The output signal of permalloy magnetic bubble detectors employing the magnetoresistance or Hall effects is directly proportional to the current density, J, through the permalloy element. Joule heating and electremigration effects limit J. Joule heating restricts J to ≤ 1×10 6 A/cm 2 in thin (500 Å – 1000 Å) permalloy detectors while microsegregation of the Ni and Fe due to electromigration causes rapid deterioration of the magnetoresistance during accelerated aging of the samples. Oxidation of the permalloy does not limit detector lifetime under normal operating conditions.