Patent Publication Number: US-2022221531-A1

Title: Method for measuring magnetic characteristics, apparatus for measuring magnetic characteristics, and method for manufacturing magnetic recording medium

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method for measuring magnetic characteristics, an apparatus for measuring magnetic characteristics, and a method for manufacturing a magnetic recording medium. 
     BACKGROUND ART 
     In a magnetic recording medium typified by a hard disk or a magnetic tape, the vibrating sample magnetometry method (the VSM method) is used to measure magnetic characteristics such as coercive force, saturation magnetization, or residual magnetization. This method is so widely used as to be regarded as an industry standard; however, this is a destructive measurement method in which a measurement sample needs to be processed to a certain size and be set on a measurement machine, and cannot obtain a measurement result rapidly in a production line. In contrast, there is known a non-contact measurement technique using the magnetic Kerr effect (for example, see Patent Documents 1 and 2), in which non-destructive, non-contact measurement can be performed on a measurement sample. For example, in a state where a measurement sample is kept at a standstill, a change of the light polarization state based on the magnetic Kerr effect is caught while the applied external magnetic field is continuously changed; thereby, the coercive force, the squareness ratio, etc. can be indirectly measured. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open No. H2-310446
 
Patent Document 2: Japanese Patent Application Laid-Open No. S61-005461
 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, in a process in which a magnetic recording medium such as a magnetic tape is produced while being continuously moved, it is difficult to measure the same place while keeping the measurement portion at a standstill and continuously changing the applied external magnetic field. Therefore, it has been impossible to rapidly measure magnetic characteristics such as coercive force while continuing production, and quick feedback to the process has been impossible. 
     An object of the present disclosure is to provide a method for measuring magnetic characteristics that can measure magnetic characteristics of a continuously moving magnetic recording medium, an apparatus for measuring magnetic characteristics, and a method for manufacturing a magnetic recording medium. 
     Solutions to Problems 
     In order to solve the above issues, a first disclosure is a method for measuring magnetic characteristics, 
     the method including: 
     applying a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measuring a light polarization state of a first reflected light that is reflected; 
     applying a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measuring a light polarization state of a second reflected light that is reflected; 
     applying a third magnetic field having an opposite direction of the second magnetic field to the continuously moving magnetic recording medium, applying a third polarized light to the surface of the magnetic recording medium to which the third magnetic field is being applied, and measuring a light polarization state of a third reflected light that is reflected; and 
     adjusting a strength of the third magnetic field so that a measurement value of the light polarization state of the third reflected light is a mean value of a measurement value of the light polarization state of the first reflected light and a measurement value of the light polarization state of the second reflected light, and obtaining the strength of the third magnetic field when the measurement value of the light polarization state of the third reflected light becomes equal to the mean value. 
     A second disclosure is 
     an apparatus for measuring magnetic characteristics, the apparatus including: 
     a first measurement section configured to apply a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, apply a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measure a light polarization state of a first reflected light that is reflected; 
     a second measurement section configured to apply a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, apply a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measure a light polarization state of a second reflected light that is reflected; 
     a third measurement section configured to apply a third magnetic field having an opposite direction of the second magnetic field to the continuously moving magnetic recording medium, apply a third polarized light to the surface of the magnetic recording medium to which the third magnetic field is being applied, and measure a light polarization state of a third reflected light that is reflected; and 
     a control section configured to control the third measurement section to adjust a strength of the third magnetic field so that a measurement value of the light polarization state of the third reflected light is a mean value of a measurement value of the light polarization state of the first reflected light and a measurement value of the light polarization state of the second reflected light, and obtain the strength of the third magnetic field when the measurement value of the light polarization state of the third reflected light becomes equal to the mean value. 
     A third disclosure is 
     a method for manufacturing a magnetic recording medium, the method including: 
     applying a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measuring a light polarization state of a first reflected light that is reflected; 
     applying a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measuring a light polarization state of a second reflected light that is reflected; 
     applying a third magnetic field having an opposite direction of the second magnetic field to the continuously moving magnetic recording medium, applying a third polarized light to the surface of the magnetic recording medium to which the third magnetic field is being applied, and measuring a light polarization state of a third reflected light that is reflected; 
     adjusting a strength of the third magnetic field so that a measurement value of the light polarization state of the third reflected light is a mean value of a measurement value of the light polarization state of the first reflected light and a measurement value of the light polarization state of the second reflected light, and obtaining, as a coercive force, the strength of the third magnetic field when the measurement value of the light polarization state of the third reflected light becomes equal to the mean value; and 
     adjusting a film formation condition for the continuously moving magnetic recording medium on the basis of the coercive force obtained. 
     A fourth disclosure is a method for measuring magnetic characteristics, the method including: 
     applying a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measuring a light polarization state of a first reflected light that is reflected; 
     applying a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measuring a light polarization state of a second reflected light that is reflected; 
     applying light to the surface of the continuously moving magnetic recording medium, and measuring a light polarization state of a third reflected light that is reflected; and 
     calculating a ratio (ΔA 20 /ΔA 10 ) of a difference ΔA 20  (=A 2 −A 0 ) between a mean value A 0  of measurement values of the light polarization states of the first reflected light and the second reflected light and a measurement value A 2  of the light polarization state of the third reflected light to a difference ΔA 10  (=A 1 −A 0 ) between the mean value A 0  and the measurement value A 1  of the light polarization state of the first reflected light. 
     A fifth disclosure is 
     an apparatus for measuring magnetic characteristics, the apparatus including: 
     a first measurement section configured to apply a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, apply a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measure a light polarization state of a first reflected light that is reflected; 
     a second measurement section configured to apply a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, apply a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measure a light polarization state of a second reflected light that is reflected; 
     a third measurement section configured to apply light to the surface of the continuously moving magnetic recording medium, and measure a light polarization state of a third reflected light that is reflected; and 
     an arithmetic section configured to calculate a ratio (ΔA 20 /ΔA 10 ) of a difference ΔA 20  (=A 2 −A 0 ) between a mean value A 0  of measurement values of the light polarization states of the first reflected light and the second reflected light and a measurement value A 2  of the light polarization state of the third reflected light to a difference ΔA 10  (=A 1 −A 0 ) between the mean value A 0  and the measurement value A 1  of the light polarization state of the first reflected light. 
     A sixth disclosure is 
     a method for manufacturing a magnetic recording medium, the method including: 
     applying a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measuring a light polarization state of a first reflected light that is reflected; 
     applying a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measuring a light polarization state of a second reflected light that is reflected; 
     applying light to the surface of the continuously moving magnetic recording medium, and measuring a light polarization state of a third reflected light that is reflected; 
     obtaining a squareness ratio by calculating a ratio (ΔA 20 /ΔA 10 ) of a difference ΔA 20  (=A 2 −A 0 ) between a mean value A 0  of measurement values of the light polarization states of the first reflected light and the second reflected light and a measurement value A 2  of the light polarization state of the third reflected light to a difference ΔA 10  (=A 1 −A 0 ) between the mean value A 0  and the measurement value A 1  of the light polarization state of the first reflected light; and 
     adjusting a film formation condition for the continuously moving magnetic recording medium on the basis of the squareness ratio obtained. 
     In the present disclosure, the light polarization states of the first reflected light, the second reflected light, and the third reflected light are, for example, the polarization axis angles (the Kerr rotation angles), ellipticities, reflection strengths, or the like of the first reflected light, the second reflected light, and the third reflected light, respectively. 
     Effects of the Invention 
     According to the present disclosure, magnetic characteristics of a continuously moving magnetic recording medium can be measured. Note that the effect written herein is not necessarily a limitative one; and any of the effects written in the present disclosure or an effect of a different nature from them is possible. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view showing a configuration of a magnetic tape. 
         FIG. 2  is a schematic diagram showing a configuration of a film formation apparatus for magnetic tapes. 
         FIG. 3  is a schematic diagram showing a configuration of an apparatus for measuring magnetic characteristics according to a first embodiment of the present disclosure. 
         FIG. 4  is a graph showing an example of a measurement result of magnetic characteristics of a magnetic tape. 
         FIG. 5  is a flow chart for describing an operation of an apparatus for measuring magnetic characteristics according to the first embodiment of the present disclosure. 
         FIG. 6  is a schematic diagram showing a configuration of an apparatus for measuring magnetic characteristics according to a second embodiment of the present disclosure. 
         FIG. 7  is a flow chart for describing an operation of an apparatus for measuring magnetic characteristics according to the second embodiment of the present disclosure. 
         FIG. 8  is a schematic diagram showing a configuration of an apparatus for measuring magnetic characteristics according to a modification example. 
         FIG. 8  is a schematic diagram showing a configuration of an apparatus for measuring magnetic characteristics according to a modification example. 
         FIG. 9  is a schematic diagram showing a configuration of an apparatus for measuring magnetic characteristics according to a modification example. 
         FIG. 10  is a schematic diagram showing a configuration of a film formation apparatus for magnetic tapes. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present disclosure will now be described in the following order. 
     1 First embodiment (apparatus for measuring magnetic characteristics) 
     2 Second embodiment (apparatus for measuring magnetic characteristics) 
     1 First Embodiment 
     [Configuration of Magnetic Tape] 
     A configuration of a magnetic tape  10  of which magnetic characteristics are measured by an apparatus for measuring magnetic characteristics according to a first embodiment will now be described with reference to  FIG. 1 . The magnetic tape  10  is a coating-type magnetic tape of a perpendicular magnetic recording system, and includes a long-length substrate  11 , a ground layer (nonmagnetic layer)  12  provided on one surface of the substrate  11 , a magnetic layer (recording layer)  13  provided on the ground layer  12 , and a back layer  14  provided on the other surface of the substrate  11 . Note that the ground layer  12  and the back layer  14  are provided as necessary, and may not be provided. In the following, out of both surfaces of the magnetic tape  10 , the surface on the side on which the magnetic layer  13  is provided is referred to as a magnetic surface  10 S 1 , and the surface on the opposite side to it, that is, the side on which the back layer  14  is provided is referred to as a back surface  10 S 2 . 
