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Recent developments in the application of Lorentz microscopy to the study of thin layered systems
j I~. ELSEVIER
Journal of Magnetism and Magnetic Materials 175 (1997) 107-108
Journalof magnetism and magnetic materials
Recent developments in the application of Lorentz microscopy to the study of thin layered systems J.P. J a k u b o v i c s Department of Materials, Universi~ of Oxford, Parks Road, Oxford OX1 3PH, UK
Many thin layered systems being developed for information storage applications have inhomogeneities in their structure, leading to properties that vary from one part of the specimen to another. It is therefore important to study not only the average properties, but also their local variations. An important indicator of local variations is the magnetic domain structure. Lorentz microscopy is a particularly useful method for domain studies in thin layered systems, because of the suitability of the geometry of the samples, and the high spatial resolution obtainable in transmission electron microscopes (TEMs). Lorentz microscopy is being carried out in Oxford using a JEOL 4000EX TEM. The microscope is equipped with an AMG40 objective pole-piece, enabling samples to be studied away from strong magnetic fields, and with a specially designed aperture mechanism, which positions the objective aperture in the correct plane for Foucault imaging of the domains [R.C. Doole, A.K. Petford-Long, J.P. Jakubovics, Rev. Sci. Instr. 64 (1993) 1038]. A specimen holder has been constructed, which incorporates a pair of coils for applying controlled magnetic fields in the plane of the specimen, for in situ magnetizing experiments. The conventional Fresnel and Foucault domain imaging methods only provide an approximate indication of the magnetization configuration, since the relationship between the magnetization distribution and the image is not straightforward. Many thin layered systems have complicated magnetization configurations, and it is important to be able to map them accurately and quantitatively. A method that has been in use for some years is the implementation [J.N. Chapman, R. Ploessl, D.M. Donnet, Ultramicroscopy 47 (1992) 331] of differential phase contrast (DPC) imaging in a scanning transmission electron microscope. Recently, the equivalent method using a conventional TEM has been developed [A.C. Daykin, A.K. Petford-Long, Ultramicroscopy 58 (1995) 365; A.C. Daykin, J.P. Jakubovics, J. Appl. Phys. 80 (1996) 3408]. The TEM implementation is based on Foucault imaging. To produce a DPC map in the TEM, four sets of Foucault images are recorded, with the incident illumination being tilted in four directions at 90 ° to each other. The images in each series are added digitally, and pairs of opposite images are subtracted to obtain maps of one component of the magnetization (strictly, the magnetic induction). The two difference images then provide the two components of the magnetization vector, from which the complete vector map can be constructed. The entire process is carried out under computer control, enabling a complete map to be obtained in about 1 min. The TEM DPC method will be illustrated with a study of a Co/Cr/Co trilayer specimen, in which the two Co layers are expected to be coupled antiferromagnetically. The specimen was taken through a complete magnetizing cycle in situ in the TEM, starting from a remanent state. In this initial state, the magnetization of two Co layers was approximately parallel. When a magnetic field was applied in approximately the opposite 0304-8853/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved Pll S0304-8853(97)00589- 1
108
J.P. Jakubovics / Journal of Magnetism and Magnetic Materials 175 (1997) 107-108
direction to the magnetization, the magnetization of the two Co layers became antiparallel in some regions. With increasing field, the antiparallel aligned regions grew, and eventually the magnetization suddenly switched to parallel alignment in the direction of the field. When the direction of the field was reversed, the process was repeated, with the antiparallel alignment appearing in the same regions as before. From each map, the average magnetization could be deduced, and a complete hysteresis loop could be constructed, showing a coercivity of about 50 Oe. The results are interpreted on the assumption that the presence of local pinning of magnetic moments prevents the antiparallel alignment in the remanent state. This assumption is supported by observations of the domain structure after AC-demagnetization of the specimen. In this state, large areas with antiparallel alignment are present, and the average magnetization is close to zero.
Journal of Magnetism and Magnetic Materials 175 (1997) 107-108
Journalof magnetism and magnetic materials
Recent developments in the application of Lorentz microscopy to the study of thin layered systems J.P. J a k u b o v i c s Department of Materials, Universi~ of Oxford, Parks Road, Oxford OX1 3PH, UK
Many thin layered systems being developed for information storage applications have inhomogeneities in their structure, leading to properties that vary from one part of the specimen to another. It is therefore important to study not only the average properties, but also their local variations. An important indicator of local variations is the magnetic domain structure. Lorentz microscopy is a particularly useful method for domain studies in thin layered systems, because of the suitability of the geometry of the samples, and the high spatial resolution obtainable in transmission electron microscopes (TEMs). Lorentz microscopy is being carried out in Oxford using a JEOL 4000EX TEM. The microscope is equipped with an AMG40 objective pole-piece, enabling samples to be studied away from strong magnetic fields, and with a specially designed aperture mechanism, which positions the objective aperture in the correct plane for Foucault imaging of the domains [R.C. Doole, A.K. Petford-Long, J.P. Jakubovics, Rev. Sci. Instr. 64 (1993) 1038]. A specimen holder has been constructed, which incorporates a pair of coils for applying controlled magnetic fields in the plane of the specimen, for in situ magnetizing experiments. The conventional Fresnel and Foucault domain imaging methods only provide an approximate indication of the magnetization configuration, since the relationship between the magnetization distribution and the image is not straightforward. Many thin layered systems have complicated magnetization configurations, and it is important to be able to map them accurately and quantitatively. A method that has been in use for some years is the implementation [J.N. Chapman, R. Ploessl, D.M. Donnet, Ultramicroscopy 47 (1992) 331] of differential phase contrast (DPC) imaging in a scanning transmission electron microscope. Recently, the equivalent method using a conventional TEM has been developed [A.C. Daykin, A.K. Petford-Long, Ultramicroscopy 58 (1995) 365; A.C. Daykin, J.P. Jakubovics, J. Appl. Phys. 80 (1996) 3408]. The TEM implementation is based on Foucault imaging. To produce a DPC map in the TEM, four sets of Foucault images are recorded, with the incident illumination being tilted in four directions at 90 ° to each other. The images in each series are added digitally, and pairs of opposite images are subtracted to obtain maps of one component of the magnetization (strictly, the magnetic induction). The two difference images then provide the two components of the magnetization vector, from which the complete vector map can be constructed. The entire process is carried out under computer control, enabling a complete map to be obtained in about 1 min. The TEM DPC method will be illustrated with a study of a Co/Cr/Co trilayer specimen, in which the two Co layers are expected to be coupled antiferromagnetically. The specimen was taken through a complete magnetizing cycle in situ in the TEM, starting from a remanent state. In this initial state, the magnetization of two Co layers was approximately parallel. When a magnetic field was applied in approximately the opposite 0304-8853/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved Pll S0304-8853(97)00589- 1
108
J.P. Jakubovics / Journal of Magnetism and Magnetic Materials 175 (1997) 107-108
direction to the magnetization, the magnetization of the two Co layers became antiparallel in some regions. With increasing field, the antiparallel aligned regions grew, and eventually the magnetization suddenly switched to parallel alignment in the direction of the field. When the direction of the field was reversed, the process was repeated, with the antiparallel alignment appearing in the same regions as before. From each map, the average magnetization could be deduced, and a complete hysteresis loop could be constructed, showing a coercivity of about 50 Oe. The results are interpreted on the assumption that the presence of local pinning of magnetic moments prevents the antiparallel alignment in the remanent state. This assumption is supported by observations of the domain structure after AC-demagnetization of the specimen. In this state, large areas with antiparallel alignment are present, and the average magnetization is close to zero.