Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator
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Transcript of Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator
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Stochastic theory of nonlinear auto-oscillator:Spin-torque nano-oscillator
Vasil TiberkevichDepartment of Physics, Oakland University,
Rochester, MI, USA
NLPV, Gallipoli, Italy June 15, 2008
In collaboration with:• Andrei Slavin (Oakland University, Rochester, MI, USA)• Joo-Von Kim (Universite Paris-Sud, Orsay , France)
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Outline
• Introduction
• Linear and nonlinear auto-oscillators
• Spin-torque oscillator (STO)
• Stochastic model of a nonlinear auto-oscillator
• Generation linewidth of a nonlinear auto-oscillator
• Theoretical results
• Comparison with experiment (STO)
• Summary
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Outline
• Introduction
• Linear and nonlinear auto-oscillators
• Spin-torque oscillator (STO)
• Stochastic model of a nonlinear auto-oscillator
• Generation linewidth of a nonlinear auto-oscillator
• Theoretical results
• Comparison with experiment (STO)
• Summary
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Models of (weakly perturbed) auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle
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Models of (weakly perturbed) auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
Unperturbed system:
)(xFx
dt
d)( 000 tXx )()2( 00 XX
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Models of (weakly perturbed) auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
)(xFx
dt
d)( 000 tXx )()2( 00 XX
),()( xfxFx
tdt
d )())((0 tt yXx
Unperturbed system:
Perturbed system:
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Models of (weakly perturbed) auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
Unperturbed system:
Perturbed system:
)(xFx
dt
d)( 000 tXx )()2( 00 XX
),()( xfxFx
tdt
d )())((0 tt yXx
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Models of (weakly perturbed) auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
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Models of (weakly perturbed) auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
• Weakly nonlinear model
0 .5 0 .5x
0 .5
0 .5
1 .0y
1 .0 0 .5 0 .5x
0 .5
0 .5
1 .0y
thII thII
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Models of (weakly perturbed) auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
• Weakly nonlinear model
0 .5 0 .5x
0 .5
0 .5
1 .0y
1 .0 0 .5 0 .5x
0 .5
0 .5
1 .0y
thII
yixc Complex amplitude:
thII
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Models of (weakly perturbed) auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
• Weakly nonlinear model
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Linear and nonlinear auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
• Weakly nonlinear model
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Linear and nonlinear auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
• Weakly nonlinear model
Nonlinear auto-oscillator: frequency depends on the amplitude
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Linear and nonlinear auto-oscillators
Auto-oscillator – autonomous dynamical system with stable limit cycle• Phase model
• Weakly nonlinear model
All conventional auto-oscillators are linear.
|| N
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Outline
• Introduction
• Linear and nonlinear auto-oscillators
• Spin-torque oscillator (STO)
• Stochastic model of a nonlinear auto-oscillator
• Generation linewidth of a nonlinear auto-oscillator
• Theoretical results
• Comparison with experiment (STO)
• Summary
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Spin-torque oscillator
Electric current I
Magnetic metal
Ferromagnetic metal – strong self-interaction – strong nonlinearity
Electric current – compensation of dissipation – auto-oscillatory dynamics
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Spin-torque oscillator
Spin-transfer torque:
p M(t)
Electric current I
J. Slonczewski, J. Magn. Magn. Mat. 159, L1 (1996)L. Berger, Phys. Rev. B. 54, 9353 (1996)
pMMTM
0S
S M
I
dt
d
VMe
g
0
B 1
2
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Equation for magnetization
Landau-Lifshits-Gilbert-Slonczewski equation:
MSGeff ][ TTMH
M
dt
d][ eff MH GT ST
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Equation for magnetization
Landau-Lifshits-Gilbert-Slonczewski equation:
– conservative torque (precession)
precession
MSGeff ][ TTMH
M
dt
d][ eff MH GT ST
][ eff MH
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Equation for magnetization
Landau-Lifshits-Gilbert-Slonczewski equation:
– conservative torque (precession)
– dissipative torque (positive damping)
precession
M
damping
SGeff ][ TTMHM
dt
d][ eff MH GT ST
][ eff MH
]][[ eff0
GG HMMT
M
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Equation for magnetization
SGeff ][ TTMHM
dt
d
Landau-Lifshits-Gilbert-Slonczewski equation:
– conservative torque (precession)
– dissipative torque (positive damping)
– spin-transfer torque (negative damping)
precession
M
damping
spin torque][ eff MH GT ST
][ eff MH
]][[ eff0
GG HMMT
M
]][[0
S pMMT M
I
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Equation for magnetization
Landau-Lifshits-Gilbert-Slonczewski equation:
precession
M
damping
spin torqueSGeff ][ TTMH
M
dt
d][ eff MH GT ST
When negative current-induced damping compensates natural magnetic damping,
self-sustained precession of magnetization starts.
