Magnetic properties have been measured for a number of samples of acicular γ-Fe2O3 particles of the type used in magnetic recording tapes. The average particle size and shape were approximately the same for all samples. However, the average crystallite sizes of the samples, as determined from x-ray line broadening, ranged from 50 to 700 Å. All magnetic properties measured showed a strong dependence on average crystallite size. Saturation magnetization at room temperature, σ, decreased sharply with decreasing crystallite size. An excellent fit to the σ vs crystallite size data was made by assuming that the crystallites were separated by a nonmagnetic grain boundary on the order of 6 Å wide. Room-temperature coercive force decreased with decreasing crystallite size, and the ratio Hc(83°K)/Hc(293°K) increased sharply with decreasing crystallite size. The data support a particle model in which the constituent crystallites interact magnetostatically across nonmagnetic grain boundaries. A critical crystallite diameter of order 400 Å may be inferred. Below this crystallite size, superparamagnetic behavior is observed; above this size, noncoherent magnetization reversals in the crystallites are suggested. This model is quite consistent with the ``fanning'' mode of reversal based on a ``chain-of-spheres'' model which has been considered by several authors to be most compatible with their data on γ-Fe2O3 particle assemblies.

Magnetic particulates in the environment, collected from surfaces and directly from the atmosphere, are reported. A significant percentage of magnetic material in many air-borne samples consists of spherical magnetite from coal-burning utilities and from iron/steel manufacturing with particle diameters 2–10 μm which correspond to average coercive fields of ∼100 Oe and remanence to saturation ratios of ∼0.1. The amount of air-borne magnetic material that settles to the ground varies inversely with distance from its source.

This paper reports on the magnetic properties of magnetosomes in the freshwater magnetotactic bacterium Aquaspirillum nlllgnetOlaClicwn. The magnetosomes are well crystallized particles of magnetite with dimensions of 40 to 50 nm, which are arranged within cells in a single linear chain and are within the single-magnetic-domain (SO) size range for. magnetite. A variety of magnetic properties have been measured for two samples of dispersions of freeze-dried cells consisting of (1) whole cells (M-l) and (2) magnetosomes chains separated from cells (M-2). An important result is that the acquisition and demagnetization of various type of remanent magnetizations are markedly diffcrent for the two samples and suggest that remanence is substantially affected by magnetostatic interactions. Interactions are likely to be much more important in M-2 because the extracted magnetosome chains are no longer separated from one another by the cell membrane and cytoplasm. Other experimental data for whole cells agree with predictions based on the chain of spheres model for magnetization reversal. This model is consistent with the unique linear arrangement of equidimensional particles in A. magnelOlacticum. The magnetic properties of bacterial and synthetic magnetites are compared and the paleomagnetic implications are discussed.

This paper describes a type of vibrating-sample magnetometer capable of sensitivity exceeding 10−8 emu. The instrument is 1000 times more sensitive than a conventional VSM with comparable working space, and is much quicker to use than a SQUID magnetometer, which generates point-by-point data. The magnetic sample is mounted on the end of a cantilevered rod that incorporates a piezoelectric element. The sample is magnetized by a dc field (variable in magnitude), and is simultaneously subjected to a small alternating field gradient. The alternating field gradient exerts an alternating force on the sample, proportional to the magnitude of the field gradient and to the magnetic moment of the sample. The resulting deflection of the cantilever rod is measured by the voltage output of the piezoelectric element. By operating at or near a mechanical resonance frequency of the cantilever, the output signal is greatly amplified. In practice, the operating frequency is 100–1000 Hz, with mechanical Q values of 25–250. Mechanical and acoustic noise in the environment limits the sensitivity. Measurements have been made with a signal-to-noise ratio of about 500 on a 25-μm sphere with a moment of 3.7×10−6 emu; this corresponds to a sensitivity of at least 10−8 emu. A complete hysteresis loop over ±10 kOe can be made in about 100 s. Measurements have been made over a temperature range from 77 to 400 K.

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