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Stray-field investigations on sharp ferromagnetic tips by electron holography
ELSEVIER
Journal of Magnetism and Magnetic Materials 133 (1994) 422-424
JH
journal of magnetism and magnetic materials
Stray-field investigations on sharp ferromagnetic tips by electron holography G. Matteucci
a,* M.
Muccini
a
U. H a r t m a n n b
a Center for Electron Microscopy, Department of Physics University of Bologna, 40126 Bologna, Italy b Institute of Thin Film and Ion Technology, KFA-Jiilich 52425 Jiilich, Germany
(Received 13 April 1993; revised 4 November 1993)
Abstract
The leakage field produced by sharp ferromagnetic probes employed for magnetic force microscopy has been investigated by electron holography. Interference fringes obtained with the double exposure technique are found to be in good qualitative agreement with calculations based on a macroscopic dipole model for the sensor tips. We show that it is possible to measure the probes' magnetic flux through the evaluation of the phase difference in the simulated map.
Magnetic force microscopy (MFM) [1], an offspring of atomic force microscopy [2], permits the imaging of near-surface magnetic microfields at sub-100 nm lateral resolution [3]. A sharp ferromagnetic tip attached to a flexible cantilever beam is employed to sense weak magnetostatic interactions between probe and sample. In almost all previous approaches to contrast interpretation it was assumed for simplicity that probe and sample are magnetostatically coupled in a completely non-perturbative way, i.e. their magnetic structures do not change upon mutual approach [4]. There is, however, significant experimental [5,6] as well as theoretical [7,8] evidence that the sharp ferromagnetic microprobes usually create a considerable magnetic leakage field which may perturb the sample's near-surface magnetic configuration, at least for fairly soft samples. Hence, a closer examination of the stray magnetic field produced near the apex of typical MFM probes is of fundamental importance [9]. Here, we report on the detailed study of MFM probes by electron holography whose potentiality has been demonstrated in a variety of applications [10-14].
* Corresponding author. Tel: + 39 (51)6305145, Fax: + 39 (51) 6305153.
Disregarding the tip shape, our attention has been focused on the possibility to display a map of the phase difference between the wave passing very next the tip apex (object wave) and a reference wave which is modulated by the field tail. From the resulting phase difference map a simulation of the actual field configuration is then derived basing on a simplified model for the tip. The MFM probes under investigation were prepared from thin nickel wires using standard electrochemical etching techniques [15]. To obtain a map of the leakage field arising by a thin magnetic tip, first a very simple model is assumed in which the apex is approximated by a macroscopic dipole of 20 txm length [16]. According to the existing experimental data [17,18], such a dipole length seems realistic for soft magnetic sensor tips. Electron holograms have been recorded using an electrostatic biprism as interferometry device [19], as sketched in Fig. 1. A coherent electron beam EB, travelling in a direction perpendicular to the tip axis, illuminates the apex which is arranged in a mutually perpendicular position with the biprism wire W. The wire splits the incoming electron beam and its electrostatic field produces a deflection and a subsequent overlapping in the observation plane OP below the wire. With this configuration the part of the wave passing next to the tip apex is the object wave O, while
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00170-V
G. Matteucci et al. /Journal of Magnetism and Magnetic Materials 133 (1994) 422-424
Fig. 1. Schematic arrangement for hologram formation.
