Rare Earth-transition Metal Thin Films Research Articles

Hydrogen-containing SiCN protective films for magneto-optical media

Three types of SiC-SiN film were investigated for protective layers of rare-earth transition-metal thin films. The SiC-SiN films were prepared by RF magnetron sputtering with varying combinations of target and reactive gas: (1) SiC/Ar and N/sub 2/, (2) Si/Ar, N/sub 2/, and CH/sub 4/, and (3) Si/Ar, NH/sub 3/, and CH/sub 4/. The SiC-SiN films of the first and second types were chemically unstable. The films of the third type (SiCH:H) were stable and had good optical properties (transmittance, >90%; refractive index, 1.9-2.1). Deposition rates of SiCN:H films were 1.5 times greater than those of SiN films. Magnetooptical media of polycarbonate substrates using the SiCN:H films had high sensitivity and reliabilities. Optimum recording powers for the media using the SiCN:H films were 1-2 mW lower than for the media with SiN films at linear velocity 4-12 m/s. For the media with SiCN:H films no appreciable changes were observed in values of carrier-to-noise ratio and bit error rate after a 75 degrees C-80% relative humidity aging test for over 1500 h and after a Z/AD test of over 40 cycles. These results can be explained on the assumption that hydrogen and carbon bound to Si and N make the SiCN:H film similar to organic structures with flexibility and hence improve affinities between the film and substrate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermomagnetic marking of rare-earth–transition-metal thin films

Analytical derivation of temperature profiles in laser-irradiated thin-film structures is hindered by the nature of the heat source terms and by the geometrical complexity that often exists. This study utilizes a combined Laplace-transform–Fourier-integral method to obtain approximate solutions for a number of simple cases. The results are used to study the thermomagnetic marking process in rare-earth–transition-metal (RE-TM) thin films, and the predictions are compared with experimental observations.

Magnetic properties of sputter-deposited amorphous rare earth-transition metal thin films

Thin films of amorphous (GdTb)×(FeCo) 1−x of different thicknesses, h, were deposited in a HUV dc planar-magnetron getter sputtering system and characterised by Hall-effect magnetic hysteresis loops. Quantitative information was obtained on the coercity, the Hall resistivity and the normal electrical resistivity for a range of compositions. The coercive force, H c , was found to vary with thickness, h. This was interpreted as a consequences of the coarsening of the microstructure with increasing thickness in amorphous sputter-deposited films. A model us presented which enables a local pinning force per unit area to be deduced from the variation of H c with h. The roughness of the substrate was not a major cause of the domain wall pinning.

On the origin of K&amp;lt;inf&amp;gt;U&amp;lt;/inf&amp;gt;in amorphous RE-TM magnetooptic recording materials

Exchange anisotropy was recently proposed as the origin of uniaxial perpendicular anisotropy in some RE-TM thin films. This magnetic anisotropy arises from the exchange interaction between the ferrimagnetic matrix and acicular shaped single domain ferromagnetic regions. Rotational hysteresis measurements on GdCo films show a non-vanishing value of the rotational magnetic hysteresis for magnetic fields greater than the magnetic anisotropy field (H k ). Such a behavior, first discovered in Co-CoO, is a necessary but not a sufficient condition for exchange anisotropy. The origin of the intrinsic uniaxial magnetic anisotropy energy density (K u ) in amorphous rare earth-transition metal (RE-TM) thin films has been attributed to pair ordering, single ion anisotropy, columnar structures that occur in the sputtering process, stress between the film and the substrate, and exchange anisotropy.

Anisotropy and coercivity of amorphous RE-TM films

The origins of magnetic anisotropy and coercivity in sputter-deposited amorphous rare earth-transition metal thin films are discussed, focussing on microscopic mechanisms. We point out that two distinct mechanisms, directional compositional short range order and bond-orientational anisotropy, have to be considered, and discuss relevant experimental results.

