ACAlejandro Ceballos

Alejandro Ceballos

University of California System
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Alejandro Ceballos has a total of 32 co-authors. Here are the top co-authors by number of shared publications:

Latest Publications

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    Characterizing the nonequilibrium response of FeRh thin films using time-domain thermoreflectance

    by Renee M. Harton, Alejandro Ceballos-Sanchez, Vivek Unikandanunni, Alexander X. Gray, Stefano Bonetti, Peter Krüger, Frances Hellman

    Time-domain thermoreflectance (TDTR) characterization of FeRh throughout its first-order antiferromagnetic (AF) to ferromagnetic (FM) transition shows that the transient reflectance ΔR(t)/R strongly depends on the magnetic order of the sample. Using TDTR, which uses optical pulses to induce small temperature excursions, we have found that ΔR(t)/R of the AF phase exhibits a large negative response, while the response of the FM phase is positive. This magnetic phase sensitivity has allowed us to study the transient response of both the AF and FM phases to the pump-pulse excitation and the mixed phase of the material. These results are significant since the ultrafast properties of antiferromagnetic materials and mixed antiferromagnetic and ferromagnetic materials are difficult to detect using other conventional techniques. We have found that the AF phase exhibits a strong subpicosecond decaying signal not observed in the FM phase. The magnetic phase dependence of the sign of ΔR(t)/R is qualitatively explained using the results of ab initio density functional theory calculations. Using the two-temperature model, we found that the change in the thermalization time across the transition is caused by differences in both the electronic heat capacity and the electron-phonon coupling factor of the AF and FM phases. The electron-phonon coupling constant in the AF phase is also determined using the two-temperature model conducted using the ntmpy code package. For the FM phase, we provide boundaries for the magnitude of the electron-phonon coupling factor for the FM phase. These results indicate that TDTR can be used to study the transient properties of magnetic materials that are otherwise challenging to probe.

    Cited by 3
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    Coexistence of soft and hard magnetic phases in single layer amorphous Tb–Co thin films

    by A. Ceballos, M. Charilaou, M. Molina-Ruiz, F. Hellman

    Amorphous, ferrimagnetic Tb–Co thin films prepared with a thin Ta underlayer and either a Ta or a Pt overlayer show evidence of both soft and hard magnetic phases. At room temperature, the films exhibit conventional ferromagnetism, but low temperature magnetometry measurements reveal the decoupling of the two magnetic phases with decreasing temperature due to increased anisotropy energy of the hard layer at lower temperatures. Decreasing the film thickness to 2 nm, slightly above the superparamagnetic limit found at 1 nm, a soft, low-density phase was isolated and found to be present in all the films as confirmed with x-ray reflectivity and Rutherford backscattering spectrometry measurements. For greater thicknesses, the bottom layer retains its soft magnetic nature, while the remainder of the film is denser and has strong perpendicular magnetic anisotropy, leading to the exchange-spring behavior when the anisotropy becomes large, either at low temperatures or via a Pt overlayer that adds a strong interfacial anisotropy to the layer. Micromagnetic simulations of a soft/hard bilayer model with the experimentally determined anisotropy and magnetization parameters into a soft/hard bilayer model reproduced the experimental hysteretic behavior very well. These findings demonstrate how the magnetic state and the response of a-Tb–Co films to external fields can be controlled, providing a high degree of tunability that is promising for high-performance nanoscale devices.

    Cited by 12
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    Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films

    by Alejandro Ceballos, Akshay Pattabi, Amal El-Ghazaly, Sergiu Ruta, Christian P. Simon, Richard F. L. Evans, Thomas Ostler, Roy W. Chantrell, Ellis Kennedy, Mary Scott, et al.

    Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous (a-) rare earth-transition metal (a-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic a−Gd22−xTbxCo78 thin films, from x=0 to 18, and elucidate the role of Gd in HI-AOS in a-RE-TM alloys and multilayers. Increasing Tb content results in increasing perpendicular magnetic anisotropy and coercivity, without modifying magnetization density, and slower remagnetization rates and higher critical fluences for switching but still shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on the two-temperature model reproduce these results qualitatively and predict that the lower damping on the RE sublattice arising from the small spin-orbit coupling of Gd (with L=0) is instrumental for the faster dynamics and lower critical fluences of the Gd-rich alloys. Annealing a−Gd10Tb12Co78 leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that accounting for element-specific damping is crucial in understanding HI-AOS phenomena. The results suggest that engineering the element-specific damping of materials can open up new classes of materials that exhibit low-energy, ultrafast HI-AOS.

    Cited by 82
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    Top Cited Papers

    Reconfigurable ferromagnetic liquid droplets

    by Xubo Liu, Noah Kent, Alejandro Ceballos, Robert Streubel, Yufeng Jiang, Yu Chai, Paul Y. Kim, Joe Forth, Frances Hellman, Shaowei Shi, et al.

    Liquid reconfigurable ferromagnetic materials: Ferromagnetic materials show a permanent magnetic dipole, whereas superparamagnetic ones only show magnetic properties under an applied field. Some materials, like ferrofluids, show liquid-like behavior but do not retain their magnetization in the absence of an applied field. Liu et al. show remnant magnetization of otherwise superparamagnetic magnetite nanoparticles at an oil-water interface of emulsion droplets (see the Perspective by Dreyfus). The permanent magnetization could be controlled by coupling and uncoupling the magnetization of individual nanoparticles, making it possible to “write and erase” shapes of the droplets or to elongate them into cylinders. Science , this issue p. 264 ; see also p. 219

    Cited by 337