IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013 4405
Switching Field Variation in MgO Magnetic Tunnel Junction Nanopillars:Experimental Results and Micromagnetic Simulations
Ana V. Silva , Diana C. Leitao , Zhiwei Huo , Rita J. Macedo , Ricardo Ferreira , Elvira Paz ,Francis Leonard Deepak , Susana Cardoso , and Paulo P. Freitas
Instituto de Sistemas e Computadores-Microsistemas e Nanotecnologias (INESC-MN), Lisboa 1000-029, PortugalInstituto Superior Técnico (IST), Lisboa 1000, Portugal
Department of Electrical and Computer Engineering, University of California–Davis, Davis, CA 95616 USAInternational Iberian Nanotechnology Laboratory (INL), Braga 4715-31, Portugal
The switching field dependence on the size of nanometric magnetic tunnel junctions was studied. CoFe/Ru/CoFeB/MgO/CoFeB nano-pillars were fabricated down to 150 300 nm and characterized, revealing a squared transfer curve with a sharp transition betweenmagnetic states. A micromagnetic finite element tool was then used to simulate the magnetic behavior of the studied nanopillar. Thesimulations indicated a single-domain like state at remanence, also displaying a sharp transition between parallel/antiparallel free-layerconfigurations. Overall, the experimentally measured switching fields were smaller than those obtained from simulations. Suchtrend was consistent with the presence of a particular free layer profile, signature of the two angle etching step used for pillar definition.Further decrease of experimental was attributed to local defects and thermal activated processes. This study was able to validatethis particular simulation tool for the control of the nanofabrication process.
Index Terms—Magnetic tunnel junctions, micromagnetic simulations, nanofabrication.
I. INTRODUCTION
N ANOMETRIC MgO-based magnetic tunnel junctions(MTJ) are expected to drive the next generation of
spintronic devices such as magnetic random access memories(MRAM), spin-transfer torque MRAM cells (STT-MRAMs)or spin-transfer nano-oscillators (STNOs) [1]–[3]. The tech-nological requirements of such devices are both challengingand heterogeneous in nature. For example, the capability tofabricate MTJs with resistance-area (R A) product lowerthan 5 m while exhibiting simultaneously a high tunnelmagnetoresistance % must to be combined withthe aptitude to properly define pillars smaller 100 nm integratedinto a device architecture [4], [5]. Consequently, the physicalproperties of the deposited materials and the nanofabricationprocess steps need to be accurately controlled. In this context,modelization arises as an expedite tool to aid on decisions, thusreducing the overall fabrication time.Such a modeling tool can be used to tune electrical charac-
teristics and magnetic properties (e.g., exchange coupling, in-duced anisotropy fields, saturation magnetization) of the ferro-magnetic layers, in addition to adequately choosing the geome-tries and sizes of the pillar. Therefore, first steps towards thevalidation of the physical scales and the convergence control ofa particular modelization tool are required.In this work we fabricate and characterize sub-micron
CoFe/Ru/CoFeB/MgO/CoFeB MTJ pillars in a current per-pendicular-to-plane architecture. The experimental results arethen compared to simulations using a micromagnetic modelwith input parameters extracted from experimental data. The
Manuscript received November 05, 2012; revised February 22, 2013; ac-cepted March 03, 2013. Date of current version July 15, 2013. Correspondingauthor: A. V. Silva (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2013.2252330
main advantage of the employed simulation method relies onthe ability to include a complex MTJ stack, in this particularcase with a synthetic antiferromagnetic pinned layer, and toaccount for all magnetic interactions present in the device. Incontrast, most simulation works and analytical calculationsfocus on simplified structures such as single layers [6] ortri-layers [7] usually overlooking the effect of magnetostaticcoupling between ferromagnetic layers. One important charac-teristic which needs to be properly controlled is the switchingfield , strongly dependent on the magnetization reversalprocess present. The latter can be affected by the pillar size, butcan also depend on the patterning process. By comparing bothexperimental and simulated data we are able to validate theemployed simulation method towards developing a proficientdecision tool.
II. EXPERIMENTAL AND SIMULATIONS DETAILS
A. Experimental Details
TheMTJfilmswere deposited at INLon thermally oxidized Siwafers (200mmdiameter),byaTimarisRFmagnetronsputteringtool. The stack consists of Si/SiO2(200)/Ta(5)/CuN(50)/Ta(3)/CuN(50) /Ta(3) / PtMn(15) /CoFe(2.3) /Ru(0.85) /CoFeB(2.5) /MgO /CoFeB(2.5)/Ta(10)/CuN(30)/Ru(7) (thickness innm), where CoFe and CoFeB stand for Co Fe and(Co Fe ) B , respectively. The deposited stack was thenannealed at 330 C in vacuum for 2 hours under a 1 T in-planeDC magnetic field. Current-in-plane tunneling (CIPT-CAPRESsystem at INL) measurements on unpatterned stacks revealedan average TMR % and R A m . Afterward,devices were fabricated and characterized at INESC-MN,starting with the deposition of a 5 nm thick TiWN protectivecoating over the stack. The samples were then patterned intosub-micron circular (nominal diameters 200 to 500 nm) andelliptical (nominal sizes 150 300 to 200 650 nm ) pillars,
4406 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013
Fig. 1. (a) Schematic illustration of the simulated MTJ pillar; (b) Orientationof simulation input vectors relative to coordinate axes.
