Table Of ContentSpin-torque driven ferromagnetic resonance
of Co/Ni synthetic layers in spin valves
W. Chen, J-M. L. Beaujour, G. de Loubens, A. D. Kent
Department of Physics, New York University, New York, NY 10003
J. Z. Sun
IBM T. J. Watson Research Center, Yorktown Heights, NY 10598
(Dated: November30th, 2007)
8
Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study thin Co/Ni synthetic
0
layers with perpendicular anisotropy confined in spin-valve based nanojunctions. Field swept ST-
0
FMRmeasurementswereconductedwithamagneticfieldappliedperpendiculartothelayersurface.
2
The resonance lines were measured under low amplitude rf excitation, from 1 to 20 GHz. These
n resultsarecompared with those obtainedusing conventionalrf fielddrivenFMR on extendedfilms
a
withthesameCo/Nilayerstructure. Thelayersconfinedinspinvalveshavealowerresonancefield,
J
anarrowerresonancelinewidthandapproximatelythesamelinewidthvs frequencyslope,implying
7 the same damping parameter. The critical current for magnetic excitations is determined from
measurements of the resonance linewidth vs dc current and is in accord with the one determined
]
from I-V measurements.
l
l
a
h
Spin-transfer torque has been theoretically predicted
-
s and experimentally demonstrated to drive magnetic ex-
e citationsinnanostructuredspinvalvesandmagnetictun- (a) (c) 4.65
m neljunctions[1,2,3,4,5]. Withanrfcurrent,spintrans- 4.60 m]
at. fTehricsatnecbheniuqsueed,tkonsotwundyasfesrproinm-taogrnqeuteicdrreisvoennanfecrero[6m,a7g]-. H V 44..5505 V/dI [Oh
m d
netic resonance (ST-FMR), enables quantitative studies 2o 4.45
- ofthemagneticpropertiesofthinlayersinaspin-transfer
d device. Specifically, the layer magnetic anisotropy and 1.56 (b) (d) 8 Oe]
on damping canbe determined[8],whichareimportantpa- Ohm] 1.55 6 eld [k
2 [c rmaemSmpeoitner-yrtsraatnhndasftrefnreomesdceimltloaotrboyerdaoeppvptiiclmiecsiaztweidoilnlisnl.ikspeliyn-itnocrlquudee-bmasaegd- dV/dI [ 11..5534 24 agnetic Fi
M
v neticlayerswithperpendicularmagneticanisotropythat -2 -1 0 1 2 -15 -10 -5 0 5 10 15
5 counteracts their shape-induced easy-plane anisotropy. Magnetic Field [kG] dc Current Bias [mA]
0 This will allow efficient use of spin current for magnetic
4
reversal with a reduced switching threshold [9] and a
0
faster switching process [10]. Recent work by Mangin et FIG. 1: (a): Sample layer structure and ST-FMR circuit.
1. al. [11] has demonstrated improvements of spin-torque (b): Zero current in-plane MR hysteresis loop for a 50×150
1 nm2 spin valve junction with t=0.4. (c): dV/dI vs I of the
efficiency in a spin valve that has perpendicularly mag-
7 samejunctionwithaperpendicularmagneticfieldof9.5kOe.
netized Co/Ni synthetic layers. For further optimization
0 (d): Contour plot of dV/dI as a function of both dc current
of perpendicular anisotropy materials, it is important to
: and perpendicular magnetic field. Data points: critical cur-
v have quantitative measurements of their anisotropy field
rents determined from ST-FMR at three different fields and
i
X anddampinginananostructureddevice,asbothofthese frequencies (see text).
parametersdirectly affect the threshold currentfor spin-
r
a transfer induced switching.
