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Register a new userExchange Interaction in Fe$_{1-x}$Ni$_{x}$Alloys: XPS Study
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S. S. Acharya
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In this paper, high Fe-concentration Fe$_{1-x}$Ni$_{x}$ alloys were investigated using high resolution X-ray photoelectron spectroscopy (XPS) down to 10K temperature. The Fe 2s core level exhibits three features, two low binding features corresponding to exchange interaction between ionized 2s core level and the unpaired 3d electrons. The high binding energy feature corresponds to the screening of 2s core hole by 4s conduction electrons. Our studies suggest high local magnetic moments on Fe sites.
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This paper reports high resolution X-ray photoelectron spectroscopy (XPS) studies on Fe$_{1-x}$Ni$_x$ (x=0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) alloys down to 10 K temperature. Core levels and Auger transitions of the alloys except the invar alloy (x=0.4) exhibit no observable temperature induced changes. The invar alloy exhibits changes in the core levels below 20 K temperature that strongly depend on the core level. Such core level dependent changes with temperature were attributed to the precipitation of spin glass like phase below 20 K only in the invar alloy. Ni L$_3$M$_{45}$M$_{45}$ Auger transition also supported such precipitation below 20 K.
Resistance of Fe$_{1-x}$Ni$_x$(x=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.9) has been measured using four probe method from 5K to 300K with and without a longitudinal magnetic field of 8T. The zero field resistivity of x=0.1 and 0.9 alloys, predominant contribution to resistivity above near room temperature is due to electron-phonon scattering, whereas for x=05 and 0.7 alloys electron-magnon scattering is dominant. Alloys with x=0.1 and 0.9 exhibit positive magnetoresistance(MR) from 5K to 300K. For x=0.5 and 0.7 alloys, magnetoresistance changes sign from positive to negative with increase in temperature. The temperature at which sign changes increase with Ni concentration in the alloy. The field dependent magnetoresistance is positive for x=0.1, 0.7 and 0.9 alloys whereas it is negative for x=0.5 alloy. MR follows linear behaviour with field for x=0.1 alloy. MR of all other alloys follow a second order polynomial in field.
Element specific ultrafast demagnetization was studied in Fe$_{1-x}$Ni$_{x}$ alloys, covering the concentration range between $0.1<x<0.9$. For all compositions, we observe a delay in the onset of Ni demagnetization relative to the Fe demagnetization. We find that the delay is correlated to the Curie temperature and hence also the exchange interaction. The temporal evolution of demagnetization is fitted to a magnon diffusion model based on the presupposition of enhanced ultrafast magnon generation in the Fe sublattice. The spin wave stiffness extracted from this model correspond well to known experimental values.
To clarify the role of the Kondo effect in screening local magnetic moments of Plutonium 5f--electrons as well as its competition to the RKKY interactions we use a combination of density functional theory with static Hartree Fock and dynamic Hubbard 1 approximations to calculate the strength of both the Kondo exchange, J_K, and of the RKKY exchange, J_RKKY, couplings for Pu{1-x}Am{x} system as a function of x. We find that J_K increases despite the atomic volume gets larger with the Am doping due to unexpected enhancement of hybridization between f and conduction electrons in the vicinity of the Fermi level. At the same time, the RKKY exchange is shown to reduce smoothly with increasing x. Our results imply that the Kondo effect should be robust against the increase in interatomic spacing of this alloy.
Magnetic susceptibility of the isostructural Ce(Ni{1-x}Cu{x})5 alloys (0< x <0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at fixed temperatures 77.3 and 300 K, using a pendulum-type magnetometer. A pronounced magnitude of the pressure effect is found to be negative in sign and to depend strongly and non-monotonously on the Cu content, showing a sharp maximum in vicinity of x = 0.4. The experimental results are discussed in terms of the Ce valence change under pressure. It has been concluded that the fractional occupation of the f-states, which corresponds to the half-integer valence of Ce ion (3.5), is favorable for the valence instability in alloys studied. For the reference CeNi5 compound the main contributions to magnetic susceptibility and their volume dependence are calculated ab initio within the local spin density approximation (LSDA), and appeared to be in close agreement with experimental data.
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