Understanding stability diagram of perpendicular magnetic tunnel junctions

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by  Witold Skowroński

Understanding stability diagram of perpendicular magnetic tunnel junctions
Witold Skowroński,1,∗ Maciej Czapkiewicz,1,† Sławomir Ziętek,1 Jakub
Chęciński,1 Marek Frankowski,1 Piotr Rzeszut,1 and Jerzy Wrona2
1AGH University of Science and Technology, Department of Electronics, Al. Mickiewicza 30, 30-059 Kraków, Poland
2Singulus Technologies, Kahl am Main, 63796, Germany
(Dated:24 stycznia 2017)
Perpendicularmagnetictunneljunctions(MTJ)withabottompinnedreferencelayerandacom-
7 posite free layer (FL) are investigated. Different thicknesses of the FL were tested to obtain an
1 optimal balance between tunneling magnetoresistance (TMR) ratio and perpendicular magnetic
0 anisotropy.Afterannealingat400◦C,theTMRratiofor1.5nmthickCoFeBsublayerreached180
2 %at room temperatureand280 % at20 Kwith anMgO tunnelbarrier thicknesscorrespondingto
the resistance area product RA = 10 Ohmµm2. The voltage vs. magnetic field stability diagrams
n
measuredinpillar-shapedMTJswith130nmdiameterindicatethecompetitionbetweenspintrans-
a
J fer torque (STT), voltage controlled magnetic anisotropy (VCMA) and temperature effects in the
switching process. An extended stability phase diagram model that takes into account all three
3
effectsandtheeffectivedampingmeasuredindependentlyusingbroadbandferromagneticresonance
2
technique enabled the determination of both STT and VCMA coefficients that are responsible for

theFL magnetization switching.
]
i
c
s
- I. INTRODUCTION STT, voltage control of magnetic anisotropy (VCMA)
l
r and temperature effects. An analytic model based on
mt Magnetic tunnel junctions (MTJs) have become a ba- work by Bernert et al.13 was extended to reproduce the
sic building block for various types of spintronics devi- experimental results.
.
t ces, such as magnetic random access memory (MRAM)
a
m cells, magnetic field sensors and microwave generators
ordetectors1.The properties ofspintronicsdevices,such II. EXPERIMENT
-
d as thermal stability of an MRAM cell2, or sensitivity
n of microwave detectors3 utilizin g MTJs can be greatly The multilayers with the following structure were de-
o improved by using magnetic layers with perpendicular posited:buffer/SyF/separator/CoFeB(1)/MgO(0.82)
c anisotropy4. Among a few ways to realize such a per- / CoFeB(tFL) / W(0.3) / CoFeB(0.5) / MgO (0.76) /
[
pendicular MTJ, taking advantage of the interface ani- capping (thicknesses in nm), with tFL rangingfrom 1 up

1 sotropy component5 yields the best results so far, espe- to 1.6 nm, using Singulus TIMARIS sputtering system.
v cially in terms of high tunneling magnetoresistance ra- The bottom Co/Pt super-lattices coupled by a thin Ru
1
tio (TMR), which is measured typically in MTJs with spacer are characterizedby high perpendicular magnetic
1
CoFeB/MgO/CoFeB trilayer. Recent studies on perpen- anisotropy(PMA).TheTa/Co/W-basedseparatorensu-
4
6 dicular MTJ showed the TMR ratio exceeding 200%6 res high ferromagnetic coupling between the top super-
0 thankstocarefuloptimizationofboththefreelayer(FL) lattice and the RL. In addition, it provides structural
. and reference layer (RL) structure7. In addition, one of transition from a face center cubic SyF14 to a body cen-
1
thekeychallengesforthecommercialdevelopmentofspin ter cubic CoFeB and contributes to the absorption of B
0
7 transfertorque(STT)-MRAMistooptimizeperpendicu- atoms from CoFeB during annealing and crystallization
1 larMTJtowithstandthetemperaturebudgetintroduced processes.
: at the back end of line CMOS fabrication process with After the deposition, the samples were annealed at
v
temperatures up to 400 ◦C. To achieve this a careful de- 400 ◦C to induce proper crystallographic orientation of
i
X signofthelayerstack,takingintoaccountallconstituent Fe-rich CoFeB and PMA of the CoFeB/MgO interfaces.
