Rare Earth-transition Metal Alloys Research Articles

Oxide formation on rare earth/transition metal and bimetallic transition metal thin films: modeling the effect of fourth element modifiers on O 2 and H 2O surface chemistries

The chemical reactions undergone by both O 2 and H 2O on (a) rare earth (Tb), (b) rare-earth transition metal alloy (FeTbCo), (c) transition metal (Fe and Ti), and (d) transition metals, modified with thin overlayers of another metal (Fe/Ti), have been studied by surface photoelectron spectroscopies, starting with the clean metals in UHV environment. Various degrees of reactivity are indicated, especially for adsorption and dissociation of H 2O, depending upon whether the pure metals are studied, or mixtures of metals, which change the type and extent of dissociation of H 2O (oxide vs. hydroxide formation).

Magneto-Optical Spectroscopy of RE/TM Alloys and TM/CO Multilayers

ABSTRACTThis paper reviews on magneto-optical spectroscopy of alloys, metallic multilayers and interference enhancement structures for magneto-optic (MO) recording. Mainly rare earth transition metal (RE-TM) alloys and Co/transition metal (Co/X) multilayers will be discussed. The read out performance in MO write/read tests is given by tile reflectivity and Kerr rotation strongly depends on the wavelength. While the magneto-optical transitions in the near infrared (IR:≈820nm) are dominated by tile 3d magnetic transition metals Fe and Co, the properties in the ultraviolet (UV: <400nm) can be dominated by either rare earths like Nd or Pr in RE-TM alloys or by exchange polarized Pt or Pd in Co/X multilayers and their respective alloys.In an attempt to correlate the magneto-optic Kerr effect to electronic properties we have systematically investigated alloys and multilayers with x-ray photoemission (XPS) and polar magneto-optical Kerr spectroscopy (0.5–5.3eV range). It is found that electronic 4f levels in Nd or Pr contribute to the Kerr elffct via Γ→, d transitions in the order of 0.3°. Exchange polarized Pd and Pt in Co/X multilayers give rise to a room temperature Kerr rotation in the same order of magnitude via excitations of the 4d and 5d bands, respectively. These effects clearly depend on film compositions and thickness ratios.,

DYNAMICS OF MAGNETIZATION REVERSAL IN AMORPHOUS FILMS OF RARE EARTH-TRANSITION METAL ALLOYS

Computer simulations of a two-dimensional lattice of magnetic dipoles are performed on the Connection Machine. The 256 x 256 hexagonal lattice is a discrete model for thin films of amorphous rare earth-transition metal (RE-TM) alloys, which have application as the storage media in erasable optical data storage systems. In these simulations the dipoles follow the dynamic equation of Landau-Lifshitz-Gilbert under the influence of an effective field arising from local anisotropy, near-neighbor exchange, classical dipole-dipole interactions, and an externally applied field. Using mean-field theory, we have calculated the temperature dependencies of the subnetwork magnetizations, the effective fields, the gyromagnetic coefficient, and the Gilbert damping parameter. These results are then used in the simulation of the thermomagnetic recording process, where a focused laser beam creates a hot spot and allows an external magnetic field to reverse the direction of local magnetization. The onset of nucleation and the dynamics of growth/contraction by domain wall motion have been studied by means of these simulations.

X-ray and neutron scattering studies of laser-annealed TbFeCo thin films

Rare earth-transition metal alloy thin films are currently the primary media for magnetooptic data storage. Although they possess very good magnetic properties they also have several undesirable qualities. In order to enhance their performance we have concentrated on attempting to allow the as-sputtered film structure to relax by use of pulsed laser beam irradiation. This technique has several advantages over thermal annealing, in particular the ability to tune the laser wavelength to a region which is absorbing in only one layer of the disc, in order to confine the effects of the anneal to that layer. By tuning a laser to an absorbing wavelength in TbFeCo media we have been able to produce local transient temperatures approaching 3000 °C, modifying the magnetic behaviour of the samples yet without significantly affecting their macroscopic layer structure. The structure has been probed by use of neutron and X-ray reflectivity measurements. In analysing the structure of samples which had corroded severely in the atmosphere, we have shown the power of complementary neutron and X-ray techniques to remove some of the ambiguity often present in the analysis of data from one of the techniques alone.

