Magnetic Field Dependence of the Noise in a Magnetoresistive by kii99990

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           Magnetic Field Dependence of the Noise in a
           Magnetoresistive Sensor having MEMS Flux
                          Concentrators
               Arif Ozbay, E.R. Nowak, A. S. Edelstein, G. A. Fischer, C. A. Nordman, and Shu Fan Cheng

                                                                               One technique to mitigate the effects of the sensor’s 1/f
   Abstract— We report the DC and AC magnetic field                         noise is to modulate the incoming magnetic signal and thus
dependence of the low frequency noise in a MEMS flux                        shift the operating frequency of a GMR sensor [5]. This can
concentrator device containing a giant magnetoresistance spin               be accomplished by depositing flux concentrators (high
valve. The noise is dominated by resistance fluctuations having a
magnetic origin. Under nominally zero magnetic field biasing
                                                                            permeability magnetic material such as Ni-Fe) on MEMS
conditions, the noise power is large and varies rapidly with small          structures that oscillate at kHz frequencies. Depending upon
changes in magnetic field. Metastability between distinct resistive         the sensor, shifting the frequency reduces the sensor’s 1/f noise
states is observed and can be suppressed with the application of a          by one to three orders of magnitude at 1 Hz. However, low
moderate longitudinal field. Stationary flux concentrators do not           frequency magnetic noise in the flux concentrators (FCs) must
contribute excess noise, rather the dominant source of noise is the         be negligible else signals of interest may be obscured. In this
spin valves themselves. This result indicates that the device is
likely to increase the sensitivity of many magnetic sensors at low          manuscript, we show that GMR elements themselves are the
frequencies by orders of magnitude.                                         dominant source of noise and that stationary FCs in our
                                                                            MEMS device do not introduce excess noise over a wide range
  Index    Terms—      field   sensor,           flux     concentrator,     of AC and DC field biasing conditions.
magnetoresistance, magnetometer, noise
                                                                                              II. DESIGN AND MATERIALS
                         I.   INTRODUCTION                                     The inset of Fig. 1 illustrates the concept of our AC MEMS

