Amorphous Rare Earth-transition Metal Alloys Research Articles

Thermomagnetic switching in amorphous rare-earth transition-metal alloys with high compensation temperature

The thermomagnetic switching process in amorphous Tb-FeCo alloys has been investigated theoretically for typical compositions with a Curie temperature of 500 K and compensation temperatures ranging from 360 to 420 K. The temperature and radial dependence of the relevant magnetic parameters such as the saturation magnetization, the exchange constant, the uniaxial anisotropy, and the domain wall energy have been calculated in terms of the respective sublattice magnetizations inferred from the mean field theory and data obtained experimentally. The condition for domain stability has been derived and the radial dependence of the forces controlling the domain wall position is calculated yielding domain diameters for writing and erasure as a function of the applied field, the wall energy, and the laser-induced temperature. Heating above the Curie temperature is shown to offer an alternative possibility to control the domain diameter and to achieve high domain regularity at low applied magnetic fields.

Basic Theories of Amorphous Ferrimagnets and Research on Amorphous Ferrimagnetic Insulators

Theoretical problems of amorphous ferrimagnets are discussed. Transition temperatures, compensation temperatures and thermal dependences of magnetizations in amorphous ferrimagnetic rare earth-transition metal alloys are investigated using the effective-field theory with correlations. Research on amorphous ferrimagnetic insulators is described. The results obtained suggest a new field of research in magnetism.

A system for measurement of domain wall motion in amorphous rare earth-transition metal alloys

A system has been developed for measuring domain-wall motion in amorphous rare-earth-transition-metal alloys. A one-dimensional scanning technique is utilized in which a focused, oscillating HeNe laser spot moves over the magnetization pattern in the film. Domain walls are detected through the polar Kerr effect as the spot scans the film. A magnetic field is applied to the film in order to move the domain walls. The maximum line-scanning rate is about 250 Hz. The spot diameter at a 632.8-nm wavelength is limited to about 8 mu m by the 0.46 numerical aperture of the focusing objective, the beam waist of the laser, and aberrations. The domain wall position can be measured to within +or-1 mu m. The apparatus is described, and the results domain-wall-motion measurements are presented. >

Numerical simulation of thermomagnetic writing in RE-TM films

In the development of high-performance magneto-optic (MO) media for data storage, it is important to understand the processes by which reversed magnetic domains are formed in thermomagnetic writing. Thin films of amorphous rare earth-transition metal (RE-TM) alloys deposited by sputtering on a nonmagnetic substrate have become a popular choice for an erasable storage medium. We have developed a simplified micromagnetics simulation of the behavior of RE-TM films that produces an equilibrium configuration of film magnetization for a specified thermal profile and applied magnetic field. This Monte Carlo-type model is useful for assessing the size, shape, and stability of written marks in MO recording. In addition, the simulation provides estimates of the minimum field strength required for nucleation and growth of reversed domains, and hence it predicts film coercivity at any temperature. An equilibrium state is determined by magnetic energy minimization. Magnetization dynamics are not incorporated into the model, but instead, a quasi-static approach to equilibrium is simulated. Spatial resolution can be arbitrarily small, and discretization down to about 20 nm can be reasonably accommodated. Readout of written domains has been incorporated as an additional feature of this model.

Thermomagnetic switching in amorphous rare-earth transition metal alloys

Conditions for writing and erasure have been investigated for different amorphous rare-earth transition metal alloys using a simple temperature distribution evaluated for a three-layer structure. The radial and time dependence of the magnetic parameters relevant for the switching process have been calculated by means of the mean-field theory based on experimental data of the saturation magnetization, uniaxial anisotropy, and the coercivity, for alloys of composition (Gd,Tb)1−xFex and Tb1−x(Fe,Co)x. From the radial profiles of the coercive field, maximum domain diameters are extracted in the limit of large domain velocities. Domain-wall stabilities are calculated from the force equation for writing and erasure conditions as a function of applied field yielding margins for stable domains and collapse fields. In addition to bubblelike domains, ring domain configurations are considered, which imply the possibility of direct overwrite for alloys with a compensation temperature above ambient temperature. The switching with a continuous read power level is discussed.

Coercivity and its role in thermomagnetic recording

A model is proposed for the mechanism of magnetization reversal in thin films of amorphous rare-earth–transition-metal alloys. The model exhibits the observed features of the magnetization reversal process, coercivity and the processes of thermomagnetic recording and erasure.

