane of incidence are rotated by 180° to 𝛼𝛼=𝜗𝜗= 180°, which corresponds to a rotation of the sample by 180°

as actually done in the experiment, the hysteresis is still sharp and square-shaped but the sense of the hysteresis curve is reversed and thus counterclockwise [see right panel in Figure 7.4(a)]. Also the absolute values of the Kerr rotation in both cases differ from each other.

A first approach to understand this data is to assume that it can be described as a superpo-sition of an in-plane and an out-of-plane magnetization component. It should be pointed out here that in section 7.6 it will become apparent that this approach cannot explain the magnetic behavior of the sample system in its entirety. Nevertheless, this first approach paves the way for a deeper understanding of the obtained data. The sketches in Figure 7.4(b) illustrate the idea of superimposed in-plane and out-of-plane magnetization com-ponents. Case A represents the situation where the external in-plane magnetic field and the plane of incidence are set up to 𝛼𝛼=𝜗𝜗= 0° (parallel to the [001] direction). In the following, a whole hysteresis cycle for case A is discussed. The initial situation is

denot-7.1 Magnetic Anisotropy of Ultrathin Fe Films on GaAs(110)

89 ed by A1 where the in-plane magnetization points into [001�] direction which is repre-sented by the dashed blue arrow. The out-of-plane component points downwards and is indicated by the dotted magenta arrow. The corresponding longitudinal (in-plane) and polar (out-of-plane) Kerr signals are represented by A1L and A1P in the dashed blue hysteresis and the dotted magenta hysteresis curves below. The overall Kerr signal is illustrated by the solid black hysteresis curve.

Figure 7.4: (a) In situ Kerr rotation signal measured in longitudinal geometry (𝛽𝛽= 90°) for a 2.5 ML thick Fe film on GaAs(1�1�0) with an angle of incidence of 𝜃𝜃= 15° and a laser wavelength of 𝜆𝜆 ≈632.8 nm. A reversal of the sense of the hysteresis curves is ob-served [52]. (b) Model of superimposed in-plane and out-of-plane magnetization which explains the reversal of the hysteresis curves [52]. The green arrows indicate the direc-tion into which the external magnetic field is applied first in the hysteresis cycle. For more details see continuous text.

The cycle begins by applying the external magnetic field in [001] direction as indicated by the green arrow. As soon as the coercive field is reached, the in-plane magnetization switches into the [001] direction and the out-of-plane magnetization switches upwards.

This is illustrated by the dashed blue arrow and dotted magenta arrow in case A2, respec-tively. The corresponding longitudinal (in-plane) Kerr signal jumps to the value A2L shown in the dashed blue hysteresis curve. In contrast, the polar (out-of-plane) Kerr

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nal falls to the value A2P as indicated by the dotted magenta hysteresis curve. Due to the relatively large polar Kerr signal the overall hysteresis curve, which is the sum of the longitudinal and polar Kerr signal, shows a downward jump to the baseline A2.

After the maximum magnetic field in [001] direction is applied and driven back to zero, the second half of the cycle starts by applying the magnetic field in [001�] direction. After reaching the coercive field, the in-plane magnetization switches back to the [001�] direc-tion and the out-of-plane component switches downwards again (case A1). The longitu-dinal Kerr signal falls back to the A1L baseline. This yields a counterclockwise hysteresis curve for the longitudinal Kerr signal. The polar Kerr signal jumps back to the A1P value which yields a clockwise polar hysteresis curve. Due to the relatively large polar Kerr signal the sum of the longitudinal and polar hysteresis curve yields an overall clockwise hysteresis curve.

In a next step the plane of incidence and the applied external magnetic field are rotated by 180° with respect to the sample (case B). (In the experiment this geometry is achieved by turning the sample itself by 180°.) This is depicted by case B1 in Figure 7.4(b). Due to its mirror symmetry the out-of-plane magnetization component yields the same polar Kerr signal no matter if the laser light impinges from the left or right hand side. Therefore, the corresponding polar Kerr signal B1P is assigned to the upper baseline of the dotted ma-genta hysteresis curve in the same way as for A1P. In contrast, the orientation of the plane of incidence is now changed with respect to the in-plane magnetization and there-fore yields a different longitudinal Kerr signal B1L if compared to A1L. Actually, the arrangement of in-plane magnetization and incident laser light in situation B1 corre-sponds to situation A2. Therefore, the longitudinal Kerr signal B1L is assigned to the upper baseline of the corresponding blue dashed hysteresis loop.

The hysteresis cycle begins by applying the external magnetic field now first along the [001�] direction corresponding to 𝛼𝛼= 180° as indicated by the green arrow. Since the in-plane magnetization already points into the [001�] direction no switching is observed.

Subsequently, the applied magnetic field is decreased to zero and then directed into the [001] direction. As soon as the coercive field is reached, the in-plane magnetization switches into the [001] direction and the out-of-plane component switches upwards (case B2). This translates into a downward jump for both the longitudinal and polar hysteresis loops to the value B2L and B2P, respectively. By repeatingly directing the external mag-netic field into the [001�] direction and reaching the coercive field, the in-plane magneti-zation switches back into the [001�] direction and the out-of-plane component switches downwards again (case B1). For both the longitudinal and the polar hysteresis loop this translates into an upward jump back to the values B1L and B1P, respectively. This also concludes an entire hysteresis cycle and reveals counterclockwise hysteresis curves for both the longitudinal and the polar Kerr signal. The sum of both signals yields the overall hysteresis curve which therefore also exhibits a counterclockwise sense.

In other words: one can say that the solid black hysteresis curves from Figure 7.4(b) and the measured Kerr signals from Figure 7.4(a) represent a superposition of a longitudinal