Scanning probe microscopy investigation of iron garnet films for magnetoplasmonics

Topography and domain structures of Bi-substituted iron garnet films proposed for magnetoplasmonic applications are presented. Investigations were carried out by scanning probe microscopy methods, including polarization near-field optical microscopy. The most suitable films with less rough surface were chosen. The period of domain structure of sputtered film of composition Bi2.8Y0.2Fe5O12 were determined, 2w = 0.5 μm.


Introduction
Synthesis and investigation of magnetoplasmonic (MP) nanostructures is relevant for construction of micro-dimensional optical isolators, modulators and switches, miniature integrated nanophotonic devices for fast multi-mode and multi-directional control, plasmonic circuit elements controlled by magnetic field [1,2]. For example, nanostructure "iron garnet (IG) -perforated Au film" experimentally demonstrated the effective modulation of transmission with contrast up to 98% using a weak external magnetic field [3]. Creation of metallic coating on IG films imposes specific surface requirements. Additionally, the films should have definite magnetic and magnetooptical properties [4]. In the work, authors present the investigation of topography and domain structures of IG films proposed for MP applications by scanning probe microscopy methods, including polarization nearfield optical microscopy [5].

Experimental
Films of bismuth-substituted IG with micro-and nanoscale thicknesses were synthesized by liquidphase epitaxy (LPE) and reactive ion beam sputtering (RIBS). For RIBS-films, the garnet phase was formed by crystallization annealing process in the air and high temperature Ta (above 650 o C) after deposition on respective substrate from ceramic target [4,[6][7][8]. Substrates of gadolinium gallium garnet (GGG), calcium niobium gallium garnet (CNGG), and fused quartz SiO2 were used for RIBS-films. All LPE-films were synthesized on GGG substrates.
Semicontact atomic force microscopy (AFM) was applied to describe the surface and measure its parameters -height range Rmax, average roughness Ra, root mean square roughness rms, and polycrystalline size Z (for RIBS-films only). To visualize the domain structure of films, semicontact two-pass or parallel magnetic force microscopy (MFM) and scanning near-field optical microscopy (SNOM) were used. The measurements were carried out by cantilevers of HA_HR ETALON, MFM01, and SNOM_NC with typical aperture diameter of (120 ± 25) nm. The scheme of used experimental equipment, scanning probe microscope (SPM) NTEGRA (NT-MDT) with configuration that allows combining the AFM and SNOM techniques, is illustrated in Figure 1. Table 2 lists the determined parameters of surface topography of investigated samples and total thicknesses of films h. Data were obtained by AFM measurements on frames with area of 2x2 μm 2 using standard AFM cantilever.   Measurements of various LPE-films showed that rms could reach values less than 0.9 nm. Figure 2 shows the typical domain structure of LPE-film No.1 with period of 16 μm. From obtained data we notice that width of domain wall of film No.1 is wdw = 895 nm measured as distance from minima to maxima of optical intensity signal. Nevertheless, the structure of domain walls and domain edges is not obvious and may be more complicated as Figure 2b,c,d demonstrates. Figure 3 shows the surface and domain structure of RIBS-film No. 4. MFM and SNOM data give the same domain period of the film that is 2w = 0.5 μm. The film before measurements was heated. We calculated the parameters of surface from the images obtained by SNOM tip. The values of Rmax, Ra, and rms decrease to 33.7, 3.9, and 4.8 nm in comparison to the same values in the Table 2. Average size of crystallites increases to Z = 270 nm. So, the surface parameters of polycrystalline films with the rough surface are not defined correctly by SNOM tip due to its pyramidal geometry, and development of SNOM tip with more appropriate forms is necessary [12,13].    Table 2). We propose these films for further experiments.

Conclusion
LPE and RIBS Bi-substituted IG films were investigated by AFM, MFM, and SNOM to choose the most suitable films with less rough surface for magnetoplasmonic applications. Typical value of rms of synthesized LPE-films is 0.8 nm. The rms of LPE-films increases with thickening the samples. The structure of domain wall of LPE-film with uniaxial anisotropy was presented.
The period of domain structure of RIBS-film of composition Bi2.8Y0.2Fe5O12 was determined as 2w = 0.5 μm. IOP