Fully epitaxial Fe „ 110 ... / MgO „ 111 ... / Fe „ 110 ... magnetic tunnel junctions : Growth , transport , and spin filtering properties

transport, and spin filtering properties J. O. Hauch, M. Fonin, M. Fraune, P. Turban, R. Guerrero, F. G. Aliev, J. Mayer, U. Rüdiger, and G. Güntherodt II. Physikalisches Institut, RWTH Aachen University, 52056 Aachen, Germany Fachbereich Physik, Universität Konstanz, 78457 Konstanz, Germany Equipe de Physique des Surfaces et Interfaces, Institut de Physique de Rennes, CNRS UMR 6251, Université de Rennes, Campus de Beaulieu, Bâtiment 11 C, 35042 Rennes Cedex, France Departamento de Fisica de la Materia, C-III, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain Gemeinschaftslabor für Elektronenmikroskopie, RWTH Aachen University, 52056 Aachen, Germany

The crucial feature of these MTJs is the conservation of spin and symmetry of the tunneling electrons.6][7]10 In turn, manifold tests of the symmetry-enforced high spin-polarized tunneling current in fully epitaxial or highly textured MTJs have been undertaken using diverse ferromagnetic ͑FM͒ electrodes and MgO barriers.For example, the use of metallic nonmagnetic or FM spacer layers at the FM/MgO interface resulted in spin filtering due to spin-polarized quantum well states 12,13 or due to spinsymmetry gaps. 14On the other hand, interfaces with a symmetry mismatch have been found to yield symmetrydependent filtering for tunneling electron spins at energies high enough to overcome the tunnel barrier. 15n this work we study the spin filtering in fully epitaxial Fe͑110͒/MgO͑111͒/Fe͑110͒ MTJs with the dense but grainy MgO layer.Epitaxially grown ͑110͒-oriented bcc Fe layers are advantageous because of a large in-plane uniaxial magnetocrystalline anisotropy, which yields well defined switching states of the magnetization. 16Moreover, they exhibit a high spin polarization value of about −80% measured near the Fermi energy ͑E F ͒ using spin-and angle-resolved photoelectron spectroscopy ͑SP-ARPES͒ with h = 21.2 eV. 178][19] For the spinpolarized tunneling, however, the ⌺ 1↓ and ⌺ 1↑ spin states along the ͓110͔ direction of Fe, both crossing E F near the N point of the Brillouin zone 18,19 ͑see Fig. 1͒, may be of relevance because of their large spatial extent.This special situation allows for a twofold test of ͑i͒ the symmetry-dependent spin filtering effect of s-like tunnel electron states 3 and ͑ii͒ the distinction between the spin-polarized states contributing to photoemission and to tunneling.For our fully epitaxial Fe͑110͒/MgO͑111͒/Fe͑110͒ MTJs we obtain TMR values of 54% at 1.5 K and 28% at RT.This evidences a reduced spin filtering by the ⌺ 1↑ spin states at E F near the N point, which are partially compensated by the ⌺ 1↓ states crossing E F a͒ Author to whom correspondence should be addressed.Electronic mail: mikhail.fonin@uni-konstanz.de.All epitaxial Fe/MgO/Fe MTJs were grown by MBE under ultrahigh vacuum conditions.A 25 nm thick Fe͑110͒ layer ͑bottom electrode͒ was deposited on Mo͑110͒ / Al 2 O 3 ͑112 ¯0͒ at RT and subsequently annealed at 640 K. MgO layers were deposited directly from bulk MgO by electron beam evaporation onto the Fe͑110͒ layer kept at RT.As magnetic top electrode a 10 nm thick epitaxial Fe͑110͒ film was deposited at RT.All samples were covered by a 5 nm thick Au cap layer to prevent oxidation during ex situ handling and measurements.The multilayer structure was patterned using a combination of optical and electron beam lithography and Ar + ion milling.All transport measurements were performed on epitaxial MTJs of two different sizes ͑10ϫ 10 m 2 and 20ϫ 20 m 2 ͒ using an ac four point method.
