We report low-temperature measurements of the frequency-dependent magnetic noise and magnetic susceptibility of nanometer-scale antiferromagnetic horse-spleen ferritin particles, using an integrated dc SQUID microsusceptometer. A sharply defined resonance near 1 MHz develops below T{~}0.2 K. The behavior of this resonance as a function of temperature, applied magnetic field, and particle concentration indicates that it results from macroscopic quantum tunneling of the N\'eel vector of the antiferromagnets.
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Awschalom et al. (1992) studied this question.
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