MICROSCOPY RESEARCH AND TECHNIQUE 23:98-99 (1992)
Rapid Communication Use of Ferrofluids to Obtain Magnetic Domain Images in AFM M. AINDOW, A.J. WILLIAMS, AND I.R. HARRIS School of Metallurgy and Materials, The University of Birmingham. Elins Road, Edghaston, R1S 27T, United Kingdom.
INTRODUCTION Conventional methods for the imaging of magnetic structure fall into two distinct categories; direct methods in which the magnetic field is mapped (e.g. Kerr effect, Secondary electron and Lorentz imaging) and decoration techniques in which a magnetic medium is deposited upon flux lines and/or domain walls. More recently high resolution images of magnetic domains at surfaces have been obtained directly using Magnetic Force Microscopy (MFM) whereby a cantilever coated with a magnetic medium is fitted to an Atomic Force Microscope (AFM) operating in non-contact mode (Saenz et al. 1987). In this communication we demonstrate that high resolution domain images can also be obtained with the more common contact-mode AFM if the specimen surface is suitably decorated. METHOD The simplest and most effective way of decorating domains is by using ferrofluids - colloidal suspensions of Fe304 particles in aqueous or organic solvents (Bitter 1932, Elmore 1938). The particles migrate through the fluid to the domain walls under the influence of stray fields and leave Bitter patterns on drying. The strong contrast observed in such patterns under the optical microscope is mainly due to a "staining" effect since the particles are dark in colour. In contact-mode the AFM operates as a very sensitive profilometer and gives purely topological information. It is, therefore, necessary to produce a smooth flat surface on the sample before applying the ferrofluid. Since in this case the role of the particles is to generate topological features which can be detected by the AFM, the fluid used must be much less dilute than is appropriate for optical microscopy. To illustrate this method we present images obtained from two polished samples of an Fe- 16%Nd-8%B alloy (atomic percentages). The first was in the as cast condition and the second had been decrepitated using hydrogen, milled, magnetically aligned and sintered to produce a strongly textured permanent magnet with a grain size of 7-8pm (McGuiness et. al. 1986). The specimen surfaces were decorated using a ferrofluid with an 8nm nominal particle size mixed with petroleum-based lapping fluid in the proportions 1:2 and 1:lO. Droplets of the mixture were placed on the specimen, distributed evenly using a microscope slide cover slip and allowed to dry for 24hrs. Images were obtained from these specimens with a Digital Instruments Nanoscope I1 contact-mode AFM using a cantilever with a force constant of 0.06Nm-1 and scanning at =4mzwith constant force. RESULTS AND DISCUSSION The domains in the magnetic Fe14Nd2B phase are known to adopt the form of elongated rods with their axis parallel to the c axis of the crystal structure and having a "cogwheel" shaped cross-section. Typical dimensions are =0.5pm in diameter and many microns in length but these dimensions fall in magnets with grain sizes below 80pm (e.g. Pastushenkov, Fork1 and Kronmuller 1991). Furthermore, the domains tend to split up as they emerge at original surfaces or grain boundaries. Typical AFM images obtained from the cast alloy specimen are shown in Figs 1-3. Fig. 1 was obtained from a region where the domains.lie with their axis parallel to the surface and exhibits arrays of rodllke features roughly parallel to one another as expected. In Fig. 2 the domains have their axis perpendicular to the surface and characteristic "swirls" are observed; these correspond to chains of cogwheels. Since the orientation of the domains is determined by that of the crystal structure, it will change across a grain boundary as shown in Fig. 3. The boundary appears as a sharp ridge protruding from the surface - it is not known whether this topology is due to ferrofluid decorating stray fields at the boundary or corrosion products. A few microns away from the boundary, a well developed domain structure can be seen but this breaks down within 3-4pm of the boundary. This could correspond to a change in the domain structure. Fig. 4 was obtained from a specimen of the processed magnet cut in such a way that the c axis of the structure is almost parallel to the surface in each grain. Rodlike domain structure can be seen within the grains and this reveals clearly the magnitude of the misorientation between individual grains. Such images could be particularly useful in assessing the extent of texturing.
