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We have confirmed that the tetragonal ternary compound DyIr2Si2 shows polymorphism; the ThCr2Si2-type structure as a low temperature phase (I-phase) and the CaBe2Ge2-type one as a high temperature phase (P-phase) exist. A comparative study on magnetic characteristics of the morphs was performed on the I- and P-phase single crystals in order to elucidate how magnetic properties are influenced by crystallographic symmetry. The magnetic behavior changes drastically depending on the structure. The DyIr2Si2(I) shows an antiferromagnetic ordering below TN = 30 K, additional magnetic transitions of T1 = 17 K and T2 = 10 K, and a strong uniaxial magnetic anisotropy with the easy [001] direction. The [001] magnetization shows four metamagnetic transitions at low temperatures. On the other hand, the DyIr2Si2(P) has comparatively low ordering temperature of TN1 = 9.4 K and an additional transition temperature of TN2 = 3.0 K, and exhibits an easy-plane magnetic anisotropy with the easy [110] direction. Two metamagnetic transitions appear in the basal plane magnetization processes. In both the morphs, the χ-T behavior suggests the existence of component-separated magnetic transitions. The ab-component of magnetic moments orders at the higher transition temperature TN1 for the P-phase compound, which is contrast to the I-phase behavior; the c-component orders firstly at TN. The crystalline electric field (CEF) analysis was made, and the difference in magnetic behaviors between both the morphs is explained by the CEF effects.

Magnetic measurements on the pseudo-ternary compounds Ho 1- x R x Rh 2 Si 2 (R = Y or La = Y or La) were performed in order to get information on the origin of “the successive component-separated magnetic transitions” which appear in HoRh 2 Si 2 . The lattice parameters a and c remain almost constant during changes to Y composition x , while they increase with increasing La composition x . The c / a ratios are also constant in the Y-system, and they increase with increasing x in the La-system especially for above around x = 0.4. The transition temperatures, T N1 = 29.1 K, T t = 27.3 K and T N2 =12.1 K at x = 0, decrease with increasing x . The rates of decrease for T N1 and T N2 in the Y-system accord with those in the La system below x = 0.4. The critical compositions for T N1 and T N2 are determined to be x N1 = 0.88 and x N2 = 0.59 in the La-system, respectively, and x N1 = 0.98 and x N2 = 0.75 in the Y-system. In both the systems, “the successive component-separated magnetic transitions” appear for the wide x regions. The magnetic ordered state persists in very dilute Ho-compounds in spite of much weak magnetic interactions. The effective magnetic moments are almost constant for the ordered compounds; μ eff = 10.6 ± 0.30 μ B /Ho. These behaviors are strange, indicating a strong correlation exists in these systems.