Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSpin waves in alloys at finite temperatures: application for FeCo magnonic crystal
195
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We study theoretically the influence of the temperature and disorder on the spin wave spectrum of the magnonic crystal Fe$_{1-c}$Co$_{c}$. Our formalism is based on the analysis of a Heisenberg Hamiltonian by means of the wave vector and frequency dependent transverse magnetic susceptibility. The exchange integrals entering the model are obtained from the emph{ab initio} magnetic force theorem. The coherent potential approximation is employed to treat the disorder and random phase approximation in order to account for the softening of the magnon spectrum at finite temperatures. The alloy turns out to exhibit many advantageous properties for spintronic applications. Apart from high Curie temperature, its magnonic bandgap remains stable at elevated temperatures and is largely unaffected by the disorder. We pay particular attention to the attenuation of magnons introduced by the alloying. The damping turns out to be a non-monotonic function of the impurity concentration due to the non-trivial evolution of the value of exchange integrals with the Co concentration. The disorder induced damping of magnons is estimated to be much smaller than their Landau damping.
rate research
Read More
In this report we present a systematic study of the magnonic modes in the disordered Fe$_{0.5}$Co$_{0.5}$ alloy based on the Heisenberg Hamiltonian using two complementary approaches. In order to account for substitutional disorder, on the one hand we directly average the transverse magnetic susceptibility in real space over different disorder configurations and on the other hand we use the coherent potential approximation (CPA). While the method of direct averaging is numerically exact, it is computationally expensive and limited by the maximal size of the supercell which can be simulated on a computer. On the contrary the CPA does not suffer from this drawback and yields a cheap numerical scheme. Therefore, we additionally compare the results of these two approaches and show that the CPA gives very good results for most of the magnetic properties, including the magnon energies and the spatial shape of the eigenmodes. However, it turns out that while reproducing the general trend, the CPA systematically underestimates the disorder induced damping of the magnons. This provides evidence that the physics of impurity scattering in this system is governed by non-local effects missing in the CPA. Finally, we study the real space eigenmodes of the system, including their spatial shapes, and analyze their temperature dependence within the random phase approximation.
Crystal structure prediction is a central problem of theoretical crystallography and materials science, which until mid-2000s was considered intractable. Several methods, based on either energy landscape exploration$^{1,2}$ or, more commonly, global optimization$^{3-8}$, largely solved this problem and enabled fully non-empirical computational materials discovery$^{9,10}$. A major shortcoming is that, to avoid expensive calculations of the entropy, crystal structure prediction was done at zero Kelvin and searched for the global minimum of the enthalpy, rather than free energy. As a consequence, high-temperature phases (especially those which are not quenchable to zero temperature) could be missed. Here we develop an accurate and affordable solution, enabling crystal structure prediction at finite temperatures. Structure relaxation and fully anharmonic free energy calculations are done by molecular dynamics with a force field (which can be anything from a parametric force field for simpler cases to a trained on-the-fly machine learning interatomic potential), the errors of which are corrected using thermodynamic perturbation theory to yield accurate ab initio results. We test the accuracy of this method on metals (probing the P-T phase diagram of Al and Fe), a refractory intermetallide (WB), and a significantly ionic ceramic compound (Earth-forming silicate MgSiO3 at pressures and temperatures of the Earths lower mantle). We find that the hcp-phase of aluminum has a wider stability field than previously thought, and the temperature-induced transition $alpha$-$beta$ in WB occurs at 2789 K. It is also found that iron has hcp structure at conditions of the Earths inner core, and the much debated (and important for constraining Earths thermal structure) Clapeyron slope of the post-perovskite phase transition in MgSiO3 is 5.88 MPa/K.
The operational characteristics of a magnonic crystal, which was fabricated as an array of shallow grooves etched on a surface of a magnetic film, were compared for magnetostatic surface spin waves and backward volume magnetostatic spin waves. In both cases the formation of rejection frequency bands was studied as a function of the grooves depth. It has been found that the rejection of the volume wave is considerably larger than of the surface one. The influences of the nonreciprocity of the surface spin waves as well as of the scattering of the lowest volume spin-wave mode into higher thickness volume modes on the rejection efficiency are discussed.
Spin-wave dispersions in the antiferromagnetic state of single crystal LiFePO$_4$ were determined by inelastic neutron scattering measurements. The dispersion curves measured from the (010) reflection along both {it a}$^ast$ and {it b}$^ast$ reciprocal-space directions reflect the anisotropic coupling of the layered Fe$^{2+}$ (S = 2) spin-system. The spin-wave dispersion curves were theoretically modeled using linear spin-wave theory by including in the spin-Hamiltonian in-plane nearest- and next-nearest-neighbor interactions ({it J}$_1$ and {it J}$_2$), inter-plane nearest-neighbor interactions ({it J}$_bot$) and a single-ion anisotropy ({it D}). A weak (010) magnetic peak was observed in elastic neutron scattering studies of the same crystal indicating that the ground state of the staggered iron moments is not along (010) direction, as previously reported from polycrystalline samples studies, but slightly rotated away from this axis.
We revise critically existing approaches to evaluation of thermodynamic potentials within the Greens function calculations at finite electronic temperatures. We focus on the entropy and show that usual technical problems related to the multivalued nature of the complex logarithm can be overcome. This results in a simple expression for the electronic entropy, which does not require any contour integration in the complex energy plane. Properties of the developed formalism are discussed and its illustrating applications to selected model systems and to bcc iron with disordered local magnetic moments are presented as well.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
