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Magnetismus Poster: Do., 13:00–15:30 D-P228
The magnetic structure of MnSi under applied field by polarized SANS
Sergey Grigoriev 1 , Sergey Maleyev 1 , Alexey Okorokov 1 , Yurii
Chetverikov 1 , Helmut Eckerlebe 2 , Klaus Pranzas 2 , Piter Böni 3 , Robert
Georgii 3
1 Petersburg Nuclear Physics Institute, Gatchina, St.Petersburg, 188350, Russia –
2 GKSS Forschungszentrum, 21502 Geesthacht, Germany – 3 ZWE FRM-II/E21, Technische
Universitat Munchen, 85747 Garching, Germany
The magnetic structure of the single crystal MnSi under applied field has been studied
by small angle diffraction with polarised neutrons below TC = 28.7 K. Experiments
have shown that in zero field the magnetic structure of MnSi consists of four lefthanded
spiral domains oriented along four 〈111〉 axes. The magnetic field, applied
along one of the 〈111〉 axes, produces the single domain helix oriented along the field
at HC1 ≈ 80 mT at low temperatures. The magnetic mosaic of the spin structure
changes with the magnetic field and has a maximum at HC1. The integral intensity
of the Bragg reflection shows a sharp minimum at Hin ≈ 160 mT attributed to the
instability of the helix structure. When the field has a component perpendicular to
the helix wavevector k, the wavevector rotates toward the field direction in the field
range H < Hin. Additionally the second harmonic of the helix structure is induced by
the perpendicular magnetic field at H < Hin. These three features are well explained
accounting for the presence of the spin wave gap ∆ ∼ Hin/ √ 2 � 11 µeV, which provides
the stability of the spin wave spectrum with respect to the perpendicular magnetic
field. Further increase of the field leads to the magnetic phase transition from conical
to ferromagnetic state near HC2 ≈ 600 mT. The critical field HC2 is related to the
spin of the spin wave stiffness A as gµHC2 = Ak 2 . Our findings are in agreement with
the recently developed theory [PRB, 73 (2006) 174402] for the cubic magnets with
Dzyaloshinskii-Moria interaction, which relates the major parameters of the spin wave
spectrum (such as the spin wave stiffness and the gap) with the features of the spin
structure of MnSi being observed under applied magnetic field.