APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 23 6 DECEMBER 1999

Nanoconstriction microscopy of the giant magnetoresistancein cobalt/copper spin valves

S. J. C. H. Theeuwen, J. Caro, K. P. Wellock, and S. RadelaarDelft Institute of Microelectronics and Submicron Technology (DIMES), Delft Universityof Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

C. H. Marrows and B. J. HickeyDepartment of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom

V. I. KozubA. F. Ioffe Physico-Technical Institute, St. Petersburg 194021, Russian Federation

~Received 10 August 1999; accepted for publication 8 October 1999!

We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance~GMR! of only a few Co domains. The measured data show strong device-to-device differences ofthe GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratiodecreases with increasing bias current. For one particular device, this is accompanied by thedevelopment of two distinct GMR plateaus, the plateau level depending on bias polarity and sweepdirection of the magnetic field. We attribute the observed behavior to current-induced changes of themagnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.© 1999 American Institute of Physics.@S0003-6951~99!01049-9#

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Currently, applications of the giant magnetoresistan~GMR! are realized, particularly in read heads for hard dis1

and magnetoresistive random access memories~MRAMs!.2

These applications are based on the spin valve. Thistrilayer structure, with a normal metal~NM! layer sand-wiched between two ferromagnetic metal~FM! layers.3 Thespin-valve resistance depends strongly on the magnetic-fiinduced relative orientation of the magnetizations of the Flayers. This GMR arises predominantly from spin-dependscattering of the electrons at the interfaces between theand FM layers.1

So far, most spin-valve studies were performed on lastructures, up to millimeters wide. Since the domain size oFM film ranges from hundreds of nanometers to severalcrometers, the GMR arises from averaging over manymains. Currently, emphasis shifts to spin valves patterinto micrometer and submicrometer structures, enteringregime of only a few domains. This has led, for examplethe direct observation4 of the correlation of domain structurand GMR and to very small memory elements for MRAM2

A very attractive way to enter the few-domain regimeto use a point contact to a macroscopic spin valve. Succontact is a nanoconstriction between a metallic electrand the spin valve. When a current flows through the devthe current density peaks strongly in the constriction~diam-eterd510– 50 nm!. As a result, in a resistance measurema hemispherical region of the spin valve is probed, in scomparable to the constriction diameter@Fig. 1~a!#. In thisregion, only two domains are expected to contribute toGMR, one from each FM layer. Due to the minute laythickness~typically, several nanometers!, the current flowsalmost perpendicularly through the layers~CPP geometry!,as in the multilayer pillars of Gijs, Lenczowski, anGiesbers5 and in the spin valves sandwiched between supconducting electrodes of Prattet al.6 In this letter, we report

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GMR measurements on Co/Cu spin-valve point contaThese devices work as a microscope, revealing effectsonly a few domains at the constriction. Further, the poicontact geometry is very suitable to search for effectselectron-spin transfer to the FM layers,7 either as predictedby Slonczewski8 or by Kozubet al.9

We fabricate the spin valves on silicon membranes.10 Onthe lower membrane face, a Cu~2 nm!/Co~3 nm!/Cu~3.5 nm!/Co~300 nm!/Cu~300 nm! structure is sputtered@Fig. 1~a!#.

FIG. 1. Schematic cross-section of a spin-valve point contact~a! and GMRof a 0.38V contact, measured atT54.2 K with an in-plane field, forI ac

5100mA ~b! and I bias510 mA ~c!.

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3678 Appl. Phys. Lett., Vol. 75, No. 23, 6 December 1999 Theeuwen et al.

The 2 nm Cu layer is a buffer and the 300 nm Cu cap shuany current-in-plane~CIP! series GMR of the trilayer film.11

The device is completed by etching a nanohole in the mbrane and deposition of Cu on the upper face, to fill the hand form the counter electrode. The different Co thicknescause magnetization reversal of the layers at different fieWe make identical layered films~but with a 3 nm cap! forCIP control measurements. The point-contact resistanmeasured in a four-wire geometry, are in the range0.3–8V.

