Elastic and anelastic anomalies associated with the antiferromagnetic ordering transition in wüstite, Fe_xO

Abstract

The elastic and anelastic properties of three different samples of Fe(x)O have been determined in the frequency range 0.1-2 MHz by resonant ultrasound spectroscopy and in the range 0.1-50 Hz by dynamic mechanical analysis in order to characterize ferroelastic aspects of the magnetic ordering transition at T(N) ~ 195 K. No evidence was found of separate structural and magnetic transitions but softening of the shear modulus was consistent with the involvement of bilinear coupling, λe(4)q, between a symmetry-breaking strain, e(4), and a structural order parameter, q. Unlike a purely ferroelastic transition, however, C(44) does not go to zero at the critical temperature, T*(c), due to the intervention of the magnetic ordering at a higher temperature. The overall pattern of behaviour is nevertheless consistent with what would be expected for a system with separate structural and magnetic instabilities, linear-quadratic coupling between the structural (q) and magnetic (m) driving order parameters, λqm(2), and T(N) > T*(c). Comparison with data from the literature appears to confirm the same pattern in MnO and NiO, with a smaller difference between T(N) and T*(c) in the former and a larger difference in the latter. Strong attenuation of acoustic resonances at high frequencies and a familiar pattern of attenuation at low frequencies suggest that twin walls in the rhombohedral phase have typical ferroelastic properties. Acoustic dissipation in the stability field of the cubic phase is tentatively attributed to anelastic relaxations of the defect ordered structure of non-stoichiometric wüstite or of the interface between local regions of wüstite and magnetite, with a rate controlling step determined by the diffusion of iron.

Domain boundary-dominated systems: adaptive structures and functional twin boundaries

“…The argument that twinning leads to smooth martensitic transformations (which untwinned material cannot do) was also proposed by Ball and James [120], who carefully characterized their symmetry conditions to provide optimal adaptation of the domain structures to the imposed boundary condition of the invariant plane. Similarly, Zhang et al [121] investigated magnetic systems and showed that smooth transformation behavior in wüstite could only be possible if the strain relaxation was optimized.…”

Multiferroic (ferroelastic/ferromagnetic/ferrimagnetic) aspects of phase transitions in RCo2Laves phases

“…Other systems in which linear-quadratic coupling is important are Fe x O, MnO, (La, Pr)AlO 3 and selected pnictides [99][100][101][102][103][104][105][106]. The topology of order parameter variations in NdCo 2 is expected to be closely similar to that of Fe x O, though the relevant instability in the latter may be due to a soft acoustic mode [99], rather than a cooperative Jahn-Teller distortion.…”

Efecto de la concentración de magnetita en la estructura, propiedades eléctricas y magnéticas de un material compuesto a base de resina de poliéster

“…7 se observa que por debajo del máximo la curva FC conserva una tendencia aproximadamente constante de la susceptibilidad magnética, lo que sugiere la ocurrencia de un comportamiento de tipo ferrimagnético [36]. El máximo valor observado en todas las muestras (M10-M40): cerca de T ≈ 120 K, se atribuye a la transición de Verwey de la magnetita [37].…”