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Surface domains in inhomogeneous yttrium iron garnet films
Journal of Magnetism and Magnetic Materials 258–259 (2003) 580–582
Surface domains in inhomogeneous yttrium iron garnet films A.G. Temiryazeva,*, M.P. Tikhomirovaa, I. Fedorovb b
a Institute of Radioengineering & Electronics RAS, Fryazino 141190, Russia Moscow Institute of Electronic and Technology, K-498, Moscow 124498, Russia
Abstract We have studied magnetic domain structures in yttrium iron garnet films with inhomogeneous anisotropy field distribution across the film thickness. Two methods of domain visualization were used: (1) magnetic force microscopy and (2) magneto-optics. The experiments have revealed an existence of surface domains. The transformations of the surface domain pattern in the external magnetic field have been also observed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic force microscopy; Magnetic domain structure; Garnet films
The objects of this study are inhomogeneous films of yttrium iron garnet (YIG). By the term inhomogeneous is meant that the films have a substantial variation of the magnetic anisotropy field Ha ðzÞ along a z-axis perpendicular to the film plane. Such films were grown epitaxially on gadolinium gallium garnet (GGG) substrates at temperature monotonically varying in time. The difference DHa in the anisotropy field at the opposite films surfaces can be as high as 500 Oe. The inhomogeneous films are of interest for microwave applications, since the variation of the magnetic parameters enables the linear excitation of exchange-dominated spin waves and shortwavelength acoustic waves propagating across the film thickness [1,2]. The aim of present study is to investigate magnetic domain structures in the inhomogeneous films. We used two methods of domain visualization: (1) magnetic force microscopy (MFM), and (2) magnetooptical (MO) microscopy based on the Faraday rotation. The MFM study was performed with scanning probe microscope Solver P47H produced by NT-MDT, Russia. The device was operating in the resonant mode. Two-pass technique [3] was used to obtain separate topographic and MFM images of the same area. The topographic line scan is first taken, and then this information is used to make the second pass at a constant user-defined scan height h: The shift in the *Corresponding author. Fax: +7-095-702-9572. E-mail address: [email protected] (A.G. Temiryazev).
cantilever phase, recorded at the second pass, is caused by the sensitivity of the magnetic tip to the sample’s stray field H: The MFM image represents the distribution of the second derivative q2 H=qz2 in the plane lying at the distance h above the sample surface. A typical value of h is about 100 nm. In contrast to MFM, the magneto-optical image shows the distribution of the z-component of magnetization inside the film volume. Note that the YIG has a smaller constant of Faraday rotation than Bi-doped garnets have. The saturation magnetization in YIG (4pMS ¼ 1750 Oe) exceeds the anisotropy field, hence MZ is small. This leads to a rather low contrast of MO images. The MO study was carried out with a polarization microscope. In order to enhance the image, a series of pictures (up to some hundreds) of the same area were taken with a CCD video camera. Then, these pictures were superimposed on one another with a computer. Let us consider the experimental results. Fig. 1a shows MO image for the YIG film with the thickness d ¼ 18:7 mm and the (1 0 0) orientation. The change in the anisotropy field DHa ; measured from spin-wave resonance spectra [1,2], was found to be equal to 400 Oe. This film will be denoted as sample 1. Fig. 1b and c have the same scale as in Fig. 1a and show MFM images of sample 1 taken at the different scan heights: h ¼ 1 mm for Fig. 1b and h ¼ 100 nm for Fig. 1c. Notice that no modulation along the stripe
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 1 5 9 - 9
A.G. Temiryazev et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 580–582
581
Fig. 1. Magneto-optical and MFM images of YIG films: (a) MO image of sample 1; (b), (c), (g) and (h) MFM images of sample 1; (d) MO image of sample 2; (e), (j) and (k) MFM images of sample 2; (f) and (i) MFM images of sample 3.
domain direction is observed optically. Fig. 1b broadly resembles the structure seen in the optic microscope. In Fig. 1c one can see a fine structure in form of the needle-
shaped pattern. This structure is the most common for the inhomogeneous YIG films while it has not been seen in uniform films. We believe that the fine structures
582
A.G. Temiryazev et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 580–582
Fig. 2. MFM images of YIG film (57 57 mm2, sample 1) in the external magnetic field He.
