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 T N =25 K and undergo first-order magnetic transitions at T ic =16.5 K. Their T-phase diagrams comprise a low temperature ( LT) 2.7 K− T ic and a high temperature ( HT) range T ic − T N with distinct wave vectors. The LT magnetic ordering of the CrB-type HoSi with the wave vector q 1=(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) μ B/Ho atom. The HT ordering, described by the wave vector q 2=( q 2 x , 0, q 2 z ) with a T-variable length, corresponds to an amplitude modulated structure. The magnetic ordering of the FeB-type HoSi requires two symmetry independent vectors q 3=(0, q 3 y , q 3 z ) for the LT- and q 4 =( q 4 x , q 4 y , 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 q 3 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) μ B/Ho atom. At T ic q 3 jumps to the wave vector q 4 =( q 4 x , q 4 y , 0) with a T-variable length. At 17 K q 4 =(0.092(1), 0.538(3), 0). Around T ic there is a narrow coexistence range of the q 3 and q 4 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.