Investigation of the paramagnetic state and itinerant electron magnetism in YCo12−xFexB6 (x = 0–4) intermetallic compounds

This work aims to investigate the delocalization degree of itinerant electrons in the YCo_12−xFe_xB₆ compounds through Takahashi’s theory of spin fluctuations. The paramagnetic state of the YCo_12−xFe_xB₆ series of compounds is first studied through the evolution of the Curie and Curie–Weiss temperatures as a function of the Fe content. The parent compound YCo₁₂B₆ is a ferromagnet with T_C = 155(3) K, \(\theta_{C}\) = 144(3) K and 0.45(4) µ_B/3d at. at 2 K, while the Fe-rich compound YCo₈Fe₄B₆ shows the behaviour of a very weakly ferromagnetic compound with T_C = 51(5) K, \(\theta_{C}\) = − 41(5) K and 0.11(4) µ_B/3d at. at 2 K, markers of dominant antiferromagnetic (AFM) interactions. The paramagnetic state was also studied through the evolution of the effective paramagnetic moment compared to the measured moment in the magnetically ordered state. We then applied Takahashi’s theory of spin fluctuations to macroscopic magnetization data to determine the physical parameters T_A and T₀, usually accessible only through local probe methods. The YCo_12−xFe_xB₆ series of compounds was placed on the Deguchi-Takahashi plot, from a slightly delocalized state (p_eff/p_s = 4.27 for x = 0) towards a fully delocalized state (p_eff/p_s = 23.54 for x = 4) by increasing the Fe content. Other markers, like the Curie–Weiss temperature, the metamagnetic transitions for x = 1.5 to 2.5, and the shape parameter s of the thermal evolution of the spontaneous magnetization indicate that this delocalization of the itinerant electron magnetism is accompanied by a progressive strengthening of the AFM interactions as Fe content increases. This study shows that magnetic properties of the YCo_12−xFe_xB₆ family of compounds can only be explained properly by accounting for the spin fluctuations, as they help to transcribe the evolution from a rather localized itinerant electron magnetism towards a completely delocalized case.