Quantum magnets BaCuSi2O6 [1] and NiCl2-4SC(NH2)2 [2] attracted significant attention in the last years, because magnetic fields can be used to tune a canted antiferromagnetic state regarded as a Bose-Einstein condensate of magnons. The coupling between spin and lattice degrees of freedom, however, was not yet studied in the ordered state. Different anisotropy in their crystallographic lattices and their spin arrangements, spin S = 1/2 Cu2+ dimers in BaCuSi2O6 and S = 1 Ni2+ in NiCl2-4SC(NH2)2, are responsible for different degree of spin-lattice coupling. While the former only displays weak lattice effects, the later shows a significant magnetostriction that changes sign as the canting angle between spins is reduced and the spin arrangement evolves gradually from AFM at the critical field Hc1, to FM at the saturation field Hc2. Recent specific heat, magnetocaloric effect and magnetostriction measurements at high magnetic fields will be discussed for these two fascinating compounds.
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