TABLE ISUMMARY OF THE INPUT PARAMETERS FOR THE SIMULATIONS. , T,AND STAND FOR SATURATION MAGNETIZATION, THICKNESS, EXCHANGE
LENGTH AND INDUCED ANISOTROPY FIELD, RESPECTIVELY
combining electron beam lithography, ion-milling etching andlift-off steps [8]. Notice that deviations between measuredand nominal sizes of the patterned devices below 10% wereobtained by SEM top view image analysis [9].The electron beam lithography and scanning electron micro-
scope (SEM) images were performed with a Raith 150 system.The magnetic characterization [M(H)] was performed by a vi-brating sample magnetometer (VSM) and the devices’ transfercurves were measured with a DC four-point probe method withthe field applied parallel to the uniaxial induced anisotropy andto the ellipses longest axis, with mV.High resolution transmission electron microscopy (HRTEM)
images of the cross section near the nanopillars were obtainedat INL with a Titan ChemiSTEM system.
B. Simulation Details and Case DefinitionSimulations of the magnetic behavior were performed using
the commercial software SpinFlow3D. This is a finite elementsimulation tool for coupled spin transport and magnetizationprocesses, where the magnetic behavior of a patterned device ismodeled by the time independent solution of the Landau-Lifs-chitz-Gilbert equation [10]. The evolution of the system towardsa static equilibrium is achieved by forcing an overdamped dy-namical regime. In the present simulation case, thermal fluctu-ations in the magnetization are not accounted for. Further de-tails concerning the program particularities can be found else-where [10].The simulated device consists of a general stack such as
bottom-lead/antiferromagnet/pinned-layer/spacer/reference-layer/barrier/free-layer/top-lead (Fig. 1(a)). Table I summarizesthe magnetic parameters considered as input.
Furthermore, one also accounts for an antiferromagnetic cou-pling strength of erg/cm for the synthetic anti-ferromagnetic (SAF) structure (PL/spacer/RL), a ferromagneticcoupling of erg/cm between the FL and theSAF and an exchange coupling strength of erg/cmbetween the PL and the AFM. and values were ob-tained using an analytical model [13], which takes into consid-eration the experimentally measured saturation fields from thePL and RL (Fig. 1(a)). These values are in accordance with pre-vious works [1], [14], [15]. Also, all magnetic layers display anuniaxial in-plane anisotropy , consistent with the inducedanisotropy during film growth, which is set parallel to the YYaxis and to the ellipses longest direction (Fig. 1(b)). The mag-netizations were initialized parallel to YY.The external magnetic field is then applied also along
the YY axis, ranging from 1 kOe to kOe, starting from a FLsaturated state and focusing only on the FL behavior.The convergence criteria used on all magnetic volumes was
, where is the magnetizationversor [10]. The entire device is then discretized in a system of
triangular vertices with an average element size of 4 nm. is computed at mesh vertices and interpolated linearly
inside each element.
III. RESULTS AND DISCUSSION
Fig. 2(a) shows the M(H) characterization of the unpatternedstack. The sample exhibits an exchange coupling field of
Oe, a free-layer (FL) coercivity of Oe, ferro-magnetic coupling field of Oe and saturation magne-tizations of 1140 emu/cm and 1070 emu/cm for CoFeBand CoFe layers, respectively. Furthermore, one is also able toextract the saturation fields of both reference (RL) and pinned(PL) layers namely, Oe,Oe and Oe, Oe.Fig. 2(b) presents the TMR versus R A for the measured
devices (total of 588). Overall, about 55% of the total numberof devices show a % and R A m .Compared to the unpatterned sample a decrease in the TMRvalue is observed, probably related to the fabrication process[16], [17]. HRTEM analysis of the MgO barrier indicates anaverage thickness of 0.5 nm, being almost defect free and with awell defined crystallography (inset of Fig. 4(b)). Such uniformMgO layer is consistent with the small dispersion obtained forR A when compared with unpatterned values.The influence of the nanopillar geometry on the magnetic
response was also evaluated. Fig. 3(a) and (b) display repre-sentative transfer curves of circular (300 300 nm ) and el-liptical (150 400 nm ) patterned devices. Squared-like TMRcurves are observed with switching fields of 85 Oe forthe dot and 58 Oe for the ellipse nanopillar devices. Although asingle sharp transition from low to high resistance is expected(and vice-versa), several devices exhibit multiple Barkhaunsenjumps in the transfer curves. Our experimental results [Fig. 3(c)]suggest a trend in increasing with pillar dimension for dotsand ellipses (with different aspect ratios and one fixed dimen-sion), following a single domainmodel [18], [19]. However, andwithin such framework, significantly larger values are pre-dicted. In addition, a slight increase of the offset field
SILVA et al.: SWITCHING FIELD VARIATION IN MGO MAGNETIC TUNNEL JUNCTION NANOPILLARS 4407
Fig. 2. (a) Normalized M(H) curve of the unpatterned sample. Inset showsHRTEM (by INL) profile of the MgO barrier. (b) TMR versus R A for allmeasured devices highlighting the cluster with %. Inset shows Rversus 1/Area with the expected linear relation.
defined as the deviation from ) with increase in pillardimensions is also visible, evolving towards the unpatternedvalue for larger areas [Fig. 3(c)]. In contrast, previous re-
ports [19] and micromagnetic simulations suggest a decrease inwith increase size for very small dimensions.
