Patent Publication Number: US-2021183550-A1

Title: Magnetic sheet, coil module having magnetic sheet, and non-contact power supply device

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetic sheet comprised of Mn—Zn ferrite, a coil module having the magnetic sheet, and a non-contact power supply device. 
     Conventionally, magnetic sheets containing ferrite have been used in fields such as short-range wireless communication, wireless power transmission, electromagnetic wave protection or the like for the purpose of electromagnetic wave blocking, electromagnetic wave absorption, magnetic wave line convergence, or the like. 
     Such a magnetic sheet is manufactured by firing a green sheet containing ferrite powder on a ceramic setter. However, the ferrite sheet obtained as a sintered body is highly brittle, and therefore physical damage such as cracks is prone to occur. In addition, when the thickness of the ferrite sheet is thin, deformation is prone to occur after firing, and it is difficult to stably manufacture the ferrite sheet. Cracks or deformation occurred in the magnetic sheet may result in deterioration in mechanical strength or magnetic characteristics. 
     To address the above problems, Japanese Patent Laid-Open No. 2016-12656 realizes stable manufacturing by preparing the composition and a grain growth inhibitor in a magnetic sheet composed of Ni—Zn ferrite. However, the invention disclosed in Japanese Patent Laid-Open No. 2016-12656 is merely a technique applied to Ni—Zn ferrite, and cannot suppress occurrence of cracks or deformation when applied to Mn—Zn ferrite. 
     Although Mn—Zn ferrite has better magnetic characteristics than Ni—Zn ferrite, it is extremely difficult to manufacture because accurate atmosphere control is required during firing. In particular, when the thickness of the magnetic sheet is thin, the manufacturing difficulty increases, and defects such as cracks and deformation are more prone to occur after firing. 
     BRIEF SUMMARY OF INVENTION 
     The present invention has been attained in view of above situation, and the object is to provide a magnetic sheet which can be stably manufactured and has excellent magnetic characteristics, a coil module having the magnetic sheet, and a non-contact power supply device. 
     In order to attain the above object, the magnetic sheet according to the present invention includes a sheet-shaped sintered body, 
     the sintered body contains Mn—Zn ferrite as a main component, and 
     a difference (|σs−σc|) between a residual stress (σs) in a surface layer portion and a residual stress (σc) in a central portion at a cross section of the sintered body is 20 MPa or less in absolute value. 
     As a result of intensive studies, the present inventors have found that the magnetic sheet of the present invention having the above-mentioned characteristics can be industrially and stably manufactured. In addition, the magnetic sheet according to the present invention has excellent magnetic characteristics (initial permeability and magnetic loss). 
     In particular, the magnetic sheet according to the present invention can be industrially and stably manufactured even when the magnetic sheet is made thin, and an average thickness of the sintered body can be set to 2500 μm or less. 
     In addition, preferably, the difference between the residual stress in the surface layer portion and the residual stress in the central portion at the cross section of the sintered body is 10 MPa or less in absolute value. The smaller the difference of residual stress between the surface layer portion and the central portion, the higher the manufacturing stability and the better the magnetic characteristics. 
     In addition, preferably, the residual stress in the surface layer portion and the residual stress in the central portion are both compressive stresses at the cross section of the sintered body. 
     The magnetic sheet according to the present invention can be manufactured by, for example, the manufacturing method shown below. That is, the method for manufacturing the magnetic sheet according to the present invention includes a firing step in which a green sheet containing ferrite powder is mounted on a setter and sintered. Besides, a porosity of the setter used in the firing step is preferably less than 30%, and more preferably 4% to 27.5%. In addition, an arithmetic mean roughness (Ra) of the surface of the setter is preferably 2.0 μm or less, and more preferably 1.5 μm or less. 
     The magnetic sheet according to the present invention can be used in fields such as short-range wireless communication, wireless power transmission, electromagnetic wave protection, and the like. In particular, the magnetic sheet according to the present invention can be preferably used as a constituent of a coil module in a non-contact power supply device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a magnetic sheet according to an embodiment of the present invention; 
         FIG. 2  is a schematic perspective view showing a mounted state of the magnetic sheet in a firing step; 
         FIG. 3  is a schematic perspective view showing a coil module according to an embodiment of the present invention; and 
         FIG. 4  is a schematic view showing a non-contact power supply device according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Hereinafter, the present invention is described in detail based on the embodiments shown in the figures. 
     First Embodiment 
     In the first embodiment, a magnetic sheet  2  according to an embodiment of the present invention is described with reference to  FIG. 1  and  FIG. 2 . The magnetic sheet  2  of the present embodiment is comprised of a sheet-shaped ferrite sintered body. 
     A shape of the magnetic sheet  2  in a plan view is not particularly limited, and may be, for example, a circular shape, an elliptical shape, a rectangle shape, a polygonal shape, or the like. The shape is appropriately determined according to the use of the magnetic sheet  2 . A dimension of the magnetic sheet  2  in a plan view is not particularly limited either, and the area of a front surface  2   a  or a back surface  2   b  can be set to 2000 mm 2  or more, and preferably 5000 mm 2  to 10000 mm 2 . 
     On the other hand, an average thickness T 0  of the magnetic sheet  2  shown in  FIG. 1  can be set to 2500 μm or less, preferably 50 μm to 2500 μm, more preferably 100 μm to 2000 μm, and further preferably 100 μm to 1000 μm. The thickness T 0  of the magnetic sheet  2  is obtained by image analysis of a cross section as shown in  FIG. 1 , and it is preferable to measure the thicknesses of at least 5 points or more and calculate the average value thereof as the thickness T 0 . 
     Moreover, because the magnetic sheet  2  can have the above-mentioned dimensions, the ratio (area/volume) of an area of the front surface  2   a  or the back surface  2   b  to a volume of the magnetic sheet  2  can be set to 0.4 mm −1  or more, preferably 0.5 mm −1  or more, and more preferably 1 mm −1  or more. 
     The magnetic sheet  2  of the present embodiment contains Mn—Zn ferrite as a main component. The Mn—Zn ferrite preferably contains iron oxide in an amount of 51 to 58 mol % in terms of Fe 2 O 3 , zinc oxide in an amount of 3 to 18 mol % in terms of ZnO, and the balance is composed of manganese oxide (MnO). The magnetic sheet  2  of the present embodiment has excellent magnetic characteristics by forming the Mn—Zn ferrite being the main component with the above composition. 
     In addition, the magnetic sheet  2  can contain a subcomponent in addition to the above-mentioned main component. The subcomponent includes, for example, zirconium oxide (ZrO 2 ), silicon oxide (SiO 2 ), calcium oxide (CaO), niobium oxide (Nb 2 O 5 ), vanadium oxide (V 2 O 5 ), tantalum oxide (Ta 2 O 5 ), nickel oxide (NiO), titanium oxide (TiO 2 ), tin oxide (SnO 2 ), cobalt oxide (CoO), and the like. The subcomponent may be one selected from the above oxides, or a combination of a plurality of the above oxides. 
     Moreover, in the above description, the content of each subcomponent is preferably within a range shown below with respect to 100 parts by weight of the main component. That is, zirconium oxide is 0.005 to 0.04 part by weight in terms of ZrO 2 , silicon oxide is 0.005 to 0.02 part by weight in terms of SiO 2 , calcium oxide is 0.02 to 0.2 part by weight in terms of CaO, niobium oxide is 0.005 to 0.075 part by weight in terms of Nb 2 O 5 , vanadium oxide is 0.005 to 0.05 part by weight in terms of V 2 O 5 , tantalum oxide is 0.005 to 0.15 part by weight in terms of Ta 2 O 5 , nickel oxide is 0.05 to 1 part by weight in terms of NiO, titanium oxide is 0.01 to 0.6 part by weight in terms of TiO 2 , tin oxide is 0.05 to 0.8 part by weight in terms of SnO 2 , and cobalt oxide is 0.02 to 0.4 part by weight in terms of CoO. By containing the subcomponents in the above range, the magnetic characteristics can be further improved. 
