Rare Earth-transition Metal Amorphous Films Research Articles

Electrodeposition of RE–TM (RE = La, Sm, Gd; TM = Fe, Co, Ni) films and magnetic properties in urea melt

Rare earth (RE)—3d transition metal (TM) films were codeposited in low temperature urea–acetamide–NaBr–KBr–RECl 3–TMCl 2 baths by potentiostatic electrolysis. Cyclic voltammetry is used to investigate the electrochemical behavior of RE(III) and TM(II) in baths on a Pt electrode. The reduction of TM(II) is an irreversible process. RE(III) cannot be reduced alone, but it can be inductively codeposited with TM(II). Some kinetic parameters are measured, e.g ., the transfer coefficient α and the diffusion coefficient D 0 for TM(II). It is reasonably suggested that the RE-induced codeposition ability and efficiency is dominated by the electronegativity difference between RE and TM atoms. Moreover, it is found that the surface morphologies, the magnetic anisotropy and preferring orientation of the as-deposited RE–TM films depend strongly on the composition and potential. At last, it is observed that, there exists a clear anisotropy with easy magnetization along the plane of the RE–TM films. The magnetic anisotropy in amorphous RE–TM films should be due to the competing contributions from several factors, such as spin–orbit coupling, stress, etc.

Properties of microstructure on amorphous film of rare earth–transition metal alloy for ultrahigh density recording

Controlling the microstructure of amorphous rare earth–transition metal films via the sputtering process was found to be an effective way of controlling their magnetic properties for applications as magneto-optical storage media. This paper describes how the relationship between a TbFeCo film’s magnetic properties and its microcolumnar structure depends on the sputtering conditions. An enhancement of electric resistance value was observed for the devices with a constriction columnar width in the 5–20nm range. The measured electrical resistance was over 1.0×10−5Ωm in this case. It is believed that the change of electrical resistance on the thin film is due to fluctuations in the density on the arranged microstructure or constriction of current induced by scattering because the film structure contains impurities. These same impurities are believed to be associated with the restriction of the trapped domain wall’s mobility. Furthermore, we observed a significant resistance change subsequent to the application of lower Xe pressure sputtering. This paper also serves as a feasibility study for high-density magneto-optical recording on these media materials.

Specific heat of amorphous rare-earth–transition-metal films

Microcalorimeters have been used to measure the temperature dependence of the specific heat ${C}_{p}(T)$ of amorphous ${R}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ $(R=\mathrm{Gd},\phantom{\rule{0ex}{0ex}}\mathrm{Tb})$ thin films prepared by both sputtering and e-beam coevaporation. $a\ensuremath{-}{\mathrm{Tb}}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ films possess large randomly oriented local magnetic anisotropy and large exchange coupling; they are considered random-anisotropy magnets. By varying growth temperature and by annealing, films of the same composition but with very different macroscopic anisotropy constant ${K}_{u}$ were prepared and studied. ${K}_{u}$ reflects the degree of nonrandomness in the local anisotropy axis directions. $a\ensuremath{-}{\mathrm{Gd}}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ films possess negligible local and macroscopic anisotropy. All samples show a relatively sharp peak in ${C}_{p}(T)$ at the Curie temperature ${T}_{c}$ determined by magnetization measurements, indicative of a phase transition, independent of the magnitude of ${K}_{u}.$ Effective critical exponents of $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=\ensuremath{-}0.6$ to -0.7 and a critical amplitude ratio of 1.5--2.5 are measured for reduced temperatures down to 0.02. Nearly all possible magnetic entropy is developed below ${T}_{c},$ unlike what is seen in spin glasses. Increased growth or annealing temperature causes a small but systematic increase in ${T}_{c},$ in the inverse high-field susceptibility \ensuremath{\chi} and in the homogeneity of the sample; ${K}_{u}$ by contrast increases with growth temperature, but decreases with annealing.

