Large magnetoresistance in Fe Õ MgO Õ FeCo „ 001 ... epitaxial tunnel junctions on GaAs „ 001 ...

We present tunneling experiments on Fe ~001!/MgO~20 Å!/FeCo~001! single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on ~001!-oriented Fe/amorphous-Al 2O3 /FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface— Fe~001! in this case—but depends on the actual electronic structure of the entire electrode/barrier system. ©2001 American Institute of Physics. @DOI: 10.1063/1.1404125 #

The magnetoresistance of magnetic tunnel junctions ͑MTJs͒ 1 is of uncontested interest for important applications with, in particular, promising perspectives for the fabrication of nonvolatile memories ͑Magnetic RAM͒. 2 Up to now, most studies have been performed on MTJs with a layer of amorphous alumina as insulating barrier between the ferromagnetic electrodes, yielding large and reproducible tunneling magnetoresistance ͑TMR͒. However, from a fundamental point of view, i.e., the understanding of the physics of spindependent tunneling, a transport study through an amorphous insulator is hardly accessible in a theoretical approach. Most numerical calculations of spin-dependent tunneling and TMR have been developed for single crystal MTJs such as Co/Al 2 O 3 /Co͑100͒, 3 Fe/ZnSe͑100͒, 4 or Fe/MgO͑100͒. 5,6 All emphasize that a correct depiction of the spin-dependent tunneling properties of epitaxial MTJs must transcend the simple potential barrier image and take into account the interplay of electronic structure between metal and insulator. A test of these models can be performed on single-crystal epitaxially grown structures. Towards this end, much work has been expended to characterize the growth and electrical behavior of ultrathin MgO layers. 7 In this letter, we present experimental results showing large TMR values in Fe͑001͒/ MgO͑001͒/FeCo͑001͒ epitaxial tunnel junctions.
FeCo/MgO/Fe epitaxial structures were grown on GaAs͑001͒ in a combined sputtering/laser ablation system with a base pressure of 2ϫ10 Ϫ9 mbar as described elsewhere. 8 We use a MgO buffer layer as an interdiffusion barrier with good electrical insulation characteristics. 9 This prevents the incorporation of As from the substrate into the Fe bottom electrode-an important precaution since, in a subsequent step, the MgO barrier is grown at 400°C in order to obtain good crystallinity. This MgO buffer layer also offers an appropriate symmetry and lattice match for the epitaxy of Fe on top, with the well known orientation relation Fe͑100͓͒001͔//MgO͑100͓͒110͔. Fe 50 Co 50 and Fe layers were deposited from individual Fe and Co targets by triode sputtering with an Ar pressure of 4ϫ10 Ϫ4 mbar and with deposition rates in the range of 0.1-0.3 Å/s. Optimal deposition temperatures were 400°C for the MgO, RT for FeCo top electrode, and RT plus annealing at 400°C for the Fe bottom electrode. This low-temperature deposition and subsequent annealing process leads to an optimal Fe electrode in terms of crystallinity, continuity, and interface sharpness. Figure 1͑a͒ shows RHEED patterns for a typical Fe 50 Co 50 /MgO/Fe structure. Fe 50 Co 50 and Fe layers are bcc structured with sharp diffraction lines for both azimuths. Similar information about epitaxial quality could be concluded from the MgO barrier pattern. Further x-ray diffraction symmetric and asymmetric scans confirm that the whole structure is epitaxial with lattice parameters close to bulk values. To check the continuity of the layers and the sharpness of the interfaces, a specific multilayered Fe/MgO test structure was grown in conditions described above with nominally constant 80-Å-thick Fe metallic layers separated by increasingly thin MgO layers, from 80 to 20 Å. Figure  1͑b͒ presents a ͓110͔ cross section transmission electron microscopy ͑TEM͒ image from this test structure covering a lateral region of about 1 m-dark regions correspond to the Fe layers while the lighter regions reflect the MgO layers. Continuous and good crystal quality MgO films with sharp a͒ Electronic mail: martin.bowen@thalesgroup.com APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 11 interfaces are obtained all the way down to the thinnest oxide layer. The electron diffraction pattern for a selected area of the TEM image shown in Fig. 1͑b͒ illustrates the aforementioned orientation relationship Fe͑100͓͒001͔// MgO͑100͓͒110͔ and is indicative of the crystallinity and high quality of the structure. A Fe͑200 Å͒/MgO͑20 Å͒/FeCo͑250 Å͒ trilayer grown epitaxially onto MgO͑001͒-buffered GaAs͑001͒ was processed by optical lithography. 10 Here we present results obtained on a tunnel junction of diameter 10 m. Transport measurements were performed in four-point voltage source mode (V ϩ ϭFe). The resistance of the Fe electrode is more than 100 times smaller than the junction resistance, thus ruling out any significant contribution from geometrical effects. 11 As shown in Fig. 2͑a͒, the resistance of our MTJ ͑at 10 mV͒ saturates below 50 K and then decreases slowly by about 25% between 50 and 300 K. This is a typical temperature dependence in which intrinsic tunneling transport processes have given way above 50 K to additional thermally assisted processes. The temperature dependence of I(V) curves places 12 the barrier height at ϭ0.9 eV. Simmons' equations 13 yield ϳ1.1 eV for a barrier thickness d ϳ15 Å, in good agreement with previous transport studies using epitaxial MgO͑111͒ by Kiyomura et al., 14 and polycrystalline MgO by Moodera et al., 15 who both report ϳ0.9 eV using this method. This value is lower than half of the 5.5 eV MgO band gap calculated for an ultrathin layer. 