     Besides, in addition to the above-mentioned oxide subcomponent, the magnetic sheet  2  may also contain a metal component or a compound containing a typical element or a transition metal element as shown below. The typical element includes boron (B), carbon (C), phosphorus (P), sulfur (S), chlorine (Cl), arsenic (As), selenium (Se), bromine (Br), tellurium (Te), iodine (I), lithium (Li), sodium (Na), magnesium (Mg), aluminum (Al), potassium (K), gallium (Ga), germanium (Ge), strontium (Sr), cadmium (Cd), indium (In), tin (Sn), antimony (Sb), barium (Ba), lead (Pb), bismuth (Bi), and the like. On the other hand, the transition metal element includes scandium (Sc), titanium (Ti), chromium (Cr), nickel (Ni), copper (Cu), yttrium (Y), zirconium (Zr), molybdenum (Mo), palladium (Pd), silver (Ag), hafnium (Hf), tantalum (Ta), and the like. The content of the metal component or the compound is not particularly limited, and is preferably in the range of about 0.0001 to 0.1 parts by weight with respect to 100 parts by weight of the main component. The subcomponent of the metal component or the compound may be intentionally added, or may be contained as an inevitable impurity in raw materials. 
     The contents of the main component and the subcomponent of the magnetic sheet  2  can be measured by various analysis methods such as component analysis by electron probe microanalyzer (EPMA), composition analysis by X-ray diffraction (XRD) or electron diffraction (ED), and various ICP analysis including laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and the analysis method is not particularly limited. Preferably, the content ratio of oxides (iron oxide, zinc oxide, and manganese oxide) constituting the main component is analyzed by EPMA. 
     Note that, as described above, the magnetic sheet  2  of the present embodiment is a sheet-shaped sintered body, and thus the inside of the magnetic sheet  2  is substantially free of resin components. “Substantially free” of resin components means that the resin component is 40 ppm by weight or less with respect to 100% by weight of the main component. It should be note that, a magnetic sheet containing a resin is more flexible than a sintered sheet, but tends to be inferior in magnetic characteristics such as magnetic permeability. 
     Specifically, as the characteristics of the magnetic sheet  2  being a sintered body, an initial permeability pi is preferably 1800 or more, more preferably 2000 or more, and further preferably 2200 or more. In addition, the magnetic loss (core loss) of the magnetic sheet  2  is preferably 1000 kW/m 3  or less, more preferably 800 kW/m 3  or less, and further preferably 600 kW/m 3  or less. 
     The magnetic sheet  2  of the present embodiment is characterized in that the residual stress is controlled by a predetermined condition in an arbitrary cross section as shown in  FIG. 1 . Specifically, in the cross section of the magnetic sheet  2 , when the residual stress in the surface layer portion is set to σs and the residual stress in the central portion is set to σc, the absolute value of the difference (|σs−σc|) between the residual stress σs in the surface layer portion and the residual stress σc in the central portion is 20 MPa or less, and more preferably 10 MPa or less. 
     Note that, in the present embodiment, the arbitrary cross section of the magnetic sheet  2  is not particularly limited as long as it is a cross section in which the thickness direction (Z-axis direction) of the magnetic sheet  2  is exposed. However, preferably, the arbitrary cross section for measuring the residual stress is an X—Z cross section or a Y—Z cross section at substantially the center of the X-Y plane of the magnetic sheet  2 . 
     In addition, in the present embodiment, the surface layer portion of the magnetic sheet  2  means a portion which is varied depending on the thickness T 0  of the magnetic sheet  2  but is within a range of about 10 μm to 50 μm in the depth direction from the outermost surface of the magnetic sheet  2 . On the other hand, the central portion of the magnetic sheet  2  means the center in the thickness direction in the arbitrary cross section, that is, the portion where the distance from the outermost surface is (½)×T 0 . Here, in the above description, the outermost surface is not distinguished from front surface and back surface, and may be either the front surface  2   a  side or the back surface  2   b  side of the magnetic sheet  2 . However, the residual stress in the surface layer portion is measured on both the front surface  2   a  side and the back surface  2   b  side, and the one side ( 2   a  or  2   d ) having the larger difference from the central portion is adopted for the calculation of |σs−σc|. 
     In the present embodiment, the residual stress G in the cross section of the magnetic sheet  2  is measured by sin 2 ψ method using XRD. In the sin 2 ψ method, the cross section of the magnetic sheet  2  in the thickness direction is irradiated with X-rays at a plurality of incident angles ψ, and a peak position  2 θ of the diffraction line at each incident angle ψ is measured. In the present embodiment, the peak position  2 θ of the diffraction line to be measured is preferably, for example, the diffraction angle on the ( 533 ) plane of Fe 2 O 3 . If residual stress exists, the peak position  2 θ of a specific diffraction line changes along with a change in the incident angle ψ of the X-ray. The residual stress σ is calculated as the inclination obtained by linear regression using the least squares method when the measurement results are plotted with the vertical axis representing the peak position  2 θ and the horizontal axis representing sin 2 ψ. 
     The residual stress σ measured by the sin 2 ψ method is a compressive stress when expressed as a negative number, and is a tensile stress when expressed as a positive number. In the magnetic sheet  2  of the present embodiment, it is preferable that the residual stress σs in the surface layer portion and the residual stress σc in the central portion are both compressive stresses. 
     Moreover, the sample used in the measurement of the residual stress σ is prepared by, for example, embedding the magnetic sheet  2  in a resin, cutting it at an arbitrary cross section, and mirror-polishing the cross section. In particular, during mirror polishing, as a finishing operation, it is preferable to perform precision polishing with a buff while dropping diamond paste to remove processing distortion during sample preparation. 
     In addition, in the measurement of the residual stress σ, an irradiation diameter D 1  of the X-rays to be irradiated (that is, a diameter D 1  of each of a measurement point  4   a  and a measurement point  4   b  shown in  FIG. 1 ) depends on the specifications of the XRD device used, and is preferably 10 μm to 50 μm. Depending on the irradiation diameter D 1  (also called collimator diameter in the device setting), the irradiated X-rays diffuse to some extent outside the irradiation diameter D 1 . Therefore, in particular, it is preferable to set the irradiation diameter D 1  to a value as small as possible (for example, 10 μm) when the average thickness T 0  of the magnetic sheet  2  is as thin as 500 μm or less. 
     Furthermore, in the measurement of the residual stress σs in the surface layer portion, it is preferable to set the surface layer side measurement point  4   a  (see  FIG. 1 ) to be irradiated with X-rays at a position separated from the outermost surface by a predetermined distance D 2 . The reason is that if the surface layer side measurement point  4   a  is set directly below the outermost surface, it is difficult to obtain accurate measurement data due to the diffusion of the irradiated X-rays. The predetermined distance D 2  is a perpendicular distance from the outermost surface to the center of the surface layer side measurement point  4   a , and is specifically set to 10 μm to 50 μm. On the other hand, in the measurement of the residual stress  6   c  in the central portion, it is preferable to set the irradiation position at a position where the perpendicular distance from the outermost surface to the center of the center side measurement point  4   b  is approximately (½)×T 0 . 
     More specifically, when the average thickness T 0  of the magnetic sheet  2  is 50 μm, the irradiation diameter D 1  is set to 10 μm, and then the surface layer side measurement point  4   a  is set at a position where the predetermined distance D 2  is 10 μm to 15 μm. In addition, in this case, the center of the center side measurement point  4   b  is set at a position having a perpendicular distance of 25 μm from the outermost surface. 