     The magnetic layer  13  contains, for example, a magnetic powder, a binder, and electrically conductive particles. The magnetic layer  13  may further contain, as necessary, additives such as a lubricant, a polisher, and an antirust. The magnetic powder is oriented in the thickness direction of the magnetic tape  10  (the perpendicular direction). As the magnetic powder, for example, ε-iron oxide magnetic powder, Co-containing spinel ferrite magnetic powder, hexagonal ferrite magnetic powder (for example, barium ferrite magnetic powder), or the like is used. 
     [Film Formation Apparatus for Magnetic Tapes] 
     A film formation apparatus  20  used for the film formation of the magnetic tape  10  described above will now be described with reference to  FIG. 2 . Herein, a case where only the magnetic layer  13  is formed as a film on one surface of the substrate  11  is described for easier description. The film formation apparatus  20  is a film formation apparatus of a roll-to-roll form, and includes rolls  21  and  22 , a film formation head  23 , a drying furnace  24 , and an apparatus for measuring magnetic characteristics  30 . 
     In the film formation apparatus  20 , a film-like substrate  11  wound in a roll form is wound out from one roll  21 , and is wound in a roll form again by the other roll  22 . The film formation head  23 , the drying furnace  24 , and the apparatus for measuring magnetic characteristics  30  are arranged in this order from the upstream side toward the downstream side on the running path of the substrate  11  that continuously moves (continuously runs) from one roll  21  toward the other roll  22 . A magnetic field orientation apparatus for orienting the magnetic field of a magnetic powder contained in a coating material  13   a  to the perpendicular direction (the thickness direction of the substrate  11 ) may be provided in the drying furnace  24 . 
     In the film formation apparatus  20  having the configuration mentioned above, the coating material  13   a  is applied by the film formation head  23  to one surface of the continuously running substrate  11 , and then the coating material (coating)  13   a  is dried by the drying furnace  24 ; thus, the magnetic layer  13  is formed. Then, magnetic characteristics of the magnetic layer  13  immediately after formation are measured by the apparatus for measuring magnetic characteristics  30 . 
     To stabilize film formation quality and improve the yield while maintaining productivity, it is desired that, in the process during production, magnetic characteristics necessary as quality be continuously measured and precise, quick feedback to the film formation process be made. To measure magnetic characteristics in this process, (A) measuring magnetic characteristics without breaking the magnetic tape  10  and (B) measuring magnetic characteristics in a state where the magnetic tape  10  continuously moves are necessary. 
     In the first embodiment, in order to enable (A) measuring magnetic characteristics without breaking the magnetic tape  10 , magnetic characteristics of the magnetic tape  10  are measured by utilizing the magnetic Kerr effect. The magnetic Kerr effect is a phenomenon in which, in a case where a magnetized surface is irradiated with polarized light, the light polarization state (the angle of the polarization axis or the ellipticity) of reflected light changes in accordance with the magnetization state of the reflection surface. An external magnetic field is applied to a measurement sample, and the light polarization state based on the magnetic Kerr effect is measured while the strength of the external magnetic field is continuously changed; thereby, data equivalent to magnetic hysteresis are obtained, and magnetic characteristics such as coercive force or magnetization can be measured in a substitutive manner without breaking the measurement sample. 
     In order to enable (B) measuring magnetic characteristics in a state where the magnetic tape  10  continuously moves, a configuration including three measurement units in each of which an electromagnet that applies an external magnetic field and a detection section that utilizes the magnetic Kerr effect, that is, uses the magnetic Kerr effect to measure the magnetization state of the magnetic tape  10  are combined is employed. 
     [Apparatus for Measuring Magnetic Characteristics] 
       FIG. 3  shows a configuration of the apparatus for measuring magnetic characteristics  30  according to the first embodiment. The apparatus for measuring magnetic characteristics  30  includes a plurality of guide rolls  31 , a negative-side saturation magnetization measurement section  32 , a positive-side saturation magnetization measurement section  33 , a magnetization measurement section  34 , and a personal computer (hereinafter referred to as a “PC”)  35 . The negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , and the magnetization measurement section  34  are arranged in this order from the upstream side toward the downstream side on the conveyance path of the moving magnetic tape  10 . 
     In the present specification, out of the thickness directions of the magnetic tape  10 , the direction from the magnetic surface  10 S 1  toward the back surface  10 S 2  is referred to as a first perpendicular direction  10 D 1 , and the opposite direction of it is referred to as a second perpendicular direction  10 D 2 . Further, a state where an external magnetic field is applied to the magnetic tape  10  in the first perpendicular direction  10 D 1  and magnetic saturation is produced is referred to as a “negative-side magnetic saturation state”, and the magnetization at this time is referred to as “negative-side saturation magnetization”. On the other hand, a state where an external magnetic field is applied to the magnetic tape  10  in the second perpendicular direction  10 D 2  and magnetic saturation is produced is referred to as a “positive-side magnetic saturation state”, and the magnetization at this time is referred to as “positive-side saturation magnetization”. 
     Each of the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , and the magnetization measurement section  34  has a configuration that can obtain hysteresis like that shown in FIG.  4  (a relationship of the voltage value equivalent to the magnetization state of the magnetic tape  10  to the strength of the external magnetic field). However, in the first embodiment, as described below, a control to obtain such a hysteresis loop is not performed in the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , or the magnetization measurement section  34 . Note that  FIG. 4  is an example of measurement in which the external magnetic field is changed while the magnetic tape  10  is kept at a standstill. 
     (Guide Rolls) 
     The plurality of guide rolls  31  is an example of a conveyance section, and is provided on the conveyance path of the magnetic tape  10 . The plurality of guide rolls  31  continuously moves (continuously runs) the magnetic tape  10  in a direction (for example, the horizontal direction) going straight relative to the direction of the external magnetic field applied by each of the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , and the magnetization measurement section  34 . One of the plurality of guide rolls  31  is connected to an encoder  31   a , and the encoder  31   a  supplies a pulse signal to the PC  35  in accordance with the rotation of the guide roll  31 . 
     (Negative-Side Saturation Magnetization Measurement Section) 
     The negative-side saturation magnetization measurement section  32  applies an external magnetic field to the continuously moving magnetic tape  10  in the first perpendicular direction  10 D 1  to magnetically saturate the magnetic tape  10  on the negative side, applies polarized light to the magnetic surface  10 S 1  of the magnetic tape  10  to which the external magnetic field is being applied, and measures the polarization axis angle θ 1  of reflected light affected by the negative-side magnetic saturation state (hereinafter referred to as “the polarization axis angle θ 1  of the negative-side magnetic saturation state”). Note that the polarization axis angle θ 1  of the negative-side magnetic saturation state is an example of a measurement value of the light polarization state of reflected light affected by the negative-side magnetic saturation state. 
     The negative-side saturation magnetization measurement section  32  includes an electromagnet  32   a , a power source  32   b , and a light polarization detection section  32   c . The electromagnet  32   a  is an example of a magnetic field generation section, and applies an external magnetic field to the magnetic tape  10  in the first perpendicular direction  10 D 1 . Specifically, the electromagnet  32   a  is capable of applying, in the first perpendicular direction  10 D 1 , an external magnetic field having enough strength to magnetically saturate the magnetic tape  10  on the negative polarity side. The power source  32   b  is a power source for an electromagnet for driving the electromagnet  32   a.    
     The light polarization detection section  32   c  includes an irradiation section  32   c   1 , a light receiving section  32   c   2 , and a polarization axis angle detection circuit  32   c   3 . The irradiation section  32   c   1  applies polarized light to the magnetic surface  10 S 1  of the magnetic tape  10  located in the external magnetic field applied by the electromagnet  32   a . The light receiving section  32   c   2  converts reflected light reflected at the magnetic surface  10 S 1  to an electrical signal by using a polarizing beam splitter, a photodetector, etc., and supplies the signal to the polarization axis angle detection circuit  32   c   3 . The polarization axis angle detection circuit  32   c   3  detects the polarization axis angle θ 1  of the reflected light on the basis of the signal supplied from the light receiving section  32   c   2 , and supplies the polarization axis angle θ 1  to the PC  35 . The polarization axis angle detection circuit  32   c   3  is an example of a light polarization state detection circuit that detects the light polarization state of reflected light on the basis of a signal supplied from the light receiving section  32   c   2 . 
     (Positive-Side Saturation Magnetization Measurement Section) 
     The positive-side saturation magnetization measurement section  33  applies an external magnetic field to the continuously moving magnetic tape  10  in the second perpendicular direction  10 D 2  to magnetically saturate the magnetic tape  10  on the positive side, applies polarized light to the magnetic surface  10 S 1  of the magnetic tape  10  to which the external magnetic field is being applied, and measures the polarization axis angle θ 2  of reflected light affected by the positive-side magnetic saturation state (hereinafter referred to as “the polarization axis angle θ 2  of the positive-side magnetic saturation state”). Note that the polarization axis angle θ 2  of the positive-side magnetic saturation state is an example of a measurement value of the light polarization state of reflected light affected by the positive-side magnetic saturation state. 
     The positive-side saturation magnetization measurement section  33  includes an electromagnet  33   a , a power source  33   b , and a light polarization detection section  33   c . The electromagnet  33   a  is an example of a magnetic field generation section, and applies an external magnetic field to the magnetic tape  10  in the second perpendicular direction  10 D 2 . Specifically, the electromagnet  33   a  is capable of applying, in the second perpendicular direction  10 D 2 , an external magnetic field having enough strength to magnetically saturate the magnetic tape  10  on the positive polarity side. The power source  33   b  is a power source for an electromagnet for driving the electromagnet  33   a.    
     The light polarization detection section  33   c  includes an irradiation section  33   c   1 , a light receiving section  33   c   2 , and a polarization axis angle detection circuit  33   c   3 . The irradiation section  33   c   1  applies polarized light to the magnetic surface  10 S 1  of the magnetic tape  10  located in the external magnetic field applied by the electromagnet  33   a . The light receiving section  33   c   2  converts reflected light reflected at the magnetic surface  10 S 1  to an electrical signal by using a polarizing beam splitter, a photodetector, etc., and supplies the signal to the polarization axis angle detection circuit  33   c   3 . The polarization axis angle detection circuit  33   c   3  detects the polarization axis angle θ 2  of the reflected light on the basis of the signal supplied from the light receiving section  33   c   2 , and supplies the polarization axis angle θ 2  to the PC  35 . The polarization axis angle detection circuit  33   c   3  is an example of a light polarization state detection circuit that detects the light polarization state of reflected light on the basis of a signal supplied from the light receiving section  33   c   2 . 
     (Magnetization Measurement Section) 
