Gd-Co Films Research Articles

Composition and temperature dependence of resistivity in amorphous Gd-Co films

The composition and temperature dependence of electrical resistivity were studied for a series of amorphous Gd-Co films prepared by an rf-sputtering method. With increasing Gd concentration the electrical resistivity at 4.2 K increased from 180 μΩ cm to 350 μΩ cm and the temperature coefficient of resistivity at 288 K decreased and became negative above 220 μΩ cm.

Bias voltage effect on electrical resistivity of GdCo amorphous sputtered films

The temperature dependence of electrical resistivity ϱ and both the effects of annealing and oxidation on the ϱ-values have been investigated in GdCo films sputtered under various substrate bias voltages. It has been found that the oxidized films or the high biased films are unstable crystallographically.

Magneto-optic disk storage

Recent researches on materials and systems for magneto-optic disk storage are reviewed. An amorphous double-layered GdCo film is suitable to use for this purpose. An experimentaI result on recording color TV signals in real time on a GdCo disk is also presented. Fairly good motion pictures were reproduced. From this work, it can be said that magneto-optic disk storage system will be put into practice in the near future.

Amorphous GdCo disk for thermomagnetic recording

Thermomagnetic recording was carried out on 150-mm-diam amorphous GdCo disks rotated at a speed of 1800 rpm. GdCo films showing good uniformity were deposited on glass disks by rf sputtering technique. Bits as small as 1 μm in width were recorded, making use of compensation temperature with a laser power of 5.6 mW and were read out with a laser power of about 2 mW. Thermal stability and dependence of the Kerr rotation angle on wavelength were also measured. It was confirmed that GdCo film is a most promising material for thermomagnetic recording.

Perpendicular anisotropy of bias-sputtered GdCo film

Some important information about the structural change of GdCo sputtered film with substrate bias is obtained by using the replica technique. It is found that the increase in perpendicular magnetic anisotropy for increasing substrate bias is related directly with the ultrafine microstructure of the GdCo film. The relative substrate potential with respect to the Ar plasma depends either on Ar pressure during sputtering or on the external substrate bias. When this relative substrate potential becomes negative by a certain amount, sputtering of surface oxygen and resputtering of Gd in the growing film will be initiated, and consequently the structure of sputter-deposited film will change. The origin of microscopic perpendicular magnetic anistropy with or without substrate bias is described by a model proposed earlier for Permalloy films. The perpendicular magnetic anisotropy based on this microscopic shape anisotropy model is Ku = 2πMs2(δ/a), where δ is the thickness of the nonmagnetic boundary and ’’a’’ is the columnar diameter of the magnetic body. The quality factor Q is expressed as Q = (δ/a). These relations explain fairly well the increase in Ku with increasing negative bias voltage.

FMR study of the oxidation of amorphous GdCo films

The FMR method ang g-factor determination in particular have been shown to be useful tools for detecting the oxidation process in some amorphous GdCo films. Typical results prove that the Gd oxidation process takes place over the whole volume of the film on annealing in air at temperatures up to 170°C. The air annealing does not lead to a marked inhomogenization of the film.

アモルファスＧｄＣｏ薄膜の磁気光学スペクトル

Spectral dependences of polar magneto-optical effect-Kerr rotation and reflectance magneto-circular dichroism (RMCD) -of rf-sputtered Gd-Co films have been studied. The Kerr rotation angle increases slightly toward longer wavelength with a hump around 11.2 μm. This result favors the use of semiconductor lasers for optical reading-out of recorded signals on the Gd-Co disk. In the RMCD spectra a sign reversal is observed at certain wavelength between 0.6 and 1.1 μm, which depends strongly on the bias voltage applied during film preparation. Influence on the RMCD spectrum of oxygen introduced during bias-sputtering has also been studied.

Measurement of Magneto-Optical Kerr Effect Using Piezo-Birefringent Modulator

An advanced magneto-optical technique is described, which allows simultaneous determination of two mutually-conjugate parameters–Kerr rotation and reflectance magneto-circular dichroism–using a piezo-birefringent modulator. This technique provides a sensitivity of 0.002° for Kerr rotation at λ=500 nm with 12 nm resolution, using a halogen lamp (150 W) as the light source. The wavelength region covered by the method can be extended from 300–2500 nm by suitable selection of gratings and light detectors. The new technique was used to measure magneto-optical hysteresis loops for several wavelengths and magneto-optical spectra on an rf-sputtered amorphous Gd-Co film, and the results are reported here.

