Magnetische Anregungen in Helimagneten über einem breiten Bereich von Spiralstrukturen
Zusammenfassung der Projektergebnisse
In our project IN 209/9-1, we have investigated low-energy magnetic excitations in both symmetric and Dzyaloshinskiitype helimagnets, including both insulating and metallic systems. Our results have uncovered several qualitatively new phenomena that are characteristic of helimagnets and have not been found in conventional magnetic materials with collinear spin structures. Most importantly, we have demonstrated the appearance of a novel emergent energy scale associated with the pseudo-Goldstone spin gap in a class of symmetric helimagnets with sufficiently high lattice symmetry, that satisfy the following conditions: (i) the magnetic order spontaneously breaks chiral symmetry, so that magnetic domains with opposite chiralities have the same energy; (ii) the magnetic ordering vector is spontaneously chosen out of several structurally equivalent possibilities. This is best realized in ZnCr2 Se4 . On the one hand, the pseudo-Goldstone spin gap results from non-linear corrections to spin-wave theory and is not directly related to any parameter that explicitly enters the spin model. On the other hand, it essentially defines the low-temperature behavior of thermodynamic and transport properties such as specific heat and thermal conductivity, leading to substantial experimentally observable effects that cannot be simply dismissed as insignificant corrections. At points of accidental degeneracy, such as pseudo-Goldstone modes, the magnon energy in linear spin-wave theory vanishes, making non-linear corrections the leading terms that define the thermodynamic properties of the system. We hope that this remarkable observation stimulates the development of future theoretical models that would be able to calculate spin-wave dispersions beyond the linear spin-wave theory, as well as intensify the search for other compounds, in particular with different spin values, different strengths of spin-orbit coupling, and different magnetic ground states, where such effects may play a decisive role. Our attempt to find similar physics Sr3 Fe2 O7 led to a complete re-evaluation of its magnetic phase diagram and to the discovery of three new unconventional ground states, including the modulated double-q helical phase with a sinusoidal component, the spin cholesteric phase that results from the melting of this sinusoidal compound, as well as the reentrant skyrmion-lattice-type double-q phase, all of which were previously unknown. The measurements on both Sr3 Fe2 O7 and its cubic counterpart SrFeO3 also revealed strong quasielastic diffuse magnetic scattering whose intensity grows with increasing temperature within the long-range ordered phase. This component is not present in the spin-excitation spectra of conventional collinear magnetic systems and could originate from the dynamics of chiral domain walls that separate magnetic domains of opposite chiralities. The important role of such domain walls is also emphasized by the slow relaxation dynamics, which we observed both in ZnCr2 Se4 and in Sr3 Fe2 O7 . Chiral domain walls could justify a separate systematic study. While domain walls in ferromagnets attracted considerable attention over several decades, domain walls in helimagnets remain largely unexplored. We expect that they would exhibit much richer behavior, especially in symmetric helimagnets, where three distinct types of domain walls are allowed by symmetry: (i) those between domains of equal chirality but different ordering vectors; (ii) those between domains of opposite chirality but the same ordering vector; and (iii) those between domains of opposite chirality and different ordering vectors. Each of these domain-wall types would be characterized by different local spin arrangements, have its different net energy, preferred orientation, and a number of other parameters, resulting in much more complex spin-wall dynamics than in conventional ferromagnets. Processes in which domain walls of one type would decay into domain walls of other types, nucleating new domains, are certainly required to explain the slow domain-wall relaxation, but they have no analogue in conventional magnetic materials. It would be particularly exciting to find a way to image such domains in real space with high spacial resolution, e.g. by exploiting the multiferroic properties of ZnCr2 Se4 . The form of such domain walls, their thickness, spatial distribution in the bulk of the sample, their relative energies, their spin structures and dynamic properties are open questions that deserve a closer investigation both experimentally and theoretically, using micromagnetic simulations. In our project, we have also outlined the way toward more accurate estimation of interaction parameters in the magnetic hamiltonian, such as Dzyaloshinskii-Moriya interactions, by relying on the topological properties of the magnon bands. We suggest using the location of magnonic Weyl points in the reciprocal space as a sensitive “zoom tool” to improve the sensitivity to Dzyaloshinskii-Moriya interactions. In particular, this emphasizes the importance of covering low-symmetry reciprocal-space directions in momentum space in contrast to measuring just one-dimensional cuts along several high-symmetry directions. Last but not least, we have established the frustration mechanism that is responsible for the formation of the doublehelix spin structure in iron phosphide. This unusual spin structure is found in several isostructural materials of the same family. One of the surprising results was that FeP has very strong ferromagnetic coupling along the crystallographic b direction, which one would not expect from it’s essentially three-dimensional crystal structure. This leads to a hierarchy of exchange interactions that reduces the effective dimensionality of the spin model.
