It is well known that the archetype chiral magnet MnSi stabilizes a skyrmion lattice, termed “A-phase,” in a narrow temperature range in the vicinity of the paramagnetic boundary around Tc ∼ 29 K and Hc ∼ 2 kOe. Recently, it has been predicted that at much lower temperatures below Tc, the conical helicoid and the forced ferromagnetic (FFM) states could be separated by a new “ unknown state.” In order to detect this “ unknown state,” we explored the phase diagram of MnSi oriented single crystals as a function of the d.c. magnetic field [Formula: see text] and the temperature ( T) by using a.c. magnetization measurements. For [Formula: see text] 〈111〉, we observed a new region, termed “B-phase,” in the magnetic phase diagram, characterized by a flat-valley-like anomaly on the in-phase component of the a.c. magnetization ( m′), over 3.5 ≤ H dc ≤ 6.2 kOe just below the low temperature ( T < 6 K) FFM boundary. The observed frequency independence over 0.3–1000 Hz and the absence of any measurable absorption in the a.c. magnetization ( m″) in the “B-phase” suggest a static nature. The “B-phase” was not observed for either [Formula: see text] 〈100〉 or 〈110〉, revealing that the magnetic anisotropy could play a role in the stabilization of the phase. The “B-phase” could be compatible with the theoretical predictions if the new magnetic state is supposedly related with a relative reorientation of the four helices in MnSi.
Small angle neutron scattering measurements in the chiral helimagnet MnSi were performed to investigate the magnetic state near the critical field at low temperature in which a unknow magnetic state was suggested theoretically. The magnetic Bragg reflections due to the conical phase were observed in the region, however, any other reflections due to a possible new magnetic state were not observed neither in the configurations H // [111] ⏊ k i or H // [111] // k i , where k i is the incident neutron beam vector. It suggests the new magnetic state has a different nature from the typical magnetic skyrmionic phase appearing in a small pocket near the magnetic transition temperature.
The nuclear and magnetic structure of the spinel MnCr2O4 is reinvestigated by magnetization, specific heat and neutron diffraction experiments at different temperatures. Four samples of this spinel synthesized under different atmospheres are analysed. Through these experiments a new magnetic phase, with propagation vector kI2 = (0.6597(1) 0.5999(1) 0.1996(2)), not previously reported, is identified below 18 K when the sample is synthesized under a reductive atmosphere. A possible explanation for the different magnetic ground states observed is given based on the competition among the main exchange interactions present in the system. Using the magnetic superspace group formalism, the symmetry of the nuclear and magnetic structures is determined. The presence of transverse conical magnetic structures in the lower-temperature phases allows for multiferroicity in this compound, and the electric polarization direction is determined for each magnetic phase.
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