Patent Publication Number: US-7590377-B2

Title: Fixing device and image forming apparatus with nonmagnetic metal layer substantially free from temperature rise due to induction heating

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, such as a copy machine, and a fixing device included therein, and more particularly to a fixing device for fixing a toner image on a transfer target in a manner based on an induction heating technique, and an image forming apparatus using the fixing device. 
     2. Description of the Related Art 
     An image forming apparatus is designed to irradiate an outer peripheral surface of a photosensitive drum in a rotating state with an image information-based light beam so as to form an electrostatic latent image on the outer peripheral surface, and supply toner serving as developer to the latent image so as to a toner image. The toner image formed on the outer peripheral surface of the photosensitive drum is transferred onto a sheet serving as a transfer target fed thereto, and then the sheet is subjected to a fixing process based on heating in a fixing device. The sheet after completion of the fixing process is ejected outside from an apparatus body. 
     Typically, the fixing device comprises a fixing roller adapted to be heated to a high temperature, and a pressing roller disposed opposed to the fixing roller in such a manner that an outer peripheral surface thereof is in contact with an outer peripheral surface of the fixing roller. The fixing process is performed by feeding a sheet into a nip zone defined between the fixing and pressing rollers. Heretofore, a built-in type halogen lamp has been employed as a heating source for the fixing roller. The halogen lamp has problems about poor thermal efficiency, and slow response (or low heat-up speed) requiring a fairly long time-period in a warming-up (initial heating) stage. While various techniques for achieving reduction in heat capacity and wall thickness of the fixing roller have been developed as measures against these problems, such approaches have limitations for themselves. 
     Recent years, great interest has been shown in an induction heating-type fixing device designed to heat a fixing roller based on an induction heating technique, as disclosed in Japanese Patent Laid-Open Publication No. 09-127810. In this induction heating-type fixing device, the fixing roller comprises a hollow roller made of a nonmagnetic metal having excellent heat conductivity, and a thin layer formed on an outer peripheral surface of the hollow metal roller and made of a magnetic metal. The fixing device is provided with an induction coil within the fixing roller, and designed to energize the induction coil so as to produce an eddy current in the magnetic metal layer and heat the fixing roller based on Joule heat generated by the eddy current. 
     As compared with the conventional halogen lamp-type fixing device, the induction heating-type fixing device allows the fixing roller to be heated up at a drastically increased speed so as to achieve a higher-speed warm-up of the fixing roller. On the other hand, the extremely high heat-up speed raises a new problem about excessive heating of the fixing roller. In order to solve this problem, it is contemplated to employ a feedback control for detecting a temperature of the fixing roller using a temperature sensor, such as a thermistor or a thermostat, and cutting off a power supply to the induction coil when the fixing roller is heated up to a predetermined temperature or more. However, the temperature sensor has difficulty in outputting a detection signal accurately in response to a temperature rise arising from the induction heating, and this time-lag or detection delay is likely to preclude prevention of excessive heating of the fixing roller. 
     Generally, heat transfer in a longitudinal direction of a fixing roller is apt to become harder as the fixing roller is reduced in wall thickness. Thus, when a sheet having a width less than a heating width of the fixing roller is continuously passed through the fixing roller (or a nip zone), heat tends to stay and accumulate at the opposite end regions of the fixing roller that a smaller number of sheets pass. In this state, if wider sheets are subjected to a fixing process, the accumulated heat will cause image defects, such as a so-called offset phenomenon that a toner image on one of the wider sheets is fusion-bonded onto the end regions of the fixing roller and then transferred onto the next wider sheet. 
     In order to solve this problem, Japanese Patent Laid-Open Publication No. 2004-151470 (hereinafter referred to as Document D2) discloses an induction heating-type fixing device comprising a fixing roller which includes a tubular-shaped temperature-sensitive metal layer made of a temperature compensator alloy, a nonmagnetic metal layer formed on an outer peripheral surface of the temperature-sensitive metal layer in a concentric manner, and an induction coil disposed inside the tubular-shaped temperature-sensitive metal layer and adapted to generate a magnetic field. In this fixing roller, the temperature-sensitive metal layer has a thickness t (m) set to satisfy the following inequality: 
     
       
         
           
             
               
                 503 
                 × 
                 
                   
                     ρ 
                     / 
                     
                       ( 
                       
                         μ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         s 
                         × 
                         f 
                       
                       ) 
                     
                   
                 
               
               &lt; 
               t 
               &lt; 
               
                 503 
                 × 
                 
                   
                     ρ 
                     / 
                     
                       ( 
                       
                         1 
                         × 
                         f 
                       
                       ) 
                     
                   
                 
               
             
             ⁢ 
             
                 
             
             , 
           
         
       
     
     wherein: ρ is a resistivity of the magnetic shunt alloy (Ω·m); f is a frequency (Hz) of a power supply for the induction coil; and μs is a relative permeability of the magnetic shunt alloy at a temperature less than a Curie temperature thereof. 
     In the above inequality, 
             503   ×       ρ   /     (     μ   ⁢           ⁢   s   ×   f     )               
is a magnetic-field penetration depth when the temperature-sensitive metal layer has a temperature less than the Curie temperature (transition temperature), and
 
             503   ×       ρ   /     (     1   ×   f     )               
is a magnetic-field penetration depth when the temperature-sensitive metal layer has a temperature equal to or greater than the Curie temperature.
 
