Competing magnetic structures in the DySi FeB-type phase diagram

The temperature magnetic phase diagrams, of the dimorphic DySi compound, have been studied in terms of wave vectors in the range 1.5-45 K, by neutron diffraction. The polycrystalline sample consists of 26% of CrB-type (Cmcm no. 63, all atoms at 4c site: (0, y, 1/4)) and of 74% of FeB-type (Pnma no. 62, all atoms at 4c site: (x, 1/4, z)). The CrB-ordering is described by the wave vector: q(1) = (0, 0, 1/2) over the entire magnetically ordered regime with a uniaxial magnetic structure along the shortest axis c. The FeB-type magnetic phase diagram reveals three distinct regions of magnetic ordering below T-N and one first order transition at T-2=23.5K (on heating). The ordering is described by two symmetry independent magnetic vectors q(2)=(0, 1/2,1/6) and q(3)=(0, q(3y), q(3z)) with a temperature variable length. At 1.5 K q(3y) approximate to 1/2 and q(3z) approximate to 1/11. The two phases coexist in the form of domains. They differ in the moment orientation of the q(3) phase that deviates by similar to 22 degrees from the b-axis in the (0, 0, 1) plane. The low temperature range (LT) 1.5 K-T-2 subdivides into two regions: (i) LT-1, between 1.5 K-T-1 where the relative amount of the two phases remains unchanged and in (ii) LT-2: T-1-T-2 where the amount of the incommensurate q3 phase increases at the cost of the commensurate q(2) amplitude modulated structure which remains unchanged but fully disappears at the first order transition at T-2=23.5 K. The q(3) phase undergoes minor changes until 22 K and gets destabilised at T-2 where the q(3z) component jumps from the LT value q(3z) approximate to 1/11 to the HT value approximate to 1/7 and the q(3y) component increases from 0.484(1) to 0.495(1). (iii) The high temperature (HT) range T-2-T-N (T-N=40 +/- 1 K) is described by a single wave vector q(3). The disproportionation of the HT magnetic phase q(3) below T-2 into two coexisting distinct phases q(2), q(3) down to 1.5 K is an unusual phenomenon, to our knowledge observed for the first time. Various mechanisms are discussed. (C) 2009 Elsevier B.V. All rights reserved.