434 research outputs found
Fluoride Thin Films: from Exchange Bias to Multferroicity
This dissertation concerns research into the growth and characterization fluoride thin films by molecular beam epitaxy. After a discussion of relevant background material and experimental procedures in the first two chapters, we study exchange bias in magnetic multilayers incorporating the uniaxial antiferromagnet FeF2, grown to varying thicknesses, sandwiched between ferromagnetic Co layers with fixed thicknesses of 5 and 20 nm. Several bilayers with only the 20 nm thick Co layer were grown for comparative study. The samples were grown on Al2O3 (112¯0) substrates at room temperature. In-situ RHEED and x-ray diffraction indicated the films were polycrystalline. The films were determined to have low surface and interlayer roughness, as determined by AFM and x-ray reflectivity. After field-cooling to below the Neel temperature of FeF2 in a magnetic field of 1 kOe, magnetic hysteresis loops were measured as a function of temperature. We found that both layers had a negative exchange bias, with the exchange bias of the thinner layer larger than that of the thicker layer. In addition, the coercivity below the blocking temperature TB of the thinner layer was significantly larger than that of the thick layer, even though the coercivity of the two layers was the same for T \u3e TB. The exchange bias effect, manifested by a shift in these hysteresis loops, showed a strong dependence on the thickness of the antiferromagnet. Anisotropic magnetoresistance measurements provided additional insight into the magnetization reversal mechanism within the ferromagnets. The thickness dependent exchange anisotropy of trilayer and bilayer samples is explained by adapting a random field model to the antiferromagnet/ferromagnet interface.;Finally, We investigate the temperature dependent growth, as well as the magnetic and ferroelectric properties of thin films of the multiferroic compounds BaMF4, where M = Fe, Co, Ni. The films were grown to thicknesses of 50 or 100 nm on single crystal Al2O3 (0001) substrates. X-ray diffraction showed that this family of films grew epitaxially in the (010) orientation, but were twinned in the plane, with three domain orientations rotated by 120 degrees relative to one another. Measurements of the remanent hysteresis via interdigitated electrodes showed that the compounds M = Co, and Ni were ferroelectric, but no switching behavior was observed in the Fe system at electric fields up to 400 kV/cm. Measurements of the field-cooled and zero-field-cooled magnetic moment confirmed low temperature antiferromagnetic behavior, and found new weak ferromagnetic phases induced by strain
Acoustic phonon dynamics in thin-films of the topological insulator Bi2Se3
Transient reflectivity traces measured for nanometer-sized films of the
topological insulator Bi2Se3 revealed GHz-range oscillations driven within the
relaxation of hot carriers photoexcited with ultrashort laser pulses of 1.51 eV
photon energy. These oscillations have been suggested to result from acoustic
phonon dynamics, including coherent longitudinal acoustic phonons in the form
of standing acoustic waves. An increase of oscillation frequency from ~35 to
~70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the
interplay between two different regimes employing traveling-acoustic-waves for
films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR)
modes for films thinner than 40 nm. The amplitude of oscillations decays
rapidly for films below 15 nm thick when the indirect intersurface coupling in
Bi2Se3 films switches the FBAWR regime to that of the Lamb wave excitation. The
frequency range of coherent longitudinal acoustic phonons is in good agreement
with elastic properties of Bi2Se3
Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3
Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in
thickness, of the topological insulator Bi2Se3 revealed a strong dependence of
the carrier relaxation time on the film thickness. For thicker films the
relaxation dynamics are similar to those of bulk Bi2Se3, where the contribution
of the bulk insulating phase dominates over that of the surface metallic phase.
The carrier relaxation time shortens with decreasing film thickness, reaching
values comparable to those of noble metals. This effect may result from the
hybridization of Dirac cone states at the opposite surfaces for the thinnest
films
Effect of carrier recombination on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3
Transient reflectivity (TR) from thin films (6 - 40 nm thick) of the
topological insulator Bi2Se3 reveal ultrafast carrier dynamics, which suggest
the existence of both radiative and non-radiative recombination between
electrons residing in the upper cone of initially unoccupied high energy Dirac
surface states (SS) and holes residing in the lower cone of occupied low energy
Dirac SS. The modeling of measured TR traces allowed us to conclude that
recombination is induced by the depletion of bulk electrons in films below ~20
nm thick due to the charge captured on the surface defects. We predict that
such recombination processes can be observed using time-resolved
photoluminescence techniques
Resonance-type thickness dependence of optical second harmonic generation in thin-films of the topological insulator Bi2Se3
Optical second harmonic generation (SHG) has been measured for the first time
in reflection from the nanometer-thick films (6 to 40 nm) of the topological
insulator Bi2Se3 using 1.51 eV (820 nm) Ti:Sapphire laser photons and revealed
a strong dependence of the integral SHG intensity on the film thickness. The
integral SHG intensity was determined by area integration of the SHG rotational
anisotropy patterns measured for different input-output light polarization
geometries. A ~100-fold enhancement of the integral SHG intensity with
decreasing film thickness has been suggested to result from the
DC-electric-field-induced SHG (EFISHG) effects. Two sources of dynamically
created DC electric field were proposed: (i) the capacitor-type DC electric
field that gradually increases with decreasing film thickness from 40 to 6 nm
due to a dynamical imbalance of photoexcited long-lived carriers between the
opposite-surface Dirac surface states and (ii) an DC electric field associated
with a nonlinearly excited Dirac plasmon, which is responsible for the resonant
enhancement of the integral SHG intensity for the 10 nm thick film with a
Lorentz-shaped resonance of ~1.6 nm full width at half maximum. Additionally to
the general SHG enhancement trends with decreasing film thickness, a relative
decrease of the out-of-plane contribution with respect to the in-plane
contribution was observed. Using a theoretical treatment of the measured SHG
rotational anisotropy patterns, this effect has been suggested to result from
the joint contributions of the linear and quadratic DC electric field effects
to the EFISHG response
Nonlinear Optical Observation of Coherent Acoustic Dirac Plasmons in Thin-Film Topological Insulators
Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient secondharmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac state
Nonlinear Optical Observation of Coherent Acoustic Dirac Plasmons in Thin-Film Topological Insulators
Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient secondharmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac state
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