Magneto structural transition in the DySi CrB- and micro-structural changes in the FeB-type compounds by XRPD and neutron diffraction

Abstract

We present the magnetic temperature phase diagrams of the CrB- and FeB-type orthorhombic phases of the DySi compound, determined from high-quality powder XRPD and neutron diffraction, as well as the sample microstructure as determined by HRTEM. Both phase diagrams comprise a HT (Tc1, Tc2−TN) and a LT range (5K–Tc1, Tc2) separated by a monoclinic phase transition at Tc1=Tc2=23.5K well below the second-order Néel transition (TN=40K). The transition paths are for CrB-type Cmcm (HT) Tc1→C2/m11 (LT), and for FeB Pnma (HT) Tc2→P21/n11 (LT). The transitions are related to non-monotonous changes of the lattice and structural parameters displaying anomalies at Tc1, Tc2 and slight volume changes. For the CrB-type the monoclinic angle decreases smoothly from Tc1 down to 5K and the maximum strain experienced by the crystal lattice in the (021) direction was found at Tc1.In the FeB-type, in addition to the magneto-elastic transition at Tc2=23.5K, minor anomalies are found at 13.5K in the temperature dependence of the monoclinic angle and the maximum strain along (011). Both temperatures mark the sequence of changes in the magnetic domain microstructure observed in FeB: below T2=23.5K the incommensurate HT magnetic phase with q3≈(01217) disproportionates into two LT phases q3≈(012111)T2 and q2≈(01216) coexisting in the form of domains with portions varying with T going from T2 down to 13.5K (q2 increasing at the cost of q3). This behaviour could be related to structural inhomogeneities below the first-order magneto-elastic transition Tc2, if one assumes a broad two phase range, where the HT (Pnma) phase coexists with (P21/n) as a metastable phase at LT in the form of domains with different magnetic behaviour.