Projektbezogene Publikationen (Auswahl)
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Emergent topological spin structures in a centrosymmetric cubic perovskite. Phys. Rev. B 101, 134406.
Ishiwata, S.; Nakajima, T.; Kim, J.-H.; Inosov, D. S.; Kanazawa, N.; White, J. S.; Gavilano, J. L.; Georgii, R.; Seemann, K. M.; Brandl, G.; Manuel, P.; Khalyavin, D. D.; Seki, S.; Tokunaga, Y.; Kinoshita, M.; Long, Y. W.; Kaneko, Y.; Taguchi, Y.; Arima, T. ... & Tokura, Y.
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Magnetic field dependence of low-energy magnons, anisotropic heat conduction, and spontaneous relaxation of magnetic domains in the cubic helimagnet ZnCr2 Se4. Phys. Rev. B 102, 184431.
Inosov, D. S.; Onykiienko, Y. O.; Tymoshenko, Y. V.; Akopyan, A.; Shukla, D.; Prasai, N.; Doerr, M.; Gorbunov, D.; Zherlitsyn, S.; Voneshen, D. J.; Boehm, M.; Tsurkan, V.; Felea, V.; Loidl, A. & Cohn, J. L.
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Magnonic Weyl states in Cu2 OSeO3. Phys. Rev. Res. 2, 013063.
Zhang, L.-C.; Onykiienko, Y. A.; Buhl, P. M.; Tymoshenko, Y. V.; Čermák, P.; Schneidewind, A.; Stewart, J. R.; Henschel, A.; Schmidt, M.; Blügel, S.; Inosov, D. S. & Mokrousov, Y.
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Frustration model and spin excitations in the helimagnet FeP. Physical Review B, 105(13).
Sukhanov, A. S.; Tymoshenko, Y. V.; Kulbakov, A. A.; Cameron, A. S.; Kocsis, V.; Walker, H. C.; Ivanov, A.; Park, J. T.; Pomjakushin, V.; Nikitin, S. E.; Morozov, I. V.; Chernyavskii, I. O.; Aswartham, S.; Wolter, A. U. B.; Yaresko, A.; Büchner, B. & Inosov, D. S.
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Incom mensurate and multiple-q magnetic misfit order in the frustrated quantum spin ladder material antlerite Cu3 SO4 (OH)4. Phys. Rev. B 106, 174431.
Kulbakov, Anton A.; Sadrollahi, Elaheh; Rasch, Florian; Avdeev, Maxim; Gaß, Sebastian; Corredor, Bohorquez Laura Teresa; Wolter, Anja U. B.; Feig, Manuel; Gumeniuk, Roman; Poddig, Hagen; Stötzer, Markus; Litterst, F. Jochen; Puente-Orench, Inés; Wildes, Andrew; Weschke, Eugen; Geck, Jochen; Inosov, Dmytro S. & Peets, Darren C.
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Rich magnetic phase diagram of putative helimagnet Sr3 Fe2 O7. Phys. Rev. B 108, 174420.
Andriushin, Nikita D.; Grumbach, Justus; Kim, Jung-Hwa; Reehuis, Manfred; Tymoshenko, Yuliia V.; Onykiienko, Yevhen A.; Jain, Anil; MacFarlane, W. Andrew; Maljuk, Andrey; Granovsky, Sergey; Hoser, Andreas; Pomjakushin, Vladimir; Ollivier, Jacques; Doerr, Mathias; Keimer, Bernhard; Inosov, Dmytro S. & Peets, Darren C.
