ИСТИНА

The family of compounds MnSi, Mn1-xFexSi, FeGe having a cubic B20-type (space group P213) crystal structure order magnetically at low temperatures in a spin helix structure with a long period of helix. The appearance of this spin helix structure is due to Dzyaloshinskii-Moriya interaction (DMI) related with the lack of inversion symmetry of a crystal lattice, in addition to the main ferromagnetic spin exchange. Recently a compound Cu2OSeO3 possessing a cubic non-centrosymmetric P213 crystal structure was found to have a magnetic structure and T-H magnetic diagram similar to those of MnSi [1]. We have studied the magnetic transition in this material by means of magnetic ac-susceptibility and ac-calorimetry at nearly hydrostatic pressure up to 6 GPa. Single crystals of Cu2OSeO3 were grown by a gas transport technique in 610–550°C temperature gradient using 2:1 CuO/SeO2 mixture and CuCl2•2H2O as a transport agent. High pressures were created in a small Teflon capsule filled with liquid and inserted in a miniature toroid-type clamped device [2]. The specific heat C(T) at high pressure was measured by ac-calorimetry technique as described earlier [3]. The coil system for magnetic ac-susceptibility measurements χ(T) was arranged inside the Teflon capsule. Pressure was measured by superconducting transition temperature of Pb located near Cu2OSeO3 sample. The temperature of the transition to the spin helix structure (TC = 57.5 K at ambient pressure) increases nearly linearly at high pressure up to 6 GPa with the initial slope 3 K/GPa (Fig.1). This is in good agreement with earlier measurements of ac-susceptibility up to 2 GPa [4]. The dependences χ(T) and C(T) near TC at ambient pressure are depicted in Fig.2. χ(T) has a sharp drop at TC and an inflection point ~1 K above TC. The midpoint of a specific heat anomaly near TC corresponds to this inflection point of χ(T) and an additional sharp peak of C(T) corresponds to a sharp drop of χ(T). The maximum of C(T) is located ~ 0.3 K above TC. The existence of a sharp peak of C(T) superimposed on a broad one was reported earlier for Cu2OSeO3 [1] and for MnSi [5,6]. All these features may be related with the development of the chiral spin fluctuations in Cu2OSeO3 near TC in the spirit of model proposed by Grigoriev et al. [7] for the related system Mn1-xFexSi on the basis of the magnetic ac-susceptibility and SANS measurements. In this case, three vertical lines in Fig.2 (right to left) correspond to the development of partially chiral spin fluctuations, strongly chiral spin fluctuations and the appearance of a static chiral magnetic order. The temperature range T between two peaks of the specific heat corresponds to the region of strong chiral fluctuations. The value of T increases nearly linearly at high pressure for Cu2OSeO3 (from T = 0.31 K at P = 0 to T = 0.54 K at 4.6 GPa) in line with the increase of TC. For MnSi T decreases and vanishes under pressure [8], again in line with the pressure effect on TC (decrease of TC). Finally we see a very close similarity between two materials MnSi (metal) and Cu2OSeO3 (insulator) which may be of importance for development of theoretical models and for better understanding of role of DMI in chiral systems. This research was supported by the Russian Foundation for Basic Research (Grants 12-02-00376-a, 12-03-00665-а and 12-03-92604-КО_а), Program of the Physics Department of RAS on Strongly Correlated Systems, and Program of the Presidium of RAS on Physics of Strongly Compressed Matter. Fig. 1 Fig. 2 Fig.1 The magnetic P-T diagram of Cu2OSeO3 based on the magnetic ac-susceptibility (TC corresponds to a sharp drop of χ(T) in Fig.2) and specific heat (TC corresponds to a narrow and sharp peak of C(T) in Fig.2). Fig.2 The dependences of the magnetic ac-susceptibility χ(T) and specific heat C(T) of Cu2OSeO3 in the vicinity of TC at ambient pressure. References [1] T. Adams, A. Chacon, M. Wagner, A. Bauer, G. Brandl, B. Pedersen, H. Berger, P. Lemmens and C. Pfleiderer, Phys. Rev. Lett. 108, 237204 (2012). [2] A.E. Petrova, V.A. Sidorov and S.M. Stishov, Physica B 359-361, 1463 (2005). [3] V.A. Sidorov, J.D. Thompson and Z. Fisk, Journal of Physics: Condensed Matter 22, 406002 (2010). [4] C.L Huang, K.F. Tseng, C.C. Chou, S. Mukherjee, J.L. Her, Y.H. Matsuda, K. Kinda, H. Berger and H.D. Yang, Phys. Rev. B 83, 052402 (2011). [5] S.M. Stishov, A.E. Petrova, S. Khasanov, G.Kh. Panova, A.A. Shikov, J.C. Lashley, D. Wu and T.A. Lograsso, Phys. Rev. B 76, 052405 (2007). [6] S.M. Stishov and A.E. Petrova, Physics-Uspekhi 54, 1117 (2011). [7] S.V. Grigoriev, E.V. Moskvin, V.A. Dyadkin, D. Lamago, T. Wolf, H. Eckerlebe and S.V. Maleyev, Phys. Rev. B 83, 224411 (2011). [8] A.E. Petrova and S.M. Stishov, Phys. Rev. B 86, 174407 (2012).