Noise in magnetic tunnel junction devices

The noise expected from an ideal magnetic tunnel junction (MTJ) head is thermal noise at low bias voltages (≪50 mV, thermally assisted barrier crossings) and shot noise at higher bias voltages (≫50 mV, field-assisted barrier crossings). The shot noise is larger than the thermal noise, therefore, at 200 mV, the signal of an MTJ head has to be a factor of 2 larger than that of a GMR head with the same resistance to realize the same SNR. This paper reports MTJ noise measurements up to the ferromagnetic resonance (1–2 GHz). An ultra low-noise pre-amplifier system is described that allows electrical biasing of the devices. Wide-band 1/f noise is observed in devices without proper hard bias. Saturation is used to assess the magnetic fluctuation noise and the resonance therein. These wide-band noise measurements demonstrate that one can electronically read out MTJ heads up to the ferromagnetic resonance and down to the noise floor.