Low-frequency magnetic noise in magnetic tunnel junctions

Abstract

We studied low-frequency noise in NiFe-Al 2 O 3 -NiFe based magnetic tunnel junctions ͑MTJ's͒ with and without a hard-axis bias field. The 1/f noise is observed to be magnetic-field dependent and reduced with the application of hard-axis bias fields, attributed to thermally activated magnetization fluctuations in the magnetic electrodes. A linear dependence of noise on derivative of magnetoresistance has been observed, and the magnetic-field noise for MTJ sensing elements is defined and evaluated to be as low as 60 nT m/Hz 1/2 .The study of noise of magnetoresistive materials is important for understanding the performance of spin-dependent electronic devices as well as the physical processes that govern their magnetic properties. There are many processes that can contribute to the noise of a magnetoresistive based device. Based on its physical origin, the noise is categorized into three types. Thermal resistance noise, also known as Johnson or Nyquist noise, results from a thermal smearing of the density of states near the Fermi level. Shot noise is due to the stochastic nature of electron transport with an applied current. The third kind of noise, 1/f noise, also called flicker noise or excess noise, arises from the coupling of electronic processes with magnetization fluctuations. 1,2 The first two noise sources are frequency independent, while 1/f noise scales inversely with frequency, as suggested by its name.Usually 1/f noise is an indirect noise and manifests itself as resistance fluctuations in different physical systems in lowfrequency range. As examples, resistance fluctuations in twodimensional electron system in semiconductor heterostructures 3 and high-T C superconductor 4 have different physical origin. Due to its nature, there is not a universal explanation to physical origin of 1/f noise.In this paper we are interested in magnetoresistance ͑MR͒ fluctuations at low fields in Py-Al 2 O 3 -Py ͑PyϭPermalloy, Ni 79 Fe 21 ) based magnetic tunnel junctions ͑MTJ's͒. This spin-dependent system shows great potential for high sensitivity, low-noise magnetic-field sensing applications, due to its low saturation field and high MR ratio ͑defined as ⌬R/R, where ⌬R and R are the maximum resistance change and the minimum resistance, respectively͒. In addition, the exponential dependence of junction resistivity on the barrier thickness allows for a large degree of control over device resistance. A typical tunnel junction consists of two ferromagnetic ͑FM͒ metal layers separated by an ultrathin insulating barrier. Changes in the relative magnetization orientations between these two FM layers in applied magnetic field͑s͒ determine the final resistance, i.e., MR. Although many fundamental aspects of this system, including magnetization reversal processes, 5 scattering mechanisms, 6 and electron transport behavior have been studied in detail, the lowfrequency noise characteristics have not been fully explored.In a recent work, Ingvarsson et al. have studied magnetic noise produced by magnetization fluctuations in MTJ's ͑...

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

“…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.…”

“…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

“…The domains can fluctuate both in size and in magnetization direction, and since the magnetoresistive response of a device is dependent on the domain configuration, and the domain magnetization direction, this causes noise in the output signal. Magnetic lowfrequency noise is strongly associated with conflicting magnetic anisotropies [10,11,17], for the PHEBs, e.g., in the sharp corners where the easy axis of the shape anisotropy changes direction and, also, in the PHEB branches where the shape anisotropy is partly opposed by the exchange bias and field-induced uniaxial anisotropies. Given the larger number of such corners, a PHEB with n>1 could have been expected to have worse detectivity than one with n=1 and equal length.…”

“…7). The magnetic part of the noise is reduced if the magnetization is directed along an anisotropy axis [10,11,17], and the detectivity should therefore have local minima at θ=θ ex and θ S . This was confirmed by the measurements.…”

“…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].…”