     In this fixing roller, when the temperature-sensitive metal layer has a temperature less than the Curie temperature, a magnetic-field penetration depth becomes less than the thickness of the temperature-sensitive metal layer. Thus, a load (electric resistance) to an eddy current generated by the magnetic field is increased (i.e., an eddy current flows through a narrow area at higher density and a load to the eddy current is increased), and thereby a magnetic flux flows through the temperature-sensitive metal layer with a large electric resistance in an axial direction thereof. The increased load to the eddy current will generate a larger quantity of heat (Joule heat) to allow the temperature-sensitive metal layer to be quickly heated up. 
     Then, when the temperature-sensitive metal layer is heated up to a temperature equal to or greater than the Curie temperature, a magnetic-field penetration depth becomes greater than the thickness of the temperature-sensitive metal layer. Thus, the magnetic field reaches the nonmagnetic metal layer with a lower resistivity than that of the temperature-sensitive metal layer, and a magnetic flux flows through the low-resistivity nonmagnetic metal layer in the axial direction. This makes it possible to reduce a heat generation rate and suppress excess heating of the fixing roller. 
     As above, this fixing roller has an effect of being able to prevent excess heating thereof without using the aforementioned control intended to suppress excess heating of a fixing roller based on detection of a temperature of the fixing roller using a temperature sensor, such as a thermistor or a thermostat (i.e., without the risk of occurrence of control lag due to output delay of a detection signal). 
     Just for reference, in the fixing roller disclosed in the Document D2, an alloy of iron (Fe) and nickel (Ni) is used as a material as the temperature-sensitive metal layer and formed to have a thickness of 0.3 mm, and aluminum (Al) is used as a material of the nonmagnetic metal layer and formed to have a thickness of 0.7 mm. 
     In a fixing roller formed with the temperature-sensitive metal layer and the nonmagnetic metal layer as disclosed in the Document D2 wherein materials and dimensions of the fixing roller are selected to satisfy the above inequality, generation of Joule heat can be reduced at a lower level, because, when a temperature of the temperature-sensitive metal layer becomes equal to or greater than its Curie temperature according to excitation of the induction coil for a fixing process, a magnetic field penetrates through the temperature-sensitive metal layer, and a magnetic flux flows across the nonmagnetic metal layer in an axial direction thereof. However, in view of meeting the need for reducing a warming-up time, a metal layer of a fixing roller is required to be further reduced in wall thickness. 
     If the nonmagnetic metal layer is reduced in thickness without reasonable limit, a load will be increased (i.e. an eddy-current density will be increased) due to reduced eddy-current generation area, to cause difficulty in suppressing generation of Joule heat even when a magnetic field flows through the nonmagnetic metal layer after the temperature-sensitive metal layer becomes equal to or greater than a Curie temperature. As a result, if a fixing process is continuously performed, even the induction heating-type fixing device disclosed in the Document D2 will be excessively heated to cause a problem about excessive temperature rise in opposite end regions of the fixing roller or a region except for a central region thereof where heat is released to sheets passing therethrough. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a fixing device capable of maximally suppressing a temperature rise thereof in a state after being heated up to a Curie temperature, so as to effectively suppress exceeding heating in opposite end regions of the fixing roller. 
     In order to achieve this object, the present invention provides a fixing device comprising a fixing member for fixing a transferred toner image onto a transfer target through a heating process, and a pressing member disposed in contact with the fixing member to define therebetween a nip zone for passing the transfer target therethrough. The fixing member includes a nonmagnetic metal layer made of a nonmagnetic metal, a temperature-sensitive metal layer made of a temperature-sensitive metal, and an induction coil for applying a magnetic field to the nonmagnetic metal layer and the temperature-sensitive metal layer to cause induction heating therein. The temperature-sensitive metal layer is disposed closer to the induction coil than the nonmagnetic metal layer, and the nonmagnetic metal layer is made of a metal having a specific resistance value less than that of aluminum and formed to have a thickness allowing the nonmagnetic metal layer to be substantially free from a temperature rise due to the induction heating. 
     The present invention further provides an image forming apparatus comprising a transfer section for transferring to a sheet a toner image based on image data, and an image fixing section for fixing the toner image transferred onto a surface of the sheet in the transfer section, to the sheet by means of heat. The image fixing section includes the above fixing device. 
     In the present invention, the wording “substantially free from a temperature rise due to induction heating” means that, even if a certain quantity of heat is generated in the nonmagnetic metal layer due to a magnetic field applied from the induction coil thereto, the quantity of generated heat is adequately balanced with a quantity of heat released from the fixing device and thereby a temperature of the nonmagnetic metal layer is not increased so greatly. 
     In the above fixing device and image forming apparatus, when the transfer target is fed to the nip zone where the fixing member and pressing member are in contact with one another, the transfer target is heated by the fixing member increased in temperature through induction heating generated by a magnetic field from an induction coil. In this manner, the transfer target can be subjected to a fixing process for melting the transferred toner on the transfer target and fusion-bonding the toner onto the transfer target. 
     Further, the fixing member may comprise the temperature-sensitive metal layer made of a temperature-sensitive metal and formed on the side of the induction coil, and the nonmagnetic metal layer made of a nonmagnetic metal and laminated on the temperature-sensitive metal layer. Thus, the temperature-sensitive metal layer can be formed to have a thickness greater than a value 
             (     503   ×       ρ   /     (     μ   ⁢           ×   f     )           )         
(wherein ρ is a specific resistance (Ω·m) of the temperature compensator alloy: f is a frequency (Hz) of an induction heating power source; and μ is a relative permeability of the temperature-sensitive metal at a temperature less than a Curie temperature) which expresses a magnetic-field penetration depth under the condition that the temperature-sensitive metal layer has a temperature less than its Curie temperature, and less than a value
 
             (     503   ×       ρ   /     (     1   ⁢           ×   f     )           )         
which expresses a magnetic-field penetration depth under the condition that the temperature-sensitive metal layer has a temperature equal to or greater than the Curie temperature. In this case, under the condition of less than the Curie temperature (or in the period where a temperature of the fixing roller is being increased in response to energization of the induction coil), the magnetic flux flows through the temperature-sensitive metal layer so that a quick temperature rise in the metal layers can be achieved based on an eddy current generated in the temperature-sensitive metal layer.
 
