Spin waves in disordered materials

Buczek, Paweł; Thomas, Stefan; Marmodoro, Alberto; Buczek, Nadine; Zubizarreta, X.; Hoffmann, Martin; Balashov, T.; Wulfhekel, Wulf; Zakeri, Khalil; Ernst, A.

doi:10.1088/1361-648x/aadefb

Cited by 20 publications

(13 citation statements)

References 84 publications

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“…In metals, the life-time of the modes can be severely limited by the interaction of these collective modes with the single particle continuum, called Landau damping [14][15][16] , but means of viable engineering of longliving magnons have been proposed, such as reducing the system's dimensionality and alloying 17 . The latter method leads to a further momentum dissipation mechanism, in which the Bloch waves cease to be the eigenstates of the magnetic Heisenberg-like Hamiltonian and acquire a finite lifetime arising from the scattering on the crystal imperfections 18,19 . This picture of the weak attenuation might break down if the magnon spectra become dominated by strongly spatially localized modes.…”

Section: Introductionmentioning

confidence: 99%

Eigenmodes of a disordered FeCo magnonic crystal at finite temperatures

Paischer

Buczek

Buczek³

et al. 2021

J. Phys.: Condens. Matter

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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 considered in this report, 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.

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Section: Introductionmentioning

confidence: 99%

Eigenmodes of a disordered FeCo magnonic crystal at finite temperatures

Paischer

Buczek

Buczek³

et al. 2021

J. Phys.: Condens. Matter

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show abstract

“…Thus, a prominent debate is the study of lattice defects in low-dimensional structure by which translational invariance is broken. In particular, non-magnetic impurities can significantly alter the properties of low-dimensional magnets [23][24][25][26]. Moreover, in addition to electron-magnon interactions, i.e., Landau damping [22], and spin-orbit coupling [27], another mechanism which affects the magnon lifetime is scattering off impurities [28].…”

Section: Introductionmentioning

confidence: 99%

The impact of defects on low-energy excitations in two dimensional bipartite uniaxial antiferromagnet insulators

Shiranzaei¹,

Fransson²

2021

Preprint

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We address scattering of spin-wave through uncorrelated defects in two-dimensional (2D) easy-axis antiferromagnet (AFM) insulators. Although an on-site magnetic anisotropy leads to that the Goldstone modes become gapped bosons such that a long-range (AFM) order can be established in 2D, lattice imperfections have a tendency to weaken, and eventually destroy, the magnetic ordering. The impact of defects is considered within two limits, single and multiple defects. We apply the Green's function approach self-consistently to find magnon properties such as density of states and lifetime of induced resonances. Our findings show that repulsive defects decrease the magnon density of states while attractive ones may enhance it. We provide a comprehensive analysis of how defects can result in a reduction, and even closing, the anisotropy induced gap, which weakens the long-range (AFM) order parameter in the 2D state. We conclude that a small concentration of random defects can fill the gap of magnon spectrum.

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“…The formulation of effective-medium approaches is rather sophisticated and it leads to nontrivial numerical implementation 20,21 . Various brute-force simulations using large supercells have thus been used as alternatives which may be efficient and reliable in a number of cases, including both the spinwave spectra and values of the spin-wave stiffness 19,[21][22][23][24] . It was found that the spin stiffness of random diluted ferromagnets is reduced due to the sites without local magnetic moments as compared to the value obtained for a clean crystal with nonrandom concentration-weighted exchange interactions of the alloy.…”

Section: Introductionmentioning

confidence: 99%

Methods of electron transport inab initiotheory of spin stiffness

2020

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We present an ab initio theory of the spin-wave stiffness tensor for ordered and disordered itinerant ferromagnets with pair exchange interactions derived from a method of infinitesimal spin rotations. The resulting formula bears an explicit form of a linear-response coefficient which involves oneparticle Green's functions and effective velocity operators encountered in a recent theory of electron transport. Application of this approach to ideal metal crystals yields more reliable values of the spin stiffness than traditional ill-convergent real-space lattice summations. The formalism can also be combined with the coherent potential approximation for an effective-medium treatment of random alloys, which leads naturally to an inclusion of disorder-induced vertex corrections to the spin stiffness. The calculated concentration dependence of the spin-wave stiffness of random fcc Ni-Fe alloys can be ascribed to a variation of the reciprocal value of alloy magnetization. Calculations for random iron-rich bcc Fe-Al alloys reveal that their spin-wave stiffness is strongly reduced owing to the atomic ordering; this effect takes place due to weakly coupled local magnetic moments of Fe atoms surrounded by a reduced number of Fe nearest neighbors.

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Spin waves in disordered materials

Cited by 20 publications

References 84 publications

Eigenmodes of a disordered FeCo magnonic crystal at finite temperatures

Eigenmodes of a disordered FeCo magnonic crystal at finite temperatures

The impact of defects on low-energy excitations in two dimensional bipartite uniaxial antiferromagnet insulators

Methods of electron transport inab initiotheory of spin stiffness

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