Magnetic friction in Ising spin systems

A new contribution to friction is predicted to occur in systems with magnetic correlations: Tangential relative motion of two Ising spin systems pumps energy into the magnetic degrees of freedom. This leads to a friction force proportional to the area of contact. The velocity and temperature dependence of this force are investigated. Magnetic friction is strongest near the critical temperature, below which the spin systems order spontaneously. Antiferromagnetic coupling leads to stronger friction than ferromagnetic coupling with the same exchange constant. The basic dissipation mechanism is explained. If the coupling of the spin system to the heat bath is weak, a surprising effect is observed in the ordered phase: The relative motion acts like a heat pump cooling the spins in the vicinity of the friction surface.Comment: 4 pages, 4 figure

Experimental application of sum rules for electron energy loss magnetic chiral dichroism

We present a derivation of the orbital and spin sum rules for magnetic circular dichroic spectra measured by electron energy loss spectroscopy in a transmission electron microscope. These sum rules are obtained from the differential cross section calculated for symmetric positions in the diffraction pattern. Orbital and spin magnetic moments are expressed explicitly in terms of experimental spectra and dynamical diffraction coefficients. We estimate the ratio of spin to orbital magnetic moments and discuss first experimental results for the Fe L_{2,3} edge.Comment: 11 pages, 2 figure

Individual variation in age‐dependent reproduction: Fast explorers live fast but senesce young?

Adaptive integration of life history and behaviour is expected to result in variation in the pace‐of‐life. Previous work focused on whether ‘risky’ phenotypes live fast but die young, but reported conflicting support. We posit that individuals exhibiting risky phenotypes may alternatively invest heavily in early‐life reproduction but consequently suffer greater reproductive senescence. We used a 7‐year longitudinal dataset with >1,200 breeding records of >800 female great tits assayed annually for exploratory behaviour to test whether within‐individual age dependency of reproduction varied with exploratory behaviour. We controlled for biasing effects of selective (dis)appearance and within‐individual behavioural plasticity. Slower and faster explorers produced moderate‐sized clutches when young; faster explorers subsequently showed an increase in clutch size that diminished with age (with moderate support for declines when old), whereas slower explorers produced moderate‐sized clutches throughout their lives. There was some evidence that the same pattern characterized annual fledgling success, if so, unpredictable environmental effects diluted personality‐related differences in this downstream reproductive trait. Support for age‐related selective appearance was apparent, but only when failing to appreciate within‐individual plasticity in reproduction and behaviour. Our study identifies within‐individual age‐dependent reproduction, and reproductive senescence, as key components of life‐history strategies that vary between individuals differing in risky behaviour. Future research should thus incorporate age‐dependent reproduction in pace‐of‐life studies

Exchange bias and interface electronic structure in Ni/Co3O4(011)

A detailed study of the exchange bias effect and the interfacial electronic structure in Ni/Co3O4(011) is reported. Large exchange anisotropies are observed at low temperatures, and the exchange bias effect persists to temperatures well above the Neel temperature of bulk Co3O4, of about 40 K: to ~80 K for Ni films deposited on well ordered oxide surfaces, and ~150 K for Ni films deposited on rougher Co3O4 surfaces. Photoelectron spectroscopy measurements as a function of Ni thickness show that Co reduction and Ni oxidation occur over an extended interfacial region. We conclude that the exchange bias observed in Ni/Co3O4, and in similar ferromagnetic metallic/Co3O4 systems, is not intrinsic to Co3O4 but rather due to the formation of CoO at the interface.Comment: 8 pages, 6 figures. Accepted for publication in Physical Review B

Giant Magnetoelectric Effect via Strain-Induced Spin-Reorientation Transitions in Ferromagnetic Films

It is shown theoretically that a giant magnetoelectric susceptibility exceeding 10^-6 s/m may be achieved in the ferromagnetic/ferroelectric epitaxial systems via the magnetization rotation induced by an electric field applied to the substrate. The predicted magnetoelectric anomaly results from the strain-driven spin-reorientation transitions in ferromagnetic films, which take place at experimentally accessible misfit strains in CoFe2O4 and Ni films.Comment: 7 pages, 3 figure

Weak decays of 4He-Lambda

We measured the lifetime and the mesonic and non-mesonic decay rates of the 4He-Lambda hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K- beam on a liquid 4He target by the reaction 4He(K-,pi-)4He-Lambda. The 4He-Lambda lifetime was directly measured using protons from Lambda p -> n p non-mesonic decay (also referred to as proton-stimulated decay) and was found to have a value of tau = 245 +/- 24 ps. The mesonic decay rates were determined from the observed numbers of pi-'s and pi0's as Gamma_pi-/Gamma_tot = 0.270 +/- 0.024 and Gamma_pi0/Gamma_tot = 0.564 +/- 0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Gamma_p/Gamma_tot = 0.169 +/- 0.019 and Gamma_n/Gamma_tot <= 0.032 (95% CL), respectively. The effects of final-state interactions and possible 3-body Lambda N N decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the non-mesonic branching fractions. The implications of the results of this analysis were considered for the empirical Delta I = 1/2 rule and the decay rates of the 4H-Lambda hypernucleus.Comment: 15 pages, 11 figures, published in PRC, revised content to match published versio

