Quadrupled nondestructive outputs from magnetoresistive memory cells using reversed word fields

Abstract

A new operating mode has been discovered in transverse magnetoresistive (MR) memory cells which increases the usable nondestructive sense output by more than a factor of 4 over the original mode described, which employed a unipolar word field. The new mode allows for increased cell density, decreased memory access time, and increased margins. This new operating mode was examined experimentally and analytically for experimental cells ranging in size from 1.5×5 μm to 1.8×18 μm. The MR material used in the cells consisted of two 150-Å-thick magnetic layers (65% Ni, 15% Fe, 20% Co) separated by a 50-Å-thick reacted tantalum layer to break the exchange. In the new operating mode, a word field of one polarity is used for writing (parallel to the edge magnetization) and a reversed polarity is used for reading. During the reading process, 120°–225° paired ‘‘walls’’ are formed at the edges. The energy in the walls is sufficient to restore the elements to their original states even if switching occurs (in a direction opposite to the normal rotation during writing). The operating margins are improved because while reading only one and not both the word field and the sense field are in a direction to cause normal switching.