r layers as well as the properties and the treatment of the Wafer-level parameters of the deposited multilayers we-
a bottom electrode, has to be performed. re investigated by current in-plane tunneling (CIPT)15,
Inthisletter,wereportontheperpendicularMTJwith vibrating sample magnetometry (VSM) and broadband
a composite CoFeB/W/CoFeB FL8,9, which is characte- ferromagnetic resonance (FMR) methods16. The latter
rizedbyhighperpendicularmagneticanisotropyandspin was performed by measuring the complex transmission
polarization resulting in up to 180 % TMR measured at coefficient (S21) in a dedicated coplanar waveguide with
room temperature and above 280 % TMR at low tem- a 10× 8mm unpatternedsample placedfacedown.The
perature. The RL is pinned to a synthetic ferromagnet frequencyofthevectornetworkanalyzeriskeptbetween
(SyF) consisting of Co/Pt super-latices10 coupled by a 4 and 16 GHz, while sweeping the perpendicular magne-
thin Ru spacer. Electrical transport measurements we- tic field in ± 550 kA/m range.
re performed in MTJs patterned into 130-nm diameter After the above mentioned wafer-level measurements,
pillar.Voltagevs.perpendicularmagneticfieldswitching selected MTJs were patterned into circular cross-section
diagrams11,12 are measuredinorder to separate between pillars with diameter ranging from 130 up to 980 nm
2
bymeansofelectron-beamlithography,ion-beametching
and lift-off process. 11.5 1 nits)
The transport properties presented in this work were -1 b. u
measured for the smallest devices with the area of A = -3 ar
0.013 µm2 in a dedicated probe station equipped with ] (
m)11.0 -5 m[
magnetic fieldsource.Four-probemethod witha voltage I
A/ -7
source was used to apply 1-ms long pulses and measure k
the resistance during this voltage-pulse application. The H ( -400 -200H (k0A/m)200 400
stability diagrams were determined by sweeping the vol- 10.5
tagepulsesamplitudeinthepresenceofagivenmagnetic
field.Selecteddeviceswerecharacterizedatlowtempera- tFL = 1.1 nm
turesofT =20Kinorderto determinethe temperature
10.0
influence on the magnetization switching properties.
17 18 19 20 21 22
f (GHz)
III. MODELLING Rysunek 1: FMR linewidth (full symbols) as a function of
theexcitationfrequencyforMTJwithtFL =1.1nmtogether
withafit(solidline)totheEq.5.Insetpresentsthemeasured
Magnetization direction of the FL (m~FL) was calcu-
imaginary partofthemagneticsusceptibility(opensymbols)
lated based on the Landau-Lifschitz-Gilbert (LLG) equ-
as a function of the magnetic field for f = 19 GHz together
ation with the following STT components taken into ac-
with a fit to theEq. 4 (solid line).
count:
dm~dtFL =−γ0m~FL×H~eff +αm~FL× dm~dtFL the temperature is represented by kt, which in the first
approximationis a square-rootfunction20.
−γ0akVRRP (m~FL×(m~FL×m~RL)) The damping factor was measured independently by
the broadband FMR technique. For each microwave fre-
2
−γ0a⊥ VRRP (m~FL×m~RL) (1) qgnueetniccyfifel,dtχhe(Hc)omispelexxtramcategdneftriocmsuSs2c1epmteibaisluitryemvse.ntmbay-
(cid:18) (cid:19)
subtracting the magnetic independent offset and time-
where γ0 = γ µ0, with the gyromagnetic ratio γ = dependent drift21:
(gµB)/¯h = 28 GHz/T, µ0 is the permeability of the free
space,g isthe Landespectroscopicsplitting factor,µB is
the Bohr magneton, ¯h is the reduced Planck’s constant, Meff(H −Meff)
χ(H)= (4)
ak and a⊥ are the parallel and perpendicular STT coef- (H −Meff)2−Hf2−i∆2H(H −Meff)
ficients expressed in T/V and T/V2 units, respectively,
α is the magnetizationdamping, R andRP are the MTJ where Meff = MS - HK is the effective magnetiza-
resistanceinagivenstateandminimal(parallelstate)re- tion,magnetizationsaturationandperpendicularmagne-
sistance, Heff is the effective magnetic field: Heff = H ± ticanisotropyfield,respectively,∆H isthelinewidthand
HW + HS, where, H is the external perpendicular field, Hf = 2πf / (γµ0). Figure 1 presents the dependence of
HW is the switching field and HS is the offset field. the ∆H on the excitation frequency fitted by the Eq. 5:
Stability diagram was modeled based on Ref.13:
4παf
∆H = +∆H0 (5)
VC = 2αaak⊥RRP −s(cid:18)2αaak⊥RRP(cid:19)2− aµ⊥0RRP22Heff (2) γ0
where, VC is the switching voltage. It was assumed that IV. RESULTS AND DISCUSSION
parallel(perpendicular)torquecomponentisalinear(qu-
adratic)functionoftheappliedcurrent17.Toaccountfor
VSM measurements of a representative sample with
the additionalphysicaleffects thatcontribute to the sta-