Transport properties of amorphous Gd-Co films

The electrical resistivity and the galvanomagnetic effect in Gd-Co sputtered films were studied as a function of temperature between 1.5 and 390 K. The logarithmic temperature dependence of electrical resistivity was dominant in the region from 1.5 to 35 K and disappeared after crystallization. The quadratic temperature dependence of electrical resistivity was observed from 50 to 390 K together with the resistivity minimum. The temperature dependence of transverse magnetoresistivity was measured to investigate the effect of magnetic domain walls to the temperature dependence of electrical resistivity. The temperature dependence of transverse magnetoresistivity was about one hundredth as large as that of electrical resistivity.

Increase in Uniformity of Amorphous GdCo Films by Control of Oxygen Quantity

The spatial uniformity of compositions and of the magnetic property of rf-sputtered amorphous GdCo thin films was investigated by Auger electron spectroscopy, electron probe microanalyzer, and polar Kerr hysteresis loops. It was found that by applying a negative dc bias to the substrate during sputtering, the quantity of oxygen incorporated into the film decreases and considerable uniformity of the oxygen and of magnetic property is obtained, though the magnetic property shows a slight increase in compensation temperature toward the edge. This slight change was found to come from a change in composition of the metals. A further increase in uniformity was achieved by intentionally introducing a small amount of oxygen during the deposition process.

The influence of strong bias voltage on the electrical and magnetic properties of the amorphous Gd-Co films

Some electrical and magnetic properties of Gd-Co films obtained by dc sputtering with bias voltage V b are discussed. We observed a decrease in the compensation temperature and in the anomalous Hall resistivity with increasing bias voltage while the specific resistivity increases. The anisotropy constant, however, increases up to V b = -200 V, where it reaches a maximum and then it decreases. Electrical conductivity of the films prepared with V b greater than -100 V showed tunneling character. We conclude that not only resputtering of Gd atoms but also oxidation of Gd takes place during deposition when bias voltage is applied.

Cross-sectional structures and depth profiles in bias sputtered GdCo films

Structures and compositions of sputtered GdCo films were carefully investigated using a carbon-replica technique and Auger electron spectroscopy. In zero-bias sputtered films, the columns are clearly observed to develop perpendicular to the substrate, and they become fine with increasing Gd content up to near the compensation composition from pure Co film. For negative-bias sputtered GdCo films having near compensation composition, the film structures become fine with increasing negative bias up to 200 V. The observed increment of perpendicular anisotropy in this negative bias region was explained by the structural change of the columns (shape anisotropy and stress induced anisotropy). Above 200 V negative bias, the column structures are damaged partially by Ar-ion-bombardment, and then the anisotropy decreases due to the damage of columns. The change in the saturation magnetization due to negative bias was interpreted by the change in the film composition; Gd-content and oxygen.

Effects of in-plane field on domain wall dynamics in amorphous GdCo magnetic films

Previously we reported the dynamics of an isolated straight domain wall near the compensation point in amorphous GdCo and GdCoAu films prepared by rf-sputtering. Nonlinear dependence of the wall velocity on drive field is observed. Here we describe the effects of externally applied in-plane field and in-plane anisotropy of the film on domain wall dynamics in amorphous GdCo magnetic films. The wall whose plane is perpendicular to the in-plane easy axis shows higher velocity and smaller wall motion coercive force compared with the wall whose plane is parallel to the in-plane easy axis. The domain wall velocity depends on the directions of pulse magnetic field and external in-plane field. The effects of external in-plane field in low field region are different from those in high field region.

Bulk and surface resonance properties of amorphous Gd-Co films

The magnetic properties of Gd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> amorphous films (0.81 < x < 0.95) were studied systematically using ferromagnetic resonance and in-plane hysteresis measurements. The various magnetic characteristics such as the magnetization and its temperature dependence, the induced anisotropy, and the ferromagnetic relaxation mechanism were studied in detail. The ferromagnetic resonance (FMR) spectra were interpreted using a model of two exchange-coupled layers: a thick one corresponding to the bulk of the film and a thin one of about 300 Å which forms at the free surface of the films by a preferential oxidation of the rare earth. The location of the surface layer at the film-air interface was demonstrated directly by a chemical etching experiment. The magnetic properties of the two layers and the changes of these layers as functions of various annealing treatments were reported. It was shown particularly that annealing in argon develops a well-defined spin-wave (SW) spectrum in the surface layer. The contribution to the line width of the surface layer arises from an impurity-type relaxation mechanism probably due to the presence of a small percentage of Co in the ionic state. It was suggested that annealing in Ar leads to an atomic reconstruction of this surface layer.