Magneto-optical recording on amorphous rare earth-transition metal thin films with an artificially layered structure

Amorphous rare earth (Cd, Tb and Dy)-transition metal (Fe, Co and FeCo alloy) thin films with an artificially layered structure have been investigated. The films were deposited in an alternating multi-layer structure by the multi-source DC-sputtering method. Films with a repeat unit composed of a monatomic layer of RE and a few atomic layers of TM show excellent magnetic properties when considered as magnetooptical recording media, such as a large magnetization and perpendicular magnetic anisotropy. RE-FeCo and RE-CoFe films used in magnetooptical recording showed carrier-to-noise ratios higher than 52 db, and Dy-FeCo films showed a high carrier-to-noise ratio over a wide frequency range that extended from 57 dB at 1 MHz to 38 db at 10 MHz.

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.

Dynamics and factors controlling regularity of thermomagnetically written domains

The reproducibility of thermomagnetically written domains in rare earth–transition metal thin films was studied and as a result films capable of supporting very regular domains were fabricated. To understand the origins of the domains’ size and shape variations, we first studied the nucleation and growth of domains by using a high-speed magneto-optical sampling camera having a 10-ns exposure time. It is observed that the domain is nucleated within 20 ns after the leading edge of the write pulse. The domain grows in size until 50–150 ns after termination of the write pulse, then shrinks, and finally reaches an equilibrium size. For very short write pulses, no shrinkage of the domain is observed after termination of the write pulse. It is found that increased laser power or longer laser pulse duration increases the final domain size, but not in proportion to total energy applied. The domain growth rate varies from film to film, indicating it is influenced by the intrinsic magnetic parameters. The reproducibility and regularity of the final stable domains were then studied as a function of writing and magnetic parameters. Although both influence the regularity of nucleated domains, the latter have been found to have more profound influences than the former. Films with compensation temperature above ambient temperature, larger perpendicular anisotropy constant, smaller magnitude of magnetization, and smaller domain aspect ratio nucleate domains with a higher degree of reproducibility. Higher coercivity films also support more regular domains. Photographs of domains nucleated with different parameters demonstrate these trends. The experimental observations can be qualitatively explained by analyzing the stiffness of the domain wall defining the domains. The wall stiffness is determined by the two competing forces derived from wall energy and demagnetizing energy. When wall energy dominates, the stiffness is large and domains tend to be circular and highly reproducible. When demagnetizing energy dominates, wall stiffness is low and domains become irregular and less reproducible in shape.

Control of parameters in rare earth-transition metal alloys for magneto-optical recording media

The techniques which can be used to control the magnetic and magneto-optical recording properties of sputtered amorphous thin films containing Fe,Co,Tb and Gd are reviewed. It is shown that by proper selection of composition one may independently vary the Curie temperature, the compensation temperature, and coercivity. On-the-other-hand it is shown that the uniaxial anisotropy perpendicular to the film plane and the magneto-optical Kerr rotation are dependent upon the substrate bias and argon pressure used during deposition. A procedure for optimizing the magnetic and magneto-optical properties of these materials for magneto-optical recording applications is outlined. Magneto-optic recording technology today employs amorphous thin films of rare earth-transition metal alloys. The films exhibit a preferred axis of magnetization perpendicular to the plane of the film. At room temperature the coercivity is high so the magnetization remains saturated perpendicular to the film plane, but at high temperatures (> 100°C) the coercivity is low, allowing the magnetization to reverse its direction in local submicrometer regions addressed by a focussed laser beam. To read out the recorded magnetization pattern, the kerr magneto-optic effect is used. This technology allows order of magnitude higher data densities than present rigid disk magnetic recording technology, freedom from head crashes, and removable media. The magnetization of the rare earth-transition metal thin films is determined by antiferromagnetic coupling between the rare earth and transition metal subnetwork magnetizations. Typically at low temperatures the rare earth magnetization dominates, whereas at high temperatures the transition metal magnetization is largest. At some intermediate temperature the two subnetwork magnetizations are equal and opposite resulting in zero net magnetization. At this so-called compensation temperature the coercivity tends toward infinity since there is no net magnetic moment to be acted upon by a magnetic field. At a sufficiently high temperature, the Curie temperature, the net magnetization and coercive force both tend toward zero. In this paper the factors which control the magnetic properties of sputtered GdTbFeCo amorphous films are discussed. Methods to independently control compensation point, Curie temperature, uniaxial anisotropy, and coercivity are explained.

