Large dynamical magnetic charges driven by exchange striction

Magnetoelectric (ME) materials are of fundamental interest and are investigated for their broad potential applications. First-principles methods have only recently been developed to calculate the full ME response tensor $\alpha$ including both electronic and ionic contributions.\footnote{A. Malashevich et al., Phys. Rev. B, {\bf 86}, 094430 (2012).} In several materials, the dominant contribution to the ME response has been shown to be the ionic term $\alpha_{\rm ion}$, which is proportional to both the Born charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$.\footnote{J. \'{I}\~{n}iguez, Phys. Rev. Lett. {\bf 101}, 117201 (2008).} Here we present a theoretical study of mechanisms that could enhance the magnetic charge $Z^{\rm m}$. The KITP\-ite structure is reported with large ME response arising from exchange striction and spin frustration.\footnote{K. Delaney et al., Phys. Rev. Lett., {\bf 102}, 157203 (2009).} Using first-principles density-functional methods, we calculate the atomic $Z^{\rm m}$ tensors in KITP\-ite and conclude that even when SOC is completely absent, the exchange striction acting on the non-collinear spin structure induces much larger magnetic charges than in the case when $Z^{\rm m}$ is driven by SOC as in $\rm{Cr_2O_3}$.