Magnetic data storage apparatus

Abstract: The apparatus employs a magnetic head which has a non-magnetic gap member (3) disposed in a magnetic body (1, 2) with positive magnetostriction constant. During reading of data from a recording medium by a coil (4) wound around the magnetic body, a dc bias magnetic field is caused to be generated in the magnetic body. This field causes a magnetostriction effect in the magnetic body which compresses the gap member thereby cancelling the tensile stress generated by the expansion rate difference between gap member and magnetic body.

Description: Technical Field of the Invention The present invention relates to a magnetic data storage apparatus and, in particular, to magnetic heads employed therein. Background of the invention In recent years, the volume of data handled by office automation equipment such as computers has increased significantly. Accordingly, there is a strong demand for high-density recording on magnetic discs commonly used as the media for storing such data. It is generally known that one way of achieving higher density recording is to narrow the clock window width. However, in digital system equipment the minimum achievable width is limited by the requirement to achieve an error rate not greater than, for example, 10 ⁻⁸. Errors result from narrowing the clock window width when the detected read pulses are incorrectly located with respect to the clock window. Sources of this misalignment of pulse and window are the shift in pulse peak caused by interference between adjacent pulses and distortion introduced by the signal processing circuit. It is therefore necessary to have sufficient window width to cope with peak shift. Various techniques have so far been proposed for eliminating the distortion caused by the signal processing circuit and by the interference between adjacent pulses on a recording medium. Even with these prior techniques the distortion of the read signal is still an obstacle to achieving ever higher density recording. One source of the distortion has been found to lie within the magnetic head device. A tensile stress is produced in the gap portion of the head, which is caused by the difference between the thermal expansion coefficients of the non-magnetic substance such as glass constituting the gap portion and the magnetic substances such as ferrite sandwiching that gap portion from both sides. This tensile stress causes a reverse magnetostriction effect in the magnetic substances, inducing strong anisotropy in the neighbourhood of the gap portion of the magnetic substances. This anisotropy generates the distortion of the read signal. JP-A-6358608 discloses a magnetic data storage apparatus in which a bias field is generated in the head body during writing to increase the gap width by the magnetostriction effect. Claim 1 is characterised against this document. Systems which alter the width of the gap using the piezoelectric effect are also known from US-A-4,398,229. Disclosure of the Invention The present invention seeks to solve the problem of read signal distortion and accordingly provides magnetic data storage apparatus as laid down in claim 1. In a preferred magnetic head device, the d.c. bias magnetic field is generated by causing a direct current of a predetermined value to flow in an additional coil wound around the magnetic body, the additional coil being separate from the coil used for generating the read signal. In an alternative embodiment, the dc bias magnetic field is generated by merely causing a direct current of a predetermined value to flow in the same coil used for generating the read signal. If a d.c. bias magnetic field is generated in the magnetic body in accordance with the present invention, a magnetostriction effect is produced in the magnetic body sandwiching the gap portion from both sides, thereby compressing the gap portion. This compression of the gap portion acts to cancel the tensile stress caused in the gap portion by the difference in thermal expansion coefficients as described above. Accordingly, no reverse magnetostriction effect is produced in the magnetic substances and also no anisotropy is induced, so that the distortion of the read signal can be suppressed. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. Brief Description of the Drawings : Figure 1 shows schematically the construction of a magnetic head employed in magnetic data storage apparatus according to the present invention; Figure 2 shows schematically a conventional magnetic head which may be employed in magnetic data storage apparatus according to a second embodiment of the present invention; and Figure 3 is a graph showing the reduction in signal distortion achieved employing heads in apparatus according to the present invention. Detailed Description of the Invention In the magnetic head device shown in figure 1, a gap portion 3 of a very small width consisting of a non-magnetic substance such as glass is provided between magnetic portions 1 and 2 having a positive magnetostriction constant, such as ferrite. A read/write coil 4 used for generating a read signal and for introducing a write signal is wound around one magnetic portion 2. In addition, a d.c. bias magnetic field coil 5 is wound around magnetic portion 2 for generating a d.c. bias field in the magnetic portions 1 and 2. When a read signal is read out from the read/write coil 4, a d.c. bias magnetic field is generated in the magnetic portions 1 and 2 by causing a d.c. current of a predetermined value to flow in the d.c. bias magnetic field coil 5. The value of the d.c. current to flow in the d.c. bias magnetic field coil 5 is preferably 0.5 mA when the value of the a.c. current of the read signal which is read out from the read/write coil 4 is 40 to 60 mA. In this embodiment, a d.c. bias magnetic field is generated in the magnetic portions 1 and 2 as described above, so a magnetostriction effect is caused between the magnetic portions 1 and 2 sandwiching the gap portion 3 from both sides. This causes the gap portion 3 to be compressed, and the tensile stress due to the difference of the thermal expansion coefficients of the gap portion 3 and the magnetic portions 1 and 2 is cancelled. Therefore, no reverse magnetostriction effect is generated in the magnetic portions 1 and 2 and no anisotropy is induced, whereby the distortion of the read signal can be suppressed. As a result, the error rate improves by one or two orders as compared with the prior techniques, or for the same error rate the storage capacity increases by 10%. Figure 2 shows the construction of the magnetic head device in accordance with a second embodiment of the invention. The magnetic head device, of Fig. 2 which is similar to the magnetic head device shown in Figure 1, comprises a gap portion 3 provided between magnetic portions 1 and 2, and a read/write coil 4 wound around one magnetic portion 2. However, there is no d.c. bias magnetic field coil 5, instead, when a read signal is read out from the read/write coil 4, a d.c. bias magnetic field is generated in the magnetic portions 1 and 2 by causing a d.c. current (offset current) of a predetermined value to flow in the read/write coil 4. The value of the d.c. current made to flow in the read/write coil 4 is similar to the embodiment shown in Figure 1. The effect of this embodiment is similar to that of the embodiment shown in Figure 1, and moreover, there is an advantage that, without effecting a design modification to the conventional magnetic head device, the present invention can be applied only with a simple modification being made to the circuit system. Fig. 3 shows the result of the measurement of the correlation between the current value producing the d.c. bias magnetic field in the magnetic head device and the distortion of the read signal and clearly indicates the beneficial effects of the invention. The line defined by the (+) symbols represents experimental results using a magnetic head device in which the distortion due to the interference between adjacent pulses on the recording medium is relatively small when there is no d.c. bias magnetic field, the line defined by open squares represents experimental results using a magnetic head device in which this distortion is large when there is no d.c. bias magnetic field, and the line defined by open diamond symbols represents experimental results using a magnetic head device in which there is no such distortion when there is no d.c. bias magnetic field. As seen from this figure when the current value generating the d.c. bias magnetic field is -0.5 mA, the distortion of each of the read signals is approximately 0ns. At the same time, since the current value generating the d.c. bias magnetic field is smaller than 1 mA, there is no problem of any adverse effect caused by this current. In addition, it has been experimentally determined that the polarity of the current generating the d.c. bias magnetic field may be either positive or negative.

