Zehra Cevher

Determination of magnetic anisotropies, interlayer coupling, and magnetization relaxation in FeCoB/Cr/FeCoB

We studied magnetic anisotropic properties, interlayer coupling, and spin wave relaxation in ten periods of CoFeB/Cr/CoFeB films grown on seed layers of Cu with a Co:Fe:B composition ratio of 2:2:1. The measurements were taken in samples with 50 A layers of CoFeB using the ferromagnetic resonance technique. The thickness of the Cr interlayers was varied from 4 to 40 A for understanding the mechanisms of interlayer coupling. We investigated the magnetic anisotropy parameters by rotating the sample with respect to the microwave magnetic field from in plane to perpendicular to the plane. We identify both the acoustic branch and the optical branch in the spin wave resonance spectra. The effective interlayer coupling constant and the out-of-plane anisotropy show an oscillatory change, while the uniaxial in-plane anisotropy increases monotonically with increasing the thickness of the spacing layers. Moreover, we show that the spin wave relaxation can be optimized by adjusting the interlayer exchange interactions.

One-pot synthesis and characterization of chalcopyrite CuInS2 nanoparticles

We synthesized tetragonal chalcopyrite CuInS2 (CIS) nanoparticles from molecular single source precursors, (Ph3P)2Cu-(μ-SEt)2In(SEt)2, by a one-pot reaction in the presence of 3-mercaptopropionic acid at reaction times of 3 hours or less with high yields. In our approach, NaCl as a by-product was used as a heat transfer agent via a conventional convective heating method. We tuned the sizes of nanoparticles through manipulation of the reaction temperature, reaction time, precursor, and thiol concentration. The sizes of nanoparticles from 1.8 nm to 5.2 nm were obtained as reaction temperatures were increased from 150 °C to 190 °C. The method developed in this study is scalable to achieve production of ultra-large quantities of tetragonal chalcopyrite CIS nanoparticles. The resulting nanoparticles were analyzed by UV-vis spectroscopy, X-ray diffraction, EDAX, and HRTEM.

Experimental demonstration of 55-fs spin canting in photoexcited iron nanoarrays

As magnetic storage density approaches 1TB/in2, a grand challenge is looming as how to read/write such a huge amount of data within a reasonable time. The ultrafast optical manipulation of magnetization offers a solution, but little is known about the intrinsic speed limit of quantum spin switching. Here, we report that low-energy 50-fs laser pulses can induce spin canting in Fe nanoparticles within 55 fs, breaking the previous record by at least one order of magnitude. Both linearly and circularly polarized light can be used to tilt spins. In our model, the incident laser field first excites the orbital angular momentum, and through spin-orbit coupling, the spin cants out-of-plane and results in a distinctive diamond hysteresis loop. The spin canting time decreases with spin angular momentum. This spin canting is not limited to Fe nanoparticles and is also observed in Fe/Pt and Fe3O4 nanoparticles. Our results demonstrate the potential of magnetic nanostructures as a viable magnetic medium for high densi...

Femtosecond Laser Pulse Induced Ultrafast Demagnetization in Fe/GaAs Thin Films

As the magnetic storage density approaches 1 TB/in2, a grand challenge is looming as how to read/write such a huge amount of data within a reasonable time. In this study, we demonstrate femtosecond demagnetization in 10-nm epitaxially grown Fe thin films by applying low-energy femtosecond laser pulses. We used time-resolved pump-probe Magneto Kerr Effect spectroscopy to record ultrafast laser induced demagnetization and its recovery. The demagnetization time was found to be 70 fs from the time-resolved hysteresis loops. The time scale is much shorter than the phonon thermalization time. Our results show that the demagnetization can be completed before electron-phonon equilibration is achieved, and therefore indicate the feasibility of ultrafast optical control of demagnetization responses for next generation femtosecond-switching magnetic storage devices.

Optimization of the defects and the nonradiative lifetime of GaAs/AlGaAs double heterostructures

We used Raman scattering and time-resolved photoluminescence spectroscopy to investigate the molecular-beam-epitaxy (MBE) growth parameters that optimize the structural defects and therefore the internal radiative quantum efficiency of MBE-grown GaAs/AlGaAs double heterostructures (DH). The DH structures were grown at two different temperatures and three different As/Ga flux ratios to determine the conditions for an optimized structure with the longest nonradiative minority carrier lifetime. Raman scattering measurements show an improvement in the lattice disorder in the AlGaAs and GaAs layers as the As/Ga flux ratio is reduced from 40 to 15 and as the growth temperature is increased from 550 to 595 °C. The optimized structure is obtained with the As/Ga flux ratio equal to 15 and the substrate temperature 595 °C. This is consistent with the fact that the optimized structure has the longest minority carrier lifetime. Moreover, our Raman studies reveal that incorporation of a distributed Bragg reflector lay...