Author name used in this publication: A. RuotoloAuthor name used in this publication: Kwok, D. T. K.2010-2011 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chu, PK

Kwok, DT

Leung, CW

Qin, W

Ruotolo, A

Wang, N

Wang, XL

Wong, HF

Xue, Q

Yeung, SH

Applied Physics Letters

PolyU Institutional Repository

Multiple-mode excitation in spin-transfer nanocontacts with dynamicpolarizerN. Wang,1 X. L. Wang,1 W. Qin,2 S. H. Yeung,2 D. T. K. Kwok,3 H. F. Wong,4 Q. Xue,2P. K. Chu,3 C. W. Leung,4 and A. Ruotolo1,a1Department of Physics and Materials Science, Device Physics Group, City University of Hong Kong,Kowloon, Hong Kong2Department of Electronic Engineering, State Key Laboratory of Millimeter Waves, City University of HongKong, Kowloon, Hong Kong3Department of Physics and Materials Science, Plasma Laboratory, City University of Hong Kong,Kowloon, Hong Kong4Department of Applied Physics, Materials Research Centre, The Hong Kong Polytechnic University,Hung Hom, Kowloon, Hong KongReceived 17 February 2011; accepted 25 May 2011; published online 16 June 2011We report our study on the emission response of a magnetic nanocontact with dynamic polarizer inperpendicular magnetic field. In this configuration three modes are accessible, two of whichcorrespond to the precessional motion of a vortex in one of the two ferromagnetic layers with theother working as a static polarizer. At high currents a third mode can be observed that is ascribedto the simultaneous precession of two vortices, one in each layer, with the other layer working as adynamic polarizer. © 2011 American Institute of Physics. doi:10.1063/1.3600328A dc spin-polarized current can excite multiple high-frequency modes in a nanodevice through spin-transfertorque STT.1,2 The highest frequency mode is representedby the excitation of the quasiuniform magnetic configurationin a nanomagnet and can reach frequencies as high as 50GHz.3 The nanomagnet is kept in a quasiuniform configura-tion by the application of a large magnetic field. The lowestfrequency mode is the excitation of the precessional motionof a magnetic vortex around its equilibrium position.4,5 Thismode has frequencies of the order of hundreds of megahertzbut no magnetic field is required,6–8 which makes it veryinteresting for applications. Recently, a new class of dynamicstates, which can be called windmill-like states,1,9 has raisedinterest.6,10 When in a spin-transfer device neither of the fer-romagnetic layers has an easy in-plane anisotropy axis, high-frequency dynamics can be simultaneously excited in bothlayers with each of them working as a dynamic polarizer forthe other. Two modes were observed, which could be ac-cessed by changing the bias current. An insight into the phys-ics behind the excitation of multiple modes in nanocontactswith dynamic polarizer has recently been gained by Kuepfer-ling et al.10 by conducting a systematic study of the dynam-ics of the system in magnetic field applied in the film plane.We here report our study on the emission response of ananocontact device with dynamic polarizer in perpendicularmagnetic field. In this configuration, the external field is notin competition with the Oersted field self-generated by thebias current and does not induce any in-plane anisotropy.This allows us to gain access to a third mode. The interpre-tation of our experimental results supports the hypothesisthat the main coupling mechanism between the two ferro-magnetic layers is mutual SST.The spin valve structure was deposited by sputteringand comprised the following layers: Cu40 nm/Py4 nm/Cu6 nm/Co15 nm/Al4 nm. The multilayer was pat-terned into a typical squared U-shaped microwave groundelectrode, the middle side of which was 40 m wide and200 m long. The nanohole was opened at the center of themiddle side by atomic force microscope nanoindentation andplasma etching into a 180 nm thick resist. A top electrode ofAl100 nm was subsequently deposited and patterned bystandard photolithography to form a coplanar waveguide. Inthe following, positive current means electrons flowing fromthe Co layer to the Py layer.In