These spin valves exhibit a clear GMR. We observe sstantial device-to-device differences in the GMR curve,taining a maximum GMR ratio of 2% (ratio5DR/Rsat; DR5R02Rsat, R05maximum resistance,Rsat5saturated resistance!. Step-like transitions in the GMR often occur. Theobservations agree with the absence of averaging, aspected when only a few domains are probed. In general,GMR ratio decreases with increasing bias current. Contrto the point contacts, the CIP films do not exhibit a GMThis indicates that 300 nm Co is thick enough to shuntCIP GMR and that, consequently, the point-contact GMarises from the constriction region, where the current mtraverse the Co/Cu interfaces. The CIP films do showanisotropic magnetoresistance~AMR!.12 The AMR of thepoint contacts is negligible (AMR,0.1%). Its dependencon the field orientation correlates in an AMR fashion wthe average current direction through the constriction~whichis along the constriction axis!, from which it thus must origi-nate. The point-contact spectrad2I /dV2(V) of the devices,i.e., the voltage dependence of the second derivative ofI –V curve, are featureless. This is unlike spectra of balliscontacts,13 which show phonon peaks, and unlike spectramultilayer contacts of Tsoiet al.,14 which show spin-waveexcitations. We conclude that electron transport in our ctacts is diffusive or between ballistic and diffusive.

This letter concentrates on one specific contact whnicely illustrates the absence of averaging over the propeof many domains. This contact hasR50.38V, which corre-sponds to a diameter of 50 nm,15 and a 1.2% GMR ratio a4.2 K. This ratio is dominated by the contribution of ththick Co layer toRsat. After correction, an upper bound othe intrinsic ratio is estimated at 5%. Figure 1~b! shows theGMR curve for the in-plane field geometry, measured aT54.2 K and with low-level ac excitation (I ac5100mA).This curve is much narrower than the perpendicular-ficurve~not shown!, in agreement with the shape anisotropythe layers. Coming from high field, the in-plane curve shoa gradual increase beforeH50, a hysteretic maximum closto H50, and a steep decrease to saturation atm0uHu56 mT. This differs from the GMR of amulti-domain areaof a spin valve, which yields a maximum afterH50 due tohysteresis of themulti-domain system. The curve of Fig. 1~b!reflects specific magnetization properties of just a feweven two domains. The curve suggests a gradual changthe magnetization of one domain to a preferential directwhen approachingH50 and an abrupt switching of the magnetization to a completely aligned state atm0uHu56 mT, in-volving the other domain. The GMR in Fig. 1~b! occurs at amuch lower field than those of Myerset al. for a very similardevice,7 fabricated with a pre-etched nanohole. This meth

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contrary to ours, for hole sizes comparable to or larger thalayer thickness, leads to strong topography at the hole adeposition of the Cu electrode. Thus, the layers are distoraffecting the field scale.

At elevated bias currents the picture becomes more ras illustrated in Fig. 1~c! for I bias5110 mA ~positive cur-rent: electrons flow from the thin Co layer to the thick one!.Two pronounced resistance plateaus develop, their heighpending on the field-sweep direction and current polarThe upper plateau occurs when coming from positiveH andbiasing with positiveI bias or from negativeH and applyingnegative I bias. The lower plateau appears for a reversesweep direction or opposite current polarity. Thus, onechoose the relative magnetization direction of the domawith the magnetic history and the current polarity. The gbetween the upper and lower plateau first opens andcloses when the bias current is increased from 100mA to 100mA, reducing the GMR ratio by a factor of 6. This behaviis plotted in Fig. 2, whereR0 is the plateau level. The plottebias range corresponds to current densities up to3109 A/cm2, i.e., 100 times higher than values in GMR reheads.3 When converted to the GMR ratio, the lower branof R0 in Fig. 2 reaches its final level of 0.2% at 20 mA, whithe upper branch decreases approximately linearly acroswhole bias range. The plateaus are stable for many hoWe never observed a spontaneous transition between thFurther, the side of the gradual change of the GMR pebroadens with increasing current, while the steep side isaffected. In Fig. 3 this is illustrated for one biasing asweeping combination.