tFL = 1.1 nm presented in Fig. 2 reveals independent
bility diagram, namely VCMA and temperature effects,
switching of the FL (at small magnetic fields below 50
HW is scaled by the factor: kA/m)andRL(athighmagneticfieldsbetween150and
300kA/m),whichensuresbistable parallel(P) andanti-
HW =HC 1−kVV − ktT (3) parallel(AP)state.TheFLmagnetizationwascalculated
(cid:16) p (cid:17) and yielded µ0MS = 1.12 T. An inset of Fig. 2 depicts
where V is the applied voltage, HC is the coercive field, the TMR ratio for different tFL measured on the wafer-
kV is the VCMA coefficient18,19 and T is the ambient level using CIPT method. An increase of the TMR from
temperature. The dependence of the switching field on 140%fortFL =1.1nmuptoTMR=180%fortFL =1.5
3

a 180 0.6
are 10 160 %)
nit 140 MR ( 0.4
per u 2 cm) 5 120 T 0.2 AP
moment -4 10emu/ 0 1.1 1.2 tF1L.3 (nm1.4) 1.5 1.6 100 V (V)0.0 T = 2PA0 -PKA-PP P/AP
c ( -0.2 Eq. (2) P
eti -5 Eq. (2) + (3)
gn tFL = 1.1 nm -0.4 T = 300 K
a P-AP
M -10 AP-P
-0.6 Eq. (2) + (3)
-800 -600 -400 -200 0 200 400 600 800 -200 -150 -100 -50 0 50 100 150 200
H (kA/m) H (kA/m)
Rysunek2:Magneticmomentperunitareavs.in-plane(open Rysunek 4: Voltage vs. magnetic field stability diagram me-
symbols) and perpendicular (full symbols) magnetic field of asured in the MTJ with tFL = 1.1 nm at T = 20 K (open
the MTJ with tFL = 1.1 nm. Inset presents the TMR ratio symbols) and T = 300 K (full symbols). Dotted lines repre-
dependenceon tFL measured using CIPT method. sentsapproximationbasedonEq.2.Anextendedmodelbased
oncorrection presentedinEq.3isrepresentedbydashed(20
K) and solid (300 K) lines.

300 T (K) 300 tFL = 1.1 nm 2.5 Ohm)
250 20 2.0 R (k 20 K To further elucidate on the properties of the fabrica-
1.5 tFL = 1.5 nm tedMTJ,current(voltage)-inducedswitchingloopswere
%)200 measured in a presence of the perpendicular magnetic
R ( 1.0 field. Inset of Fig. 3 presents a representative resistance
M150 -0.6 -0.3 0.0 0.3 0.6 vs. voltage loop measured in a magnetic field compensa-
T
V (V) ting the offset field in the MTJ with tFL = 1.1 nm. It
100
300 K, tFL (nm) hasbeenalreadyestablishedthatapartfromtheconven-
1.1 tional STT effect observed in MTJs with relatively thin
50
1.3 MgO barriers, the switching process can be also affec-
0 1.5 ted by the VCMA effect in a thin FL22,23. To investiga-
-250 -200 -150 -100 -50 0 50 100 150 te the switching process in more detail, we repeated the
H (kA/m) R(V)loopmeasurements,withdifferentconstantmagne-
ticfield.Thestabilitydiagramobtainedinthiswayboth
Rysunek3: TMR vs. magnetic field loop of MTJs with diffe-
at room temperature (T = 300 K) and low temperature
renttFLmeasuredatroomtemperature(300K).Significantly
smallercoercivefieldHC ismeasuredfortFL =1.5nm,which (T = 20 K) for the MTJ with tFL = 1.1 nm is presented
increases at T=20 K.Aninset presentsaresistance vs.vol- in Fig. 4. To understand these diagrams, the following
tageswitchingloopmeasuredinanexternalmagneticfieldof fitting procedure was used. First, low-temperature data
H =25 kA/m,obtained at T =300 K(squares) andT = 20 were modeled using Eq. 2 to obtain HW, HS (being the
K (circles). offset field measured at low bias voltage) and STT co-
efficients, which are little affected by heat. The slope of
V(H)dependsmostlyona ,whereas,theverticaloffsetis
k
nm is explained by an increase of the spin polarization adjustedbyHW -solidlinesinFig.4.Next,tocompensa-
for the thicker ferromagnetic layer. A rapid reduction of te the offsetbetween AP-P andP-AP switching voltages
the TMR for tFL = 1.6 nm is causedby the transitionof (whichtakeplaceatopposite electricfieldappliedto the
the FL magnetization vector to the sample plane. After MgO/CoFeB interface) kV was introduced according to
the patterning process the TMR ratio dropped by about the Eq. 3, without temperature influence so far (kt =
5-15%,whichisexplainedbytheappearanceofthesmall 0). Finally, thermal reduction of HW was introduced by
serialparasiticresistancethathasnegligibleinfluence on adjusting kt to fit the stability diagram obtained at ro-
the MTJ parameters derived afterwards. om temperature. In addition, for the precise derivation
of the STT coefficients, the magnetization damping was
Figure 3 presents the TMR vs. magnetic field depen-
dence measured in the MTJ with different tFL. Increase calculated based on an independent FMR measurement
in tFL leads to an increase in TMR ratio and decrease presented above and included in Tab. I.