ChemInform Abstract: Magneto‐Optical Recording Materials and Technologies

Abstract Thin films of amorphous rare-earth transition-metal alloys represent a suitable class of materials for magneto-optical information storage. They are ferrimagnets and exhibit a uniaxial magnetic anisotropy. The magnetic and magneto-optical properties can be well tailored by the composition as well as the deposition conditions. The information is stored via magnetic domains, which can be written by a thermomagnetic switching process. The reading process utilizes the magneto-optical Kerr effect. At present, the most prominent candidates for device applications are GdTb-Fe and Tb-FeCo alloys. High read-out efficiencies and a long-term stability of the active magneto-optical medium can be realized using a multileyer structure. Thus carrier-to-noise values up to 61 dB (30 kHz bandwidth) have been achieved. Other media like ferrimagnetic oxides and Pt-Co multilayers are also discussed for magneto-optical recording.

Simulation of the magnetization-reversal dynamics on the Connection Machine

Large two-dimensional lattices of interacting magnetic dipoles representing thin films of amorphous rare-earth–transition-metal alloys are studied on the Connection Machine. The accuracy of the results is established by comparing the simulated dynamics of isolated Bloch walls with theoretical predictions. The effects of random axis anisotropy on the process of magnetization reversal are then investigated, and the nucleation and growth of reverse-magnetized domains are found to be the mechanism of reversal. The obtained hysteresis loops have very high squareness, and sample coercivities are well within the range of experimental values.

Magneto-Optical Storage Materials

Magneto-optical (MO) recording combines the advantages of optical recording and magnetic recording. It offers very high storage densities, it is a noncontact technique, and it allows an unlimited number of read/write cycles. Although the potential of magneto-optical recording was recognized nearly 25 years ago, suitable materials did not exist at that time. Since then, substantial efforts have been made optimizing existing materials and searching for new ones. In 1973 an important development was started by Chaudhari et al., who discovered amorphous GdCo with perpendicular magnetic anisotropy as a possible MO recording material. Today amorphous ternary rare-earth (RE) transition-metal (TM) alloys like GdTbFe and TbFeCo are the recording layers in the MO disks which appear on the market. Although these materials show good recording performance, they also exhibit some drawbacks, mainly caused by their susceptibility to oxidarion and corrosion. However, two new classes of suitable MO materials with good oxidation resistance are emerging.

Recent advances in thin films for magneto-optic recording

Magneto-optic storage technology based on amorphous rare-earth transition metal alloys has recently been introduced on the market and is viewed as a competitor for conventional magnetic recording. We will review the preparation, microstructure, magnetic and magneto-optic properties of amorphous rare-earth transition-metal films. Their advances with respect to thermo-magnetic switching and magneto-optic recording as well as their improved long term stability are discussed. Two new classes of materials, oxides and, recently, metallic multilayers, are at the focus of research for magneto-optic recording, promising advantages over the rare-earth transition-metal alloys. Their properties and possible advantages with respect to future issues of magneto-optic recording are discussed.

Magneto-optical recording materials and technologies

Thin films of amorphous rare-earth transition-metal alloys represent a suitable class of materials for magneto-optical information storage. They are ferrimagnets and exhibit a uniaxial magnetic anisotropy. The magnetic and magneto-optical properties can be well tailored by the composition as well as the deposition conditions. The information is stored via magnetic domains, which can be written by a thermomagnetic switching process. The reading process utilizes the magneto-optical Kerr effect. At present, the most prominent candidates for device applications are GdTb-Fe and Tb-FeCo alloys. High read-out efficiencies and a long-term stability of the active magneto-optical medium can be realized using a multileyer structure. Thus carrier-to-noise values up to 61 dB (30 kHz bandwidth) have been achieved. Other media like ferrimagnetic oxides and Pt-Co multilayers are also discussed for magneto-optical recording.

