Enhancement of ferromagnetism by oxygen isotope substitution in strontium ruthenate SrRuO(3)

The oxygen isotope effect of the ferromagnetic transition in itinerant ferromagnet strontium ruthenate SrRuO(3) with a Curie temperature T(c) of 160 K is studied. We observed for the first time a shift of ∆T(c) ~ 1 K by oxygen isotope substitution of (16)O → (18)O in SrRuO(3) by precise measurements of DC and AC magnetizations. The results surprisingly lead to the noteworthy inverse isotope effect with negative coefficient α = −∂ lnT(c)/∂ lnM. The Raman spectra indicate that the main vibration frequency of (16)O at 363 cm(−1) shifts to 341 cm(−1) following oxygen isotope substitution (18)O. This shift is remarkably consistent with the Debye frequency being proportional to ∝ 1√M where M is the mass of an oxygen atom. The positive isotope shift of ∆T(c) can be understood by taking account of the electron-phonon interaction.