Magnetoresistance and Magnetic Dynamics on the Nanoscale Jack Sankey, Ilya Krivorotov, Nathan Emley, Greg Fuchs, Kiran Thadani, Bob Buhrman, D

Magnetoresistance and Magnetic Dynamics on the Nanoscale Jack Sankey, Ilya Krivorotov, Nathan Emley, Greg Fuchs, Kiran Thadani, Bob Buhrman, D. C. Ralph

Kirill Bolotin, Ferdinand Kuemmeth, D. C. Ralph

Jacob Grose, Moon-Ho Jo, Abhay Pasupathy, R. Bialczak, K Baheti, M. M. Deshmukh, J. J. Sokol, E. M. Rumberger, Jan Martinek, Paul McEuen, D. N. Hendrickson, J. R. Long, Hongkun Park, D. C. Ralph

.Using spin-transfer torques to control spin dynamics in 100-nm-scale pillar devices. Spin-transfer-driven FMR in Single Nanomagnets .Can these techniques be extended to the molecular scale?

Spin-transfer dynamics Single-molecule transistors Cu Source Drain Pt SiO2 V I Py Aluminum Gate

Vg Cu 100 nm Slonczewski/Berger Model of Spin-Transfer Torques:

An alternative to manipulating magnetic moments with magnetic fields:

When a Magnetic Layer Acts as a Spin-Filter, it Can Feel a Torque

Spin-polarized Current Torque z θ

Magnetic Layer Magnetic Dynamics with Spin Transfer

Effective Field switching steady-state Damping precession Spin-Transfer

Precession M

(fixed) (free ) y y g g r r e e n n e e

electron flow 0 π angle 0 π angle Controlling Dynamics in 100-nm-Scale Magnetic Devices Using Spin Transfer Torques

8.3 Magnetic ) Ω (

Switching I 8.2 d

/ spin

d 8.1 valves tunnel junctions

8.0

-0.6 0.0 0.6 I (mA) DC-driven Microwave Oscillations

FWHM = 0.65 MHz NIST group 8.0 f/Δf = 1800 f = 35 GHz f= 34.4 GHz 6.0f/Δf = 17,500 r /Hz) !f= 1.9 MHz e 2

w Q= 18100 4.0 o P Power (nV 2.0

0.0 Frequency (GHz) 34.38 34.39 34.40 Frequency (GHz) Nanopillar Structures Point Contact Other Recent Experimental Milestones Phase Locking to an External Microwave Drive

Rippard et al., PRL 95, 067203 (2005) M. Tsoi et al., Nature 406, 46 (2000)

Phase Locking of Two Spin-Transfer-Driven Oscillators

Kaka et al., Nature 437, 389 (2005). Mancoff et al., Nature 437, 393 (2005).

Spin-Transfer Excitations of Single Magnetic Layers Ozyilmaz et al., PRL 93, 176604 (2004). Spin-Transfer Excitations in Magnetic Semiconductor Devices (talk by Hideo Ohno, earlier today) Resistive Detection of Spin-Transfer-Driven Magnetic Resonance

S R(t) = #R cos("t +! )

V DC mix circuitry

V(t) = I(t)R(t) ~ IRF cos("t)#Rcos("t + $) 1 = IRF#R[cos($) + cos(2"t + $)] 2 1 V ~ I #Rcos($) mix 2 RF Resonant resistance oscillations generate a DC voltage component by mixing

! (similar technique used for radio-frequency detection by Tulapurkar et al., Nature 438, 339 (2005)) Measuring Normal Modes in a Single Nanomagnet

Sankey et al., submitted to PRL (cond-mat/0602105) What are the Expected Normal Modes?

from numerical modeling

Measured frequencies and frequency spacings are in reasonable qualitative agreement with simulation.

(Detailed modeling of our sample geometry has not been done yet.)

