Study of Interfaces in Metal/ C60 Bilayer under X-ray Standing Wave Condition; Theoretical Simulations

In the present work, we demonstrated the depth selectivity of FeCoB/C60 interface on basis of theoretical simulations by generating x-ray standing wave (XSW) using a waveguide structure Si/Pt/C60/FeCoB (interface)/FeCoB (bulk)/C60/Pt. Fekfluorescence (XRF) spectra were simulated theoretically for increasing angle of incidencefor the different values of electron density and thickness of interface FeCoB layer. It is found that the fluorescence peaks corresponding to the XSW antinodes crossing theinterface layer is highly sensitive to the electron density and thickness of interface FeCoB layers. Present study demonstrates that by measuring electron density profiles through XRF measurement under XSW, interface resolved structural information can be achieved by moving antinode region and selecting appropriate incident angle. INTRODUCTION Fullerene (C60) based ferromagnetic (FM) multilayered (ML) nanostructures are the imperative candidates for organic spintronics (OS) devices because of the low spin-orbit coupling due to lower atomic weight, longer spin lifetime, good thermal stability and reasonably good mobility [1-2]. In such devices, organic semiconductor (OSCsuch as C60, Alq3 etc, sandwiched between two ferromagnetic (FM) layers are used to mediate or control a spin-polarized electron transport but mechanical softness of OSC create challenges owing to penetration, diffusion and possible chemical reaction of metal atoms at the interfaces, which creates complicated magnetism and transport in these structures. Recently, combination of different FM layers such as FeCoB, Co have been grown on C60 [1,2, 3] and unusual magnetic properties at the interfaces, significantly different from their constituent layers were obtained. In view of this, a clear picture of how effective organic spin valve (OSV) could be achieved for an efficient spin injection has not been experimentally realized till date [3-4]. The main difficulty is to get genuine and depth resolved interface structure using conventional lab-based characterization techniques such as x-ray reflectivity (XRR), secondary ion mass spectroscopy (SIMS) etc., either do not have enough depth resolution so as to resolve the interfaces or may not be probing the real interfaces. Furthermore, techniques like XRR is sensitive to all the individual parameters of the layers (thickness, roughness, and electron density etc). Hence to get genuine information from a particular interface is difficult. On the other hand, after creating XSW in the wave guide structure and placing the metal layer in the wave guide cavity, element sensitive fluorescence depends on mainly illuminated area by XSW antinodes and very sensitive to the particular layer [5,6]. In the present work, understanding of interface diffusion and deep penetration of FeCoB atom into C60is understood through theoretical simulation, where use of XRF under XSW will be demonstrated to be a suitable method for FM-C60 systems otherwise it is difficult in the systems where deep penetration of the FM atom and spread over up to hundreds of angstroms in OSC layer. Present simulation has been used to demonstrate increased sensitivity from the interface by generating x-ray standing wave (XSW) using a waveguide structure (WGS). Interface selectivity could be used to follow the evolution of the interfaces with thermal annealing to get insight the interface structure and diffusion. Reported XRF simulations under XSW condition seems to be the promising in order to develop better understanding at FM/C60 interface and to tune the properties for the desired functionality. FORMATION OF XSWIN WAVE GUIDE STRUCTURE In case of wave guide structure, the XSW can be generated by total reflection of x-rays from an under-layer of high Z element (e.g., Au, Pt). Interference between the incident and the reflected wave-fronts form a standing wave above the surface of the reflecting layer, with separation between the successive antinodes given byD=1/q, where q is the scattering vector. Figure 1(a) gives a waveguide structure, where a lowdensity layer (FullereneC60) is sandwiched between two dense layers (say Pt). Simulated electric field intensity (EFI) profile inside the cavity (along the depth),as shown in fig. 1(b-c), clearly confirm formation of XSW in wave guide structure. It may be noted that, when angle of incidence changes, the position of antinode shifts and formation of different standing wave modes (TE01, TE02, TE03 etc.,) takes place along the depth of the cavity. X-ray measurements such as XRF, under XSW condition would provide selective information from the region of antinodes, when it crosses the layer inside the cavity [5,7]. (a)(b)(c) FIGURE 1.(a) Schematic ofwave guide structure: where one low density layer is sandwiched between two high density layers, (b) Rearrangement of x-ray field intensity (EFI) inside the C60 cavity as function of the scattering vector q; simulated for Si substrate /Pt(30nm)/C60/(35nm)/Pt (2nm) structure (c) Variation of EFI along the depth at q1 = 0.0304 Å-1 (TE0 mode), q2 =0.04 Å-1 (TE1 mode) and q3 =0.0536 Å-1(TE2 mode). Interface Selectivity of Metal Layer in The C60Waveguide Cavity: In order to show the formation of XSW, even in the presence of metal layer inside the cavity, EFI profile is theoretically generated for the same wave guide structure in the presence of 50 Åthick FeCoB. Simulated EFI profiles are shown in fig. 2 (a-d), which were generated by placing FeCoB layerat the different depth in the cavity; (a)140 Å, (b) 120 Å,(c) 170 Å,(d) 220 Å.As the angle of incidence is changed, the formation of the different XSW modes takes place at the different fixed q values. In the present case, the position of antinodes(TE1 and TE2) crosses top and bottom side of the FeCoB layer at different q values.X-ray measurements such as fluorescence, XRD, XAFS etc. understanding wave condition would provide selective information from the region of antinodes. It can clearly be observed thaton introducing a FeCoB layer, the field inside the waveguide cavity gets perturbed and hence, formation of XSW waves takes place in an asymmetric way as compared to Fig. 1(b). It is important to note that for positions140 Å (depth), TE1 and TE2 XSW antinodes coincide with the FeCoB layer at the top and bottom side of the interface at different q values (q2=0.0432Å and q3=0.0552Å). It suggests that interface sensitivity for both interfaces can be achieved by performing other X-ray based measurements at q2=0.0432Å and q3=0.0552Å. 0 100 200 300 400 500 0.03 0.04 0.05 0.06 0.07