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Experimental studies of spin-torque oscillators
S.I. Kiselev et al., Nature 425, 380 (2003)
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Experimental studies of spin-torque oscillators
M.R. Pufall et al., Appl. Phys. Lett. 86, 082506 (2005)
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Specific features of spin-torque oscillators
S.I. Kiselev et al., Nature 425, 380 (2003)
M.R. Pufall et al., Appl. Phys. Lett. 86, 082506 (2005)
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Specific features of spin-torque oscillators
S.I. Kiselev et al., Nature 425, 380 (2003)
M.R. Pufall et al., Appl. Phys. Lett. 86, 082506 (2005)
• Strong influence of thermal noise (spin-torque NANO-oscillator)
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Specific features of spin-torque oscillators
S.I. Kiselev et al., Nature 425, 380 (2003)
M.R. Pufall et al., Appl. Phys. Lett. 86, 082506 (2005)
• Strong influence of thermal noise (spin-torque NANO-oscillator)
• Strong frequency nonlinearity
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Outline
• Introduction
• Linear and nonlinear auto-oscillators
• Spin-torque oscillator (STO)
• Stochastic model of a nonlinear auto-oscillator
• Generation linewidth of a nonlinear auto-oscillator
• Theoretical results
• Comparison with experiment (STO)
• Summary
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Nonlinear oscillator model
2|| cp – oscillation power
)arg(c – oscillation phase
0)()()( cpcpcpidt
dc cp)( cp)(cpi )(
precession
M
damping
spin torque
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Nonlinear oscillator model
0)()()( cpcpcpidt
dc cp)( cp)(cpi )(
precession
2|| cp – oscillation power
)arg(c – oscillation phase
precession
M
damping
spin torque
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Nonlinear oscillator model
0)()()( cpcpcpidt
dc cp)( cp)(cpi )(
precession positive damping
2|| cp – oscillation power
)arg(c – oscillation phase
precession
M
damping
spin torque
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0)()()( cpcpcpidt
dc cp)( cp)(cpi )(
Nonlinear oscillator model
negative damping
positive dampingprecession
2|| cp – oscillation power
)arg(c – oscillation phase
precession
M
damping
spin torque
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How to find parameters of the model?
0)()()( cpcpcpidt
dc cp)( cp)(cpi )(
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How to find parameters of the model?
0)()()( cpcpcpidt
dc cp)( cp)(cpi )(
• Start from the Landau-Lifshits-Gilbert-Slonczewski equation
• Rewrite it in canonical coordinates
• Perform weakly-nonlinear expansion
• Diagonalize linear part of the Hamiltonian
• Perform renormalization of non-resonant three-wave processes
• Take into account only resonant four-wave processes
• Average dissipative terms over “fast” conservative motion
• and obtain analytical results for
)( p )( p)( p
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Deterministic theory
0)()()( cpcpcpidt
dc
Stationary solution: )exp()( 0g0 itiptc
2|| cp – oscillation power
Stationary power is determined from the condition of vanishing total damping:
)()( 00 pp
Stationary frequency is determined by the oscillation power:
)( 0g p
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Deterministic theory: Spin-torque oscillator
Stationary power is determined from the condition of vanishing total damping:
)()( 00 pp
Qp
1
0
Stationary frequency is determined by the oscillation power:
)( 0g p
thII – supercriticality parameter
0th I – threshold current
QNNp
1
000g
0 – FMR frequency
N – nonlinear frequency shift
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Generated frequency
0 2 4 6 8 100
5
10
15
20
25
30
35
Ge
ne
ratio
n f
req
ue
ncy
g /2 (
GH
z)
Bias magnetic field H0 (kOe)
0 30 60 900
5
10
15
20
25
30
35
H0 = 5 kOe
Ge
nera
tion
fre
quen
cy
g /2 (
GH
z)
Magnetization angle 0 (deg)
H0 = 8 kOe
Frequency as function of applied field
Frequency as function of magnetization angle
Experiment: W.H. Rippard et al., Phys. Rev. Lett. 92, 027201 (2004)
Experiment: W.H. Rippard et al., Phys. Rev. B 70, 100406 (2004)
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Stochastic nonlinear oscillator model
)()()()( nn tfDcpcpcpidt
dc
– thermal equilibrium power of oscillations:
Random thermal noise:
Stochastic Langevin equation:
– scale factor that relates energy and power: dpppE )()(
)(2)()( nn tttftf
)()()()()( B
n p
TkppppD
00
2|| pc
)(nn tfD
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Fokker-Planck equation for a nonlinear oscillator
Probability distribution function (PDF) P(t, p, ) describes probability that oscillator has the power p = |c|2 and the phase = arg(c) at the moment of time t.