the reference wave R is the part of the illuminating wave which travels a few i~m distant. Since also the latter is modulated by the leakage field the phase difference between the object and the reference waves will be recorded in the holograms [20]. A Philips EM 400T F E G electron microscope with the electrostatic biprism at the selected area level was used to take off-axis image holograms. The objective lens was switched off not to perturb the leakage field
423
arising from the tip and the microscope operated in the diffraction mode. In this way the diffraction lens was used to focus the sample on the photographic plate; under our experimental conditions the final magnification was about 2000 x . Phase difference maps of the magnetic leakage field of the tip were recorded using the double exposure technique [21]. An optical reconstructed double exposure hologram is reported in Fig. 2. The interference field, whose vertical width is about 4 p~m, is at 1 Ixm distance from the tip apex, whose axis forms an angle of about 30 ° with the interference field. The lateral extension of the latter was limited by the diameter of the illuminating laser beam. The black lines represent the loci of the points with constant phase difference of an odd multiple of -rr between the two exposures. The curvature of the lines on the left part is different with respect to that of the lines on the right side. This is because the magnetic tip had still some influence on the electron beam during the second exposure that, in order to be performed, the tip was moved along its axis but slightly shifted on the left side. This experimental result clearly shows that it is not possible to associate a one to one correspondence among the lines of this map and the magnetic field around the tip apex. In order to get the actual field trend we must first show that, starting from the field model, we can simulate the experimental phase difference map of Fig. 2. Simulations of the phase difference distribution
Fig. 2. Reconstructed double exposure hologram around the tip vertex whose image was filtered out by the reconstruction process.
Fig. 3. Simulation of the phase difference distribution of Fig. 2. The vertical width of the interference field is 4 Ixm.
424
G. Matteucci et al. /Journal of Magnetism and Magnetic Materials 133 (1994) 422-424
arising by the apex leaking field was p e r f o r m e d by using an IBM P C / A T equipped with a video board able to display 512 × 512 pixels at 256 grey levels. This information is displayed as a set of curves with phase difference of 2av, between two successive black and white ones, which correspond to an enclosed magnetic flux h / e . In Fig. 3 the resulting pattern is shown in which three simulations are assembled. They display the trend of the equi-phase lines on a laterally wider region with respect to Fig. 2. As it can be seen the experimental result is in good agreement with the calculated equi-phase lines. Therefore we can conclude that the field generated by the tip can be represented, as a good approximation, as that arising from a macroscopic dipole. For M F M the possibility to evaluate the magnetic flux leaking from the tip apex is of basic importance. Having already noticed that the number of black lines in Fig. 3 is the same as that of Fig. 2, it follows that we can simulate, using the same magnetic charge of Fig. 3 the equivalent dipole field [20]. An evaluation of the total phase difference around the tip apex allows us to estimate the total flux in the region of interest. In the present case there is a magnetic flux leaving the tip apex towards the sample of about 1.6 × 10 13 Wb. Summing up, electron holography has been used for the first time to study the leaking field of M F M probes. Modeling the tip by a macroscopic dipole we obtained good qualitative agreement between theoretical and experimental data. We showed that electron holography, through the double exposure method, can provide quantitative information on the leakage flux of magnetic microprobes. Since the long-range stray field of the tip influences also the reference electron beam, great attention must be paid to the interpretation of experimental results. Work is now in progress to map the leakage field of thin film magnetic probes. This kind of tip is a promising candidate for M F M since it is expected to have a reduced leakage magnetic field [3] thus keeping the perturbation of the sample structure very low.
Acknowledgements. The skillful technical assistance of S. Patuelli is gratefully acknowledged. This
work has been supported by funds from M U R S T coordinated by Consorzio I N F M and C N R - G N S M .