Exchange stiffness constant and effective gyromagnetic factor of Gd, Tb and Nd containing, amorphous rare earth-transition metal films

The exchange stiffness constant and the effective gyromagnetic factor of amorphous Gd/sub 13.5/Tb/sub 6.2/Fe/sub 80.3/, Gd/sub 21.2/Co/sub 78.8/, Gd/sub 19.4/Nd/sub 6.1/Fe/sub 74.5/ and Gd/sub 19.6/Nd/sub 10.3/Fe/sub 70.1/ alloy films have been determined by Brillouin light scattering and from measurements of the domain wall energy. All samples, which show a constant Curie temperature of 465 K, exhibit the same exchange stiffness constant, as measured by Brillouin light. The obtained values of both methods agree Gd/sub 13.5/Tb/sub 6.2/Fe/sub 80.3/ and Gd/sub 21.2/Co/sub 78.8/ films, but not for the Nd containing films. The latter is attributed to a breakdown of the standard Bloch domain wall equation in Nd containing systems. >

Magnetic field modulation recording on Pt/Co magneto-optical disks

Recording properties have been investigated for a magnetooptical disk made of Pt/Co multilayers. The disk exhibits good recording properties for both light intensity modulation and field modulation methods. Reasonable recording properties, namely a carrier-to-noise ratio (CNR) of 46 dB and a block error rate of 1.9%, have been obtained for the field modulation method which seems to satisfy usual standards. However, in the case of low modulation fields, the Pt/Co disk exhibits lower CNR than conventional disks made of rare earth-transition metal amorphous films because of lower recording carrier.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the origin of the perpendicular anisotropy in rare earth-transition metal amorphous films

The induced anisotropy in amorphous (Co 93Zr 7) 100− x (RE) x films has been studied. The sputter deposition was performed in a magnetic field. For RE = Nd, Pr, Dy and Tb, the formation of a well-defined in-plane uniaxial anisotropy K u is related to the single-ion anisotropy of RE atoms. When RE = Gd and Sm, a biaxial anisotropy develops, comprising K u and a perpendicular anisotropy K p. This magnetic structure is expected to be related to the simultaneous formation of two distinct local structural configurations when the deposition temperature is higher than the critical value.

Exchange Coupled Films for Magnetooptic Recording Applications

The magnetic properties of exchange-coupled films are discussed from the viewpoint of applications in magnetooptic recording. Various magnetic properties, such as the magnetization process, coercive force, spin orientation and Curie temperature, can be controlled through the exchange coupling. The most important and useful application involves control of the temperature dependence of the coercive force in rare earth-transition metal amorphous films. Highly controlled thermomagnetic responses bring high performance as well as new functions to magnetooptic recording media. High recording sensitivity, overwriting capability and super-high magnetic resolution all become possible through the use of exchange-coupled multilayer films. Exchange-coupled films are also expected to be prime candidates for use in future magnetooptic recording systems employing blue-light lasers.

Boltzmann distribution of bond orientations and perpendicular anisotropy in amorphous rare-earth-transition-metal films.

We show that the Boltzmann distribution of bond orientations induced by a perpendicularly magnetized layer in the growth process of an amorphous rare-earth--transition-metal film always leads to the bulk perpendicular magnetic anisotropy, irrespective of the specific functional form of the bond energy. The initial perpendicularly magnetized layer can arise from the surface anisotropy produced by antiparallel dipoles or even from spontaneous magnetization.

Generic source of perpendicular anisotropy in amorphous rare-earth-transition-metal films.

We show that the magnetic dipolar energy of antiparallel dipoles in the surface layers contributes to the intrinsic uniaxial perpendicular anisotropy in amorphous rare-earth-transition-metal films. The volume anisotropy energy density K u of this contribution depends inversely on the thickness of the film. For an amorphous GdCo film with a thickness of 1000 A the calculated K u is on the order of 10 3 -10 4 ergs/cm 3 . Dependence of K u on temperature and atomic concentration is also calculated

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.

