The pseudo‐brookite spin‐glass system studied by means of muon spin relaxation

Zero-field muon spin relaxation (µSR) experiments have been performed on the spin glass Fe1.75Ti1.25O5. Above the spin-glass temperature of 44 K a distinct exponential µSR rate (¿) is observed, while below Tg a square-root exponential decay occurs, indicating fast spin fluctuations. Near 8 K, a maximum in ¿ is indicative of transverse spin ordering. The low ¿ values and the sharp ¿ peak at Tg are very promising for the study of spin freezing models like the Vogel–Fulcher law or the power law

Altruistic behaviour in Dictyostelium discoideum explained on the basis of individual selection

It is often argued that natural selection acting at the level of the individual may not be sufficient to explain the evolution of altruism. We suggest that before accepting such a point of view in any specific instance, the parsimonious course would be to examine all possible ways in which individual-level selection might act and rule out its sufficiency only when the postulated means for its action are either inherently improbable or experimentally disproven. As an illustration we propose an evolutionary model, based on the individual cell as the unit of selection, for the maintenance of 'altruistic' behaviour by pre-stalk cells in the social amoeba Dictyostelium discoideum

Altruistic behaviour in Dictyostelium discoideum explained on the basis of individual selection

It is often argued that natural selection acting at the level of the individual may not be sufficient to explain the evolution of altruism. We suggest that before accepting such a point of view in any specific instance, the parsimonious course would be to examine all possible ways in which individual-level selection might act and rule out its sufficiency only when the postulated means for its action are either inherently improbable or experimentally disproven. As an illustration we propose an evolutionary model, based on the individual cell as the unit of selection, for the maintenance of 'altruistic' behaviour by pre-stalk cells in the social amoeba Dictyostelium discoideum

Magnetic anisotropy in the cubic Laves REFe2 intermetallic compounds

In the past, the Callen–Callen (1965 Phys. Rev. 139 A455–71; 1966 J. Phys. Chem. Solids 27 1271–85) model has been highly successful in explaining the origin and temperature dependence of the magneto-crystalline anisotropy in many magnetic compounds. Yet, despite their high ordering temperatures of ~650 K, the Callen–Callen model has proved insufficient for the REFe2 compounds. In this paper, we show that it is possible to replicate the values of the phenomenological parameters K1, K2, and K3 given by Atzmony and Dariel (1976 Phys. Rev. B 13 4006–14), by extending the Callen–Callen model to second order in HCF. In particular, explanations are provided for (i) the unexpected changes in sign of K1 and K2 in HoFe2 and DyFe2, respectively, and (ii) the origin and behaviour of the K3 term. In addition, it is demonstrated that higher order terms are required, and that K4 exceeds K3 at low temperatures. Revised estimates of K1, K2, K3, K4, and K5 are given. Finally, an alternative 'multipolar' approach to the problem of magnetic anisotropy is also provided. It is shown that the latter confers significant advantages over the older phenomenological method. In particular, all the multipolar coefficients (\tilde {K}_N , N = 4, 6, 8, 10, 12) decrease monotonically with increasing temperature, with \tilde {K}_N decreasing faster than \tilde {K}_{N-2} etc. These observations are in accord with expectations based on the original Callen–Callen model