The change of the hysteresis curves by oxidation at room temperature of the sample with TOPO is imaged in figure 9.9. The initial curve is drawn in black, the red curves denote later measurements. The saturation magnetic moment decreases with time. The progress of all five samples of this batch looks similar, therefore only one image is shown here as an example.
In figure 9.10 the change of the saturation magnetic moment over time is depicted for all samples. For better comparison the saturation magnetic moment values are normalized to the associated initial value. These curves are displayed in figure 9.11.
All curves show an increase prior to a decrease of the saturation magnetic moment. The
9.4 Oxidation during the first thirty minutes after fabrication
Figure 9.9: Hysteresis curves recorded at different times during the oxidation process for the particles stabilized with TOPO.
Figure 9.10: Change of the saturation mag-netization during oxidation.
Figure 9.11: Change of the saturation mag-netization during oxidation - normalized data.
increase is about 2% in the case of TOA, for the other samples the increase is between 0.9% and 1.4%.
This can be explained by an induced magnetic moment in a monolayer of dissociatively chemisorbed oxygen atoms on the particle surface. For low coverage couples the oxygen layer to the ferromagnetic substrate. An exchange splitting of the 2p level of the oxygen and an increase of the magnetic moment of the surface atoms were described for a oxygen-on-iron-system. In [79, 80, 81] an induced magnetic moment between 0.2 µB and 0.7 µB for each oxygen atom is described for the case of iron. "Magneticlly dead" layers at the surface begin to form with the incorporation of the oxygen atoms into the bulk and the oxidation process, respectively [82]. A similar exchange splitting of the oxygen 2p level was found in [83, 82] for the system oxygen on cobalt which implies an induced magnetic moment in the adsorbed oxygen layer as well. 3 A detail of the first 700s of figure 9.11 is displayed in figure 9.12, because the course of the curves differs significantly from the ones of the previous measurements. To emphasize point of decrease below the initially measured values a black line at the normalized magnetic moment of 1 is added. The corresponding values are displayed in table 9.2.
Figure 9.12: Detail of the first 700s of figure 9.11.
After a steep increase to 101.4% of the initial value, the particles covered with TOPO exhibit a steep decrease of the saturation magnetic moment. The decrease rate of the sat-uration magnetic moment is reduced after t=420s. The further course of the curve shows a slight decrease.
The curves for the particles with surfactants with equal headgroups show a lot of similari-ties. The samples of the particles stabilized with oleylamine and TOA undergo a less steep increase than in the case of TOPO. After t=180s (TOA) and 150s-180s (oleylamine) the
3In the case of cobalt coexistence of chemisorbed oxygen and CoO occurs, because oxygen diffuses into the bulk and forms antiferromagnetic CoO. Therefore oxygen exists at the surface as well and exhibits hybridization between adsorbate and substrate. With ongoing oxidation descreases the the exchange splitting by further formation of CoO at the surface [83].
9.4 Oxidation during the first thirty minutes after fabrication
maximum values of 102.0% (TOA) and 101.2% (oleylamine) are reached, directly followed by a decrease. From t=420s to t=720s the curves run parallel and between t=420s-480s the curves fall below the initial value. Around t=840s the curves separate and a steeper decrease in the case of oleylamine occurs, while the curve becomes shallower in the case of TOA, where it finally runs parallel to the later part of the TOPO curve.
The samples of the particles stabilized with oleic acid and 1-pyrenebutyric acid undergo the same increase as the particles stabilized with an amine headgroup at first. The increase stops earlier and leads to a plateau. The maximum values of 101.0% (oleic acid) and 100.9%
(1-pyrenebutyric acid) are slightly below the values for the amine headgroup samples. The plateaus stretch from t=120s to t=360s with a small maximum from t=240s-300s in the case of oleic acid and from t=120s to t=270s with a slightly higher maximum at t=240s for 1-pyrenebutyric acid. Afterwards they undergo a slight decrease, which becomes steeper in the case of oleic acid, and runs parallel to the curve of oleylamine finally. In the case of 1-pyrenebutyric acid, the curve becomes shallower and this curve runs finally parallel to the curves of TOA and TOPO. Both curves of samples with a carboxyl headgroup fall below the initial values between t=540s-600s.
(ms)max (ms)initial (ms)max (ms)max treturn
name after [memu] in in to (ms)initial
t [s] att=60 s [memu] % [s]
TOPO 120 2066 2094 101.4 240-270
TOA 180 2495 2545 102.0 420-480
oleylamine 150-180 1368 1385 101.2 420-480
oleic acid 240-300 1735 1753 101.0 540-600
1-pyr. acid 240 1794 1811 100.9 540-600
Table 9.2: Increase and decrease of the saturation magnetic moment ms.
9.4.1 Conclusion
The initial similarity of the curves in the case of equal headgroups marks a dependence on the headgroups of the surfactants for the first period of the oxidation. The further development of the curves exhibits a dependence on the tailgroups of the surfactant, as the curves of oleylamine and oleic acid run parallel after some time. The curves of TOA and TOPO underline this result.
The correlation to 1-pyrenebutyric acid cannot be explained by this. But if it is regarded that oleylamine and oleic acid are rather string- or line-like molecules and the other sur-factants are rather area-covering molecules, an explanation for the parallel development of the curves is given.
The oxidation is influenced by the surfactant only during the first minutes of oxidation.
The headgroup appears to be of greater importance during the first seconds, followed by a chaingroup influenced part. In the longer term only the size, shape and crystallinity of the particles exhibit an influence on the oxidation, as the oxidation depends on the diffusion of oxygen in the nanoparticles, then.