ferromagnetic josephson junctions for cryogenic memory · 2017. 7. 28. · josephson junction –...
TRANSCRIPT
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Tu-I-SSP-04
Ferromagnetic Josephson Junctions for
Cryogenic Memory
Norman Birge, Michigan State University
in collaboration with Northrop Grumman Corporation
and Arizona State University
(with thanks to D. Scott Holmes, Booz Allen Hamilton & IARPA)
The project depicted was or is supported in part by the
U.S. Army Research Office and/or the Department of
Energy. The information depicted does not necessarily
reflect the position or the policy of the Government, and
no official endorsement should be construed.
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
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The group Eric Gingrich, Bethany Niedzielski, Joseph Glick, Yixing Wang, Bill Martinez,
Josh Willard, Sam Edwards, Reza Loloee, William P. Pratt, Jr.,
plus many earlier students!
An old picture…
2
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteThis talk will feature the work of Eric Gingrich, Bethany Niedzielski, and Joseph Glick, all of whom are in the photo below.
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Outline
• The need for energy-efficient computing
• Superconducting memory: JMRAM
• Superconducting/ferromagnetic hybrid systems
• Demonstration of phase control of an S/F/S
Josephson junction – the basic memory device
• Future prospects
3
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
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The need for energy-efficient computing
• Opened in 2013
• Cost: ~760 M$
• Nearby Lule River generates
9% of Sweden's electricity (~4.23 GW)
• Average annual temperature: 1.3 C
Copyright
Facebook
Specifications
Performance* 27-51 PFLOP/s
Memory* 21-27 PB RAM
1900-6800 PB disk
Power 84 MW avg* (120 MW max)
Space 290,000 ft2 (27,000 m2)
Cooling* ~1.07 PUE
* estimated
Slide courtesy of Scott Holmes
Facebook Data Center, Luleå, Sweden
4
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteThis facility was built near the Arctic Circle because it is close to a hydroelectric power plant, and the air conditioning is free! The need for energy-efficient computing is clear.
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Superconducting computing looks promising
Tianhe2
Titan
K-Computer
Sequoia
Mira
0.01
0.1
1
10
100
1 10 100 1000
Top Computers
DOE Exascale Goal
Coral Goal (2017)
Superconducting,Projected
Tianhe-2
Titan K-Computer
Sequoia
Mira
Coral
Po
we
r (M
W)
Performance (PFLOP/s)
D.S. Holmes et al.,
IEEE Trans. Appl. Supercond.
23, 1701610 (2013)
Slide courtesy of Scott Holmes 5
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteSome researchers believe that superconducting computing is a promising alternative to CMOS. We'll find out for sure only when a prototype superconducting computer has been built.
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Our approach to superconducting memory:
Josephson Magnetic Random Access Memory
(JMRAM) Anna Y. Herr & Quentin P. Herr, US Patent 8,270,209 (2012)
A.Y. Herr, Q.P. Herr & Ofer Naaman, US Patent 9,208,861 (2015)
Northrop Grumman Corporation
Memory cell is a SQUID loop
One junction has two stable
phase states for “0” and “1”
Magnetic states are written
using standard MRAM
techniques
Bit read
Word write
Bit write
Word read
L1 L2
Ic1 Ic2IcM
6
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteJMRAM, invented by researchers at Northrop Grumman Corporation, uses a ferromagnetic Josephson junction to insert a pi phase shift into a SQUID loop. The SIS junctions in the loop provide fast switching speeds.
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JMRAM is a Superconducting MRAM
Memory cell is a Josephson junction containing a magnetic spin valve
memory state – spin-valve state sets junction phase
write – magnetization reversal
read – Josephson effect
www.everspin.com
MRAM (Everspin)
ON for
sensing,
OFF for
programmingI Ref
Isense
Write Line 1
I
Bottom
Electrode
Top
Electrode
Magnetic Tunnel Junction
H
H
Write Line 2I
No idle/static power dissipation, read energy is dissipated only for logical “1”
Bit write
Word write
Magnetic Josephson Junctions
Word read
Bit read
Ground plane
JMRAM (Northrop Grumman)
7
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteThe write operation of the JMRAM cell is similar to that of standard MRAM (not the newer spin-torque MRAM). The magnetization of one of the ferromagnetic layers stays fixed, while the other magnetization is rotated 180 degrees by an applied magnetic field. The read operation is totally different, as it relies on the Josephson effect. The read operation is extremely fast and energy efficient.