Joule heating might be responsible for the above behior, in particular, the decrease of the GMR peak with incre

FIG. 2. Dependences ofR0 andRsat on bias current, for the combinations osweep direction and current polarity, for the in-plane field geometry anT54.2 K. The opening and closing of a gap between plateaus can beLines guide the eye.

FIG. 3. GMR for different positive currents.T54.2 K. The arrow indicatesthe sweep direction. For high currents, the GMR is reduced and broadon one side. The inset shows the temperature dependence of the GMR

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3679Appl. Phys. Lett., Vol. 75, No. 23, 6 December 1999 Theeuwen et al.

ing current and its broadening. To check this, we measthe temperature dependence~1.5–293 K! of the zero-bias(I ac5100mA) GMR curve, yielding a constant GMR ratiup to 100 K and a decrease to 0.6% at 293 K~inset, Fig. 3!.Further, with increasing temperature the gradual side ofGMR curve is hardly affected, while the steep side shtowardsH50. This is contrary to the bias dependence,that Joule heating is clearly excluded. The zero-field retanceR0(T54.2 K, I bias) decreases in the range 0–50 mwhereafter it increases~Fig. 2!. Again, this differs from themeasuredR0(T,I ac5100mA), which increases monotonously. The measured bias dependenceRsat(T54.2 K, I bias)also shows a monotonic increase. We attribute the lattercrease to extra scattering due to phonon and magnon creenabled by a bias-induced nonequilibrium electrodistribution function. The resulting resistance increase acontributes toR0(T54.2 K, I bias). However, we argue thabelow 50 mA R0(T54.2 K,I bias) is dominated by anotheeffect, viz. current-induced magnetization changes. Tharise from the self-field~i.e., the field generated by the curent according to Ampe`re’s law! or from spin transfer to theFM layers. Spin transfer leads to coherent rotation ofmagnetization,8 resulting in switching of the resistance,7 orto incoherent emission of magnons,9 as discussed below.

We estimate the self-field by approximating the pocontact by a hyperboloid of revolution.16 For I 55 mA, themaximum ofm0Hself in the thin ~thick! Co layer is 24~12!mT. This is comparable to the field scale of the zero-bGMR. Thus, the self-field should affect the GMR for mostthe bias range. For example, it should broaden the Gpeak. Broadening is indeed observed for the gradual sidthe peak, but the steep side is unaffected. Apparently, oand stronger driving forces dominate certain features ofGMR, for example, a uniaxial anisotropy inducedfabrication-related effects. This, combined with the self-fiewhich depending on the bias polarity, can guide the magtization in either direction of the anisotropy axis and can grise to plateaus. With increasing bias, the self-field will teto impose a vortex character upon the magnetization of bCo layers, eventually leading to almost congruent vorticethe Co layers, and consequently, a reduction of the GMwhich is what we observe.

Spin transfer influences the thin Co layer much mothan the thick Co layer, since the latter is strongly exchacoupled to its surroundings.8 According to the coherent rotation model,8 the magnetization of the thin layer, and thus tmagnetoresistance, should switch abruptly at a thresholdrent. Our contacts do not show switching, apparently excling this mechanism. Rather, the GMR evolves gradually wbias. This can be explained9 by incoherent emission of nonequilibrium magnons, giving rise to a magnon ‘‘hot spot’’the thin Co layer, characterized by an effective magnon teperatureTm . Since the probability of electron–magnon cration ~a spin-flip process! is proportional to the density ofinal electron states, which in turn is spin dependent,Tm

depends on the current polarity and the relative orientatiothe magnetizations of the layers, angles close to 0° or 1favoring high Tm . Further,Tm is proportional to the biasDeviations of the magnetization of the thin layer from a prerential direction are assumed to be thermally activated

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Tm . Apart from a gradual decrease of the GMR with currethis model also gives a current-direction-dependent GMplateau. However, the plateau level does not depend onsweep direction, contrary to the measurements. This incates that the model in its present form does not catchmagnetic ingredients of the spin valve. In addition, a coplete treatment should include the interplay of spin transand the self-field effect. Similar to the Slonczewski’s sptransfer, also the spin-transfer effect put forward here canused to set the bits of a MRAM, but with the additionadvantage of write capability for parallel and antiparallel oentations of the magnetizations.