M     AGNETORESISTIVE materials are candidates for
      developing low-power, miniature magnetic field sensors
for the detection of sub-nanoTesla magnetic fields at
                                                                            flux concentrator device. A GMR spin valve (SV) sensor is
                                                                            sandwiched between two trapezoidal-shaped Permalloy FCs
                                                                            that are deposited on MEMS flaps. The FCs have, roughly, a
milliHertz frequencies. Spin-electronic devices are of interest             80 µm front face, 150 µm rear face, 100 µm height, and a
due, in part, to their compatibility with standard silicon                  0.25 µm thickness. The MEMS flaps are driven to oscillate at
microelectronics processing and their large magnetoresistance               frequencies of order of 10 kHz by electrostatic comb drives.
[1, 2]. For example, giant-magnetoresistance (GMR) and                      Silicon springs couple the two flaps and ensure the oscillations
tunneling-magnetoresistance (TMR) devices offer much larger                 are in-phase. Further details of the device can be found in
signals in low fields (< 10 G) compared to anisotropic                      Refs. [5, 6]. In our study, the FCs were stationary.
magnetoresistive (AMR) sensors [1, 3]. However, signal                         Two sets of samples were investigated: one set consisted of
resolution at sub-Hertz frequencies is often limited by low                 only SVs and the other of SVs with adjacent Permalloy FCs.
frequency noise having a power spectrum that varies inversely               The SVs were 2 µm x 90 µm and had the following
with frequency, namely 1/f noise. Although GMR and TMR                      composition:      Si/Si3N4//Ta/35      NiFeCo/50      Ta/    42.5
devices can have more sensitivity, they also tend to exhibit                NiFeCo/12.5 CoFe/ 27.5 Cu/43.5 CoFe/325 CrPtMn, where
larger 1/f resistance noise. For this reason, AMR sensors                   the numbers denote layer thickness in Angstroms. The double
currently provide the highest magnetic field resolution at                  free-layer is used to reduce the hysteresis and improve the
frequencies below 1 Hz [4].                                                 linearity [7]. A transverse (x-axis) field, Ht, and a longitudinal
                                                                            (y-axis) field, Hl, could be applied using our apparatus. For
                                                                            sensing, the signal is applied along the x-axis, transverse to the
   Manuscript received March 10, 2006. This work was supported in part by
the National Science Foundation under grant #0405136, the donors of The     free layer’s easy axis which lies along the length of the SV.
American Chemical Society Petroleum Research Fund, and the Cottrell
Scholar Program of the Research Corporation.                                                 III. RESULTS AND DISCUSSION
   Arif Ozbay and E. R. Nowak are with the Department of Physics and
Astronomy, University of Delaware, Newark DE 19711 USA (phone: 302-            The main panel of Fig. 1 shows the normalized
831-1087; e-mail: aozbay@udel.edu, nowak@udel.edu).                         magnetoresistance of SVs with and without the FCs. The
   A. S. Edelstein and G. A. Fischer are with U.S. Army Research            resistance is greatest when the free and fixed layer are
Laboratory, Adelphi, MD 20783 USA.
   C. A. Nordman is with NVE Corp., Eden Prairie, MN 55344 USA
                                                                            antiparallel (AP) and lowest in the parallel (P) orientation.
   Shu Fan Cheng is with U.S. Naval Research Laboratory, Washington, DC     FCs enhance the linear magnetoresistive response near Ht = 0
20375 USA.
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G. When the MEMS flaps are in motion, the separation                            Further details on the field dependence of the noise will be
between the FC changes, and the enhancement factor oscillates                   presented in a subsequent paper. Here, we are interested in
between 1.8 to 3.0. The magnetoresistive response and the                       comparing noise properties of SVs with and without FCs.
noise properties of the SVs were closely matched among many
SVs both within and between the two sets of samples: sample
to sample variations were of order ±10% for
magnetoresistance and ±100% for the 1/f noise at Ht = 0.




                                                                                Fig. 2. Transverse magnetic field dependence of the normalized resistance
                                                                                noise power in a GMR spin-valve. Data is for f = 34 Hz. Inset shows an
                                                                                example of random telegraph noise observed at sharp spikes in the noise
                                                                                power. Telegraph noise is attributed to metastability in the free layer.