     Further, it is preferable to measure the residual stresses of at least 5 points and calculate the average value thereof as the residual stresses σs and σc. 
     Next, an example of the method for manufacturing the magnetic sheet  2  shown in  FIG. 1  is described. 
     First, a ferrite raw material constituting the magnetic sheet  2  is prepared. Specifically, the raw material of the main component and the raw material of the subcomponent are weighed so as to have the above-mentioned predetermined composition ratio and then mixed to obtain a raw material mixture. The mixing method is not particularly limited, and may include wet mixing with a ball mill and dry mixing with a dry mixer. Moreover, the average particle size of each raw material is preferably about 0.1 to 3 μm. 
     Iron oxide (α-Fe 2 O 3 ), zinc oxide (ZnO), manganese oxide (Mn 3 O 4 ), or a composite oxide thereof can be used as the raw material of the main component. In addition, various compounds that become the above oxides or composite oxides by firing can also be used. The compound that becomes the above oxide by firing includes a metal simple substance, carbonate, oxalate, nitrate, hydroxide, halide, an organic metal compound, and the like. Moreover, although the content of manganese oxide in the main component is converted in terms of MnO, Mn 3 O 4  is preferably used as the raw material. 
     In addition, similar to the main component, not only an oxide but also a composite oxide or a compound which becomes an oxide after firing can be used as the raw material of the subcomponent. Alternatively, the raw material of the subcomponent may be added in the first mixing step as described above, or may be added after a calcination step described later. 
     Next, the raw material mixture obtained in the above mixing step is calcined to obtain a calcined material. As the conditions for this calcination, it is preferable that the holding temperature is 800 to 1100° C., and the holding time is 1 to 3 hours. In addition, the processing atmosphere during calcination may be an air atmosphere or an atmosphere having a higher oxygen partial pressure than in the air atmosphere. 
     After the calcination step, the calcined material is pulverized to obtain the pulverized material. The pulverization is performed in order to break the aggregation of the calcined material and obtain powder having appropriate sinterability. When the calcined material forms a large lump, it is preferable to perform coarse crush and then perform wet pulverization using a ball mill, an attritor, or the like. In addition, an average particle size of the pulverized material is preferably about 0.1 to 2 μm. 
     Subsequently, a sheet-shaped molded body (green sheet) is manufactured using the pulverized material obtained in the previous step. The method for obtaining the green sheet is not particularly limited, and may include various molding methods such as a doctor blade method, extrusion molding, powder compression molding, and the like. For example, it is preferable to perform molding using the doctor blade method when the average thickness T 0  of the magnetic sheet  2  is 300 μm or less, and perform extrusion molding when the average thickness T 0  is 300 μm or more. 
     To obtain the green sheet using the doctor blade method, first, the pulverized material is kneaded together with a binder, a solvent, a dispersant, or the like to obtain a ferrite paste. The type and the content of the binder, the solvent, or the like in the ferrite paste are optional, and known specifications can be adopted. Then, the ferrite paste is applied onto a carrier tape to form a sheet and thereby obtain a green sheet. 
     On the other hand, to obtain the green sheet using extrusion molding, first, the pulverized material is kneaded together with a binder to obtain a ferrite kneaded soil. The type and the content of the binder used at this time are optional, and known specifications can be adopted. Then, a green sheet is obtained by introducing this ferrite kneaded soil into a mold of an extruder, applying pressure to the ferrite kneaded soil while appropriately heating it and extruding it through a gap having a predetermined cross-sectional shape. 
     The magnetic sheet  2  can be obtained by appropriately drying the green sheet obtained in the above step and punching it to obtain product dimensions, and then firing the green sheet. As shown in  FIG. 2 , in the firing step, after a green sheet  3  is mounted on a setter  6  and the setter  6  is put into a firing furnace, the green sheet  3  is sintered by heating on setter while controlling the atmosphere inside the furnace and the temperature inside the furnace. In the manufacture of the magnetic sheet  2  according to the present embodiment, firing is a key step. In particular, the difference of residual stress |σs−σc| in the cross section of the magnetic sheet  2  is controlled by adjusting various conditions such as the holding temperature, the holding time, the oxygen partial pressure in the atmosphere during firing, and the characteristics of the setter used. Hereinafter, preferable firing conditions are described. 
     First, as the setter  6 , ceramic materials such as alumina (Al 2 O 3 ), zirconia (ZrO 2 ), mullite (3Al 2 O 3 /2SiO 2 -2Al 2 O 3 /SiO 2 ) and the like can be used, and it is preferable to use high-purity alumina having a purity of 99% or more. In addition, the setter  6  is preferably denser and smoother. Specifically, the porosity of the setter  6  is preferably less than 30%, and more preferably 4% to 27.5%. Besides, the surface roughness of the setter  6  is preferably 2.0 μm or less, and more preferably 1.5 μm or less, in terms of arithmetic average roughness Ra. As described above, the difference of residual stress |σs−σc| at the cross section can be reduced by using the setter  6  which is dense and has a smooth surface  6   a.    
     In addition, the firing furnace may be any furnace that can control the atmosphere inside the furnace, and a batch type firing furnace or a continuous type firing furnace can be used. Besides, the holding temperature during firing is preferably 1150° C. to 1300° C., and the holding time is preferably 0.5 to 3 hours. Moreover, in the above description, the holding temperature means a temperature at which the material temperature (the temperature of the sheet itself) is stable at the highest point. 
     Furthermore, the oxygen partial pressure is preferably controlled so that the maximum oxygen concentration in the atmosphere inside the furnace is preferably 6.0 vol % or less, and more preferably 0.5 vol % to 6.0 vol %. In particular, in the present embodiment, it is preferable that the oxygen partial pressure is controlled so that the oxygen concentration is within the above range not only in a temperature holding process, but also in a temperature rising process from 900° C. to the holding temperature and a temperature lowering process from the holding temperature to 900° C. 
     Alternatively, in addition to the above-mentioned conditions, a treatment for increasing the vapor pressure of zinc in the atmosphere may be carried out during firing. The treatment includes, for example, a treatment of placing a bulk of zinc oxide on the setter  6 , a treatment of covering a front surface  3   a  of the green sheet  3  with zinc oxide powder, a treatment of covering the green sheet  3  being a product with a ferrite box and firing, and the like. 
     In addition, in  FIG. 2 , one green sheet  3  is mounted on one setter  6 . Alternatively, a plurality of green sheets  3  may be mounted at the same time. However, when a plurality of green sheets  3  are mounted at the same time, it is preferable that the plurality of green sheets  3  are arranged side by side on the X-Y plane instead of being laminated in the Z axis. The reason is that in the case of Mn—Zn ferrite, the plurality of green sheets may be fused to each other when they are laminated and fired. 
     Furthermore, a protective layer may be formed on the front surface  2   a  or/and the back surface  2   b  (the back surface  2   b  is the surface that is in contact with the setter  6 ) after the fired magnetic sheet  2  is removed from the setter  6 . However, it is preferable that the surface  2   a  after firing and the back surface  2   b  after firing can be used as fired surfaces without performing machining such as blasting, polishing, cutting, or the like. 
