Low-Frequency Magnetic Noise in Micron-Scale Magnetic Tunnel Junctions

Abstract

We report that low frequency (up to 200 kHz) noise spectra of magnetic tunnel junctions with areas 10 −10 cm 2 at 10 Kelvin deviate significantly from the typical 1/f behavior found in large area junctions at room temperature. In most cases, a Lorentzian-like shape with characteristic time between 0.1 and 10 ms is observed, which indicates only a small number of fluctuators contribute to the measured noise. By investigating the dependence of noise on both the magnitude and orientation of an applied magnetic field, we find that magnetization fluctuations in both free and reference layers are the main sources of noise in these devices. At small fields, where the noise from the free layer is dominant, a linear relation between the measured noise and angular magnetoresistance susceptibility can be established.

Absence of magnetic state dependent low-frequency noise in spin-valve systems

“…These two independent methods directly support our conclusions that for all the samples we have measured the magnetic aftereffect is responsible for the observations of increased noise in the transition regions similar to those described in a number of reports [6][7][8][9][10][11][12]14] and the increase in the spectral exponent of the noise from 1 to 2 in other reports [12,13]. We conclude that the magnetic relaxation in the measured devices will distort the apparent low frequency noise level and thus it is clear that care needs to be taken to avoid the possible artifact when measuring noise spectra in magnetic systems when a possible magnetic aftereffect is present.…”

“…Like the previous noise studies [6][7][8][9][10][11][12][13], the unexpected PSD behaviors are observed in the transition regions when either the free layer or hard/pinned layer is in the process of switching. As can be seen, in these regions the PSD at 1 Hz can be over 1000 times larger for the MTJ sample and approximately 100 times larger for the GMR sample compared to the noise in either the P or the AP states.…”

“…Regarding this disagreement, we note that our devices have dimensions of either 300 μm×300μm or 10μm×10μm for the MTJ devices and 1.8mm×9.0mm for the GMR devices, a point we will address further in the conclusions with regards to the work on smaller devices [6,8].…”

“…In the case of disk drive read heads and magnetic field sensors the need to minimize the noise is important while at the same time understanding the noise provides a window to various physical processes in magnetic materials. In the case of MTJs there have been a number of studies that focus on the noise [6][7][8][9][10][11][12][13]. In most of these [6][7][8][9][10][11][12] is reported a significant increase in the low frequency (1/f) noise when dR/dH is large which occurs when a magnetic layer is reversing which is attributed to magnetic fluctuations.…”

Low-frequency noise in planar Hall effect bridge sensors

“…While it has been possible to achieve close to picotesla sensitivity at high frequencies (typically at frequencies higher than some tens of kHz), it has been proven more difficult to achieve the sensitivity goal at low frequencies due to 1/f resistance noise. The 1/f noise can have both magnetic origins, due to coupling between transport properties and magnetization fluctuations, and electronic origins [8][9][10][11][12]. For TMR devices, the latter contribution is often attributed to defects in the tunnel barrier resulting in electron trapping with thermally activated kinetics and a broad distribution of activation energies, but it should be noted that 1/f noise is also found in metals where carrier scattering by extrinsic defects has been pointed out as the source of the resistivity fluctuations [13].…”