Temperature dependence of the magnetic properties of amorphous TbFe thin films determined from hall effect measurements

Measurements of the extraordinary Hall effect as a function of temperature for amorphous TbFe thin films of various thicknesses and compositions were performed to investigate the thickness dependence of the magnetic properties previously observed for rare earth-transition metal thin films. It is found that a change in the composition alters the temperature dependence of the extraordinary Hall resistivity ρ H( T). The change in ρ H( T) with film thickness is found to be much greater than that which can be ascribed to a change in the composition alone. These experimental results support our previously proposed model that the microstructure-induced variations in the short-range order are the dominant reason for the change in the magnetic properties with film thickness.

Homogeneity studies of amorphous TbFe films by Kerr and Hall effect methods

The homogeneity of amorphous TbFe thin films was examined, utilizing variable phase retardation in polar Kerr effect measurements in conjunction with extraordinary Hall effect studies. These techniques were suggested as very useful tools to study film homogeneity in amorphous rare earth-transition metal thin films with composition close to the compensation composition.

Sputtering-induced recrystallization on the target and its effect on magnetic properties of amorphous rare-earth transition metal thin films

An investigation was made to evaluate the variation in the magnetic hysteretic properties of rf-sputtered amorphous Tb-Fe thin films as a function of sputtering parameters, such as Ar pressure and rf input power. It was discovered that, during sputtering from a composite target which consists of rare-earth disks placed on a transition metal plate, the average surface composition of the target changed by formation of intermetallic RE-TM compounds through a recrystallization process on the RE disk surface. Using x-ray diffraction, electron microprobe, and scanning electron microscopy, analysis of the surface revealed that the amount and morphology of intermetallic compounds formed had a strong dependence on Ar pressure but a lesser dependence on power. The magnetic properties of the films deposited under different sputtering conditions were measured using Kerr magneto-optic and VSM hysteresis loop tracers. The results show that the changes in the magnitude of magnetization, coercivity, and polarity of the hysteresis loop of the films obtained from a given sputtering target with various sputtering parameters are the consequence of changes in film composition which arise from the altered surface composition and morphology of the sputtering target.

Properties of amorphous rare earth-transition metal thin films relevant to thermomagnetic recording

Properties of amorphous rare earth-transition metal thin films relevant to thermomagnetic recording

Microstructure and magnetism in amorphous rare-earth–transition-metal thin films. II. Magnetic anisotropy

Amorphous thin films of GdxCo1−x, HoxCo1−x, GdxFe1−x, and YxFe1−x (0.1&amp;lt;x&amp;lt;0.7) possess an anisotropic microstructure when prepared by vapor or sputter deposition onto room-temperature substrates. The structure consists of mutually parallel columnar regions of high density that are surrounded by a network of low-density material of 10–30 Å thickness. This structure is produced by geometrical shadowing of the incident vapor atoms by the growing film. If separate vapor sources are used to deposit each alloy constituent, compositional inhomogeneities are also formed by shadowing effects. These produce an in-plane magnetic anisotropy that may be eliminated by deposition onto rotating substrates. Annealing at temperatures between 250 and 400 °C, deposition in oxygen-contaminated environments, or bias-sputter deposition produces compositional inhomogeneities at the surface of the columns. Development of this inhomogeneity is accompanied by production of an easy axis of magnetization perpendicular to the film surface in Gd-Co alloys, and development of a diffuse diffraction maximum at 0.38 Å−1. Upon annealing at 500 °C, this diffuse ring is transformed into a set of closely spaced Bragg maxima that correspond to Gd2O3 crystallites. All of the films exhibit large biaxial tensile stresses in the as-deposited state. These are reduced by deposition in oxygen and by bias-sputter deposition. Tensile stress and perpendicular magnetic anisotropy are linearly correlated in Gd-Co alloys, but the magnetostrictive contribution to perpendicular anisotropy is only 0.01 of the observed value.

