Magnetic-field induced instability surrounding the A-phase of MnSi:: Bulk and SANS measurements

We have determined the boundary of the magnetic A-phase of MnSi from distinct changes of slope in the AC susceptibility for magnetic field B applied parallel to the crystallographic (100), (110) and (111) directions. The specific heat displays an anomaly of a few percent at the border of the A-phase characteristic of a well-defined phase transition. In addition to the previously reported change of orientation of the helical modulation in the A-phase, we observe a gradual appearance of a ring of neutron small-angle scattering intensity that extends over the field and temperature regime of the A-phase. The estimated integrated intensity of this ring is a sizable fraction of the intensity in the helically ordered state, thus indicating that the change of the magnetic structure near the A-phase cannot be interpreted in terms of an abrupt change of orientation of a simple helical modulation alone. (c) 2006 Elsevier B.V. All rights reserved.