References
[1] Y. Martin and H.K. Wickramasinghe, Appl. Phys. Lett. 50 (1987) 1455. [2] G. Binning, C.F. Quate and C. Gerber, Phys. Rev. Lett. 56 (1986) 930. [3] U. Hartmann, J. Magn. Magn. Mater. 101 (1991) 263. [4] U. Hartmann, J. Vac. Sci. Technol. A 8 (1990) 411. [5] T. G6ddenhenrich, U. Hartmann, M. Anders and C. Heiden, J. Microsc. 152 (1988) 527. [6] H.J. Mamin, D. Rugar, J.E. Stern, R.E. Fontana, Jr. and P. Kasiraj, Appl. Phys. Lett. 55 (1989) 318. [7] U. Hartmann, J. Appl. Phys. 64 (1988) 1561. [8] M.R. Scheinfein, J. Unguris, D.T. Pierce and R.J. Celotta, J. Appl. Phys. 67 (1990) 5932. [9] S. McVitie and U. Hartmann, in: Proc. 49th Ann. Mtg. of the Electron Microscopy Society of America (San Francisco Press, San Francisco, 1991) p. 92. [10] A. Tonomura, Rev. Mod. Phys. 59 (1987) 639. [11] S. Frabboni, G. Matteucci, G. Pozzi and M. Vanzi, Phys. Rev. Lett. 55 (1985) 2196. [12] G. Matteucci, G.F. Missiroli, E. Nichelatti, A. Migliori, M. Vanzi, G. Pozzi, J. Appl. Phys. 69 (1991) 1835. [13] H. Lichte, in Advances in Optical and Electron Microscopy vol. 12, ed. by T. Mulvey (Springer, Berlin, 1991) p. 25. [14] G. Matteucci, G.F. Missiroli, M. Muccini and G. Pozzi, Ultramicroscopy 45 (1992) 77. [15] H. Lemke, T. G6ddenhenrich, H.P. Bochem, U. Hartmann and C. Heiden, Rev. Sci. Instrum. 61 (1990) 2538. [16] G. Matteucci and M. Muccini, in: Proc. X European Congr. on Electron Microscopy, vol. 1, Granada (1992) p. 657. [17] D. Rugar, H.J. Mamin, P. Guethner, S.E. Lambert, J.E. Stern, I. McFadyen and T. Yogi, J. Appl. Phys. 68 (1990) 1169.
[18] C. Sch6nenberger and S.F. Alvarado, Z. Phys. B 80 (1990) 373. [19] G. M611enstedt and H. Diiker, Z. Phys. 145 (1956) 377. [20] G. Matteucci, M. Muccini and U. Hartmann, Appl. Phys. Lett. 62 (1993) 1839. [21] G. Matteueci, G.F. Missiroli, J.W. Chen and G. Pozzi, Appl. Phys. Lett. 52 (1988) 176.
Journal of Magnetism and Magnetic Materials 133 (1994) 422-424
JH
journal of magnetism and magnetic materials
Stray-field investigations on sharp ferromagnetic tips by electron holography G. Matteucci
a,* M.
Muccini
a
U. H a r t m a n n b
a Center for Electron Microscopy, Department of Physics University of Bologna, 40126 Bologna, Italy b Institute of Thin Film and Ion Technology, KFA-Jiilich 52425 Jiilich, Germany
(Received 13 April 1993; revised 4 November 1993)
Abstract
The leakage field produced by sharp ferromagnetic probes employed for magnetic force microscopy has been investigated by electron holography. Interference fringes obtained with the double exposure technique are found to be in good qualitative agreement with calculations based on a macroscopic dipole model for the sensor tips. We show that it is possible to measure the probes' magnetic flux through the evaluation of the phase difference in the simulated map.
Magnetic force microscopy (MFM) [1], an offspring of atomic force microscopy [2], permits the imaging of near-surface magnetic microfields at sub-100 nm lateral resolution [3]. A sharp ferromagnetic tip attached to a flexible cantilever beam is employed to sense weak magnetostatic interactions between probe and sample. In almost all previous approaches to contrast interpretation it was assumed for simplicity that probe and sample are magnetostatically coupled in a completely non-perturbative way, i.e. their magnetic structures do not change upon mutual approach [4]. There is, however, significant experimental [5,6] as well as theoretical [7,8] evidence that the sharp ferromagnetic microprobes usually create a considerable magnetic leakage field which may perturb the sample's near-surface magnetic configuration, at least for fairly soft samples. Hence, a closer examination of the stray magnetic field produced near the apex of typical MFM probes is of fundamental importance [9]. Here, we report on the detailed study of MFM probes by electron holography whose potentiality has been demonstrated in a variety of applications [10-14].
* Corresponding author. Tel: + 39 (51)6305145, Fax: + 39 (51) 6305153.
