Papers by Keyword: Exchange Coupling

Abstract: In this work, theoretical investigation in coherent manipulation throughout local density of states calculation for serially coupled double quantum dots embedded between ferromagnetic leads (FM-QD1-QD2-FM) by using the non-equilibrium Green's function approach. Since the local density of states are formulated incorporating the spin polarization and the type of spin configuration on the leads. Our model incorporates the inter-dot hopping, the intra-dot Coulomb correlation, the spin exchange energy and the coupling interactions between the quantum dots and leads. The results concerned to the parallel configuration at strong inter-dot coupling regime shows that the spin down electrons in the quantum dots may be more coupled coherently if the regime is tuned. The local density of states of the two dots for spin up electrons shows a broad hump with small splitting i.e. the case is decoherent for spin up electrons. In the case of weak interdot coupling it is obvious that the spin dependent density of states on the quantum dots show that the resonances are not well splitted. For the antiparallel configuration in the strong coupling regime, the spin dependent density of states of the double quantum dots show four peaks but with broaden and overlapping. In the case of weak coupling regime, the total spin dependent density of states, which have two peaks with certain board, one can conclude that the states are not coupled coherently. The case of the antiferromagnetic nature of the spin exchange interaction, our calculations for the parallel and antiparallel configurations (in strong and weak regimes) show a decoherence state.

Abstract: The existence of exchange coupling or magnetostatic coupling between two phases, one magnetically hard and other magnetically soft is a way for obtaining higher maximum energy product (BH_max). In this study, it is discussed the microstructures for obtaining either exchange coupling or magnetostatic coupling in real materials. One relevant condition is that the assumptions of the Stoner-Wohlfarth model should be obeyed, with both phases, hard and soft, with dimensions less than single domain particle size. A real possibility is enveloping a spherical grain of hard phase with a first shell of soft phase, and another second shell or layer of a paramagnetic phase. The paramagnetic phase may play relevant role promoting magnetically decoupling between the grains.

Abstract: The effect of physical and technological factors on the effectiveness of the exchange coupling in two-layer film structures such as Fe₂₀Ni₈₀/Fe₅₀Mn₅₀ has been investigated. The dependences of the coercive force and the exchange bias field of permalloy layer on presence of a buffer Ta layer and its thickness, the order of sputtering and the thickness of Fe₂₀Ni₈₀ and Fe₅₀Mn₅₀ layers, substrate temperature and the features of radio-frequency (RF) substrate bias during deposition were investigated. It has been found that the substrate RF bias improves homogeneity of microstructure and reduces the surface roughness of permalloy layer. This in turn increases the magnetic pinning of permalloy layer and creates the prerequisites for the practical application of layered structures with higher thickness of the ferromagnetic layers.

Abstract: Nanocrystalline magnets (SmCo5)94(Cr3C2)6 were prepared using melt-spinning and their magnetic behaviors were investigated by studying their structures and magnetic properties. The alloys prepared using rapid solidification consisted of SmCo5 matrix phase, MgCu2-type SmCo2 secondary phase, and small traces of Sm2Co7 phase. The solidification with higher wheel speed were found to be preferable for the formation of single SmCo2 secondary phase. Relatively high coercive values of 28–36 kOe and high reduced remanence of 0.78–0.79 were observed for the SmCo5/SmCo2 nanocrystalline magnets. The shape of the corresponding magnetization curves revealed that two magnetization processes, nucleation and pinning of domain walls, took place in these magnets. The Henkel plots indicated strong inter-grain exchange coupling effect in these ribbons, consequently resulted in the phenomena of domains interacting with each other and enhancement of the remanence in the ribbons.

Abstract: Nanocomposite (Nd,Dy)₂Fe₁₄B/α-Fe magnets were prepared by directly solidification (DS). The effect of wheel speed on the magnetic properties, microstructure and exchange coupling interaction has been studied. It was found that a uniform R₂Fe₁₄B/α-Fe nanocomposite structure with fine α-Fe grains can be developed at an optimum wheel speed of about 18 m/s. Without any heat treatment, the optimal ribbons (v = 18 m/s) show a strong exchange coupling interaction and good magnetic properties, e.g. _iH_c=1027 kA/m, m_r =0.71, (BH)_max=174 kJ/m³.

Abstract: The uniform of the heat treatment process and NdFeB alloy powder metallurgy technology on the α-Fe soft phase effects. Experimental results show that the uniform of the heat treatment process can significantly reduce the α-Fe soft phase of the content, and the thinning, also found that the remaining α-Fe soft phase sintering and after annealing and hard to deal with after the occurrence of magnetic exchange coupling And its magnetic properties can have a greater impact, so this experiment for the Development of the fourth generation of rare earth permanent magnets laid the foundation.

Abstract: FMR and SWR spectra of exchange-coupled structures NiFe/Cu/NiFe, NiFe/Ag/NiFe and NiFe/DyCo/NiFe have been investigated. We revealed that the optical satellites of the exchange spin-wave modes are characterized not by the standard Kittle-dependence but by their own resonance field dependence on the mode number: H_r^opt(n) ~ n^5/2.

Abstract: Fe19Ni81/Tb₃₅Co₆₅ magnetic films having unidirectional anisotropy were investigated with focus on the influence of the Fe19Ni81 layer and Ti spacer thicknesses on magnetic and magnetoresistive properties of the multilayered structures. Principles of the optimization of the properties of magnetoresistive multilayers with unidirectional anisotropy were proposed.

Abstract: This study continues previous work on off-stoichiometric Fe-Pd alloys using a combined reaction strategy during thermomechanical processing [1,2]. Severe plastic deformation of the initial disordered fcc gamma phase (γ) of compostion Fe-35at.%Pd, followed by heat treatment in the two phase field produces a nano-composite ferromagnet comprised of soft alpha phase/ferrite (α) in a high-anisotropy L1₀ FePd matrix. The length scale and morphology of the transformation products have been characterized using x-ray diffraction, and scanning electron microscopy. The transformed microstructures exhibit strong texture retention similar to the stoichiometric alloy suggesting a massive ordering mode. The alloy has shown a proclivity to exchange couple at a length scale not in agreement with proposed theories of exchange coupling [3,4]. The magnetic properties were measured using standard vibrating sample magnetometry (VSM). This research has been supported by the National Science Foundation (NSF-DMR).

Abstract: The exchange bias phenomena in Tb/Cr films, where the Curie temperature (TC) of ferromagnetic Tb is much less than the Néel temperature (TN) of antiferromagnetic Cr, are systematically investigated. For the Tb/Cr bilayers, the exchange bias field HE changes from negative to positive as the temperature goes up. The unusual behavior of HE(T) is due to the spin-density waves spin structure in the Cr layer. For the Tb/Cr/Tb/Cr film, however, HE is negative throughout the temperature range in our experiment and its absolute value decreases monotonously with the temperature, which is ascribed to the coexistence of the interlayer coupling between Tb layers and the interfacial exchange coupling between Tb and Cr layers.