     The magnetization measurement section  34  applies an external magnetic field to the continuously moving magnetic tape  10  in the first perpendicular direction  10 D 1  to demagnetize the magnetic tape  10 , applies polarized light to the magnetic surface  10 S 1  of the magnetic tape  10  to which the external magnetic field is being applied, and measures the polarization axis angle θ 3  of reflected light (hereinafter referred to as “the polarization axis angle θ 3  of a demagnetization state”). Note that the polarization axis angle θ 3  of the demagnetization state is an example of a measurement value of the light polarization state of reflected light reflected at the magnetic surface  10 S 1  in the demagnetization state. 
     The magnetization measurement section  34  includes an electromagnet  34   a , a power source  34   b , and a light polarization detection section  34   c . The electromagnet  34   a  is an example of a magnetic field generation section, and applies an external magnetic field to the magnetic tape  10  in the first perpendicular direction  10 D 1  to demagnetize the magnetic tape  10  magnetized by the positive-side saturation magnetization measurement section  33 . The power source  33   b  is a power source for an electromagnet for driving the electromagnet  34   a.    
     The light polarization detection section  34   c  includes an irradiation section  34   c   1 , a light receiving section  34   c   2 , and a polarization axis angle detection circuit  34   c   3 . The irradiation section  34   c   1  applies polarized light to the magnetic surface  10 S 1  of the magnetic tape  10  located in the external magnetic field applied by the electromagnet  34   a . The light receiving section  34   c   2  converts reflected light reflected at the magnetic surface  10 S 1  to an electrical signal by using a polarizing beam splitter, a photodetector, etc., and supplies the signal to the polarization axis angle detection circuit  34   c   3 . The polarization axis angle detection circuit  34   c   3  detects the polarization axis angle θ 3  of the reflected light on the basis of the signal supplied from the light receiving section  34   c   2 , and supplies the polarization axis angle θ 3  to the PC  35 . The polarization axis angle detection circuit  34   c   3  is an example of a light polarization state detection circuit that detects the light polarization state of reflected light on the basis of a signal supplied from the light receiving section  34   c   2 . 
     (PC) 
     The PC  35  is an example of a control section, and controls the whole of the apparatus for measuring magnetic characteristics  30 . Specifically, the PC  35  controls the plurality of guide rolls  31 , the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , and the magnetization measurement section  34 . Further, the PC  35  controls, in addition to the apparatus for measuring magnetic characteristics  30 , the whole of the film formation apparatus  20  for the magnetic tape  10 . Although herein a case where the PC  35  controls both the apparatus for measuring magnetic characteristics  30  and the film formation apparatus  20  for the magnetic tape  10  is described, the apparatus for measuring magnetic characteristics  30  and the film formation apparatus  20  for the magnetic tape  10  may be controlled by different control apparatuses. 
     The PC  35  includes a pulse counter board  35   a ; the pulse counter board  35   a  counts pulse signals supplied from the encoder  31   a , and calculates the movement distance of the continuously moving magnetic tape  10 . The PC  35  includes a D/A board  35   b , and controls the power sources  32   b ,  33   b , and  34   b  via the D/A board  35   b  to adjust the output magnetic field strengths of the electromagnets  32   a ,  33   a , and  34   a . The PC  35  includes an A/D board  35   c , and takes in the polarization axis angle θ 1  of the negative-side magnetic saturation state, the polarization axis angle θ 2  of the positive-side magnetic saturation state, and the polarization axis angle θ 3  of the demagnetization state that are supplied from the polarization axis angle detection circuits  32   c   3 ,  33   c   3 , and  34   c   3 , respectively, via the A/D board  35   c . At the time of the data taking-in, the PC  35  associates data and the measurement position on the magnetic tape  10  together on the basis of the count value of the pulse counter board  35   a.    
     The PC  35  calculates the mean value θ 0  (=(θ 1 +θ 2 )/2) of the polarization axis angle θ 1  of the negative-side magnetic saturation state supplied from the negative-side saturation magnetization measurement section  32  and the polarization axis angle θ 2  of the positive-side magnetic saturation state supplied from the positive-side saturation magnetization measurement section  33 . Then, the output magnetic field strength of the electromagnet  34   a  is adjusted via the D/A board  35  and the power source  34   b  so that the polarization axis angle θ 3  of the demagnetization state supplied from the magnetization measurement section  34  is equal to the mean value θ 0 . In the following, this control is referred to as “demagnetization control”, as appropriate. Specifically, the value of current to be supplied to the electromagnet  34   a  is controlled via the D/A board  35  and the power source  34   b , and thereby the output magnetic field strength of the electromagnet  34   a  is adjusted. Since the magnetic tape  10  continuously moves without stopping, the PC  35  continues the demagnetization control mentioned above while constantly managing the position of the magnetic tape  10 . 
     The PC  35  adjusts the strength of the magnetic field in the magnetization measurement section  34  by using data acquired in the same position of the continuously moving magnetic tape  10  (that is, the polarization axis angle θ 1  of the negative-side magnetic saturation state, the polarization axis angle θ 2  of the positive-side magnetic saturation state, and the polarization axis angle θ 3  of the demagnetization state). That is, the PC  35  performs demagnetization control in the magnetization measurement section  34  on the basis of data that is acquired when the magnetic tape  10  existing in the magnetization measurement section  34  is located in the negative-side saturation magnetization measurement section  32  and the positive-side saturation magnetization measurement section  33  (that is, the polarization axis angle θ 1  of the negative-side magnetic saturation state and the polarization axis angle θ 2  of the positive-side magnetic saturation state). By performing an arithmetic operation by using data of the same position on the magnetic tape  10 , a light polarization variation resulting from a difference in position on the magnetic tape  10  (in a case where film thickness or film quality varies, also this variation is included) can be canceled. Thereby, a result that can substitute for part of conventional measurement in a standstill state is obtained even for a continuously moving magnetic tape  10 . 
     There is a case where, immediately after demagnetization control is started, agreement with the mean value θ 0  mentioned above is not made fully (that is, demagnetization is not made fully); however, feedback whereby the demagnetization state can be maintained is achieved by continuously performing demagnetization control. The PC  35  acquires, as the coercive force, the strength of the magnetic field applied by the electromagnet  34   a  in a state where the demagnetization state is maintained successfully. Specifically, the PC  35  assesses whether the polarization axis angle θ 3  of the demagnetization state is equal to the mean value θ 0  or not; the PC  35  converts the value of current that is supplied to the electromagnet  34   a  when the polarization axis angle θ 3  of the demagnetization state becomes the mean value θ 0  to magnetic field strength, and acquires the coercive force. The PC  35  may display the measurement result of coercive force on a display apparatus of the PC  35 , or may output the measurement result to an external device, as necessary. 
     [Operation of Apparatus for Measuring Magnetic Characteristics] 
     Hereinbelow, an operation of the apparatus for measuring magnetic characteristics  30  having the configuration described above is described with reference to  FIG. 5 . 
     First, a worker uses the PC  35  to execute a manipulation of the start of film formation of the magnetic tape  10 ; then, in step S 1 , the PC  35  drives the plurality of guide rolls  31  to continuously move the magnetic tape  10  in a direction (for example, the horizontal direction) going straight relative to the direction of the external magnetic field applied by each of the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , and the magnetization measurement section  34 . 
     Next, in step S 2 , the PC  35  controls the negative-side saturation magnetization measurement section  32  to apply a sufficiently large external magnetic field (for example, −15 kOe or the like) to the continuously moving magnetic tape  10  in the first perpendicular direction  10 D 1 , and magnetically saturates the magnetic tape  10  on the negative side. Further, polarized light is applied to the magnetic surface  10 S 1  of the magnetic tape  10  existing in the external magnetic field, and the polarization axis angle θ 1  of reflected light of the polarized light (that is, the polarization axis angle θ 1  of the negative-side magnetic saturation state) is measured and is supplied to the PC  35 . 
     Next, in step S 3 , the PC  35  controls the positive-side saturation magnetization measurement section  33  to apply a sufficiently large external magnetic field (for example, +15 kOe or the like) to the continuously moving magnetic tape  10  in the second perpendicular direction  10 D 2 , and magnetically saturates the magnetic tape  10  on the positive side. Further, polarized light is applied to the magnetic surface  10 S 1  of the magnetic tape  10  existing in the external magnetic field, and the polarization axis angle θ 2  of reflected light of the polarized light (that is, the polarization axis angle θ 2  of the positive-side magnetic saturation state) is measured and is supplied to the PC  35 . 
     Next, in step S 4 , the PC  35  controls the magnetization measurement section  34  to apply an external magnetic field to the continuously moving magnetic tape  10  in the first perpendicular direction  10 D 1 , and demagnetizes the magnetic tape  10 . Specifically, the value of current to be supplied to the electromagnet  34   a  is controlled via the D/A board  35  and the power source  34   b , and thereby the magnetic tape  10  is demagnetized. Further, polarized light is applied to the magnetic surface  10 S 1  of the magnetic tape  10  existing in the external magnetic field applied by the magnetization measurement section  34 , and the polarization axis angle θ 3  of reflected light of the polarized light (that is, the polarization axis angle θ 3  of the demagnetization state) is measured and is supplied to the PC  35 . 
     Next, in step S 5 , the PC  35  calculates the mean value θ 0  (=(θ 1 +θ 2 )/2) of the polarization axis angle θ 1  measured in step S 2  and the polarization axis angle θ 2  measured in step S 3 . Then, it is assessed whether the polarization axis angle θ 3  measured in step S 4  is equal to the calculated mean value θ 0  or not. Whether the magnetization of the magnetic tape  10  is zero or not can be assessed by thus assessing whether the polarization axis angle θ 3  is equal to the mean value θ 0  or not. 
     In a case where it is assessed that the polarization axis angle θ 3  measured in step S 4  is equal to the mean value θ 0 , in step S 6 , the PC  35  converts the value of current that is supplied to the electromagnet  34   a  when it is assessed that the values are equal as mentioned above to magnetic field strength, and acquires the coercive force. 
     On the other hand, in a case where in step S 6  it is assessed that the polarization axis angle θ 3  measured in step S 5  is not equal to the mean value θ 0 , in step S 7 , the PC  35  adjusts the magnetic field strength of the magnetization measurement section  34  so that the polarization axis angle θ 3  measured in step S 4  is equal to the mean value θ 0 . Specifically, the value of current to be supplied to the electromagnet  34   a  is controlled via the D/A board  35  and the power source  34   b  so that the polarization axis angle θ 3  measured in step S 4  is equal to the mean value θ 0 , and thereby the output magnetic field strength of the electromagnet  34   a  is adjusted. 
     Note that, since the magnetic tape  10  continuously moves, the processing of steps S 1  to S 7  is set to be continuously performed without a break. Further, the encoder  31   a , etc. are used to manage the position of the magnetic tape  10  so that the movement distance of the magnetic tape  10  can be grasped; thus, the measurement, comparison, adjustment, etc. of steps S 2  to S 7  are allowed to be performed in the same position on the magnetic tape  10 . 
     Table 1 shows a result of measurement in which the coercive force of a magnetic tape  10  with the coercive force set to 2.5 [kOe] was measured repeatedly 10 times by using the apparatus for measuring magnetic characteristics  30  according to the first embodiment. From Table 1, it can be seen that, in the apparatus for measuring magnetic characteristics  30  according to the first embodiment, the coercive force of the continuously moving magnetic tape  10  can be measured with good precision in a non-destructive, non-contact manner. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Number of times of  
                 Measurement value of  
               
