MAGNETO-OPTICAL RECORDING ELEMENT AND A MAGNETO-OPTICAL RECORDING DEVICE

Abstract: A magneto-optical recording element having a substrate (1) and a magneto-optical recording layer (4) which is crystalline and comprises a compound of the formula R₂T₁₄B, wherein R is a rare earth metal or a combination of two or more rare earth metals and T is a transition metal or a combination of two or more transition metals, in particular Fe or Co or a mixture thereof, as well as a magneto-optical recording element which comprises a magneto-optical recording element (18).

Description: The invention relates to a magneto-optical recording element which comprises a substrate and a thin recording layer provided thereon of an alloy of a rare earth metal, a transition metal and boron, the alloy having an intrinsic uniaxial anisotropy of magnetization and an easy axis of magnetization which is perpendicular to or substantially perpendicular to the plane of the layer. Such an optical recording element is known from the published Netherlands patent Application No. 7900921 in the name of the Applicants. In the known recording element an amorphous recording layer is used which comprises a comparatively high content of 15-30 at.% of the element boron. The content of the transition metal, in particular Fe, is approximately 50-70 at.%. The rare earth metal, in particular Gd, is present in a quantity of 14-26 at.%. According to the above-mentioned Netherlands Patent Application the addition of B presents the advantage that the alloy crystallises at a higher temperature which lies above the Curie temperature. Herewith the stability of the recording layer is enhanced and notably a greater resistance against irreversible structural changes which change the magnetic properties of the layer is obtained. Although this advantage enables the practical usability of the recording element, it should be borne in mind that an amorphous layer by nature is an unstable layer. When high requirements are imposed upon a non-lasting stability and a long period in use of the recording element the known element does not give good satisfaction. In the long run structural changes nevertheless occur in the amorphous layer in particular as a result of the ever recurring local heating to the Curie temperature. Another disadvantage is that the amorphous layer of the known recording element is rather sensitive to oxidation. This also adversely influences the life of the element. It is an object of the invention to provide a magneto-optical recording element which does not exhibit the disadvantages mentioned hereinbefore. According to one object of the invention a magneto-optical recording element is provided which remains stable and useful for many years. In particular no annoying structural changes occur in the recording layer. Another object of the invention is that the recording layer of the magneto-optical element shows a large polar Kerr rotation, hence a large ϑ k . This quantity, as will be explained in detail hereinafter, is of essential importance for the operation of the magneto-optical element. The reading process is based on it. A large Kerr rotation contributes to a large signal-to-noise ratio upon reading the recorded information. A large signal-to-noise ratio of, for example, 55 dB and higher at a bandwidth of 30 kHz enables reading of video information. Still another object is that the recording layer of the magneto-optical element shows a favourable ellipticity (ε k ) because the value of the measured signal depends not only on the Kerr rotation but is also proportional to the ellipticity according to the formula The value of the Kerr rotation furthermore depends on the wavelength of the light used during the reading process. A further object of the invention is to provide a magneto-optical recording element the recording layer of which produces a very good Kerr rotation of light originating from an AlGaAs laser. This laser, sometimes termed semiconductor laser, has an emission wavelength of approximately 870 nm. The semiconductor laser has small dimensions and can therefore be incorporated easily and advantageously in the apparatus required for the practical use of a magneto-optical recording element. According to the invention, these objects are achieved by means of a magneto-optical recording element of the type mentioned in the opening paragraph which is characterized in that the recording layer is a crystalline layer of a compound which satisfies the formula R₂T₁₄B wherein R is a rare earth metal, or a combination of two or more rare earth metals, and T is a transition metal or a combination of two or more transition metals, in particular Fe or Co or mixtures thereof. When T is the metal Fe or Co or mixtures thereof, at most 50 at.