Disregarding the tip shape, our attention has been focused on the possibility to display a map of the phase difference between the wave passing very next the tip apex (object wave) and a reference wave which is modulated by the field tail. From the resulting phase difference map a simulation of the actual field configuration is then derived basing on a simplified model for the tip. The MFM probes under investigation were prepared from thin nickel wires using standard electrochemical etching techniques [15]. To obtain a map of the leakage field arising by a thin magnetic tip, first a very simple model is assumed in which the apex is approximated by a macroscopic dipole of 20 txm length [16]. According to the existing experimental data [17,18], such a dipole length seems realistic for soft magnetic sensor tips. Electron holograms have been recorded using an electrostatic biprism as interferometry device [19], as sketched in Fig. 1. A coherent electron beam EB, travelling in a direction perpendicular to the tip axis, illuminates the apex which is arranged in a mutually perpendicular position with the biprism wire W. The wire splits the incoming electron beam and its electrostatic field produces a deflection and a subsequent overlapping in the observation plane OP below the wire. With this configuration the part of the wave passing next to the tip apex is the object wave O, while
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00170-V
G. Matteucci et al. /Journal of Magnetism and Magnetic Materials 133 (1994) 422-424
Fig. 1. Schematic arrangement for hologram formation.
the reference wave R is the part of the illuminating wave which travels a few i~m distant. Since also the latter is modulated by the leakage field the phase difference between the object and the reference waves will be recorded in the holograms [20]. A Philips EM 400T F E G electron microscope with the electrostatic biprism at the selected area level was used to take off-axis image holograms. The objective lens was switched off not to perturb the leakage field
423
arising from the tip and the microscope operated in the diffraction mode. In this way the diffraction lens was used to focus the sample on the photographic plate; under our experimental conditions the final magnification was about 2000 x . Phase difference maps of the magnetic leakage field of the tip were recorded using the double exposure technique [21]. An optical reconstructed double exposure hologram is reported in Fig. 2. The interference field, whose vertical width is about 4 p~m, is at 1 Ixm distance from the tip apex, whose axis forms an angle of about 30 ° with the interference field. The lateral extension of the latter was limited by the diameter of the illuminating laser beam. The black lines represent the loci of the points with constant phase difference of an odd multiple of -rr between the two exposures. The curvature of the lines on the left part is different with respect to that of the lines on the right side. This is because the magnetic tip had still some influence on the electron beam during the second exposure that, in order to be performed, the tip was moved along its axis but slightly shifted on the left side. This experimental result clearly shows that it is not possible to associate a one to one correspondence among the lines of this map and the magnetic field around the tip apex. In order to get the actual field trend we must first show that, starting from the field model, we can simulate the experimental phase difference map of Fig. 2. Simulations of the phase difference distribution
Fig. 2. Reconstructed double exposure hologram around the tip vertex whose image was filtered out by the reconstruction process.
Fig. 3. Simulation of the phase difference distribution of Fig. 2. The vertical width of the interference field is 4 Ixm.
424
G. Matteucci et al. /Journal of Magnetism and Magnetic Materials 133 (1994) 422-424
arising by the apex leaking field was p e r f o r m e d by using an IBM P C / A T equipped with a video board able to display 512 × 512 pixels at 256 grey levels. This information is displayed as a set of curves with phase difference of 2av, between two successive black and white ones, which correspond to an enclosed magnetic flux h / e . In Fig. 3 the resulting pattern is shown in which three simulations are assembled. They display the trend of the equi-phase lines on a laterally wider region with respect to Fig. 2. As it can be seen the experimental result is in good agreement with the calculated equi-phase lines. Therefore we can conclude that the field generated by the tip can be represented, as a good approximation, as that arising from a macroscopic dipole. For M F M the possibility to evaluate the magnetic flux leaking from the tip apex is of basic importance. Having already noticed that the number of black lines in Fig. 3 is the same as that of Fig. 2, it follows that we can simulate, using the same magnetic charge of Fig. 3 the equivalent dipole field [20]. An evaluation of the total phase difference around the tip apex allows us to estimate the total flux in the region of interest. In the present case there is a magnetic flux leaving the tip apex towards the sample of about 1.6 × 10 13 Wb. Summing up, electron holography has been used for the first time to study the leaking field of M F M probes. Modeling the tip by a macroscopic dipole we obtained good qualitative agreement between theoretical and experimental data. We showed that electron holography, through the double exposure method, can provide quantitative information on the leakage flux of magnetic microprobes. Since the long-range stray field of the tip influences also the reference electron beam, great attention must be paid to the interpretation of experimental results. Work is now in progress to map the leakage field of thin film magnetic probes. This kind of tip is a promising candidate for M F M since it is expected to have a reduced leakage magnetic field [3] thus keeping the perturbation of the sample structure very low.
