Hall-magnetometry on ferromagnetic dots and dot arrays

The switching of electroplated Nickel-pillars with diameters smaller 170 nm and heights greater 300 nm was investigated. The nanomagnets were placed on top of the sensor either individually or in groups of 5 up to 13 in different patterns. The individual pillars show two types of hysteresis loops: one type resembles a perfect rectangular hysteresis trace over a wide temperature range, the other type shows two steps in the Hall-voltage when magnetization direction is reversed. The switching field H_sw of a pillar with rectangular hysteresis trace was measured between 1.6 K and 225 K. H_sw decreases linearly with increasing temperature. Both results as well as the variation of switching fields of different “single domain” particles suggest, that oxide layers on the surface of the pillars strongly influence the magnetization reversal. Despite this complication small arrays of Ni pillars placed on the sensor display a well defined switching behavior. Different steps in the Hall-voltage reflect the switching of the individual magnets