Magnetoelectricity in the ferrimagnetic Cu2OSeO3: symmetry analysis and Raman scattering study

Gnezdilov, V. P.; Lamonova, K. V.; Pashkevich, Yu. G.; Lemmens, P.; Berger, H.; Bussy, F.; Gnatchenko, S. L.

doi:10.1063/1.3455808

Cited by 44 publications

(62 citation statements)

References 19 publications

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“…Having established the effective trillium lattice model of Cu 2 OSeO 3 , we now proceed to the long-wavelength magnetic continuum theory that allows us to describe the helimagnetism of Cu 2 OSeO 3 on the mesoscopic scale. To this end, we follow Gnezdilov et al 24 and rewrite the four tetrahedral entities in each unit cell in terms of four coarsegrained fields, the total spin per site (in the effective model)…”

Section: Resultsmentioning

confidence: 99%

The quantum nature of skyrmions and half-skyrmions in Cu2OSeO3

et al. 2014

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The Skyrme-particle, the skyrmion, was introduced over half a century ago in the context of dense nuclear matter. But with skyrmions being mathematical objects-special types of topological solitons-they can emerge in much broader contexts. Recently skyrmions were observed in helimagnets, forming nanoscale spin-textures. Extending over length scales much larger than the interatomic spacing, they behave as large, classical objects, yet deep inside they are of quantum nature. Penetrating into their microscopic roots requires a multi-scale approach, spanning the full quantum to classical domain. Here, we achieve this for the first time in the skyrmionic Mott insulator Cu 2 OSeO 3 . We show that its magnetic building blocks are strongly fluctuating Cu 4 tetrahedra, spawning a continuum theory that culminates in 51 nm large skyrmions, in striking agreement with experiment. One of the further predictions that ensues is the temperature-dependent decay of skyrmions into half-skyrmions.

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Section: Resultsmentioning

confidence: 99%

The quantum nature of skyrmions and half-skyrmions in Cu2OSeO3

et al. 2014

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“…Although we are unable to resolve the details of the high-energy dispersion expected according to the model, our measurements also prove to be sensitive to the energy scale and overall bandwidth of the modes at higher energies, which fix the relationship between J electron spin resonance data [23], Raman data [16], and far-infrared data [18] and the model-calculated dispersion on two independent grids throughout two-dimensional (2D) (J near the various minima in five-dimensional parameter space and comparing best-local-fit SSE as well as full predicted spectra, we have found a set of best-global-fit parameters, which are detailed in Table I. A mean-field approximation for the high-temperature susceptibility of this model gives…”

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“…These Cu 4 "molecules," with an effective spin of S = 1, are the elementary magnetic building blocks of Cu 2 OSeO 3 instead of the single Cu ions. The effective spins of the Cu 4 tetrahedra are ferromagnetically coupled and form a trillium lattice, just as the Mn and Fe ions do in the B20 structure of the metallic skyrmion compounds MnSi and FeGe.Prior to the undertaking of the present work, previous studies of the magnetic excitation spectra of Cu 2 OSeO 3 were conducted using Raman scattering [16] and microwave resonance absorption [17], i.e., techniques that are sensitive only to excitations in the center of the Brillouin zone. In contrast, inelastic neutron scattering (INS) is able to measure at finite momentum transfer and is therefore uniquely suited to probe the magnetic excitation spectra of Cu 2 OSeO 3 throughout reciprocal space.…”

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confidence: 99%

“…Prior to the undertaking of the present work, previous studies of the magnetic excitation spectra of Cu 2 OSeO 3 were conducted using Raman scattering [16] and microwave resonance absorption [17], i.e., techniques that are sensitive only to excitations in the center of the Brillouin zone. In contrast, inelastic neutron scattering (INS) is able to measure at finite momentum transfer and is therefore uniquely suited to probe the magnetic excitation spectra of Cu 2 OSeO 3 throughout reciprocal space.…”

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Spin excitations in the skyrmion hostCu2OSeO3

