Unconventional enhancement of ferromagnetic interactions in Cd-doped Gd Fe 2 Zn 20 single crystals studied by ESR and Fe 57 Mössbauer spectroscopies

Single crystals of $\mathrm{Gd}{\mathrm{Fe}}_{2}{\mathrm{Zn}}_{20\ensuremath{-}x}{\mathrm{Cd}}_{x}$ ($0.0l\phantom{\rule{4pt}{0ex}}x\phantom{\rule{4pt}{0ex}}l1.4$) were grown and characterized through structural, magnetic, and electronic properties using x-ray diffraction, field- and temperature-dependent magnetization, specific heat, $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy, and electron spin resonance (ESR). A negative chemical pressure effect is accompanied by an unexpected increase of ${T}_{C}$ from 86 to 96 K, together with a reduction of the magnetic effective moment and saturation magnetic moment, as evidenced by all of the experimental techniques. From the microscopic point of view, probing at the $4f$ electron level and the Fe nucleus has allowed the extraction of important information about the configuration and the effective role of the partial Cd substitution for Zn in this ferromagnetic system. Our $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy experiments show a negligible variation of the hyperfine field at the Fe site, and ESR experiments reveal an enhancement of the Korringa-type relaxation and a molecular field effect as Cd is incorporated. This complex behavior is assigned to a possible reconstruction of the Fermi surface and/or a new distribution of the $d$ type of conduction electrons in response to the negative chemical pressure, leading to an enhancement of the ferromagnetic transition temperature in a generalized Ruderman-Kittel-Kasuya-Yosida interaction scenario.