Spin-specific heat determination of the ratio of competing first- and second-neighbor exchange interactions in frustrated spin- 12 chains

The magnetic susceptibility χ(T) of spin-12 chains is widely used to quantify exchange interactions, even though χ(T) is similar for different combinations of ferromagnetic J1 between first neighbors and antiferromagnetic J2 between second neighbors. We point out that the spin-specific heat C(T) directly determines the ratio α=J2/|J1| of competing interactions. The J1-J2 model is used to fit the isothermal magnetization M(T,H) and C(T,H) of spin-12 Cu(II) chains in LiCuSbO4. By fixing α,C(T) resolves the offsetting J1,α combinations obtained from M(T,H) in cuprates with frustrated spin chains.