     Then, when the temperature of the temperature-sensitive metal layer becomes equal to or greater than the Curie temperature, the magnetic-field penetration depth becomes greater than the thickness of the temperature-sensitive metal layer (or 503×√{square root over (ρ/(1×f))}). Thus, the magnetic field passes over the temperature-sensitive metal layer and reaches the nonmagnetic metal layer, and the magnetic flux flows through the nonmagnetic metal layer. Further, in this state, the nonmagnetic metal layer made of a metal having a specific resistance value less than that of aluminum and formed to have a thickness allowing the nonmagnetic metal layer to be substantially free from a temperature rise due to the induction heating can reduce generation of Joule heat at lower level as compared with a temperature-sensitive metal layer made of aluminum. 
     As above, in the present invention, the nonmagnetic metal layer is made of a metal having a specific resistance value less than that of aluminum, and formed to have a thickness allowing the nonmagnetic metal layer to be substantially free from a temperature rise due to the induction heating. Thus, as compared with a case where aluminum is used as a material of the nonmagnetic metal layer, the fixing device can effectively suppress a temperature rise after the fixing device reaches a given temperature, while ensuring a high heat-up rate of the fixing member by means of induction heating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory sectional front view showing an outline of an internal structure of a printer as one example of an image forming apparatus incorporating a fixing device of the present invention. 
         FIG. 2  is a schematic partly cutout perspective view showing a fixing device according to a first embodiment of the present invention. 
         FIG. 3  is a sectional view taken along the line III-III in  FIG. 2 . 
         FIG. 4  is a sectional view taken along the line IV-IV in  FIG. 2 . 
         FIGS. 5A and 5B  are sectional front views schematically showing a fixing member, for the purpose of explaining functions of the present invention, wherein  FIG. 5A  shows a state when a heating layer has a temperature less than a Curie temperature, and  5 B shows a state when the heating layer has a temperature equal to or greater than the Curie temperature. 
         FIGS. 6A and 6B  are schematic explanatory diagrams of a fixing device according to a second embodiment of the present invention, wherein  FIG. 6A  is a sectional front view showing the fixing device, and  FIG. 6B  is an enlarged sectional view showing a fixing belt. 
         FIG. 7  is a schematic explanatory diagram showing a testing device used in a functional verification test. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , a printer as one example of an image forming apparatus incorporating a fixing device of the present invention will be firstly described.  FIG. 1  is a sectional front view showing an outline of an internal structure of the printer. As shown in  FIG. 1 , the printer (image forming apparatus)  10  comprises: an apparatus body  11  which is internally provided with a sheet storage section  12  for storing a sheet (transfer target) P to be subjected to a printing process, a transfer section  13  for subjecting each of the sheets P fed from a sheet stuck P 1  stored in the sheet storage section  12 , to an image transfer process, and a fixing section  14  for subjecting the sheet P subjected to the transfer process through the transfer section  13 , to a fixing process; and a sheet ejection section  15  formed at a top portion of the apparatus body  11  and adapted to receive the sheet P subjected to the fixing process through the fixing section  14 . 
     The sheet storage section  12  includes a given number (one in  FIG. 1 ) of sheet cassettes  121  detachably inserted into the apparatus body  11 . A pickup roller  122  is disposed at an upstream end (right end in  FIG. 1 ) of the sheet cassette  121  to pick up the sheets P from the sheet stack P 1  on a one-by-one basis. The sheet P picked up from the sheet cassette  121  by driving of the pickup roller  122  is fed to the transfer section  13  through a sheet feeding passage  123  and a registration roller pair  124  disposed at a downstream end of the sheet feeding passage  123 . 
     The transfer section  13  is provided as a means to subject the sheet P to the transfer process based on image information transmitted from a computer or the like. A photosensitive drum  131  is designed to be rotatable about a drum axis extending in a longitudinal direction (in a direction orthogonal to the drawing sheet of  FIG. 1 ). An electrostatic charger  132  is disposed immediately above the photosensitive drums  131 , and then a light exposure device  133 , an image-development device  134 , a transfer roller  135  and a cleaning device  136  are disposed along an outer peripheral surface of the photosensitive drums  131  clockwise in this order. 
     The photosensitive drum  131  is designed to allow an electrostatic latent image and a toner image corresponding to the toner image to be formed on the outer peripheral surface thereof. For this purpose, the outer peripheral surface of the photosensitive drum  131  is formed of an amorphous silicon layer to provide a surface suitable for forming these images. 
     The electrostatic charger device  132  is operable to form a uniform charge layer on the outer peripheral surface of the photosensitive drum  131  which is being rotated clockwise about the drum axis. The electrostatic charger device  132  employed in the printer illustrated in  FIG. 1  is a type designed to give charges onto the outer peripheral surface of the photosensitive drum  131  by means of corona discharge. The electrostatic charger device  132  serving as a means to give charges onto the outer peripheral surface of the photosensitive drum  131  may be substituted with an electrostatic charge roller designed to give charges onto the outer peripheral surface of the photosensitive drum  131  while being rotationally driven by the photosensitive drum  131  through an outer peripheral surface thereof in contact with the outer peripheral surface of the photosensitive drum  131 . 
     The light-exposure device  133  is operable to irradiate the outer peripheral surface of the photosensitive drum  131  in a rotating state, with a laser light having intensity varied based on image data transmitted from an external device, such as a computer, so as to eliminate charges in a region of the outer peripheral surface of the photosensitive drum  131  scanningly irradiated with the laser light to form an electrostatic latent image on the outer peripheral surface of the photosensitive drum  131 . 
     The image-development device  134  is operable to supply toner onto the outer peripheral surface of the photosensitive drum  131  so as to attach the toner on a region formed as the electrostatic latent image to form a toner image on the outer peripheral surface of the photosensitive drum  131 . 
     The transfer roller  135  is operable to transfer the positively-charged toner image formed on the outer peripheral surface of the photosensitive drum  131 , to the sheet P fed to a position immediately below the photosensitive drum  131 . The transfer roller  135  is designed to give to the sheet P negative charges having a reverse polarity relative to charges of the toner image. 
     Thus, the sheet P reaching the position immediately below the photosensitive drum  131  is pressed and nipped between the transfer roller  135  and the photosensitive drum  131 , and the positively-charged toner image on the outer peripheral surface of the photosensitive drum  131  is peeled toward a surface of the negatively-charged sheet P. In this manner, the sheet P is subjected to the transfer process. 