2
2n
n 2
)()(2)()()(2
P
p
pD
p
PppD
p
PpPppp
pdt
dP
Deterministic terms describe noise-free
dynamics of the oscillator
Stochastic terms describe thermal diffusion in the
phase space
P
pPpppp
)()()(2 2
2n
n 2
)()(2
P
p
pD
p
PppD
p
)()()()()( B
n p
TkppppD
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Stationary PDF
Stationary probability distribution function P0:• does not depend on the time t (by definition);• does not depend on the phase (by phase-invariance of FP equation).
dp
dPppD
dp
dPppp
dp
d 0n0 )(2)()(2
Ordinary differential equation for P0(p):
Stationary PDF for an arbitrary auto-oscillator:
Constant Z0 is determined by the normalization condition 1)(0
0
dppP
p
pdp
pp
TkZpP
0B00 )(
)(1)(exp)(
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Analytical results for STO
“Material functions” for the case of STO:
)1()( G Qpp )1()( pIp Gth III
Analytical expression for the stationary PDF:
Analytical expression for the mean oscillation power
Here 2)1( QQ
1
)( dttexE xt – exponential integral function
2
2
0 )(
)1)((exp
)1()(
QQE
QQpQ
Qp
QpP
QQQE
Q
Qp
1
)(
)(exp1
2
2
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Power in the near-threshold region
4 5 6 7 8 90
1
2
3
Osc
illat
or p
ower
p (
a.u.
)
Bias current I (mA)
Experiment: Q. Mistral et al., Appl. Phys. Lett. 88, 192507 (2006).
Theory: V. Tiberkevich et al., Appl. Phys. Lett. 91, 192506 (2007).
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Outline
• Introduction
• Linear and nonlinear auto-oscillators
• Spin-torque oscillator (STO)
• Stochastic model of a nonlinear auto-oscillator
• Generation linewidth of a nonlinear auto-oscillator
• Theoretical results
• Comparison with experiment (STO)
• Summary
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Linewidth of a conventional oscillator
C L
+R0 –Rs
~e(t)
Classical result (see, e.g., A. Blaquiere, Nonlinear System Analysis (Acad. Press, N.Y., 1966)):Full linewidth of an electrical oscillator
20
200B2
U
RTk
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Linewidth of a conventional oscillator
C L
+R0 –Rs
~e(t)
Classical result (see, e.g., A. Blaquiere, Nonlinear System Analysis (Acad. Press, N.Y., 1966)):Full linewidth of an electrical oscillator
20
200B2
U
RTk
Physical interpretation of the classical result
L
R
20
0 200 2
1CUE
LC
10
Frequency Equilibrium half-linewidth Oscillation energy
0
B02E
Tk
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Linewidth of a laser
Quantum limit for the linewidth of a single-mode laser (Schawlow-Townes limit) [A.L. Schawlow and C.H. Townes, Phys. Rev. 112, 1940 (1958)]
out
200
22
P
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Linewidth of a laser
Quantum limit for the linewidth of a single-mode laser (Schawlow-Townes limit) [A.L. Schawlow and C.H. Townes, Phys. Rev. 112, 1940 (1958)]
out
200
22
P
Physical interpretation of the classical result
0
B02E
Tk
0out0 PE2
0B
Tk
Effective thermal energy Oscillation energy
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Linewidth of a spin-torque oscillator
35.850 35.875 35.900 35.925
0.0
0.1
0.2
0.3 Q(V)=2650Q(W)=4235 f=35.89 GHz
f = 13.5 MHz
Am
plit
ud
e (
nV
/Hz1/
2 )
Frequency (GHz)
Experiment by W.H. Rippard et al., DARPA Review (2004).
