Positive magnetoresistance and large magnetostriction at first-order antiferro-ferromagnetic phase transitions in RMn₂Si₂ compounds

The magnetostriction and magnetoresistance associated with the field-induced and spontaneous first-order antiferro-ferromagnetic (AF-F) phase transitions have been studied for quasi-single-crystalline samples of La _0.25Sm_0.75Mn₂Si₂, La _0.25Y_0.75Mn₂Si₂ and La _0.27Y_0.73Mn₂Si₂ compounds with natural layered ThCr₂Si₂-type structure. It was found that both the spontaneous and field-induced AF-F transitions are accompanied by a large volume magnetostriction ΔV/V≈2 × 10^-3 and anisotropic linear changes of the lattice parameters Δa/a≈1.6 × 10^-3, Δc/c≈-0.75 × 10^-3. The field-induced AF-F magnetic phase transition has been observed in magnetic fields applied both along the c-axis and in the basal plane, and the magnetostriction value is virtually independent of the direction of applied field. It has been found also that the magnetoresistance is positive in these compounds (the value of the electrical resistance in the ferromagnetic state is higher than that in the antiferromagnetic state) for the fields applied both along the c-axis and in the basal plane. The value of the magnetoresistance observed along the c-axis is 30 times as high as that in the basal plane. The obtained results indicate that the electronic band structure changes are likely responsible for the AF-F magnetic phase transitions observed in the RMn₂X₂ compounds.