Acknowledgements. The skillful technical assistance of S. Patuelli is gratefully acknowledged. This
work has been supported by funds from M U R S T coordinated by Consorzio I N F M and C N R - G N S M .
References
[1] Y. Martin and H.K. Wickramasinghe, Appl. Phys. Lett. 50 (1987) 1455. [2] G. Binning, C.F. Quate and C. Gerber, Phys. Rev. Lett. 56 (1986) 930. [3] U. Hartmann, J. Magn. Magn. Mater. 101 (1991) 263. [4] U. Hartmann, J. Vac. Sci. Technol. A 8 (1990) 411. [5] T. G6ddenhenrich, U. Hartmann, M. Anders and C. Heiden, J. Microsc. 152 (1988) 527. [6] H.J. Mamin, D. Rugar, J.E. Stern, R.E. Fontana, Jr. and P. Kasiraj, Appl. Phys. Lett. 55 (1989) 318. [7] U. Hartmann, J. Appl. Phys. 64 (1988) 1561. [8] M.R. Scheinfein, J. Unguris, D.T. Pierce and R.J. Celotta, J. Appl. Phys. 67 (1990) 5932. [9] S. McVitie and U. Hartmann, in: Proc. 49th Ann. Mtg. of the Electron Microscopy Society of America (San Francisco Press, San Francisco, 1991) p. 92. [10] A. Tonomura, Rev. Mod. Phys. 59 (1987) 639. [11] S. Frabboni, G. Matteucci, G. Pozzi and M. Vanzi, Phys. Rev. Lett. 55 (1985) 2196. [12] G. Matteucci, G.F. Missiroli, E. Nichelatti, A. Migliori, M. Vanzi, G. Pozzi, J. Appl. Phys. 69 (1991) 1835. [13] H. Lichte, in Advances in Optical and Electron Microscopy vol. 12, ed. by T. Mulvey (Springer, Berlin, 1991) p. 25. [14] G. Matteucci, G.F. Missiroli, M. Muccini and G. Pozzi, Ultramicroscopy 45 (1992) 77. [15] H. Lemke, T. G6ddenhenrich, H.P. Bochem, U. Hartmann and C. Heiden, Rev. Sci. Instrum. 61 (1990) 2538. [16] G. Matteucci and M. Muccini, in: Proc. X European Congr. on Electron Microscopy, vol. 1, Granada (1992) p. 657. [17] D. Rugar, H.J. Mamin, P. Guethner, S.E. Lambert, J.E. Stern, I. McFadyen and T. Yogi, J. Appl. Phys. 68 (1990) 1169.
[18] C. Sch6nenberger and S.F. Alvarado, Z. Phys. B 80 (1990) 373. [19] G. M611enstedt and H. Diiker, Z. Phys. 145 (1956) 377. [20] G. Matteucci, M. Muccini and U. Hartmann, Appl. Phys. Lett. 62 (1993) 1839. [21] G. Matteueci, G.F. Missiroli, J.W. Chen and G. Pozzi, Appl. Phys. Lett. 52 (1988) 176.