Abstract
We report measurements in natural horse-spleen ferritin that provide a detailed mapping of the blocking temperature, ${T}_{B},$ as a function of magnetic field over a broad range up to 20 kOe. Unlike most superparamagnetic materials where it decreases with applied field, ${T}_{B}$ increases with increasing field at small fields, reaching a maximum at $\ensuremath{\approx}3$ kOe before exhibiting the expected decrease. The hysteresis loops are anomalously ``pinched'' near zero field. Both observations are consistent with an effective energy barrier that is smaller at zero field than in small finite fields. This may arise from tunneling between pairs of states on opposite sides of the anisotropy barrier that are in resonance in zero magnetic field, regardless of particle size. However, direct measurements of the magnetic viscosity yield ambiguous results, leaving open other possible explanations.
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