Magnetic reading device with alternating magnetic biasing means

This magnetic medium reading device comprises means for the induction of an AC magnetic excitation field in the reading zone of the magnetic head. It can be applied notably to magneto-optical reading heads. Applications: reading of magnetic media (magnetic tapes and disks).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a magnetic reading device and, notably, to a 
reading head for magnetic recording media. It can be applied preferably in 
a magnetic reader of computer peripherals or in professional video tape 
recorders. 
More particularly, the invention can be applied to a Kerr effect type of 
magneto-optical reading head with a view to linearizing its operation. 
In the general context of a system for the recording and reading of data on 
magnetic media, the reading sensor may be magneto-optical and may use the 
"Kerr" effect to read the data elements. 
The magneto-optical Kerr effect is a property of certain materials used in 
the industry of magnetic sensors: a linearly polarized optical beam that 
gets reflected on the surface of a material possessing the Kerr effect has 
its direction of polarization rotated as a function of the magnetic flux 
present in the material, at the position of the reflection. 
As shown in FIGS. 1a and 1b, a reading sensor working by Kerr effect is 
organized in two layers of magnetic material 1, 2 separated by a 
non-magnetic insulator 3: this structure is optimized to channel the 
magnetic flux coming from the information elements recorded on the medium 
as is shown in FIG. 1a. 
In the zone 5 of the sensor, this sensor is in contact or almost in contact 
with a magnetic medium 4 to be read. A linearly polarized optical beam FI 
gets reflected on one of the magnetic layers of the head and the 
information elements are read by detection of the rotation of polarization 
(the Kerr effect) of the reflected beam FR. 
This device is the object of the French patent application No. 89 17313. 
However, this device has the following drawbacks: 
The Kerr effect is a surface effect, and to obtain high sensitivity of the 
Kerr sensor, the layers forming the magnetic circuit have to be as thin as 
possible; 
In structures such as these, the magnetization is organized in "magnetic 
domains" that shift randomly as a function of the magnetic information 
elements read back from the magnetic medium. The noise due to these random 
reconfigurations reduces the signal-to-noise ratio and makes the signal 
unusable; 
Furthermore, the magnetic layers always display magnetic anisotropy in the 
plane and a residual coercive field that takes the form of non-linearities 
in the signal that is read back. 
One technique used to stabilize the domains of the magnetic layers in 
magneto-resistive sensors consists of the addition, to the head, of a 
magnetized layer which "anchors" the domains by its field. 
This technique is aimed at overcoming these defects. 
SUMMARY OF THE INVENTION 
The invention therefore relates to a magnetic reading device comprising a 
reading head having at least one magnetic thin layer comprising a reading 
zone, wherein said device also comprises means for the induction of a 
magnetic excitation field superimposed in the reading zone. 
The superimposed magnetic excitation field is preferably an alternating 
field. 
In this device, the magneto-optical reading head is a device wherein the 
magneto-optical reading head is a magneto-optical head with Kerr effect 
comprising two layers of magnetic material separated by a gap layer made 
of non-magnetic material and comprising the reading zone designed to be 
located in a magnetic field to be detected, the reading light beam being 
polarized and being reflected by one of the magnetic layers, an analyzer 
of polarizations receiving the read beam and transmitting it to a 
photodetector.

DETAILED DESCRIPTION OF THE INVENTION 
The invention consists of a low-amplitude AC magnetic field generator 
positioned in the vicinity of the head: this device homogenizes the Kerr 
effect reading head from the magnetic viewpoint. 
In FIG. 3, this generator is represented by a device 6 inducing a 
superimposed AC magnetic field (magnetic polarization) in the zone 5 where 
the magnetic head is under the influence of a magnetic field to be 
detected. This device can take different forms: what is essential is that 
it must induce an almost uniform magnetic field throughout the zone 5. 
This is also necessary if the medium is a multiple-track medium and if the 
zone 5 enables the simultaneous reading of several recorded information 
elements. 
In FIG. 3, this magnetic field induction device has been represented by 
device 6. However, this device may be placed in any other location so as 
to induce a magnetic field in the zone 5, for example the locations 6' or 
6". 
FIG. 4 shows an exemplary embodiment applied to the reading of a magnetic 
tape. This device has the Kerr effect reading head 10 which itself 
corresponds to the elements 1, 2, 3 of FIG. 3. The light beam for the 
reading of the magnetic head has not been shown. 
The reading head 10 is in contact (or almost in contact) with the magnetic 
medium to be read which moves past regularly during the reading stage. 
A solenoid 62 equipped with a ferrite core 6, positioned in a straddling 
position on the reading head, generates an AC field in the layers. This 
field, which is made homogeneous throughout the active part of the sensor 
by the geometry of the ferrite, "agitates" the magnetic domains. 
