Magnetic phase diagrams of the CrB- and FeB-type HoSi compounds

Abstract The temperature magnetic phase diagrams of the dimorphic HoSi compound were studied by neutron diffraction. The sample comprises 35.5% CrB- (Cmcm) and 64.5% FeB-type (Pnma) of structure. Both phases order antiferromagnetically below TN=25 K and undergo first-order magnetic transitions at Tic=16.5 K. Their T-phase diagrams comprise a low temperature (LT) 2.7 K−Tic and a high temperature (HT) range Tic−TN with distinct wave vectors. The LT magnetic ordering of the CrB-type HoSi with the wave vector q1=(1/2, 0, 1/2) corresponds to a uniaxial magnetic structure, with the Ho moments along the shortest axis c. At 2.7 K the ordered moment value is 8.6(2) μB/Ho atom. The HT ordering, described by the wave vector q2=(q2x, 0, q2z) with a T-variable length, corresponds to an amplitude modulated structure. The magnetic ordering of the FeB-type HoSi requires two symmetry independent vectors q3=(0, q3y, q3z) for the LT- and q4=(q4x, q4y, 0) for the HT range. Both vectors correspond to sine wave modulated structures with the Ho magnetic moments confined along the shortest axis b. The q3 vector has an almost invariable length vs. T close to ≈(0, 9/17, 1/11). At 2.7 K the amplitude of the wave is 10.9(1) μB/Ho atom. At Tic q3 jumps to the wave vector q4=(q4x, q4y, 0) with a T-variable length. At 17 K q4=(0.092(1), 0.538(3), 0). Around Tic there is a narrow coexistence range of the q3 and q4 competing phases. Various models are discussed and compared with the isomorphic RSi (R=rare earth) compounds counterparts of HoSi, a comparison that has led us to briefly review the magnetic structures available in the literature for this interesting class of compounds.