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Investigations of nanoscale variations in spin and charge transport in manganites and organic semiconductors using spin polarized scanning tunneling spectroscopy

Citation

Hughes, Cameron Richard (2010) Investigations of nanoscale variations in spin and charge transport in manganites and organic semiconductors using spin polarized scanning tunneling spectroscopy. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:02082010-153048901

Abstract

Spintronics is a new class of spin-dependent electronics with great potential for nonvolatile memory and logic technology. Additionally, spintronics may be combined with optoelectronic applications to achieve higher efficiency and novel capabilities. All of these developments require growth and characterization of new materials to polarize and transport electron spin currents. In this context, spin-polarized and non spin-polarized spatially resolved conductance measurements performed by scanning tunneling microscopy (STM) are effective means to investigate the spin and charge quantum transport in magnetic and organic systems, particularly for systems that are prone to phase separations and complex magnetic properties, such as the colossal magnetoresistive (CMR) manganites La_(1-x)Ca_xMnO_3 (LCMO) that are known to exhibit intrinsic electronic heterogeneity due to strong electronic correlation and competing orders in the ground state. Additionally, STM measurements can provide direct information about the band structure and mobility of the organic semiconductor 8-hydroxyquinoline aluminum (Alq_3) in the Alq_3/LCMO heterostructures to further understand their performance in spintronic devices. The manganite compound La_(1-x)Ca_xMnO_3 (LCMO) with a bulk doping level x = 0.3 is a ferromagnetic metal with a relatively high Curie temperature T_c = 270K. This system is promising for spintronic device applications, and may be used as a spin current injector because of the gapped band structure for minority spins, a property known as half-metallicity. On the other hand, even in this bulk ferromagnetic metallic phase, inherent electronic inhomogeneity at microscopic scales is expected. To further study this effect, we have investigated x = 0.3 LCMO thin films using scanning tunneling microscopy in spectroscopic mode under varied temperature, magnetic field and spin polarization of the tunneling current. Spatially resolved maps of tunneling conductance taken with non polarized Pt/Ir tip show variations on the scale of a few hundred nanometers in size in the bulk ferromagnetic state, which are believed to be the result of intrinsic inhomogeneity of the manganites due to their tendency toward phase separation. Maps of tunneling conductance taken with spin-polarized Cr coated tips are consistent with the convolution of the LCMO and Cr density of states, and below the T_c of LCMO the spin-polarized tunnel junction can be described as a spin valve configuration. The electronic homogeneity in the material increases above the magnetic ordering temperature, or with application of magnetic field in the bulk ferromagnetic state. We identified gaps in the conductance at two separate characteristic energies. The first gap of energy approximately 0.6 eV is believed to arise from a ferromagnetic insulator (FI) surface phase due to its disappearance above the Curie temperature (T_c) and the dependence of gap energy on relative tip and sample magnetic orientation. The surface phase may be stabilized by Ca deficiency at the LCMO surface, corroborated by x-ray photoemission spectroscopy (XPS). Second, we observe a nearly temperature independent and spatially varying gap of approximately 0.4 eV for all zero-field tunneling spectra, which is believed to be associated with the psuedogap (PG) phenomena in the manganites. Application of a magnetic field converts the regions of PG phenomena to FI, in conjunction with an increase in the homogeneity of the lm conductance. These findings suggest that the PG phenomena arise from electronic inhomogeneity in the manganite film, in agreement with theoretical investigations, and that the vertical and lateral electronic inhomogeneity, along with its dependence on temperature and applied magnetic field, has important implications for use of these materials in high-density nanoscale spintronic devices. We have also successfully deposited and investigated Alq_3/LCMO heterostructures of varying thicknesses to investigate charge transport in Alq_3. Bulk Alq_3 structural properties are preserved down to 10 nm in thickness with a -0.3 eV offset in band energies. The lack of band bending between LCMO and Alq_3 is suggestive of a shift in the preferred isomer from meridinial to facial at the interface. The absence of polaron states from our STM studies implies the relative unimportance of polarons in Alq_3 for this heterostructure. In addition, the measured mobilities on the order of 10^(-5)cm^2(Vs)^(-1) for electrons and holes in Alq^3 lms deposited on heated LCMO substrates more closely resemble values of the intrinsic mobility estimated from the muon spin relaxation measurements than those from studies of the bulk LED structures, suggesting that superior film conductivity close to the fundamental limit is possible with a heated substrate during sublimation.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Spin Polarized, STM, Manganite, LCMO, Organic Semiconductor, Alq3
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Yeh, Nai-Chang
Thesis Committee:
  • Yeh, Nai-Chang (chair)
  • Eisenstein, James P.
  • Bockrath, Marc William
  • Refael, Gil
Defense Date:29 January 2010
Author Email:chughes (AT) its.caltech.edu
Record Number:CaltechTHESIS:02082010-153048901
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:02082010-153048901
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5546
Collection:CaltechTHESIS
Deposited By: Cameron Hughes
Deposited On:19 Mar 2010 17:04
Last Modified:26 Dec 2012 03:21

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