R.f.-diode sputtered amorphous rare earth-transition metal films: Microstructure, stability, and magneto-optical properties

We report the strong correlation of deposition parameters, microstructure and stability of r.f.-diode sputtered magneto-optical thin films studied by transmission electron microscopy and a magneto-optical hysteresis loop tracer. Both argon bleeding pressure and substrate bias strongly affect the microstructure and oxidation resistance of the films. Films sputtered using moderate argon pressures and substrate bias voltages are dense, smooth and morphologically featureless. They are strongly resistant to oxidation. In contrast, low or high argon pressures and substrate bias result in films of high porosity, columnar island-like structure or “sponge-like” morphology. They degrade rapidly in air.

Single ion model for magnetostriction in rare-earth–transition-metal amorphous films

The mechanism of magnetostriction in rare-earth–transition-metal amorphous films was investigated based on a single ion anisotropy model for rare-earth atoms using a point charge approximation. The theory was compared with the observed magnetostriction in (Gd0.75R0.25)19Co81 films with various rare-earth elements R. The dependence of the observed magnetostriction on substitutional rare-earth elements R is well explained with this model. Screening of an electric field was found to be essential for magnetostriction, and a screening function was determined from the observed magnetostriction. The screening was found to be of the type in which an electric field decreases more rapidly with an increase of distance than a Coulombic interaction.

Single ion model for perpendicular magnetic anisotropy in RE-TM amorphous films

To investigate the mechanism of perpendicular magnetic anisotropy K u in rare earth-transition metal amorphous films, K u was measured for (Gd 0.75 R 0.25 ) 19 Co 81 films with varying the rare earth element R. Results were analysed based on a single ion anisotropy model using a point charge approximation. The dependence of K u on rare earth element is well explained with the single ion anisotropy. A model was proposed in which anelastic deformation during deposition of the film is responsible for the induction of K u . Magnetostriction can also be explained with a point charge model with screening.

Amorphous rare-earth–transition-metal thin films with an artificially layered structure

We investigated the origin of magnetic anisotropy in rare-earth–transition-metal (RE-TM) thin films with an artificially layered structure and their magnetic and crystallographic properties. The various RE-TM films such as Gd-Fe, Tb-Fe, and Dy-Fe were deposited in an alternating multilayer structure by a multisource dc-sputtering method. We made films with an anisotropic distribution of RE and TM atoms parallel to the film plane, where the monatomic layering of RE and TM was confirmed by small angle x-ray diffraction analysis. The films were thermally stable to temperature up to 400 °C. The films composed of monatomic layer of RE and several atomic layers of TM show a large perpendicular uniaxial anisotropy and a large magnetization over a wide range of RE composition. It is concluded that the anisotropic distribution of RE-TM pairs is the most important origin of the magnetic anisotropy in amorphous RE-TM films.

Magneto-optical recording.

This paper is a review article for magneto-optical (MO) recording. Principles of writing (erasing) and reading and an outline of a MO recording apparatus are described firstly. Secondly, recording media (including material requirements and properties of rare earth-transition metal amorphous films), methods of film preparation, and the construction and properties of the MO disk are illustrated shortly. Lastly, future problems to be solved for practical use are discussed.

Stability phenomena in amorphous rare earth-transition metal films

TbFe films have been prepared with a range of compositions near to the room temperature compensation point and subjected to annealing at 100°C for up to 800 hours. Measurements of magnetic properties as a function of anneal time revealed a decrease in coercivity coupled with an increase in saturation magnetisation for all compositions. Changes in M s appeared to conform to a Johnson-Mehl-Avrami. exponent of approximately 0.5. Rutherford back-scattering spectrometry and electron microscopy revealed that the most likely cause of the observed changes in magnetic properties was oxygen diffusion resulting in the formation of TbO n oxides in turn leading to local diffusion, segregation and crystallisation of b.c.c, α-iron. The addition of an aluminium protective overlayer stablised the films against these changes, at least for the time and temperature ranges studied.

