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Noise in Magnetoresistive Devices
Introduction
In 2005 INESC-MN installed an
experimental setup for noise measurements using a spectrum analyzer operating
from DC up to 8 GHz. There are two main targets: 1) to characterize the high
frequency magnetic noise spectrum (caused by thermally excited spin ringing) of
sub-micron sized Spin Valve and Magnetic Tunnel Junction stacks. The results
will help to optimize the devices electrical and magnetic properties in the high
frequency range, while spin dynamics parameters (such as the phenomenological
damping parameter α) can be extracted from the high frequency noise spectrum. 2)
Measure the magnetic sensitivity of magnetic field sensors in the thermal and
shoot noise background. Excite spin valve and magnetic tunnel junction devices
with local magnetic fields created and measure and optimize the minimum
detectable field of the magnetoresistive sensors.
For more
information on this topic, contact Paulo
Freitas or Susana Cardoso Freitas.
Main Results
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Setup
for noise measurements using a spectrum analyzer (from DC to 8GHz),
installed at INESC-MN since 2005.
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Noise measurements on AlOx and MgO-based junctions, as
well as in spin valve sensors have been carried out.
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Noise
spectrum of MgO tunnel junctions,
obtained at several bias currents. The curves were fitted to the
theoretical 1/f noise expression and the Hooge constant αH=1.24x10-9
μm2 was obtained. Noise background levels in the range
of 10-12 T/Hz0.5 can be achieved at room
temperature. Such small sensitivities are currently only at the reach of
squid sensors, which require low temperatures in order to operate.
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Noise
measurements on hybrid devices, consisting
of spin valve sensors in CoZrNb flux guide structures. It is observed
that down to 60 pT/Hz0.5, no excess noise is observed coming
from the flux guide.
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Results
on commercial device incorporating a spin valve sensor,
done in collaboration with Neocera (USA). Neocera fabricates
non-contact, non-destructive imaging tools for the Semiconductor
industry's devices, advanced packages, and full assemblies, using
scanning SQUID microscope
imaging systems, and has successfully incorporated spin valve
sensors in their most recent products (2006). Several wafers have been
microfabricated at INESC-MN, either using spin valves and tunnel
junctions. Then, at Neocera, the polishing of the spin-valve probes
yielded tips of less than 50μm x 50μm cross section; at a height of 100
μm from the sensor, the cross section can be less than 100μm x 75μm.The
magnetic noise characteristic for a polished device is shown. The
noise level near 100 kHz is below 10 nT/Hz0.5.
Projects and
Collaborations 2004 - present
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“Noise control of magnetoresistive linear devices for
biologic, mapping and read head applications”, POCTI/CTM/59411/2004,
INESC-MN (2005-2006).
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“Ultra sensitive magnetic sensors for medical
applications”, BIOMAGSENSE, EC FP6
NMP4-CT-2005-017210, INESC-MN (PT), CEA (Orsay, FR), Neuromag OY (F),Siemens
(DE), Trinity College (Dublin, Ir), CEDRAT Technologies SA (FR), (2005-2008)
Industrial
Collaborations
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Singulus (Germany)
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Neocera (USA)
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