Isotope shift of the ferromagnetic transition temperature in itinerant ferromagnets

Abstract

We present a theory of the isotope effect of the Curie temperature $T_{\mathrm{c}}$ in itinerant ferromagnets. The isotope effect in ferromagnets occurs via the electron-phonon vertex correction and the effective attractive interaction mediated by the electron-phonon interaction. The decrease of the Debye frequency increases the relative strength of the Coulomb interaction, which results in a positive isotope shift of $T_{\mathrm{c}}$ when the mass $M$ of an atom increases. Following this picture, we evaluate the isotope effect of $T_{\mathrm{c}}$ by using the Stoner theory and a spin-fluctuation theory. When $T_{\mathrm{c}}$ is large enough as large as or more than 100K, the isotope effect on $T_{\mathrm{c}}$ can be measurable. Recently, precise measurements on the oxygen isotope effect on $T_{\mathrm{c}}$ have been performed for itinerant ferromagnet SrRuO${}_3$ with $T_{\mathrm{c}}\sim 160$K. A clear isotope effect has been observed with the positive shift of $T_{\mathrm{c}}\sim 1$K by isotope substitution (${}^{16}O{\to }^{18}O$). This experimental result is consistent with our theory.

arXiv comment6 pages, 7 figures

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