3799. Microstructure and magnetism in amorphous rare earth-transition metal thin films. (USA): A G Dirks and H J Leamy,J appl Phys, 49 (3), 1978, 1735–1737

3799. Microstructure and magnetism in amorphous rare earth-transition metal thin films. (USA): A G Dirks and H J Leamy,J appl Phys, 49 (3), 1978, 1735–1737

Magnetic and structural properties of some amorphous rare earth-transition metal thin films

With a transmission electron microscope (TEM) we have studied various amorphous rare earth (RE) - transition metal (TM) and (Fe, Co)-B thin films. The greater part of the films has been produced by ultra high vacuum (UHV) vapour deposition. A series of amorphous Gd-Co films are made by sputter deposition as well. All films show an anisotropic microstructure. It consists of columnar regions surrounded by a network of less dense material. Using electron small angle scattering (SAS) and microfractography the columnar structure has been studied in more detail. This microstructure is caused by self shadowing of the obliquely incident vapour atoms. The column growth direction and the vapour flux orientation are correlated. So, the microstructure obtained will be influenced by the angle of vapour incidence. Vapour deposition from separate RE and TM sources produces in-plane anisotropy of the magnetization having its origin in the same mechanism that is responsible for the column formation. Our results indicate that perpendicular magnetic anisotropy may be related with 1) stress relief by oxygen take up during heating in the TEM, during vapour deposition in an oxygen contaminated vacuum, or during bias voltage sputtering and 2) composition fluctuations on the scale of the column size (50 - 250 Å) by RE-oxide formation at the low-density sites surrounding the rods.

Microstructure and magnetism in amorphous rare-earth–transition-metal thin films. I. Microstructure

A microscopic examination of vapor- or sputter-deposited amorphous rare-earth–transition-metal thin films reveals that they possess anisotropic microstructure. The morphology of the structure is similar in all of the films. It consists of small (50–250 Å in diameter) rod-shaped regions of high density that are surrounded by a network of less-dense material (10–25 Å thickness). The column axes are mutually parallel and are oriented at an angle β that is described by the relationship: tanβ= (1/2) tanα, where α is the angle of incidence of the vapor beam, measured with respect to the substrate normal. The columnar structure is formed during deposition by self-shadowing of the incident atoms by the atoms in the growing film. The columnar structure persists when multiple or distributed vapor sources are used to deposit the film. The structure is stable upon heating to temperaures in excess of the crystallization temperature of the amorphous deposits. Vapor deposition at large α leads to the formation of rowlike column aggregates in a direction perpendicular to the plane of incidence.

Microstructure and magnetism in amorphous rare earth–transition metal thin films

We have examined UHV vapor deposited and sputtered films of various amorphous alloys by transmission electron microscopy (TEM). All of the films exhibited an anisotropic microstructure composed of columnar regions surrounded by a low density network. The structure is produced by shadowing of the incident vapor by the growing film. Films produced by vapor deposition from separate rare earth (RE) and transition metal (TM) sources may possess an easy axis of magnetization in the film plane that is not present when the substrate is rotated or when the film is produced by sputtering. All of the films possess a high tensile stress which may be reduced and eventually converted to a compressive stress by vapor deposition in an oxygen containing ambient or by negative bias sputtering. The appearance of compressive stresses is coincident with the development of a perpendicular easy axis in Gd-Co. A perpendicular easy axis is also induced in vapor deposited and zero volt bias sputtered Gd-Co films upon heating within the TEM. Our results indicate that perpendicular anisotropy may be produced by the formation of an inhomogeneous structure via removal of ferromagnetic Gd, either by oxidation or preferential resputtering.