reproduced by the reproduction of the sample with the same coverages but is also sensitive to the pinning and defect density present in the sample as suggested in the previous section.
5.8
Varying lateral separation of nanowires Au(5ML) / Co(0.26ML)
/ Pt(13 13 11).
In order to alter the lateral spacing between the nanowires of a certain width on Pt vicinal surfaces the terrace width has to altered, this can only be achieved by changing the off- cut angle with respect to the planar surface so that the terrace width is then altered. The goal of this sample is to study the effect that a different lateral spacing will have on the magnetic behaviour of the wires as a result of the change in the coupling strength between neighbouring nanowires.
5.8.1 Introduction
For the final sample grown for this study, a newly acquired Pt crystal with two differing Pt(111) offcuts on the same crystal as out lined in detail in chapter 3. As a reminder instead of being a single surface it had two different vicinal surfaces on the opposite sides of the circular crystal face the Pt(13 13 11) and the Pt(889) surface were created by cleaving the crystal at 4.371◦ and -3.237◦ with respect to the Pt(111) surface normal. The Pt(13 13 11) surface is off cut from the Pt(111) in the same directions as the Pt(997) surface and has the same step edge structure of (11¯1) as the Pt(997) with a terrace width of 12 atoms 32 time the Pt(997) terrace. The Pt(889) surface is offcut in the opposite direction and has a different step edge structure of (001) surface and a terrace width of 16 atoms twice that of the Pt(997) terrace.
As the temperature dependence for the self-assembled growth of the nanowires on the Pt(997) surface goes to higher temperature the Co atoms gain enough energy to move over the step edges moving on to a different terrace and going to a lower temperature the Co atoms do not have enough energy to diffuse along the step edge to allow row by row growth, and only in between these two regimes does row by row growth occur. As the step- edge structure on the Pt(13 13 11 ) surface is the same as the Pt(997) surface the sample grown on this crystal was prepared using the same procedure as the samples produced on the Pt(997) crystals.
When the Pt(13 13 11) surface was cleaned, a 0.26 ML Co coverage was deposited which corresponds to 3 atom wide wires on this surface and was then capped with 5 ML of Au (Au(5 ML)/(0.26 ML)Co/Pt(13 13 11)). As this sample was grown to study the effects
5.8. Varying lateral separation of nanowires Magnetic behaviour
of an increased lateral separation between the wires this sample will be compared to the ”3w5ML” sample grown on the Pt(997) surface presented earlier in this chapter. The earlier samples exhibited an increased Curie temperature compared to bare wires which was close to or above room temperature. Any significant changes in behaviour could be due to the modification in the strength of the coupling between the neighbouring nanowires via the capping layer and substrate material.
The coverage deposited on the Pt(889) surface was also studied but only as a ”bonus” sample and thus only when measurement time allowed. As the step edge structure of the Pt(889) differs for the step structure of the Pt(997) the growth dynamics on this surface are not know to the same. Time did not allow for an in depth study of the growth dynamics or the magnetic properties from this ”bonus” sample and thus the data presented from it here should be taken with caution.
5.8.2 MOKE
Variable temperature MOKE was performed on both samples, at the full temperature range available from room temperature to the lowest temperatures obtainable in the cryostat was
≈80K. No MOKE hysteresis loops were observed for either the samples, at all temperatures obtainable in the set up with a maximum field of 122 mT.
The absence of an observable MOKE loop could have a few possibilities, the first and most obvious is that the sample is not ferromagnetic at these temperature indicating that the Curie temperature is much lower than that previously observed from the other ”3w5ML” samples which were grown on Pt(997) surfaces or possibly that it is so slightly ferromagnetic that the loop is lost is the background subtraction procedure.
Another possibility is that the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) sample is mag- netically harder than the previous Au(5ML)/(0.39 ML)Co/Pt(997) samples and that the 120 mT is not sufficient to magnetically saturate the wires, so the MOKE signal from the sample will not change as the magnetic field is varied.
5.8.3 XMCD
5.8.3.1 MAX-lab data
XMCD was performed on the two samples at 79 K and room temperature at the MAX-lab beamline I1011. The room temperature XMCD showed no signal at the maximum field available (500 mT), indicating that neither regions are ferromagnetic at room temperature. The 79 K XMCD spectra obtained from the two regions, when several XAS spectra with the same combination of polarisation and direction of applied field were averaged, then all
5.8. Varying lateral separation of nanowires Magnetic behaviour
the possible combinations were used to obtain XMCD spectra which were then averaged to produce the XMCD spectra shown in figure 5.35 for the ”bonus” and in figure 5.36 for the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11).
The ”bonus” sample on the Pt(889) surface shows a possible very small XMCD shown in figure 5.35, which would indicate that the sample is starting to be magnetic (most likely paramagnetic) at this temperature.
For the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) sample on the (13 13 11) the XAS spectra were averaged again giving a larger and more definite XMCD signal. The average of the available XMCD spectra is shown in figure 5.36. This is definitely indicating that the sample is becoming magnetic at this temperature.
Figure 5.35: XMCD spectrum from (889) surface ”bonus” sample, indicating that there is some magnetic behaviour of this sample which is small.
Figure 5.36: XMCD observed from Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) at 79K this spectra is the result of averaging the individual XAS and then averaging the XMCD spectra then obtained to produce the spectra and integral show.
5.8.3.2 ALS
After these MOKE (in TCD) and XMCD (in MAX-lab) experiments were performed, we were lucky enough to obtain some experimental time at beamline 6.3.1 at the ALS due to collaboration with Dr Alpha N’Diaye, who performed the experiments for the results shown here. Liquid He was also available for these experiments enabling lower temperature to be achieved than for earlier experiments either via MOKE in TCD or XMCD at MAX-lab.