     The above is the manufacturing method of the magnetic sheet  2  in the present embodiment. Moreover, in the manufacture of the magnetic sheet  2 , the following processing may be performed in addition to the above steps. For example, the magnetic sheet  2  may be divided into multiple small pieces to improve impact resistance. In this case, the green sheet  3  before firing is precut to form grid-like grooves. A resin film such as polyethylene terephthalate (PET) is laminated and attached to the front surface  2   a  and the back surface  2   b  of the magnetic sheet  2  obtained after firing the green sheet  3 . Then, by applying a roller to the magnetic sheet  2  to which the film is attached, the magnetic sheet  2  is divided into regular small pieces while being sandwiched between the films. 
     Summary of the First Embodiment 
     The magnetic sheet  2  of the present embodiment is comprised of a sheet-shaped sintered body and contains Mn—Zn ferrite as a main component. Besides, in the magnetic sheet  2 , the difference of residual stress (|σs−σc|) between the surface layer portion and the central portion at the cross section is 20 MPa or less, and more preferably 10 MPa or less. 
     Conventionally, Mn—Zn ferrites have been practically used as massive ferrite cores of drum type, E type, I type, or the like, but it has been extremely difficult to put the Mn—Zn ferrites into practical use for a sheet. The reason is that in the case of the sheet shape, various defects occur in the manufacturing process, and it is difficult to manufacture by the conventional technique. The possible reasons are as follows. 
     For example, in the case of a sheet-shaped ferrite sintered body, because the ratio of the plane (area of the front surface  2   a  and the back surface  2   b  of the magnetic sheet  2 ) to the volume is large, deformation such as warpage or undulations and internal cracks are prone to occur after firing. In addition, in the case of the Mn—Zn ferrite sheet, it is considered that deterioration of magnetic characteristics and fusion defect are prone to occur due to occurrence of dezincification on the front surface  3   a  and the back surface  3   b  of the green sheet  3  during the firing process. 
     More specifically, it is considered that zinc oxide contained in the green sheet  3  is reduced to metallic zinc on the front surface  3   a  of the green sheet  3 . Because metallic zinc has a lower sublimation temperature than zinc oxide, the generated metallic zinc easily volatilizes on the front surface  3   a  of the green sheet  3  (dezincification on the front surface  3   a ), leading to a deterioration in magnetic characteristics. On the other hand, it is considered that on the back surface  3   b  of the green sheet  3 , the zinc component on the green sheet  3  side diffuses into the setter  6  and reacts with the ceramic component of the setter  6  during firing (dezincification on the back surface  3   b ). When the dezincification occurs on the back surface  3   b , the obtained magnetic sheet  2  and the setter  6  are fused, leading to defects such as deformation, cracks, breakage, and the like. 
     The deterioration of magnetic characteristics and fusion defect as described above scarcely occur in Ni—Zn ferrite and are defect modes peculiar to Mn—Zn ferrite. The reason is considered to be that Ni—Zn ferrite can be fired in the air atmosphere, but Mn—Zn ferrite needs to be fired under low oxygen partial pressure in order to ensure magnetic characteristics. In addition, in the case of a massive ferrite core, even if an abnormal portion is formed on the surface of the core due to the dezincification, the abnormal portion can be removed by polishing or the like. However, in the case of a sheet-shaped ferrite sheet, because the thickness is thin, the abnormal portion cannot be removed by polishing or the like. The reason is that the sheet is damaged when machining is applied. 
     As a result of intensive studies on the above-mentioned problems, the present inventors have found that the difference of residual stress at the cross section of the magnetic sheet  2  is varied depending on the shrinkage rate of the green sheet  3  and the degree of occurrence of the dezincification on the front surface  3   a  and the back surface  3   b  during the firing process. In addition, the present inventors have found that the difference of residual stress can be controlled by the firing conditions and the characteristics of the setter  6  used during firing. Furthermore, the present inventors have found that when the residual stress of the magnetic sheet  2  is 20 MPa or less, the above-mentioned deformation defects, crack defects, fusion defects and the like can be suppressed in the manufacturing process and the mass productivity is good. 
     The reason why the mass productivity is good is not exactly clear, and it is considered that one of the reasons is, for example, the characteristics of the setter  6  used during firing. In general, it has been considered preferable to use a material having a rough surface and a high porosity (30% or more) as the setter  6  so as to reduce the contact area between the green sheet and the setter. 
     However, according to the experiments of the present inventors, it becomes clear that the difference of residual stress becomes large when a setter having a high porosity and a rough surface is used. On the other hand, when the setter  6  having high denseness and smoothness is used as in the present embodiment, the difference of residual stress |σs−σc| becomes as small as 20 MPa or less. It is considered that when the porosity of the setter is high and the surface is rough, the zinc component is easily diffused deep inside the setter, and the dezincification on the back surface  3   b  is promoted. On the other hand, it is considered that when the setter  6  has high denseness and smoothness, the diffusion of the zinc component is kept within a necessary minimum range on the outermost surface of the setter  6 . It is considered that the magnetic sheet  2  of the present embodiment has good mass productivity (that is, the magnetic sheet  2  can be industrially and stably manufactured) because the fusion defects and the like are suppressed in the manufacturing process based on the above principle. 
     Moreover, conventionally, measures to prevent poor fusion to the setter  6  include a treatment of coating the surface  6   a  of the setter  6  with zinc oxide or zirconium oxide as a release agent, a treatment of pre-containing zinc oxide and zirconium oxide inside the setter  6 , and the like. However, in the manufacture of a sheet-shaped ferrite sintered body such as the magnetic sheet  2 , the above-mentioned conventional measures are insufficient. The reason is that when the ferrite sintered body has a sheet shape, the ratio of the area of the back surface  3   b  to the volume of the green sheet  3  is large, and the dezincification on the back surface  3   b  is activated. In addition, deformation or defect occurs in the magnetic sheet  2  when zinc oxide or zirconium oxide is present on the surface  6   a  of the setter  6 . Therefore, in the case of a thin magnetic sheet, reduction of fusion defects cannot be achieved by the above-mentioned conventional technique, but can be preferably realized by the manufacturing method of the present embodiment. 
     In addition, in the magnetic sheet  2  of the present embodiment, as a result of the suppression of various defects as described above, deterioration of the magnetic characteristics has been avoided, thus enhancing the initial permeability and reducing the magnetic loss. In particular, the magnetic sheet  2  has good mass productivity and excellent magnetic characteristics even when the average thickness T 0  is reduced to 2500 μm or less or the sheet area is widened to 2000 mm 2  or more. In other words, the magnetic sheet  2  of the present embodiment can be made thinner or wider. 
     The magnetic sheet  2  of the present embodiment can be mounted on a smartphone, a tablet, a non-contact IC card, or the like for electromagnetic wave blocking. In addition, the magnetic sheet  2  can also be used as a noise filter or an electromagnetic wave absorber, and further can be preferably used as a constituent of a non-contact power supply device by being incorporated in a coil module. 
     Second Embodiment 
     In a second embodiment, a coil module  10  ( FIG. 3 ) and a non-contact power supply device  100  ( FIG. 4 ) having the magnetic sheet  2  are described as an example of the use of the magnetic sheet  2  described in the first embodiment. Note that, the configuration in the second embodiment common to the first embodiment is designated by the similar signs, and the description thereof is omitted. 
       FIG. 3  is a schematic perspective view showing the coil module  10  according to the second embodiment. As shown in  FIG. 3 , the coil module  10  has the magnetic sheet  2  and a coil  12 . 