Received June 18, 1992; accepted June 26, 1992
0 1992 WILEY-LISS, INC
DOMAIN IMAGES OBTAINED WITH AFM
99
Whilst the resolution of the AFM is less than lnm, the resolution of the domain images obtained will be limited by the decoration technique and thus the ultimate resolution of such images will be equal to the size of an individual dried ferrofluid particle. It is unlikely that this can be achieved experimentally as there is usually some tendency for the colloid particles to agglomerate. Furthermore, since the accumulation of a significant number of particles is required to generate the topological features, it is not clear how well very fine structure in domain walls will be revealed. Despite these objections, the resolution of domain images obtained by the method described in this communication should be similar to that which can be achieved using MFM (i.e. = 30-50 nm).
Images obtained from Nd-Fe-B samples decorated with diluted ferrofluid. 1 and 2. Alloy sample - dilution 1:2. 3. Alloy sample - dilution 1:lO. 4.Magnet sample - dilution 1:2.
REFERENCES Bitter, F., (1932) Experiments on the Nature of Ferromagnetism, Phys. Rev. 41; 507. Elmore, W.C., (1938) Ferromagnetic Colloid for Studying Magnetic Structures, Phys. Rev. 54, 309. McGuiness, P.J., Harris, I.R., Rozendaal, E., Ormerod, J., and Ward, M., (1986) The Production of a Nd-Fe-B Permanent Magnet by a Hydrogen Decrepitation / Attritor Milling Route, J. Mater. Sci. 21; 4107. Pastushenkov, J., Forkl, A., and Kronmiiller, H., (1991) Magnetic Domain Structure of Sintered Fe-Nd-B Type Permanent Magnets and Magnetostatic Grain Interaction, J. Magn. Magn. Mater. 101; 363. Saenz, J.J., Garcia, N., Grutter, P., Meyer, E., Heinzlmann, H., Hidber, H.-J., and Giintherodt, H.-J., (1987) Observation of Magnetic Forces by the Atomic Force Microscope, J. Appl. Phys. 70; 4293.
Rapid Communication Use of Ferrofluids to Obtain Magnetic Domain Images in AFM M. AINDOW, A.J. WILLIAMS, AND I.R. HARRIS School of Metallurgy and Materials, The University of Birmingham. Elins Road, Edghaston, R1S 27T, United Kingdom.
INTRODUCTION Conventional methods for the imaging of magnetic structure fall into two distinct categories; direct methods in which the magnetic field is mapped (e.g. Kerr effect, Secondary electron and Lorentz imaging) and decoration techniques in which a magnetic medium is deposited upon flux lines and/or domain walls. More recently high resolution images of magnetic domains at surfaces have been obtained directly using Magnetic Force Microscopy (MFM) whereby a cantilever coated with a magnetic medium is fitted to an Atomic Force Microscope (AFM) operating in non-contact mode (Saenz et al. 1987). In this communication we demonstrate that high resolution domain images can also be obtained with the more common contact-mode AFM if the specimen surface is suitably decorated. METHOD The simplest and most effective way of decorating domains is by using ferrofluids - colloidal suspensions of Fe304 particles in aqueous or organic solvents (Bitter 1932, Elmore 1938). The particles migrate through the fluid to the domain walls under the influence of stray fields and leave Bitter patterns on drying. The strong contrast observed in such patterns under the optical microscope is mainly due to a "staining" effect since the particles are dark in colour. In contact-mode the AFM operates as a very sensitive profilometer and gives purely topological information. It is, therefore, necessary to produce a smooth flat surface on the sample before applying the ferrofluid. Since in this case the role of the particles is to generate topological features which can be detected by the AFM, the fluid used must be much less dilute than is appropriate for optical microscopy. To illustrate this method we present images obtained from two polished samples of an Fe- 16%Nd-8%B alloy (atomic percentages). The first was in the as cast condition and the second had been decrepitated using hydrogen, milled, magnetically aligned and sintered to produce a strongly textured permanent magnet with a grain size of 7-8pm (McGuiness et. al. 1986). The specimen surfaces