Staff directory Blai Casals Montserrat

Blai Casals Montserrat

Postdoctoral Researcher
JDC-I 2020
Oxide Nanophysics



  • Energy exponents of avalanches and Hausdorff dimensions of collapse patterns

    Casals B., Salje E.K.H. Physical Review E; 104 (5, 054138) 2021. 10.1103/PhysRevE.104.054138. IF: 2.529

    A simple numerical model to simulate athermal avalanches is presented. The model is inspired by the "porous collapse"process where the compression of porous materials generates collapse cascades, leading to power law distributed avalanches. The energy (E), amplitude (Amax), and size (S) exponents are derived by computer simulation in two approximations. Time-dependent "jerk"spectra are calculated in a single avalanche model where each avalanche is simulated separately from other avalanches. The average avalanche profile is parabolic, the scaling between energy and amplitude follows E∼Amax2, and the energy exponent is ϵ = 1.33. Adding a general noise term in a continuous event model generates infinite avalanche sequences which allow the evaluation of waiting time distributions and pattern formation. We find the validity of the Omori law and the same exponents as in the single avalanche model. We then add spatial correlations by stipulating the ratio G/N between growth processes G (linked to a previous event location) and nucleation processes N (with new, randomly chosen nucleation sites). We found, in good approximation, a power law correlation between the energy exponent ϵ and the Hausdorff dimension HD of the resulting collapse pattern HD-1∼ϵ-3. The evolving patterns depend strongly on G/N with the distribution of collapse sites equally power law distributed. Its exponent ϵtopo would be linked to the dynamical exponent ϵ if each collapse carried an energy equivalent to the size of the collapse. A complex correlation between ϵ,ϵtopo, and HD emerges, depending strongly on the relative occupancy of the collapse sites in the simulation box. © 2021 American Physical Society.


  • Disentangling Highly Asymmetric Magnetoelectric Effects in Engineered Multiferroic Heterostructures

    Menéndez E., Sireus V., Quintana A., Fina I., Casals B., Cichelero R., Kataja M., Stengel M., Herranz G., Catalán G., Baró M.D., Suriñach S., Sort J. Physical Review Applied; 12 (1, 014041) 2019. 10.1103/PhysRevApplied.12.014041. IF: 4.532

    One of the main strategies to control magnetism by voltage is the use of magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic-ferroelectric heterostructures. When such heterostructures are subjected to an electric field, piezostrain-mediated effects, electronic charging, and voltage-driven oxygen migration (magnetoionics) may simultaneously occur, making the interpretation of the magnetoelectric effects not straightforward and often leading to misconceptions. Typically, the strain-mediated magnetoelectric response is symmetric with respect to the sign of the applied voltage because the induced strain (and variations in the magnetization) depends on the square of the ferroelectric polarization. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. By engineering a ferromagnetic-ferroelectric hybrid consisting of a magnetically soft 50-nm thick Fe75Al25 (at. %) thin film on top of a (110)-oriented Pb(Mg1/3Nb2/3)O3-32PbTiO3 ferroelectric crystal, a highly asymmetric magnetoelectric response is obtained and the aforementioned magnetoelectric effects can be disentangled. Specifically, the large thickness of the Fe75Al25 layer allows dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of Fe75Al25, leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. The origin of this asymmetric strain-induced magnetoelectric effect arises from the asymmetry of the polarization reversal in the particular crystallographic orientation of the ferroelectric substrate. These results are important to optimize the performance of artificial multiferroic heterostructures. © 2019 American Physical Society.