% hereof may be replaced by one or more other d-transition metals, in particular by Ni, Cr, Ru or Mn. In a favourable embodiment of the recording element in accordance with the invention the compound satisfies the formula R′₂Fe₁₄B, wherein R′ is a rare earth metal or a combination of rare earth metals, in particular selected from the group consisting of Nd, Gd, and La. In a further favourable embodinent the compound satisfies the formula Nd₂Fe₁₄B. In the magneto-optical recording element according to the invention binary (digital) information is recorded thermo-magnetically. For this purpose, the recording layer is exposed to a light beam preferably originating from an AlGaAs laser focused on the layer. The light beam is pulsated in accordance with the information to be recorded. The pulse time is low and is, for example, from 50-500 ns. The power of the laser is, for example, from 0.5 to 5 mW. In the exposed areas the recording layer is heated to the Curie temperature. Upon cooling the magnetization direction of the heated area is reversed under the influence of the magnetic interference fields of adjacent non-heated areas of the recording layer. The reversal of the direction of magnetization can also be produced by an external magnetic field which is directed oppositely to the initial direction of magnetisation. The areas (info-bits) of opposite direction of magnetization have small dimensions of, for example, from 0.5 to 5 µm. The exposure is preferably carried out via the substrate. In that case the substrate must be transparent to the light of the light beam used. Herewith it is achieved that any dust particles or other contaminations on the surface of the substrate do not adversely influence the quality of reading and also of writing because these impurities fall beyond the depth of focus of the objective with which the light beam is focused on the recording layer. The transparent substrate is manufactured, for example, from glass, quartz, or a transparent synthetic resin, for example polycarbonate or polymethylmethacrylate. The surface of the substrate, on the side of the recording layer, may comprise an optically scannable groove, a so-called servo track, which usually is spiral-like. The groove may comprise a so-called heading, i.e. info-areas which are situated alternately at a higher level and at a lower level and which can be read optically by means of laser light and comprise information with which, for example, the writing process is controlled. The magneto-optical recording of information preferably takes place in the groove. The groove may also be provided in a separate layer of a synthetic resin provided on the substrate and consisting, for example, of a light-cured layer of acrylic acid esters. The recording layer is provided over said layer. The recording layer may be coated by means of a protective layer of a synthetic resin. Alternatively, two elements according to the invention may be adhered together, the recording layers facing each other. The recorded info-bits are read magneto-optically by means of a weak, continuous, linearly polarized light beam which is focused on the recording layer. At the area of an info-bit the plane of polarization of the light bean is rotated over a given angle as a result of the magnetization, which can be established and measured by means of a detector. The rotation of the plane of polarization occurs both in a reflected light beam, and is then termed Kerr rotation, and in a transmitted light beam, which is known as the Faraday rotation. The recording element according to the invention is preferably reed in reflection via the substrate. The Kerr rotation can be indicated in two manners. There is a single Kerr rotation if the measurement is carried out with respect to an area which has no magnetization, there is a double Kerr rotation if the measurement is carried out with respect to an area the magnetization of which has an opposite sign, that is the direction of magnetization of which has been rotated over 180°. The information stored in the magneto-optical recording element can be erased by reversing the direction of magnetization of the info-areas to the original direction by using an external magnetic field which is directed oppositely to the direction of magnetization of the info-areas. The crystalline recording layer of R₂T₁₄B used according to the invention is provided by means of an electroless deposition process, in particular a vapour deposition process or a sputtering process. In the vapour deposition process an (ultra) high vacuum is used, for example a vacuum of 10⁻⁸ or 10⁻¹⁰ mbar, the individual elements being deposited on the substrate surface in the desired ratio. A sputtering process with, for example, the sputtering gas argon, in preferably used. During the deposition a magnetic field may be used having a field strength of, for example, 160 kA/m which is perpendicular to the plane of depositon. The substrate is preferably kept at an elevated temperature during the deposition. It is to be noted that it is known per se from, for example, paper 4 G-10, Int. Conf. on Magnetism, San Francisco, August 26-30, 1985 by F.J. Cadieu et al. to use the compound Nd₂Fe₁₄B as a permanent magnetic material in the form of a thin film. From Journal de Physique, Collogue C 6, 1985, pp. 45-48 the potential use is stated of compounds of the formula R₂Fe₁₄B for magnetic recording purposes. These known uses are based on phenomena and properties quite different from the magneto-optical recording described hereinbefore. Important factors for magneto-optical recording are inter alia the Curie temperature of the