     The cleaning device  136  is operable to remove toner remaining on the outer peripheral surface of the photosensitive drum  131  after completion of the transfer process, so as to clean the outer peripheral surface of the photosensitive drum  131 . The outer peripheral surface of the photosensitive drum  131  cleaned by the cleaning device  136  will be rotated toward the electrostatic-charge device  132  again to perform a next image forming process. 
     The fixing section  14  serves as a means to heat the toner image on the sheet P subjected to the transfer process through the image forming section  13 , so as to subject the sheet P to the fixing process. The fixing section  14  includes a fixing member  30  for giving heat to the sheet P, and a pressing member  40  disposed below the fixing member  30  in opposed relation to the fixing member  30 . The sheet P after completion of the transfer process is fed into a nip zone N defined between the fixing member  30  and the pressing member  40 , and heated by the fixing member  30  while passing through the nip zone N so as to subject the sheet P to the fixing process. The sheet P subjected to the fixing process will be ejected to the sheet ejection section  15  through a sheet-ejecting passage  143 . 
     The sheet ejection section  15  is formed by concaving the top portion of the apparatus body  11  to define a concaved depression with a bottom serving as a sheet tray  151  for receiving the ejected sheet P. 
       FIG. 2  is a schematic partly cutout perspective view showing a fixing device  20  according to a first embodiment of the present invention.  FIG. 3  is a sectional view taken along the line III-III in  FIG. 2 , and  FIG. 4  is a sectional view taken along the line IV-IV in  FIG. 2 . In these figures, each thickness dimension of a fixing roller  31  and a pressing roller  42  is illustrated in an exaggerated manner. As shown in  FIG. 2 , the fixing device  20  comprises a fixing member  30 , a pressing member  40  and a box-shaped housing  21  housing the fixing member  30  and the pressing member  40 . 
     The fixing member  30  includes a tubular-shaped fixing roller  31  disposed in an upper region of an inner space of the housing  21 , and an induction coil  34  housed in the fixing roller  31 . More specifically, the fixing roller  31  is mounted to an upper portion of the housing  21  rotatably about a tube axis  310  (see  FIG. 3 ) extending in a sheet-width direction orthogonal to a sheet-feeding direction (indicated by the two-dot-chain-lined arrow in  FIG. 2 ). The fixing roller  31  is drivenly rotated clockwise about the tube axis  310  by a driving force of a drive motor (not shown) disposed outside the housing  21 . While the fixing member  30  in this embodiment is formed to have an outer diameter of 40 mm, the outer diameter of the fixing member  30  is not limited to 40 mm, but may be set at an optimal value depending on the situation. 
     The pressing member  40  is disposed in a lower region of the inner space of the housing  21 , and in parallel relation to the fixing roller  31  while allowing an outer peripheral surface of the pressing member  40  to be in contact with an outer peripheral surface of the fixing roller  31 . The pressing member  40  includes a pressing roller shaft  41  mounted to each of opposite side walls of the housing  21  to extend therebetween in a rotatable manner about an axis thereof, and a pressing roller  42  concentrically supported by the pressing roller shaft  41  in a rotatable manner about the pressing roller shaft  41 . 
     The pressing roller  42  is made of an elastomer, such as elastic silicon rubber. As shown in  FIG. 3 , the pressing roller  42  is disposed in press contact with the outer peripheral surface of the fixing roller  31 , and elastically deformed radially inward. The pressing roller  42  is rotationally driven by the fixing roller  31  rotated of about the tub axis  310 . A nip zone Z for passing the sheet P therethrough while nipping the sheet P is defined at a position where the pressing roller  42  is in contact with the fixing roller  31 . Thus, a front surface of the sheet P fed from the image forming section  13  in a state when the fixing roller  31  and the pressing roller  42  are rotated, respectively, in opposite directions, is pressed onto the fixing roller  31  by the elastically deformed pressing roller  42 , and heated by the fixing roller  31  while passing through the nip zone Z. In this manner, the sheet P is subject to the fixing process for fusion-bonding molten toner onto the front surface of the sheet P. 
     As shown in  FIG. 2 , the induction coil  34  is wound along a longitudinal direction of a pair of upper and lower flanges of a core  341  made of a magnetic material and mounted to the fixing roller  31  to extend a longitudinal direction of the fixing roller  31 . The fixing device  20  is designed to supply a power to the induction coil  34  from a high-frequency generator circuit (not shown) serving as an induction-heating power source. In response to supplying the induction-heating power to the induction coil  34 , lines of magnetic force (magnetic flux) are output from one of the flanges of the core  341  of the induction coil  34 . The magnetic flux flows through the fixing roller  31  toward the other flange of the core  341  of the induction coil  34 , as indicated by the arrows in  FIG. 5 . This flow of the magnetic flux generates an eddy current in the fixing roller  31 , and the fixing roller  31  is heated by Joule heat arising from the eddy current. 
     The fixing roller  31  includes a heating layer  32  made of a metal (metal layer) and adapted to heat the fixing roller  31  by means of induction heating, and a resin layer  33  laminated around an outer peripheral surface of the heating layer  32 . The resin layer  33  is provided as a means to protect the outer peripheral surface of the heating layer  32  and ensure peelability or releasability relative to the sheet P. The resin layer  33  includes an elastic layer  331  made of an elastic material, such as silicon rubber, and a release layer  332  made, for example, of PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether polymer). In this embodiment, the elastic layer  331  is formed to have a thickness of about 100 μm, and the release layer  332  is formed to have a thickness of about 50 μm. 
     As shown in  FIGS. 3 and 4 , the heating layer  32  includes an annular-shaped temperature-sensitive metal layer  321  made of a temperature-sensitive metal, and a nonmagnetic metal layer  322  made of a nonmagnetic metal and laminated around an outer peripheral surface of the temperature-sensitive metal layer  321 . As used in this specification, the term “temperature-sensitive metal” means a metal having magnetic characteristics to be changed depending on temperatures. In this embodiment, the temperature-sensitive metal layer  321  is made of an alloy of iron (Fe) and nickel (Ni). The temperature-sensitive metal has a property where a magnetic-field penetration depth is changed at a magnetic transition temperature (Curie temperature) as a transition point of magnetic characteristics. In this embodiment, respective composition ratios of iron (Fe) and nickel (Ni) in the alloy are adjusted to set a Curie temperature of the temperature-sensitive metal layer  321  at about 200° C. In the present invention, the above property of the temperature-sensitive metal is utilized to prevent excess heating of the fixing roller  31  due to induction heating. 
     A magnetic-field penetration depth in a temperature-sensitive metal will be described below. At a temperature less than a Curie temperature, a magnetic-field penetration depth σ in a temperature-sensitive metal is expressed by the following formula (1): 
                     σ   =     503   ×       ρ   /     (     μ   ⁢           ×   f     )             ⁢           ,           (   1   )               
wherein σ is a magnetic-field penetration depth (m); ρ is a specific resistance (Ω·m):
 