Fig. 1 we show the power spectral density in an H=24 mT perpendicular applied field. Three modes are acces-sible. Although in the experiment the maximum current wasfirst applied and the map was taken by stepping down thecurrent, let us consider first the low frequency/low currentmode. The interpretation of this mode is straightforward.While large currents can stabilize a vortex in each of theferromagnetic layers, at low currents the vortex in the thin Pyis not stable and the magnetic configuration in this layer is ina single domain state under the contact. The vortex in the Coprecesses around the contact at frequencies that are consis-tent with those reported in nanocontacts with uniformly mag-netized polarizer.5,11 As a matter of fact, this mode is the onewith the largest power see Fig. 2.aElectronic mail: antonio.ruotolo@cityu.edu.hk.10 20 30 400.20.40.60.81.00.3 1.5P (pW/mA /GHz, log scale)2H = 24 mT⊥f(GHz)Idc (mA)FIG. 1. Color online Map of power spectrum amplitude vs frequency andapplied current bias Idc. A field of 24 mT was applied perpendicular to thesample plane.APPLIED PHYSICS LETTERS 98, 242506 20110003-6951/2011/9824/242506/3/$30.00 © 2011 American Institute of Physics98, 242506-1Downloaded 20 Jun 2011 to 158.132.183.58. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsLet us now consider the mode observed at the highestcurrents. In agreement with Kuepferling et al.,10 we assumethat the large current stabilizes two vortices, one in each ofthe two ferromagnetic layers. We will verify this assumptiona posteriori. Since the local STT is inversely proportionalto MSd Refs. 1 and 9 with MS and d the saturation mag-netization and layer thickness, respectively, the vortex in thePy is subject to a stronger STT as compared to the vortex inthe Co. The vortex in the Py precesses and since the fre-quency of precession is inversely proportional to the filmthickness,5,12 it is not surprising that this mode has fairlyhigher frequency as compared to that due to the precessionalmotion of the vortex in the thick Co at low currents. Yet, thevortex in the Co cannot be static because, in the absence ofin-plane field, its resting position would be the contact centerand, therefore, no signal would be generated on the deviceelectrodes. Local STT in the Co must be not negligible andthe vortex in this layer precesses at lower frequency. In thisrange of currents the system is in a windmill-like precession,with the angle between the magnetization vectors not beingconstant.We can exclude that the Co is in a single domain stateunder the contact. First, the observation of a vortex preces-sion in the Co at low currents suggests that, most likely, thevortex was nucleated at higher currents. Besides, if one as-sumed that the Co is in a single domain state under thecontact, a gyrating vortex in the Py should, either generatethe same power as in the case, previously discussed, of pre-cession of Co vortex with static and uniform Py polarizer, ifthe vortex gyrates far away from the contact, or the outputpower should dependent on the bias current, if the vortexgyrates under the contact area.12 Instead, the output power isabout six times smaller see Fig. 2 and does not change withthe current in the range where only this mode is detected. Wecan unambiguously conclude that the Co is in a vortex stateunder the contact and, since no field is applied in the plane,the vortex in this layer precesses under the action of the STTexerted by Py. With the same argumentations one can con-clude that the opposite is true. The Py is in the vortex stateand is precessing under the action of the STT exerted by theCo. The Co is behaving as a dynamic polarizer for the Pyand vice versa.In the windmill-like state the two vortices are within theelectrically active area of the device because of the strongrestoring force generated by the large current. With the cur-rent decreasing, the vortex in the thin Py would be the firstone to be expelled if the approximation of infinite straightconductor could be considered reasonably valid for the cur-rent distribution. Yet, the appearance of a