Perpendicular anisotropy in rare earth-transition metal amorphous films prepared by dual ion beam sputtering

The effect of resputtering on perpendicular magnetic anisotropy has been investigated for amorphous Gd-Co, Gd-Fe, Tb-Co and Tb-Fe films using a dual ion beam sputtering system. In Tb-Co and Tb-Fe films, perpendicular anisotropy increases remarkably owing to resputtering, while in Gd-Co and Gd-Fe films, it changes only through compositional change caused by resputtering. It is likely that the different behaviors between two cases are due to the fact that Tb is anisotropic and Gd is isotropic atom. However, its mechanism is not clear.

Effects of Additive Elements on Corrosion Resistance of RE-TM Films

Rare earth-transition metal amorphous films are promising for use in erasable optical memory media. However, they have the drawback of poor reliability due to poor corrosion resistance. Corrosion of films is classified as either surface oxidation, selective oxidation of RE metals, or pitting corrosion. We have studied the corrosion behavior and succeeded in improving the corrosion resistance. To raise the latter, one of the elements Co, Ni, Pt, Al, Cr, or Ti was added to Tb-Fe and Tb-Co films by simultaneous sputtering. The films (no protection) were subjected to an accelerated aging test. The elements were classified into two groups, Co, Ni, and Pt, on the one hand, Al, Cr, and Ti, on the other. Elements in the former group are only effective in preventing surface oxidation and pitting corrosion. The latter group of elements, however, is effective in inhibiting corrosion in all of the above modes. This high corrosion resistance can be explained in terms of the elements' ability to bind oxygen and to form passive states. The elements are effective in improving corrosion resistance in the order Cr, Ti, Al. These additives cause no appreciable change in the magneto-optical properties.

Effects of Additive Elements on Corrosion Resistance of RE-TM Films

Rare earth-transition metal amorphous films are promising for use in erasable optical memory media. However, they have the drawback of poor reliability due to poor corrosion resistance. Corrosion of films is classified as either surface oxidation, selective oxidation of RE metals, or pitting corrosion. We have studied the corrosion behavior and succeeded in improving the corrosion resistance. To raise the latter, one of the elements Co, Ni, Pt, Al, Cr, or Ti was added to Tb-Fe and Tb-Co films by simultaneous sputtering. The films (no protection) were subjected to an accelerated aging test. The elements were classified into two groups, Co, Ni, and Pt, on the one hand, Al, Cr, and Ti, on the other. Elements in the former group are only effective in preventing surface oxidation and pitting corrosion. The latter group of elements, however, is effective in inhibiting corrosion in all of the above modes. This high corrosion resistance can be explained in terms of the elements' ability to bind oxygen and to form passive states. The elements are effective in improving corrosion resistance in the order Cr, Ti, Al. These additives cause no appreciable change in the magneto-optical properties.

ＲＥ‐ＴＭ膜の耐食性に及ぼす第３元素添加効果

Rare earth-transition metal amorphous films are promising for erasable optical memory media. However, they have the drawback of poor reliability due to the poor corrosion resistance. The corrosion of the films is classified as being surface oxidation, selective oxidation of RE metals, and pitting corrosion. We have studied the corrosion behavior and succeeded in improving the corrosion resistance. To improve corrosion resistance, one of the elements, Co, Ni, Pt, Al, Cr, and Ti, is added to Tb-Fe and Tb-Co films by simultaneous sputtering. The films (no protection) are subjected to an accelerated aging test. The elements are classified into two groups: (Co, Ni, and Pt) and (Al, Cr, and Ti). The former group elements are only effective in preventing surface oxidation and pitting corrosion. The latter group elements, however, are effective in inhibiting corrosion of all the above modes. This high corrosion resistance can be explained in terms of to their ability to getter oxygen and to form passive states. The degree of these effects, in order of strength, is Cr Ti Al. These additives cause no appreciable change in the magnetooptical properties.