f is a frequency (Hz) of an induction heating power source; and μ is a relative permeability at a temperature less than the Curie temperature.
 
     As seen in the formula (1), the magnetic-field penetration depth σ is proportional to a square root of the specific resistance ρ of the temperature-sensitive metal, and inversely proportional to a square root of the relative permeability μ and the induction heating power source frequency f. Thus, in a temperature-sensitive metal, the magnetic-field penetration depth σ is increased as the specific resistance ρ is increased. Further, the magnetic-field penetration depth σ is reduced as the relative permeability μ and the induction heating power source frequency f are increased. Generally, at a temperature less than a Curie temperature, the relative permeability μ is fairly greater than 1. 
     At a temperature equal to or greater than a Curie temperature, a magnetic-field penetration depth σ in a temperature-sensitive metal is expressed by the following formula (2): 
                     σ   =     503   ×       ρ   /     (     1   ⁢           ×   f     )             ⁢           ,           (   2   )               
wherein σ is a magnetic-field penetration depth (m); ρ is a specific resistance (Ω·m):
 
f is a frequency (Hz) of an induction heating power source; and μ=1 is a relative permeability at a temperature equal to or greater than the Curie temperature.
 
     That is, when a temperature-sensitive metal has a temperature equal to or greater than the Curie temperature, the specific resistance ρ has a minimum value of 1, and thereby the magnetic-field penetration depth σ is increased. In this embodiment, the induction heating power source frequency f is set at 25 kHz. 
     An eddy current load or a load to an eddy current generated by a magnetic field applied to a metal will be described below. The concept of “eddy current load” is introduced by the inventors for the purpose of adequately describing the present invention. An eddy current load R is expressed by the following formula (3):
 
 R=ρ/z   (3),
 
wherein R is an eddy current load (Ω); ρ is a specific resistance (Ω·m); and z is a depth in the range of which an eddy current is generated.
 
     That is, the eddy current load R is proportional to the specific resistance ρ of the metal, and inversely proportional to the eddy current generation depth z. Thus, in metals having the same specific resistance ρ, the eddy current load R becomes lower as the eddy current generation depth z is increased. 
     The eddy current generation depth z is equal to the magnetic-field penetration depth σ in the formulas (1) and (2) (z=σ). Thus, the eddy current load R becomes lower as the magnetic-field penetration depth σ is increased. This means that a thickness of a temperature-sensitive metal can be set at a value greater than the magnetic-field penetration depth σ in the formula (2) to reduce the eddy current load R defined by the formula (3) so that heat generation due to Joule heat generated by an eddy current is limited to a low value (or excess heating of the fixing roller  31  is avoided) even if the temperature-sensitive metal is heated up to a temperature equal to or greater than the Curie temperature. However, this approach leads to substantial increase in thickness of the temperature-sensitive metal  321 , which is against the needs for reduction in wall thickness of the fixing roller  31 . 
     In the temperature-sensitive metal layer  321  formed to have a thickness less than the eddy current generation depth z (or the magnetic-field penetration depth σ), the eddy current load R is expressed by the following formula (4):
 
 R=ρ/d   (4),
 
wherein d is a thickness of the temperature-sensitive metal layer (d&lt;z=σ).
 