Magnetization reversal, coercivity, and the process of thermomagnetic recording in thin films of amorphous rare earth–transition metal alloys

A model is proposed for the mechanism of magnetization reversal in thin films of amorphous alloys with perpendicular magnetic anisotropy. Examples of these alloys are TbFe, GdCo, DyFe, and GdTbFeCo, which are currently under investigation as storage media for erasable optical recording applications. The model exhibits the observed behavior of the media such as nucleation and growth of reverse-magnetized domains under external magnetic fields; square hysteresis loops; temperature dependence of coercivity; formation and stability of domains under conditions of thermomagnetic recording; and incomplete erasure with insufficient applied fields.

Direct overwrite in amorphous rare-earth transition-metal alloys

The thermomagnetic writing and erasure process has been investigated, paying special attention to the case of zero applied magnetic field. Concerning bubblelike domains, writing can easily be performed by the demagnetizing field while erasure requires an external magnetic field larger than the collapse field. However, in alloys exhibiting a compensation temperature above ambient temperature erasure can be achieved in the absence of an external field via the creation and annihilation of a ring domain which opens the capability of direct overwrite.

MAGNETO-OPTICAL PROPERTIES OF AMORPHOUS RE-FeCo ALLOYS

Amorphous rare-earth transition metal alloys of composition RE1-xTMx (RE = Gd, Tb, GdTb; TM = Fe, Co, FeCo) were prepared by evaporation onto rotating glass substrates. The magnetic and magneto-optical properties have been investigated as a function of composition and temperature ranging between 4.2 K C. Also, the spectral dependence of the Kerr rotation, θK, has been measured at T = 295 K. The compositional variation of θF and θK for cobalt- and iron-based alloys reveal significant differences which can be interpreted in terms of the different concentration dependences of the Curie temperatures. The temperature dependence of θF and θK for GdFe and GdCo alloys can be described in terms of the RE and TM sublattice magnetizations inferred from the fit of the mean field theory to the measured saturation magnetizations. At low temperatures θF and θK are controlled by both the RE and TM sublattice contributions, while for high temperatures the RE contribution is significantly smaller for most alloys.

非晶質フェリ磁性体の基礎理論と非晶質フェリ磁性絶縁体の研究

Theoretical problems of amorphous ferrimagnets are discussed. Transition temperatures, compensation temperatures and thermal dependences of magnetizations in amorphous ferrimagnetic rare earth-transition metal alloys are investigated using the effective-field theory with correlations. Research on amorphous ferrimagnetic insulators is described. The results obtained suggest a new field of research in magnetism.

Magneto-optics and amorphous metals: an optical storage revolution

In the last five years excitement has been developing about the realization of a magneto-optic storage technology, based on amorphous rare-earth transition metal alloys. The goal of this paper is to identify important physics' and materials' issues that arise out of the storage, erasure and readout processes. Particular attention is paid to the origin of the magneto-optic properties in highly disordered metal alloys.

Magnetic and magneto-optic properties of amorphous Gd-FeAu films

Amorphous rare-earth transition metal alloys of composition (Gd1−xFex)1−yAuy with x=0.74, 0.77, and 0≤y≤0.40 were prepared by evaporation onto rotating glass substrates. The magnetic and magneto-optic properties have been investigated in the temperature range 4.2≤T≤TC revealing a decrease of the compensation and Curie temperature, the uniaxial anisotropy, and the Faraday and Kerr rotation with increasing gold content. The temperature behavior can be interpreted in terms of a mean-field analysis indicating a pronounced influence of the gold on the iron moment and the exchange constants. The temperature dependence of the Faraday rotation can be described in terms of the sublattice magnetizations inferred from the fit of the mean-field theory to the measured saturation magnetization.

Mean-field analysis of amorphous rare earth-transition metal alloys for thermomagnetic recording

A mean-field model is developed for amorphous ferromagnetic materials with potential applications in thermomagnetic recording/magneto-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.

Hydrogen in amorphous magnetic rare earth-transition metal alloys

A wide range of hydrogenated amorphous rare earth-transition metal (R-T) alloys has been synthesized using a newly developed vapor deposition synthesis procedure. Measured H/R ratios vary from 2 to 5 depending on composition. The R-T films do not fracture or disintegrate when activated as do crystalline alloys. Magnetic and electrical results are given for the a-Gd xFe 1−x system. The inclusion of hydrogen sharply reduces the compensation temperature by 200K and the Curie temperature by 50K at a given composition. The dominant magnetic effect is a reduction of the GdFe exchange interaction. Potential application advantages of hydrogenated amorphous R-T alloys are also discussed.

