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Register a new userSwitching times of nanoscale FePt: finite size effects on linear reversal mechanism
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The linear reversal mechanism in FePt grains ranging from 2.316 nm to 5.404 nm has been simulated using atomistic spin dynamics, parametrized from ab-initio calculations. The Curie temperature and the critical temperature (T*), at which the linear reversal mechanism occurs, are observed to decrease with system size whilst the temperature window T* < T < TC increases. The reversal paths close to the Curie temperature have been calculated, showing that for decreasing system size the reversal path becomes more elliptic at lower temperatures, consistent with the decrease in the Curie temperature arising from finite size effects. Calculations of the minimum pulse duration show faster switching in small grains and is qualitatively described by the Landau-Lifshitz-Bloch equation with finite size atomistic parameterization, which suggests that multiscale modeling of FePt down to a grain size of ~ 3.5 nm is possible.
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A detailed investigation of electronic-transport properties of Nd0.5Sr0.5CoO3 has been carried out as a function of grain size ranging from micrometer order down to an average size of 28 nm. Interestingly, we observe a size induced metal-insulator transition in the lowest grain size sample while the bulk-like sample is metallic in the whole measured temperature regime. An analysis of the temperature dependent resistivity in the metallic regime reveals that the electron-electron interaction is the dominating mechanism while other processes like electron-magnon and electron-phonon scatterings are also likely to be present. The fascinating observation of enhanced low temperature upturn and minimum in resistivity on reduction of grain size is found due to electron-electron interaction (quantum interference effect). This effect is attributed to enhanced disorder on reduction of grain size. Interestingly, we observed a cross over from positive to negative magnetoresistance in the low temperature regime as the grain size is reduced. This observed sign reversal is attributed to enhanced phase separation on decreasing the grain size of the cobaltite.
Electrical manipulation of magnetization is essential for integration of magnetic functionalities such as magnetic memories and magnetic logic devices into electronic circuits. The current induced spin-orbit torque (SOT) in heavy metal/ferromagnet (HM/FM) bilayers via the spin Hall effect in the HM and/or the Rashba effect at the interfaces provides an efficient way to switch the magnetization. In the meantime, current induced SOT has also been used to switch the in-plane magnetization in single layers such as ferromagnetic semiconductor (Ga,Mn)As and antiferromagnetic metal CuMnAs with globally or locally broken inversion symmetry. Here we demonstrate the current induced perpendicular magnetization switching in L10 FePt single layer. The current induced spin-orbit effective fields in L10 FePt increase with the chemical ordering parameter (S). In 20 nm FePt films with high S, we observe a large charge-to-spin conversion efficiency and a switching current density as low as 7.0E6 A/cm2. We anticipate our findings may stimulate the exploration of the spin-orbit torques in bulk perpendicular magnetic anisotropic materials and the application of high-efficient perpendicular magnetization switching in single FM layer.
A wide range of unconventional transport phenomena have recently been observed in single-crystal delafossite metals. Here, we present a theoretical framework to elucidate electron transport using a combination of first-principles calculations and numerical modeling of the anisotropic Boltzmann transport equation. Using PdCoO$_2$ as a model system, we study different microscopic electron and phonon scattering mechanisms and establish the mean free path hierarchy of quasiparticles at different temperatures. We treat the anisotropic Fermi surface explicitly to numerically obtain experimentally-accessible transport observables, which bridge between the diffusive, ballistic, and hydrodynamic transport regime limits. We illustrate that distinction between the quasi-ballistic, and quasi-hydrodynamic regimes is challenging and often needs to be quantitative in nature. From first-principles calculations, we populate the resulting transport regime plots, and demonstrate how the Fermi surface orientation adds complexity to the observed transport signatures in micro-scale devices. Our work provides key insights into microscopic interaction mechanisms on open hexagonal Fermi surfaces and establishes their connection to the macroscopic electron transport in finite-size channels.
We analyze in detail the fluctuations and correlations of the (spatial) Fourier modes of nano-scale single-layer islands on (111) fcc crystal surfaces. We analytically show that the Fourier modes of the fluctuations couple due to the anisotropy of the crystal, changing the power spectrum of the fluctuations, and that the actual eigenmodes of the fluctuations are the appropriate linear combinations of the Fourier modes. Using kinetic Monte Carlo simulations with bond-counting parameters that best match realistic energy barriers for hopping rates, we deduce absolute line tensions as a function of azimuthal orientation from the analyses of the fluctuation of each individual mode. The autocorrelation functions of these modes give the scaling of the correlation times with wavelength, providing us with the rate-limiting kinetics driving the fluctuations, here step-edge diffusion. The results for the energetic parameters are in reasonable agreement with available experimental data for Pb(111) surfaces, and we compare the correlation times of island-edge fluctuations to relaxation times of quenched Pb crystallites.
Understanding the role played by the microstructure of materials on their macroscopic failure properties is an important challenge in solid mechanics. Indeed, when a crack propagates at a heterogeneous brittle interface, the front is trapped by tougher regions and deforms. This pinning induces non-linearities in the crack propagation problem, even within Linear Elastic Fracture Mechanics theory, and modifies the overall failure properties of the material. For example crack front pinning by tougher places could increase the fracture resistance of multilayer structures, with interesting technological applications. Analytical perturbation approaches, based on Bueckner-Rice elastic line models, focus on the crack front perturbations, hence allow for a description of these phenomena. Here, they are applied to experiments investigating the propagation of a purely interfacial crack in a simple toughness pattern: a single defect strip surrounded by homogeneous interface. We show that by taking into account the finite size of the body, quantitative agreement with experimental and finite elements results is achieved. In particular this method allows to predict the toughness contrast, i.e. the toughness difference between the single defect strip and its homogeneous surrounding medium. This opens the way to a more accurate use of the perturbation method to study more disordered heterogeneous materials, where the finite elements method is less adequate. From our results, we also propose a simple method to determine the adhesion energy of tough interfaces by measuring the crack front deformation induced by known interface patterns.
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