Exchange-induced frustration in Fe/NiO multilayers

Using spin-polarized low-energy electron microscopy to study magnetization in epitaxial layered systems, we found that the area vs perimeter relationship of magnetic domains in the top Fe layers of Fe/NiO/Fe(100) structures follows a power-law distribution, with very small magnetic domain cutoff radius (about 40 nm) and domain wall thickness. This unusual magnetic microstructure can be understood as resulting from the competition between antiferromagnetic and ferromagnetic exchange interactions at the Fe/NiO interfaces, rather than from mechanisms involving the anisotropy and dipolar forces that govern length scales in conventional magnetic domain structures. Statistical analysis of our measurements validates a micromagnetic model that accounts for this interfacial exchange coupling.Comment: 15 pages, 2 figure

Fecundity and the demographic strategies of coral morphologies

JM, AB and SC were supported by fellowships from the Australian Research Council (FT110100609, FT0990652 and DP0880544 respectively). MD was supported by the ERC (BioTIME 250189) and the Scottish Funding Council (MASTS - HR09011).Understanding species differences in demographic strategies is a fundamental goal of ecology. In scleractinian corals, colony morphology is tightly linked with many demographic traits, such as size-specific growth and morality. Here, we test how well morphology predicts the colony size-fecundity relationship in eight species of broadcast-spawning corals. Variation in colony fecundity is greater among morphologies than between species with a similar morphology, demonstrating that colony morphology can be used as a quantitative proxy for demographic strategies. Additionally, we examine the relationship between size-specific colony fecundity and mechanical vulnerability (i.e. vulnerability to colony dislodgment). Interestingly, the relationship between size-specific fecundity and mechanical vulnerability varied among morphologies. For tabular species, the most fecund colonies are the most mechanically vulnerable, while the opposite is true for massive species. For corymbose and digitate colonies, mechanical vulnerability remains relatively constant as fecundity increases. These results reveal strong differences in the demographic trade-offs among species of different morphologies. Using colony morphology as a quantitative proxy for demographic strategies can help predict coral community dynamics and responses to anthropogenic change.Publisher PDFPeer reviewe

Life history evolution, species differences and phenotypic plasticity in hemiparasitic eyebrights (Euphrasia)

Premise of the study: Species delimitation in parasitic organisms is challenging as traits used in the identification of species are often plastic and vary depending on the host. Here, we use species from a recent radiation of generalist hemiparasitic Euphrasia to investigate trait variation and trait plasticity. We test whether Euphrasia species show reliable trait differences, investigate whether these differences correspond to life history trade-offs between growth and reproduction, and quantify plasticity in response to host species. Methods: We perform common garden experiments to evaluate trait differences between eleven Euphrasia taxa grown on a common host, document phenotypic plasticity when a single Euphrasia species is grown on eight different hosts, and relate our observations to trait differences recorded in the wild. Key results: Euphrasia exhibit variation in life history strategies; some individuals transition rapidly to flower at the expense of early season growth, while others invest in vegetative growth and delay flowering. Life history differences are present between some species, though many related taxa lack clear-cut trait differences. Species differences are further blurred by phenotypic plasticity—many traits are plastic and change with host type or between environments. Conclusions: Phenotypic plasticity in response to host and environment confounds species delimitation in Euphrasia. When grown in a common garden environment it is possible to identify some morphologically distinct taxa, though others represent morphologically similar shallow segregates. Trait differences present between some species and populations demonstrates the rapid evolution of distinct life history strategies in response to local ecological conditions."Manyhosts.csv" contains morphological measurements from one Euphrasia arctica population from North Berwick, Scotland, grown with eight hosts. "Manyspecies.csv" contains morphological measurements of five Euphrasia species and six natural Euphrasia hybrids grown on a single host, Trifolium repens. "Earlylate.csv" contains repeated growth measurements at different times of year, used in correlations of height at end of season. "Wildcommon.csv" contains Euphrasia species grown in the common garden experiment and wild collected plants for trait comparisons.Data collection is detailed in the associated manuscript. Post collection data processing can be viewed at: https://github.com/Euphrasiologist/phenotypic_plasticity_euphrasi

Negative Domain Wall Resistance in Ferromagnets

The electrical resistance of a diffusive ferromagnet with magnetic domain walls is studied theoretically, taking into account the spatial dependence of the magnetization. The semiclassical domain wall resistance is found to be either negative or positive depending on the difference between the spin-dependent scattering life-times. The predictions can be tested experimentally by transport studies in doped ferromagnets.Comment: 4 pages, 2 figures, accepted Phys. Rev. Let