Optical recording aspects of rf magnetron-sputtered iron-garnet films

The intrinsic magneto-optical readout performance in reflection is calculated for bismuth and cobalt-substituted iron-garnet films on a multilayer interference mirror at 800-, 633-, 488-, and 420-nm wavelengths and is compared with that of a trilayer medium composed of an antireflection layer, a rare-earth transition-metal film, and a metallic mirror. It is found, when disregarding inhomogeneities, like irregular domain shape, ripple of the magnetic anisotropy, and surface roughness, that iron garnets are superior to rare-earth transition-metal films at blue to near-ultraviolet wavelengths if operated at thicknesses where optical interference occurs in the magnetic layer. Optical transmittance at these thicknesses is sufficiently high so that multilevel recording media can be conceived. In contrast, the optical absorption of rare-earth transition-metal alloys is much higher so that only thicknesses much above interference conditions are feasible, thus precluding them from multilevel recording. This comparative study is supplemented by calculating the magneto-optical performance in reflection of a recently reported multilayer medium composed of an antireflection coating and a periodically repeated sandwich of 4-Å Co and 9-Å Pt layers. In contrast to conventional rare-earth transition-metal films, the magneto-optical Kerr effects of this material do not degrade when decreasing the wavelength from 800 to 400 nm, but still do not reach the performance of bismuth-iron garnets in the green to ultraviolet spectrum. For the garnet system Y3−xBixFe5O12 the spectra of the real and imaginary parts of the diagonal and off-diagonal component of the dielectric tensor εij are reported in the range of photon energies between 1 and 5 eV, i.e., 1240- and 248-nm wavelengths and a bismuth concentration up x=1.4 Bi3+ atoms per garnet formula. In addition, the off-diagonal components ε′12 and ε■12 are parametrized in terms of paramagnetic optical transitions, taking the spectra for x=1.25 as a typical example. Furthermore, optical and magneto-optical spectra are presented for Co2+- and Co3+- substituted iron garnets and barium hexaferrite BaFe12O19. Finally, the spectral dependence of the magneto-optical figure of merit 2ΘF/α of (Y,Bi)3Fe5O12 and amorphous TbFe is compared. Furthermore, high-resolution transmission electron micrographs and x-ray double-crystal diffractograms are presented that elucidate the perfect epitaxial alignment of single-crystalline iron-garnet films and the columnar morphology of polycrystalline iron-garnet films prepared by rf magnetron sputtering. The initial nucleation period of polycrystalline garnet films can be influenced by low-energy ion bombardment for improving the film texture. Under favorable sputtering conditions single- and polycrystalline bismuth-iron garnet films develop a perpendicular magnetic anisotopy. It is not yet clear whether sputtered iron-garnet films can meet the critical requirements on magnetic wall coercivity and magnetic remanence.

Temperature dependence of the critical radius of domains in rare earth-transition metal alloys for magnetic media

We have investigated the dependence of the critical radius Rc of cylindrical domains as a function of temperature in GdTbFe and GdFe alloys. For terbium-based alloys, Rc increases with rising temperature and can lead to erasure of the magnetic data, whereas for GdFe samples better stability is observed. The experimental results are compared with theoretical models based on a thermally activated process.

Magnetic and magneto-optical properties of rare-earth transition-metal alloys containing Gd, Tb, Fe, Co

Binary and ternary amorphous rare-earth transition-metal alloys of general composition RE1−x(Fe1−yCoy)x with RE=Gd, Tb and 0<x<1, 0≤y≤1 were prepared by evaporation. The saturation magnetization, uniaxial anisotropy, and Faraday rotation were investigated as a function of composition and temperature. Also, from the temperature-dependent measurements the compositional variation of the Curie temperature and the compensation temperature were determined. The experimental data have been analyzed in terms of the mean-field theory yielding spin values and effective exchange interaction energies. The compositional variation of the spin value was described by the environment model. The anisotropy data were fitted with a dipolar equation using the sublattice magnetizations inferred from the mean-field analysis. The magneto-optical effects are primarily caused by the transition metals. For the binaries GdFe and GdCo the compositional variation of the Kerr rotation typically peaks between 1000 and 1500 nm. The lowest magneto-optical effects for binary RE-Th alloys with respect to composition are reached at about x≊0.5 for T=4.2 K. The temperature dependence of the Faraday rotation can be well described in terms of the sublattice magnetizations.

ChemInform Abstract: Magneto‐Optical Recording and Data Storage Materials

AbstractReview: (emphasis on amorphous rare‐earth transition metal alloys.

The effects of the sputtering conditions on bismuth doped gadolinium iron garnet films

The effects of sputtering conditions on the magnetic and magnetooptic properties of bismuth-substituted gadolinium-iron-garnet (GdIG) films were studied. GdIG films with uniaxial perpendicular anisotropy and room-temperature coercivities greater than 1 kOe were deposited on glass substrates by RF magnetron sputtering. These films have Faraday rotations ( theta /sub f/) between 0.6 degrees / mu m and 1.3 degrees / mu m and temperature-dependent coercivities similar to those of rare earth-transition metal alloys. Increasing the RF power or argon bleeding pressure results in an increase in the compensation temperature with little change in the Curie temperature (T/sub c/). A negative substrate bias lowers T/sub c/, lowers theta /sub f/, and raises the crystallization temperature. >

Mean-field analysis of ternary and quaternary rare earth-transition metal alloys for thermomagnetic recording

