Identification of two patterns in magnetic force microscopy of shape memory alloys B. R. A. Neves a) and M. S. Andrade Laborato ´rio de Nanoscopia, CETEC, Av. Jose ´ Ca ˆndido da Silveira, 2000 Belo Horizonte, MG, CEP:31170-000, Brazil ~Received 13 January 1999; accepted for publication 9 February 1999! In this work, we report on the observation of two coexisting patterns in magnetic force microscopy ~MFM! images of shape memory alloys. The MFM signal of both patterns presents similar behavior with tip–surface separation. An investigation on the origin of these patterns presents strong evidence that both are of magnetic nature only and, furthermore, can be assigned as bulk and surface-related, respectively. © 1999 American Institute of Physics. @S0003-6951~99!04214-X# Magnetic force microscopy ~MFM! is, nowadays, a well-established technique used in the investigation of mag- netic materials and their properties. After its introduction during the 1980s, the MFM technique has rapidly spread due to, mainly, some useful characteristics such as high spatial resolution and minimum sample preparation. 1,2 Imaging of different materials, from artificially created domains of re- cording heads 1 and media 3 to naturally occurring domains on hard and soft magnetic materials, 4,5 have been routinely ac- complished. The initial spatial resolution, of the order of 100 nm, 1–5 has improved by one order of magnitude during this decade by the use of electron beam deposited spike tips 6 and specially devised MFM operation modes. 7 However, for most commercially available tips and operation modes, the MFM resolution is of order of 50 nm. 8 The influence of tip–surface separation on resolution and other properties of MFM images has also been discussed by several authors. 1,3,5,9 Shape memory alloys are a class of materials that exhibit martensitic transformation. 10 The shape memory effect, which may be defined as the property of recovering its origi- nal shape during a thermal cycle after a material has been apparently deformed in a permanent way, was first discov- ered in a AuCd alloy in the early 1950s. 10 Since then, various alloys exhibiting this effect have been developed, including some stainless steels, for example, the Fe–Mn–Cr–Ni–Si alloy. 11 In this work, a Fe–Mn–Cr–Ni–Si stainless steel was imaged using the MFM technique. The tip–surface separa- tion was varied from 25 to 300 nm, and two coexisting pat- terns in MFM images were observed. An analysis of the images showed that both patterns presented qualitatively the same behavior with the separation between the magnetic tip and the sample surface, although one of them exhibited a weak variation of the MFM signal with separation and the other displayed a strong variation. A study of these results, which included the use of a simple model for the origin of the MFM image, resulted in a strong evidence that both pat- terns are of magnetic nature only. Moreover, they were found to originate from the magnetic properties of the bulk and surface of the sample, respectively. The investigated sample consisted of a Fe–Mn–Cr–Ni– Si–C alloy with the following composition ~% in weight!: 59.46–16.70–9.70–8.00–6.10–0.04, respectively. Before imaging, the sample was thermally treated at 600 °C for 5 min. Its surface was polished with a diamond paste and then etched with an acetic gliceregia ~AG! solution. The etching step was employed to reveal ferritic-phase islands sur- rounded by the austenitic-phase matrix, which constitute the structure of this alloy after employing such thermal treatment. 12 All imaging was carried out below the marten- sitic transformation temperature with the sample submitted to no deformation. The MFM images were acquired with a MultiMode Scanning Probe Microscope using a Nanoscope IIIa control- ler with Phase Extender Box from Digital Instruments. The magnetic information was sensed by scanning the oscillating tip over the surface at a chosen distance and, then monitoring frequency shifts on the natural oscillation frequency of the cantilever. The tips employed in this work were standard 225 mm cantilever Si tips coated with a high coercivity Co 85 Cr 15 film. 8 Before MFM imaging, each tip was magnetized paral- lel to the tip axis. Some nonmagnetized tips and also some noncoated tips with 225 mm cantilever were also utilized during this work. Figure 1 presents a series of images of the same sample region. Figure 1~a! shows the topographic image of the sur- face acquired using atomic force microscopy ~tapping mode!. One can clearly see a relatively large ferrite island ~lighter region! embedded in the austenitic matrix ~dark re- gion!. Due to the etching process, the ferrite surface is found to be ;10 nm higher than the austenite surface. It is also clear in the image that the surface of the ferrite contains sub-islands forming a somewhat geometric design. The ori- gin of these sub-islands is related to the sample preparation process ~thermal treatment, polishing and etching!, but a de- tailed discussion on this subject will be presented elsewhere. 12 However, the bulk of the ferrite, i.e., the part of the island under the surface, is believed to be formed by a homogeneous and continuous material. 11,12 Finally, the white spots found in the upper left corner of the image and the white bump ~indicated by an arrow! in the center are most probably dust particles adhered in the sample surface. a! Corresponding author. Present address: Analytical Instrumentation Facil- ity, North Carolina State University, EGRC Room 318, 1010 Main Cam- pus Drive, Box 7531, Raleigh, NC 27695-7531. Electronic mail: bneves@unity.ncsu.edu APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 14 5 APRIL 1999 2090 0003-6951/99/74(14)/2090/3/$15.00 © 1999 American Institute of Physics Downloaded 18 Apr 2007 to 150.164.15.81. 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