     The coil  12  is a flat plate-shaped spiral coil. In  FIG. 3 , the outer edge shape of the coil  12  in a plan view is a rounded square shape, but the outer edge shape of the coil  12  is not limited thereto, and may be a circular shape, an elliptical shape, a polygonal shape, or the like. 
     In addition, the coil  12  is formed by winding a conductive wire such as an insulation-covered copper wire or aluminum wire in a plane and spiral shape. The cross-sectional shape of the conductive wire to be used is not particularly limited, and may be circular, elliptical, triangular, quadrangular, or the like. Note that, the method of forming the coil  12  is not limited to the above method; alternatively the coil  12  may also be formed by a thin film method. The thin film method is a method of forming a coil by, for example, metal foil, conductive paste, plating transfer, sputtering, vapor deposition, or screen printing. 
     In addition, as shown in  FIG. 3 , a pair of lead terminals  13  is electrically connected to the coil  12 . The lead terminals  13  are formed by pulling out the front end and the rear end of the conductive wire constituting the coil  12  to the outside of the coil  12 . Moreover, the pulled-out position of the lead terminal  13  is not particularly limited. 
     In the second embodiment, the coil  12  is formed on the magnetic sheet  2  described in the first embodiment via an adhesive layer (not shown) such as an adhesive or a double-sided tape. The thickness of the adhesive layer is not particularly limited and can be, for example, 10 μm to 100 μm. Moreover, a resin film (not shown) may be formed as a protective layer on the front surface  2   a  and the back surface  2   b  of the magnetic sheet  2 . 
     In addition, the coil module  10  may have an alignment magnet in addition to the above-mentioned constituents. The alignment magnet is placed at, for example, the center of the coil  12 . Furthermore, in  FIG. 3 , the coil module  10  is shown to have a single coil  12 . Alternatively, the coil module  10  may also be a coil array type module formed by combining a plurality of coils  12 . 
     The coil module  10  having the magnetic sheet  2  is preferably used in the non-contact power supply device  100  as shown in  FIG. 4 . Hereinafter, the non-contact power supply device  100  according to the second embodiment is described. 
     The non-contact power supply device  100  mainly has a power transmission side coil module  10   a  and a power receiving side coil module  10   b . The power transmission side coil module  10   a  and the power receiving side coil module  10   b  each have coils  12   a  and  12   b  and the magnetic sheet  2 , and correspond to the coil module  10  shown in  FIG. 3 . Here, in the present embodiment, the magnetic sheet  2  can be mounted on both the power transmission side and the power receiving side as described above, and may also be mounted only on either the power transmission side or the power receiving side. Particularly, the magnetic sheet  2  is preferably used for the power transmission side coil module  10   a.    
       FIG. 4  shows a state in which the power transmission side coil module  10   a  and the power receiving side coil module  10   b  are positioned in order to perform non-contact power supply. As shown in  FIG. 4 , during non-contact power supply, the power transmission side coil  12   a  and the power receiving side coil  12   b  are arranged so as to face each other at a predetermined interval. And, the magnetic sheet  2  is arranged so as to cover the back surfaces of the coils  12   a  and  12   b.    
     The power transmission side coil module  10   a  is incorporated in a charger side. Besides, a power transmission side internal circuit  20   a  including an AC/DC converter, an LC resonance circuit and the like is connected to the coil  12   a  on the power transmission side via the lead terminal  13  (not shown). Furthermore, a power supply  22  is connected to the power transmission side internal circuit  20   a . By having the above-mentioned configuration, the coil  12   a  on the power transmission side is supplied with an alternating voltage having a predetermined frequency during power supply. When an alternating voltage is supplied to the coil  12   a  on the power transmission side, a magnetic field is generated around the coil  12   a.    
     On the other hand, the power receiving side coil module  10   b  is incorporated in a charging object such as a mobile terminal or a PC. Similar to the power transmission side, in the power receiving side coil module  10   b , a power receiving side internal circuit  20   b  including a converter, a resonance circuit and the like is connected to the coil  12   b  on the power receiving side via the lead terminal  13  (not shown). Besides, a secondary battery  24  is connected to the power receiving side internal circuit  20   b . When the coil  12   b  on the power receiving side is brought close to the magnetic field generated on the power transmission side, an alternating current is generated in the coil  12   b  on the power receiving side by electromagnetic induction. The alternating current generated on the power receiving side is converted into a direct current via the power receiving side internal circuit  20   b  and used for charging the secondary battery  24 . 
     In the non-contact power supply device  100  as described above, the magnetic sheet  2  is arranged mainly for preventing leakage of magnetic flux and forming a magnetic path. As described in the first embodiment, the magnetic sheet  2  according to the present invention has excellent magnetic permeability characteristics, and thus the non-contact power supply device  100  having the magnetic sheet  2  can efficiently converge the magnetic flux, and the power transmission efficiency is improved. 
     In addition, in recent years, the non-contact power supply device  100  as shown in  FIG. 4  is required to have high output and small size. In order to satisfy these requirements, heat generation during power supply is especially problematic. As described in the first embodiment, the magnetic sheet  2  according to the present invention has a small magnetic loss, and the heat generation of the non-contact power supply device  100  can thus be reduced. 
     As described above, the non-contact power supply device  100  according to the present embodiment has the magnetic sheet  2  which has excellent magnetic characteristics, and thus the power transmission efficiency can be improved and the size and thickness can be reduced. 
     Although the embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications can be made within the scope of the present invention. For example, the non-contact power supply device  100  may have a sensor for detecting the position of the charging object (that is, the power receiving side coil  12   b ), a drive device for moving the power transmission side coil  12   a  to the position of the power receiving side coil  12   b , or the like, in addition to the above-mentioned constituents. 
     EXAMPLE 
     Hereinafter, the present invention is further described based on detailed examples, but the present invention is not limited to these examples. 
     Experiment 1 
     In Experiment 1, the magnetic sheets  2  according to Examples 1 to 3 were prepared by adjusting the firing conditions and the like so that the difference of residual stress at the cross section is within a predetermined range. In each of Examples 1 to 3, 500 magnetic sheets  2  were prepared, and manufacturing yield rate, initial permeability pi, and magnetic loss Pcv were evaluated. The detailed experimental conditions are described below. 
     Examples 1 to 3 
     First, the raw materials of the main component and the raw materials of the subcomponents were weighed at a predetermined blending ratio and mixed, and then calcined at 900° C. for 2 hours in an air atmosphere. At this time, powders having an average particle size of 0.1 to 3.0 μm were used as the raw materials of the main component, and were weighted so that the final main component composition was Fe 2 O 3 : 54.0 mol %, ZnO: 9.0 mol %, and the balance MnO. In addition, SiO 2 , CaCO 3 , Nb 2 O 5 , and V 2 O 5  were added as the subcomponents and were blended so that the final content of each subcomponent was SiO 2 : 0.01 part by weight, CaO: 0.08 part by weight, Nb 2 O 5 : 0.02 part by weight, V 2 O 5 : 0.01 part by weight with respect to 100 parts by weight of the main component. 
     Next, the calcined material obtained in the previous step was pulverized with a ball mill for 14 hours to obtain a pulverized material having an average particle size of 1.0 to 2.0 μm. Then, the pulverized material was kneaded together with a binder, pure water and the like to obtain ferrite kneaded soil, and green sheets were formed by an extrusion molding method using this ferrite kneaded soil. Moreover, in this molding step, the diameter of the mold was adjusted so that the average thickness of the magnetic sheet after firing is 500 μm. 
