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Register a new userMagnetic order of intermetallic FeGa$_{3-y}$Ge$_y$ studied by $mu$SR and $^{57}$Fe Mossbauer spectroscopy
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Temperature dependent magnetization, muon spin rotation and $^{57}$Fe Mossbauer spectroscopy experiments performed on crystals of intermetallic FeGa$_{3-y}$Ge$_{y}$ ($y=0.11,0.14,0.17,0.22,0.27$, $0.29,0.32$) are reported. Whereas at $y=0.11$ even a sensitive magnetic microprobe such as $mu$SR does not detect magnetism, all other samples display weak ferromagnetism with a magnetic moment of up to 0.22 $mu_B$ per Fe atom. As a function of doping and of temperature a crossover from short range to long range magnetic order is observed, characterized by a broadly distributed spontaneous internal field. However, the $y=0.14$ and $y=0.17$ remain in the short range ordered state down to the lowest investigated temperature. The transition from short range to long range order appears to be accompanied by a change of the character of the spin fluctuations, which exhibit spin wave excitations signature in the LRO part of the phase diagram. Mossbauer spectroscopy for $y=0.27$ and 0.32 indicates that the internal field lies in the plane perpendicular to the crystallographic $c$ axis. The field distribution and its evolution with doping suggest that the details of the Fe magnetic moment formation and the consequent magnetic state are determined not only by the dopant concentration but also by the way the replacement of the Ga atoms surrounding the Fe is accomplished.
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One initial and essential question of magnetism is whether the magnetic properties of a material are governed by localized moments or itinerant electrons. Here we expose the case for the weakly ferromagnetic system FeGa$_{3-y}$Ge$_y$ wherein these two opposite models are reconciled, such that the magnetic susceptibility is quantitatively explained by taking into account the effects of spin-spin correlation. With the electron doping introduced by Ge substitution, the diamagnetic insulating parent compound FeGa$_3$ becomes a paramagnetic metal as early as at $ y=0.01 $, and turns into a weakly ferromagnetic metal around the quantum critical point $ y=0.15 $. Within the ferromagnetic regime of FeGa$_{3-y}$Ge$_y$, the magnetic properties are of a weakly itinerant ferromagnetic nature, located in the intermediate regime between the localized and the itinerant dominance. Our analysis implies a potential universality for all itinerant-electron ferromagnets.
We have performed detailed $^{57}$Fe Mossbauer spectroscopy measurements on Ba$_{0.78}$K$_{0.22}$Fe$_2$As$_2$ and BaFe$_{2-x}$Ni$_x$As$_2$ single crystal mosaics showing antiferromagnetic ordering below $T_N$ with superconductivity below $T_C$. Analysis of the Mossbauer spectra shows a decrease in the magnetic hyperfine (hf) field but no change in the magnetic volume fraction below $T_C$. This clearly indicates the coexistence of magnetism and superconductivity in these compounds. The decrease in the magnetic hf field below $T_C$ depends on the difference between $T_N$ and $T_C$, being the largest for $T_N$ close to $T_C$. Two different explanations for this observation are given. We also find that the non-magnetic volume fraction below $T_N$ correlates with the Ni doping $x$, being large for high $T_C$ and small for high $T_N$.
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