In this Letter, we present ST-FMR studies of bilayer Cr| 100 nm Cu| 20 nm Pt| 10 nm Cu| [t nm Co| 2t nm
nanopillars, where the thin (free) layer is composed of a Ni]× 1.2/t |10 nm Cu|12 nm Co|200 nm Cuk. We var-
Co/Ni synthetic layer and the thick (fixed) layer is pure iedthe Cothickness t from0.1to 0.4,tuning the magni-
Co. Themagnetic anisotropyanddamping ofthe Co/Ni tudeoftheCo/Nicompositelayer’snetanisotropy,while
have been determined by ST-FMR. We compare these keeping the total magnetic moment and thickness of the
resultswith thoseobtainedfromextended films withthe free layer constant. For ST-FMR measurements, an rf
sameCo/Nilayerstackmeasuredusingtraditionalrffield currentgeneratedby ahighfrequency sourceis addedto
driven FMR. a dc current using a bias-T (the dashed-line box in Fig.
Pillar junctions with submicron lateral dimensions 1(a)). Positivedccurrentsaredefinedsuchthatelectrons
(Fig. 1(a)) were patterned on a silicon wafer using a flow from the free layer to the fixed layer.
nanostencil process [12]. Junctions were deposited us- The magnetoresistance (MR) was measured with a
ing metal evaporation with the layer structure k 1.5 nm magnetic field applied in the film plane using a 4-point
2
geometry. AtypicalMRhysteresisloopofa50×150nm2
junction with t=0.4 is shown in Fig. 1(b). The magne-
toresistance MR= (RAP − RP)/RP is ≃2.3±0.3 % for 20 GHz 20 1.0
all junctions, independent of t, within the range inves- (a) (b)
tigated. Here RAP (RP) represents the static junction 6 z] e] 0.8
resistance when the free layer and fixed layer magneti- V] 15GH kO
zsnwuaeatrasieromlanypespnpalteirseredpwaee2nnr◦tdeifipccraoournmlaadlrlutehctlteo(epdfitalhmwreailtnlsheaolmr)am.pmaClelaugtrsonrueerpntfriatco-cvdefiouel(cltTdeaghaaeepspmmfilieeaeladdl-l V [(cid:80)lock-in 24 10requency [ Linewidth [ 00..46
F
in-plane field along the easy axis of the junction. This 0.2
5
was done to suppress vortex states in the magnetic lay-
3 GHz
0 0.0
ers.) Measurements were conducted in a 2-point geome-
2 4 6 8 0 10 20 30
try where lead resistances are included. Fig. 1(c) shows
Magnetic Field [kOe] Frequency [GHz]
dV/dI vs I of the same junction in a 9.5 kOe applied
field. A peak without hysteresis is observed at 9.1 mA,
whichweinterpretasthecriticalcurrentI forexcitation
c
of the free layer[13]. A contour plot of2-point dV/dI as FIG. 2: (a): Lock-in voltage signal as a function of applied
the functionofbothcurrentandperpendicular magnetic perpendicular magnetic field at different rf frequencies from
field is shownin Fig. 1(d). The peak in dV/dI is seenas 3 up to 20 GHz in 1 GHz steps. N: Hres of a 50×150 nm2,
t=0.4Co/Nisyntheticfreelayerinaspinvalve;blackdashed
the bright color at high field and current.
line: corresponding linear fit; gray dashed line: a linear fit
At resonance, the rf current and spin valve resistance of Hres vs f of an extended film with the same Co/Ni layer
oscillate at the same frequency resulting in a dc voltage stack. (b): ∆H vs f for the spin valve junction (N) and the
(V =<I(t)R(t)>) [6, 7]. This voltage can be expressed extended film ((cid:4)), together with their corresponding linear
as V = 1(R − R )I sinβsinθ. Here β is the an- fits.
