Microbes, magnetism, and microscopy

An accurate quantification of magnetic force microscope images has been accomplished. The magnetosomes produced by magnetotactic bacteria, an ideal micromagnetic model system, were the specimens used for the quantification (a moment on the order of 10 IJ emu). Magnetic force microscopes (MFMs) are a variant of the non-contact AFM in which magnetic fields have been imaged with submicron resolution. These instruments consist of a small magnetic probe, typically on the end of a vibrating cantilever, which is scanned above a magnetic sample. The interactions of the magnetic probe with the stray micromagnetic fields from the sample are mapped to form an image that yields information about the micromagnetic structure of the sample. Although imaging is straightforward, quantification of the images, i.e., a determination of the magnetization of the specimen under investigation, has been difficult. The main difficulties are a lack of well characterized tips, a well defined, simple magnetic moment to investigate, and the invasive nature of the measurements [1], i.e., the fields of the tip can alter the specimen magnetization and vice versa. An added complication in the MFM is sensitivity to the second derivative with respect to distance of the sample generated magnetic fields. In recent work [1] to be summarized here, we were successful in circumventing these difficulties and successfully quantifying MFM images of the magnetic field produced by a chain of magnetosomes in a magnetotactic bacterium. Magnetotactic bacteria (MTB) orient and migrate along the geomagnetic field towards favorable habitats, a behavior known as magnetotaxis [2]. MTB produce intracellular chains of permanent single magnetic domain particles of magnetite (Fe30 4 ), or gregite (Fe3S4 ). The mineral particles and their enveloping membrane, called magnetosomes, are characterized by a narrow size distribution, specific crystallographic orientations, and species-specific crystal morphologies. Magnetosomes are usually organized in one or more linear chains, with the crystallographic magnetic easy axes «Ill) for Fe3 0 4 ) of each particle also aligned along the chain axis. The size specificity and crystallographic orientation of the chain assembly is optimally designed for magnetotaxis [3]. The magnetic dipole moments of individual magnetotactic bacteria have previously been inferred by a variety of techniques to be in the range of 1013 to 1012 emu. Interest in the biomineralization of magnetite by bacteria has initiated research in several different fields including microbiology, physics, and paleomagnetism. For paleomagnetism, biogenic magnetic minerals can be deposited in sediments and preserve a record of the ancient geomagnetic field [4]. For physics, biogenic magnetic minerals provide a novel source of single domain particles for experimental studies in fine particle magnetism [5]. In our work, we used the magnetosome chains as very simple magnetic dipole moments for quantification of the images obtained by MFM. An added bonus of this work is a measurement of the moment of the bacteria which provides a detailed characterization of the chains and represents a measurement of a single moment of on the order of 1013 emu. In what follows, we will briefly discuss the experimental techniques, the background necessary for understanding the MFM imaging data, and detail the model used for calculating the moment of the magnetosome chains from the MFM images. Freeze-dried cells of the magnetotactic bacterial strain MV1 were used in our study [6]. Individual magnetite particles in MV1 are truncated hexahedral prisms with average dimensions of 53 × 35 × 35 nm. Particles are arranged in a single linear chain with an average number of magnetosomes/cel l of 10 [7]. The freeze-dried cells had a bulk coercivity of 385 Oe at room temperature. Specimens for the MFM study were prepared by rehydrating freeze-dried cells, and allowing them to dry on a glass slide [8] prior to AFM and MFM studies. For MFM imaging, the tip is scanned over the surface such that only long range forces, e.g., the magnetic forces, are acting on the cantilever. To separate the long range or magnetic forces from the short range topographic forces, several non-contact MFM images at different interaction strengths (or equivalently, different t ip-sample separa tions) were made. Analysis of these scans allowed separa tion of the topographic and magnetic forces. The MFM image of three bacteria showing a clear magnetic image is shown in Fig. la. This image, taken with a high coercivity tip, shows both attractive and repulsive magnetic interactions as expected. In the situation when imaging with magnetically soft tips, the fields from the magnetosomes can affect the image. For example, shown in Fig. 2 is an MFM image made with a a low coercivity magnetic tip which resulted in the interaction between the tip and dipolar field from the cell always being attractive. Most models of ac mode MFM assume that the cantilever acts as a simple harmonic oscillator. Magnetic interactions enter in as perturbations, shifting the equilibrium position of the cantilever up or down and shifting the resonant frequency up or down by an amount proportional to F ' , the z component of the force gradient acting between the tip and sample [9]. In analyzing the behavior