In conclusion, using Co/Cu spin-valve point contacts,have studied the CPP GMR of only a few domains. Duethe absence of averaging over many domains, the Gcurve shows strong device-to-device differences, the mmum GMR ratio being 2%. For one device we studieddetail the dependence of the GMR curve on the bias curup to 100 mA and at a temperature up to 293 K. The GMratio decreases with increasing bias, while two distinct GMplateaus develop aroundH50. We attribute this behavior tocurrent-induced changes of the magnetization, involving stransfer due to incoherent emission of magnons and self-fieffects, while we exclude Joule heating.

The authors gratefully acknowledge G. E. W. Bauer,P. van Gorkom, and T. M. Klapwijk for stimulating discussions and N. N. Gribov for expert support in preparingmembranes.

1See the collection of articles in IBM J. Res. Dev.42, ~1998!.2S. Tehrani, E. Chen, M. Durlam, M. DeHerrera, J. M. Slaughter, J. Sand G. Kerszykowski, J. Appl. Phys.85, 5822~1999!.

3For a review on spin valves, see B. Dieny, J. Magn. Magn. Mater.136,335 ~1994!.

4X. Portier, E. Yu. Tsymbal, A. K. Petford-Long, T. C. Anthony, and J.Brug, Phys. Rev. B58, R591~1998!.

5M. A. M. Gijs, S. K. J. Lenczowski, and J. B. Giesbers, Phys. Rev. L70, 3343~1993!.

6W. P. Pratt Jr., S. D. Steenwyk, S. Y. Hsu, W.-C. Chiang, A. C. SchaeR. Loloee, and J. Bass, IEEE Trans. Magn.MAG-33, 3505~1997!.

7E. B. Myers, D. C. Ralph, J. A. Katine, R. N. Louie, and R. A. BuhrmaScience285, 867 ~1999!.

8J. C. Slonczewski, J. Magn. Magn. Mater.159, L1 ~1996!; 195, L261~1999!.

9V. I. Kozub et al. ~unpublished!.10N. N. Gribov, S. J. C. H. Theeuwen, J. Caro, E. van der Drift, F.

Tichelaar, T. R. de Kruijff, and B. J. Hickey, J. Vac. Sci. Technol. B16,3943 ~1998!.

11K. P. Wellock, S. J. C. H. Theeuwen, J. Caro, N. N. Gribov, R. P. vGorkom, S. Radelaar, F. D. Tichelaar, B. J. Hickey, and C. H. MarroPhys. Rev. B60, 10291~1999!.

12T. R. McGuire and R. I. Potter, IEEE Trans. Magn.MAG-11, 1018~1975!.

13P. A. M. Holweg, J. Caro, A. H. Verbruggen, and S. Radelaar, Phys. RB 45, 9311~1992!.

14M. Tsoi, A. G. M. Jansen, J. Bass, W.-C. Chiang, M. Seck, V. Tsoi, aP. Wyder, Phys. Rev. Lett.80, 4281~1998!.

15For our layersrCo /rCu'8. Thus, neglecting interface resistances, thelayers dominate the resistance, so that to a good approximation the rtance is the Maxwell resistance:R'rCo/4a. The value rCa(4.2 K)540 nVm then gives 2a'50 nm, in agreement with the lithographic siz

16We apply Ampe`re’s law to the hyperboloid geometry@L. K. J. Van-damme, Appl. Phys.11, 89 ~1976!#, using the oblate spherical coordinateh and j. This yields the~by nature inhomogeneous! self-field m0Hself

5F(h,j)(m0I bias/2pa). The maximum value of the spatial functioF(h,j) in the thin ~thick! Co layers at the constriction is 0.6~0.3!.

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