Glossary

Exchange correlation length One of the two most relevant magnetic characteristic lengths. It gives the typical distance along which a small local departure from a homogeneous magnetization configuration extends due to exchange interactions. It coincides, in a uniaxial system, with the thickness of a wall separating two domains with antiparallel magnetizations and having the so-called Bloch-type structure (corresponding to the rotation of the moments inside the wall in a plane perpendicular to that containing the moments of the neighboring domains).

Magnetic anisotropy In the most general sense, the dependence of the magnetization process of a given magnetic material on the applied field direction. In the particular case of single crystal, that dependence is linked both to the spin orbit coupling and to the symmetry of the crystal field at the site of the ions bearing magnetic moments (magnetocrys-talline anisotropy). In other cases, the occurrence of magnetic anisotropy could be linked to the occurrence of large anisometries (shape anisotropy), either applied or residual stresses (magnetoelastic anisotropy) and/or chemical disorder ("pair ordering" anisotropy).

Magnetic domains Homogeneous magnetization regions formed in a sufficiently large magnetic system in order to minimize the magnetostatic energy. Two neighboring domains are separated by a transition structure named the domain wall. In a nanostructured system, the role of the domains in the magnetostatic energy minimization often corresponds to extense heterogeneous magnetization structures as those appearing in an isolated nanoparticle at the remanent state ("S" and "C" states, "flower state," etc.). Magnetic interactions In a localized moments picture short- (a few interatomic distances) and long-range moment couplings associated with the exchange coupling and with the magnetostatic fields, respectively.

Magnetic viscosity It is commonly observed as a time decay of the magnetization of an ordered magnetic material, measured at constant applied demagnetizing field and temperature. In a nanostructured magnetic material, and due to distribution of magnetic properties, thermally activated decay takes place in a time scale ranging from picoseconds up to several tens of years. The effects of the temperature fluctuations have become important in relation to the high-density magnetic recording, where they could cause thermal instability of the recorded information. Magnetization dynamics Magnetization processes taking place at short time scale (from pico- to nanoseconds), including the magnetization precession around the local field direction. The precession leads to important effects in fast magnetization dynamics, such as spinwave phenomenology, relevant to magnetic recording applications. Magnetization reversal process Process corresponding to the applied field induced evolution from the saturation remanence state to the fully reversed one (in which the local magnetization points parallel to the demagnetizing field). It can take place either locally or in a collective way, and, in the most general case, involves a sequence including reversed nucleus formation, nucleus expansion, nucleus steady propagation, and nucleus wall pinning/depinning stages. In a given material, the relationship between the characteristics of the reversal process and the material nanostructure (including here, dimensions, shape, and defect structure) determines the possibilities of tailoring its hysteretic behavior. Magnetoelasticity Modification of the magnetization process of a magnetically ordered material associated to the presence in it of mechanical stresses.

Magnetostatic correlation length One of the two most relevant magnetic characteristic lengths. It gives the typical distance along which a small local departure from a homogeneous magnetization configuration extends due to the magnetostatic interactions. It coincides, in a uniaxial system, with the thickness of a wall separating two domains with antiparallel magnetizations and having the so-called Neel-type structure (corresponding to the rotation of the moments inside the wall in a plane parallel to that containing the moments of the neighboring domains). Magnetostriction Variation of the dimensions of a magnetically ordered material submitted to different applied fields. Micromagnetic modeling Theoretical modeling of magnetic materials at nanoscale based on total magnetic energy minimization considerations. This allows one to predict qualitatively the variation, in a given magnetic material, of the hysteretic properties (mainly those of the coercivity and the remanence) associated to both intrinsic and extrinsic parameters.

Remanence Residual magnetization value measurable at zero applied magnetic field in a magnetically ordered material after submitting it to a saturating field. Saturation coercive force or saturation coercivity Applied demagnetizing field value required to reduce to zero the magnetization of a magnetically ordered material after saturating it.

Soft and hard magnetic materials Traditional classification of the technologically relevant magnetic materials based on their coercive force values. Soft magnetic materials exhibit reduced coercivities (from 1 A/m up to a few KA/m), whereas, the hard ones are difficult to demagnetize and are characterized by coercive force values in excess of the MA/m. Spontaneous magnetization In a ferromagnetic material formed by localized moments, the magnetization corresponding to the complete parallelism of all the moments in the system. In the case of a ferromagnetic system exhibiting order linked to itinerant electrons, the magnetization corresponding to the largest unbalance between the populations of the majority (spin up) and the minority (spin down) bands. The spontaneous magnetization at a given temperature coincides fairly well with that measured when the material is submitted to sufficiently large (saturating) fields. In the case of entities having nanometric dimensions, the division into domains is not favorable and, therefore, the local remanent magnetization is approximately equal to the spontaneous one.

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