Ahtract-A mean-field model is developed for amorphous ferromagnetic materials with potential applications in thermomagnetic recordinglmagneto-optical readout systems. The emphasis is on the reduction of the number of adjustable parameters, so that important variables and their effects on magnetic properties can be investigated. The available experimental data on GdCo-, GdFe-, and TbFe-based alloys is compared with the model predictions and good agreement is obtained in all cases. Expressions for the exchange stiffness coefficient and macroscopic anisotropy energy constant are derived and the latter is compared with available experimental data. The results have been used to study domain wall characteristics of the three material systems. A I. INTRODUCTION MORPHOUS rare earth-transition metal (RE-TM) alloys have proved extremely suitable for thermomagnetic recording and magneto-optical readout applications [ 11-[6]. In thin film form, these media exhibit strong perpendicular anisotropy, which makes them particularly useful for polar Kerr or Faraday effect readout. Being ferromagnetic, they possess a compensation point temperature that can be brought to the vicinity of room temperature by proper choice of composition. This feature preserves the uniform magnetic alignment of the media by preventing the magnetization from breaking into oppositely oriented domains. Moreover, the high coercivity around the compensation point protects the recorded data from stray magnetic fields. The amorphous nature of the films eliminates a significant source of noise previously encountered in polycrystalline media [7]. Surface roughness and grain boundary noise are no longer-limiting factors in the readout performance of the RE-TM alloys. The first step in the study of thermomagnetic recording and erasure processes in the RE-TM alloys is the development of a model that can explain the behavior of magnetization versus temperature [8], [9]. Mean-field theory provides a simple solution to this problem, although its usefulness has been marred in the past by the existence of too many adjustable parameters [lo]-[15]. .Our goal in this paper is to develop a mean-field model for amorphous RE-TM alloys that can explain the available data with as Manuscript
Articles you may be interested inPhotoemission electron microscopy study of remanent magnetic domain states in ferromagnetic wedge films deposited on substrates with micrometer-sized square plateaus J. Appl. Phys. 99, 063904 (2006); 10.1063/1.2174119 Variation of domain-wall structures and magnetization ripple spectra in permalloy films with controlled uniaxial anisotropy J. Appl. Phys. 98, 053905 (2005); 10.1063/1.2033152 Magnetic domain wall transitions based on chirality change and vortex position in thin Permalloy™ films Cross-tie walls and magnetic singularities on the surface of permalloy films (abstract)The Bloch to Néel wall transition is investigated in Permalloy films between 160 and 10 nm thickness using direct integration of the Landau-Lifshitz-Gilbert equation in a three-dimensional Cartesian lattice. At 80 nm, the wall is a symmetric Bloch wall characterized by two adjoining vortices with the magnetization at the wall center pointing perpendicular to the plane of the material throughout the thickness. The Bloch to Néel transition takes place between 35 and 30 nm, below which the wall becomes a symmetric Néel wall. For the Bloch walls, our wall energy per unit area calculations match reasonably well the results of A. Hubert's Ritz method calculations ͓Magnetic Domains ͑Springer, New York, 1998͒, p. 251͔ and A. E. Labonte's numerical calculations ͓J. Appl. Phys. 40, 2450 ͑1969͔͒. For the Néel walls, however, our results indicate an approximately 70% higher energy for thicknesses of 30 nm and below, since the Néel wall tails are included. For thicknesses below 160 nm, the anisotropy energy component is low, and both C-shaped and symmetric Bloch walls are dominated by exchange interaction. As the wall transforms from Bloch to Néel below 35 nm, the energy contribution changes from 76% exchange and 24% demagnetization to 70% demagnetization and 30% exchange, respectively. Wall widths are computed for thicknesses between 10 and 640 nm along with the out-of-plane magnetization due to the presence of the vortex.
Three-dimensional magnetic structures of pi-vertical-Bloch line (VBL) and 2pi-VBL are investigated in 80–320-nm-thick Permalloy films using direct integration of the Landau–Lifshitz–Gilbert equation in an 128×128×80 point Cartesian lattice. A pi-VBL reflects the shapes of its adjacent walls, which change with film thickness. The pi-VBL conducts the flux between walls of opposite chirality by letting the magnetization rotate out of the plane of the walls via a vortex structure. The Néel caps switch chirality via a “converging point,” or cross-tie, flux at one surface and a vortex flux, or a swirl, at the other surface. The pi-VBL energy per unit area is 0.44, 0.20, and 0.30 erg/cm2 in 80, 160, and 320 nm films, respectively. The corresponding pi-VBL widths are 88, 60, and 86 nm. A stable winding 2pi-VBL structure was also computed by combining two pi-VBL structures of appropriate chirality. The Néel caps intersect via the pair-(swirl, converging point) like flux at both surfaces. The width of the 2pi-VBL is 140 nm and its energy per unit area is 0.57 erg/cm2.
Domain wall mobility in Permalloy films has been calculated as a function of thickness at 10, 80, and 160 nm which reflects the structure change of Néel, symmetric Bloch and C-shaped (asymmetric Bloch) domain walls. The mobility has been derived from the dynamics of a single nonperiodic domain wall using direct integration of the Landau–Lifshitz–Gilbert equation in a Cartesian lattice. This investigation allows for a detailed examination of spin precession, wall motion and overall magnetization distortion as the wall is moved in the presence of fields ranging from 0.5 to 5 Oe applied in the easy axis direction. At 10-nm-thick films, the mobility of a Néel wall is 30 m/s Oe. Wall motion takes place without noticeable distortion in the magnetization distribution in the vicinity of the Néel wall. For 80 nm-thick films, the mobility of a symmetric Bloch wall is 5 m/s Oe, or 20% less than the theoretical prediction for the mobility of a 180° domain wall model. At dynamic equilibrium, the symmetric Bloch wall has been slightly distorted into a C-shaped wall. For 160 nm-thick films, the mobility of a C-shaped wall is 12 m/s Oe, or 29% less than the theoretical predicted value. Shearing-type magnetization distortion is observed in this composite structure of a Bloch component at the center and Néel caps of opposite chirality at the surfaces.
The three possible transitions of a wall involving a change of chirality and position of a vortex were previously identified in Permalloy™ using the Kerr effect. These transitions have now been simulated using direct integration of the Landau–Lifshitz–Gilbert equation in a 1 300 000 point Cartesian lattice. One transition is between two C-shaped, same chirality walls whose vortices are on opposite sides. The transition is done via a 32-nm-long pi-VBL structure at the surfaces and at the center via a shape transition 117 nm long. The pi-vertical Bloch line (pi-VBL), which changes chirality along the wall, conducts the flux between the walls of opposite chirality via a vortex structure by letting the magnetization rotate out of the plane of the walls at the center of this vortex. The Néel caps switch chirality via an antivortex flux at one surface and a vortex flux at the other surface. Another transition is a pi-VBL that takes place between two C-shaped, opposite chirality walls whose vortices are on opposite sides of the walls. The transition between the Néel caps of same chirality is a jog at the top surface, while it is a vortex–antivortex pair at the bottom surface.
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