TbFeCo Films Research Articles

Optical and magneto-optical characterization of TbFeCo and GdFeCo thin filmsfor high-density recording

Thin, optically semi-infinite films of amorphous TbFeCo and GdFeCo, suitable formagneto-optical recording, have been deposited by DC magnetron sputteringonto glass. Ellipsometric techniques have been used to determine thecomplex refractive index and complex magneto-optical parameter of the filmsin the wavelength range 400–900 nm, thus characterizing the materials.A review of the literature is presented and shows that the results forthe TbFeCo films compare favourably with published results obtainedfrom measurements conducted in situ, with the films protected with ZnSbarrier layers. It is found that GdFeCo and TbFeCo are optically verysimilar, but magneto-optically the materials are quite different.

ＴｂＦｅＣｏ膜における磁気特性のスパッタガス種依存性

This paper investingates what the magnetic characteristics of TbFeCo films would become if Ar and Kr were used as sputtering gases. The sample structure was glass / underlayer SiNx(u) / TbFeCo / protective layer SiNx(p), and the top three layers were deposited by magnetron sputtering. When Kr gas was used, the coercive force HC and effective perpendicular anisotropy Keff increased. The increase was not dependent on the difference in Curie temperature. The type of sputtering gas used was changed for deposition of the SiNx(u) and TbFeCo layers. Consequently, HC increased when TbFeCo was deposited by using Kr gas. Changing the kind of sputtering gas used seldom affected SiNx(u).

Temperature dependence of magnetization reversal in TbFeCo films

The magnetization reversal process has been studied in a series of four TbFeCo films over the temperature range 300–460 K. The films were characterized by time decay, dynamic coercivity, Kerr imaging, and torque measurements. For a film with a sputter-etched SiN underlayer, the reversal process at low temperatures was dominated by nucleation followed by rapid domain wall motion. The low temperature relaxation curves for this sample were analyzed using a Fatuzzo model. At high temperatures, the Fatuzzo model did not apply. For the other films, with no etching, relaxation curves were logarithmic at all temperatures. All activation volumes increased with increasing temperature and decreased with increasing sputtering pressure. The magnetic anisotropy direction changed from perpendicular to in-plane as the argon sputtering pressure was increased.

Effect of Si dielectric materials on the magnetic coupling of Tb–Fe–Co multilayer thin films

Magnetization behavior of TbFeCo thin film materials was systematically examined. In a single layer system, the sputtering parameters were observed to significantly affect the M– H hysteresis of the TbFeCo thin films, which is ascribed to the modification on the thin film composition. Moreover, the sputtering conditions were observed to change the stress condition of the Si 3N 4 passivation layer, which, in turn, also pronouncedly alters the M– H hysteresis behavior of the TbFeCo thin films. TbFeCo/Si/TbFeCo multilayer thin films, which were prepared carefully to assure the correct composition of each TbFeCo films, were observed to behave similarly with pseudo-spin-valve like magnetic materials system. The coercive field ( H c) of the low- H c TbFeCo films was modified markedly due to TbFeCo–Si–TbFeCo magnetic coupling effect and was observed to fluctuate periodically with the thickness of Si-intermediate layer, a behavior similar to RKKY phenomenon.

Magnetic Characteristics of TbFeCo/FePt Double Layer for MO Recording

To achieve ultra high-density Magneto-optical (MO) recording, it is necessary to increase the domain wall coercivity (HW) to freeze the shape of a written domain. TbFeCo film is fabricated on FePt crystalline film, which has large crystalline magnetic anisotropy, to increase the HW of TbFeCo film by the exchange coupling effect. The TbFeCo film is rare earth (RE) dominant ranging from room temperature (Troom) to Curie temperature (Tc). The FePt is deposited as multilayers and then it is subjected to vacuum annealing at 400°C for 2 hours. The TbFeCo/FePt film shows higher HW, coercivity (Hc), and the thermal stability than those characteristics of the single TbFeCo film. Those characteristics can be attributed to the exchange coupling between magnetic moments of Fe and Co in TbFeCo and FePt films.

Superior film composition uniformity and adjustable composition for the next generation MO technology

This paper describes the details of a new co-sputtering device (Multi-Source Trio) designed for the SPRINTER 750 sputtering system for MO production (MiniDisk, 3.5″ or smaller formats). Focussing on TbFeCo films we present data for changing the magnetic properties of the film by adjusting the sputter power level on the individual targets with different elemental composition. Special emphasis is placed on the composition uniformity throughout the disk which has been shown to be better than 0.2%.

Thermal stability of written bits in high density magneto-optical recording media

Masurements of magnetic domains in TbFeCo films currently used as magneto-optical recording media indicate a shrinkage in domain size under accelerated conditions. A criterion factor (E/sub a//kT) for thermal stability is found to be about 40 for domains with a diameter of about 0.8 /spl mu/m, where E/sub a/ is an activation energy estimated on the basis of Arrenhius plot of the shrinkage with time. For high density recording applications, a higher wall pinning field is necessary.