0eff0
B0G2
pVM
Tk
H
Theoretical result [J.-V. Kim, PRB 73, 174412 (2006)]:
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Linewidth of a spin-torque oscillator
35.850 35.875 35.900 35.925
0.0
0.1
0.2
0.3 Q(V)=2650Q(W)=4235 f=35.89 GHz
f = 13.5 MHz
Am
plit
ud
e (
nV
/Hz1/
2 )
Frequency (GHz)
Experiment by W.H. Rippard et al., DARPA Review (2004).
0eff0
B0G2
pVM
Tk
H
Physical interpretation:
0
B02E
Tk
Theoretical result [J.-V. Kim, PRB 73, 174412 (2006)]:
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Linewidth of a spin-torque oscillator
35.850 35.875 35.900 35.925
0.0
0.1
0.2
0.3 Q(V)=2650Q(W)=4235 f=35.89 GHz
f = 13.5 MHz
Am
plit
ud
e (
nV
/Hz1/
2 )
Frequency (GHz)
Experiment by W.H. Rippard et al., DARPA Review (2004).
0eff0
B0G2
pVM
Tk
H
Physical interpretation:
MHz5.13222 exp f MHz35.0222 theor f
Theoretical result [J.-V. Kim, PRB 73, 174412 (2006)]:
0
B02E
Tk
![Page 51: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/51.jpg)
Linewidth of a spin-torque oscillator
35.850 35.875 35.900 35.925
0.0
0.1
0.2
0.3 Q(V)=2650Q(W)=4235 f=35.89 GHz
f = 13.5 MHz
Am
plit
ud
e (
nV
/Hz1/
2 )
Frequency (GHz)
Experiment by W.H. Rippard et al., DARPA Review (2004).
0eff0
B0G2
pVM
Tk
H
Physical interpretation:
This theory does assumes that the frequency is constant
const)( 0 p
MHz5.13222 exp f MHz35.0222 theor f
Theoretical result [J.-V. Kim, PRB 73, 174412 (2006)]:
0
B02E
Tk
![Page 52: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/52.jpg)
How to calculate linewidth?
Power spectrum S() is the Fourier transform of the autocorrelation function K():
two-time function
Autocorrelation function K() can not be found from the stationary PDF. Ways to proceed:
• Solve non-stationary Fokker-Planck equation [J.-V. Kim et al., Phys. Rev. Lett. 100, 167201 (2008)];
• Analyze stochastic Langevin equation.
deKS i)()( )()()( tctcK
)()()()( nn tfDcpcpcpidt
dc
![Page 53: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/53.jpg)
Power and phase fluctuations
1)( 0
B
0
pE
Tk
p
p
)(0
)()()( titi epetptc
Two-dimensional plot of stationary PDF
)Re(c
)Im(c
|| cp
)arg(c
Power fluctuations are weak
Oscillations – phase-modulated process:
Autocorrelation function:
)0()(exp)( 0 iipK
Linewidth of any oscillator is determined by the phase fluctuations
![Page 54: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/54.jpg)
Linearized power-phase equations
)()()()( nn tfDcpcpcpidt
dc Stochastic Langevin equation:
![Page 55: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/55.jpg)
Linearized power-phase equations
)()()()( nn tfDcpcpcpidt
dc
)(0 )()( tietpptc
Stochastic Langevin equation:
Power-phase ansatz: 0|| pp
![Page 56: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/56.jpg)
Linearized power-phase equations
)()()()( nn tfDcpcpcpidt
dc
)(0 )()( tietpptc
pNetfp
Dp
dt
d
etfpDpdt
pd
i
i
)(Im)(
)(Re22
n0
n0
n0neff
pN
Stochastic Langevin equation:
Power-phase ansatz: 0|| pp
Stochastic power-phase equations:
0eff pGG Effective damping: dppdG )(
Nonlinear frequency shift coefficient: dppdN )(
Nonlinear frequency shift creates additional source of the phase noise
Linear system of equations.Can be solved in general case.