The inset of Fig. 4(b) displays the simulated free-layersquared M(H) curve for the 150 300 nm pillar with a sharpreversal of the magnetization at the coercive field . Themicromagnetic distributions revealed a single-domain like statein the FL magnetization configuration. Experimentally, for apillar of nominal dimensions 150 300 nm , a Oeis obtained [Fig. 4(a)], being smaller than the 176 Oe expectedfrom simulations.These results can be understood upon analysis of the HRTEM
image of the patterned pillar [top inset of Fig. 4(b)] which re-veals a tapered profile in the FL with dimensions at leasttimes larger than nominal. The latter is a signature of a 2-stepbased ion beam milling process used for nanopillar definition.For the case of ellipses with fixed aspect ratio, simulations showa decrease of with increase in pillar dimensions [Fig. 4(b)],supporting the trend observed in experimental and beingconsistent with the decrease in the demagnetizing field [18].The magnetic behavior of a tapered FL pillar was also
addressed. Fig. 4(c) presents the variation of with a ta-pered FL profile. For the considered dimensions the taperedprofile reduces in times. The expected valuefor the 150 300 nm nominal pillar should then beOe, still larger than the experimentally obtained. Therefore,
Fig. 3. MR(H) representative transfer curves of (a) 300 300 nm circular and(b) 150 400 nm elliptical pillars. Inset shows SEM images of the patternedresist prior to ion-milling etch. (c) and versus pillars diameters fordevices with %, namely dots and ellipses with fixed width of 200nm. Each data point displays the average and values obtained frommeasurements of several devices and corresponding maximum deviation (errorbars).
differences between nominal and effective size and geometryof the FL cannot fully account for the discrepancy betweenexperimental and simulation results. Nevertheless, the visibleBarkhausen jumps in the devices transfer curves suggest thepresence of complex magnetization reversal processes withinthe FL. Notice that such accentuated discrepancies, betweentheoretically predicted and experimentally obtained reversalfield values, have already been observed in nanometric systemsand related to the presence of localized reversal [20], [21] dueto low anisotropy sites [21], [22]. In this particular case, edgeroughness or local defects [17], [23], can energetically favorthe nucleation and propagation of domain-walls, resulting ina smaller than theoretically expected. On the other hand,thermal activated processes are well known to decrease ,[19]–[21], although not included in these simulations.
4408 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013
Fig. 4. (a) Experimental MR(H) of a 150 300 nm elliptical pillar. (b) Simulated versus pillar dimensions. Bottom inset shows simulatedM(H) for 150 300nm elliptical pillar. Top inset shows HRTEM (by INL) cross section of a representative MTJ device. (c) Simulated versus pillar dimensions, for a taperedpillar and respective steep profiles of corresponding dimensions. The layers included in this finite element model are: pinned-layer/spacer/reference-layer/barrier/free-layer/metal 1/metal 2/metal 3/metal 4.
IV. CONCLUSION
The magnetic behavior of nanofabricated CoFe/Ru/CoFeB/MgO/CoFeB MTJs was studied for elliptical and circularnanopillars. Overall % of the total processed devicesshowed a % and R A m , con-sistent with the characterization performed in the unpatternedsample. Squared-like transfer curves were observed, typicallydisplaying a single sharp transition between magnetic states.For dots and ellipses with different aspect ratios and one fixeddimension, an increase in with increase in pillar size isobserved. On the other hand, for ellipses with fixed aspect ratio,a decrease in was obtained for larger sizes, consequenceof smaller demagnetizing fields. Micromagnetic simulation ofcorresponding nanopillars revealed a consistently larger re-versal field than the experimentally obtained. Such differenceswere here attributed to edge roughness or local defects andthermal activated processes.This work allows one to validate this particular simulation
tool towards providing a device response closer to real fabri-cated devices.
ACKNOWLEDGMENT
This work was supported in part by FCT projects PTDC/CTM-NAN/112672/2009, PTDC/CTM-NAN/110793/2009and PTDC/CTM-NAN/118236/2010. A. V. Silva and D. C.Leitao are thankful to FCT for grants SFRH/BD/74975/2010and SFRH/BPD/72359/2010, respectively. Z. Hou acknowl-edges funding from IMAGIC-EU-FP7-ICT-288381. INLacknowledges partial funding from the ON2 project from PONorte. INESC-MN acknowledges FCT funding through theInstituto de Nanociência e Nanotecnologia (IN) AssociatedLaboratory.
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