Intrinsic instability of the helix spin structure in MnGe and order-disorder phase transition

E. Altynbaev, S.-A. Siegfried, V. Dyadkin, E. Moskvin, D. Menzel, A. Heinemann, C. Dewhurst, L. Fomicheva, A. Tsvyashchenko, and S. Grigoriev
Phys. Rev. B 90, 174420 – Published 17 November 2014

Abstract

The magnetic structure of the cubic helimagnet MnGe was studied by small-angle neutron scattering in a wide temperature range from 10 to 300 K. We show that MnGe undergoes a complex order-disorder phase transition covering more than 100 K above the critical temperature TN=(130±2) K. Moreover, the helical structure is intrinsically unstable below TN, since the profile of the reflection at kh2 nm1 associated with the spin helix can be described by the sum of Gaussian and Lorentzian contributions, indicating a stable helix and spin-helix fluctuations, respectively. The Gaussian behavior determines the system at low temperature, whereas the Lorentzian is negligible. With increasing temperature, however, the Lorentzian starts to contribute and dominates at TN. The spin-helix fluctuations are accompanied by intensive spin excitations observed in small-angle neutron scattering as Q-independent scattering at Q<kh. The integral intensities of both the spin excitations and spin fluctuations obey exactly the same temperature law proportional to exp(T/Ta) with the activation temperature Ta=(80±1) K. These two contributions coexist above TN, being well distinguished up to TDM=150 K, where they are intermixed with and transform into diffuse scattering at TSR=(175±5) K. Above TSR the diffuse scattering is well described by a Gauss law, ascribed to static short-range ferromagnetic inhomogeneities with an inverse correlation length κ2 nm1.

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  • Received 10 September 2014

DOI:https://doi.org/10.1103/PhysRevB.90.174420

©2014 American Physical Society

Authors & Affiliations

E. Altynbaev1,2, S.-A. Siegfried3, V. Dyadkin1,4, E. Moskvin5,2, D. Menzel5, A. Heinemann3, C. Dewhurst6, L. Fomicheva7, A. Tsvyashchenko7, and S. Grigoriev1,2

  • 1Petersburg Nuclear Physics Institute, Gatchina, 188300 Saint Petersburg, Russia
  • 2Faculty of Physics, Saint Petersburg State University, 198504 Saint Petersburg, Russia
  • 3Helmholtz Zentrum Geesthacht, 21502 Geesthacht, Germany
  • 4Swiss-Norwegian Beamlines at the ESRF, Grenoble 38000, France
  • 5Technische Universität Braunschweig, 38106 Braunschweig, Germany
  • 6Institute Laue-Langevin, F-38042 Grenoble Cedex 9, France
  • 7Institute for High Pressure Physics, 142190, Troitsk, Moscow Region, Russia

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Issue

Vol. 90, Iss. 17 — 1 November 2014

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