     That is, the eddy current load R becomes larger as the temperature-sensitive metal layer  321  is reduced in thickness to achieve the need for reduction in wall thickness of the fixing roller  31 , and resulting increased Joule heat will make it difficult to effectively prevent excess heating of the fixing roller  31 . Moreover, a temperature-sensitive metal originally has a relatively large specific resistance. Thus, as long as an eddy current is generated in the temperature-sensitive metal layer  321 , it is difficult to expect a desirable excess-heating suppressive effect. 
     In the present invention, as shown in  FIG. 3 , a thickness of the temperature-sensitive metal layer  321  made of an alloy of iron (Fe) and nickel (Ni) is firstly minimized (specifically, the thickness is set at a value slightly greater than the magnetic-field penetration depth σ calculated by the formula (1); in this embodiment, the thickness is set at 250 μm). Then, the nonmagnetic metal layer  322  made of copper (Cu) having a specific resistance value less than that of aluminum is laminated around the outer peripheral surface of the temperature-sensitive metal layer  321 . Thus, when the temperature-sensitive metal layer  321  is heated up to a temperature equal to or greater than its Curie temperature, a magnetic field is introduced into the nonmagnetic metal layer  322  having a low specific resistance (i.e., generation of Joule heat in the temperature-sensitive metal layer  321  is eliminated based on the eddy current load calculated by the formula (4)) to allow a magnetic flux to flow through the nonmagnetic metal layer  322  having a low specific resistance. 
     Thus, in a state when the heating layer  32  has a temperature equal to or greater than a Curie temperature (specifically, 200° C.), Joule heat will be generated in the nonmagnetic metal layer  322  having a low specific resistance without generation of Joule heat in the temperature-sensitive metal layer  321  having a large specific resistance. However, copper (Cu) forming the nonmagnetic metal layer  322  has a specific resistance less than aluminum (Al) (just for reference, a specific resistance of aluminum (Al) is 0.027 μΩm, and a specific resistance of copper (Cu) is 0.017 μΩm). While aluminum (Al) is hardly heated up by Joule heat, copper (Cu) is more hardly heated up. Thus, the nonmagnetic metal layer made of copper (Cu) makes it possible to more reliably prevent excess heating of the fixing roller  31  as compared with the conventional nonmagnetic metal layer made of aluminum. 
     Further, in this embodiment, the nonmagnetic metal layer  322  is formed to have a thickness of “30 μm” as the thickness for the “substantially (practically) free from a temperature rise due to induction heating”. This value “30 μm” has been determined through various functional verification tests based on comparison with aluminum (Al). The state “substantially free from a temperature rise due to induction heating” means that, even if a certain quantity of heat is generated in the nonmagnetic metal layer  322  due a magnetic field applied from the induction coil  34  thereto, the quantity of generated heat is adequately balanced with a quantity of heat released from the fixing device  20  and thereby a temperature of the nonmagnetic metal layer  322  is not increased so greatly. Thus, the state is practicable in preventing excess heating of the fixing member  30 . 
     In this embodiment, a thickness of the temperature-sensitive metal layer  321  is set at a value ten times greater than a thickness calculated by the formula (1) (about 25 μm). The reason is to adequately maintain a mechanical strength of the temperature-sensitive metal layer  321  so as to allow the fixing roller  31  to serve as a roller. 
       FIG. 5  is a sectional front view schematically showing the fixing member  30 , for the purpose of explaining functions of the present invention, wherein  FIG. 5A  shows a state when the heating layer  32  has a temperature less than the Curie temperature, and  FIG. 5B  shows a state when the heating layer  32  has a temperature equal to or greater than the Curie temperature. In  FIG. 5 , the resin layer  33  is not illustrated. 
     In the state of  FIG. 5A  when the heating layer  32  has a temperature less than the Curie temperature, the penetration depth σ (see the formula (1)) of a magnetic field from the induction coil  34  is not greater than the thickness d of the temperature-sensitive metal layer  321 . Thus, as indicated by the arrows, the magnetic flux from the induction coil  34  flows through the temperature-sensitive metal layer  321  without reaching the nonmagnetic metal layer  322 , to generate Joule heat based on an eddy current induced in the temperature-sensitive metal layer  321  so as to allow the temperature-sensitive metal layer  321  to be quickly heated. 
     Then, when the temperature of the temperature-sensitive metal layer  321  becomes equal to or greater than 200° C. set as a Curie temperature of the temperature-sensitive metal layer  321 , the penetration depth σ (see the formula (2)) of the magnetic field from the induction coil  34  becomes greater than the thickness d of the temperature-sensitive metal layer  321 . Thus, as shown  FIG. 5B , the magnetic flux from the induction coil  34  passes over the temperature-sensitive metal layer  321  and reaches and flows through the nonmagnetic metal layer  322 . 
     In this state, Joule heat based on an eddy current is generated in the nonmagnetic metal layer  322 . However, copper (Cu) having an extremely low specific resistance is used as a nonmagnetic material of the nonmagnetic metal layer  322 , and heating power based on Joule heat is reduced because the magnetic field is concentrated in a region of the nonmagnetic metal layer  322  having a temperature less than the Curie temperature. Further, the power supply to the induction coil  34  is cut off in response to detection of overload by load detection means provided in the high-frequency power supply. This makes it possible to prevent excess heating or a problem that the fixing roller  31  is heated up to a temperature fairly greater than the Curie temperature. 
     Then, when the temperature of the heating layer  32  becomes less than 200° C. or the Curie temperature, the penetration depth σ of the magnetic field from the induction coil  34  becomes less than the thickness d of the temperature-sensitive metal layer  321  as shown in  FIG. 5A . Thus, the temperature-sensitive metal layer  321  is re-heated based on Joule heat. 
     In this manner, the flowpath of the magnetic flux is changed at the Curie temperature, and a cycle of heating and cooling of the fixing roller  31  will be repeated. Thus, a temperature of the fixing roller  31  can be controlled within an allowable range without the need for the feedback control using a temperature sensor. This can also contribute to reduction in cost of the fixing device. 