Claims: Magnetic data storage apparatus having a magnetic head for reading data stored as changes of magnetisation on a magnetisable record medium, the head comprising: a magnetic body (1, 2); a gap member (3) of non-magnetic material interposed in the magnetic body, and a coil (4) wound around the magnetic body in which read signals are generated from data stored on said record medium; wherein the apparatus further comprises means (5) for generating a bias magnetic field in the magnetic body to produce dimensional changes by magnetostriction characterised in that said bias magnetic field generating means generates said bias magnetic field during generation of the read signal such as to produce dimensional changes in the magnetic body by magnetostriction which oppose dimensional changes resulting from thermal expansion arising from different thermal expansion coefficients of the magnetic body and the gap member respectively. Apparatus as claimed in claim 1 wherein the bias field generating means includes a source of direct current connected to flow around the magnetic body during generation of the read signal. Apparatus as claimed in claim 2 wherein the direct current source is connected to said coil in which read signals are generated. Apparatus as claimed in claim 2 in which the bias field generating means includes an additional coil (5) wound around the magnetic body, the direct current source being connected to said additional coil. Apparatus as claimed in any preceding claim in which the magnetic body has a positive magnetostriction constant.

Inventors: KURACHI KOJI, OGASAWARA KENJI, TOBARI ATUSHI, KURACHI, KOJI, OGASAWARA, KENJI, TOBARI, ATUSHI
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B5
IPC codes: G11B5