were decorated using a ferrofluid with an 8nm nominal particle size mixed with petroleum-based lapping fluid in the proportions 1:2 and 1:lO. Droplets of the mixture were placed on the specimen, distributed evenly using a microscope slide cover slip and allowed to dry for 24hrs. Images were obtained from these specimens with a Digital Instruments Nanoscope I1 contact-mode AFM using a cantilever with a force constant of 0.06Nm-1 and scanning at =4mzwith constant force. RESULTS AND DISCUSSION The domains in the magnetic Fe14Nd2B phase are known to adopt the form of elongated rods with their axis parallel to the c axis of the crystal structure and having a "cogwheel" shaped cross-section. Typical dimensions are =0.5pm in diameter and many microns in length but these dimensions fall in magnets with grain sizes below 80pm (e.g. Pastushenkov, Fork1 and Kronmuller 1991). Furthermore, the domains tend to split up as they emerge at original surfaces or grain boundaries. Typical AFM images obtained from the cast alloy specimen are shown in Figs 1-3. Fig. 1 was obtained from a region where the domains.lie with their axis parallel to the surface and exhibits arrays of rodllke features roughly parallel to one another as expected. In Fig. 2 the domains have their axis perpendicular to the surface and characteristic "swirls" are observed; these correspond to chains of cogwheels. Since the orientation of the domains is determined by that of the crystal structure, it will change across a grain boundary as shown in Fig. 3. The boundary appears as a sharp ridge protruding from the surface - it is not known whether this topology is due to ferrofluid decorating stray fields at the boundary or corrosion products. A few microns away from the boundary, a well developed domain structure can be seen but this breaks down within 3-4pm of the boundary. This could correspond to a change in the domain structure. Fig. 4 was obtained from a specimen of the processed magnet cut in such a way that the c axis of the structure is almost parallel to the surface in each grain. Rodlike domain structure can be seen within the grains and this reveals clearly the magnitude of the misorientation between individual grains. Such images could be particularly useful in assessing the extent of texturing.
Received June 18, 1992; accepted June 26, 1992
0 1992 WILEY-LISS, INC
DOMAIN IMAGES OBTAINED WITH AFM
99
Whilst the resolution of the AFM is less than lnm, the resolution of the domain images obtained will be limited by the decoration technique and thus the ultimate resolution of such images will be equal to the size of an individual dried ferrofluid particle. It is unlikely that this can be achieved experimentally as there is usually some tendency for the colloid particles to agglomerate. Furthermore, since the accumulation of a significant number of particles is required to generate the topological features, it is not clear how well very fine structure in domain walls will be revealed. Despite these objections, the resolution of domain images obtained by the method described in this communication should be similar to that which can be achieved using MFM (i.e. = 30-50 nm).
Images obtained from Nd-Fe-B samples decorated with diluted ferrofluid. 1 and 2. Alloy sample - dilution 1:2. 3. Alloy sample - dilution 1:lO. 4.Magnet sample - dilution 1:2.
REFERENCES Bitter, F., (1932) Experiments on the Nature of Ferromagnetism, Phys. Rev. 41; 507. Elmore, W.C., (1938) Ferromagnetic Colloid for Studying Magnetic Structures, Phys. Rev. 54, 309. McGuiness, P.J., Harris, I.R., Rozendaal, E., Ormerod, J., and Ward, M., (1986) The Production of a Nd-Fe-B Permanent Magnet by a Hydrogen Decrepitation / Attritor Milling Route, J. Mater. Sci. 21; 4107. Pastushenkov, J., Forkl, A., and Kronmiiller, H., (1991) Magnetic Domain Structure of Sintered Fe-Nd-B Type Permanent Magnets and Magnetostatic Grain Interaction, J. Magn. Magn. Mater. 101; 363. Saenz, J.J., Garcia, N., Grutter, P., Meyer, E., Heinzlmann, H., Hidber, H.-J., and Giintherodt, H.-J., (1987) Observation of Magnetic Forces by the Atomic Force Microscope, J. Appl. Phys. 70; 4293.