Figure 2͑a͒ shows a scanning tunneling microscopy ͑STM͒ image of the atomically flat Fe͑110͒ surface of the bottom electrode.The ͑1 ϫ 1͒ low-energy electron diffraction ͑LEED͒ pattern of twofold symmetry ͓inset in Fig. 2͑a͔͒, typical for the bcc Fe͑110͒ surface, confirms its high quality.The surface morphology of a 1 nm thick MgO film deposited at RT on Fe͑110͒ is shown in Fig. 2͑b͒.The MgO grains visible in the STM image give clear evidence of a threedimensional growth mode of MgO.After the deposition of 3 nm of MgO a well-ordered hexagonal ͑1 ϫ 1͒ LEED pattern corresponding to MgO͑111͒ was observed ͓inset of Fig. 2͑b͔͒.Because of the grainy structure the MgO barrier thickness for all MTJs has been chosen rather thick, i.e., nominally 4 nm.A 10 nm thick epitaxial Fe͑110͒ top electrode was deposited at RT onto the MgO layer.The epitaxial relationship of the MgO͑111͒/Fe͑110͒ system as well as the growth of the Fe͑110͒ top electrode were monitored using reflection high-energy electron diffraction ͑not shown͒.
A typical cross section transmission electron microscopy ͑TEM͒ image of the epitaxial MTJ is shown in Fig. 3͑a͒.The interfaces between the different layers are clearly discernible as abrupt and without any indication of interdiffusion.The MgO barrier shows no indication of defects ͑pinholes͒ in spite of the three-dimensional growth mode on Fe͑110͒.Thus the MgO layer is a dense, though grainy insulating barrier.Figure 3͑b͒ shows a typical hysteresis loop of the epitaxial MTJ with the magnetic field applied parallel to the ͓001͔ in-plane direction ͑magnetic easy axis͒ of the Fe͑110͒ electrodes.The switching of the first layer at approximately 65 Oe already results in a reversal of the magnetization.Therefore, the thicker ͑bottom͒ electrode has the lower coercivity.The magnetic hard, thinner ͑top͒ electrode switches at approximately 145 Oe.A typical temperature dependence of the tunneling resistance R͑T͒ of the epitaxial MTJ is presented in Fig. 3͑c͒.The tunneling resistance decreases from 1.2 M⍀ at 4.2 K to 700 k⍀ at 250 K, showing an insulatorlike behavior of the MTJ.This gives proof of a pinholefree barrier. 20The current-voltage ͑I-U͒ characteristics of the epitaxial MTJ measured at 4.2 K show a typical nonlinear dependence ͓see Fig. 3͑d͔͒.The tunneling barrier height ⌰ and barrier width d estimated by a numerical fitting procedure using the Simmons model 21 are ⌰ = 2.0 eV and d = 1.5 nm, respectively.The estimated barrier width is significantly smaller than the nominal 4 nm thick MgO layer.We suggest that this has to do partly with the grainy structure of the MgO͑111͒ layer allowing for some limited intrusion of Fe into the oxide layer, leading to an on average smaller than nominal MgO barrier thickness.An effectively reduced ͑about halved͒ barrier thickness may also be accounted for by resonant tunneling via localized states dominantly in the middle of the barrier. 22igure 4 shows the TMR curves measured for a micro- structured MTJ ͑20ϫ 20 m 2 ͒.The TMR is defined as ͑R AP -R P ͒ / R P with tunneling resistances for antiparallel ͑R AP ͒ and parallel ͑R P ͒ magnetization directions.The epitaxial MTJ shows a TMR ratio of about 54% at 1.5 K, which drops gradually with increasing temperature, yielding about 28% at RT. On the basis of the simple Jullière model 23 we determined from the TMR value at 1.5 K a rough estimate of the absolute value of the tunneling spin polarization of the Fe͑110͒ electrodes of about P t = 46.1%.This value is of the order of P t =40% of polycrystalline Fe, 24 but way below P t =74% of annealed, ͑100͒-textured Fe, yielding 290% TMR at low temperature. 7The magnitude of our 54% TMR points to a reduced spin filtering by the ⌺ 1↑ spin states, crossing E F nearest to the N point.The symmetry-dependent spin filtering by these states will be partially compensated by the ⌺ 1↓ states, crossing E F less close to the N point with a roughly 50% smaller group velocity.We argue that the ⌺ 1↑ states at E F and nearest to the N point have about half the density of states compared to the ⌺ 1↓ states. 19A crude estimate of the spin polarization due to the ⌺ 1↑ and ⌺ 1↓ states near E F yields ͉P t ͉Х33% in reasonable agreement with P t estimated by the Jullière model above.