observed in MFM studies are associated with the sharp changes of domain structure in a narrow near-surface layer. Because of the small thickness, this layer is unable to make a substantial contribution to the Faraday rotation, thus we fail to observe optically these peculiarities. On the other hand, the surface domains form the magnetic field distribution in the immediate vicinity of the film boundary. It is just the pattern that is observed by the MFM. Fig. 1d shows the MO image and Fig. 1e represents the MFM image for the sample 2. It has the following parameters: d ¼ 34 mm, the (1 1 1) orientation, and DHa ¼ 42 Oe. It should be noted that there are two types of domain patterns in the MFM scan. Either pattern corresponds to the region having one of two directions of stripe domains. The MO image in Fig. 1d does not show such peculiarities. In some of the investigated YIG films we could not see any MO domains, while the MFM proved an existence of the surface domains. As already noted, the needleshaped pattern is the most typical. The one exception is exhibited in Fig. 1f, where the MFM image for sample 3 (d ¼ 23 mm, the (1 0 0) orientation, and DHa ¼ 370 Oe) is shown. There are regions where the needles of the neighboring branches merge together and a honeycomb surface domain structure is formed. The MFM images of the above-listed samples are shown in Fig. 1g–j on an enlarged scale. It is interesting to note that the scans in Fig. 1g and h were made on the same YIG film (sample 1).
Fig. 2 represents a series of domain structure transformations, that were observed when we applied an external magnetic field He, lying in plane of the film. Sample 1 was studied. The scans were made at successive increasing in the magnetic field strength. As indicated in the picture, the external magnetic field leads to the rotation of domain structure as well as to the change of its period and shape. We conclude that the MFM investigations of the inhomogeneous YIG films have demonstrated the existence of rather complicated surface domain structures, while the MO investigations of the same films has not shown the occurrence of any modulation along the stripe domain structure (biperiodic stripe domain structures) observed previously in Bi-doped garnet films [4]. The work was supported by ISTC (Grant #1522).
References [1] A.G. Temiryazev, M.P. Tikhomirova, P.E. Zil’berman, J. Appl. Phys. 76 (1994) 5586. [2] P.E. Zil’berman, A.G. Temiryazev, M.P. Tikhomirova, JETP 108 (1995) 151. [3] Y.E. Martin, H.K. Wickramasinghe, Appl. Phys. Lett. 50 (1987) 1455. [4] G.V. Arzamastseva, F.V. Lisovskii, E.G. Mansvetova, JETP 118 (2000) 1011.
Surface domains in inhomogeneous yttrium iron garnet films A.G. Temiryazeva,*, M.P. Tikhomirovaa, I. Fedorovb b
a Institute of Radioengineering & Electronics RAS, Fryazino 141190, Russia Moscow Institute of Electronic and Technology, K-498, Moscow 124498, Russia
Abstract We have studied magnetic domain structures in yttrium iron garnet films with inhomogeneous anisotropy field distribution across the film thickness. Two methods of domain visualization were used: (1) magnetic force microscopy and (2) magneto-optics. The experiments have revealed an existence of surface domains. The transformations of the surface domain pattern in the external magnetic field have been also observed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic force microscopy; Magnetic domain structure; Garnet films
The objects of this study are inhomogeneous films of yttrium iron garnet (YIG). By the term inhomogeneous is meant that the films have a substantial variation of the magnetic anisotropy field Ha ðzÞ along a z-axis perpendicular to the film plane. Such films were grown epitaxially on gadolinium gallium garnet (GGG) substrates at temperature monotonically varying in time. The difference DHa in the anisotropy field at the opposite films surfaces can be as high as 500 Oe. The inhomogeneous films are of interest for microwave applications, since the variation of the magnetic parameters enables the linear excitation of exchange-dominated spin waves and shortwavelength acoustic waves propagating across the film thickness [1,2]. The aim of present study is to investigate magnetic domain structures in the inhomogeneous films. We used two methods of domain visualization: (1) magnetic force microscopy (MFM), and (2) magnetooptical (MO) microscopy based on the Faraday rotation. The MFM study was performed with scanning probe microscope Solver P47H produced by NT-MDT, Russia. The device was operating in the resonant mode. Two-pass technique [3] was used to obtain separate topographic and MFM images of the same area. The topographic line scan is first taken, and then this information is used to make the second pass at a constant user-defined scan height h: The shift in the *Corresponding author. Fax: +7-095-702-9572. E-mail address: [email protected] (A.G. Temiryazev).
