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 SrRuO3 with a Curie temperature Tc of 160 K is studied. We observed for the first time a shift of ∆Tc ~ 1 K by oxygen isotope substitution of 16 O → 18 O in SrRuO3 by precise measurements of DC and AC magnetizations. The results surprisingly lead to the noteworthy inverse isotope effect with negative coefficient α = -∂ lnTc /∂ 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 ∆Tc can be understood by taking account of the electron-phonon interaction.