Etapa de Planificación

The bulk characterisations were performed using the in-house facilities within the School of Physics and Astronomy at the University of St. Andrews.

4.4.1 Magnetisation Measurements

The magnetisation measurements were taken using a 5T MPMS SQUID with a base temper- ature of operation at 1.8K. The hysteresis behaviour in the normal state was measured for an in-plane applied field at 15K as illustrated in Figure4.4.1(a). The small volume of the FM material in the structure accounts for slim nature of the loop. The spin valve behaviour of the structure can be confirmed from the regions marked by the blue circles in Figure4.4.1(a). These regions have been zoomed in Figure4.4.1(b). The switching behaviour of the two F blocks can be deduced from the forward branch of the magnetisation curve in part (a) of Figure4.4.1(b). The arrows marked the field values at which switching occurs. The thin Py layer switches first and the thick layer switches at a higher field. Similar switching occur in the reverse direction of the hysteresis loop as shown in the inset of Figure4.4.1(b). On cooling below the superconducting transition temperature (Tc), a superconducting signal can be detected in the sample in the form of a change of slope of magnetisation at higher fields where the diamagnetic contribution (M = −µ0H) from the Nb layer is significant at higher fields, although the spin valve behaviour is still present which can be seen in Fig- ure4.4.2(a). The superconducting nature of the sample can also be confirmed from the tem- perature dependence of the magnetisation measured in the ZFC and FC states of the sample for an in-plane applied field of 100G in Figure4.4.2(b). A clear difference between the two branches of the magnetisation curves (ZFC and FC) indicates a strong presence of bulk su- perconductivity in the sample and in both these cases magnetisation go through a sharp superconducting transition at around 7K (Tcfor pure Nb is 7.2K) indicating good quality of the film (Figure4.4.2(b)).

(a) (b)

Figure 4.4.1:(a) Magnetic hysteresis behaviour of Py/Nb/Py trilayered structure in the normal state measured at 10K for an external in-plane applied field, thebluecircles marked the regions over which magnetic switching occurs, (b) The switching behaviour of F layers can be seen from the Zoomed version of thebluecircled regions. Thegreenarrows indicate the direction of measurement and thepurplearrows indicate the switching fields. Switching behaviour observed in region(a) has been shown in the top o the figure and region(b) in the inset.

(a) (b)

Figure 4.4.2:(a) Magnetic hysteresis behaviour of a Py/Nb/Py sample measured in the superconducting state (3K) for an in-plane external applied magnetic field. (b) Temperature dependence of the magnetisation in the ZFC and FC states measured under an in-plane applied field of 100G.

CHAPTERIV BULKCHARACTERIZATIONS

4.4.2 Surface Microscopy Measurements

4.4.2.1 Experimental

The surface microscopy measurements were taken using a Vecco NanoScope Scanning Probe Microscope at the Centre for Mesoscience and Nanotechnology at the University of Manch- ester. The measurements were performed in the amplitude feedback mode with a drive frequency of∼80kHz. The scans were taken typically over a 40µm×40µmarea with a scan rate of∼40µm/s. The tip can detect surface information over a range of 2.5µm.

4.4.2.2 Results

The surface microscopy data have been plotted in Figure 4.4.3 and Figure 4.4.4. In Fig- ure4.4.3(a), the AFM topography of the sample describes minimal effect of overall rough- ness in the structure. The line plot in Figure4.4.3(b) describes the variation of the roughness across a specific scan direction. The maximum roughness observed is ∼2Å which can be accounted for the growth inhomogeneities associated with this structure. The surface mag- netic structure of the same has been plotted in Figure 4.4.4(a). The weak inhomogeneity of the magnetic flux structure indicates the presence of domain boundaries, although none of them are quite strong. Magnetic domains can arise in the case of an unsaturated state of the ferromagnet which accounts for the weak magnetic inhomogeneities observed like randomised moments in the plane of the film giving rise to surface states detected in this case. The line plot across a specific line of scan has been illustrated in Figure4.4.4(b), which quantifies the height of maximum inhomogeneity. It is to be noted that the line plot in Fig- ure4.4.4(b) did not show any sharp contrast across the the scan direction. In the presence of a significant magnetic domain structure, the contrast is more prominent across a domain boundary which is not the situation in the present case. It is to be noted that these samples were grown in the presence of an external magnetic field to control the growth direction and minimising the presence of the domain states. However, the sample measured in the present case was not saturated before the measurement which might account for the weak magnetic modulations observed.

(a) (b)

Figure 4.4.3: (a) AFM image of Py/Nb/Py sample measured over a40µm×40µm area at room temperature. (b) Average roughness behaviour of the sample measured across a specific scanning direction as taken from Fig.4.4.3(a).

(a) (b)

Figure 4.4.4:(a) MFM image of Py/Nb/Py sample measured over a40µm×40µmarea at room temperature. (b) Typical linescan illustrating spread in the values of magnetic flux contrast on the sample measured as taken from Fig.4.4.4(a).

CHAPTERIV LE-µSR MEASUREMENTS