rippard_kitp
TRANSCRIPT
Microwave Dynamics and Phase Locking in Spin Transfer
Nanocontacts
W. H. Rippard, M. R. PufallS. Kaka*, S. E. Russek, T. J. Silva
National Institute of Standards and Technology,Boulder, CO
J. A. KatineHitachi Global Storage Technologies
San Jose, CASupported by: NIST Nanomagnetodynamics
DARPA SpinS programNIST OMP Office
* Now at Seagate Research Labs,Pittsburgh, PA
Outline
IntroductionSpin transfer effectsDevice geometry
Spin transfer induced dynamicsin plane fieldsout of plane fields
Phase locking of devices
Injection locking to source
Mutual synchronization of oscillators
Summary
0.1 0.2 0.3 0.4 0.5 0.6 0.7
-200
-100
0
100
200
V Osc
(µV)
Time (ns)
Idc= 7.85 mA
Idc= 7.8 mA
Idc= 7.6 mA
Idc= 7.4 mA
Magnetodynamics
Larmor term:precession
My
Mz
Mx
M
H
10 1 eff
dm m Hdt
µ γ= − ×r rr
Precession
H
Spin Torque
Damping
M
Spin Torque Term
Spin torque cancounteract damping
1 1 21
J( )
2inj
z s
m m mel Mε γ
+ × ×h r r v
1Larmor Damping SpinTransfer
dm T T Tdt
= + +r
M Initial
My
Mz
Mx
0 1 1( )effm m Hµ γα− × ×rr r
MFinal
H
Damping term:aligns M with HEFF
MInitial
Slonczewski 1996
TSpin Transfer ≅ TDampingJ ~ 107A/cm2
Device Schematic
V+
Au
Cu“Fixed” layer
“Free” layerInsulator
~50 nmV-
5 nm
20 nm
Measurement setup
Detection:40 GHz Spectrum
Analyzeror
Sampling scopeDC current in
Device
bias teehigh bwprobes
Spinwave radiation
Micromagnetics
Measured signal results from IDC and GMR effect
Nanocontact Dynamics Device Schematic
• Step DC current• Measure DC R, microwave
power output
5 6 7 8 924.8
25.0
25.2
25.4
Res
ista
nce
(Ω)
Current (mA)
9.6 9.7 9.8 9.9 10.0
0.0
0.1
0.2
0.3
0.4
6 mA
6.5 mA 7 mA
7.5 mA
8 mA
8.5 mA
9 mA
Pow
er (p
W)
Frequency (GHz)
Devices are nanoscale current controlled microwave oscillators
Au
Cu
~50 nmT = 300K
“Free” layer“Fixed” layer
In-Plane Applied Fields
0.0 0.2 0.4 0.6 0.8 1.00
10
20
30
40
Field (T)
26.2 ±0.5 GHz/T
Freq
uenc
y (G
Hz)
Small-angle FMR frequency7.5
6.9
7.2
7.8
5 6 7 8
-0.23 GHz/mA
Current (mA)
Freq
uenc
y (G
Hz)
Current Response Field Response
Consistent with large angle version of FMRQualitative behavior in accordance with
Slonczewski equation
φ ≈ 80˚H
M
Out of Plane Fields
Precession occurs about the applied field directionlarger tunablity with current
Low IDemag ≈ Ms
Heff=Happ-Hdemag
Demag ≈ 0 THigh I
0.5 0.6 0.7 0.8 0.9 1.0
20
25
30
35
40
Freq
uenc
y (G
Hz)
Field (T)
Tunability:20 GHz to >40 GHz
Field Response
Device Output
Integrated voltage output:
V = 850 µV
Maximum GMR response:
∆R = 250 mΩI = 10 mA
∆V = 2.5 mV
Lower bound: Impedance mismatch not accounted forBoth frequency and power indicate approaching ~80° precessional angles
θ ∼ 85˚H
13.10 13.15 13.20
0
2000
4000
6000
8000
Spec
tral D
ensi
ty (n
V2 /Hz)
Frequency (GHz)
f = 13.15 GHz∆f = 7.1 MHzIdc= 10 mA
Injection LockingSynchronized Fireflies
Huygens (1665)Coupled pendulum clocks
http://www.pojman.com/NLCD-movies/NLCD-movies.html
General characteristic of all nonlinear oscillators:
Circadian rhythms, Huygens’ clocks, power grid, Josephsonjunctions, fireflies…….