               
                   
                 measurement 
                 coercive force (kOe) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 1 
                 2.57 
               
               
                   
                 2 
                 2.62 
               
               
                   
                 3 
                 2.67 
               
               
                   
                 4 
                 2.57 
               
               
                   
                 5 
                 2.41 
               
               
                   
                 6 
                 2.41 
               
               
                   
                 7 
                 2.41 
               
               
                   
                 8 
                 2.36 
               
               
                   
                 9 
                 2.41 
               
               
                   
                 10 
                 2.36 
               
               
                   
                 Average 
                 2.48 
               
               
                   
                 3 sigma 
                 0.35 
               
               
                   
                   
               
            
           
         
       
     
     [Effects] 
     In the apparatus for measuring magnetic characteristics  30  according to the first embodiment, the coercive force (magnetic characteristics) of the magnetic tape  10  can be measured without bringing the continuously moving magnetic tape  10  to a standstill or breaking the magnetic tape  10 . 
     In a roll-to-roll film formation process, magnetic characteristics of the magnetic tape  10  can be measured quickly without interrupting running-based film formation or breaking the magnetic tape  10 ; therefore, a measurement result of magnetic characteristics can be fed back to the film formation process rapidly and appropriately. Thus, the yield can be improved. Further, the occurrence of defects can be further suppressed by controlling film formation conditions on the basis of feedback of a measurement result of magnetic characteristics so that the characteristics fall within a range narrower than the standard value range of the process. 
     2 Second Embodiment 
     [Apparatus for Measuring Magnetic Characteristics] 
       FIG. 6  shows a configuration of an apparatus for measuring magnetic characteristics  30 A according to a second embodiment. The apparatus for measuring magnetic characteristics  30 A differs from the apparatus for measuring magnetic characteristics  30  according to the first embodiment in that the apparatus for measuring magnetic characteristics  30 A includes a residual magnetization measurement section  36  in place of the magnetization measurement section  34 . 
     (Residual Magnetization Measurement Section) 
     The residual magnetization measurement section  36  applies polarized light to the magnetic surface  10 S 1  of the magnetic tape  10  to which an external magnetic field is not applied, and measures the polarization axis angle θ 4  of reflected light (hereinafter referred to as “the polarization axis angle θ 4  of a residual magnetization state”). Note that the polarization axis angle θ 4  of the residual magnetization state is an example of a measurement value of the light polarization state of reflected light affected by residual magnetization. 
     The residual magnetization measurement section  36  includes a light polarization detection section  36   c . The light polarization detection section  36   c  includes an irradiation section  36   c   1 , a light receiving section  36   c   2 , and a polarization axis angle detection circuit  36   c   3 . The irradiation section  36   c   1  applies polarized light to the magnetic surface  10 S 1  of the magnetic tape  10  whose the external magnetic field is not applied by the electromagnet. The light receiving section  36   c   2  converts reflected light reflected at the magnetic surface  10 S 1  to an electrical signal by using a polarizing beam splitter or a photodetector, and supplies the signal to the polarization axis angle detection circuit  36   c   3 . The polarization axis angle detection circuit  36   c   3  detects the polarization axis angle θ 4  of the reflected light on the basis of the signal supplied from the light receiving section  36   c   2 , and supplies the polarization axis angle θ 4  to the PC  35 . The polarization axis angle detection circuit  36   c   3  is an example of a light polarization state detection circuit that detects the light polarization state of reflected light on the basis of a signal supplied from the light receiving section  36   c   2 . 
     (PC) 
     The PC  35  is an example of an arithmetic section, and controls the whole of the apparatus for measuring magnetic characteristics  30 A. Specifically, the PC  35  controls the plurality of guide rolls  31 , the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , and the residual magnetization measurement section  36 . The PC  35  takes in the polarization axis angle θ 1  of the negative-side magnetic saturation state, the polarization axis angle θ 2  of the positive-side magnetic saturation state, and the polarization axis angle θ 4  of the residual magnetization state that are supplied from the polarization axis angle detection circuits  32   c   3 ,  33   c   3 , and  36   c   3 , respectively, via the A/D board  35   c . At the time of the data taking-in, the PC  35  associates data and the measurement position on the magnetic tape  10  together on the basis of the count value of the pulse counter board  35   a.    
     The PC  35  calculates the mean value θ 0  (=(θ 1 +θ 2 )/2) of the polarization axis angle θ 1  of the negative-side magnetic saturation state supplied from the negative-side saturation magnetization measurement section  32  and the polarization axis angle θ 2  of the positive-side magnetic saturation state supplied from the positive-side saturation magnetization measurement section  33 . Then, the difference Δθ 10  (=θ 1 −θ 0 ) between the polarization axis angle θ 1  of the negative-side magnetic saturation state and the mean value θ 0  and the difference Δθ 40  (=θ 4 −θ 0 ) between the polarization axis angle θ 4  of the residual magnetization state and the mean value θ 0  are calculated. When performing the calculation, since data and the measurement position on the magnetic tape  10  are associated together beforehand, it is preferable that an arithmetic operation be performed using data of the same position on the magnetic tape  10  and a light polarization variation resulting from a difference in position on the magnetic tape  10  (in a case where film thickness or film quality varies, also this variation is included) be canceled. 
     Difference Δθ 10  means the polarization axis angle θ 1 ′ (=θ 1 −θ 0 ) of the negative-side magnetic saturation state with the mean value θ 0  as a reference. Difference Δθ 40  means the polarization axis angle θ 4 ′ (=θ 4 −θ 0 ) of the residual magnetization state with the mean value θ 0  as a reference. Difference Δθ 10  is an example of a measurement value of the light polarization state of the negative-side magnetic saturation state using, as a reference, the mean value of the measurement value of the light polarization state of the negative-side magnetic saturation state and the measurement value of the light polarization state of the positive-side magnetic saturation state. Difference Δθ 40  is an example of a measurement value of the light polarization state of the residual magnetization state using, as a reference, the mean value of the measurement value of the light polarization state of the negative-side magnetic saturation state and the measurement value of the light polarization state of the positive-side magnetic saturation state. 
     The PC  35  uses the calculated differences Δθ 10  and Δθ 40  to calculate the ratio (Δθ 40 /Δθ 10 ) of difference Δθ 40  to difference Δθ 10 , and obtains the squareness ratio. Note that the PC  35  calculates the squareness ratio described above by using data (that is, the polarization axis angle θ 1  of the negative-side magnetic saturation state, the polarization axis angle θ 2  of the positive-side magnetic saturation state, and the polarization axis angle θ 4  of the residual magnetization state) acquired in the same position of the continuously moving magnetic tape  10 . As described above, the PC  35  takes in and stores measurement data and the measurement position on the magnetic tape  10  while associating them together, and can therefore calculate the squareness ratio by using measurement data in the same position of the magnetic tape  10 . 
     [Method for Measuring Magnetic Characteristics] 
     Hereinbelow, an operation of the apparatus for measuring magnetic characteristics  30 A having the configuration described above is described with reference to  FIG. 7 . 
     First, a worker uses the PC  35  to execute a manipulation of the start of film formation of the magnetic tape  10 ; then, in step S 11 , the PC  35  continuously moves the magnetic tape  10  like in step S 1  in the first embodiment. 
     Next, in step S 12 , the PC  35  measures the polarization axis angle θ 1  (that is, the polarization axis angle θ 1  of the negative-side magnetic saturation state) in a similar manner to step S 2  in the first embodiment, and supplies the polarization axis angle θ 1  to the PC  35  itself 
     Next, in step S 13 , the PC  35  measures the polarization axis angle θ 2  (that is, the polarization axis angle θ 2  of the positive-side magnetic saturation state) in a similar manner to step S 3  in the first embodiment, and supplies the polarization axis angle θ 2  to the PC  35  itself 
     Next, in step S 14 , the PC  35  applies polarized light to the magnetic surface  10 S 1  of the continuously running magnetic tape  10  to which an external magnetic field is not applied by an electromagnet, measures the polarization axis angle θ 4  of reflected light of the polarized light (that is, the polarization axis angle θ 4  of the residual magnetization state), and supplies the polarization axis angle θ 4  to the PC  35  itself 
     Next, in step S 15 , the PC  35  calculates the mean value θ 0  (=(θ 1 +θ 2 )/2) of the polarization axis angle θ 1  measured in step S 12  and the polarization axis angle θ 2  measured in step S 13 . 
     Next, in step S 16 , the PC  35  calculates the difference Δθ 10 =θ 1 −θ 0  between the polarization axis angle θ 1  measured in step S 12  and the mean value θ 0  calculated in step S 15 . Further, the PC  35  calculates the difference Δθ 40 =θ 4 −θ 0  between the polarization axis angle θ 4  measured in step S 14  and the mean value θ 0  calculated in step S 15 . 
     Next, in step S 17 , the PC  35  uses the differences Δθ 10  and Δθ 40  calculated in step S 16  to calculate the ratio (Δθ 40 /Δθ 10 ) of difference Δθ 40  to difference Δθ 10 , and obtains the squareness ratio. 