in the coercive field. The offset field of about Hs = 25 FittingtheexperimentalstabilitydiagramtotheEq.2
kA/m originates from the stray field, which depends on and3 yieldedthe temperaturecoefficients ofkt = 0.0014
the MTJ lateral size. 1/K. This parameter was kept constant for MTJs with
4
it is more susceptible to the anisotropy change induced
TabelaI:SummaryoftheobtainedperpendicularMTJpara- by the electric field29. Moreover, in the same thickness
meters.
regime, where the transition between perpendicular and
tFL HC κ τk α Meff in-plane magnetization occurs, the effective damping in-
(nm) (kA/m) (fJ/Vm) (Nm) no units (kA/m)
1.1 264 46 4.5×10−19 0.038 -450 creases, which may be attributed to an increase in the
1.3 280 73 4.5×10−19 0.044 -15 level of magnetization disorder30.
1.5 72 66 5.9×10−19 0.087 -0.5
V. SUMMARY
different tFL. Remaining parameters of the stability dia-
grams for each tFL were modeled independently. For tFL Insummary,weinvestigatedperpendicularMTJswith
= 1.1 nm, the following STT components were obtained
composite CoFeB/W/CoFeB FL of different thickness
a = 0.024 T/V and a = 0.02 T/V2, however, we note
k ⊥ and SyF Co/Pt/Ru-pinned RL. In the investigated FL
thatthemodeledstabilitydiagramisonlylittle sensitive
thickness range we observed an increase of the effecti-
toa ,whichagreeswithanothermacrospinapproachba-
⊥ ve damping extracted from the broadband FMR measu-
sedonLLGequationpresentedinRef.24.In-planetorque
rements with increasing FL thickness, which is mainly
τ was thereafter recalculated using Eq. 6:
k caused by the reduction of the effective magnetization.
After patterning MTJs into nano-meter scale pillars, we
MSυτLLG measuredtheresistancevs.voltageloopsandcreatedthe
τk = γ (6) stability diagrams for each FL thickness. To model the
experimental data, we included the thermal and VCMA
termsintothe theoreticalSTT-switchingphasediagram.
where υ is the FL volume and τLLG = - γak . As the
result we obtained τ = 4.5×10−19 Nm/V, which agre- Basedonthefittingprocedure,weobtainedSTTcompo-
k (cid:0) (cid:1) nents together with the VCMA coefficient. Our findings
es well with literature values of STT in case of an in-
plane MTJ17,25,26. Regarding the VCMA, the best re- shine more light on the switching process of MTJs ap-
plied in future MRAM technologies.
sults for MTJ with tFL = 1.1 nm were obtained for
kV = 0.12 1/V. Based on the following relation: kV =
κ/µ0HCMStFLtB, where tB = 0.82nm is the tunnel bar-
rier thickness, VCMA coefficient of κ = 46 fJ/Vm was Acknowledgments
calculated, which fits well the commonly measured valu-
es for CoFeB/MgO devices27,28.VCMA and STT coeffi- The authors would like to express their gratitude to
cients of all investigated MTJs are gathered in Tab. I. Prof. T. Stobiecki for a fruitful discussion and his cri-
Thein-planetorquecomponentobtainedfromthesta- tical remarks. The project is supported by Polish Na-
bility diagram is almost constant as a function of tFL, tional Center for Research and Development grant No.
which is explained by little dependence of the TMR ra- LIDER/467/L-6/14/NCBR/2015. Nanofabrication pro-
tio, and thus the spin polarization on the ferromagnetic cesswasperformedatAcademicCenterforMaterialsand
layer thickness in the investigated regime. The VCMA Nanotechnology of AGH University. J.Ch. acknowledges
coefficient is comparable for MTJs with tFL = 1.3 nm the scholarshipunderMarianSmoluchowskiKrakowRe-
and1.5nmandgreaterthaninMTJ with tFL =1.1nm. search Consortium KNOW programme. Numerical cal-
This behavior is expected, as for thicker tFL the abso- culations were supported by PL-GRID infrastructure.
lute value of the effective magnetization is reduced and ïż£
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