McMichael and Stiles, J. Appl. Phys. 97, 10J901 (2005) The width of the resonance curves gives a measure of the damping coefficient for the oscillations

2Δ0 t T b t ( h h T y h V e e h

i a s

e e c m

r u r m

e a y r e s r g e f e

u f n a W e n l e s t c t

t u - a t e i ﬁ i c r v g e a e

Limits ontheEffective-FieldContribution ofSpinTransfer e r l l d a k e d

ﬁ

y e

Less than1%shiftin

s p e e D s h h l r

d i s w e f a

t , f s i

p r t i a s e h o e s

m Resonance frequency (GHz) p d c

n t r o i h a o s d n < 15%ofthe e g b p c i e r a i r l n a

u n b r m e s t l c r y i l a . o a

e d n n t i

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c c e b e h u n y a s r

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s u ( a n m

p n s

A l g F ( i g N 5 i ) n r 3 h Y T g e 5 t l U

orque y q d m , ) i

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b o e r

a l b e n s y

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n t d y h t e

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l t ﬁ y w e s u e i l s d r e

, r o n

e n f o n t t Nonlinearities at Large Precession Angle

Largest precession angle observed experimentally ~ 40º

Demag Field

H Applied Field The DC-driven spectral peaks can be much narrower than the FMR resonances

DC-driven T = 10 K FMR

370 mT

Different IRF

HWHM = 13 MHz

HWHM = 250 MHz

What modes are excited in DC-Driven Precession?

DC-driven signals FMR spectra

10 K Summary: Spin-Transfer-Driven Dynamics

Magnetic Switching •Considerable recent progress on understanding switching mechanisms and on reducing switching currents to the level required for MRAM applications (More on this from Bob Buhrman)

Spin-Transfer Driven FMR •A new technique allows detailed characterization of magnetic normal modes in samples 1000 times smaller than demonstrated by other techniques •Provides rapid measurements of magnetic damping

Magnetic Precession Driven by DC Spin-Transfer Currents •By comparing to FMR measurements, can identify which magnetic mode is precessing •Excellent linewidths have been achieved (f/Δf ~ 1800 in nanopillars, ~17,000 in point contacts (NIST)) •DC-driven linewidths can be narrower than predictions of macrospin models Can Similar Control of Spin-Transfer-Driven Dynamics be Achieved on Smaller Length Scales?

Need: . Nanoscale Magnetic Electrodes with Controllable Moments . Ability to Insert Magnetic Nanostructures

Caution: Mechanical stability needs to be a big concern. Previous measurements of huge “Ballistic Magnetoresistance” at room temperature in magnetic point contacts (Garcia et al., PRL 82, 2923 (99); Hua and Chopra, PRB 67, 060401 (2003)) have been challenged due to artifacts from magnetostriction and magnetostatic forces (Gabureac et al., PRB 69, 100401; Yang et al., APL 84, 2865 (2004); Egelhoff et al., J. Magn. Magn. Mater. 287, 496 (2005)). Our Strategy: . No suspended magnetic parts . Measure only at low temperature Magnetic Electrode Fabrication 150nm 200nm

. We fabricate permalloy contacts connected by a narrow constriction using e-beam lithography

Py Py . The constriction is than narrowed down using controlled electromigration* at 4K *Strachan et al.APL 86, 043109 (2005) Au Au . By monitoring the resistance of the junction we can estimate the size of the constriction. Al2O3

145 Ω Electromigration in a test gold device live

Narrowing the contact using 150 Ω electromigration

{154 Ω The Design of the Magnetic Electrodes • We design the shape of the electrodes to enable controlled studies with both parallel and antiparallel moment conﬁgurations, with clean switching.

• Use of Permalloy -- low magnetostriction, high polarization, small crystalline anisotropy

With conventional “bowtie” electrodes, dipole ﬁelds act to destabilize the antiparallel conﬁguration.

With this shape for electrodes, accurate antiparallel and parallel orientations are both accessible. An Unanticipated Effect: Large Tunneling Anisotropic Magnetoresistance in Nanometer-Scale Junctions

When the moments in the two electrodes are forced parallel by a large applied magnetic ﬁeld, the resistance still depends on the ﬁeld angle.

4 MΩ, AMR=25% . Large AMR is observed even for the samples in the tunneling regime

. The resistance changes smoothly and reproducibly. Indicates that the large AMR is not a result of mechanical artifacts

6 kΩ, AMR=14%

4.2 K, ﬁeld magnitude = 800 mT

K. I. Bolotin et al., submitted to PRL (cond-mat/0602251) Dependence of the AMR/TAMR on Resistance

AMR vs. resistance

. Probable mechanism: the orbital wavefunction contributing to tunneling is not spherically symmetric -- and it rotates as a function of the angle of the applied field due to spin-orbit coupling.

