Complex magnetoelastic properties in the frustrated kagome-staircase compounds (Co1-xNix)(3)V2O8

High-resolution heat-capacity and thermal-expansion measurements on single crystals of the kagome-staircase compounds (Co1-xNix)(3)V2O8 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 alpha(i) (i = a, b, and c) show a strong anisotropy for all (Co1-xNix)(3)V2O8 compounds. The low temperature expansivities indicate that Co doping (Ni doping) yields changes similar to uniaxial pressures along a or b (c). Linear Gruneisen parameters Gamma(i) are extracted for the three main axes i and are found to exhibit a complex temperature and doping dependence. For each axis, Gamma(i) and alpha(i) exhibit a sign change at low temperature at the critical concentration x(c) similar or equal to 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)(3)V2O8