et al. 2016

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We have used inelastic neutron scattering to measure the magnetic excitation spectrum along the high-symmetry directions of the first Brillouin zone of the magnetic skyrmion hosting compound Cu 2 OSeO 3 . The majority of our scattering data are consistent with the expectations of a recently proposed model for the magnetic excitations in Cu 2 OSeO 3 , and we report best-fit parameters for the dominant exchange interactions. Important differences exist, however, between our experimental findings and the model expectations. These include the identification of two energy scales that likely arise due to neglected anisotropic interactions. This feature of our work suggests that anisotropy should be considered in future theoretical work aimed at the full microscopic understanding of the emergence of the skyrmion state in this material. [8], and in the polar magnetic semiconductor GaV 4 S 8 [9]. To understand the formation and the microscopic origin of these skyrmion phases, one needs a multiscale approach that covers the macroscopic domain of the skyrmion as well as the quantum scale of the local spins. This, however, breaks down in the above-mentioned metals because the low-energy delocalized electrons and magnetic degrees of freedom are mixed, intrinsically involving multiple energy and spatial scales.Among cubic helimagnets, Cu 2 OSeO 3 is the only insulator with magnetoelectric properties in the ground state [8,[10][11][12][13][14]. It offers an ideal laboratory to explore the microscopic ingredients that lead to skyrmion formation in a quantitative manner, since its Bloch-type ground-state properties and low-energy excitations are fully governed by the magnetic interactions between localized spins and are not affected by the presence of itinerant carriers. Exchange pathway considerations, susceptibility measurements, and ab initio calculations reveal that two magnetic energy scales divide the system into weakly coupled Cu 4 tetrahedra [15]. These Cu 4 "molecules," with an effective spin of S = 1, are the elementary magnetic building blocks of Cu 2 OSeO 3 instead of the single Cu ions. The effective spins of the Cu 4 tetrahedra are ferromagnetically coupled and form a trillium lattice, just as the Mn and Fe ions do in the B20 structure of the metallic skyrmion compounds MnSi and FeGe.Prior to the undertaking of the present work, previous studies of the magnetic excitation spectra of Cu 2 OSeO 3 were conducted using Raman scattering [16] and microwave resonance absorption [17], i.e., techniques that are sensitive only to excitations in the center of the Brillouin zone. In contrast, inelastic neutron scattering (INS) is able to measure at finite momentum transfer and is therefore uniquely suited to probe the magnetic excitation spectra of Cu 2 OSeO 3 throughout reciprocal space. The additional information afforded by INS therefore provides more rigorous tests of theoretical models aimed at describing the excitation spectra of Cu 2 OSeO 3 .Single crystals of Cu 2 OSeO 3 (cubic P 2 1 3 space group, a = 8.82Å) were gro...

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“…Performing measurements with the magnetic field applied along the different crystallographic axes 100 , 110 , and 111 , we find a series of characteristic resonances in all three magnetic phases. The spin dynamics of multiferroic compounds are of particular interest as they offer electric-field control of magnetic states, avoiding large power consuming magnetic fields [ [23][24][25][26][27][28][29][30].…”

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Skyrmion dynamics under uniaxial anisotropy

et al. 2016

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Broadband microwave spectroscopy has been performed on single-crystalline GaV4S8, which exhibits a complex magnetic phase diagram including cycloidal, Néel-type skyrmion lattice, as well as field-polarized ferromagnetic phases below 13 K. At zero and small magnetic fields two collective modes are found at 5 and 15 GHz, which are characteristic of the cycloidal state in this easy-axis magnet. In finite fields, entering the skyrmion lattice phase, the spectrum transforms into a multimode pattern with absorption peaks near 4, 8, and 15 GHz. The spin excitation spectra in GaV4S8 and their field dependencies are found to be in close relation to those observed in materials with Bloch-type skyrmions. Distinct differences arise from the strong uniaxial magnetic anisotropy of GaV4S8 not present in so-far known skyrmion hosts. The occurence of nontrivial topology in the spin pattern of magnets has gained considerable interest in condensed matter physics. Recent research focuses on magnetic skyrmions which are thermodynamically stabilized in compounds with noncentrosymmetric crystal structures, in a limited region of the magnetic field versus temperature phase diagram [1][2][3]. Skyrmions are whirllike objects of spins which can crystallize in skyrmion lattices (SkLs) with typical lattice constants from ten to hundred nanometers and give rise to emergent electrodynamics, like the topological Hall effect [4][5][6] or magnetic monopoles [7]. Individual skyrmions have been proposed as building blocks for novel nanomagnetic storage devices [8,9]. The SkL has raised high interest for microwavetechnology applications after collective spin excitations predicted in the GHz range [10] were evidenced in the insulating chiral magnet Cu 2 OSeO 3 [11][12][13][14][15][16]. Later it was found that different metallic, semiconducting, and insulating chiral magnets support the same set of characteristic excitations, i.e., three SkL modes characterized as clockwise (CW), counterclockwise (CCW) and breathing (BR) modes, that all follow a universal behavior [17].Besides the Bloch-type skyrmions reported in the aforementioned works, a Néel-type SkL has recently been discovered in GaV 4 S 8 [18], where the spins rotate radially towards the vortex core. In this semiconductor characterized by V 4 S 4 clusters with spin S = 1 2 [19], a structural Jahn-Teller transition [20] at 44 K is followed by the onset of magnetic order at the Curie temperature T C = 13 K. At the structural transition the lattice is stretched along one of the four body diagonals, resulting in a strongly anisotropic easy-axis magnet. The magnetic multi-domain structure strongly depends on the orientation and strength of the applied magnetic field and gives rise to complex magnetic phase diagrams [see Figures 1(a), 1(c) and 2(a)] including cycloidal (Cyc), SkL, and ferromagnetic (FM) regions. Specifically, the skyrmions do not follow the external magnetic field but are confined to the magnetic easy axes. The phases have been interpreted in terms of a competition of symmetric and ...

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Magnetoelectricity in the ferrimagnetic Cu2OSeO3: symmetry analysis and Raman scattering study

Cited by 44 publications

References 19 publications

The quantum nature of skyrmions and half-skyrmions in Cu2OSeO3

The quantum nature of skyrmions and half-skyrmions in Cu2OSeO3

Spin excitations in the skyrmion hostCu2OSeO3

Skyrmion dynamics under uniaxial anisotropy

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