Sensing magnetic fields using superparamagnetic nanomagnets

An experimental investigation using magnetooptical magnetometry is described into the dependence of susceptibility and hysteresis on the lateral size (30-500 nm), thickness (3-7.5 nm), and geometric shape (triangular, square, and pentagonal) of supermalloy nanomagnets made by electron beam lithography. We show that as the lateral size of the nanomagnets is reduced, magnetic softness is at first reduced and then increases abruptly. We show that this increase in softness is due to thermal fluctuations overcoming the anisotropy imposed by the geometric shape of the nanomagnets, leading to superparamagnetism. Nanometer scale magnetic elements with high susceptibility, zero hysteresis, and saturation fields of a few oersteds can thus be made. Implications for field sensing technology are discussed. (C) 2000 American Institute of Physics. [S0021-8979(00)35108-8].