Without high frequency bias, the signal read from the tape is affected by 
the magnetic non-linearities of the layers and by the noise of 
reconfiguration of the domains described above. 
In the presence of bias, the AC field applied by the solenoid to the head 
gives rise to a field that gets superimposed on the useful signal coming 
from the medium. Thus, the sensor delivers a "carrier" signal at the 
frequency of the AC field, and to this signal there is added the useful 
signal coming from the tape that is read back. 
The frequency of this activation must be greater than the useful spectrum 
of the signal read back so that it can be eliminated by filtering. Since 
the noise of reconfiguration in domains and the non-linearities occur in 
synchronism with the bias, these phenomena are also rejected by filtering. 
In FIG. 5, the zone 5 of the magnetic head 10 is in the vicinity of the 
magnetic tape 4 to be read. The AC magnetic field induction device has two 
poles 60, 61 placed in the vicinity of the magnetic head 10 in such a way 
that the flux induced by the coil 62 gets closed by the magnetic head and 
notably in such a way that the magnetic flux is induced in the zone 5. 
In the case of a multiple-track magnetic tape, the active part of the 
magnetic head 10 is placed transversally to the direction of the tape and 
the zone 5 enables the simultaneous reading of several tracks. The poles 
60, 61 are placed on the sides of the magnetic head transversally with 
respect to the magnetic tape. In this way, an AC magnetic field (for 
example, a high frequency field) is induced in the plane of the magnetic 
layers of the magnetic head, this being done in the zone 5 in parallel to 
the plane of the magnetic tape. 
FIG. 5 shows a variant of an embodiment of the device of FIG. 4. The 
magnetic circuit 6 and the Kerr effect reading head 10 are located on one 
and the same side of the magnetic medium 4 to be read. What is essential 
is that the AC magnetic field should be applied in parallel to the plane 
of the magnetic layers of the reading head, and in the zone 5 subjected to 
the magnetic field to be read on the magnetic medium. 
Furthermore, the application of a low-amplitude DC magnetic field (DC 
magnetic polarization) makes it possible to optimize the efficiency of the 
high frequency polarization. The solenoid valve 62 furthermore fulfills 
this role of DC polarization. An electrical diagram shown in FIG. 6 
illustrates this possibility by enabling the supply of the solenoid valve 
62 by means of a DC current given by a high frequency generator G2. 
Should the signal of the sensor be discreetly sampled, it is advantageous 
to place the frequency of the magnetic polarization at an even multiple of 
the sampling frequency. The filtering is then naturally carried out 
through the synchronism of the frequencies, without spectral aliasing. For 
example, the frequency of the magnetic polarization could range from 500 
KHz to 1 MHz for a sampling frequency of 50 KHz. 
The invention brings the following advantages: 
The alternating polarization increases the signal-to-noise ratio of the 
magneto-optical sensor in the useful frequency band. 
The high frequency polarization prompts an additional noise that is 
filtered in the following part of the reading system, and the useful 
signal is increased up to 6 decibels. Everything happens as if the 
alternating polarization were to act as a "magnetic fluidifier" which 
fosters the propagation of the field induced by the tape in the reading 
magnetic layer. 
The gain in signal-to-noise ratio is 2 to 3 dB as a function of the 
frequency of the signal that is read back. 
The square waveform for the magnetic polarization enables a better 
signal-to-noise ratio to be achieved than the sinusoidal waveform. 
Through its role of a "magnetic fluidifier", the magnetic polarization 
promotes the propagation of the field in the layers, and the adjusting of 
the optical elements becomes less of an intricate task, and hence easier 
to achieve. 
In the foregoing description, the invention was applied to a 
magneto-optical reading head using the Kerr effect. However, it can also 
be applied to any other reading head in which a light beam has its 
characteristics modified by the magnetic field. The essential object is to 
be provided for an additional magnetic field that has the role of 
stabilizing the magnetic layers. 
Also, the invention can be applied to all types of magnetic heads in which 
there is a need to overcome instabilities or non-linearities of the head. 
Notably, it can be applied to all magnetic sensors constituted by thin 
layers such as magneto-resistive heads. 
The technique presently used to stabilize the magnetic thin layers of these 
sensors consists of the addition, of a magnetized layer to the sensors 
that makes the layers become magnetic mono-domain layers. This DC field 
provides for a magnetic stability of the layers to the detriment of the 
sensitivity of the sensor. 
The application of an AC field by an external coil or a coil integrated 
with the magnetic layers of the magneto-resistive sensors would have the 
same advantages for the amplitude of the signal and for the linearization 
of the working of the sensor.