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1Nonlinear optical microscopy of epitaxial garnet films
A.I. Maydykovskiy1, T. Murzina1*
1- Lomonosov Moscow State University, Department of Physics, Leninskie Gory 1, 119991, Moscow, Russia
The composition and properties of the domain structure of various specimen are of high interest, as they are the main building blocks for such important media as ferroelectrics and magnetics. A proper place here belongs to magnetic domains, which reveal a wide variety of types and shapes depending on the constituent materials, e.g. metals or dielectrics, as well as on the structural parameters [1]. It is also well recognized that the domain structure in the bulk of a material and at its interfacial layers can also be quite different. Thus the search and development of efficient non-destructive diagnostics methods is very much desired. Commonly magnetic domain composition in the volume of the transparent dielectrics is revealed by optical polarization and magneto-optical microscopy, while the surface layers can be studied in much detail by magnetic force microscopy (MFM) and the Lorentz microscopy [2,3].
It has been shown recently that nonlinear optical microscopy based on second harmonic generation (SHG) can provide additional information about the organization of magnetic domains in crystalline iron-garnet films [3]. The functional potential of the SHG probe is based on unique sensitivity of the second-order nonlinear optical response to any kind of symmetry breaking in a medium, including the surface and nanostructure effects in the case of centrosymmetric materials, as well as the electric- or magnetic field-induced subjection. Importantly that the characteristic values of the magnetization induced effects in the SHG response (the nonlinear optical analogues of the magneto-optical Faraday and Kerr effects) are one-two orders of magnitude larger than in the linear optics. This allows for a high contrast in the SHG images of the magnetic domain structures. In combination with the confocal microscopy scheme and the use of a common femtosecond titanium-sapphire laser operating in the near IR wavelength range, the SHG probe provides a submicron in-plane spatial resolution, while along the direction normal to the surface it is about 1-2 microns. Symmetry analysis of the SHG response induced by the nonlinear susceptibility tensor allows to estimate the orientation of the magnetic moment of a medium not only along the normal to the surface as in the case of the MFM, while in the other directions as well. Consequently, noninvasive and relatively simple SHG microscopy technique can be applied for the studies of a 3D microstructure of magnetic domains in epitaxial garnet films of several micrometers in thickness both in the bulk of the film and at their surface layers, as is shown in our experiments.
We estimated the mutual orientation of the magnetization in the surface and bulk stripe domains, and showed that the thickness of the surface ones is about 1-1.5 ^m in the studied garnet film. The SHG microscopy also showed the changes in the magnetization processes of a free garnet layer and of that decorated by regular arrays of ferromagnetic particles. Further extension of the nonlinear optical microscopy in application to the garnet films is the use of the third harmonic generation (THG) as the probe; in these experiments we demonstrate even higher resolution of this method for the studies of magnetic domains [4].
This work was supported by RSCF, grant 19-72-20103-P. The use of the equipment of the Center for Collective Use "Physics and Technology of Micro- and Nanostructures" is greatly acknowledged.
[1] A. Hubert, R. Schafer, Magnetic Domains: The Analysis of Magnetic Microstructures (Springer, 1998).
[2] X. Yu, J.P. DeGrave, Y. Hara, T. Hara, S. Jin, Y. Tokura, Observation of the magnetic skyrmion lattice in a MnSi nanowire by Lorentz TEM, Nano Lett. 14, 3755-3759 (2013).
[3] M.P. Temiryazeva, E.A. Mamonov, A.I. Maydykovskiy, A.G. Temiryazev, T.V. Murzina, Magnetic Domain Structure of Lu2.1Bi0.9Fe5O12 Epitaxial Films Studied by Magnetic Force Microscopy and Optical Second Harmonic Generation, Magnetochemistry 8(12), 180(10) (2022).
[4] A.I. Maydykovskiy, N.S. Popov, T.V. Murzina, Third harmonic generation microscopy of magnetic domains in garnet films, Las. Phys. Letters 21, 025401 (6pp) (2024).
