Electronic transport and magnetization dynamics in low-dimensional nanostrucutres made of bismuth and ferromagnetic metals

Resumen

The discovery of giant magnetoresistance (GMR) supposed a major breakthrough in Electronics that opened the way to new devices. At this time there was a considerable research effort devoted to the implementation of the phenomenon of anisotropic magnetoresistance (AMR) [1] in read-back heads used to read out the information in hard disk drives, but the AMR effect is small, of the order of a few percent, thus, the GMR effect reported by A. Fert of 50% change in resistance under magnetic field in Fe/Cr multilayers [2] had a very strong impact in recording industry. By December 1997, IBM introduced the first hard disk product using GMR heads and this substitution supposed an immediate growth rate up to 100% per year. GMR opened the door to a new field of science, spin electronics, where both properties of the electron, its charge and its spin are manipulated simultaneously, not only its charge as traditional Electronics does. It is likely that development of spin electronics will trigger a revolution in modern electronics. The second generation of spin-electronic devices is based on magnetic tunnel junctions (MTJs)-devices, such as magnetic random access memories (MRAMs), programmable logic circuits and novel magnetic field sensors. For example, MRAMs have been proposed to replace dynamic RAMs (DRAMs), the currently dominant memory technology [3]. The fundamental advantage of MRAMs is that they are non-volatile as compared to DRAMs that require constant refreshing. From a practical point of view, the ability of spin-polarized current to reverse the magnetization orientation of a nanomagnet (spin-transfer torque switching) should also enable a variety of devices based on magnetic nanostructures [4]. On the other hand, spin-transfer torques not only permit magnetization switching of individual nanomagnets but can generate current-induced domain wall motion in magnetic nanowires [5]. In this thesis we have studied several low-dimensional nanostructures that are promising for applications in Spintronics. A deep understanding of the electronic transport properties and magnetization dynamics of low-dimensional nanostructures is the key for developing new devices that can be useful for applications. In the following the different nanostructures studied in this thesis are described and the main results obtained are presented: - Epitaxial Fe thin films: We have grown epitaxial Fe thin films on single-crystalline MgO(001) substrates by pulsed laser deposition (PLD) and sputtering. We have studied the electronic transport properties (longitudinal resistivity, magnetoresistance and Hall effect) of MgO(001)//Fe(t)/MgO epitaxial heterostructures with Fe thicknesses down to 1.3 nm, finding an increase in the longitudinal resistance for thicknesses below 2 nm due to finite size effects and in its temperature dependence we have observed a minimum at a temperature that strongly depends on the thickness of the Fe ultrathin films. We have focused on the anomalous Hall resistivity, which strongly depends on the thickness of the Fe layer. By comparing our Hall effects measurements with others in similar samples published in the literature, we have found that the thickness and the roughness of the Fe layer are control parameters tuning both the longitudinal and anomalous Hall conductivities, ¿xx and ¿xy, respectively [6]. We indeed have found upon reducing the Fe thickness a crossover from the intermediate region of conductivities where the anomalous Hall conductivity is constant to the dirty region of poorly conducting materials (¿xx < 104 S/cm), where the relation ¿xy ~ ¿xxn with n = 1.66(4) holds in reasonable good agreement with theory. Therefore, a single compound such as bcc Fe can span all regions of different behaviour of the AHE, providing additional support to the unified theory by Onoda et al [7]. - Fe/MgO/Fe multilayers: We have grown Fe(4.5 nm)/MgO(t)/Fe(10.0 nm) heterostructures using a combined PLD-sputtering system. We have focused on the magnetization dynamics properties, that have been measured by vector network analyzer-ferromagnetic resonance (VNA-FMR), of these multilayer stacks and how they vary as a function of MgO barrier thickness, in which we have found a clear dependence of the FMR spectra on the MgO barrier thickness. We have also developed a theoretical model to calculate the dispersion relations of the multilayers. We expect that the ongoing numerical analysis of the experimental magnetic-field dependence of the resonance frequencies according with the theoretical model will provide us with the value of the interlayer exchange coupling for each Fe/MgO/Fe samples studied. - Bi thin films and nanowires: We have performed a systematic investigation of the structural and magnetotransport properties of 300-nm-thick Bi thin films grown on different substrates. The non-linear magnetic-field dependence of the Hall resistivity is qualitatively explained using a simple 2-band model with both, electron and hole bands where concentrations of electrons and holes are not compensated. The carrier density ratio between holes and electrons depends on temperature, varying from an electron majority situation