AVS2011 Session MI-WeM: Fundamental Problems in Magnetism

In this talk I will present results from a systematic study to understand the role of LiF, Al₂O₃, and MgO tunnel barriers in organic spin-valves. The overall aim of this work is to better quantify the degree of spin-polarized electron injection and extraction at ferromagnetic metal/organic semiconductor interfaces. In general we find that spin-valves made with two ferromagnetic transition metals has a positive magnetoresistance. However, when one of the ferromagnetic metals is exchanged with La_0.67Sr_0.33MnO₃, the sign of the magnetoresistance is inverted. In addition the spin-polarized tunneling measurements the structural, electronic, and magnetic properties of these interfaces have been thoroughly investigated by cross-sectional transmission microscopy, photoelectron spectroscopy, and polarized neutron reflectometry. These results will be compared to other findings in the literature in order to summarize the current status of spin-polarized electron transport across organic semiconductor/insulator/ferromagnetic metal interfaces.

Heusler compounds are a remarkable class of intermetallic materials with 1:1:1 (often called Half-

Heusler) or 2:1:1 composition comprising more than 1500 members [1]. Today, more than a century after their discovery by Fritz Heusler, they are still a field of active research. New properties and potential fields of applications emerge constantly; the prediction of topological insulators is the most recent example [2]. Surprisingly, the properties of many Heusler compounds can easily be predicted by the valence electron count or within a rigid band approach. Their extremely flexible electronic structure offers a toolbox which allows the realization of demanded but apparently contradictory functionalities based on a virtual lab approach. The subgroup of more than 250 semiconductors is of high relevance for the development of novel materials for energy technologies. Their band gaps can readily be tuned from zero to 4 eV by changing the chemical composition. Thus, great interest has been attracted in the fields of thermoelectrics and topological insulator research. Ternary materials based on multifunctional properties, i.e. the combination of two or more functions such as superconductivity and topological edge states will revolutionize technological applications. The design scheme for topological insulstors from the view point of bands and bond will be presented.

The wide range of the multifunctional properties of Heusler compounds is reflected in extraordinary magneto-optical, magneto-electronic, and magneto-caloric properties. Tetragonal Heusler compounds Mn₂YZ as potential materials for STT applications can be easily designed by positioning the Fermi energy at the van Hove singularity in one of the spin channels [3]. A high calculated magnetic anisotropy energy (MAE) is the sufficient condition for a material with perpendicular magnetocrystalline anisotropy (PMA). Materials with saturation magnetizations of 0.2 – 4.0 µ_B, high Curie temperatures of 380 – 800 K, high spin polarizations, PMA, and required lattice constant matching with MgO can be realized with ferri- or ferromagnetic Heusler-related compounds. Such materials are strongly recommended for the spin transfer torque magnetic random access memory (STT-MRAM) data storage and the spin torque oscillators (STO) for telecommunication. Additionally the first spin gapless semiconductor is realized in Mn₂CoZ.

Core-shell nanoparticles have been receiving an increasing attention in order to practically employ magnetic clusters in devices [1]. The shell permits to protect the magnetism of the core, which readily oxidize in environmental conditions. Moreover, the shell can donate to the particle new chemico-physical functionalities which, combined with magnetism, permit to obtain multifunctional systems. Cobalt self-organized supported nanodots are promising candidates for applications as very high density magnetic recording media. However, it is necessary to improve the magnetic stability of such small nanostructures against thermal excitations. The capping with a non-magnetic metal of magnetic nanostructures induces different interfacial phenomena which together lead to a modification of the magnetic behavior of the system.

In the present work, the growth and the magnetism of Co/Pt core-shell have been studied. Pt has been deposited as over layer on Co self-organized on Au(111) template. The structural properties have been addressed by combining Scanning Tunneling Microscopy (STM) and Surface Extended X-ray Absorption Fine Structure (SEXAFS) measurements with molecular dynamics calculations. Magneto-Optic Kerr Effect (MOKE) and X-ray Magnetic Circular Dichroism (XMCD) have been coupled in order to identify the main magnetic phenomena acting at the Co/Pt interface. In the submonolayer regime, Pt forms metal rims around Co nanodots, and the Co magnetic anisotropy decreases. On the other hand, if more than one monolayer of Pt is deposited, the Co dots are completely covered and their magnetic anisotropy is enhanced. Furthermore, the Pt capping is found to have a minor effect of the cobalt magnetic moments. By changing the deposition conditions and by comparing the effect of Pt and Au capping [2], we identified three principal phenomena at the Co/Pt interface: inter-mixing, magnetoelasticity and band hybridization. Our results indicate that the principal one is band hybridization, which is responsible for the observed increasing of magnetic anisotropy. Intermixing and magnetoelasticity have rather the opposite effect and tend to decrease the magnetic anisotropy energy. The knowledge of the interplay between these different phenomena is fundamental in order to tune the magnetic properties of nanoparticles for precise applications, from data storage to biomedical research.

[1]A. Lu, E. Salabas, and F. Schüth, Angew. Chem. Int. Ed. 46, (2007) 1222.

[2] Y. Nahas, V. Repain, C. Chacon, Y. Girard, J. Lagoute, G. Rodary, J. Klein, S. Rousset, H. Bulou, and C. Goyhenex, Phys. Rev. Lett., 103 (2009) 067202.

For fundamental and technological reasons materials with a spin-split electronic band structure in proximity of the Fermi level are highly attractive. The possibility of separately manipulating the two spin channels introduces novel functional behaviors without counterpart in the corresponding spin-degenerate systems. A promising approach in this field consists in the exploitation of the spin-orbit interaction, which couples spin and orbital degrees of freedom. The Rashba interaction offers particularly interesting perspectives: at the surface of a crystalline material the breaking of the full translational symmetry gives rise to an effective (Rashba) electric field which splits in k-space the valence electrons with opposite spin orientations. For heavy elements, such as Bi, the Rashba interaction results in spin-polarized surface bands, detectable in angle-resolved photoemission experiments [1]. In this talk various cases of such systems will be presented. Bi-Ag(110) surface alloy allows to study the anisotropy of splitting induced by the anisotropic electric fields of the (110) – surface. The system Bi-Ag(111)/ Fe(110) provides direct evidence of the interplay between the Rashba splitting of the Bi-Ag alloy and the ferromagnetic splitting of the Fe bands.

[1] Yu. M. Koroteev, G. Bihlmayer, J. E. Gayone, E. V. Chulkov, S. Blügel, P. M. Echenique, and Ph. Hofmann, Phys. Rev. Lett. 93, 046403 (2004); T. Hirahara, T. Nagao, I. Matsuda, G. Bihlmayer, E. V. Chulkov, Yu. M. Koroteev, P. M. Echenique, M. Saito, and S. Hasegawa, Phys. Rev. Lett. 97, 146803 (2006).