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Superconductor/Ferromagnet proximity effect
FexFF vEQkk 2
ex
FF
E
D
ex
FF
E
vQ
2
1 ballistic
diffusive
S N
D = diffusion constant
k
E
-kF
-kF
kF
kF
2Eex
NF
mTk
D
B
NN
1
2
nm
E
D
ex
FF 1~
S F
“FFLO-type” physics
8
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteThis is a quick review of the superconducting proximity effect in S/N and S/F systems. In the S/N case, the pair correlations extend over long distances in N. In the S/F case, the pair correlations oscillate and decay over very short length scales due to the exchange energy in F. Note that the ballistic and diffusive forms for the pair correlation coherence length, xi_F, are similar to the corresponding forms for the superconducting coherence length, but with the gap Delta replaced by the exchange energy E_ex..
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Consequence: S/F/S Josephson junctions oscillate between
0 and π junctions as dF increases:
Ryazanov et al., PRL 86, 2427 (2001); PRL 96, 197003 (2006).
Weak F: Cu48Ni52 alloy
Robinson, Piano, Burnell, Bell, Blamire, PRL 97, 177003 (2005)
Strong F: Co
0.0 1.0 2.0 3.0 4.0 5.0
dCo (nm)
I cR
N (
mV
)
0-state: Is = Ic sin(f2-f1)
-state: Is = Ic sin(f2-f1+)
S F S
dF
f1 f2
ex
FF
E
D
Buzdin, Bulaevskii, & Panyukov (1982).
Can we control Ic or phase state of a single Josephson junction? 9
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteA consequence of the oscillating pair correlations is that S/F/S Josephson junctions exhibit alternating zero and pi ground states as the F-layer thickness is increased. But note that a single S/F/S junction is always in either the 0 or pi state; it cannot be switched between states.
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Add a second ferromagnetic layer: S/F1/F2/S
Ic and phase depend on relative magnetization direction
Parallel state:
Antiparallel state:
S
S
dF1
dF2
Electron pair accumulates phase
f while traversing junction
S
S
dF1
dF2
2
2
1
1
F
F
F
F dd
f
2
2
1
1
F
F
F
F dd
f
Ic exp[-(dF1/F1 + dF2/F2)] cos(dF1/F1 dF2/F2)
10
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteIn a junction containing two ferromagnetic layers with appropriate thicknesses, the phase state of the junction can be switched between pi and zero by changing the relative orientations of the two magnetizations from parallel to antiparallel. The graph on the left shows a simplified representation of the critical current as a function of the accumulated phase acquired by a pair of electrons traversing the junction. Putting the magnetic layers into the parallel (P) state produces a pi junction, while putting them in the antiparallel (AP) state produces a standard 0-junction.
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S/F1/N/F2/S Josephson Junction Composition
Nb
Nb
F2 Cu
Cu F1
Cu F1 = Ni (1.2 nm)
fixed layer
F2 = NiFe (1.5 nm)
free layer
Nb(100)/Cu(5)/NiFe(1.5)/Cu(10)/Ni(1.2)/Cu(5)/Nb(150)
Choose NiFe thickness to put F2
close to 0 - transition.
Ni provides additional small push
to right or left.
11
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteWe used Ni for the fixed magnetic layer and NiFe (Permalloy) for the free layer, with thicknesses chosen to approximate the situation in the cartoon drawn below.
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Two junctions in a SQUID loop used to
measure relative junction phase
Switching field H1
Switching field H2
H1 < H2
Schematic:
Junction sizes:
Both have Area = 0.5 m2
Aspect Ratios = 2.2 and 2.8 Switch magnetization with in-plane field
Cartoon of Actual Device:
On-chip current line couples magnetic flux into
SQUIDs
Different aspect ratio pillars have different switching fields 12
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteTo measure the phase of a junction, we must perform an interference experiment, so we put two junctions in a SQUID. The junctions are elliptical in cross-section, with different aspect ratios, which should give them different switching fields.