     Note that, since the magnetic tape  10  continuously moves, the processing of steps S 11  to S 17  is set to be continuously performed without a break. Further, the encoder  31   a , etc. are used to manage the position of the magnetic tape  10  so that the movement distance of the magnetic tape  10  can be grasped; thus, the measurement of steps S 12  to S 14  are allowed to be performed in the same position on the magnetic tape  10 . 
     [Effects] 
     In the apparatus for measuring magnetic characteristics  30 A according to the second embodiment, the squareness ratio (magnetic characteristics) of the magnetic tape  10  can be measured without bringing the continuously moving magnetic tape  10  to a standstill or breaking the magnetic tape  10 . 
     Modification Examples 
     Hereinabove, the first and second embodiments of the present disclosure are specifically described; however, the present disclosure is not limited to the first or second embodiment described above, but may make various modifications based on the technical idea of the present disclosure. For example, the configurations, methods, processes, shapes, materials, numerical values, etc. shown in the first and second embodiments described above are only examples, and configurations, methods, processes, shapes, materials, numerical values, etc. different from them may be used as necessary. Further, configurations, methods, processes, shapes, materials, numerical values, etc. of the first and second embodiments described above may be combined with each other without departing from the spirit of the present disclosure. 
     Although the first and second embodiments described above have described a case where magnetic characteristics of a continuously running (continuously moving) magnetic tape  10  are measured, the present disclosure is not limited to the measurement of magnetic characteristics of the magnetic tape  10 , but can be applied also to the measurement of magnetic characteristics of a magnetic recording medium other than the magnetic tape  10 . For example, the present disclosure can be applied also to the measurement of magnetic characteristics of a continuously rotating magnetic disk (for example, a hard disk). In this case, magnetic characteristics (for example, the coercive force, the squareness ratio, or the like) of the continuously rotating magnetic disk can be measured in a non-destructive, non-contact manner. 
     Although the first and second embodiments described above have described apparatuses for measuring magnetic characteristics  30  and  30 A in which a magnetic tape  10  is caused to pass through gap portions of electromagnets  32   a ,  33   a , and  34   a  and the light polarization state of the magnetic surface  10 S 1  of the magnetic tape  10  passing through the gap portions is measured, the configuration of the apparatus for measuring magnetic characteristics is not limited to this. For example, as shown in  FIG. 8 , the electromagnets  32   a ,  33   a , and  34   a  may be provided only on one surface side of the magnetic tape  10  (for example, the magnetic surface  10 S 1  side), and the light polarization state of the magnetic surface  10 S 1  of the magnetic tape  10  to which an external magnetic field is being applied by the electromagnet  32   a ,  33   a , or  34   a  may be measured. 
     Although the first and second embodiments described above have described a case where the apparatuses for measuring magnetic characteristics  30  and  30 A are controlled by a PC  35 , the apparatuses for measuring magnetic characteristics  30  and  30 A may be controlled by a dedicated control apparatus or the like in place of the PC  35 . 
     Although the first embodiment described above has described a method in which the coercive force of the magnetic tape  10  is measured by the apparatus for measuring magnetic characteristics  30 , it is also possible to employ a method in which the saturation magnetization of the magnetic tape  10  is measured by the apparatus for measuring magnetic characteristics  30  in the following manner. That is, a relationship between the amount of magnetization of the magnetic tape  10  and the amount of change of the polarization axis angle based on magnetization is found in advance, and a conversion factor is prepared and is stored in the storage section of the PC  35  in advance. The PC  35  compares the two polarization axis angles of the negative-side saturation magnetization measurement section  32  and the positive-side saturation magnetization measurement section  33  by using measurement values of the same position on the magnetic tape  10 , and uses the conversion factor mentioned above to convert the difference between the two values to the amount of magnetization; thus, obtains the amount of saturation magnetization. Note that it is also possible to calculate the difference between the mean value of the measurement values of the negative-side and positive-side saturation magnetizations as a reference and the measurement value of the negative-side or positive-side saturation magnetization measurement section and use the difference for conversion. 
     Although the first embodiment described above has described a configuration of an apparatus for measuring magnetic characteristics  30  that measures the coercive force of the magnetic tape  10 , also residual magnetization can be measured by employing the following configuration. That is, as shown in  FIG. 9 , the irradiation section  32   c   1  and the light receiving section  32   c   2  are provided in positions more on the downstream side of the conveyance path of the magnetic tape  10  than the electromagnet  32   a  and more on the upstream side of the conveyance path of the magnetic tape  10  than the electromagnet  33   a . The irradiation section  33   c   1  and the light receiving section  33   c   2  are provided in positions more on the downstream side of the conveyance path of the magnetic tape  10  than the electromagnet  33   a . A relationship between the amount of magnetization of the magnetic tape  10  and the amount of change of the polarization axis angle based on magnetization is found in advance, and a conversion factor is prepared and is stored in the storage section of the PC  35  in advance. 
     The apparatus for measuring magnetic characteristics  30  having the configuration described above operates in the following manner. First, the PC  35  uses the electromagnet  32   a  to magnetically saturate the magnetic tape  10  on the negative side, and then uses the light polarization detection section  32   c  to measure the polarization axis angle in a position outside the magnetic field area of the electromagnet  32   a . Subsequently, further on the downstream side, the magnetic tape  10  is magnetically saturated on the positive side by the electromagnet  33   a , and then the polarization axis angle is measured by the light polarization detection section  33   c  in a position outside the magnetic field area of the electromagnet  33   a . After that, the PC  35  compares the two polarization axis angles obtained by the light polarization detection section  32   c  and the light polarization detection section  33   c  by using measurement values of the same position on the magnetic tape  10 , and uses the conversion factor mentioned above to convert the difference between the two values to the amount of magnetization; thus, obtains the amount of residual magnetization. Note that it is also possible to calculate the difference between the mean value of the measurement values in the light polarization detection section  32   c  and the light polarization detection section  33   c  as a reference and the measurement value in the light polarization detection section  32   c  or the light polarization detection section  33   c  and use the difference for conversion. 
     It is also possible to measure the squareness ratio with the apparatus for measuring magnetic characteristics  30  according to the first embodiment. That is, it is also possible to measure the squareness ratio by a method in which the electromagnet  34   a  is not electrically energized but set in a nonuse state and an operation similar to the operation of the apparatus for measuring magnetic characteristics  30 A according to the second embodiment is performed. 
     Although the first embodiment described above has described an example in which the value of current that is supplied to the electromagnet  34   a  when the polarization axis angle θ 3  of the demagnetization state becomes equal to the mean value θ 0  is converted to magnetic field strength to obtain the coercive force, the method for measuring the coercive force is not limited to this. For example, it is also possible to employ a method in which the magnetization measurement section  34  includes a magnetic field measurement section (for example, a Hall element or the like) for measuring the magnetic field strength of the electromagnet  34   a  and the magnetic field strength of the electromagnet  34   a  when the polarization axis angle θ 3  of the demagnetization state becomes equal to the mean value θ 0  is measured by the magnetic field measurement section to obtain the coercive force. 
     Although the first and second embodiments described above have described an example in which a continuously moving magnetic tape  10  is brought first into the negative-side magnetic saturation state and then into the positive-side magnetic saturation state, the magnetic tape  10  may be brought first into the positive-side magnetic saturation state and then into the positive-side magnetic saturation state. That is, the placement positions of the negative-side saturation magnetization measurement section  32  and the positive-side saturation magnetization measurement section  33  may be reversed. 
     Although the first and second embodiments described above have described a case where magnetic characteristics of a magnetic tape  10  of a perpendicular magnetic recording system are measured, it is also possible to measure magnetic characteristics of a magnetic tape  10  of a horizontal magnetic recording system (an in-plane magnetic recording system). In this case, each of the electromagnets  32   a ,  33   a , and  34   a  is capable of applying an external magnetic field in the longitudinal direction of the magnetic tape  10 . Note that the directions of application of the magnetic fields of the electromagnets  32   a  and  34   a  and the electromagnet  33   a  are opposite directions like in the first and second embodiments. 
     In the first and second embodiments described above, the PC  35  may feed a measurement result of magnetic characteristics (the coercive force, the squareness ratio, or the like) back to the film formation process. That is, the PC  35  may adjust a film formation condition for the magnetic layer  13 , etc. on the basis of a measurement result of magnetic characteristics so that the magnetic characteristics of the magnetic tape  10  fall within a prescribed range. 