third mode at in-termediate currents that has larger power, larger frequencyand larger tunability as compared to the windmill-like modesuggests that not only is the vortex in the Co expelled firstbut the Co layer behaves as a static polarizer for the vortex inthe Py. In order to understand the reasons of this behavior wecomputed the current distribution across the nanocontact.The simulations13 showed that the current distributions in thetwo ferromagnetic layers are substantially different. Even as-suming ideal interfaces, the current spreads in the spacerbecause of the smaller resistivity of the Cu as compared tothat of Co and Py. As a result, the electrically active area inthe Py is at least two times larger than that in the Co. Sincethe Py is sandwiched between two Cu layers, the in-planecomponent of the current distribution is negligible in thislayer. Instead, in agreement with the computations performedon a similar system,14 the in-plane component of the currentdistribution in the Co is not negligible at the edge of thenanocontact. When the vortex in the Co approaches the bor-der of the electrically active area, this in-plane componentexerts an additional torque that favors expulsion of thevortex.12 Finally, the simulations show that the Joule heatdensity in the Py is about three times smaller than that in theCo, under the contact. This is because the Py is sandwichedbetween two Cu layers that work as heat sinks. At high cur-rents, when the two vortices are both under the contact, thevortex in the Co sees a substantially larger increase in tem-perature as compared to that in the Py.All these numerical results support the following sce-nario. The vortex in the Co is ejected from the contact areaand leaves a quasiuniform magnetic configuration under thecontact, which results in an increase in the power. The powernever reaches the same value as the low current mode but inFig. 2 a plateau is reached where the power is about half ofthe maximum. This confirms that the vortex in the Py is stillwithin the electrically active area. Since the Co is no longerfollowing the Py, the frequency and the tunability of thismode are higher as compared to the windmill-like mode.This mode is metastable and coexist with the windmill-like mode. This means that the vortex in the Co is periodi-cally renucleated. It has been experimentally found that, innanocontacts with similar diameters and current amplitudes,vortex nucleation occurs in a few nanoseconds.15 Since thevortex in the Py is gyrating at a frequency 1 GHz see Fig.1, which means the period of oscillation is 1 ns, the vor-tex in the Py has the time to make several turns, before thewindmill-like state is temporarily re-established. Since thespectrum is acquired on a timescale much longer than thecycling time, the two modes appear simultaneously in thepower spectrum but they are not coexistent in time.6,8When the current is further reduced below a criticalvalue about 27 mA in Fig. 1 for stabilization/destabilizationof the vortex in the Py, this vortex annihilates leaving a uni-form configuration under the contact area. The STT on theCo increases and this stabilizes the low-current mode inwhich the Py behaves as a static polarizer and the vortex inthe Co gyrates outside the electrically active area. In a smallrange of currents around this critical value, the three modesappear simultaneously in the spectrum. This is not surprisingbecause nucleation of the vortex in the Py, as well as in theCo, is a stochastic process and thermal effects play an im-portant role.15 In any case, the value of the integrated powerin Fig. 2, in the range of currents where multiple modes are10 20 30 400.00.10.20.3Idc (mA)P(pW/mA2)FIG. 2. Power integrated on the entire spectrum at fixed applied currentbias Idc.242506-2 Wang et al. Appl. Phys. Lett. 98, 242506 2011Downloaded 20 Jun 2011 to 158.132.183.58. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionssimultaneously detected, should be taken into account bear-ing in mind that in frequency-domain measurements it is notpossible to estimate the fraction of time the system spends oneach mode and, hence, the contribution to the total integratedpower.The characteristics and the existence itself of the modeat highest frequencies strongly dependent on the specific ge-ometry of the device, i.e., thickness and relative position ofthe two ferromagnetic layers in the stack, and on the specificmeasurement configuration. For the device geometry usedhere, this mode is metastable and never exists alone see Fig.1. The application of an in-plane field see Fig. 3 as smallas 0.5 mT completely suppresses it. An in-plane field elon-gates the orbit of the vortices and off-center them.8 The dis-placement is inversely proportional to the product MSd,10,16therefore the vortex in the Py is particularly sensitive to thefield. At intermediate currents and in absence of in-planefield, when the vortex in the Co is expelled from the contact,the STT on the Py suddenly increases. As a consequence, theorbital radius of the vortex in the Py increases and can ap-proach the edge of the electrically active area. When thein-plane field is applied, the elongation and the displacementof the orbit favor expulsion and subsequent annihilation ofthe vortex in the Py. Therefore, the metastable mode is sup-pressed.It is interesting to note that a nonlinearity between thefrequency and the current in the windmill-like mode can benow appreciated see Fig. 3. The tunability decreases withthe current. This was not evident in Fig. 1, where this modeexists alone in a smaller range of currents. This behavior is inagreement with the theoretical prediction12 of a nonlineardependence of the radial distance of the vortex with the cur-rent in the small amplitude limit and enforces the assumptionthat the vortices are just below the contact.The work described in this letter was fully supported bya grant from City University of Hong Kong Project No.7008081.1J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 1996.2L. Berger, Phys. Rev. B 54, 9353 1996.3S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, N. C. Emley, R. J. Schoe-lkopf, R. A. Buhrman, and D. C. Ralph, Nature London 425, 3802003.4V. S. Pribiag, I. N. Krivorotov, G. D. Fuchs, P. M. Braganca, O. Ozatay, J.C. Sankey, D. C. Ralph, and R. A. Buhrman, Nat. Phys. 3, 498 2007.5Q. Mistral, M. van Kampen, G. Hrkac, J.-V. Kim, T. Devolder, P. Crozat,C. Chappert, L. Lagae, and T. Schrefl, Phys. Rev. Lett. 100, 2572012008.6V. S. Pribiag, G. Finocchio, B. J. Williams, D. C. Ralph, and R. A. Buhr-man, Phys. Rev. B 80, 180411 2009.7T. Devolder, J.-V. Kim, P. Crozat, C. Chappert, M. Manfrini, M. vanKampen, W. Van Roy, L. Lagae, G. Hrkac, and T. Schrefl, Appl. Phys.Lett. 95, 012507 2009.8A. Ruotolo, V. Cros, B. Georges, A. Dussaux, J. Grollier, C. Deranlot, R.Guillemet, K. Bouzehouane, S. Fusil, and A. Fert, Nat. Nanotechnol. 4,528 2009.9Ya. B. Bazaliy, D. Olaosebikan, and B. A. Jones, J. Nanosci. Nanotechnol.8, 2891 2008.10M. Kuepferling, C. Serpico, M. Pufall, W. Rippard, T. M. Wallis, A.Imtiaz, P. Krivosik, M. Pasquale, and P. Kabos, Appl. Phys. Lett. 96,252507 2010.11M. R. Pufall, W. H. Rippard, M. L. Schneider, and S. E. Russek, Phys.Rev. B 75, 140404 2007.12J.-V. Kim and T. Devolder, arXiv:1007.3859v1 unpublished.13Performed with Quickfield, www.quickfield.com.14E. Jaromirska, L. Lopez-Diaz, A. Ruotolo, J. Grollier, V. Cros, and D.Berkov, Phys. Rev. B 83, 094419 2011.15T. Devolder, J.-V. Kim, M. Manfrini, W. van Roy, L. Lagae, and C. Chap-pert, Appl. Phys. Lett. 97, 072512 2010.16R. S. Tebble, Proc. Phys. Soc. London, Sect. B 68, 1017 1955.10 20 30 400.20.40.60.81.0Idc (mA)f(GHz)0.3 1.5P (pW/mA /GHz, log scale)2H = 24 mT⊥H = 0.5 mT||FIG. 3. Color online Map of power spectrum amplitude vs frequency andapplied current bias Idc. A field of H ,H= 24,0.5 mT was applied.242506-3 Wang et al. Appl. Phys. Lett. 98, 242506 2011Downloaded 20 Jun 2011 to 158.132.183.58. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Multiple-mode excitation in spin-transfer nanocontacts with dynamic polarizer

Multiple-mode excitation in spin-transfer nanocontacts with dynamic polarizer

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