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Hext Φ
Hext
AP
P
Φ Hext
AP
AP
Initialize with large field in -z direction, then slowly increase Hext in +z direction
Destructive Interference Constructive Interference Constructive Interference
H1 < Hext < H2 Hext > H2
Φ P
P
At Relatively Low External Fields, Two
Phase Changes Should Be Observable
13
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteHere is how the experiment should work. The sample is initialized with a large magnetic field, H_ext, pointing downward (lower left figure), which aligns both the Ni and NiFe layers in both junctions. H_ext is returned to zero for all interference measurements. The SQUID shows constructive interference at zero applied flux. When an applied upward field is large enough to flip the NiFe layer in one junction (middle picture), that junction switches from the P to AP state, and the SQUID now shows destructive interference at zero applied flux. Applying a larger upward field (right picture) switches the NiFe layer in the second junction, and the SQUID shows constructive interference again. The field can then be applied in the downward direction to reach a 4th state and finally the initial state.
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Data show clean switching between the
four expected states
Switching Fields: +30 Oe, +50
Oe, -35 Oe, -100 Oe
0-0
0-π
π-π
0-0
π-0
π-π
Gingrich, Niedzielski, Glick, et al., Nat. Phys. 12, 564 (2016) 14
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteThese data are real! They demonstrate oscillations of the critical current Ic+ of the SQUID as a function of flux current I_phi, after different in-plane set fields H_in are applied. (Note that the set field called H_in on this slide is the same as H_ext on the previous slide.) All measurements take place in zero field. The upper 3D plot shows the sequence of states obtained with positive set fields. The states follow the pattern described on the previous slide. The lower 3D plot shows the states obtained with negative set fields. The final pi-pi state is identical to the initial state. For more details, see our paper cited at the bottom.
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Data cuts for the four magnetic states
Icave = (Ic+ - Ic-)/2 Ic+ , Ic-
-
0 -
0 - 0
- 0
Gingrich, Niedzielski, Glick, et al., Nat. Phys. 12, 564 (2016) 15
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteThe plot on the left shows high-resolution data for each of the four magnetic states. The solid lines are fits that are described on the next slide. The plot on the right shows the critical current averaged over the positive and negative current directions. The pi phase shifts are immediately apparent.
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Ic() curves have tilted ratchet shape when loop inductances and/or critical currents are asymmetric
L1 Ic1
L2
Ic2 -30 0 30
-0.3
0.0
0.3
Vo
ltag
e (V
)
Current (A)
Ic+ Ic-
Ic+ and Ic
- shift by equal amounts and
in opposite directions along the axis
Ic+
Ic-
Ic
Φ
Ic+() and Ic
-() oscillations are
asymmetric when L1 ≠ L2 & Ic1 ≠ Ic2
Analyze Ic+ and Ic
- peak shifts to
extract JJ phase shifts
16
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteAn astute reader may have noticed that the SQUID oscillation curves have a tilted ratchet shape. That is due to asymmetries in the SQUID design (the inductances of the two arms are different) and in the junction critical currents (the 0-state and pi-state critical currents are quite different in this sample). Those asymmetries are well understood -- see Chapter 2 of The SQUID Handbook. We wrote a Mathematical program that enabled us to fit the data and extract the critical currents of both junctions as well as the inductances of the two arms of the loop.
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Quantitative fits to SQUID modulation data
for the four magnetic states
Icave = (Ic+ - Ic-)/2 Ic+ , Ic-
-
0 -
0 - 0
- 0
Gingrich, Niedzielski, Glick, et al., Nat. Phys. 12, 564 (2016) 17
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteThe solid lines are fits using the Mathematica program. The agreement with the data is extremely good.
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Quantitative Analysis Consistently Assigns the
Inductance and Critical Currents of Each State
state Ic1 (mA) Ic2 (mA) L1 (pH) L2 (pH)
- 0.292 0.217 5.73 11.38
0 - 0.565 0.203 5.64 11.33
0 - 0 0.567 0.419 5.63 11.55
- 0 0.294 0.420 5.71 11.56
Fitting parameters from independent fits of 4 magnetic states are highly
consistent • Exception: critical current of JJ #2 changes slightly in state when JJ #1
switches from to 0 state
ave 5.68 11.46
0.05 0.12
FastHenry simulations:
L1 7 pH, L2 13 pH
18 Gingrich, Niedzielski, Glick, et al., Nat. Phys. 12, 564 (2016)
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteThe data in the table shows that the fits to the four magnetic states provide a consistent set of fit parameters: junction critical currents and SQUID inductances. Note that only one junction switches at each transition between states, except for the first transition where Ic2 changes slightly, but noticeably, whereas junction 1 undergoes the major switch in that transition.