cantilever phase, recorded at the second pass, is caused by the sensitivity of the magnetic tip to the sample’s stray field H: The MFM image represents the distribution of the second derivative q2 H=qz2 in the plane lying at the distance h above the sample surface. A typical value of h is about 100 nm. In contrast to MFM, the magneto-optical image shows the distribution of the z-component of magnetization inside the film volume. Note that the YIG has a smaller constant of Faraday rotation than Bi-doped garnets have. The saturation magnetization in YIG (4pMS ¼ 1750 Oe) exceeds the anisotropy field, hence MZ is small. This leads to a rather low contrast of MO images. The MO study was carried out with a polarization microscope. In order to enhance the image, a series of pictures (up to some hundreds) of the same area were taken with a CCD video camera. Then, these pictures were superimposed on one another with a computer. Let us consider the experimental results. Fig. 1a shows MO image for the YIG film with the thickness d ¼ 18:7 mm and the (1 0 0) orientation. The change in the anisotropy field DHa ; measured from spin-wave resonance spectra [1,2], was found to be equal to 400 Oe. This film will be denoted as sample 1. Fig. 1b and c have the same scale as in Fig. 1a and show MFM images of sample 1 taken at the different scan heights: h ¼ 1 mm for Fig. 1b and h ¼ 100 nm for Fig. 1c. Notice that no modulation along the stripe
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 1 5 9 - 9
A.G. Temiryazev et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 580–582
581
Fig. 1. Magneto-optical and MFM images of YIG films: (a) MO image of sample 1; (b), (c), (g) and (h) MFM images of sample 1; (d) MO image of sample 2; (e), (j) and (k) MFM images of sample 2; (f) and (i) MFM images of sample 3.
domain direction is observed optically. Fig. 1b broadly resembles the structure seen in the optic microscope. In Fig. 1c one can see a fine structure in form of the needle-
shaped pattern. This structure is the most common for the inhomogeneous YIG films while it has not been seen in uniform films. We believe that the fine structures
582
A.G. Temiryazev et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 580–582
Fig. 2. MFM images of YIG film (57 57 mm2, sample 1) in the external magnetic field He.
observed in MFM studies are associated with the sharp changes of domain structure in a narrow near-surface layer. Because of the small thickness, this layer is unable to make a substantial contribution to the Faraday rotation, thus we fail to observe optically these peculiarities. On the other hand, the surface domains form the magnetic field distribution in the immediate vicinity of the film boundary. It is just the pattern that is observed by the MFM. Fig. 1d shows the MO image and Fig. 1e represents the MFM image for the sample 2. It has the following parameters: d ¼ 34 mm, the (1 1 1) orientation, and DHa ¼ 42 Oe. It should be noted that there are two types of domain patterns in the MFM scan. Either pattern corresponds to the region having one of two directions of stripe domains. The MO image in Fig. 1d does not show such peculiarities. In some of the investigated YIG films we could not see any MO domains, while the MFM proved an existence of the surface domains. As already noted, the needleshaped pattern is the most typical. The one exception is exhibited in Fig. 1f, where the MFM image for sample 3 (d ¼ 23 mm, the (1 0 0) orientation, and DHa ¼ 370 Oe) is shown. There are regions where the needles of the neighboring branches merge together and a honeycomb surface domain structure is formed. The MFM images of the above-listed samples are shown in Fig. 1g–j on an enlarged scale. It is interesting to note that the scans in Fig. 1g and h were made on the same YIG film (sample 1).
Fig. 2 represents a series of domain structure transformations, that were observed when we applied an external magnetic field He, lying in plane of the film. Sample 1 was studied. The scans were made at successive increasing in the magnetic field strength. As indicated in the picture, the external magnetic field leads to the rotation of domain structure as well as to the change of its period and shape. We conclude that the MFM investigations of the inhomogeneous YIG films have demonstrated the existence of rather complicated surface domain structures, while the MO investigations of the same films has not shown the occurrence of any modulation along the stripe domain structure (biperiodic stripe domain structures) observed previously in Bi-doped garnet films [4]. The work was supported by ISTC (Grant #1522).
References [1] A.G. Temiryazev, M.P. Tikhomirova, P.E. Zil’berman, J. Appl. Phys. 76 (1994) 5586. [2] P.E. Zil’berman, A.G. Temiryazev, M.P. Tikhomirova, JETP 108 (1995) 151. [3] Y.E. Martin, H.K. Wickramasinghe, Appl. Phys. Lett. 50 (1987) 1455. [4] G.V. Arzamastseva, F.V. Lisovskii, E.G. Mansvetova, JETP 118 (2000) 1011.