* Corresponding author. Fax: #44-1752-2325-83. E-mail address: jwindmill@plymouth.ac.uk (J.F.C. Windmill). Journal of Magnetism and Magnetic Materials 226}230 (2001) 1267}1269 A new theoretical probe for the magnetic force microscope J.F.C. Windmill*, W.W. Clegg, D.F.L. Jenkins, P.J. Davey CRIST, DCEE, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, UK Abstract The magnetic force microscope (MFM) is established as a valuable tool for the analysis of magnetic structures. The standard design of MFM incorporates a silicon tip coated with a magnetic material. However, these tips are subject to several inherent problems, e.g. changing characteristics over time due to damage or magnetic hysteresis. A new theoretical electromagnetic MFM probe is introduced here. Although electromagnetic MFM has been discussed before by Zhou et al. (J. Vac. Sci. Technol. A 17 (1999) 2233), the design presented here is a di!erent approach. Two di!erent probe iterations and their magnetic "eld intensity distribution are modelled. The probe imaging capability is compared using the reciprocity principle (Wright and Hill, Appl. Phys. Lett. 68 (1996) 1726) to image the simulated force interaction between a sample and the probe "elds. Thus, images of a sample's magnetic distribution are produced by the convolution of the di!erent probe gradient "eld distributions and the sample magnetisation. Both perpendicular and longitudinal magnetisation patterns were simulated with the di!erent probe iterations. This clearly showed the improvement of the second probe iteration, particularly for longitudinal patterns. The practical use of the new probe is also discussed, and future work outlined.  2001 Published by Elsevier Science B.V. Keywords: MFM; Magnetic modelling; Electromagnetic MFM probe The design of magnetic force microscope (MFM) probes is a continuing area of research interest. The standard MFM probe uses a silicon tip, magnetically sensitised using a ferromagnetic coating. This design has several inherent problems, for example, a range of probes are required to image di!erent samples, and a probe's characteristics can change over time. As such it is di$cult to obtain quantitative data using MFM. A possible an- swer to these problems is to use an electromagnetic probe to interact with a magnetic sample. An electromagnetic MFM probe has been reported before; however, it is a relatively new idea, and the design presented here is completely novel [1]. A "eld would be created about a micro-sized aperture in a conductor at the end of a cantilever. This follows the Biot}Savart law, where, by changing the current density in a conductive material, the magnetic "eld intensity normal to its surface is altered. So the current must #ow around the aperture, changing the current density local to the aperture, generating a "eld. The probe has the advantage that the "eld intensity is variable, controllable, and should provide repeatable re- sults. The "rst probe "eld model, shown in Fig. 1, followed work done in the 1970s on current-access bubble mem- ory, where a circular aperture was modelled [2]. This was then con"rmed using three-dimensional "nite element modelling. The next step was to simulate the imaging performance of the new probe. Most previous work has simulated the interaction between the probe tip magnet- isation and a sample's stray "eld, and thus the force acting on the tip. However, the reciprocity principle can be used [3]. In this case the force on the sample due to the interaction of the probe stray "eld and the sample magnetisation is considered, i.e. the inverse of the force on the probe. Magnetisation distribution models for perpendicular and longitudinal magnetic media bit transitions have been used to simulate the imaging capa- bility of the new probe [4]. A FFT convolution of the probe's gradient "eld and the sample magnetisation dis- tribution then creates simulated DC-MFM images. The simulated images shown in Fig. 2a and b were created using the above technique for a circular aperture 0304-8853/01/$- see front matter  2001 Published by Elsevier Science B.V. PII:S0304-8853(01)00074-9