     More specifically, the PC  35  compares, on a real time basis, a measurement result of magnetic characteristics measured by the apparatus for measuring magnetic characteristic  30  or  30 A with prescribed magnetic characteristics stored in the storage section of the PC  35 , and makes feedback to the film formation process so that the magnetic characteristics fall within the range of the prescribed magnetic characteristics. That is, a film formation condition for the magnetic layer  13 , etc. is adjusted so that the magnetic characteristics fall within the range of the prescribed magnetic characteristics. As the film formation condition to be adjusted, for example, at least one of the amount of the coating material  13   a  discharged (that is, the thickness of the magnetic layer  13 ), a drying condition for the coating material  13   a , the magnetic field strength at the time of orienting the magnetic field, or the like is given. Specifically, for example, the squareness ratio can be changed by adjusting the magnetic field strength and adjusting the orientation state of the magnetic powder immediately after the application of the coating material  13   a . Further, the amount of saturation magnetization can be adjusted by adjusting the thickness of the magnetic layer  13 . 
     Although the first and second embodiments described above have described an example in which magnetic characteristics of a coating-type magnetic tape  10  in which a magnetic layer  13 , etc. are produced by a coating process (a wet process) are measured, it is also possible to measure magnetic characteristics of a vacuum thin film-type magnetic tape in which a magnetic layer, etc. are produced by a technology for producing a vacuum thin film (a dry process). As the method for producing a vacuum thin film, for example, the sputtering method, the vapor deposition method, or the like is used, but the method is not limited to these. 
       FIG. 10  shows a configuration of a film formation apparatus  40  for magnetic tapes that uses the sputtering method to perform film formation to obtain a magnetic layer, etc. The film formation apparatus  40  for magnetic tapes is a continuous winding-type sputtering apparatus used for the film formation of a seed layer, a ground layer, and a magnetic layer (a recording layer), and includes a film formation chamber  41 , a drum  42  that is a metal can (a rotation body), cathodes  43   a  to  43   c , a supply reel  44 , a winding reel  45 , a plurality of guide rolls  47   a  to  47   c  and  48   a  to  48   c , and an apparatus for measuring magnetic characteristics  49 . The film formation apparatus  40  is, for example, a sputtering apparatus of a direct current (DC) magnetron sputtering system, but the sputtering system is not limited to this system. The apparatus for measuring magnetic characteristics  49  is the apparatus for measuring magnetic characteristics  30  according to the first embodiment or the apparatus for measuring magnetic characteristics  30 A according to the second embodiment. However, the PC  35 , which is a control section, is placed outside the film formation apparatus  40  for magnetic tapes, and serves also as a control section that controls the film formation apparatus  40 . 
     Although herein an example in which the film formation apparatus  40  includes three cathodes  43   a  to  43   c  is described, the number of cathodes is not limited to this, but may be one, two, or four or more. Further, although herein an example in which a seed layer and a ground layer are formed as films as sputtered layers other than the magnetic layer is described, at least one kind of layer of a soft magnetic backing layer (a SUL layer), an intermediate layer, etc. may be formed as a film in place of the seed layer and the ground layer or in combination with the seed layer and the ground layer. 
     The film formation chamber  41  is connected to a not-illustrated vacuum pump via an air outlet  46 , and the atmosphere in the film formation chamber  41  is set at a prescribed degree of vacuum by the vacuum pump. The drum  42 , the supply reel  44 , and the winding reel  45 , each of which has a rotatable configuration, are placed in the interior of the film formation chamber  41 . In the interior of the film formation chamber  41 , the plurality of guide rolls  47   a  to  47   c  for guiding the conveyance of the substrate  11  between the supply reel  44  and the drum  42  is provided, and further the plurality of guide rolls  48   a  to  48   c  for guiding the conveyance of the substrate  11  between the drum  42  and the winding reel  45  is provided. At the time of sputtering, the substrate  11  wound out from the supply reel  44  is wound around the winding reel  45  via the guide rolls  47   a  to  47   c , the drum  42 , and the guide rolls  48   a  to  48   c . The drum  42  has a circular columnar shape, and the long-length substrate  11  is conveyed in agreement with the circular columnar circumferential surface of the drum  42 . A not-illustrated cooling mechanism is provided in the drum  42 , and performs cooling to, for example, approximately 20° C. at the time of sputtering. In the interior of the film formation chamber  41 , the plurality of cathodes  43   a  to  43   c  is arranged facing the circumferential surface of the drum  42 . A target is set in each of the cathodes  43   a  to  43   c . Specifically, targets for forming a seed layer, a ground layer, and a magnetic layer as films are set in the cathodes  43   a ,  43   b , and  43   c , respectively. A plurality of kinds of films, that is, a seed layer, a ground layer, and a magnetic layer are simultaneously formed as films by the cathodes  43   a  to  43   c.    
     The film formation apparatus  40  having the configuration described above can continuously form a seed layer, a ground layer, and a magnetic layer as films by a roll-to-roll method. 
     The PC  35  compares, on a real time basis, a measurement result of magnetic characteristics measured by the apparatus for measuring magnetic characteristic  49  with prescribed magnetic characteristics (for example, the holding force and the squareness ratio, or the like) stored in the storage section of the PC  35 , and makes feedback to the film formation process so that the magnetic characteristics fall within the range of the prescribed magnetic characteristics. That is, a film formation condition for the magnetic layer  13 , etc. is adjusted so that the magnetic characteristics fall within the range of the prescribed magnetic characteristics. As the film formation condition to be adjusted, for example, at least one of the sputtering electric power, the film running speed, the amount of gas introduced, the kind of gas introduced, the degree of vacuum, or the like is given. Specifically, for example, the coercive force and the squareness ratio can be changed by adjusting the degassing state in the film formation chamber  41 . Note that, in a case where film formation is performed by a vapor deposition method, examples of the film formation condition to be adjusted include the strength of an electron beam, and the like. 
     Products falling within the range of process standard values can be continuously produced by providing the apparatus for measuring magnetic characteristics  49  in the film formation apparatus  40  for magnetic tapes, setting the range of the relevant magnetic characteristics narrower than the standard value range of the process, and performing feedback control as described above. 
     In the first embodiment described above, each of the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , and the magnetization measurement section  34  may be provided facing the magnetic surface  10 S 1  of the magnetic tape  10 , and may further include a reflecting mirror (a reflection section) that reflects, toward the magnetic surface  10 S 1 , polarized light reflected at the magnetic surface  10 S 1 . In this case, polarized light incident on the magnetic surface  10 S 1  is repeatedly reflected between the magnetic surface  10 S 1  and the reflecting mirror, that is, is reflected multiple times at the magnetic surface  10 S 1  of the magnetic tape  10 , and is then received by the light receiving section  32   c   2 ,  33   c   2 , or  34   c   2 . Thus, changes of the light polarization state based on the magnetic Kerr effect can be accumulated. Therefore, the measurement sensitivity of the light polarization state can be improved. 
     Similarly, in the second embodiment described above, each of the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , and the residual magnetization measurement section  36  may be provided facing the magnetic surface  10 S 1  of the magnetic tape  10 , and may further include a reflecting mirror (a reflection section) that reflects, toward the magnetic surface  10 S 1 , polarized light reflected at the magnetic surface  10 S 1 . 
     Although the first and second embodiments described above have described a case where each of the negative-side saturation magnetization measurement section  32 , the positive-side saturation magnetization measurement section  33 , the magnetization measurement section  34 , and the residual magnetization measurement section  36  measures a polarization axis angle as the light polarization state of reflected light, it is also possible to measure the ellipticity, the intensity of reflection, etc. instead of the polarization axis angle. In this case, ellipticity measurement circuits, reflection intensity measurement circuits, etc. are used in place of the polarization axis angle detection circuits  32   c   3 ,  33   c   3 ,  34   c   3 , and  36   c   3 . 
     In addition, the present disclosure may be configured by the following configuration. 
     (1) 
     A method for measuring magnetic characteristics, the method including: applying a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measuring a light polarization state of a first reflected light that is reflected; 
     applying a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measuring a light polarization state of a second reflected light that is reflected; 
     applying a third magnetic field having an opposite direction of the second magnetic field to the continuously moving magnetic recording medium, applying a third polarized light to the surface of the magnetic recording medium to which the third magnetic field is being applied, and measuring a light polarization state of a third reflected light that is reflected; and 
     adjusting a strength of the third magnetic field so that a measurement value of the light polarization state of the third reflected light is a mean value of a measurement value of the light polarization state of the first reflected light and a measurement value of the light polarization state of the second reflected light, and obtaining the strength of the third magnetic field when the measurement value of the light polarization state of the third reflected light becomes equal to the mean value. 
     (2) 
     The method for measuring magnetic characteristics according to (1), in which the magnetic recording medium is continuously moved in a direction going straight relative to a direction of each of the first magnetic field, the second magnetic field, and the third magnetic field. 
     (3) 