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New result!
Controllable 0- switching with spin-triplet supercurrent
19
F’’
F’
S
S
F
F
Ru
[Pd(0.9)/Co(0.3)]n
[Co(0.3)/Pd(0.9)]n
Ru(0.95)
Ni(1.6)
Cu
Cu
Cu
Cu
{ SAF
All units in nm
NiFe(1.25)
Spin-triplet supercurrent
decays very slowly in F
0 - switching occurs by
spin rotation rather than
accumulated pair phase
Spin-triplet JJ requires three
F layers with non-collinear
magnetizations between
adjacent layers
Data are not yet available for public dissemination, but we plan to submit
them for publication soon: J.A. Glick et al. (2017)
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteIn early June we demonstrated a phase-controllable junction that carries spin-triplet supercurrent. This is a major milestone that we have been chasing for several years! We will submit a paper to a peer-reviewed journal soon; I apologize that we cannot show the data here.
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What needs to be done
• Memory (see talk Fr-C-DIG-03 by Ofer Naaman)
– Optimize performance of magnetic materials • Lower Msat lower Eswitch
• Reduce extrinsic sources of anisotropy in thin films
• Find better material for fixed layer (Ni has issues)
• Minimize underlayer roughness
– Develop read/write electronics & interface to SFQ logic • (see poster We-SDM-08 by Quentin Herr)
• Make the rest of the computer! (see talk Fr-I-DIG-02
by Anna Herr)
20
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
birgeSticky NoteNow that we have demonstrated the feasibility of a phase-controllable Josephson junction, are we all done? No! Making a working memory requires getting thousands or millions of devices to behave similarly. We will continue to work toward that goal with our collaborators at Northrop Grumman and Arizona State.
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Conclusions
• Magnetic Josephson junctions have demonstrated
potential for ultra-low-power cryogenic memory
• Much more work needs to be done!
21
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
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Bibliography
• E.C. Gingrich, B.M. Niedzielski , J.A. Glick , Y Wang , D.L. Miller , R. Loloee R, W.P.
Pratt Jr, N.O. Birge, “Controllable 0- Josephson junctions containing a
ferromagnetic spin valve,” Nature Phys. 12, 564 (2016). DOI: 10.1038/nphys3681
• D.S. Holmes, A.M. Kadin, M.W. Johnson, “Superconducting Computing in Large-
Scale Hybrid Systems”, Computer, 48, 34, December 2015. DOI:
10.1109/MC.2015.375
• D.S. Holmes, A.L. Ripple, and M.A. Manheimer, “Energy-efficient superconducting
computing – power budgets and requirements”, IEEE Trans. Appl. Supercond.,
23, 1701610 (2013). DOI: 10.1109/TASC.2013.2244634
• Q.P. Herr, A.Y. Herr, O.T. Oberg, and A.G. Ioannidis, “Ultra-low-power
superconductor logic”, J. Appl. Phys. 109, 103903 (2011). DOI: 10.1063/1.3585849
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IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2017. This invited oral presentation Tu-I-SSP-04 was given at ISEC 2017.
2016-04_BeyondCMOS_SuperconComp_Holmes_v01.pptx2016-04_BeyondCMOS_SuperconComp_Holmes_v01.pptx2016-04_BeyondCMOS_SuperconComp_Holmes_v01.pptx2016-04_BeyondCMOS_SuperconComp_Holmes_v01.pptxhttp://dx.doi.org/10.1109/MC.2015.375http://dx.doi.org/10.1109/MC.2015.375http://dx.doi.org/10.1109/MC.2015.375http://dx.doi.org/10.1109/MC.2015.375http://dx.doi.org/10.1109/TASC.2013.2244634http://dx.doi.org/10.1109/TASC.2013.2244634http://dx.doi.org/10.1109/TASC.2013.2244634http://dx.doi.org/10.1109/TASC.2013.2244634http://dx.doi.org/10.1063/1.3585849http://dx.doi.org/10.1063/1.3585849http://dx.doi.org/10.1063/1.3585849http://dx.doi.org/10.1063/1.3585849