Moving toward an atomistic reader model

With the move to recording densities up to and beyond 1 Tb/in/sup 2/, the size of read elements is continually reducing as a requirement of the scaling process. The expectation is for read elements containing magnetic films as thin as 1.5 nm, in which finite size effects, and factors such as interface mixing might be expected to become of increasing importance. Here, we review the limitations of the current (micromagnetic) approach to the theoretical modeling of thin films and develop an atomistic multiscale model capable of investigating the magnetic properties at the atomic level. Finite-size effects are found to be significant, suggesting the need for models beyond the micromagnetic approach to support the development of future read sensors

Multiscale modeling of magnetic materials: Temperature dependence of the exchange stiffness

For finite-temperature micromagnetic simulations the knowledge of the temperature dependence of the exchange stiffness plays a central role. We use two approaches for the calculation of the thermodynamic exchange parameter from spin models: (i) based on the domain-wall energy and (ii) based on the spin-wave dispersion. The corresponding analytical and numerical approaches are introduced and compared. A general theory for the temperature dependence and scaling of the exchange stiffness is developed using the classical spectral density method. The low-temperature exchange stiffness A is found to scale with magnetization as m(1.66) for systems on a simple cubic lattice and as m(1.76) for an FePt Hamiltonian parametrized through ab initio calculations. The additional reduction in the scaling exponent, as compared to the mean-field theory (A similar to m(2)), comes from the nonlinear spin-wave effects

Electronic structure, exchange interactions and Curie temperature in diluted III-V magnetic semiconductors: (GaCr)As, (GaMn)As, (GaFe)As

We complete our earlier (Phys. Rev. B, {\bf 66}, 134435 (2002)) study of the electronic structure, exchange interactions and Curie temperature in (GaMn)As and extend the study to two other diluted magnetic semiconductors (GaCr)As and (GaFe)As. Four concentrations of the 3d impurities are studied: 25%, 12.5%, 6.25%, 3.125%. (GaCr)As and (GaMn)As are found to possess a number of similar features. Both are semi-metallic and ferromagnetic, with similar properties of the interatomic exchange interactions and the same scale of the Curie temperature. In both systems the presence of the charge carriers is crucial for establishing the ferromagnetic order. An important difference between two systems is in the character of the dependence on the variation of the number of carriers. The ferromagnetism in (GaMn)As is found to be very sensitive to the presence of the donor defects, like As$_{\rm Ga}$ antisites. On the other hand, the Curie temperature of (GaCr)As depends rather weakly on the presence of this type of defects but decreases strongly with decreasing number of electrons. We find the exchange interactions between 3d atoms that make a major contribution into the ferromagnetism of (GaCr)As and (GaMn)As and propose an exchange path responsible for these interactions. The properties of (GaFe)As are found to differ crucially from the properties of (GaCr)As and (GaMn)As. (GaFe)As does not show a trend to ferromagnetism and is not half-metallic that makes this system unsuitable for the use in spintronic semiconductor devices

Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press

Finite-temperature anisotropy of magnetic alloys

The temperature dependence of the magnetic anisotropy of ferromagnetic materials is analyzed. Simple ferromagnets, such as Fe and Co, obey the m=n (n+1)/2 power laws predicted by the Callen and Callen [Phys. Rev. 129, 578 (1963)] theory, but in alloys, the applicability of the theory is an exception rather than the rule. Many alloys, such as the rare-earth transition-metal intermetallics and L10 magnets, violate a basic assumption of the theory, namely, that the single-ion anisotropy and the spontaneous magnetization have the same origin. This is the reason for significant deviations from the Callen and Callen behavior, such as the m=2 law we obtained for L10 alloys

Field-controlled domain-wall resistance in magnetic nanojunctions

The electrical resistance of a constrained domain wall in a nanojunction is investigated using micromagnetic modeling and ballistic conductance calculations. The nanojunction represents two ferromagnetic electrodes connected by a ferromagnetic wire of 10 nm in length and of a few nanometers in cross section. We find that the anisotropy of the electrodes favors a localization of the domain wall within the constriction (wire) revealing a positive domain-wall resistance. An applied magnetic field moves the domain wall toward one of the electrodes and reduces its width. This compression of the domain wall leads to a sizeable enhancement of the domain-wall resistance

Ultrafast generation of ferromagnetic order via laser-induced phase transformation in FeRh thin films

It is demonstrated that ultrafast generation of ferromagnetic order can be achieved by driving a material from an antiferromagnetic to a ferromagnetic state using femtosecond optical pulses. Experimental proof is provided for chemically ordered FeRh thin films. A subpicosecond onset of induced ferromagnetism is followed by a slower increase over a period of about 30 ps when FeRh is excited above a threshold fluence. Both experiment and theory provide evidence that the underlying phase transformation is accompanied, but not driven, by a lattice expansion. The mechanism for the observed ultrafast magnetic transformation is identified to be the strong ferromagnetic exchange mediated via Rh moments induced by Fe spin fluctuations

Ultrafast generation of ferromagnetic order via laser-induced phase transformation in FeRh thin films

It is demonstrated that ultrafast generation of ferromagnetic order can be achieved by driving a material from an antiferromagnetic to a ferromagnetic state using femtosecond optical pulses. Experimental proof is provided for chemically ordered FeRh thin films. A subpicosecond onset of induced ferromagnetism is followed by a slower increase over a period of about 30 ps when FeRh is excited above a threshold fluence. Both experiment and theory provide evidence that the underlying phase transformation is accompanied, but not driven, by a lattice expansion. The mechanism for the observed ultrafast magnetic transformation is identified to be the strong ferromagnetic exchange mediated via Rh moments induced by Fe spin fluctuations