at room temperature to a hole majority situation at low temperature [8]. Furthermore, we have studied the thickness and temperature dependence of the magnetotransport properties of Bi ultrathin films grown on SiO2. The remarkable changes we have observed in the magnetotransport properties as the thickness of the films is reduced below about 30 nm can be interpreted as due to the appearance of surface states which are found to play an important role in determining the transport properties of these ultrathin Bi films [9]. By decreasing the film thickness, we have been able to completely reduce the classical contribution to the MR and to perform a detailed study of the contribution from weak anti-localization (WAL) that has allowed us to extract the thickness and temperature dependence of the diffusion lengths associated with the different scattering processes. The spin-orbit scattering length, LSO, extracted from the perpendicular MR indicates that is an intrinsic parameter of our films, regardless of the thickness of the films and the ratio ¿e/¿SO increases rapidly when decreasing thickness, we associate this increase with the large spin-orbit splitting present in the Bi surface states [10]. To perform four-probe measurements on individual Bi nanowires we have pattern nanoelectrodes on individual Bi nanowires of 100 nm in diameter, achieving ohmic contacts. We have studied the temperature dependence of the perpendicular MR and found two different contributions, the classical and the correction due to WAL, at temperatures below 10 K [11]. - Ferromagnet-superconductor nanocontacts: We have developed an innovative method to create ferromagnet-superconductor nanocontacts by using focused electron/ion beams and in-situ control of the resistance during growth for current-in-plane measurements. This method has been applied to the measurement of current-in-plane Andreev reflection (AR) in nanocontacts between a ferromagnetic Co-based nanodeposit and a superconducting W-based nanodeposit created with a dual beam equipment. From the differential-conductance measurements we have extracted the spin polarization of the ferromagnetic nanodeposit, about 35% and the value of the superconducting gap of the W nanodeposit at the contact region [12]. We have also studied the magnetic-field dependence of the differential conductance in these nanocontacts from which we have obtained the magnetic-field dependences of the superconducting gap and the broadening parameter. By considering both dependences, we have developed a specific approach for determining the density of states of superconductors in high-magnetic fields and validated the results by comparison with the direct measurement of the density of states by scanning tunnelling spectroscopy [13]. References [1]. I. A. Campbell and A. Fert, ¿Transport Properties of Ferromagnets¿ in Ferromagnetic Materials, ed. E. P. Wohlfarth (North-Holland, Amsterdam, 1982), Vol. 3, p. 747. [2]. M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich and J. Chazelas, ¿Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices¿, Physical Review Letters 61, 2472 (1988). [3]. J. C. Scott, ¿Is There an Immortal Memory¿, Science, 304 62, (2004). [4]. J. A. Katine and E. E. Fullerton, ¿Device implications of spin-transfer torques¿, Journal of Magnetism and Magnetic Materials 320, 1217 (2008). [5]. S. S. P. Parkin, M. Hayashi and L. Thomas, ¿Magnetic Domain-Wall Racetrack Memory¿, Science 320, 190 (2008). [6]. S. Sangiao, L. Morellón, G. Simón, J. M. De Teresa, J. A. Pardo, J. Arbiol and M. R. Ibarra, ¿Anomalous Hall effect in Fe(001) epitaxial thin films over a wide range in conductivity¿, Physical Review B 79, 014431 (2009). [7]. S. Onoda, N. Sugimoto and N. Nagaosa, ¿Intrinsic Versus Extrinsic Anomalous Hall Effect in Ferromagnets¿, Physical Review Letters 97, 126602 (2006). [8]. N. Marcano, S. Sangiao, J. M. De Teresa, L. Morellón, M. R. Ibarra, M. Plaza and L. Pérez, ¿Structural and magnetotransport properties of Bi thin films grown by thermal evaporation¿, Journal of Magnetism and Magnetic Materials 322, 1460 (2010). [9]. N. Marcano, S. Sangiao, C. Magén, L. Morellón, M. R. Ibarra, M. Plaza, L. Pérez and J. M. De Teresa, ¿Role of the surface states in the magnetotransport properties of ultrathin bismuth films¿, Physical Review B 82, 125326 (2010). [10]. S. Sangiao, N. Marcano, J. Fan, L. Morellón, M. R. Ibarra and J. M. De Teresa, ¿Quantitative analysis of the weak anti-localization effect in ultrathin bismuth films¿, EPL 95, 37002 (2011). [11]. N. Marcano, S. Sangiao, M. Plaza, L. Pérez, A. Fernández-Pacheco, R. Córdoba, M. C. Sánchez, L. Morellón, M. R. Ibarra and J. M. De Teresa, ¿Weak-antilocalization signatures in the magnetotransport properties of individual electrodeposited Bi nanowires¿, Applied Physics Letters 96, 082110 (2010). [12]. S. Sangiao, L. Morellón, M. R. Ibarra and J. M. De Teresa, ¿Ferromagnet-superconductor nanocontacts grown by focused electron/ion beam techniques for current-in-plane Andreev Reflection measurements¿, Solid State Communications 151, 37 (2011). [13]. S. Sangiao, J. M. De Teresa, M. R. Ibarra, I. Guillamón, H. Suderow, S. Vieira and L. Morellón, ¿Andreev reflection under high magnetic fields in ferromagnet-superconductor nanocontacts¿, Physical Review B, in press.