Relationship between Faraday Rotation and Hall Effect in Amorphous Rare-Earth—Transition-Metal Alloys

The Faraday rotation and spontaneous Hall effect were investigated in amorphous thin films with compositions ${(\mathrm{GdFe})}_{1\ensuremath{-}x}{M}_{x}$ ($M=\mathrm{B}\mathrm{i},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{n},\phantom{\rule{0ex}{0ex}}\mathrm{A}\mathrm{u}$). Bismuth and tin are found to increase the Faraday rotation, ${\ensuremath{\phi}}_{\mathrm{F}}$, the Hall resistivity, ${\ensuremath{\rho}}_{\mathrm{H}}$, and the sample resistivity, $\ensuremath{\rho}$, while Au decreases ${\ensuremath{\phi}}_{\mathrm{F}}$, ${\ensuremath{\rho}}_{\mathrm{H}}$, and $\ensuremath{\rho}$ as compared to values observed for GdFe. Specifically, ${\ensuremath{\phi}}_{\mathrm{F}}$ is proportional to the Hall angle ($\frac{{\ensuremath{\rho}}_{\mathrm{H}}}{\ensuremath{\rho}}$). A model first proposed by Voigt is developed to account for this proportionality.

Pair ordering and perpendicular anisotropy in RE-TM amorphous thin films

Atomic pair ordering may play an important role in determining the uniaxial anisotropy in thin amorphous rare earth-transition metal alloys. We present results for annealed Gd-Co rf sputtered films. After annealing at a temperature of at least 100°C alignment of the easy axis perpendicular to the film plane and an increase of Gd-Gd pairs occur.

Electrical characteristics of polymer thick film resistors, Part I: experimental results: Shen-Li Fu, Mong-Song Liang, Toyotaro Shiramatsu and Tien-Shou Wu. IEEE Trans. Components Hybrids Mfg Technol.CHMT-4 (3), 283 (1981)

Atomic pair ordering may play an important role in determining the uniaxial anisotropy in thin amorphous rare earth-transition metal alloys. We present results for annealed Gd-Co rf sputtered films. After annealing at a temperature of at least 100°C alignment of the easy axis perpendicular to the film plane and an increase of Gd-Gd pairs occur.

Electronic and magnetic properties of rare-earth-transition-metal glasses

We have studied the magnetic and electrical properties of several amorphous rare-earth—transition-metal alloys obtained by splat cooling. Low-field susceptibility, high-field magnetization and electrical resistivity measurements were made between 1.6 and 300 K. Mössbauer results are reported for (Gd 65Fe 35) 90B 10 at the temperatures of 80 and 300 K. Resistivity measurements show both positive and negative coefficients and in some cases maxima or minima that are associated with magnetic ordering or Kondo-type scattering, respectively.

Magnetic ordering in amorphous Nd-Co, Gd-Co and Er-Co alloys

Previous studies of magnetism in amorphous rare earth–transition metal alloys have emphasized alloys rich in transition metal. We report here results on three rare earth rich amorphous alloys: Nd64Co36, Gd65Co35, and Er65Co35. Low field susceptibility, χ, was measured for T in the range 4.2 K⩽T⩽300 K and magnetization was measured for 1.3⩽T⩽200 K and applied fields, H, of 0⩽H⩽80 kOe. Curie-Weiss fits were made to high temperature χ (T) data to determine average effective moments and Weiss temperatures. Hysteresis loops were used to determine the coercive force and the ordering temperatures, TC. The effective moments and saturation moments are compared with values calculated on the assumption of local moments on both the rare earth and Co ions. TC values for Nd, Gd, and Er alloys are 38 K, 170 K, and 12 K, respectively. The data are consistent with a ferrimagnetic structure in Gd65Co35. Local random anisotropy appears to play an important role in the Nd-Co and the Er-Co alloys.

Crystalline and amorphous rare-earth transition metal alloys

A large body of experimental data is available on the magnetic properties of crystalline and amorphous rare-earth transition metal compounds or alloys. These include the results of measurements of magnetization, nuclear magnetic resonance and Mössbauer effect and also results of measurements of magnetovolume effects. Some of the experimental data point to strongly localized 3d moments; others, however, point to itinerant 3d electron magnetism. In this paper we show that the 3d electron interaction between nearest neighbour atoms has a dominating influence on the magnetic properties of the crystalline compounds and amorphous alloys. Implications of this on the 3d electron bandwidth and the exchange interaction between the 3d electrons are discussed.