Influence of the Interface Quality

As was discussed in detail in the previous subsection, the large experimental Kerr rota-tions for ultrathin (< 3ML) Fe films cannot be explained in the framework of a canted film magnetization. In this context it is important to raise the question where the appar-ently large out-of-plane magnetization component of these thin films finds its origin.

Thus far, in this chapter the magnetic properties of solely LT grown ideal Fe/GaAs{110}

interfaces have been investigated. However, in chapter 6 we saw already that “slowly”

RT grown interfaces exhibit signs of intermixing as predicted by DFT calculations [80]

(see also section 1.6). From these findings it has been concluded that the growth of an initial submonolayer Fe film at RT has a significant impact on the atomic and electronic structure of the Fe/GaAs{110} interface. It therefore only seems logical to investigate the impact of the “slow” RT growth and the associated higher degree of intermixing on the magnetic properties of the ultrathin Fe films on GaAs{110}.

Figure 7.18: Experimentally obtained Kerr signal [52] for a sample that was prepared in the following way: First, 0.5 ML Fe were grown on a GaAs(11�0) surface at RT. Then the sample was cooled down to ~130 K and additional 2 ML Fe were deposited on the sur-face. The MOKE measurements were conducted at 𝜃𝜃= 15° and a laser wavelength of 𝜆𝜆= 632.8 nm.

In a first attempt to do so, 0.5 ML Fe are grown on a GaAs(11�0) surface at RT. This should cause intermixing of the Fe atoms with the topmost layer of the GaAs and there-fore significantly reduce the quality of the GaAs(11�0) surface. Subsequently, the sample is cooled down to ~130 K and additional 2 ML Fe are deposited on the surface so that the total Fe coverage adds up to 2.5 ML. After that the sample is annealed to RT and in-vestigated by in situ MOKE. The obtained Kerr signals measured in the longitudinal (𝛽𝛽= 90°) geometry for 𝛼𝛼=𝜗𝜗= 0° and 𝛼𝛼=𝜗𝜗= 180° are shown in Figure 7.18.

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Figure 7.19: Dependency of the Kerr rotation on the Fe film thickness at an angle of inci-dence of 𝜃𝜃= 15° as in Figure 7.11. In addition, here also the Kerr signals of the differently grown samples are plotted. The 𝜑𝜑⁺,𝜑𝜑^{𝐿𝐿, and} 𝜑𝜑⁻ Kerr rotation values of the “0.5 ML Fe

@RT + 2 ML Fe @LT” sample are represented by the upper, middle, and lower magenta diamonds, respectively. The Kerr rotation of the “0.4 ML @RT + 2.3 ML @RT” sample measured along the <110> easy axis is represented by the orange triangle [52].

The hysteresis curves can be directly compared to the data of the purely LT grown 2.5 ML Fe film in Figure 7.4(a) from section 7.1. The interchanged sense of the hysteresis curves can be attributed to the reversed anisotropy of the GaAs(11����0) and GaAs(11�0) along the [001] direction (see section 7.2 for details). More interestingly, the absolute values of the Kerr signals (height of the hysteresis curves) are significantly reduced for the “0.5 ML @RT + 2 ML @LT” grown interface compared to the purely LT grown in-terface and amount to 𝜑𝜑⁺≈+8.3 mdeg and 𝜑𝜑⁻≈ −0.7 mdeg. This is nicely shown in Figure 7.19 where the corresponding values are plotted as magenta diamonds. The 𝜑𝜑⁺, 𝜑𝜑^𝐿𝐿, and 𝜑𝜑⁻ Kerr rotation values of the “0.5 ML Fe @RT + 2 ML Fe @LT” sample are represented by the upper, middle, and lower magenta diamonds, respectively. Apparently, the modification of the surface quality induced by intermixing has a significant impact on the detected magnetic momentum. The longitudinal component can be calculated accord-ing to equation (7.2) and amounts to 𝜑𝜑^𝐿𝐿≈+3.8 mdeg which is very close to the value of 𝜑𝜑^𝐿𝐿≈+2.7 mdeg for the purely LT grown sample. For the polar component, equation

7.6 Discussion

113 (7.3) yields 𝜑𝜑^𝑃𝑃≈4.5 mdeg which differs by more than 10 mdeg from the corresponding value of 𝜑𝜑^𝑃𝑃≈14.9 mdeg for the purely LT grown sample. Therefore, one can say that the modification of the interface quality has a particularly strong impact on the detected polar Kerr component. However, one has to keep in mind that the specified film thick-nesses have an uncertainty of ~20 % due to variations in the deposition flux and the cali-bration of the electron beam evaporator. As can be seen in Figure 7.19, the Kerr rotation values of the “0.5 ML Fe @RT + 2 ML Fe @LT” sample are very similar to the values for the purely LT grown 2.7 ML sample. Therefore, when interpreting the data one should exercise great caution.