Micron-scale magnetic structures for atom optics

Magnetic structures with periodicities on the scale of a micron have been constructed by magneto-optical recording on perpendicularly magnetised TbFeCo film with a focussed diode laser beam in the presence of an external field, and by microfabricating perpendicularly magnetised grooved CoCr structures. Strong reflection signals and predominantly specular reflection have been realised for beams of laser-cooled caesium atoms normally incident on both types of structure. Preliminary measurements of the specularity for a grooved a=1 μm magnetic structure are particularly encouraging.

Simulation of Thermomagnetic Recording Using Extended Huth's Equation.

Thermomagnetic recording process for various TbFeCo films on PMMA has been simulated in the case of the irradiation of the laser spot with 1.0 and 0.2 μm by using a modified Huth’s equation. In the case of TM-rich alloy, it is found that the domains smaller than 0.1 μm in diameter are formed at temperature below the Curie point because of the large demagnetizing energy, which results from the large magnetization. On the other hand, the large temperature gradient makes it difficult to stabilize the small size domain in RE-rich film.

Different mechanisms of spin reorientation in exchange coupled double rare earth-transition metal layers with in-plane and perpendicular magnetic anisotropy

The objective of this article is to clarify the spin reorientation effects in exchange coupled layers based on rare earth and transition metals with in-plane and perpendicular anisotropy. These systems are proposed for high density magneto-optical recording with super resolution readout. Two mechanisms are found to be responsible for this change of the magnetization direction with the temperature. The first mechanism is an intrinsic spin reorientation in the GdFeCo layer and the second is the exchange coupling force between the perpendicularly magnetized TbFeCo film and the in-plane GdFeCo film. With appropriate adjustment of the compensation temperatures and the thicknesses of the films, it is possible that the one or the other mechanism dominates. The experimental results are in agreement with a numerical calculation of the magnetization profiles in the coupled system.

Dynamics of first- and second-order phase transitions in amorphous magnetooptic TbFeCo films

The dynamics of phase transformations in thin amorphous TbFeCo films under the action of ∼ 1 ps laser pulses is investigated. The films are heated to the Curie temperature in the amorphous state (T C1), to the crystallization temperature (T ac), and to the Curie temperature in the crystalline phase (T C2). The change in magnetization is detected by Faraday magnetooptic effect during and after the action of the heating pulse. A static external magnetic fieldH∼1−12 kOe, whose flux lines are directed perpendicular to the plane of the film, is used in the experiments. Amorphous TbFeCo films possess a perpendicular magnetic anisotropy, which on crystallization becomes reoriented in the plane of the film. It is observed that crystallization and magnetization reorientation occur during the heating pulse (within ∼ 1 ps). The spin subsystem is heated to the Curie temperature several picoseconds after the end of the laser pulse. The characteristic spin relaxation time is ∼ 10 ps. A model of the dynamics of the electronic, spin, and phonon subsystems that makes it possible to explain the experimental results is proposed on the basis of the data obtained.

ランド／グルーブ基板への均一スパッタ成膜の検討

Uniformity of the thickness of magnetic film on land/groove substrate was improved using collimated-sputtering method. The magnetic properties of TbFeCo films and interface wall energy of exchange-coupled double layer DyFeCo/TbFeCo films were compared with conventional sputtering method. There was not much difference of the magnetic properties and interface wall energy between two sputtering methods. It was confirmed that this method was effective in production of high recording density MO disk using land/groove substrate.

Magnetisation of thin films under oblique field conditions

Magneto-optical measurements have been made of a single-layer film of TbFeCo and a multilayer Pt/Co film both with perpendicular anisotropy. Magnetising fields were applied at angles in the range θ = 0°-60° with respect to the film normal. Switching fields ( H sw) and the time dependence of magnetization were measured as functions of θ. Values of the magnetic viscosity parameter ( A) were determined from the time-dependent magnetization measurements. The angular variation of both H sw and A are given. The distributions of regions of reversed magnetization in the films have been examined by magnetic force microscopy (MFM) and Kerr effect microscopy. The fractal dimensions of these patterns have been determined. In the TbFeCo film the magnetization distributions and fractal dimension change significantly as the obliquity of the magnetising field changes. In contrast, the obliquity of the magnetising field has little effect on the characteristics of magnetised regions of the Pt/Co film.

SIMULATION OF THERMOMAGNETIC RECORDING IN RARE EARTH-TRANSITION MAGNETIC FILM USING VERY SMALL LASER SPOT

Thermomagnetic recording process has been simulated for transition metal(TM)-rich and rare earth(RE)-rich TbFeCo films assuming heam spot size of 1 and 0.1 micron, where very simple temperature dependence is assumed for the wall energy. For the TM-rich Tb18(Fe78Co22)82 film, domains smaller than 0.1 micron can be written owing to the large stray field. On the other hand, for the RE-rich Tb25.6(Fe78Co22)74.4 film, small domains cannot be written due to the large effective field which, originating from the wall energy and its spatial gradient, forces to shrink the domain.