recording material, the intrinsic, uniaxial anisotropy of the material having an easy axis of magnetization which is perpendicular to the plane of deposition, the light absorption coefficient, the heat conduction coefficient and the specific heat of the material, the change of the direction of magnetization upon heating by means of laser light, the value of the demagnetization field and the rotation of the plane of polarization of linearly polarized light caused by the material upon reflection and transmission (Ken and Faraday rotation). In the known uses the above phenomena and properties do not play a role but, for example, the energy product of the material, the BHmax, is a very important datum. Herein, B is the magnetic flux density and H is the strength of the magnetic field. The invention furthermore relates to a magneto-optical recording device having a recording element which comprises a substrate and a thin recording layer provided thereon, reading and writing means which comprise a source of radiation for providing a linearly polarized light beam, means to focus the light beam at a desired area of the recording layer, and a detector to measure the rotation of the plane of polarization of the light beam after reflection against or transmission through the recording layer, and having a magnet coil for generating a magnetic field which is perpendicular to the recording layer, which device is characterized in that the recording layer is a crystalline layer of a compound of the formula R₂T₁₄B, wherein R is a rare earth metal or a combination of two or more rare earth metals, and T is a transition metal or a combination of two or more transition metals, in particular Fe or Co, or a mixture thereof. The invention will be described in greater detail with reference to the specific example and the drawing, in which Figure 1 is a cross-sectional view of a magneto-optical recording element according to the invention. Figure 2 is a graph showing the double Kerr rotation of the substances used in the recording element as a function of the wavelength of the light used, and Figure 3 shows diagrammatically a recording device according to the invention. Example Reference numeral 1 in Figure 1 denotes a substrate plate of glass comprising a layer 2 of a light-cured acrylate synthetic resin in which a spiral-like groove 3 has been provided. The groove may be provided with information areas situated alternately at a higher level and at a lower level, the so-called heading, which can be read in reflection by means of laser light via substrate plate 1 on the basis of phase differences. Layer 2 comprises a vapour-deposited layer of SiO₂, not shown. A magneto-optical layer 4 having the composition R₂T₁₄B is provided hereon. Layer 4 is a polycrystalline layer provided by an RF sputtering process performed in an argon atmosphere at 0.15 mbar in the presence of a magnetic field which is perpendicular to the surface of the layer 4. The thickness of the layer 4 is approximately 150 nm. The easy axis of magnetization of the layer 4 is perpendicular to the major surface of the substrate 1. Layer 4 is coated by a protective layer 5 of a synthetic resin. Information is recorded in the groove parts of the layer 4 by means of pulsated laser light indicated by arrow 6 which is focused on the recording layer 4 via substrate plate 1. The pulse time is 100 ns. The laser light originates from an AlGaAs laser having an emission wavelength of 870 nm. During the exposure an external magnetic field is applied which is directed oppositely to the direction of magnetization of layer 4. In the exposed areas 7 of layer 4 heated to the Curie temperature a reversal of the direction of magnetization takes place. The information bits thus obtained have a diameter of approximately 1 µm. The information bits are read by means of weak laser light which is linearly polarized. The light is focused on the recording layer 4 via the substrate plate 1. Upon reflection against an information bit a rotation of the plane of polarization takes place (Kerr rotation) which is measured by means of a detector (seeFigure 3). The information bits 7 can be erased by means of an external magnetic field which is directed oppositely to the direction of magnetization of the information bit. The magneto-optical properties of a number of polycrystalline compounds which satisfy the formula R₂T₁₄B have been measured and are recorded in the Table below. In the Table, λ is the wavelength of the light given in nm; 2 ϑ k is the double Kerr rotation given in degrees (degr.) and measured in a field of 925 kA/m at the first negative peak in the rotation spectrum, σ s is the total magnetization measured in a field of 925 kA/m; σ 3d is the 3 d sublattice magnetization and 2 ϑ k /σ 3d is the reduced Kerr rotation. On the vertical (Y) axis of Figure 2 the double Kerr rotation (2 ϑ k ) is recorded given in degrees of a number of crystalline compounds of the formula R′₂Fe₁₄B as a function of the wavelength (λ), expressed in nm. The Kerr rotation has been measured at room temperature in a magnetic field of 925 kA/m. The various compounds of the formula R′₂Fe₁₄B are indicated in the Figure by stating the rare