Ana L. Dantas

Giant magnetocaloric effect of thin Ho films

We report a theoretical study of the impact of finite size on the magnetocaloric effect (MCE) of thin Ho films. For strong external field strengths, the adiabatic temperature change ΔTad is comparable to the value found in bulk Ho, reaching about 12 K for an external field strength change ΔH = 50 kOe. For thicknesses below the helix period, there is a large enhancement. In this thickness range, the helical state does not form, leading to a giant MCE reaching ΔT/ΔH = 6.5 K/T for ΔH = 2 kOe.

Interface roughness effects on coercivity and exchange bias

We report model calculations of the hysteresis loops of exchange-coupled ferromagnet/antiferromagnet bilayers with monolayer-scale roughness and show that the loops are affected by the combined effect of the interface field strength, the degree of magnetic roughness and magnetostatic effects. The magnetization reversal may occur via domain-wall nucleation at the edges of monoatomic interface steps or coherent magnetization rotation. A magnetic phase diagram is constructed for a 10-nm-thick Fe film, subjected to nanometer-scale interface roughness.

Nucleation of vortex pairs in exchange biased nanoelements

We report a theoretical investigation of interface effects in the magnetic order of interface biased iron and Permalloy™ elliptical nano-elements. Contrary to intuition, there is a partial pinning of the interface layer, favoring double vortex states along the hysteresis loop. Interface biasing affects the relative chirality and the distance of the vortices. Unbiased nanoelements may nucleate vortex pairs with the same chirality separated by an antivortex. For interface biased nanoelements the vortex pair forms with opposite chirality separated by a magnetic domain.

Controlling the vortex core of thin Permalloy nano-cylinders dipolar coupled to Co polarizers

We report a theoretical study of the vortex profile of in-plane magnetized PyTM nano-cylinders subjected to the stray field of perpendicular anisotropy Co nano-cylinders. We consider 6 nm thick PyTM cylinders dipolar coupled to 60 nm thick Co cylinders, at distances from 1.5 nm to 30 nm, with diameters (D) ranging from 45 nm to 105 nm. We find considerable reduction of critical diameter for stable PyTM magnetic vortices and spiral-vortex phases, as well as vortex core diameters twice as large as the bulk value.

Controlling the core-to-core distance of vortex pairs in exchange-biased iron elliptical nanoelements

We report a theoretical study of vortex pairs in exchange-biased elliptical iron nanoelements. We show that the remanent state may be tailored to fit vortex pairs with opposite chiralities separated by a diamond-like domain. Flat nanoelements with lateral dimensions ranging from 115 nm × 425 nm to 195 nm × 425 nm have the core-to-core distance tunable by the interface field strength.

Depinning field of a periodic domain wall array in vicinal nanowires

We report a theoretical investigation of the magnetic states and depinning field of a periodic array of head-to-head domain walls of flat Fe rectangular nanowires, exchange coupled with a vicinal two-sublattice uniaxial antiferromagnetic substrate. We show that for strong interface exchange energy, domain walls are pinned at interface steps perpendicular to the antiferromagnetic easy axis, separating terraces with opposite interface exchange field. The array sequence, which alternates head-to-head and tail-to-tail domain walls, may form a structure with alternate chirality or with the same chirality. The domain wall dipolar field affects the chirality sequence, which is tunable by the geometrical constraints and the strength of the interface exchange field. The depinning field of 10 nm thick, 1 μm long wires, with widths of 100 and 200 nm, is of the order of the interface field strength, and the depinning process involves domain wall motion and the transversal displacement of a periodic array of vortices.

Thermal hysteresis of interface biased ferromagnetic dots

We present a theoretical investigation of the thermal hysteresis of iron dots exchange-coupled to an antiferromagnetic substrate. We consider a temperature interval bounded by the Neel temperature of the substrate, and we calculate the heating and cooling curves in the presence of an external field oriented opposite to the interface exchange field. The thermal hysteresis is due to the temperature variation of the interface field and the influence of the geometrical shapes and sizes of the dots on the magnetic states and switching mechanisms. We show that Fe dots on an uncompensated NiO substrate exhibit large thermal hysteresis at room temperature, and external fields of a few kOe. The width of the hysteresis loops depends on the dimensions of the ferromagnetic dot, and can be significant for dots elongated in the direction of the interface field.

Thermal Hysteresis of Thin Dy Films

We report a theoretical study of thermal hysteresis of thin Dy films. Large differences between the isofield magnetization curves during heating and cooling are found. The temperature width of the thermal hysteresis is tunable by the external field strength, ranging from 40 to 100 K, for external fields in the 50 Oe-1.5 kOe interval

Thermal hysteresis of interface biased dipolar coupled nanoelements

We report a theoretical investigation of thermal hysteresis of a pair of interface biased elliptical iron nanoelements, separated by an ultrathin layer of nonmagnetic material. The thermal hysteresis originates in the strong dipolar interaction, and is tunable by the nature of the low temperature state and the eccentricity of the nanoelements. The width of the thermal hysteresis varies from 500 K to 100 K for lateral dimensions of 125 nm × 65 nm and 145 nm × 65 nm.

Excitations of domain walls pinned at F/AF interface steps

Summary form only given. The magnetic properties of thiii ferromagnetic (F) films grown on antiferromagnetic (AF) substrates may be controlled to a large extent by the nature of the interface. Interface roughness in F/AF bilayers is a subject of current interest and despite the research effort dedicated to the subject and the large amount of experimental data accumulated so far, there are still unsolved questions. In this contribution we show that the step defect may produce strong reduction of the domain wall width and that the frequency of rigid displacement domain wall oscillations is determined by the interface exchange energy within the domain wall region. We consider a Neel wall of a thin uniaxial ferromagnetic film on a two-sublattice AF substrate. The uniaxial direction of the substrate is supposed to coincide with the easy axis direction (z-axis) of the ferromagnet. The normal to the surface is along the y-axis and a step, running along the z-direction, divides the substrate into two regions, each containing spins from opposite sublattices of antiferromagnet.