Innovations may lead to magnetic sensors with superior performance. Examples of this are the chip scale atomic
magnetometer, magnetic tunnel junctions with MgO barriers, and a device for minimizing the effect of 1/<i>f</i> noise, the
MEMS flux concentrator. In the chip scale atomic magnetometer, researchers have been able to fabricate the light
source, optics, heater, optical cell, and photodiode detector in a stack that passes through a silicon wafer. Theoretical and
subsequent experimental work has led to the observation of magnetoresistance values of 400% at room temperature in
magnetic tunnel junctions with MgO barriers. The MEMS flux concentrator has the potential to increase the sensitivity
of magnetic sensors at low frequencies by more than an order of magnitude. The MEMS flux concentrator does this by
shifting the operating frequency to higher frequencies where the 1/<i>f</i> noise is much smaller. The shift occurs because the
motion of flux concentrators on MEMS flaps modulates the field at kHz frequencies at the position of the sensor.
Microfabricated magnetoresistive zigzag-shaped elements based on the anisotropic magnetoresistance effect were studied for use as magnetic field sensors. Images taken using scanning electron microscopy with polarization analysis show that the magnetization in the devices tends to follow the edges of the device, thereby providing a geometrical (45°) bias that alternates along length of the sensor. It was found that these devices are primarily sensitive to magnetic fields applied along the long axis; a flat response is observed for perpendicular fields. The alternating magnetization bias provides the directionality of the sensor because the angles in adjacent zigzag blocks scissor for fields parallel to the long axis and rotate for perpendicular fields. This results in resistances that either add or cancel, respectively. A single-domain, coherent rotation description provides an estimate for the qualitative behavior of these zigzag structures and indicates the possible role of exchange in the shape of the transfer curves. Noise measurements were also taken on these devices. Thermal resistance noise was the dominant noise source above about 10 kHz. At low frequencies the resistance noise was found to be dominated by a 1/f contribution that depends on the applied magnetic field. The 1/f noise is relatively low and field independent when the element is in a saturated magnetic state and contains a relatively large and field dependent excess contribution when the magnetic field is in the sensitive field range of the element. The 1/f noise level observed in the saturated state depends in a nontrivial way on the quality and processing of the magnetic element, showing a trend for lower normalized noise in elements having higher sensitivity. In the most sensitive elements (magnetoresistance > 1%) the 1/f noise level is comparable to that found in nonmagnetic metals. We attribute the origin of noise to defect motion. In the unsaturated state, the excess noise is found to track the dc resistance susceptibility. For particular values of applied field we also observed large random telegraph signals in the time domain. The telegraph noise was extremely sensitive to the applied field, becoming active and inactive in our measurement bandwidth for changes in field of only a few Oersteds. This behavior indicates a magnetic origin to the excess noise. The variation of the excess noise level with applied dc magnetic field can be explained qualitatively using a model based on thermal excitation of the magnetization direction and/or domain wall hopping between pinning sites.
We report on the voltage fluctuations in exchange-biased, micron-scale magnetic tunnel junctions. We find that the spectral power density is 1/f-like at low frequencies and becomes frequency independent at high frequencies. The frequency-independent background noise is due to thermal and shot noise mechanisms. The nature of the 1/f noise has its origin to two different mechanisms. In the magnetic hysteresis loops this noise power is strongly field-dependent and is due to thermal magnetization fluctuations in both the 'free' and 'fixed' magnetic layers. We attribute these magnetic fluctuations to thermally excited hopping of magnetic domain walls between pinning sites. A second mechanism for the 1/f noise, connected with defects in the tunnel barrier but not related to the overall magnetization fluctuations, was found at fields for which the magnetic structure in the free and fixed layers is well aligned. This noise is associated with electron trapping processes having thermally activated kinetics and a broad distribution of activation energies. Below ~ 25 K the noise power is temperature independent suggesting that the kinetics are dominated by tunneling. Our results show that the thermal stability of the both magnetic layers and the quality of the tunnel barrier are important factors in reducing the low-frequency noise in magnetic tunnel junctions.
The relatively large and linear magnetoresistance found in nonstoichiometric silver chalcogenides makes them attractive candidates for studying the mechanisms of linear magnetoresistance and for field sensing applications. After a brief review of the magnetoresistive properties of these materials, we report on the intrinsic electrical noise in bulk, polycrystalline Ag<sub>2+δ</sub>Te. Low-frequency noise is due to resistance fluctuations having a 1/f-like power spectrum. The temperature dependence of the noise magnitude and its spectral slope indicate thermally activated kinetics which we attribute to some form of charge trapping-detrapping process occurring in or near the intergranular regions. The effective magnetic field noise in Ag<sub>2+δ</sub>Te is also compared to other materials systems used in field sensing applications.
Conference Committee Involvement (3)
Fluctuations and Noise in Materials II
24 May 2005 | Austin, Texas, United States
Fluctuations and Noise in Materials
26 May 2004 | Maspalomas, Gran Canaria Island, Spain