On the other hand, under our experimental conditions of the SP-ARPES experiment 17 the allowed electronic states contributing to the photoemission spin polarization along the ͓110͔ direction may be of ⌺ 1 , ⌺ 3 , and ⌺ 4 symmetries, 18,25,26 as derived from the ⌫ 25↓ Ј and ⌫ 12↑ states.For h = 21.2 eV photon energy and an energy resolution of 100 meV a dominant contribution to the photoemission spin polarization near E F arises from the ⌺ 1↓ states closer to the zone center ͑see Fig. 1͒. 17,18The latter account mainly for the −80% spin polarization in SP-ARPES, but do not play a role for the spin-polarized tunneling.A possible small contribution may be due to the ⌺ 3↓ states, 18 which are crossing E F close to the zone center ͑see Fig. 1͒.The latter, however, are less favored for tunneling because of their high angular momentum. 27espite the fact that for Fe͑110͒ both the ⌺ 1↓ and ⌺ 1↑ states are crossing E F , there is a slight asymmetry in spinpolarized tunneling conductance because of some prevailing d-character of the minority states at E F compared to the prevailing s-character of the majority states. 27The tunneling from these states of Fe͑110͒ with k ʈ 0 may in principle be impeded by the 4 nm thick MgO barrier.However, an effectively smaller barrier thickness ͑see above͒ may alleviate this problem.However, tunneling through the MgO͑111͒ barrier via its ⌳ 1 states 28 with ͑s , p , d͒-character, 27,28 starting from symmetry-mismatched ⌺ 1 states of Fe͑110͒, may present another obstacle.Hence the transmission probability or conductance of the majority electrons will be severely attenuated as compared to the case of the Fe͑100͒/MgO͑100͒/Fe͑100͒ MTJs.The attenuation will be determined by the imaginary part of the wavevector along the ͓111͔-direction of the complex band structure in the gap region of MgO.This may explain the rather small TMR values found in our work.
In conclusion, the ͑110͒ orientation of the Fe electrodes in epitaxially MBE grown Fe͑110͒/MgO͑111͒/Fe͑110͒ MTJs appears as an intricate test case for TMR in terms of symmetry-enforced spin filtering of off-normal tunneling contributions.For the ͑110͒ orientation of Fe, the spin filtering effect in tunneling of ⌺ 1 majority spin electrons with s-character is partially compensated by the states of the same symmetry, crossing E F for the minority channel.However, the roughly 50% smaller group velocity of the latter states favors the tunneling from ⌺ 1↑ states.A reduced effective tunnel barrier thickness and resonant tunneling may enable the required off-normal tunneling.However, the ͑111͒ orientation of the MgO barrier with symmetry-mismatched ⌳ 1 states with s , p , d-character may severely attenuate the transmission probability compared to the ͑100͒ orientation.In this light the measured TMR values as high as 54% at 1.5 K ͑28% at RT͒ are somewhat unexpected.The difference in spin polarization in tunneling and SP-ARPES has been identified as due to, respectively, near-zone-boundary s-type ⌺ 1↑ states at E F , partly compensated by d-like ⌺ 1↓ states, and due to mainly near-zone-center ⌺ 1↓ states slightly below E F .This work was supported by BMBF, Grant No. FKZ 13N7329.Work in Madrid was supported by Spanish MEC ͑MAT2006-07196͒.The authors would like to thank A. Bückins for the help with the TEM measurements.

FIG. 1 .
FIG. 1. ͑Color online͒ Schematic spin-split band structure of Fe along the ͓110͔, ⌫-⌺-N direction adapted according to Refs.18 and 19; spin-up ͑-down͒ states: solid ͑dashed͒ lines.The free electron final band is shifted by E = 21.2 eV with its energy displayed along the top scale.