Poster presented in TNT 2005 "Trends in Nanotechnology", Oviedo, Spain, 29 August-02 September, 2005

Fernández-Rossier, Joaquín

Jacob, David

Palacios Burgos, Juan José

Ballistic spin-valves with Ni nanocontacts

Repositorio Institucional de la Universidad de Alicante

Ballistic spin-valves with Ni nanocontactsD. Jacob, J. Ferna´ndez-Rossier, and J.J. PalaciosDepartamento de Fsica Aplicada, Universidad de Alicante, 03690 Alicante, Spaindavid.jacob@ua.esThe strong sensitivity of the current flow between two ferromagnetic metals (FM’s) sep-arated by a non-magnetic region to the relative orientation of their magnetization vectorsis a fundammental physical phenomenon with a huge impact in the magneto-electronics in-dustry[1]. The quantity characterizing this phenomenon is the so-called magneto-resistance(MR) which is defined as the difference in resistance between antiparallel (AP) and paral-lel (P) relative magnetic orientations of the FM’s normalized to the maximal or minimalresistance of the two orientations. Recently a number of groups [2-4] have studied MR inbreak junction systems, where two sections of a Ni wire are connected through an atomic-size contact. In this arrangement the intermediate region connecting the two bulk FM’shas a different geometry but the same chemical composition, in contrast to GMR [5] andTMR [6] systems. Some groups have obtained extremely high values of MR comparedto GMR[2] while others obtain moderate or even negative values[3,4]. Mainly, two differ-ent mechanisms have been proposed so far to account for the large values of MR, whenobserved: Ballistic domain-wall (DW) scattering [7] and magnetostriction[4]. Previoustheoretical works present mutually conflicting results with methodologies that either usedan oversimplified description of Ni electronic structure [8] or idealized geometries [9]. Thusthe fundamental question of whether MR is dramatically enhanced in atomic sized ferro-manetic contacts due to the presence of a DW remains open and is the subject of thiswork.We present transport calcuations [10] of Ni nanocontacts describing the electronic,magnetic, and atomic structure with ab-initio calculations (Fig. 1 and Fig. 2) using bothLSDA and the hybrid functional B3LYP. Our results lead us to conclude that intrinsicballistic MR is certainly not large in Ni nanocontacts [11]. Finally, we report on ab-initio calculations of oxidized Ni nanocontacts where an oxygen atom forms an atomicbridge between the two Ni tip atoms. Our calculations indicate that the Ni-O-Ni bridgeacts as an insulating barrier which strongly suppresses the current via the Ni s-orbitals.This nanoscale tunneling union should exhibit enhanced MR compared to the pure Ninanocontact and could thus serve as a nanoscopic ballistic spin valve.References:[1] S. A. Wolf et al., Science 294, 1488 (2001).[2] N. Garc´ıa et al., Phys. Rev. Lett. 82, 2923 (1999). M. R. Sullivan et al., cond-mat/0404348.[3] H. Oshima and K. Miyano, Appl. Phys. Lett. 73,2203 (1998). T. Ono et al., Appl. Phys.Lett. 75, 1622 (1999). M. Viret et al., Phys. Rev. B 66, 220401 (2002).[4] W. F. Egelhoff et al., J. Appl. Phys. 95, 7554 (2004).[5] M. N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988).[6] M. Julliere, Phys. Lett. 54A, 225 (1975).[7] G. Tatara et al., Phys. Rev. Lett. 83, 2030 (1999).1Oral TNT2005                           29 August - 02 September, 2005                                 Oviedo-Spain  BALLISTIC SPIN-VALVES WITH Ni NANOCONTACTS Poster  [8] G. Tatara and H. Fukuyama, Phys. Rev. Lett. 78, 3773 (1997) A. R. Rocha and S. Sanvito,Phys. Rev. B 70, 094406 (2004). J. Velev and W. H. Butler, Phys. Rev. B 69, 094425 (2004).[9] J. B. A. N. van Hoof et al., Phys. Rev. B 59, 138 (1999). A. Smogunov et al., Surf. Sci. 532,549 (2003).[10] The ab-initio transport calculations on Ni nanocontacts were done with our new ALACANTcode which is a further development in Fortran90 of our previous Gaussian Embedded ClusterMethod (see J. J. Palacios et al., Phys. Rev. B 64, 115411 (2001)). The code and documentationwill be available soon at this web-site: http://www.dfa.ua.es.[12] D. Jacob et al., cond-mat/0406249.Figures:-2 0 2Energy (eV)spin upspin down-2 0 2Energy (eV)00.511.522.53G /(e2 /h)majority spinminority spin(a) (b)Figure 1: Left: Ni nanocontact consisting of two pyramids formed along the (001) directionhaving bulk atomic distances and perfect crystalline order and with the two tip atoms being 2.6A˚ apart. Right: Conductance for P (a) and AP (b) alignment of the electrodes’ magnetizationsfor the idealized geometry on the left calculated with LSDA.0 A˚ 1.5 A˚ 3 A˚0 0.5 1 1.5 2 2.5 3displacement (Å)00.511.522.53G(EF)/(e2/h)majority channel Pminority channel Pup channel APdown channel APFigure 2: Left: Ab-initio simulation of the breaking of a Ni nanocontact. We start with a slightlycompressed contact compared to Fig. 1 and displace the outer planes in steps of 0.5 A˚ and performab-initio structural relaxations of the inner atoms at each step. Right: The conductances of thetwo spin channels for the P and AP alignement of the electrodes’ magnetizations as a function ofthe stretching of the Ni nanocontact.2Oral TNT2005                           29 August - 02 September, 2005                                 Oviedo-Spain  Poster  

Ballistic spin-valves with Ni nanocontacts

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