5.8. Varying lateral separation of nanowires Magnetic behaviour
Figure 5.37: XAS spectra with different applied field as shown in the upper panel wherein as obvious difference can be seen. XMCD spectra obtained from data, with the resultant integral is shown in the lower panel. All are obtained data from the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) sample at 50 K.
XMCD
Ferromagnetic XMCD spectra were observed at temperature of ≤ 100K and below for the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) sample as shown in figure 5.37. The ml/ms
ratio obtained from these XMCD (which were not averaged to produce one spectra as was carried out for the MAX-lab spectra) spectra was 0.11±.04, this value is slightly higher than the bulk value of 0.102, however with the error on the measurement the ratio could be consistent with the ratio at the Au(5 ML)/Co(0.39 ML)/Pt(997) sample which has a value of 0.151±0.02 as seen in table 5.1. The ml/ms data presented suggest though not conclusively that the orbital re-hybridisation and orbital anisotropy is not as strong in this sample as for previous samples. Whether these are affected by local (symmetry) or non-local (interaction between neighbouring wires) effects is left for future work.
ESHL
Temperature dependent ESHLs were also collected from the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) sample, with the same temperature dependent hysteresis behaviour as observed in the other samples. Open hysteresis loops were observed for temperature to 100 K as show in
5.8. Varying lateral separation of nanowires Magnetic behaviour
figure 5.38 from the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) but no hysteresis was observed at 140K, therefore the Curie temperature of this sample is 100K ≥Tc≤140K.
The Gaunt strong pinning model was applied to the observed temperature dependence as shown in figure 5.39. It was observed that the ESHL collected at 100K has a 7mT
difference between theHc from each of the arms of the loop. This difference is well outside
the error obtained from the fit and the typical difference observed in the hysteresis loops observed from this entire study, which is normally <1mT. The ESHL collected at 100 K being close to the TC the loop has become sufficiently narrow that the remanence in the
electromagnets pole pieces are now representing a significant component of the measured
Hc. Also when the magnitude of the non-normalised loops is examined they were found to be 1.18, 1.07, 1.13, 0.6 for the 20K, 25K, 50K and 100K loops respectively, which shows that the saturation magnetisation of the sample is decreasing at 100K because of its proximity to the TC. The field steps in the 100 K loop are also large which result in a poor sampling in
this loop as can be seen in figure 5.38 which is another reason to exclude this loop from the Gaunt fitting. When the strong pinning model was applied to the data, the fit obtained with the inclusion and exclusion of the 100 K point was investigated and are shown in figure 5.39. The red line shows the fit for all the temperature data points and the green line showing the fit when the 100 K point is excluded. The latter fit shows a significant improvement in agreement and is most likely to be a better representation of the behaviour of the sample.
The parameters extracted from the fits are show in table 5.10 where these values are compared to the values extracted from Au(5 ML)/Co(0.39 ML)/Pt(997) samples as show in tables 5.4, 5.8 and 5.9. It will be noticed that the values of Hc0 are similar to the values obtained for the other three or multi atom wires, however the values of 4bf are significantly smaller, which could be indicative that the coupling between the wires is reduced because of the increased spacing between the wires causing the pinning sites on the individual wires to have less effect on the domain wall movement as they are now acting more independently. As the RKKY interaction decays with increasing distance in addition to oscillating, so with the increase in the nanowires lateral separation from 8 atom on the (997) surface to 12 atoms on the (13 13 11) surface, the coupling between the wires will be significantly weakened and it is this weakened coupling between the wires that could stop the pinning site on one nanowire effecting the domain wall motion on the neighbouring nanowires. The exclusion of the point at 100 K seems justified by the Gaunt fit itself as the sample was observed to no longer be ferromagnetic at 140 K with a non-open hysteresis loop; the extrapolated TC
for the green fit in figure 5.39 gives a TC of 131±14, while the red fit from figure 5.39 give
5.8. Varying lateral separation of nanowires Magnetic behaviour
Figure 5.38: ESHL for all the temperatures at which hysteresis was observed from the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) sample. The loop at 100 K was observed to be asymmetric
with a 7mT difference betweenHc from each of the arm of the ESHL.
There was no explicit data acquired to investigate angular dependence. However the XMCD and ESHL were collected at normal incident to the optical surface of the (13 13 11) surface and as a magnetic signal was observed, the magnetic easy axis is at least not in plane, and there is a significant component out of plane. Clearly, further study is desirable to confirm PMA and whether this is indeed along the Pt[111] direction.
Fitted col- our Hc0(T) 4bf (eV) f(10−10N) Intercept TC(K) Red 0.27±.03 1.25 ±0.2 3.00±0.5 193±31 Green 0.32±.02 0.85 ±0.1 2.27±0.2 131±14 ”3w5ML” * 0.26±.01 4.1±.2 10.9±.4 701±23
Table 5.10: The physical values extracted from the Gaunt strong pinning model fits to the temperature dependent hysteresis from the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11) sample. * Au(5 ML)/Co(0.39ML)/Pt(997) from table 5.4.
5.8. Varying lateral separation of nanowires Magnetic behaviour
Figure 5.39: Temperature dependent hysteresis observed from the Au(5 ML)/Co(0.26 ML)/Pt(13 13 11). The Gaunt strong pinning model was applied with the red line showing the fit for the all the points and the green fitting if the point at 100 K which is both close
to the TC and has interference from the remanence in the poles of the electromagnet is