     Subsequently, the green sheets obtained in the above step were placed on the surface of the setters and then the setters were put into a continuous firing furnace for firing. Moreover, in Experiment 1, the experiments were carried out by changing the firing conditions in Examples 1 to 3. Hereinafter, the firing conditions of Examples 1 to 3 were described. 
     In Example 1, firing was performed under condition 1 shown in Table 1. Specifically, in Example 1, the holding temperature was set to 1200° C. to 1250° C. and the holding time was set to 1 to 2 hours during firing. At this time, the maximum oxygen concentration in the atmosphere was controlled to a range of 0.01 to 4.0 vol % in the temperature rising process from 900° C. to the holding temperature, the temperature holding process at 1200° C. to 1250° C., and the temperature lowering process from the holding temperature to 900° C. In addition, high-purity alumina substrates having a porosity of 26 to 29% and a surface roughness Ra of 1.7 to 2.0 μm were used as the setters. 
     On the other hand, in Example 2, firing was performed under condition 2 shown in Table 1. Specifically, in Example 2, a setter having different characteristics from that in Example 1 was used. That is, in Example 2, high-purity alumina substrates having a porosity of 20 to 23% and a surface roughness Ra of 1.4 to 1.6 μm were used. Other conditions not mentioned above (holding temperature, holding time, or the like) were the same as in Example 1. 
     Furthermore, in Example 3, firing was performed under condition 3 shown in Table 1. Specifically, in Example 3, a setter having higher denseness and smoothness than in Examples 1 and 2 was used. That is, in Example 3, high-purity alumina substrates having a porosity of 12 to 15%, a surface roughness Ra of 1.1 to 1.4 μm, and a purity of 99.5% or more were used. Other conditions not mentioned above (holding temperature, holding time, or the like) were the same as in Example 1. 
     The magnetic sheets according to Examples 1 to 3 were obtained by firing under the above conditions. Moreover, in each example, each of the magnetic sheets had a plate shapes having a dimension of 100 mm×50 mm in the plan view. In addition, the obtained magnetic sheets were evaluated as shown below. 
     Measurement of Residual Stress 
     The residual stress σs in the surface layer portion and the residual stress σc in the central portion at the cross section of the magnetic sheet were measured using XRD (manufactured by Rigaku Co., Ltd.: SmartLab Epsilon). The measurement samples were obtained by cutting the magnetic sheet at a substantially central position on the X-Y plane, embedding it in resin, and mirror-polished the cross section of the magnetic sheet. Further, the residual stresses σs and σc were calculated as the average value of the residual stresses of five points in the cross sections of at least two samples in each example. Furthermore, the residual stresses in the surface layer portion were measured on both the front surface  2   a  side and the back surface  2   b  side, and the one side ( 2   a  or  2   d ) having the larger difference from the central portion was adopted for σs. 
     Calculation of Manufacturing Yield Rate 
     The manufacturing yield rate was calculated in order to evaluate the mass productivity of the magnetic sheets according to Examples 1 to 3. In Experiment 1, 500 magnetic sheets were prepared for each example, and each of the 500 magnetic sheets was subjected to appearance inspection to evaluate the presence or absence of various defects such as deformation defects, crack defects, fusion defects, or the like. Then, the ratio of the number of non-defective products to the number of manufactured products, that is, the manufacturing yield rate was calculated based on the result of the appearance inspection. The manufacturing yield rate is acceptable when it is 97% or more, and in this case, it is determined that the mass productivity is good. 
     Evaluation of Magnetic Characteristics 
     As a preliminary step for evaluating the magnetic characteristics, first, the magnetic sheets of Examples 1 to 3 were cut into a toroidal shape having an outer diameter of 20 mm and an inner diameter of 10 mm by laser processing. Then, a conductive wire having a wire diameter of 0.35 mm was wound around this evaluation sample for 10 turns, and the initial permeability μi (dimensionless quantity) and the magnetic loss Pcv (unit: kW/m 3 ) were measured. 
     The initial permeability μi was measured using an LCR meter (Keysight Technologies, Inc.: E4980A). As for the measurement conditions, the measurement temperature was set to room temperature (25° C.) and the measurement frequency was set to 100 kHz. In addition, the initial permeability μi was measured four times for each of Examples 1 to 3, and the average value was calculated. The initial permeability μi is determined to be good when it is 2000 or more, and is determined to be better when it is 2200 or more, with 1800 as a reference value. 
     The magnetic loss Pcv was measured using a BH analyzer (manufactured by Iwatsu Electric Co., Ltd.: SY-8218). As for the measurement conditions, the measurement temperature was set to room temperature (25° C.), the measurement frequency was set to 100 kHz, and the exciting magnetic flux density was set to 200 mT. The magnetic loss Pcv was also measured 8 times for each example, and the average value was calculated. The magnetic loss Pcv is determined to be good when it is 800 kW/m 3  or less, and is determined to be better when it is 600 kW/m 3  or less, with 1000 kW/m 3  as a reference value. 
     Moreover, in the evaluation of the magnetic characteristics, a plurality of magnetic sheets obtained as non-defective products were selected from among the 500 magnetic sheets manufactured in each example and each comparative example, and were used as evaluation samples. 
     Comparative Example 1 
     In Comparative example 1, unlike Examples 1 to 3, firing was performed under conventional condition 1 shown in Table 1 to prepare a magnetic sheet according to Comparative example 1. Specifically, in Comparative example 1, the holding temperature during firing was set to 1300° C. to 1350° C., and the holding time was set to 3 to 5 hours. In addition, the oxygen partial pressure in the firing atmosphere was controlled only in the temperature holding process and the temperature lowering process, and the maximum oxygen partial pressure at that time was set to 0.03 to 3.0 vol %. In addition, a mullite substrate having a porosity of 38 to 42% and a surface roughness Ra of 3.0 to 4.0 μm was used as the setter. The other experimental conditions in Comparative example 1 were the same as those in Example 1, and the same evaluations as in Example 1 were carried out. 
     Comparative Example 2 
     In Comparative example 2, unlike Examples 1 to 3, firing was performed under conventional condition 2 shown in Table 1 to prepare a magnetic sheet according to Comparative example 2. Specifically, in Comparative example 2, the holding temperature during firing was set to 1300° C. to 1350° C., and the holding time was set to 3 to 5 hours. In addition, the oxygen partial pressure in the firing atmosphere was controlled only in the temperature holding process and the temperature lowering process, and the maximum oxygen partial pressure at that time was set to 0.03 to 3.0 vol %. In addition, an alumina substrate having a porosity of 30 to 32%, a surface roughness Ra of 2.1 to 2.6 μm, and a purity of about 95% was used as the setter. The other experimental conditions in Comparative example 2 were the same as those in Example 1, and the same evaluations as in Example 1 were carried out. 
     Evaluation 1 
     Table 2 shows the evaluation results of Examples 1 to 3 and Comparative examples 1 and 2 in Experiment 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Atmosphere control 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Temperature control 
                 Oxygen 
                 Temperature range having 
                 Characteristics of setter 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Holding 
                 Holding 
                 concentration 
                 controlled oxygen partial 
                 Roughness 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Condition 
                 temperature 
                 time 
                 range 
                 pressure 
                 Material 
                 Porosity 
                 (Ra) 
               