5 This difference may be due to metal-induced gap states in the MgO barrier 4 although we cannot completely rule out the presence of stoichiometric and/or thickness inhomogeneities in the insulating film. A somewhat higher value of has been reported by Wulfhekel et al. through STM measurements in which both the MgO and the vacuum barriers are taken into account. 7 Figure 2͑b͒ shows an R(H) cycle taken at 30 K for an applied bias of ϩ10 mV. We find a TMR of ϩ60%, using the definition TMRϭ(R AP ϪR P )/R P . The rise in resistance to the antiparallel ͑AP͒ state in a decreasing field before reaching Hϭ0, and more generally the symmetry of the R(H) curve around Hϭ0, is the result of overmilling into the bottom Fe electrode during the junction mesa definition, thus creating a stray field-induced antiparallel state. As temperature increases, the TMR decreases in almost linear fashion to 27% at 300 K. The low-temperature TMR value is in agreement with expectations from Jullière's expression 16 if P Fe ϭ45% and P FeCo ϭ51%, which are the highest values of P found in recent experiments, are used. 17 It is worth noting that these polarization values were obtained in tunnel junctions with polycrystalline electrodes and an amorphous barrier. Yuasa et al. have studied Fe/Al 2 O 3 /FeCo tunnel junctions 18 with ͑100͒-, ͑110͒-, and ͑211͒-oriented singlecrystal electrodes and amorphous Al 2 O 3 yielding up to 40% TMR for the ͑211͒ orientation but only 13% for the ͑100͒ orientation. Yuasa et al. ascribed this weak TMR to a small ͑7%͒ spin polarization of the calculated density of states ͑DOS͒ at the ͑100͒ surface of Fe. The 60% TMR we find for Fe͑100͒ demonstrates that the spin polarization of tunneling electrons cannot be directly correlated with the spinpolarized DOS of a free metal surface, but depends on the actual electronic structure of the barrier/electrode system and can be quite different for Fe͑100͒/Al 2 O 3 and Fe͑100͒/ MgO͑100͒ interfaces.
In this vein, recent spin-polarized tunneling experiments 19 have emphasized the role of the metal-oxide interface in favoring a particular spin polarization and electronic character of the tunneling current. Co/Al 2 O 3 interfaces result in a positive polarization which, in an oversimplified picture, can be ascribed to a predominant tunneling of s-character electrons due to a specific bonding mechanism at the interface. 3 On the other hand, a Co/SrTiO 3 interface leads to a negative polarization corresponding to the sign of the polarization at E F in the d band of Co. The complex bias dependence of the TMR with a SrTiO 3 barrier also differs from the rapid and symmetric decrease observed in any junction with an Al 2 O 3 barrier. We present a bias dependence study of the TMR obtained for our Fe/MgO/FeCo junction in Fig. 3. The TMR decreases almost symmetrically from a value of 60% at 10 mV to nearly 0 around 1.4 V. This result is confirmed with 2 mV resolved I(V) curves taken in the parallel and antiparallel states. Given the similarity of the symmetric decrease of the TMR with bias for MgO and Al 2 O 3 , we surmise that s-character electron transmission occurs predominantly for a MgO barrier. The lack of d elements in MgO and Al 2 O 3 lends credence to this hypothesis. As corroborating evidence, Butler et al. point out in recently published first principles conductance calculations 5 for Fe͑100͒/MgO͑100͒/Fe͑100͒ trilayers that, despite a strongly negative spin polarization on the interfacial Fe layer, the spin polarization of tunneling electrons should be positive due to wave function symmetry matching in the Fe electrodes and in the MgO barrier. Tunneling conductance in the parallel alignment of Fe͑100͒ electrodes is dominated by the majority spin channel owing to a state (⌬ 1 ) of significant s character which decays much more slowly than other states and does not exist in the minority spin channel. This state will further accentuate the tunneling spin polarization in the parallel state, and the resulting TMR, as barrier thickness is increased-a discrepancy to Jullière's time-proven model which is only governed by the spin polarization of the tunnel junction electrodes. Numerically, the TMR values calculated by Butler et al. are much higher ͑2000% for 20 Å MgO͒ than our experimental value of 60% ͑with a FeCo top electrode͒, as is generally the case when comparing theory with experiment. More interestingly, in future experiments we will aim to confirm the predicted relative increase of the TMR with MgO barrier thickness.
In conclusion, we have observed tunneling MR on Fe͑001͒/MgO͑20 Å/FeCo͑001͒ grown by a combination of laser ablation and triode sputtering onto MgO-buffered GaAs͑001͒. As evidenced by RHEED, x-ray diffraction and TEM analyses, optimized growth conditions result in entirely epitaxial samples of high crystalline quality with flat, sharp interfaces. Transport measurements show 27% TMR at 300 K which increases to 60% at 30 K. We construe from the bias dependence of the TMR, previous experimental results and recent calculations 5 that s-character electrons are predominantly tunneling in the case of a 20 Å MgO barrier.
The Spanish Commission of Science and Technology ͑CICyT͒, Comunidad de Madrid and Acciones Integradas Hispano Francesas ͑Picasso Program͒ are acknowledged for financial support.