                                                                                   For clarity, Fig. 3 shows noise data taken in a longitudinal
Fig. 1. Normalized resistance as a function of magnetic field for GMR spin-     field, Hl = 35 G. The longitudinal field highlights the
valve sensors with (open symbols) and without (solid) flux concentrators.
                                                                                background 1/f noise by suppressing the occurrence of spikes
Insets show the concept of the AC MEMS flux concentrator device: flux
concentrator (a), sensor (b), comb drive (c), and silicon springs (d). In the   resulting from the telegraph noise. The 1/f noise is largest in
upper inset the sensor is between the electrodes labeled by (b).                the AP state and decreases monotonically as the free and fixed
                                                                                layers become parallel at positive field values. This trend was
   In a GMR sensor, the dominant sources of intrinsic low-                      observed in all SVs, independent of FCs. Moreover, the
frequency noise are Johnson (Nyquist) noise and resistance                      magnitude of the noise is equivalent to within statistical
fluctuations [8]. In general, the resistance noise has both                     variance between the two sets of samples. The curve for the
electronic and magnetic contributions [9, 10]. Under constant                   FC data is compressed along the field axis due to flux
current bias, resistance fluctuations in the SV give rise to                    concentration. The conclusion is that FCs contribute negligible
voltage fluctuations having a power spectral density, SV ( f ) ,                1/f noise under DC magnetic field conditions.
that scales approximately as I 2/f, where I is the dc current bias.                In practice, a sensor will be exposed to AC fields that could
All reported noise data is for I L 1 mA.                                        introduce additional low-frequency noise by affecting the
   Fig. 2 shows the dependence of the noise power on Ht in a                    magnetization dynamics in the SV and/or the FCs; for
SV without FCs. Similar behavior is observed in SVs with                        example, if there is hysteresis on some magnetic field scale. To
FCs (not shown). The field dependence of the noise is                           address this issue, we investigated to what extent the
characterized by a smoothly varying background and a series                     background 1/f noise is affected by a large sinusoidal magnetic
of narrow spikes superimposed on this background. The                           field perturbation.
spectrum of the noise is 1/f except at fields corresponding to
the occurrence of a spike in noise power where a Lorentzian-
like spectrum is observed. Lorentzian spectra are associated
with random telegraph noise in the time domain, see Fig. 2
inset. Noise spikes are prominent about Ht = 0 G and are
absent for sufficiently high Ht at which the free layer is
saturated parallel to Ht. The kinetics of the telegraph noise are
highly sensitive to the applied field which explains the rapid
onset and decrease in noise power in given octave frequency.
The dependence on field indicates that the telegraph noise and                  Fig. 3. Dependence of the noise power on transverse field with a constant
background 1/f noise are, in large part, magnetic in origin [8,                 longitudinal field, Hl =35 G. The noise has a 1/f-like spectrum. Behavior is
9]. The precise fields at which noise spikes occur varies among                 similar for SVs with (a) and without (b) flux concentrators.
samples and depends somewhat on the magnetic history of the
SV. However, the general location of their occurrence in
relation to the magnetoresistance curve is highly reproducible.
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                                                                                Fig. 5. AC transverse field dependence of low-f under nominally zero
                                                                                magnetic field biasing conditions for a SV without (a) and with (b) flux
                                                                                concentrators. The detailed dependence varies among different samples.
Fig. 4. AC transverse field dependence of low frequency noise under various     Symbols represent noise power at different frequencies.
dc magnetic field biasing conditions. The noise is independent of Hac for SVs
without (a) and with (b) flux concentrators.                                    contributing excess noise. These results establish benchmarks
                                                                                for comparing future noise measurements on oscillating FCs in
   Fig. 4a shows that the noise power in a bare SV is                           the AC MEMS device. If moving FCs do not contribute
unaffected by field perturbations at 85 Hz and as large Hac L 5                 excess noise then the operating frequency can be shifted into a
Grms along the x-axis. Fig. 4b shows data for a SV with FCs.                    regime where sensor noise is dominated by Johnson rather
Again, a dependence of the noise on Hac is not evident. The                     than 1/f noise. This crossover occurs at roughly 10 kHz when
biasing field conditions for these data were Ht = 0 G and Hl =                  these SVs are operated at a 1 mA bias current.
35 G, and Hl = 0 G and Ht of order ±75 G. The latter case
corresponds to saturation of the free layer. Varying the                                                       REFERENCES
perturbation frequency between 60 Hz and 3 kHz had no
                                                                                [1]   J. M. Daughton, “Spin-dependent sensors”, Proc. of the IEEE 91, 681
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                                                                                      (2003).
   The situation is very different if the external biasing fields,
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Ht and Hl, are small (< 5 G). Fig. 5 shows that SVs with and
                                                                                     future”, Science 294, 1488 (2001).
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denote different octave frequencies, f < fac. All octaves show                  [4] N. A. Stutzke, S. E. Russek, and D. P. Pappas, “Low-frequency noise
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without FCs.                                                                         and C. A. Nordman, “Progress toward a thousand-fold reduction in 1/f
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occurrence of telegraph noise under the nominally zero field                         Appl. Phys. (to be published).
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                                                                                     sensitivity GMR spin-valve magnetometer”, IEEE Trans. Magn. 33,
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without telegraph noise, 1/f noise is substantial, suggesting that                   M. C. de Nooijer, M. A. M. Gijs, et al., “1/f noise in anisotropic and
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