               
                 No. 
                 ° C. 
                 h 
                 vol % 
                 — 
                 — 
                 % 
                 μm 
               
               
                   
               
               
                 Conventional 
                 1300~1350 
                 3~5 
                 0.03~3.0 
                 Temperature holding process, 
                 Mullite 
                 38~42 
                 3.0~4.0 
               
               
                 condition 1 
                   
                   
                   
                 temperature lowering process 
               
               
                 Conventional 
                 1300~1350 
                 3~5 
                 0.03~3.0 
                 Temperature holding process, 
                 Alumina 
                 30~32 
                 2.1~2.6 
               
               
                 condition 2 
                   
                   
                   
                 temperature lowering process 
               
               
                 Condition 1 
                 1200~1250 
                 1~2 
                 0.01~4.0 
                 Temperature rising process, 
                 High-purity 
                 29~26 
                 1.7~2.0 
               
               
                   
                   
                   
                   
                 temperature holding process, 
                 alumina 
               
               
                   
                   
                   
                   
                 temperature lowing process 
               
               
                 Condition 2 
                 1200~1250 
                 1~2 
                 0.01~4.0 
                 Temperature rising process, 
                 High-purity 
                 20~23 
                 1.4~1.6 
               
               
                   
                   
                   
                   
                 temperature holding process, 
                 alumina 
               
               
                   
                   
                   
                   
                 temperature lowing process 
               
               
                 Condition 3 
                 1200~1250 
                 1~2 
                 0.01~4.0 
                 Temperature rising process, 
                 High-purity 
                 12~15 
                 1.1~1.4 
               
               
                   
                   
                   
                   
                 temperature holding process, 
                 alumina 
               
               
                   
                   
                   
                   
                 temperature lowing process 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Average 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 thickness 
                 Residual stress 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Composition of main 
                 of 
                 Surface 
                 Central 
                   
                 Evaluation characteristics 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 components 
                 magnetic 
                 layer 
                 portion 
                 Difference 
                 Yield 
                 Initial 
                 Magnetic 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Fe 2 O 3   
                 ZnO 
                 MnO 
                 sheet (T0) 
                 portion σs 
                 σc 
                 |σs − σc| 
                 rate 
                 permeability 
                 loss 
               
               
                 Sample No. 
                 mol % 
                 mol % 
                 mol % 
                 μm 
                 MPa 
                 MPa 
                 MPa 
                 % 
                 — 
                 kW/m 3   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 54.0 
                 9.0 
                 37.0 
                 500 
                 −34.4 
                 −5.0 
                 29.4 
                 65.2 
                 1597 
                 1210 
               
               
                 example 1 
               
               
                 Comparative 
                 54.0 
                 9.0 
                 37.0 
                 500 
                 −42.8 
                 −22.3 
                 20.5 
                 78.8 
                 1888 
                 898 
               
               
                 example 2 
               
               
                 Example 1 
                 54.0 
                 9.0 
                 37.0 
                 500 
                 −39.2 
                 −19.5 
                 19.7 
                 97.2 
                 1871 
                 754 
               
               
                 Example 2 
                 54.0 
                 9.0 
                 37.0 
                 500 
                 −40.8 
                 −31.2 
                 9.6 
                 98.6 
                 2223 
                 619 
               
               
                 Example 3 
                 54.0 
                 9.0 
                 37.0 
                 500 
                 −37.4 
                 −42.7 
                 5.3 
                 99.2 
                 2524 
                 588 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, in Comparative examples 1 and 2, magnetic sheets having a difference of residual stress |σs−σc| in the cross section exceeding 20 MPa were obtained. In Comparative examples 1 and 2, many crack defects and fusion defects were observed after firing, and the manufacturing yield rate was low. That is, it is difficult to industrially and stably manufacture the magnetic sheets of Comparative examples 1 and 2. 
     On the other hand, in Examples 1 to 3, magnetic sheets having a difference of residual stress |σs−σc| at the cross section of 20 MPa or less were obtained. In Examples 1 to 3, defects scarcely occurred after firing and the manufacturing yield rate was greatly improved as compared with Comparative examples 1 and 2. In particular, when comparing Examples 1 to 3, it could be confirmed that the smaller the difference of residual stress |σs−σc| at the cross section, the higher the manufacturing yield rate. From this result, it was proved that the magnetic sheet having a difference of residual stress of 20 MPa or less, more preferably 10 MPa or less can be industrially and stably manufactured. 
     In addition, all of Examples 1 to 3 satisfied the reference values of the initial permeability μi and the magnetic loss Pcv, and were excellent in mass productivity and magnetic characteristics. In particular, when comparing Examples 1 to 3, it could be confirmed that the smaller the difference of residual stress, the higher the manufacturing yield rate and the more the magnetic characteristics can be increased. 
     Note that, in Examples 1 to 3, the residual stress σs in the surface layer portion and the residual stress σc in the central portion were both compressive stresses. 
     Experiment 2 
     Examples 11 to 16 
     In Experiment 2, magnetic sheets according to Examples 11 to 16 were prepared by changing the average thickness T 0  of the magnetic sheets. In particular, in Examples 11 and 12, the green sheets were formed by the doctor blade method and the thicknesses of the obtained magnetic sheets were adjusted by changing the amount of ferrite paste applied at that time. On the other hand, in Examples 13 to 16, the green sheets were formed by the extrusion molding method and the thicknesses of the obtained magnetic sheets were adjusted by changing the diameter (extrusion diameter) of the mold used at that time. Table 3 shows the average thickness T 0  in each of Examples 11 to 16. Further, the other experimental conditions in Experiment 2 were the same as those in Experiment 1, and in particular, firing was carried out under condition 3 which was the same as in Example 3 of Experiment 1. 
     Comparative Examples 11 to 15 
     In Comparative examples 11 to 15, magnetic sheets having different average thicknesses T 0  were manufactured by changing the molding conditions. Table 3 shows the average thicknesses T 0  in Comparative examples 11 to 15. The other experimental conditions in Comparative examples 11 to 15 were the same as those in Comparative examples 1 and 2 of Experiment 1 (wherein, the firing conditions of Comparative examples 11 and 12 were set to conventional condition 2, and the firing conditions of Comparative examples 13 to 15 were set to conventional condition 1). 
     Evaluation 2 
     Table 3 shows the evaluation results of Examples 11 to 16 and Comparative examples 11 to 15 in Experiment 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Average 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 thickness 
                 Residual stress 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Composition of main 
                 of 
                 Surface 
                 Central 
                   
                 Evaluation characteristics 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 components 
                 magnetic 
                 layer 
                 portion 
                 Difference 
                 Yield 
                 Initial 
                 Magnetic 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Fe 2 O 3   
                 ZnO 
                 MnO 
                 sheet (T0) 
                 portion σs 
                 σc 
                 |σs − σc| 
                 rate 
                 permeability 
                 loss 
               
               
                 Sample No. 
                 mol % 
                 mol % 
                 mol % 
                 μm 
                 MPa 
                 MPa 
                 MPa 
                 % 
                 — 
                 kW/m 3   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 54.0 
                 9.0 
                 37.0 
                 50 
                 −42.0 
                 −10.4 
                 31.6 
                 72.6 
                 1753 
                 1192 
               
