The evolution of magnetic order in CrFe alloys. II. Onset of ferromagnetism

Proximity-enhanced magnetocaloric effect in ferromagnetic trilayers

The demagnetization and associated magnetocaloric effect (MCE) in strong-weak-strong ferromagnetic trilayers, upon a reorientation of the strong ferromagnets from parallel to antiparallel (AP) magnetization, is simulated using atomistic spin dynamics. The simulations yield non-trivial spin distributions in the AP state, which in turn allows entropy to be calculated directly. The influence of longer-range spin-spin interactions and of variable strength of the external switching field are investigated. Finally, we find that the MCE in the system can be significantly improved by allowing the local exchange to vary through the spacer, which in practice can be implemented by spatially tailoring the spacer's magnetic dilution.

Debye temperature of disordered bcc-Fe-Cr alloys

The Debye temperature, Θ(D), of Fe(100-x)Cr(x) disordered alloys with 0 ≤ x ≤ 99.9 was determined from the temperature dependence of the centre shift of (57)Fe Mössbauer spectra recorded in the temperature range of 60-300 K. Its compositional dependence shows an interesting non-monotonous behaviour. For 0<x ≤ ∼ 45, as well as for ∼ 75 ≤ x ≤ ∼ 95, the Debye temperature is enhanced relative to its value of a metallic iron, and at x ≈ 3 there is a local maximum having a relative height of ∼ 12% compared to a pure iron. For ∼ 45 ≤ x ≤ ∼ 75 and for x ≥ ∼ 95 the Debye temperature is smaller than the one for the metallic iron, with a local minimum at x ≈ 55 at which the relative decrease of Θ(D) amounts to ∼ 12%. The first maximum coincides quite well with that found for the spin-waves stiffness coefficient, D(o), while the pretty steep decrease observed for x ≥ ∼ 95, which is indicative of a decoupling of the probe Fe atoms from the underlying chromium matrix, is likely related to the spin-density waves which constitute the magnetic structure of chromium in that interval of composition and show also anomalous dynamic behaviour. The harmonic force constant calculated from the effective Debye temperature of the least Fe-concentrated alloy (x ≥ 99.9) amounts to only 23% of the one characteristic of a pure chromium as determined from the heat capacity experiment.

Finite-size scaling in band ferromagnets with non-universal critical behavior

The ultra-high sensitivity of the ferromagnetic resonance (FMR) technique has been fully exploited to study the finite-size effects in the critical region near the ferromagnetic to paramagnetic phase transition in Cr(75-x)Fe(25+x) (x = 0, 5) thin films of high structural and magnetic quality. Conclusive experimental evidence is provided for the validity of finite-size scaling. Irrespective of the film thickness and composition, the critical exponents β, γ and ν for spontaneous magnetization, initial magnetic susceptibility and spin-spin correlation length retain their bulk values so that no dimensionality crossover occurs within the film thickness range covered in the FMR experiments. The present results indicate that (i) like Cr(75-x)Fe(25+x), the previously studied Fe, Co, Ni, and CoNi(3) thin films behave as itinerant-electron (band) ferromagnets in which the isotropic long-range interactions between spins decay as J(r)∼r(-(d+σ)) (σ>0), and (ii) the lattice dimensionality d, spin dimensionality m, and range of spin-spin interactions (via the material-specific parameter σ) decide the (non-universal) values of the critical exponents.