     The method for measuring magnetic characteristics according to (1) or (2), in which the measurement value of the light polarization state of the first reflected light, the measurement value of the light polarization state of the second reflected light, and the measurement value of the light polarization state of the third reflected light used to adjust the strength of the third magnetic field are acquired in the same position of the continuously moving magnetic recording medium. 
     (4) 
     The method for measuring magnetic characteristics according to any one of (1) to (3), in which each of the first polarized light, the second polarized light, and the third polarized light is reflected multiple times at the surface of the magnetic recording medium. 
     (5) 
     The method for measuring magnetic characteristics according to any one of (1) to (4), in which the light polarization state of the first reflected light, the light polarization state of the second reflected light, and the light polarization state of the third reflected light are a polarization axis angle of the first reflected light, a polarization axis angle of the second reflected light, and a polarization axis angle of the third reflected light, respectively. 
     (6) 
     An apparatus for measuring magnetic characteristics, the apparatus including: 
     a first measurement section configured to apply a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, apply a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measure a light polarization state of a first reflected light that is reflected; 
     a second measurement section configured to apply a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, apply a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measure a light polarization state of a second reflected light that is reflected; 
     a third measurement section configured to apply a third magnetic field having an opposite direction of the second magnetic field to the continuously moving magnetic recording medium, apply a third polarized light to the surface of the magnetic recording medium to which the third magnetic field is being applied, and measure a light polarization state of a third reflected light that is reflected; and 
     a control section configured to control the third measurement section to adjust a strength of the third magnetic field so that a measurement value of the light polarization state of the third reflected light is a mean value of a measurement value of the light polarization state of the first reflected light and a measurement value of the light polarization state of the second reflected light, and obtain the strength of the third magnetic field when the measurement value of the light polarization state of the third reflected light becomes equal to the mean value. 
     (7) 
     The apparatus for measuring magnetic characteristics according to (6), the apparatus further including: a conveyance section configured to continuously move the magnetic recording medium in a direction going straight relative to a direction of each of the first magnetic field, the second magnetic field, and the third magnetic field. 
     (8) 
     The apparatus for measuring magnetic characteristics according to (6) or (7), in which the control section adjusts the strength of the third magnetic field by using the measurement value of the light polarization state of the first reflected light, the measurement value of the light polarization state of the second reflected light, and the measurement value of the light polarization state of the third reflected light that are acquired in the same position of the continuously moving magnetic recording medium. 
     (9) 
     The apparatus for measuring magnetic characteristics according to any one of (6) to (8), the apparatus further including: reflection sections provided facing the surface of the magnetic recording medium, 
     in which the first polarized light, the second polarized light, and the third polarized light are repeatedly reflected between the surface of the magnetic recording medium and the reflection sections. 
     (10) 
     The apparatus for measuring magnetic characteristics according to any one of (6) to (9), in which the third measurement section includes a magnetic field measurement section configured to measure the strength of the third magnetic field, and 
     the control section uses the magnetic field measurement section to measure the strength of the third magnetic field when the measurement value of the light polarization state of the third reflected light becomes equal to the mean value. 
     (11) 
     The apparatus for measuring magnetic characteristics according to any one of (6) to (9), in which the third measurement section includes a magnetic field generation section configured to apply the third magnetic field to the continuously moving magnetic recording medium, 
     the control section adjusts the strength of the third magnetic field by controlling a value of current to be supplied to the magnetic field generation section, and 
     the control section converts the value of current that is supplied to the magnetic field generation section when the measurement value of the light polarization state of the third reflected light becomes equal to the mean value to a strength of a magnetic field, and calculates the strength of the third magnetic field when the measurement value of the light polarization state of the third reflected light became equal to the mean value. 
     (12) 
     The apparatus for measuring magnetic characteristics according to any one of (6) to (11), in which the light polarization state of the first reflected light, the light polarization state of the second reflected light, and the light polarization state of the third reflected light are a polarization axis angle of the first reflected light, a polarization axis angle of the second reflected light, and a polarization axis angle of the third reflected light, respectively. 
     (13) 
     A method for manufacturing a magnetic recording medium, the method including: 
     applying a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measuring a light polarization state of a first reflected light that is reflected; 
     applying a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measuring a light polarization state of a second reflected light that is reflected; 
     applying a third magnetic field having an opposite direction of the second magnetic field to the continuously moving magnetic recording medium, applying a third polarized light to the surface of the magnetic recording medium to which the third magnetic field is being applied, and measuring a light polarization state of a third reflected light that is reflected; 
     adjusting a strength of the third magnetic field so that a measurement value of the light polarization state of the third reflected light is a mean value of a measurement value of the light polarization state of the first reflected light and a measurement value of the light polarization state of the second reflected light, and obtaining, as a coercive force, the strength of the third magnetic field when the measurement value of the light polarization state of the third reflected light becomes equal to the mean value; and 
     adjusting a film formation condition for the continuously moving magnetic recording medium on the basis of the coercive force obtained. 
     (14) 
     A method for measuring magnetic characteristics, the method including: 
     applying a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measuring a light polarization state of a first reflected light that is reflected; 
     applying a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measuring a light polarization state of a second reflected light that is reflected; 
     applying light to the surface of the continuously moving magnetic recording medium, and measuring a light polarization state of a third reflected light that is reflected; and 
     calculating a ratio (ΔA 20 /ΔA 10 ) of a difference ΔA 20  (=A 2 −A 0 ) between a mean value A 0  of measurement values of the light polarization states of the first reflected light and the second reflected light and a measurement value A 2  of the light polarization state of the third reflected light to a difference ΔA 10  (=A 1 −A 0 ) between the mean value A 0  and the measurement value A 1  of the light polarization state of the first reflected light. 
     (15) 
     An apparatus for measuring magnetic characteristics, the apparatus including: 
     a first measurement section configured to apply a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, apply a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measure a light polarization state of a first reflected light that is reflected; 
     a second measurement section configured to apply a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, apply a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measure a light polarization state of a second reflected light that is reflected; 
     a third measurement section configured to apply light to the surface of the continuously moving magnetic recording medium, and measure a light polarization state of a third reflected light that is reflected; and 
     an arithmetic section configured to calculate a ratio (ΔA 20 /ΔA 10 ) of a difference ΔA 20  (=A 2 −A 0 ) between a mean value A 0  of measurement values of the light polarization states of the first reflected light and the second reflected light and a measurement value A 2  of the light polarization state of the third reflected light to a difference ΔA 10  (=A 1 −A 0 ) between the mean value A 0  and the measurement value A 1  of the light polarization state of the first reflected light. 
     (16) 
     A method for manufacturing a magnetic recording medium, the method including: 
     applying a first magnetic field to a continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a first polarized light to a surface of the magnetic recording medium to which the first magnetic field is being applied, and measuring a light polarization state of a first reflected light that is reflected; 
     applying a second magnetic field having an opposite direction of the first magnetic field to the continuously moving magnetic recording medium to magnetically saturate the magnetic recording medium, applying a second polarized light to the surface of the magnetic recording medium to which the second magnetic field is being applied, and measuring a light polarization state of a second reflected light that is reflected; 
     applying light to the surface of the continuously moving magnetic recording medium, and measuring a light polarization state of a third reflected light that is reflected; 
     obtaining a squareness ratio by calculating a ratio (ΔA 20 /ΔA 10 ) of a difference ΔA 20  (=A 2 −A 0 ) between a mean value A 0  of measurement values of the light polarization states of the first reflected light and the second reflected light and a measurement value A 2  of the light polarization state of the third reflected light to a difference ΔA 10  (=A 1 −A 0 ) between the mean value A 0  and the measurement value A 1  of the light polarization state of the first reflected light; and 
     adjusting a film formation condition for the continuously moving magnetic recording medium on the basis of the squareness ratio obtained. 
     REFERENCE SIGNS LIST 
     
