Strength and scales of itinerant spin fluctuations in<mml:math xmlns:mml="" title="undefined" rel="noopener noreferrer">http://www.w3.org/1998/Math/MathML">mml:mrowmml:mn3mml:midparamagnetic metals (original) (raw)

Spin-density fluctuations and the fluctuation-dissipation theorem in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">mml:mrowmml:mn3mml:mid ferromagnetic metals

Physical review, 2017

Spatial and time scales of spin density fluctuations (SDF) were analyzed in 3d ferromagnets using ab initio linear response calculations of complete wavevector and energy dependence of the dynamic spin susceptibility tensor. We demonstrate that SDF are spread continuously over the entire Brillouin zone and while majority of them reside within the 3d bandwidth, a significant amount comes from much higher energies. A validity of the adiabatic approximation in spin dynamics is discussed. The SDF spectrum is shown to have two main constituents: a minor low-energy spin wave contribution and a much larger high-energy component from more localized excitations. Using the fluctuation-dissipation theorem (FDT), the on-site spin correlator (SC) and the related effective fluctuating moment were properly evaluated and their universal dependence on the 3d band population is further discussed.

Ab initio calculation of spin fluctuation spectra using time-dependent density functional perturbation theory, plane waves, and pseudopotentials

Physical Review B, 2018

We present an implementation of time-dependent density functional perturbation theory for spin fluctuations, based on planewaves and pseudopotentials. We compute the dynamic spin susceptibility self-consistently by solving the time-dependent Sternheimer equation, within the adiabatic local density approximation to the exchange and correlation kernel. We demonstrate our implementation by calculating the spin susceptibility of representative elemental transition metals, namely bcc Fe, fcc Ni and bcc Cr. The calculated magnon dispersion relations of Fe and Ni are in agreement with previous work. The calculated spin susceptibility of Cr exhibits a soft-paramagnon instability, indicating the tendency of the Cr spins to condense in a incommensurate spin density wave phase, in agreement with experiment.

First-principle based simulations of longitudinal spin-fluctuations in metals and integration in classical space with variable spin amplitude

AIP Advances

The use of the classical Heisenberg model which incorporates only transverse spin degrees of freedom has only limited success for description of the metallic magnetism at finite temperature, since temperature and magnetic disorder induced longitudinal variations of the atomic spin moments might become large in the itinerant electron systems away from the limit of localized moments. In order to incorporate the longitudinal spin fluctuations in finite temperature simulation schemes a simple extended version of the Heisenberg model which allows for an on-site spin magnitude variation controlled by the one-site energy terms is widely used during the recent decade for ab-initio mapping and statistical simulations. Here, we apply and discuss such ab-initio based scheme for the canonical itinerant ferromagnetic metals (Fe, Co, Ni) and recently discovered high temperature antiferromagnet-V 3 Al, in conjunction with standard spherical integration metrics in classical spin state and the recently proposed linear one. We also examine the dependence of the results on the choice of the exchange and correlation potential in ab-initio total energy calculations. We compare the respective uncertainties in the calculated values of the magnetic ordering temperature and temperature dependent spin moment magnitude to the difference in the results which relate to the choice of the metrics.

Anisotropic and incommensurate spin fluctuations in hcp iron and some other nearly magnetic metals

2002

We present an ab initio theoretical formalism for the static paramagnetic spin susceptibility of metals at finite temperatures. Since relativistic effects, e.g. spin-orbit coupling, are included we can identify the anisotropy or easy axes of the spin fluctuations. Our calculations find hcp-iron to be unstable to in ab-plane, incommensurate anti-ferromagnetic (AFM) modes (linked to nested Fermi surface) below T N =69K for the lowest pressures under which it is stable. T N swiftly drops to zero as the pressure is increased. The calculated susceptibility of yttrium is consistent with the helical, incommensurate AFM order found in many rare-earth-dilute yttrium alloys. Lastly, in line with experimental data, we find the easy axes of the incommensurate AFM and ferromagnetic spin fluctuations of the normal state of the triplet superconductor Sr 2 RuO 4 to be perpendicular and parallel with the crystal c-axis respectively. 75.50Bb,75.40Cx,71.15Rf,75.25+2

