1/f Magnetic Noise Dependence on Free Layer Thickness in Hysteresis Free MgO Magnetic Tunnel Junctions

The noise spectrum of MgO magnetic tunnel junctions with linear transfer curves for varying CoFeB free layer thickness was studied. For continuous ferromagnetic free layers (t_CoFeB ges 15.5 A), the noise spectrum in the linear range is dominated by 1/f magnetic noise and random telegraph noise. When superparamagnetic behavior sets on the free layer (t_CoFeB < 14.5 A) the junctions do not present 1/f magnetic noise. This cancellation of 1/f magnetic noise is done at the expense of field sensitivity. In terms of field detection limit, the junctions with thin and thick free layer show the same detection limit of 25 nT/Hz^1/2 low frequency (500 Hz). At high frequency (100 kHz), however, junctions with thicker free layer show detection limit of 1.5 nT/Hz^1/2, which is three times better than for thin free layer.