Characterizing the Noise in Magnetic Tunnel Junctions

1
Characterizing the Noise in Magnetic Tunnel
Junctions
Lai Jiang Presentation 1 for spintronics class
2
Introduction

• Why is noise interesting
• - Technological Aspects
• limits sensitivity of devices (field
  sensor)
• - A tool for studying material
• provides valuable information about
• .defects in the tunnel barrier
• .magnetic dynamics

3
Introduction

• structure of a Magnetic Tunnel Junction(MTJ)

4
Introduction
MagnetoResistance (MR)
5
Introduction
typical magnetoresistance loop of MTJs
L. Jiang, E. R. Nowak et. al., Physical Review B
69, 054407 (2004).
6
Schematic of Experiment
7
Voltage fluctuations

• A crossover from I/f noise to f independent
  noise
• 1/f noise reflects the Charge trapping and
  detrapping process in the AlO barrier.
• ?I 2Rd2/Af
• White noise has origin in thermal and shot
  noise mechanisms
• 2eIRdcoth(eV/2kT)
• eVltltkT thermal noise 4kTRd
• eVgtgtkT Shot noise 2eIRd2

Sv?2eIRdcoth(eV/2kT)?I 2Rd2/Af
L. Jiang, E. R. Nowak et. al., Physical Review B
69, 054407 (2004).
8
Voltage fluctuations
predicted shot noise 2eIRd2
the frequency independent portion of Sv scales
in agreement with the
L. Jiang, E. R. Nowak et. al., Physical Review B
69, 054407 (2004).
9
Thermal magnetization fluctuations

• The noise and dR/dH are observed to have similar
  field dependence. In qualitative agreement with
  the Fluctuation Dissipation (FD) relation
• ?R/?H2mm0 /kTDR (R2/A) ln(fmax/fmin)a
• H.T.Hardner et al. Phys. Rev. B48, 16156(1993)

L. Jiang, E. R. Nowak et. al., Physical Review B
69, 054407 (2004).
10
Telegraph Noise

• A dramatic spike in noise is seen as the free
  layer undergoes reversal

• Random telegraph noise is observed in the time
  domain.
• The signals duty cycle is extremely sensitive to
  the applied field.

L. Jiang, E. R. Nowak et. al., Physical Review B
69, 054407 (2004).
11
Telegraph Noise

• The probability distributions for the time spent
  in the up and down states for the telegraph
  noise
• D(t)? exp(-t/t)

• The transition rates are assumed to follow
• 1/ti?j(H)1/t0 exp(-E?Dm?H/kT)
• Dm can be estimated from the ratio of lifetimes
• ti?j(H) /tj?i(H) exp(2Dm?H/kT)

L. Jiang, E. R. Nowak et. al.,Physical Review B
69, 054407 (2004).
12
Conclusion

• Noise Mechanisms
• 1. Johnson-Nyquist(Thermal)
• 2. Shot
• set ultimate sensitivity
• 3. Magnetic
• field fluctuation
• 4. Telegraph
• free layer reversal
• 5. Resistance - charge trapping processes
• 1/f-ish