Quantum oscillations in quasi-one-dimensional metals with spin-density-wave ground states

Physical Review B, 1999

We consider the magnetoresistance oscillation phenomena in the Bechgaard salts (TMTSF) 2 X, where X ϭClO 4 , PF 6 , and AsF 6 in pulsed magnetic fields to 51 T. Of particular importance is the observation of a new magnetoresistance oscillation for XϭClO 4 in its quenched state. In the absence of any Fermi-surface reconstruction due to anion order at low temperatures, all three materials exhibit nonmonotonic temperature dependence of the oscillation amplitude in the spin-density-wave ͑SDW͒ state. We discuss a model where, below a characteristic temperature T* within the SDW state, a magnetic breakdown gap opens.

Quantum and thermal fluctuations in a two-dimensional correlated band ferromagnet: Goldstone-mode-preserving investigation with self-energy and vertex corrections

Physical Review B, 2007

Ferromagnetism in the t-t ′ Hubbard model is investigated on a square lattice. Correlation effects in the form of self-energy and vertex corrections are systematically incorporated within a spinrotationally-symmetric scheme which explicitly preserves the Goldstone mode and is therefore in accord with the Mermin-Wagner theorem. Interplay of band dispersion and correlation effects on ferromagnetic-state stability are highlighted with respect to both long-and short-wavelength fluctuations, which are shown to have substantially different behaviour. Our approach provides a novel understanding of the enhancement of ferromagnetism near van Hove filling for t ′ ∼ 0.5 in terms of strongly suppressed saddle-point contribution to the destabilizing exchange part of spin stiffness. Finite-temperature electron spin dynamics is investigated directly in terms of spectralweight transfer across the Fermi energy due to electron-magnon coupling. Relevant in the context of recent magnetization measurements on ultrathin films, the role of strong thermal spin fluctuations in low dimensions is highlighted, in the anisotropy-stabilized ordered state, by determining the thermal decay of magnetization and Tc within a renormalized spin-fluctuation theory.

On the energetics of transversal and longitudinal fluctuations of atomic magnetic moments

Journal of Magnetism and Magnetic Materials, 2012

By constrained spin-density functional calculations we estimate the relative role of the longitudinal and transversal fluctuations of the magnetic moments in the series of 3d metals (bcc Fe, hcp and fcc Co, and fcc Ni) for weak excitations from the ferromagnetic ground state. It is shown that the importance of longitudinal fluctuations strongly varies from relatively small in bcc Fe to large in fcc Ni. This means that a consistent adiabatic treatment of the low-energy spin fluctuations should include independent longitudinal fluctuations.

Longitudinal and transverse static spin fluctuations in layered ferromagnets and antiferromagnets

Physical Review B, 2012

We analyse the momentum dependence of static non-uniform susceptibilities of layered localmoment systems below Curie (Neel) temperature within the 1/S expansion, the renormalizationgroup approach, and first order of 1/N expansion. We argue that the previously known results of the spin-wave theory and renormalization-group approach for the transverse spin susceptibility acquire strong corrections already at sufficiently low temperatures, which appear due to the interaction of the incomping magnon having momentum q with magnons with momenta k < q. Such corrections can not be treated in the standard renormalization-group approach, but can be described by both, 1/S and 1/N expansions. The results of these expansions can be successfully extrapolated to T = TM , yielding the correct weight of static spin fluctuations, determined by the O(3) symmetry. For the longitudinal susceptibility, the summation of leading terms of 1/S expansion within the parquet approach allows to fulfill the sumrule for the weights of transverse and longitudinal fluctuations in a broad temperature region below TM outside the critical regime. We also discuss the effect of longitudinal spin fluctuations on the (sublattice) magnetization of layered systems.