               
                 example 11 
               
               
                 Comparative 
                 54.0 
                 9.0 
                 37.0 
                 500 
                 −42.8 
                 −22.3 
                 20.5 
                 78.8 
                 1888 
                 898 
               
               
                 example 12 
               
               
                 Comparative 
                 54.0 
                 9.0 
                 37.0 
                 2500 
                 −32.4 
                 −10.9 
                 21.5 
                 96.6 
                 2424 
                 441 
               
               
                 example 13 
               
               
                 Comparative 
                 54.0 
                 9.0 
                 37.0 
                 2600 
                 −36.2 
                 −12.6 
                 23.6 
                 97.8 
                 2408 
                 454 
               
               
                 example 14 
               
               
                 Comparative 
                 54.0 
                 9.0 
                 37.0 
                 5000 
                 −34.4 
                 26.8 
                 61.2 
                 99.2 
                 2602 
                 400 
               
               
                 example 15 
               
               
                 Example 11 
                 54.0 
                 9.0 
                 37.0 
                 50 
                 −40.4 
                 −33.4 
                 7.0 
                 97.2 
                 2040 
                 952 
               
               
                 Example 12 
                 54.0 
                 9.0 
                 37.0 
                 100 
                 −45.3 
                 −35.7 
                 9.6 
                 98.4 
                 2364 
                 729 
               
               
                 Example 13 
                 54.0 
                 9.0 
                 37.0 
                 500 
                 −37.4 
                 −42.7 
                 5.3 
                 99.2 
                 2524 
                 588 
               
               
                 Example 14 
                 54.0 
                 9.0 
                 37.0 
                 1000 
                 −42.7 
                 −36.4 
                 6.3 
                 99.4 
                 2418 
                 540 
               
               
                 Example 15 
                 54.0 
                 9.0 
                 37.0 
                 2000 
                 −41.0 
                 −46.1 
                 5.1 
                 99.2 
                 2320 
                 517 
               
               
                 Example 16 
                 54.0 
                 9.0 
                 37.0 
                 2500 
                 −35.7 
                 −28.5 
                 7.2 
                 99.0 
                 2198 
                 560 
               
               
                   
               
            
           
         
       
     
     Comparison of the Comparative examples 11-15 shows that the manufacturing yield rate was deteriorated and the magnetic characteristics were also deteriorated in Comparative examples 11 and 12 in which the thickness T 0  is thin. From this result, it could be confirmed that the deformation and cracks after fired and the defects due to fusion were prone to occur as the thickness of the magnetic sheet decreases. In addition, in Comparative example 13, although the magnetic characteristics of the sample obtained as a non-defective product were ensured, the manufacturing yield rate was 97% or less, and the mass productivity was poor. In the case of the thickness of the sheet is 2.0 mm or more as in Comparative example 13, the magnetic characteristics of the sheet obtained as a non-defective product could be ensured to some extent by optimizing the firing temperature or the like. However, improvement in mass productivity cannot be realized. 
     In contrast, in Examples 11 to 16 of the present invention, the difference of residual stress was 20 MPa or less when the T 0  is 1.0 mm to 2.5 mm (Examples 14 to 16) as well as when the T 0  is as thin as 50 μm to 1 mm (Examples 11 to 13). And therefore high manufacturing yield rate and stable manufacturing could be realized. In addition, Examples 11 and 12 had a higher initial permeability μi and a smaller magnetic loss Pcv than Comparative examples 11 and 12. From this result, it could be confirmed that in the magnetic sheet in which the difference of residual stress was controlled within a predetermined range, mass productivity could be ensured and high magnetic characteristics can be obtained even if the thickness is reduced. 
     Note that, the results of Comparative examples 14 and 15 are supplemented. In Comparative examples 14 and 15, the results show that although the difference of residual stress |σs−σc| exceeds 20 MPa, the manufacturing yield rate was high and the magnetic characteristics were also high. It is considered that when the thickness T 0  exceeds 2.5 mm, the magnetic sheet becomes a lump shape rather than a sheet shape, the ratio of the plane to the volume decreases, and as a result, mass productivity can be maintained even if the difference of residual stress exceeds 20 MPa. 
     Experiment 3 
     In Experiment 3, the magnetic sheets according to Examples 21 to 23 were prepared by changing the composition ratio of the main components. Table 4 shows the composition of the main components in each of Examples 21 to 23. Moreover, the other experimental conditions in Experiment 3 were the same as those in Experiment 1. In particular, the firing in each of Examples 21 to 23 of Experiment 3 was carried out under condition 3 which was the same as Example 3 of Experiment 1. 
     Evaluation 3 
     Table 4 shows the evaluation results of Examples 21 to 23 in Experiment 3. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                 Average 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 thickness 
                 Residual stress 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Composition of main 
                 of 
                 Surface 
                 Central 
                   
                 Evaluation characteristics 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 components 
                 magnetic 
                 layer 
                 portion 
                 Difference 
                 Yield 
                 Initial 
                 Magnetic 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Fe 2 O 3   
                 ZnO 
                 MnO 
                 sheet (T0) 
                 portion σs 
                 σc 
                 |σs − σc| 
                 rate 
                 permeability 
                 loss 
               
               
                 Sample No. 
                 mol % 
                 mol % 
                 mol % 
                 μm 
                 MPa 
                 MPa 
                 MPa 
                 % 
                 — 
                 kW/m 3   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 21 
                 51.0 
                 18.0 
                 31.0 
                 500 
                 −50.2 
                 −36.0 
                 14.2 
                 97.6 
                 3653 
                 907 
               
               
                 Example 22 
                 54.0 
                 9.0 
                 37.0 
                 500 
                 −37.4 
                 −42.7 
                 5.3 
                 99.2 
                 2524 
                 588 
               
               
                 Example 23 
                 58.0 
                 3.0 
                 39.0 
                 500 
                 −32.3 
                 −29.3 
                 3.0 
                 99.6 
                 1885 
                 382 
               
               
                   
               
            
           
         
       
     
     As shown in Table 4, it could be confirmed that stable manufacturing could be realized even when the ratio of the main components was changed, as long as the difference of residual stress was controlled within the predetermined range. Moreover, when comparing Examples 21 to 22, it could be confirmed that when the ratio of ZnO increased, the initial permeability μi was improved, but the magnetic loss Pcv was greater. In addition, it could also be confirmed that when the ratio of ZnO decreased, the magnetic loss Pcv could be reduced, but the initial permeability μi was rather reduced. As for the composition of Mn—Zn ferrite, when Fe 2 O 3  was in the range of 51 to 58 mol % and ZnO was in the range of 3 to 18 mol %, stable manufacturing could be realized and excellent magnetic characteristics could be obtained. 
     NUMERICAL REFERENCES 
     
         
           2  . . . . Magnetic sheet 
           2   a  . . . Front surface 
           2   b  . . . . Back surface 
           3  . . . Green sheet 
           4   a  . . . Surface layer side measurement point 
           4   b  . . . Center side measurement point 
           6  . . . Setter 
           6   a  . . . . Surface of the Setter 
           10  . . . Coil module 
           12  . . . Coil 
           13  . . . Lead terminal 
           100  . . . Non-contact power supply device 
           10   a  . . . . Power transmission side coil module 
           10   b  . . . . Power receiving side coil module 
           20   a  . . . . Power transmission side internal circuit 
           20   b  . . . Power receiving side internal circuit 
           22  . . . Power supply 
           24  . . . Secondary battery