A mean-field model developed to analyze ternary (TbFeCo) and quaternary (TbGdFeCo, TbDyFeCo) alloys of interest in thermomagnetic recording is based on the localized mean-field theory and emphasizes the reduction of adjustable parameters. The experimental magnetization versus temperature curves are compared with the model predictions, and good agreement is obtained in all instances. The exchange parameters and atomic moment values obtained are consistent with the current understanding of these alloys. The possibility of including an itinerant model to account for the 3d transition-metal magnetic electrons is considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Atomic structure and thermal stability in amorphous rare-earth–transition-metal magneto-optic alloys (abstract)

The technique of EXAFS has been used to study local atomic structure and thermal stability in amorphous thin-film alloys of TbFe, TbFeCo, TbFeCoAu, and DyFe. The rare-earth–transition-metal (RE-TM) films used were coated on the underside and topside by additional dielectric layers of Si3N4 to prevent oxidation during annealing. The EXAFS data were obtained by direct measurement of sample current in a total electron yield technique of data collection. The applicability of this method of analysis to the particular magneto-optic sample structure used is discussed with results of atomic local site geometry and limitation of the technique. Calculations from Fe K-edge data in TbFe and TbFeCo samples yield an Fe nearest-neighbor distance of 2.50±0.10 Å and an average coordination number of 11.0 representative of a dense random-packed amorphous structure. On annealing to 400 °C a second-nearest-neighbor distance and second shell structure is observed in the radial distribution functions as bccFe precipitates into the amorphous matrix. The addition of small amounts of Au into the RE-TM alloy is shown to stabilize the amorphous structure and prevent this process of atomic rearrangement of Fe through a process of stearic pinning. Analysis of Tb Liii–edge data is also discussed in addition to measurements of magnetic parameters Hc and Ku relevant to application in optical storage media.

Ellipsometric comparisons of rare earth-transition metal alloy films in connection with magneto-optical recording

The results of a comprehensive set of ellipsometric observations of 17 rare earth-transition metal amorphous alloy films are presented in terms of the optical and magneto-optical constants of the film materials. The films were prepared independently in two American and two European laboratories, one academic and the other industrial in each case. The measurements are used to emphasize the sensitivity of ellipsometry and the ability of this technique of observation to provide insight into structural features of thin films, as well as to assess the potential of the given thin film materials for the magneto-optical read-out of magnetically stored information. The advantages of measuring basic constants are also stressed.

Magnetization reversal dynamics in the media of magneto-optical recording

Domain wall dynamics in thin films of amorphous rare-earth–transition-metal alloys are investigated using numerical integration of the Landau–Lifshitz–Gilbert equation. A two-dimensional square lattice of dipoles with nearest-neighbor exchange interaction and random axis anisotropy is simulated and the structure of domain walls (with and without Bloch lines) is determined. The motion of domain walls under applied fields is then considered and the effects of random spatial variations of magnetic parameters upon wall mobility are discussed.

Materials for magneto-optic data storage (invited) (abstract)

The optimization of magneto-optic media for data storage applications involves a detailed consideration of the impact of material properties on the overall performance and reliability characteristics of the complete media design, which includes both the active layer structure and the substrate material. Currently, the most highly developed materials for the active layer are the amorphous rare earth-transition metal (RE-TM) alloys. The RE-TM composition determines both the Curie temperature and the compensation temperature, both of which parameters affect the sensitivity, and therefore the data rate which can be achieved using currently available diode lasers. The thermal and optical characteristics of the active layer thin-film structure affect the sensitivity, and also determine the degree of enhancement of the shot-noise-limited SNR during playback. Reliability of magneto-optic media is affected by uniform surface oxidation of RE-TM alloys, which leads to reduced Kerr rotation through the formation of an absorptive surface layer with only in-plane anisotropy. The proper choice of dielectric passivation layers and deposition process can significantly limit this and other degradation mechanisms. The choice of substrate material must be compatible with the coating process for the active layer structure, provide a stable mechanical interface to the drive mechanisms, and also possess good optical properties, most notably low birefringence. This talk will review the current directions in materials research on magneto-optic media, and relate the physical characteristics of the materials involved to the major requirements of the data storage application.

Magnetization reversal dynamics in magneto-optic media (invited)

Dynamics of magnetization reversal in thin films of amorphous rare earth-transition metal alloys are investigated. Using computer simulations for a two-dimensional square lattice of dipoles with nearest-neighbor exchange interaction and random axis anisotropy, the Landau–Lifshitz–Gilbert equation is numerically integrated to yield the static wall structure and its motion under an applied field. The simulated results show excellent agreement with theory in the special cases where analytic solutions exist.