       FIG. 6  is a schematic explanatory diagrams of a fixing device  20 ′ according to a second embodiment of the present invention, wherein  FIG. 6A  is a sectional front view showing the fixing device  20 ′, and  FIG. 6B  is an enlarged sectional view showing a fixing belt  37 . As shown in  FIG. 6A , in the fixing device  20 ′ according to the second embodiment, a fixing member  30 ′ comprises a tension roller (first support roller)  35 , a fixing roller (second support roller)  36  disposed below and in opposed relation to the tension roller  35 , a fixing belt  37  wound around between the tension roller  35  and the fixing roller  36  in a tensioned manner, and an induction coil  34 ′ disposed above and in opposed relation to the fixing belt  37 . The remaining structure of the fixing device  20 ′ is the same as that in the first embodiment. 
     The tension roller  35  includes a tension roller shaft  351 , and a tubular-shaped nonmagnetic metal body  352  formed concentrically around the tension roller shaft  351  and rotatably together with the tension roller shaft  351 . The tension roller shaft  351  is drivenly rotated clockwise by a driving force of a drive motor (not shown), and then the tubular-shaped nonmagnetic metal body  352  is integrally rotated by the tension roller shaft  351 . In this embodiment, the tubular-shaped nonmagnetic metal body  352  is made of stainless steel (SUS304) and formed to have a thickness of 0.1 mm. 
     The fixing roller  36  includes a fixing roller shaft  361  disposed parallel to the tension roller shaft  351  to extend in the same direction as that of the tension roller shaft  351 , and a fixing roller body  362  formed on an outer peripheral surface of the fixing roller shaft  361  concentrically and integrally. In this embodiment, the fixing roller body  362  is formed of so-called “silicon sponge” consisting of foamed silicon rubber. The fixing roller body  362  is disposed in press contact with a pressing roller  42 , and elastically deformed radially inward. 
     As shown in  FIG. 6B , the fixing belt  37  includes a metal layer  38  formed on the side of an inner surface thereof, and a resin layer  39  laminated on an outer surface of the metal layer  38 . The metal layer  38  includes a nonmagnetic metal layer  381  made of copper (Cu) and formed on the side of an inner surface thereof, and a temperature-sensitive metal layer  382  made of a temperature-sensitive metal consisting of an alloy of iron (Fe) and nickel (Ni) and laminated on an outer surface of the nonmagnetic metal layer  381 . In this embodiment, the nonmagnetic metal layer  381  made of copper (Cu) is formed to have a thickness of 30 μm, and the temperature-sensitive metal layer  382  is formed to have a thickness of 25 μm slightly greater than the magnetic-field penetration depth σ (24.6 μm) calculated by the formula (1). The nonmagnetic metal layer  381  and the temperature-sensitive metal layer  382  has substantially the same function, respectively, as those of the nonmagnetic metal layer  322  and the temperature-sensitive metal layer  321  in the first embodiment. 
     The resin layer  39  includes an elastic layer  391  made of silicon rubber, and a release layer  392  made of PFA. The elastic layer  391  has the same thickness (100 μm) as that of the elastic layer  331  in the first embodiment and substantially the same function as that of the elastic layer  331 . The release layer  392  has the same thickness (50 μm) as that of the release layer  332  in the first embodiment and substantially the same function as that of the release layer  332 . 
     In the fixing device  20 ′ according to the second embodiment, when the fixing belt  37  is circulatingly moved between the tension roller  35  and the fixing roller  36  by a rotational driving force of the tension roller  35 , a magnetic flux is supplied from the induction coil  34 ′ to an outer surface of the fixing belt  37 . Therefore, in a state before the metal layer  38  does not reach the Curie temperature (200° C.), the temperature-sensitive metal layer  382  is quickly heated up to the Curie temperature by Joule heat generated by an induced eddy current. 
     Thus, when a sheet P is fed to a nip zone N, the sheet P is moved leftward in  FIG. 6A  while being pressed and nipped between the pressing roller  42  and the fixing belt  37  circulating along the fixing roller body  362  which is elastically deformed. During this movement, the sheet P is subjected to the fixing process based on heat from the fixing belt  37 . 
     Then, when the temperature of the temperature-sensitive metal layer  382  becomes equal to or greater than the Curie temperature, a magnetic field from the induction coil  34 ′ passes over the temperature-sensitive metal layer  382  and reaches the nonmagnetic metal layer  381  having a low specific resistance. Thus, a quantity of heat to be generated based on Joule heat is reduced, and the magnetic field is concentrated in a region of the nonmagnetic metal layer  381  having a temperature less than the Curie temperature to cause reduction in heating power. Further, the power supply to the induction coil  34 ′ is cut off in response to load detection in a high-frequency power supply. This makes it possible to prevent excess heating of the fixing belt  37 . When the fixing belt  37  becomes less than the Curie temperature, the temperature-sensitive metal layer  382  is induction-heated again, and subsequently the temperature of the fixing belt  37  will be varied up and down within an allowable range on the basis of the Curie temperature. 
     In the second embodiment, a mechanical strength is not required for the fixing belt  37 . Thus, the thickness of temperature-sensitive metal layer  382  can be reduced to a lower limit value (25 μm) so as to ensure a high heat-up speed. 
     As descried above, the fixing device ( 20 ,  20 ′) of the present invention comprises a fixing member ( 30 ,  30 ′) designed to be heated up by means of induction heating based on a magnetic field from an induction coil ( 34 ,  34 ′), and a pressing member ( 40 ) disposed in contact with the fixing member ( 30 ,  30 ′) to define a nip zone (N) for passing a sheet (P) therethrough. Thus, when the sheet (P) is fed to the nip zone (N) where the fixing member ( 30 ,  30 ′) and the pressing member ( 40 ) are in contact with one another, the sheet (P) is heated up by the fixing member ( 30 ,  30 ′) increased in temperature through induction heating generated by the magnetic field from an induction coil ( 34 ,  34 ′). In this manner, the sheet P can be subjected to a fixing process for melting transferred toner on the sheet P and fusion-bonding the toner onto the sheet P. 
     Further, the fixing member ( 30 ,  30 ′) comprises a heating layer ( 32 ,  38 ) which includes a temperature-sensitive metal layer ( 321 ,  382 ) made of a temperature-sensitive metal and formed on the side of the induction coil ( 34 ,  34 ′) and a nonmagnetic metal layer ( 322 ,  381 ) made of a nonmagnetic metal and laminated on the temperature-sensitive metal layer ( 321 ,  382 ). Thus, the temperature-sensitive metal layer ( 321 ,  382 ) can be formed to have a thickness (d) greater than a value 
             (     σ1   =     503   ×       ρ   /     (     μ   ⁢           ×   f     )             )         
calculated by the formula (1) expressing a magnetic-field penetration depth at a temperature less than the Curie temperature (d&gt;σ1) and less than a value
 