4 AP P rf
gle between the free and fixed layers before applying the
rf current and θ is the precession angle. I represents
rf
the rf current amplitude. This is a simplified formula We also measuredresonance lines on an extended film
that assumes smallangle precessionand a sinusoidalan- with the same Co/Ni synthetic layer stack sandwiched
gular dependence of junction resistance between parallel between 10 nm Cu on each side. These measurements
and antiparallel states. With a perpendicular magnetic were conducted with a traditional rf field driven FMR
field greater than the free layer’s easy-plane anisotropy using a flip-chip method [14]. Broader resonance peaks
field, the free layer magnetization is normal to the sur- were not found in extended films with t=0.1, 0.2, and
face, while the fixed layer, which has a larger easy-plane 0.3.
anisotropy field, is still mainly magnetized in the film The resonance field H of the Co/Ni element in the
res
plane. This non-collinear arrangement of the layer mag- spin valve increases linearly with f above 4 GHz, as
netizations (β . π/2) enhances the ST-FMR signal. To shown in Fig. 2(a) (N symbols). At lower frequencies,
furtherincreasethesignal(typicallyinthesub-µVrange) the free layer magnetization tilts into the plane, lead-
to noise ratio, we modulate the rf current on and off at ing to a lower resonance field. A linear fit of H vs f
res
800 Hz and use a lock-in amplifier to detect the voltage of the extended film is also plotted with a gray dashed
at this frequency. line (to the right of the N symbols) in Fig. 2(a). A
ST-FMR measurements were conducted with the cir- linear relationshipbetween f and Hres in extended mag-
cuit shown in Fig. 1(a). Resonance lines under low am- netic filmsisexpectedwhenthemagnetizationisnormal
plitude rf current at zero dc current and different rf fre- to the film surface µhBf = g(Hres −4πMeff) [15]. Here
quencies f are plotted in Fig. 2(a) versus perpendicular g is the Land´e g factor and the easy-plane anisotropy
magneticfield. Differentfrequencies(3∼20GHzin1GHz is 4πM = 4πM − H , where M and H repre-
eff s P s P
steps) are plotted with each adjacent curve offset by 0.4 sentthe saturationmagnetizationandthe perpendicular
µV. The voltage signals are shown on the left vertical anisotropyfield. Alinearfitofeachdataset(dashedlines
axis. From the peak height V and I , we estimate in Fig. 2(a)) gives g=2.17 and 4πM = 2.58 kOe for
peak rf eff
◦
the precession angle to be ∼4 . We verified that this the extended film, and a slightly larger slope (2.28) and
set of data was taken in a linear response regime with a smaller field-axis intercept (1.92 kOe) for the Co/Ni
V /I2 independent of I . These data are typical of element confined in the spin valve. This consistency be-
peak rf rf
all junctions with t=0.4. However, much broader reso- tween data sets confirms that the main peak of the ST-
nance peaks and multiple peaks were found on samples FMR signal is associated with the Co/Ni synthetic free
with t=0.1, 0.2 and 0.3. This is likely associated with layer rather than the other magnetic layers. The differ-
the excitation of higher order spin wave modes, but is ences are associated with the static dipolar fields from
not presently understood. Therefore, data analysis and other magnetic layers and finite size effects on the spin
discussion mainly focus on samples with t=0.4. wavemodes,whichisdiscussedindetailinaforthcoming
3
larizationfactor and V is the volume of the magnetic el-
ement. So with a dc bias, ∆H-∆H =2αhf(1− I ), and
0.75 0 gµB Ic
(a) 6 mA (b) therefore at fixed frequency ∆H-∆H depends linearly
0.6 0
on current and goes to zero at the critical current. We
0.50
0.5 plot resonance lines of the spin valve with f = 18 GHz
V] atdifferentdccurrentsfrom-4to6mAin2mAstepsin
[(cid:80)n 0.25 0.4 Oe] Fig. 3(a). ∆H vs dc current bias is shown in Fig. 3(b).
Vlock-i 0.00 0.3 H [k dTehcereiansteesrctehpetinotfer∆ceHptvbsyIniosm7.o8remtAha.nIn0c.l2umsioAn,bofec∆auHs0e
(cid:39)
0.2 ∆H is small compared to the linewidth at the dc cur-
0
-0.25 rentsstudied. Criticalcurrentsdeterminedforf =10,14
-4 mA 0.1
and18GHz areplottedinFig. 1(d), andagreewellwith
-0.50 0.0 those obtained from the I-V measurements. Further, Ic
6.0 6.5 7.0 7.5 8.0 -4 -2 0 2 4 6 8
is quantitatively consistent with the Slonczewski model
Magnetic Field [kG] Current Bias [mA] taking a spin polarization factor P ∼0.3.
