1.  Field of the Invention
The present invention relates to a magneto-optical recording medium, and more particularly, to a magneto-optical recording medium of a magnetic domain enlarging/reproducing system in which a magnetic domain of a recording layer is copied to a reproducing layer and enlarged to reproduce a signal. 
2.  Description of the Related Art
A magneto-optical recording medium, which is noted as a rewritable and highly reliable recording medium having a large storeage capacity, has been put into practice as a computer memory or the like.  Further, standardization of a magneto-optical recording medium having a storage capacity of 6.0 Gbytes is recently progressed and would be available for practical use.  The signal reproduction from such a magneto-optical recording medium with high density is performed by MSR (Magnetically Induced Super Resolution) method, which copies a magnetic domain of a recording layer of the magneto-optical recording medium to a reproducing layer while forming a detecting window in the reproducing layer so as to detect only the copied magnetic domain and detecting the magnetic domain copied from the detecting window. 
Further, in the signal reproduction from the magneto-optical recording medium, the technology of a magnetic domain enlarging/reproducing has been developed, which applies an alternating magnetic field and enlarges the magnetic domain copied from the recording layer to the reproducing layer by the alternating magnetic field to reproduce a signal, and a magneto-optical recording medium, which can record and/or reproduce a signal of 14 Gbyte on the disk of 12 cm in diameter by the technology, has also been proposed. 
Such a magneto-optical recording medium of the magnetic domain enlarging/reproducing system generally includes a reproducing layer, a non-magnetic layer formed on the reproducing layer, and a recording layer formed on the non-magnetic layer.  In the signal reproduction from the magneto-optical recording medium obtained by enlarging the magnetic domain, a laser beam is directed from the reproducing layer side, and an external magnetic field for enlarging and erasing of the magnetic domain is applied from the side of the recording layer, whereby the magnetic domain of the recording layer is copied to the reproducing layer and enlarged, by a magneto-static coupling via the non-magnetic layer.  The magnetic domain copied to the reproducing layer and enlarged is detected by the laser beam directed from the side of the reproducing layer, whereby the signal in the recording layer is reproduced. 
The above-described copy of the magnetic domain by the magneto-static coupling occurs in response to a leakage magnetic field from the recording layer and the reproducing layer.  However, because of the random magnetic domain length of the recording layer, a problem arises in that the profile of the leakage magnetic field differs depending on the magnetic domain length.  That is, for a short magnetic domain length, the intensity of the leakage magnetic field becomes maximum at the center of the magnetic field, whereas for a long magnetic field length, it considerably weakens at the center of the magnetic field.  Thus, the long magnetic domain may not be copied while the short magnetic domain is reliably copied. 
It is an object of the present invention to provide a magneto-optical recording medium in which a magnetic domain can be reliably copied from a recording layer to a reproducing layer independently of the magnetic domain length of the recording layer. 
According to the present invention, the magneto-optical recording medium includes a reproducing layer, a blocking layer formed on the reproducing layer, a gate/magnetic field reinforcement layer formed on the blocking layer, and a recording layer formed on the gate/magnetic field reinforcement layer.  The gate/magnetic field reinforcement layer selectively extracts each magnetic domain within the recording layer and reinforces a leakage magnetic field reaching there from to the reproducing layer to copy the extracted magnetic domain into the reproducing layer.  The blocking layer blocks an exchange-coupling force from the gate/magnetic field reinforcement layer to the reproducing layer. 
Preferably, the above-described gate/magnetic field reinforcement layer includes a gate layer and a magnetic field reinforcement layer.  The gate layer selectively extracts each magnetic domain.  The magnetic field reinforcement layer is formed between the blocking layer and the gate layer to reinforce the leakage magnetic field. 
Alternatively, the above-described gate/magnetic field reinforcement layer has a saturation magnetization increasing from the side of the recording layer toward the side of the blocking layer at a reproducing temperature. 
In the above-described magneto-optical recording medium, the magnetic domain in the recording layer is not selectively extracted depending on its length, so that the extracted magnetic domain can reliably be copied to the reproducing layer.  Further, because the leakage magnetic field reaching the reproducing layer is reinforced, the extracted magnetic domain can more reliably be copied into the reproducing layer.  The blocking layer is also provided so as to block the exchange-coupling force to the reproducing layer.  As a result, the magnetic domain copied into the reproducing layer can smoothly be enlarged.