![Page 57: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/57.jpg)
Power fluctuations
Correlation function for power fluctuations:
||2
0
B
eff
20
eff
)()()(
epE
Tkptptp
![Page 58: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/58.jpg)
Power fluctuations
Correlation function for power fluctuations:
||2
0
B
eff
20
eff
)()()(
epE
Tkptptp
Power fluctuations are small
)( 0
B
pE
Tk
![Page 59: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/59.jpg)
Power fluctuations
Correlation function for power fluctuations:
||2
0
B
eff
20
eff
)()()(
epE
Tkptptp ||2 eff e
)( 0
B
pE
Tk
Power fluctuations are small
Non-zero correlation time of power fluctuations
Additional noise source –N p for the phase fluctuations is Gaussian, but not “white”
![Page 60: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/60.jpg)
Phase fluctuations
Averaged value of the phase difference:
)()()( 0ptt
![Page 61: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/61.jpg)
Phase fluctuations
Averaged value of the phase difference:
)()()( 0ptt Determines mean oscillation frequency
![Page 62: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/62.jpg)
Phase fluctuations
Averaged value of the phase difference:
)()()( 0ptt Determines mean oscillation frequency
Variance of the phase difference:
GG
NNp
eff
0 – dimensionless nonlinear frequency shift
eff
||22
0
B2
2
1||||
)()(
eff
e
pE
Tk
GG
NNp
eff
0
![Page 63: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/63.jpg)
Phase fluctuations
Averaged value of the phase difference:
)()()( 0ptt Determines mean oscillation frequency
Variance of the phase difference:
eff
||22
0
B2
2
1||||
)()(
eff
e
pE
Tk Determines oscillation linewidth
GG
NNp
eff
0 – dimensionless nonlinear frequency shift
Auto-correlation function:
)(
2
1exp)( 2)(
00 piepK
GG
NNp
eff
0
![Page 64: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/64.jpg)
Low-temperature asymptote
||)1()(
)( 2
0
B2 pE
Tk||
“Random walk” of the phase
![Page 65: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/65.jpg)
Low-temperature asymptote
Lorentzian lineshape with the full linewidth
||)1()(
)( 2
0
B2 pE
Tk
Frequency nonlinearity broadens linewidth by the factor2
2 11
GG
N
lin2
0
B2 2)1()(
)1(2 pE
Tk
||
“Random walk” of the phase
![Page 66: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/66.jpg)
Low-temperature asymptote
Lorentzian lineshape with the full linewidth
lin2
0
B2 2)1()(
)1(2 pE
Tk
||)1()(
)( 2
0
B2 pE
Tk||
Frequency nonlinearity broadens linewidth by the factor2
2 11
GG
N
Region of validity K300~1
)(2
0effB
pETk
“Random walk” of the phase
![Page 67: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/67.jpg)
High-temperature asymptote
2eff
2
0
B2
)()( pE
Tk 2
“Inhomogeneous broadening” of the frequencies
![Page 68: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/68.jpg)
High-temperature asymptote
Gaussian lineshape with the full linewidth
Different dependence of the linewidth on the temperature:
T~2
2eff
2
0
B2
)()( pE
Tk 2
)(||22
0
Beff pE
Tk
“Inhomogeneous broadening” of the frequencies
![Page 69: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/69.jpg)
High-temperature asymptote
Gaussian lineshape with the full linewidth
Different dependence of the linewidth on the temperature:
T~2
Region of validity
2eff
2
0
B2
)()( pE
Tk 2
)(||22
0
Beff pE
Tk
K300~)(
20eff
B pE
Tk
“Inhomogeneous broadening” of the frequencies
![Page 70: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/70.jpg)
Outline
• Introduction
• Linear and nonlinear auto-oscillators
• Spin-torque oscillator (STO)
• Stochastic model of a nonlinear auto-oscillator
• Generation linewidth of a nonlinear auto-oscillator
• Theoretical results
• Comparison with experiment (STO)
• Summary
![Page 71: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/71.jpg)
Temperature dependence of STO linewidth
0 50 100 150 2000
100
200
300
400
500
600
700
low-temperatureasymptote
Lin
ew
idth
(M
Hz)
Temperature (K)
device #1second harmonic
high-temperatureasymptote
0 50 100 150 200 250 3000
100
200
300
400
500
600
700
low-temperatureasymptote
Lin
ew
idth
(M
Hz)
Temperature (K)
device #2first harmonic
high-temperatureasymptote
Low-temperature asymptote gives correct order-of-magnitude estimation of the linewidth.