To further investigate the impact of the interface quality on the magnetic properties of the sample, a second RT grown interface is prepared but this time all growth steps are carried out at RT. In a first step 0.4 ML Fe are deposited on a GaAs(1�10) surface. About 3 hours later additional 2.3 ML Fe are grown on this pretreated surface again at RT. After each preparation step LEED measurements of the surface are conducted [52]. After the deposi-tion of the 0.4 ML Fe the intensity of the LEED spots from the GaAs(1�10) surface is decreased. This can be ascribed to the formation of disordered three-dimensional Fe nu-clei [65]. After the deposition of additional 2.3 ML Fe, LEED patterns characteristic of bcc Fe are observed. Höllinger et al. [65] do not observe Fe LEED patterns for films thinner than 4 ML that they deposited at RT and at a rate of 0.5 ML/min. Seemingly, the submonolayer deposition and the subsequent 3 hours waiting time prior to further deposi-tion altered the surface in such a manner that the addideposi-tional 2.3 ML Fe was grown in a 2D-like mode.

Figure 7.20: Experimentally obtained Kerr signal [52] for a sample that was prepared in the following way: First, 0.4 ML Fe were grown on a GaAs(1�10) surface at RT. After some hours additional 2.3 ML Fe were deposited on the surface at RT. The easy axis is now directed along <110> (corresponding to 𝛼𝛼=𝜗𝜗= 90°/270°) and no reversal of the hysteresis loops is observed (indicated by the arrows). The MOKE measurements were conducted at 𝜃𝜃= 15° and a laser wavelength of 𝜆𝜆= 632.8 nm.

Moreover, the in situ MOKE measurements on the “0.4 ML Fe @RT + 2.3 ML Fe @RT”

sample exhibit interesting magnetic properties. The square-shaped hysteresis curves with

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small coercive fields taken at 𝛼𝛼=𝜗𝜗= 90° and 𝛼𝛼=𝜗𝜗= 270° show that for this sample the easy axis is now oriented along the in-plane <110> direction (see Figure 7.20). In contrast, the easy axis for the LT grown 2—3 ML Fe films on GaAs{110} appears to be oriented along the <001> in-plane direction as was discussed in the previous sections.

Furthermore, the MOKE measurements at 𝛼𝛼=𝜗𝜗= 90° and 𝛼𝛼=𝜗𝜗= 270° on the

“0.4 ML Fe @RT + 2.3 ML Fe @RT” sample do not exhibit a reversal of the sense of the two hysteresis curves as can be clearly seen in Figure 7.20. No sense reversal is observed in any arbitrary direction 𝜗𝜗. Furthermore, the Kerr rotation for the cases of 𝛼𝛼=𝜗𝜗= 90°

and 𝛼𝛼=𝜗𝜗= 270° amount to 𝜑𝜑^90° ≈2.1 mdeg and 𝜑𝜑^270°≈2.3 mdeg, respectively. This indicates that the “0.4 ML Fe @RT + 2.3 ML Fe @RT” sample is purely in-plane mag-netized and no out-of-plane component is present.

In summary, a higher degree of intermixing at the interface, induced by RT deposition of a submonolayer Fe on the GaAs{110} surface, significantly decreases or even completely quenches the large polar Kerr component that has been observed for purely LT grown 2—3 ML Fe films. Apparently, the magnetic properties of the Fe/GaAs{110} interface are directly connected with the quality of the interface itself. Together with the finding from the previous subsection that a canted film magnetization cannot explain the experi-mental data this indicates that the polar Kerr component of the “LT grown 2—3 ML Fe on GaAs{110}” samples is somehow induced by the interface and the magnetic proper-ties might be governed by interface magnetism.