1,721,120 research outputs found
A wide temperature range susceptometer
No full textA new type of high sensitivity magnetic
susceptometer is proposed. It utilises a long thin vibrating wire
as a sample support, which is driven at resonance by an alternating
field gradient, with no applied dc field. The gradient is
generated by a tiny soft ferromagnetic rod close to the sample.
The chosen configuration is particularly suitable to perform
measurements on thin films and small samples in a wide temperature
range (up to 1200 K). A realised prototype for high
temperatures is described, with a few examples of measurements.
An analysis of the moment sensitivity and considerations
on the noise are given, with indications of the physical
limits of the technique. A moment sensitivity of the order of
10^-11 Am2/(Hz)^(1/2), with an alternating field of 100 Nm, is estimated
for the present prototype
Thermal stability in exchange-spring chains of spins
Full text linkThermal stability and switching behaviour have been compared in pure-hard and soft–hard Heisenberg linear spin chains of the same total length and equal magnetic parameters (except for magnetic anisotropy) with the anisotropy axis and external magnetic field parallel to the chain direction. The zero-temperature energy barriers and finite-temperature transition rates between remanent equilibrium states have been calculated by utilizing the string method and the forward flux sampling (FFS) method, respectively. Depending on the assumed interfaces, the FFS method could in fact fail to correctly sample the characteristic transition paths at interfaces at which these paths have probabilities much lower than those associated with other non-characteristic transition paths. This can especially occur in the case of the asymmetric energy landscapes and multiple asymmetric minimum energy paths (MEPs) of soft–hard systems. Therefore, a proper interface definition is needed in order to deduce the correct transition rates. In particular, we show that the thermal switching of soft–hard chains starting in the soft or in the hard part turns out to occur with an equal rate provided that the interfaces of the FFS method are defined on the basis of the corresponding zero-temperature MEPs. The thermal stability of a soft–hard chain in the remanent equilibrium state could be to some extent lower with respect to that of a pure-hard chain, due to the shorter hard-part length crossed by the domain wall formed in the chain and also to the related slightly smaller energy barrier. However, its switching field at zero temperature is verified to be widely lower than that of the pure-hard chain. Analytical expressions of switching fields and energy barriers have been deduced in various cases
Discontinuous free rotations in uniaxial ferrimagnets
No full textThe magnetic-phase diagram of the free-powder anisotropic Néel ferrimagnet is presented in terms of the reduced sublattice anisotropy fields, x and y. It marks the boundaries of the regions of field-induced first-order transitions involving the collinear and the non-collinear states. The transition from the ferrimagnetic to the canted phase is always second order
Alternating field gradient susceptometer
No full textA new type of high sensitivity susceptometer has been developed. It is based on the principle of alternating field gradient, using a thin metallic wire as a resonating system. Test measurements up to 750°C have been performed on samples with a mass of few μg, like Ni foils, Fe-Al multilayers, Ba-ferrite disks and single crystals of ZnCo-W Ba-ferrite
Inclusion of surface anisotropy in the micromagnetic analysis of exchange-coupled hard/soft bilayers
Full text linkThe effects of surface anisotropy and interface anisotropy in soft and hard layers have been considered in the frame of a previously developed micromagnetic continuum model for the study of demagnetization processes in exchange-coupled bi- and multi-layers of soft/hard-type with planar and perpendicular anisotropy, in order to extend and generalize it. In particular, fields applied along the easy direction and strong interface ferromagnetic coupling have been considered. The theoretical results have been applied to a bilayer with perpendicular anisotropy in the hard layer and positive perpendicular surface anisotropy in the soft layer, in order to compare them with those obtained by considering the equivalent volume anisotropy in the soft layer. Moreover, the separate effects of positive perpendicular surface and interface anisotropies in the bilayer have been compared. We have deduced the nucleation field equation and obtained plots of the nucleation field as a function of the soft layer thickness and of the surface and interface anisotropies. Demagnetization curves have been also calculated. The nucleation process can be reversible (exchange-spring (ES) regime) or irreversible (rigid magnet (RM) regime) depending on magnetic and geometrical parameters. The critical equation, which allows determining the boundary between RM and ES regions, has been deduced. Corresponding phase diagrams in the plane of soft and hard layer thickness have been obtained as a function of surface and interface anisotropies
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