Experiment: J. Sankey et al., Phys. Rev. B 72, 224427 (2005)
![Page 72: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/72.jpg)
Angular dependence of the linewidth
)(1
)()1(2
0
B
2
0
B2
pE
Tk
GG
N
pE
Tk
Nonlinear frequency shift coefficient N strongly depends on the orientation of the bias magnetic field
0 30 60 90-20
0
20
40
No
nlin
ea
r fr
eq
ue
ncy
sh
ift N
/2 (
GH
z)
Out-of-plane magnetization angle 0 (deg)
0 30 60 90-10
-8
-6
-4
-2
0
No
nlin
ea
r fr
eq
ue
ncy
sh
ift N
/2 (
GH
z)
In-plane magnetization angle 0 (deg)
Isotropic film,out-of-plane magnetization
Anisotropic film,in-plane magnetization
N
![Page 73: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/73.jpg)
Angular dependence of the linewidth
)(1
)()1(2
0
B
2
0
B2
pE
Tk
GG
N
pE
Tk
Nonlinear frequency shift coefficient N strongly depends on the orientation of the bias magnetic field
0 30 60 90-20
0
20
40
No
nlin
ea
r fr
eq
ue
ncy
sh
ift N
/2 (
GH
z)
Out-of-plane magnetization angle 0 (deg)
0 30 60 90-10
-8
-6
-4
-2
0
No
nlin
ea
r fr
eq
ue
ncy
sh
ift N
/2 (
GH
z)
In-plane magnetization angle 0 (deg)
Isotropic film,out-of-plane magnetization
Anisotropic film,in-plane magnetization
N
![Page 74: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/74.jpg)
Angular dependence of the linewidth
45 50 55 60 65 70 75 80 85 900
50
100
150
200
250
Experiment
10 x (|N| = 0)
Ge
ne
ratio
n li
ne
wid
th
(M
Hz)
Out-of-plane magnetization angle 0 (deg)
(|N| > 0)
Experiment: W.H. Rippard et al., Phys. Rev. B 74, 224409 (2006)
Out-of-plane magnetization
Theory: J.-V. Kim, V. Tiberkevich and A. Slavin, Phys. Rev. Lett. 100, 017207 (2008)
![Page 75: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/75.jpg)
Angular dependence of the linewidth
)(1
)()1(2
0
B
2
0
B2
pE
Tk
GG
N
pE
Tk
Nonlinear frequency shift coefficient N strongly depends on the orientation of the bias magnetic field
0 30 60 90-20
0
20
40
No
nlin
ea
r fr
eq
ue
ncy
sh
ift N
/2 (
GH
z)
Out-of-plane magnetization angle 0 (deg)
0 30 60 90-10
-8
-6
-4
-2
0
No
nlin
ea
r fr
eq
ue
ncy
sh
ift N
/2 (
GH
z)
In-plane magnetization angle 0 (deg)
Isotropic film,out-of-plane magnetization
Anisotropic film,in-plane magnetization
N
![Page 76: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/76.jpg)
Angular dependence of the linewidth
0 30 60 90 1200
1000
2000
3000
4000
|N| = 0(x 10)
Lin
ew
idth
(M
Hz)
Field angle (deg)
|N| > 0
0 30 60 900
500
1000
1500
2000
2500
|N| = 0(x 10)
Lin
ew
idth
(M
Hz)
Field angle (deg)
|N| > 0
Experiment: K. V. Thadani et al., arXiv: 0803.2871 (2008)
In-plane magnetization
![Page 77: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/77.jpg)
Outline
• Introduction
• Linear and nonlinear auto-oscillators
• Spin-torque oscillator (STO)
• Stochastic model of a nonlinear auto-oscillator
• Generation linewidth of a nonlinear auto-oscillator
• Theoretical results
• Comparison with experiment (STO)
• Summary
![Page 78: Stochastic theory of nonlinear auto-oscillator: Spin-torque nano-oscillator](https://reader036.fdocuments.in/reader036/viewer/2022081514/56814527550346895db1ecaf/html5/thumbnails/78.jpg)
Summary
• “Nonlinear” auto-oscillators (oscillators with power-dependent frequency) are simple dynamical systems that can be described by universal model, but which have not been studied previously
• There are a number of qualitative differences in the dynamics of “linear” and “nonlinear” oscillators:
• Different temperature regimes of generation linewidth (low- and high-temperature asymptotes)• Different mechanism of phase-locking of “nonlinear” oscillators [A. Slavin and V. Tiberkevich, Phys. Rev. B 72, 092407 (2005); ibid., 74, 104401 (2006)]
• Possibility of chaotic regime of mutual phase-locking [preliminary results]
• Spin-torque oscillator (STO) is the first experimental realization of a “nonlinear” oscillator. Analytical nonlinear oscillator model correctly describes both deterministic and stochastic properties of STOs.