earth metal present in the compound. It appears from the Figure that the compound Nd₂Fe₁₄B shows the largest negative Kerr rotation at all wavelengths between 2500 and 250 nm. It may also be derived from the Figure that the Kerr rotation shows the highest absolute values in a wavelength range from approximately 1650 nm to 830 nm. When an AlGaAs laser is used having an emission wavelength of 870 nm the Kerr rotation shows a very good value. Reference numeral 8 in Figure 3 a denotes an AlGaAs laser which provides a pulsated light beam 9 having a wavelength of 870 nm. The light beam 9 passes through an objective 10 with a numerical aperture (NA) of 0.3. The astigmatism of the laser beam is corrected by the passage of a cylinder lens 11. The parallel beam 12 then traverses a partially transparent mirror 13 and a polarizing beam splitter 14. This passes the parallel polarization component of the laser light while the perpendicular component is reflected. The passed linearly polarized light passes through a Faraday rotator 15 which rotates the direction of polarization over a small angle (1/2 β) (see also Figure 3 b ). The linearly polarized light is then focused on the recording layer 17 of the magneto-optical recording element 18 by means of an objective 16 having a numerical aperture (NA) of 0.6. The recording layer is a crystalline layer of a compound of the formula R₂T₁₄B. The recording layer is provided on a quartz substrate 19. The exposure of the recording layer takes place via the substrate 19. In the exposed areas a rise in temperature to approximately the Curie temperature takes place by light absorption. By the use of a magnetic field having a strength of 4 x 10⁴ A/m generated by coil 20 the direction of the magnetization of the recording layer in an exposed area is rotated through 180°. Upon reading the recorded information a weak continuous light beam is used originating from the AlGaAs laser 8 which follows the same light path as described hereinbefore and is shown in Figure 3 a . Upon reflection of the linearly polarized light beam against an information bit of the recording layer 17, the plane of polarization is rotated over an angle ϑ k (Kerr rotation) as a result of the locally varied direction of magnetization (see Figure 3 b ). The reflected light beam again passes the objective 16, the Faraday rotator 15 (rotates again over 1/2 β) and then reaches the polarizing beam splitter 14 in which the parallel component is passed. After reflection against the partially transmitting mirror 13 said parallel component 21 is returned to the control device 22 for positioning and focusing the laser light beam on the recording layer 17 upon writing information. The perpendicular light component is reflected by the polarizing beam splitter. Via an objective 23 the perpendicular component 24 is focused and detected in a detector 25 in the form of, for example, an avalanche photo diode. The amplitude fluctuation (A⁻ - A ⁺) shown in Figure 3 b as a result of the rotation of the plane of polarization (2 ϑ k ) provides the reading signal after quadratation.

Claims: 1. A magneto-optical recording element which comprises a substrate and a thin recording layer provided thereon of an alloy of a rare earth metal, a transition metal and boron, the alloy having an intrinsic uniaxial anisotropy of magnetization and an easy axis of magnetization which is perpendicular to or substantially perpendicular to the plane of the layer, characterized in that the recording layer is a crystalline layer of a compound which satisfies the formula R₂T₁₄B wherein R is a rare earth metal, or a combination of two or more rare earth metals, and T is a transition metal or a combination of two or more transition metals, in particular Fe or Co or a mixture thereof. 2. A magneto-optical recording element as claimed in Claim 1, characterized in that the compound satisfies the formula R′₂Fe₁₄B wherein R′ is a rare earth metal or a combination of rare earth metals, in particular selected from the group consisting of Nd, Gd, and La. 3. A magneto-optical recording element as claimed in Claims 1 and 2, characterized in that the compound satisfies the formula Nd₂Fe₁₄B. 4. A magneto-optical recording device having a recording element which comprises a substrate and a thin recording layer provided thereon, reading and writing means which comprise a source of radiation for providing a linearly polarized light beam, means to focus the light beam on a desired area of the recording layer, and a detector to measure the rotation of the plane of polarization of the light beam after reflection against or transmission through the recording layer, and having a magnet coil for generating a magnetic field which is perpendicular to the recording layer, characterized in that the recording layer is a crystalline layer of a compound of the formula R₂T₁₄B, wherein R is a rare earth metal or a combination of two or more rare earth metals and T is a transition metal, in particular Pe or Co.

Inventors: BUSCHOW, KURT HEINZ JUERGEN, VAN ENGELEN, PETRUS PAULUS JACOBUS, Buschow, Kurt Heinz Jürgen
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: G03G5
IPC codes: H01F10, G11B11, C23C14, G02F1