Magnetic properties of multilayered amorphous magnetic films

The magnetic properties of multilayered films consisting of an amorphous Gd-Co layer having a uniaxial magnetic anisotropy perpendicular to the film plane, and a Permalloy or a Tb-Fe layer having an in-plane anisotropy are studied. The thickness dependence of the shift of compensation temperature is studied in the amorphous Gd-Co film coated with Permalloy. When the Permalloy film is thicker than 600A, the shift of compensation temperature becomes constant (about 5°C). Above a thickness of 2500A of the amorphous Gd-Co film, the shift of compensation temperature becomes constant ( about 5°C). On the other hand, if the amorphous Gd-Co film is 1500A thick, the change of compensation temperature does not occur. In the multilayered film having a nonmagnetic layer between the amorphous Gd-Co and the Permalloy film, no shift of compensation temperature occurs. The magnetic properties of thin Tb-Fe films coated on amorphous Gd-Co films have also been studied. When the thickness of the Tb-Fe is greater than about 100A and thinner than about 500A, the polarity of the Kerr hysteresis loops in double layered films changes.

Rate control in dual source evaporation

Two-component thin films are deposited in a high-vacuum system from two close sources, heated by an electron beam which is deflected between them. By using quartz-crystal monitors the evaporation rates are measured separately, which is usually considered to be problematical. One rate signal is used to regulate the appropriate deposition rate by controlling the emission current. The other deposition rate is regulated by using the rate signal to control the dwell-time ratio of the electron beam. With this system it is possible to deposit any composition with a good homogeneity, as in the case Gd-Fe and Gd-Co films.

Inhomogeneities and FMR spectra in some dc sputtered amorphous Gd-Co films

Inhomogeneities and FMR spectra in some dc sputtered amorphous Gd-Co films

Abstract: Influence of oxygen on perpendicular anisotropy in amorphous Gd-Fe and Gd-Co films

Influence of oxygen adsorption on the perpendicular anisotropy in vacuum deposited amorphous Gd–Fe and Gd–Co films was studied. The films of typically 500 Å thickness were prepared by co-evaporation technique. As for Gd–Fe, the perpendicular anisotropy of 3×105 erg/cm3 was found in oxygen-free films but the perpendicular magnetization was not found in oxygen-adsorbed films. This difference is not only due to the increase of the magnetization resulting from the oxidation of the surface layer, but also due to the decrease of the anisotropy by the adsorption of oxygen. It was further found that Gd–Fe films of only 100 Å thickness exhibit the perpendicular magnetization if the films are not exposed to the atmosphere. For Gd–Co films, the magnetization was in-plane in as-deposited samples, but slight increase of the anisotropy was found in the sample containing oxygen during annealing at 200–300 C.

Short-range order in annealed Gd-Co films with perpendicular anisotropy

Amorphous Gd-Co films prepared by sputtering at low dc bias fields show perpendicular magnetic anisotropy after they have been annealed in a vacuum of 10 -6 mbar. Anisotropy is not induced in the films if they are annealed at 10 -3 mbar. Determination of the radial distribution function of the atoms before annealing and after various annealing steps showed that heat-treatment activates oxygen diffusion into the film. The oxygen bonds preferentially to Gd atoms and does not react with Co atoms. The former model of annealing-induced perpendicular anisotropy in RE-TM amorphous films is supported by the results of the present investigation.

Effects of Third Elements on the Magnetic Anisotropy of Sputtered Amorphous GdCo Film

The effects of third elements on the magnetic uniaxial anisotropy of sputtered GdCo films were studied. The magnetic anisotropy induced by bias sputtering is explained from the anisotropic distribution of Co–Co pairs produced by environment-dependent resputtering. It is concluded that the rapid decrease of the positive uniaxial anisotropy on doping with Mo arises from the low resputtering yield of Mo. The thermal stability of the anisotropy of GdCoMo is also related to the low resputtering yield of Mo. Since Cu atoms as well as Gd are easily resputtered in the bias sputtering of GdCoCu films, the anisotropy of GdCoCu is larger than that of GdCoMo and changes greatly on annealing. It is shown that the magnetic anisotropy of the ternary alloy system GdCoM strongly depends on the resputtering probability of the third element M.