1 Introduction
Extensive research has been carried out on amorphous rare earth (RE) - transition metal (TM) thin films from both fundamental and applications points of view[1-4]. These amorphous films offer unique advantages in magnetic and magneto-optical properties compared to crystalline phases. As for the magnetic properties of the amorphous phases, the ferri-magnetic structure with two subnetworks of RE and TM atoms enables the saturation magnetization to be controlled continuously by changing the compositions of RE and TM. Such a behavior can not be achieved in corresponding crystalline alloys (only some certain alloy compositions are possible to crystallize, such as (RE)TM2). Further, a large perpendicular magnetic anisotropy, in the order of 106-107 ergs/cc, is induced. The perpendicular magnetic anisotropy enables the amorphous films to posses a relatively high polar Kerr activity. These properties have been considered to be very attractive for the application as magneto-optical recording materials.
Magneto-optical recording techniques of today offer high areal storage density, a few Gbits per square inch, on a removable storage medium. Since the read and write processes are performed by a laser at comparably large distance, the magneto-optical drives possesses a freedom from head crashes caused by the small head-to-media flying hights used in conventional magnetic recording. The media of choice in current magneto-optical recording technology are the amorphous RE-TM alloys such as TbFeCo thin films. In order to increase the areal recording density to obtain a future high density storage media the wavelengths of the read and write laser will have to decrease from the current wavelengths (650-830 nm) to obtain a smaller focused spot size. However, the current media such as TbFeCo amorphous films exhibit a significant reduction in the polar Kerr effect at shorter wavelengths. Following this, future high-density magneto-optical recording technology requires novel materials with high polar Kerr activities at short wavelengths.
Amorphous films exhibits another unique feature compared to crystalline films, the absence of grain boundaries enables the domain walls to move smoothly under the influence of an external field. This behavior gives rise to a better recording performance, since the noise due to wall pinning and non-uniformity of the magnetization distribution is much smaller than in crystalline media. It has been reported that the carrier-to-noise ratio of TbFeCo discs, even though the disadvantage in Kerr rotation still remained at the same level as of the Co/Pt multilayer disc at short wavelength (more than 62 dB at 488 nm)[5]. It is therefore believed that TbFeCo may continue to be chosen as fundamental material for shorter wavelengths, although the addition of some other element is required to improve the readout sensitivity.
There has been a growing interest for ultra-high density MO recording using ultra-violet light, which may enable densities beyond 50 Gbits per square inch. However, there have been very little systematic studies as to the magneto-optical polar Kerr characteristics of these RE-TM over a wide range of wavelength including ultra-violet. This is due to of the lack of appropriate measuring apparatuses capable to do such measurements. Therefore, it is not clear whether or not these amorphous films of TbFeCo and other RE-TM are suitable for the future recording techniques.
In the present laboratory, Information Storage Materials Laboratory (ISML), a novel apparatus has been developed which is called ?Wide Wavelengths Magneto-optical Kerr spectroscope Apparatus?. This system is capable of measuring the magneto-optical Kerr rotation and ellipticity, as well as the optical constants n and k over a wide wavelength from 180 nm to 900 nm at temperatures from 80 K to 600K in fields up to 20 kOe. Therefore, it is now possible to measure a detailed behavior of the polar Kerr activity using this system.
Polarized Pd and Pt atoms are known to enhance the magneto-optical Kerr effect at short wavelengths[6]. Awano et al.[7] showed the effect of Pt on the Kerr rotation of TbFeCo films. However, their measurements were made for a wavelength down to only 400nm, and thus no information was available about the ultra-violet region. Neither the magnetic properties nor the magneto-optical properties were discussed as a function of layer thicknesses of Pt and TbFeCo, and thus very little systematic information was obtained.
In view of this background, the present study has been aimed to understand the magnetic properties and magneto-optical properties of TbFeCo multilayers with Pt and Pd elements and to examine the role of Pt and Pd in these characteristics.