Biogenic magnetic minerals also provide a novel source of material for fundamental studies in magnetism. For example, biogenic magnetite has been used to study the effects of magnetic interactions on isothermal (IRM) and anhysteretic (ARM) remanent magnetization. Because magnetosomes are of uniform size and shape, one does not have to untangle these effects from magnetic interaction effects, a common and often limiting problem in fine-particle studies using synthetic samples. An interaction test based on IRM and ARM behavior and ``calibrated'' against whole cells was used to study compaction, cementation, and dolomitization of platform carbonates [ Diaz Ricci et al., 1991; McNeill and Kirschvink, 1993]. Proksch and Moskowitz [1994] used a variation of the Wohlfarth-Cisowski interaction test in which intact chains and extracted magnetosomes were prepared in different initial remanent states before the IRM acquisition curve was measured. By analyzing a series of IRM curves, it was possible to separate the effects of both positive interactions along a chain and negative interactions among chains or clumped magnetosomes. This approach may prove useful for detecting biogenic chain fractions in sediments.
As mentioned previously, some magnetotactic bacteria produce ``large'' magnetosomes (up to 200 nm) with particle dimensions that are larger than the theoretical SD size range for magnetite [ Farina et al., 1994]. Neglecting the biological implications of non-SD magnetosomes and engaging in some speculation, the existence of these magnetosomes has several fascinating magnetic implications. Either the magnetosomes are indeed two domain; or (1) the magnetosomes are really SD and the theoretical groundstate SD-TD transition size needs to be slightly revised; or (2) the magnetosomes are uniformly magnetized in an SD state but it is a higher energy metastable SD state within the equilibrium TD range. Possibility (2) is the most intriguing and, if true, these bacteria can provide validation of micromagnetic models as well as provide a source of metastable SD magnetite particles for study. Interestingly, the magnetosome dimensions are consistent with recent theoretical grain size limits for metastable SD magnetite particles [e.g. Dunlop, 1990].
Contact and non-contact scanning force microscopy with a
magnetic tip was used to simultaneously image topography and
magnetic forces from the magnetosome chain assembly in a single MTB
cell [ Proksch et al., 1994; Farina et al., 1994]. The
MFM estimate of the dipole moment of a single cell agreed well with
the average dipole moment of the cell population from which the
cell came, as determined with a superconducting (SQUID)
magnetometer. However, the MFM result is a direct magnetic
measurement of a single cell that represents 
10
improvement in sensitivity over conventional SQUID magnetometers
[ Proksch et al., 1994].
The MFM can also be a useful magnetic probe to study individual SD and ``large'' magnetosomes. It should be possible to decide among the alternative micromagnetic explanations for ``large'' magnetosomes discussed above by either directly imaging a domain wall, spin vortex structures, or inducing the nucleation of a domain wall from the field of the MFM tip. The ``conventional'' SD magnetosomes also can provide a useful magnetic system for studying single particle switching behavior and time-dependent phenomena. Finally, MFM studies of magnetosomes can be useful for comparison with current computer modelling of the micromagnetic spin structures of fine magnetic particles from first-principle calculations and quantitative predictions based on those models [e.g., Dunlop, 1990]. One potential payoff of these models will be to predict the grain size dependent behavior of remanence, coercivity, and susceptibility (ie., hysteresis properties) for the different magnetic minerals found in nature. Most importantly, as ``grain-size proxies,'' these magnetic parameters form the basis for interpreting the magnetic record of paleoclimate change and the magnetic fingerprinting of remagnetization in limestones. The study of biogenic magnetic minerals can provide critical experimental validation of these micromagnetic models and advance our understanding of the magnetic behavior of magnetite.
Acknowledgments. Support for the Institute for Rock Magnetism is provided by grants from the Keck Foundation and the National Science Foundation. This is contribution 9405 of the Institute for Rock Magnetism.