         
           10  Magnetic tape 
           10 S 1  Magnetic surface 
           10 S 2  Back surface 
           11  Substrate 
           12  Ground layer 
           13  Magnetic layer 
           13   a  Coating material 
           14  Back layer 
           20 ,  40  Film formation apparatus 
           21 ,  22  Roll 
           23  Film formation head 
           24  Drying furnace 
           30 ,  30 A Apparatus for measuring magnetic characteristics 
           31  Guide roll (conveyance section) 
           31   a  Encoder 
           32  Positive-side saturation magnetization measurement section (first measurement section) 
           33  Negative-side saturation magnetization measurement section (second measurement section) 
           34  Magnetization measurement section (third measurement section) 
           35  PC (control section and arithmetic section) 
           36  Residual magnetization measurement section (third measurement section) 
           32   a ,  33   a ,  34   a  Electromagnet (magnetic field generation section) 
           32   b ,  33   b ,  34   b  Power source 
           32   c ,  33   c ,  34   c ,  36   c  Light polarization detection section 
           32   c   1 ,  33   c   1 ,  34   c   1 ,  36   c   1  Irradiation section 
           32   c   2 ,  33   c   2 ,  34   c   2 ,  36   c   2  Light receiving section 
           32   c   3 ,  33   c   3 ,  34   c   3 ,  36   c   3  Polarization axis angle detection circuit