Complex magnetoelastic properties in the frustrated kagome-staircase compounds (Co1-xNix)3V2O8
Résumé
High-resolution heat-capacity and thermal-expansion measurements on single crystals of the kagome-staircase compounds (Co1−xNix)3V2O8 are presented. The parent compounds Co3V2O8 and Ni3V2O8 undergo a sequence of first- and second-order magnetic phase transitions. The low-temperature (T⩽40 K) magnetic entropy evolves monotonically with the doping content x from the full value expected for Ni2+ S=1 magnetic moments in Ni3V2O8 to only half the value expected for Co2+ S=3/2 moments in Co3V2O8. The thermal-expansion coefficients αi (i=a, b, and c) show a strong anisotropy for all (Co1−xNix)3V2O8 compounds. The low-temperature expansivities indicate that Co doping (Ni doping) yields changes similar to uniaxial pressures along a or b (c). Linear Grüneisen parameters Γi are extracted for the three main axes i and are found to exhibit a complex temperature and doping dependence. For each axis, Γi and αi exhibit a sign change at low temperature at the critical concentration xc≃0.25 where the incommensurate magnetic propagation vector changes. Our study motivates further investigations to understand how the multiple and complex parameters such as magnetic frustration, magnetic anisotropy, and mixture of S=1 and 3/2 ions affect the rich magnetoelastic properties of (Co1−xNix)3V2O8.