Injection Locking Measurements
6.0 6.5 7.0 7.5 8.0
15.6
15.8
dV/d
I (Ω
)
10.7
10.8
10.9
11.0
f (G
Hz)
Current (mA)
No Modulation
15.6
15.8
dV/d
I (Ω
)
10.7
10.8
10.9
11.0
f (G
Hz)
6.0 6.5 7.0 7.5 8.0
-5
0
5
10
V rect (µ
V)
Idc
(mA)
Indirect evidence of phaselocking in frequency domain
Tsoi et al. 2001
locked
Modulation f = 10.86 GHz
nV2 /H
z
0
100
DC rect. voltage
Spectral Measurement of LockingSpectral measurement of locking
PS
D (n
W/H
z)
During locking the device takes on thenoise characteristics of the input signal
-oscillator linewidth approaches Hz
Time Domain MeasurementsSampling oscilloscope
Microwavesource
30 dB
Powersplitter Bias
tee
SMTdevice
DC Bias
0.1 0.2 0.3 0.4 0.5 0.6 0.7
-50
-25
0
25
50
V out (µ
V)
Time (ns)
f = 10.86 GHz
10.84 10.86 10.880
25
50
75
100
Am
plitu
de (n
V2/H
z)
Frequency (GHz)
f = 10.62 GHz
∆f = 7 MHzAmp = 44 µV
Spectral Measurement Time-domain Measurement
Electronic Phase Control of Oscillations
0.1 0.2 0.3 0.4 0.5 0.6 0.7
-200
-100
0
100
200
V Osc
(µV
)
Time (ns)
Idc= 7.4 mA
Idc= 7.6 mA
Idc= 7.8 mA
Idc= 7.85 mA
7.4 7.5 7.6 7.7 7.8
-100
-50
0
50
100
Rel
. Pha
se S
hift
(deg
rees
)
Idc (mA)
locked
Time-domain Measurements Relative Phase Change
Relative phase change over locking region is approximately 180 degreesDemonstrates electronic phase control of oscillators
Injection Locking of DevicesPhase relationship
General theory (Adler, 1973):
The Adler Equation
osc
injlinewidthlock V
Vff ∆=∆ Generally expect linear relationship
between locking range and oscillator-linewidth-injected signal
Injection Locking ResultsInject Vac at 14 GHz
No injection
27 28 29 30
13.8
14.0
14.2
14.4
Current (mA)
-2E-11
2.8E-10
27 28 29 30
13.8
14.0
14.2
14.4
Current (mA)
Freq
uenc
y (G
Hz)
0.00 0.15 0.300
100
200 Locking Range ∆f(Iac) = -27.4 + 685 Iac
Lock
ing
Ran
ge (M
Hz)
ac Injection Amplitude (mA)
Inject Vac at 12.7 GHz
21 22 23 24 25 26 27
12.4
12.6
12.8
Current (mA)
Freq
uenc
y (G
Hz)
22 24 26
12.4
12.6
12.8
Current (mA)
-2E-11
2E-10
0.00 0.15 0.300
100
200
Lock
ing
Ran
ge (M
Hz)
ac injection Amplitude (mA)
Locking Range ∆f(Iac) = -4.3 + 238 Iac
Lack quantitative understanding of locking range
Mutual Phase Locking
Measurement circuit:Device schematic:
500 nm Magneticinteraction
ac to power combiner
Ic2 Vac, c2
S.A.