             (     σ2   =     503   ×       ρ   /     (     1   ⁢           ×   f     )             )         
calculated by the formula (2) expressing a magnetic-field penetration depth at a temperature equal to or greater than the Curie temperature (d&lt;σ2). In this case, under the condition of less than the Curie temperature (or in the period where a temperature of the fixing roller is being increased in response to energization of the induction coil ( 34 ,  34 ′)), the magnetic flux flows through the temperature-sensitive metal layer ( 321 ,  382 ) so that a quick temperature rise in heating layer ( 32 ,  38 ) can be achieved based on an eddy current generated in the temperature-sensitive metal layer ( 321 ,  382 ).
 
     Then, when the temperature of the temperature-sensitive metal layer ( 321 ,  382 ) becomes equal to or greater than the Curie temperature, the magnetic-field penetration depth becomes greater than the thickness of the temperature-sensitive metal layer ( 321 ,  382 ). Thus, the magnetic field passes over the temperature-sensitive metal layer ( 321 ,  382 ) and reaches the nonmagnetic metal layer ( 322 ,  381 ), and the magnetic flux flows through the nonmagnetic metal layer having a low specific resistance. Further, the nonmagnetic metal layer ( 322 ,  381 ) is made of a metal having a specific resistance value less than that of aluminum and formed to have a thickness allowing the nonmagnetic metal layer to be substantially free from a temperature rise due to the induction heating. Thus, as compared with the conventional nonmagnetic metal layer made of aluminum, the nonmagnetic metal layer ( 322 ,  381 ) can suppress generation of Joule heat at lower level. 
     As above, as compared with a case where aluminum is used as a material of the nonmagnetic metal layer ( 322 ,  381 ) as in the conventional technique, the nonmagnetic metal layer ( 322 ,  381 ) made of a metal having a specific resistance value less than that of aluminum and formed to have a thickness allowing the nonmagnetic metal layer to be substantially free from a temperature rise due to the induction heating makes it possible to further effectively suppress a temperature rise after the fixing device reaches a given temperature, while ensuring a high heat-up rate of the fixing member ( 30 ,  30 ′) by means of induction heating, and reliably prevent occurrence of an offset phenomenon which would otherwise be caused by such an abnormal high temperature. 
     In the above embodiments, copper is used as the metal having a specific resistance value less than that of aluminum, and its lower limit of thickness is set at 30 μm. The specific resistance value of copper is about 0.017 μΩm which is less than the specific resistance value (0.027 μΩm) of aluminum. Thus, as compared with the nonmagnetic metal layer made of aluminum, the nonmagnetic metal layer made of copper can reliably suppress generation of Joule heat at lower level in a state after the temperature-sensitive metal layer  321 ,  382  of the fixing member  30 ,  30 ′ is heated up to a temperature equal to or greater than the Curie temperature. Further, “30 μm” as the lower limit thickness of the copper layer is a finding obtained by various actual functional verification tests. The copper layer set at the lower limit thickness of 30 μm makes it possible to maximally reduce a thickness of the nonmagnetic metal layer  322 ,  381  while suppressing a temperature rise of the heating layer  32 ,  38 . 
     In the first embodiment, the heating layer  32  is used as a component of the tubular-shaped fixing roller  31  designed to be rotatable about the tube axis  310 . In this case, the induction coil  34  can be housed in the tubular-shaped fixing roller to achieve reduction in size of the fixing member  30 . 
     In the second embodiment, the heating layer  38  is used as a component of the fixing belt  37  wound around between the tension roller  35  and the fixing roller  36  in a tensioned manner. In this case, a structural strength is not required for the fixing belt  37 . Thus, the thickness of temperature-sensitive metal layer  382  can be reduced to a lower limit value so as to achieve a maximized heat-up speed. 
     The present invention is not limited to the above embodiments, but may include the following modifications. 
     While the fixing devices according to the above embodiments are employed in the printer  10  as an image forming apparatus, the image forming apparatus is not limited to the printer  10 , but may be a copying machine for transferring onto a sheet P a toner image based on image information scanned by a scanner, or a facsimile machine for transferring onto a sheet P a toner image based on transmitted image information. 
     While copper having a specific resistance value less than that of aluminum is used as a material of the nonmagnetic metal layer  322 ,  381  in the above embodiments, a material of the nonmagnetic metal layer ( 322 ,  381 ) of the present invention is not limited to copper, but may be any other alloy prepared to have a specific resistance value less than that of silver or aluminum. 
     In the second embodiment, the fixing belt  37  may be composed only of the temperature-sensitive metal layer  382 , and the outer peripheral surface of the tension roller  35  may be formed with a nonmagnetic metal layer made of copper or silver as a nonmagnetic metal. In this structure, the fixing belt  37  made only of a temperature-sensitive metal is circulatingly moved between the tension roller  35  and the fixing roller  36  while being induction-heated based on a magnetic field from the induction coil  34 ′ disposed outside and in opposed relation to the fixing belt  37 , and subjects the sheet P to the fixing process in the nip zone Z. 
     Then, when the fixing belt  37  made only of the temperature-sensitive metal is heated up to a temperature equal to or greater than the Curie temperature, the magnetic field from the induction coil  34 ′ passes through the fixing belt  37  and penetrates into the low-resistance nonmagnetic metal formed on the outer peripheral surface of the tension roller  35 . Thus, subsequently, generation of Joule heat can be suppressed to prevent excess heating of the fixing belt  37 . 
     The above structure where the heating layer  37  is divided into the fixing belt  37  made of a temperature-sensitive metal and the nonmagnetic metal layer formed on the outer peripheral surface of the tension roller  35  wounded by the fixing belt  37  in a tensioned manner makes it possible to further reduce a thickness of the fixing belt  37  as compared with the heating layer where the temperature-sensitive metal layer and the nonmagnetic metal layer are integrally laminated. This makes it possible to achieve a further increased heat-up rate in the belt-type fixing member  30 ′. Further, an amount of bending in the fixing belt  37  can be increased. This allows the tension roller  35  and the fixing roller  36  to be reduced in diameter so as to contribute to reduction in size of the fixing device. 
     While the fixing belt  37  in the above embodiment is wound around the tension roller  35  and the fixing roller  36  in a tensioned manner, the present invention is not limited to this type where the fixing belt  37  is wound around the tension roller  35  and the fixing roller  36  in a tensioned manner, but a given number of idlers may be optionally interposed between the tension roller  35  and the fixing roller  36 , and the fixing belt  37  may be additionally wound around the idlers. 
     The following functional verification test was conducted to check to what extent the thickness of the nonmagnetic metal layer  322 ,  381  can be more reduced when copper is used as a material of the nonmagnetic metal layer  322 ,  381 , as compared with aluminum (Comparative Example) which is used as a material of the conventional nonmagnetic metal layer. 
       FIG. 7  is a schematic explanatory diagram showing a testing device used in the functional verification test. As show in this figure, the testing device  50  comprises an induction-heating power supply  51  internally having a load detection circuit  511 , and an induction-heating coil  52  for generating high-frequency magnetic field lines based on an induction-heating power supplied from the induction-heating power supply  51 . This testing device  50  was designed to supply a high-frequency power of 25 kHz from the induction-heating power supply  51  to the induction-heating coil  52 . 
     In the above testing device  50 , a test piece  53  serving as Inventive Example (copper) was disposed above the induction heating coil  52 . Then, the induction-heating power supply  51  was activated to supply magnetic field lines to the test piece  53 , and a resulting temperature rise of the test piece  53  was measured to check an excess-heating suppressive effect. As to Comparative Example (aluminum), a test piece  53  having the same size was prepared. Then, the test piece was disposed above the induction heating coil  52 , and a comparative test was conducted in the same manner. 
     The test piece  53  was comprised of a temperature-sensitive metal layer  531  made of an alloy of iron (Fe) and nickel (Ni) and formed to have a square shape having a planar dimension of 100 mm×100 mm, and a thickness of 25 μm, and a nonmagnetic metal layer  532  made of a nonmagnetic metal (Inventive Example: copper, Comparative Example: aluminum) and laminated onto the temperature-sensitive metal layer  531 . 
     As to the nonmagnetic metal layer  532 , six types having different thicknesses of 10 μm, 20 μm, 30 μm, 40 μm, 50 μm and 60 μm was prepared. Each of the six type of test pieces  53  was induction-heated, and it was determined whether a temperature-rise suppressive effect is observed when a load of the test piece  53  detected by the load detection circuit  511  becomes less than 30% of a normal load (load of the temperature-sensitive metal layer  531 ). The reason for using “30%” as a criterion is as follows. Through actual test results using various types of fixing devices, it was verified that a quantity of heat generated by induction heating at a load of about 30% is balanced with a quantity of heat released in an actual fixing device  20 , and a fixing roller  31  is not heated up to a temperature fairly greater than a Curie temperature (about 200° C. in this embodiment) at the load 30%. The test result is shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Test Result 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Conditions 
                 Thickness of 
                 25 
               
               
                   
                 temperature-sensitive 
               
               
                   
                 metal layer (μm) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Thickness of nonmagnetic 
                 10 
                 20 
                 30 
                 40 
                 50 
                 60 
               
               
                   
                 metal layer (μm) 
               
               
                 Test Result 
                 Inventive Examples 
                 X 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
                 (nonmagnetic metal 
               
               
                   
                 layer: copper) 
               
               
                   
                 Comparative Examples 
                 X 
                 X 
                 X 
                 X 
                 ◯ 
                 ◯ 
               
               
                   
                 (nonmagnetic metal 
               
               
                   
                 layer: aluminum) 
               
               
                   
               
               
                 Note) 
               
               
                 ◯: Temperature-rise suppressive effect was observed 
               
               
                 X: No temperature-rise suppressive effect was observed 
               
            
           
         
       
     
     As shown in Table 1, in the Comparative Examples, a temperature-rise suppressive effect is observed only if a thickness of the nonmagnetic metal layer  532  is increased to 50 μm or more. In contrast, the Inventive Examples exhibit a temperature-rise suppressive effect when a thickness of the nonmagnetic metal layer  532  is increased to 30 μm or more. Through this test, it could be verified that the Inventive Example using copper as a material of the nonmagnetic metal layer  532  can be more reduced in thickness than the Comparative Example using aluminum as a material of the nonmagnetic metal layer  532 . 
     This application is based on patent application No. 2005-089174 filed in Japan, the contents of which are hereby incorporated by references. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to embraced by the claims.