Brillouin light-scattering study of TbFeCo thin films

Spin-wave excitations have been observed, using Brillouin light scattering, in thin films of TbFeCo, grown by sputtering on silicon substrates, with thicknesses in the range 4.5–250 nm. The variations of spin-wave frequency with applied in-plane magnetic-field H (100 Oe⩽H⩽4.5 kOe) and in-plane magnon wave-number k (4.24×104⩽k⩽2.41×105 cm−1) were obtained. For these films, oxidation during the deposition process produces a bulk film, which is very weakly ferrimagnetic and, therefore, largely magnetically dead. However, further oxidation, at the film surface after deposition, due to exposure to air, increases the magnetization to produce a magnetically active surface layer. The results were modeled as a Damon–Eshbach surface mode, and the TbFeCo films considered as a magnetic film on a nonmagnetic layer, the thicknesses of which were both unknown. The frequency versus field measurements enabled values for the saturation magnetization and gyromagnetic ratio to be determined. The measurements of the frequency versus wave number showed evidence that, for the thicker films, the magnetically active layer was only a small fraction of the total thickness, and the active layer thicknesses were determined. The values of magnetizations are compared with those obtained from vibrating sample magnetometry measurements.

Oblique sputtering of amorphous TbFeCo thin films on glass substrates and the effect of deposition angle on perpendicular magnetic anisotropy

We measured the magnetic hysteresis loops for several obliquely deposited amorphous TbFeCo films. The experimental results show that the direction of the average magnetic anisotropy (i.e., the easy axis of magnetization) is no longer along the surface normal. With the help of computer simulations, we have quantified the effects of oblique deposition in terms of the deviations of local anisotropy directions from the surface normal. We also found that, with the increasing of the deposition angle, the compensation point shifts toward the Fe-rich side and the films become thinner.

Effect of AlN Film on the Magnetic Properties of Rare Earth-Transition Metal Film

The amorphous TbFeCo films covered with the protective AlN films have been prepared by the rf magnetron sputtering system. It is found that the protective AlN films not only protect rare earth elements from oxidation and enhance magneto-optical Kerr effect but also affect the magnetic properties of the rare earth-transition metal films. The coercivity Hc and perpendicular magnetic anisotropy energy constants K'u1 and Ku2 of the films are changed with the thickness of the protective AlN films. It can be explained according to the stress mechanism and the model of single ion anisotropy.

Optical Properties and Reactive Sputtering Conditions of AIN and AlSiN Thin Films for Magneto- Optical Applications

The dependence of the refractive index and film thickness of AlN and AlSiN thin films on the reactive sputtering conditions (N2 partial pressure, total pressure, and rf sputtering power) have been studied in detail and analyzed in terms of the mechanism of reactive sputtering and the target poisoning phenomenon. On the basis of the results, the sputtering conditions of AlN and AlSiN films have been optimized for magneto-optical applications. The AlN and AlSiN films after optimization of the sputtering conditions had very high transmissivity and refractive index, and were suitable for magneto-optical Kerr effect enhancement. A marked enhancement of the magneto-optical Kerr effect for a TbFeCo film coated with AlN and AlSiN films was observed. The improvement of the signalto-noise ratio obtained with the AlSiN film was greater than the AlN film.

Dielectric tensor characterization of Mn0.53Bi0.47 and Mn0.52Bi0.44Sb0.04 films

We have grown Mn0.53Bi0.47 and Mn0.52Bi0.44Sb0.04 alloy films on glass substrates under UHV conditions. These films exhibit good magneto-optical (MO) properties. Unfortunately, the measurements of Kerr rotation and ellipticity cannot by themselves provide reliable evaluation of the MO properties of a layer capped with a dielectric SiO film. The purpose of the present work was to determine the dielectric tensor in the MnBi1−xSbx films within a multilayer stack, using a combination of ellipsometric, reflection/transmission, and polar MO Kerr effect measurements in the wavelength range of 360–860 nm. We have evaluated the Mn0.53Bi0.47 and Mn0.52Bi0.44Sb0.04 films based on the intrinsic MO figure of merit (FOM) defined by FOM=‖εxy‖/(2 Im εxx), where εxy and εxx are the diagonal and off-diagonal elements of the dielectric tensor of the MO material. For short wavelengths (360–550 nm) the measured FOM in the Mn0.53Bi0.47 and Mn0.52Bi0.44Sb0.04 films is significantly larger (≥factor of 2) than that commonly observed in TbFeCo films (∼0.01).

Barkhausen jumps during domain wall motion in thin magneto-optical films

A method with high spatial resolution is developed to study wall motion in thin magnetic films with perpendicular anisotropy. Barkhausen jumps during domain wall motion in amorphous TbFeCo films and polycrystalline Co/Pd multilayer films have been observed. The average distance between strong pinning sites in Co/Pd was found to be around 0.3–0.4 μm. Maximum pinning times as long as several seconds were observed. Magnitudes of major jumps in TbFeCo were around 0.7 μm.