Improved Detectivity in Tunnel Magnetoresistance Sensors by Controlling Free Layer Thickness and Annealing process

Detectivity is an important figure-of-merit for evaluating the magnetic field resolution of a magnetic sensor. In this work, MgO based tunnel magnetoresistance sensors with an amorphous-CoFeSiB free layer were fabricated. A two-step annealing technique was utilized to achieve good linearity with a small magnetic hysteresis. The dependances of magnetic and noise properties of these sensors on the free layer thickness and annealing process have been studied. An improvement in the sensitivity and a lowest detectivity of 1.06 nT/Hz ^0.5 at 10 Hz was obtained for devices with free layer thickness of 80 nm and annealed at 180 °C. The detectivity value further decreased to 0.9 nT/Hz ^0.5 for the devices for which the second annealing was performed at 225 °C. Another set of devices annealed at the same temperature, but after the microfabrication process, exhibited a minimum detectivity of 0.4 nT/Hz ^0.5 .