AMR, (%) (Similar to effects in GaMnAs junctions, Gould et al., PRL 86, 043109 (2005).)

Resistance (Ω) Tunneling Magnetoresistance of Bare Junctions

40 We can tune the tunneling gap: ) %

( % p

P • Wires can be “rebroken” to change the tunneling R R 20 / / 20 gap R R ! Δ

• The magnetoresistance can vary with the gap

0 • The switching ﬁelds in general remain the same -200 -100 0 100 200 H (mT)

RAP-RP 80 15.5 kkΩ 25 MR= ! RP

60 20t n %

49 k u p k!Ω 15

15o R 4 (%) 0

/ 40 C P

Counts R R 10 / Δ 2R 0

" 5

20 k! 0 0 -10 10 30 50 70 90 95 k! MR(%) -150 -100 -50 0 50 H (mT) Bolotin et al., Nano Lett. 6, 123 (2006) see also Keane, Lu, Natelson, APL 88, 062514 (2006) Domain walls in atomic-scale contacts

60 Ω sample . AMR explains the domain wall resistance in low-resistance samples

. Only when DW gets narrow we observe larger contribution not accounted for by AMR

7 kΩ sample

domain wall scattering electrons Kondo effect in C60 molecules with Magnetic Electrodes

• Ni – C60 – Ni ferromagnetic sample

Ni-C60-Ni

• Au – C60 – Au B=0

parallel B=0 electrodes 1.5 K

Splitting of zero-bias anomaly due to exchange B=10T interaction Cannot be due to magnetic field. 5 meV splitting would require >50 T. No splitting for B=0 (see also Natelson group, Nano Lett. 4, 79 (2004)) A. N. Pasupathy et al., Science 306, 86 (2004). Kondo splitting depends on electrode orientation

• Parallel

• Large splitting for parallel moments • Reduction of splitting when moments are antiparallel • Gradual change corresponds to noncollinear geometry

• Antiparallel Good agreement with theory of Kondo effect with magnetic electrodes (J. Martinek et al., PRL 2003) Large, Inverted Tunneling Magnetoresistance

2.0 ) S

µ 1.7 ( V d / 1.5 dI

1.2

1.0 500 250 0 -250 -500 B (mT)

• Julliere magnetoresistance for simple Ni tunnel Junction = 21% •Our observed MR has much greater magnitude, up to -80% Adding Magnetic Molecules: Mn12

V g S = 10

V I

(ﬁrst experiments: nonmagnetic Au electrodes)

(B = 0)

r r H " D S2 + gµ B #S N z B

(see also Heersche et al., cond-mat/0510732) Signatures of Magnetic States I: Zero-Field Splittings 4 of 16 devices that exhibit Coulomb-blockade signals have magnetic excitations with zero-ﬁeld splittings. (0.25 meV to 1.34 meV)

Yelow state: magnetic excitation with zero-ﬁeld splitting Green state: phonon excitation, no shift with ﬁeld

N e- (N ± 1) e- Spin S Spin S+1/2

Moon-Ho Jo, Jacob E. Grose et al., submitted to Nature Physics (cond-mat/0603276) Signatures of Magnetic States II: Nonlinear Field Dependence 5.0 ) V 2.5 m .In modeling, nonlinear evolution of levels vs. )

( v magnetic ﬁeld is generic when the strength of

m 0.0

( magnetic anisotropy is different for N and N±1

V V electrons and the ﬁeld is applied at an angle -2.5 relative to the magnetic easy axis.

-5.0 . No hysteresis vs. B. Can tunneling electrons can enhance magnetic relaxation? -7.5 -5.0 -2.5 0.0 2.5 5.0 7.5 (Waintal and Brouwer, PRL 91, 247201 (2003)) B (T)

0 100 200 dI/dV (nS) Summary of Molecular-Scale Spintronics

• Nanoscale Magnetic Electrodes: Can use electromigration to fabricate magnetic electrodes with a nanoscale gap appropriate for single molecule studies. Even simple bare electrodes exhibit some unexpected new physics: large tunneling anisotropic magnetoresistance and ﬂuctuations in magnetoresistance values in the tunneling regime.

• Transport Measurements on Single Magnetic Molecules with Non-Magnetic Electrodes: Accomplished energy-level spectroscopy and found signatures of molecular magnetism.

Spin Transfer measurements of magnetic dynamics in single magnetic molecules are planned.