The frequency independent term ∆H originates from
0
film inhomogeneities: roughness, polycrystalline struc-
FIG. 3: (a): ST-FMR signal as a function of applied field at
ture,as wellasdefects. The scaleof the inhomogeneities
different dc currents. The rf frequency was set at 18 GHz,
and the rf amplitudes were 595, 595, 470, 470, 315 and 315 is likelythe filmgrainsize,5∼10nm. Inasimple model,
µArespectively foreachdccurrentfrom -4to6mAin2mA fluctuations in Hres from grain to grain result in an in-
steps. Each adjacent curve is offset by 0.20 µV. Solid lines homogeneously broadened resonance line [18]. However,
are Lorentzian fits of each data set. (b): ∆H (full width at it is likely that the exchange coupling between grains is
half maximum) vs dccurrent. important to a detailed understanding of the linewidth
[19].
The free layer in the nanostructured device contains
publication [16].
Here wefocus onthe resonancelinewidth∆H. ∆H vs at most a few hundred grains, therefore one expects less
inhomogeneity than that in extended film. More im-
f at zero dc bias is plotted in Fig. 2(b). ∆H of both
portantly, the lateral magnetic confinement results in
the Co/Ni layer in spin valve (N) and the same-stack
strongly varying internal field in the plane of the nanos-
extended film ((cid:4)) increases linearly with f. Linear fits
tructure that lifts the degeneracy between different spin
areshownassolidlinesinFig. 2(b),andgiveanintercept
wave modes. Numerical and analytical calculations of
and slope:
normalmodes in the Co/Nirectangularelement arepre-
2αh sentedandcomparedwithourST-FMRdatainRef. [16].
∆H =∆H + f (1)
0 gµ The separation between them is more than the inhomo-
B
geneous broadening ∆H in the extended film, therefore
0
wherehisthePlanckConstantandµB istheBohrMag- we expect that the linewidth measured on an individ-
neton. The first term ∆H0 describes the inhomogeneous ual mode is close to its intrinsic value (the term propor-
broadening,andthe second termis relatedto the damp- tionaltof inEq. 1)[20]. Theremaininginhomogeneous
ing α[17]. ∆H vs f ofthe spin valveandthatofthe ex- broadeninginthenanostructuremaybeattributedtothe
tendedfilmhaveasimilarslope,implyingasimilardamp- quasi-degeneracy subsisting between some very closely
ingparameter(α=0.036±0.002fortheextendedfilmand spaced modes resulting from film inhomogeneities.
0.033±0.003 for the magnetic layer in the spin valve).
Insummary,ST-FMRhasbeenusedtostudythemag-
However,theinterceptsarequitedifferent: ∆H =24±15
0 neticpropertiesofCo/Nisyntheticlayerswithperpendic-
Oe in the spin valve, which is much lower than that of
ular anisotropy in spin valves. The ST-FMR resonance
the extended film, 284±30 Oe.
lines were compared with those of traditional FMR on
Whenadccurrentbiasisappliedtoaspin-value,there
same-stackextendedfilm. ThedampingoftheST-device
is an additional spin transfer torque that modifies the
free layer is essentially the same as that of an unpat-
free layer’s effective damping, α = α(1 − I ), where
eff Ic ternedfilmandthe criticalcurrentsdeterminedfromthe
I is the dc current. Thus α decreases with increasing
eff ST-FMR homogeneous linewidth are in agreement with
positive current up to a critical current I , that defines
c those of quasistatic I-V measurements.
the threshold for magnetic excitation of the free layer.
The criticalcurrentinthe Slonczewskimodel [1]is given This researchis supportedby NSF-DMR-0706322and
by I = 2e αMsV(H −4πM ), where P is the spin po- an NYU-Research Challenge Fund award.
c ~P cosβ eff
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