ac to power combiner
Ic1
power combiner
Vac, c1
φ
Dual Contacts On Single Mesa
Insulator
Magnetic mesa
Leads to c1, c2
nanocontacts
500nm
Individual Device Outputs
8 9 10 1115.5
16.0
Current (mA)
Freq
uenc
y (G
Hz)
Contact 2
8 9 10 1115.5
16.0
Current (mA)
Freq
uenc
y (G
Hz)
Contact 1
Bias point
Swee
p C2
Bias contact 1 and sweep current through contact 2
15 16 170
5
2.5
Pow
er (n
W/H
z)
Frequency (GHz)
Contact 1
Contact 2
Output from Both DevicesCombined Output
Contact 1 = Constant biasContact 2 = Swept bias
8 9 10 1115
16
17
Current (mA)
Freq
uenc
y (G
Hz)
Mutual Phase Locking
Locking range
20
0
Log
(pow
er)
Contact 1
Contact 2
0.0
0.2
0.4
0.6
Pow
er (n
W/H
z)
0
10
20
Pow
er (n
W/H
z)
15 16 170.0
0.5
1.0
Pow
er (n
W/H
z)
Frequency (GHz)
Mutual Phase Locking: Power & Phase
8.0 8.5 9.0 9.5 10.0 10.5 11.00
20
40
60
80
100
120
140
160
180
Pc1 Pc2
8.0 8.5 9.0 9.5 10.0 10.5 11.00
20
40
60
80
100
120
140
160
180
Pc1 Pc2 Pc1 + Pc2
8.0 8.5 9.0 9.5 10.0 10.5 11.00
20
40
60
80
100
120
140
160
180
Pc1 Pc2 Pc1 + Pc2 Pboth measure
21 ccout PPP +=
Pow
er (p
W)
Current (mA)
Incoherent sum:
8.0 8.5 9.0 9.5 10.0 10.5 11.00
20
40
60
80
100
120
140
160
180
Pc1 Pc2 Pc1 + Pc2 Pboth measure PCoherent sum
( )φcos2 2121 ccccout PPPPP ++=
Coherent sum:
Rel. Phase φ during lock ≈ 0o
…Phase locking a route to higher powers (n2), phased arrays
Mutual Phase Locking
8.5 9.0 9.5 10.0 10.5 11.0106
107
108
Line
wid
th (H
z)Current (mA)
Linewidth
8.5 9.0 9.5 10.0 10.5 11.015.5
16.0
16.5
17.0
Freq
uenc
y (G
Hz)
Current (mA)
Frequency
Contact 1
Contact 2
Linewidth decreases below individual oscillators when lockedNo clear master/slave relationship between devices
-stabilization against thermal effects?
Possible Locking Mechanisms Contact 1 Contact 2
• AC Dipole fieldsEstimate coupling field on theorder of several Oe
Possible mechanism
• Mode OverlapSimulations do not support such largearea of precession
Improbable mechanism
Contact 1 Contact 2
• Spin wave radiation between contactsSimulations support spinwave coupling between devices
Possbile mechanism
FIB cutting of magnetic mesa:
Cut Mesa
FIB cut
nanocontact
Cut between two contacts:Look for change in interaction
5 6 7 8 9 109.5
10.0
10.5
11.0
11.5
(log scale)
Current (mA)
Freq
uenc
y (G
Hz)
Phase locking Pre-FIB
5 6 7 8 9 109.5
10.0
10.5
11.0
11.5 mc2s310c10b
Current (mA)
Freq
uenc
y (G
Hz)
Bias point
Contact 1 Contact 2
5 6 7 8 9 10 119.5
10.0
10.5
11.0
11.5 mc2sc1m8b
Current (mA)
Freq
uenc
y (G
Hz)
Locking range
Combined output
5 6 7 8 9 109.5
10.0
10.5
11.0
11.5
(log scale)
Current (mA)
Freq
uenc
y (G
Hz)
Post FIB Loss of Phase Locking
5 6 7 8 9 109.5
10.0
10.5
11.0
11.5
(log scale)
Current (mA)
Freq
uenc
y (G
Hz)
Bias point
5 6 7 8 9 10 119.5
10.0
10.5
11.0
11.5
(log scale)
Current (mA)
Freq
uenc
y (G
Hz)
No phase locking
Indicates spinwaveradiation responsiblefor synchronizationat large distances
Combined output
Contact 1 Contact 2
SummaryObserve coherent microwave dynamics resulting from spin
torque effect
Qualitative behavior consistent with Slonczewski equation
Oscillators can be injection locked to:Injected AC currentApplied AC fields Proximate devices through spinwave interactions
(possibly through dipole fields at very close spacings)Qualitative behavior consistent with non-linear oscillator analysis
Remaining questions:
What determines locking range:How do we quantify the strength of the spinwave interaction.
What determines linewidth and phase relation of synchronized devices:No clear master/slave relationship.
Quantitative agreement with current/field dependence:Not given by Slonczewski equation.
Likely all point to understanding of the mode structure