fig1 shows an embodiment in which a magnetoresistive head using the composite magnetic film is fabricated . a composite magnetic film of 2 μm obtained by adding 4 mol % of sio 2 to nife is formed and used as a lower shielding layer 10 . the surface roughness of the lower shielding layer 10 is 0 . 8 nm . al12o3 is applied as a lower gap film 20 of 0 . 15 μm . a spin valve film 100 is formed in such a manner that a rotatable magnetizing direction layer of ta having a thickness of 5 nm and nife having a thickness of 10 nm , a fixed magnetizing direction layer of cu having a thickness of 5 nm , co having a thickness of 3 nm , and nife having a thickness of 8 nm , are sequentially laminated , and continuously , nimn of 15 nm is formed . nimn and nife in the fixed magnetizing direction layer are exchanged and coupled by a heat treatment with field applied to the height direction of the mr sensor at 260 ° c . for four hours . a magnetic domain control film 60 and an electrode 70 are formed by a hard bias structure . al 2 o 3 of 0 . 1 μm is formed and used as an upper gap film 30 . after that , the composite magnetic film ( upper shielding layer ) 90 of 3 μm , a write head gap film 40 of al 2 o 3 , an inductive coil 120 , an insulating layer 130 , an upper magnetic core 110 of the write head , and a protective layer 50 made of al 2 o 3 , and terminals are sequentially formed . a magnetic field is applied in the direction of a track width and a heat treatment is given at 180 ° c . for one hour . the magnetic field strength is 3 koe . the write head is formed by plating . the magnetic field is first applied to the height direction of the mr sensor and magnetization is oriented to the magnetic path of the write head and is turned toward the track width direction by the final heat treatment , thereby controlling the magnitude of the anisotropic magnetic field . on the other hand , in a magnetoresistive head ( not shown ) as a target to be compared with the magnetoresistive head made of the composite magnetic material according to the invention , an amorphous film of conbzr is formed as a lower shielding layer and a heat treatment is given at 250 ° c . for three hours just after the formation . the same spin valve part , magnetizing conditions , magnetic domain control part , and electrode as those in the embodiment of fig1 are used . nife alloy of 3 μm is formed as the upper shielding layer . the final heat treatment at 180 ° c . for one hour after the formation of the write head and the terminals is given by the magnetic field applied in the track width direction . fig2 shows cross - sections at a position on a wafer and where the air bearing surface is supposed to be positioned , to illustrate fabricating steps . the lower shielding layer 10 is formed on a substrate 8 and an insulating layer 9 ( step 1 ). after forming the lower gap film 20 ( step 2 ), the spin valve layer 100 , an electrode , and the magnetic domain control layer 70 are formed ( step 3 ). after forming an upper gap film 30 , the upper shielding layer 90 is applied ( step 4 ). the write head gap 40 is formed , and after that , the inductive coil , insulating layer , and upper magnetic core 110 are formed ( step 5 ). the protective layer 50 , terminals , and the like are formed ( step 6 ). by performing the procedures of the steps 1 to 6 , the wafer step is completed . after that , a post step such as an air bearing surface process , cutting , gimbal adhesion , and the like is performed , thereby obtaining the head . fig3 shows the sensor part 100 in detail , which has the spin valve structure shown in fig1 . in the example , the upper magnetic film is used as a fixed magnetizing direction layer and the lower magnetic film is used as a rotatable magnetizing direction layer . the sensor part 100 has a hard bias structure in which a rotatable magnetizing direction layer 53 such as nife alloy is formed on a proper underlayer 52 . a non - magnetic interlayer 54 , a magnetoresistive change auxiliary layer 55 not necessarily needed , a fixed magnetizing direction layer 56 , and an antiferromagnetic layer 66 are sequentially formed . an mr sensor layer is limited only in the sensor part , and simultaneously , the magnetic domain control layer 60 and the electrode 70 are formed . fig4 shows a bias relation of the spin valve structure . it is necessary to direct the magnetization of the rotatable magnetizing direction layer 53 so as to be perpendicular to the height direction of the mr sensor by a magnetic field obtained by synthesizing a current bias field hi , a magnetostatic coupling field hbt from the end of the fixed magnetizing direction layer 56 , an interlayer coupling field he and a bias field hb1 from the magnetic domain control layer 60 by a sensing current supplied . consequently , the magnetizing direction of the fixed magnetizing direction layer has to be in parallel to the height direction of the mr sensor . the recording magnetic field invades the height direction of the mr sensor and vertically turning the magnetization of the rotatable magnetizing direction layer . in this instance , a signal in which the resistance is minimum when an angle θ with the fixed magnetizing direction layer is 0 and the resistance is maximum when the angle is 180 ° is obtained . it is , therefore , necessary to direct the magnetization of the rotatable magnetizing direction layer to the track width direction . fig5 a shows a method of measuring activation energy of the composite magnetic material used for the shielding layer . a pre - annealing ( heat treatment ) process was performed on a sample by applying a magnetic field in the direction as shown in the diagram , and after that , the magnetizing direction was inclined by 45 ° and the heat treatment was given . the heat treatment was given at temperatures from 100 ° c . to 250 ° c . for three hours , respectively . the magnetizing direction after the heat treatment is shown with temperatures . the heat treatment was given to the composite film according to the invention shown by dotted lines , in which 4 mol % of sio 2 is added to nife . for comparison , results of the conbzr amorphous film are also shown by solid lines . in the diagram , even the pre - annealing temperature is increased to 250 to 300 ° c ., the rotation in the composite film at the temperature of 250 ° c . is approximately 45 ° and there is little influence by the pre - annealing . on the other hand the rotation in the amorphous film is about 23 ° which is approximately one - half that of the composite film after the pre - annealing at 250 ° c . and the heat treatment at 250 ° c . according to &# 34 ; thin magnetic film engineering &# 34 ; compiled by iida , publishing division of maruzen co . ltd ., p . 136 , a rotation angle β at this time is expressed by the following equation . where , ku1 is magnetic anisotropy remained in the original direction and ku2 is magnetic anisotropy generated in the 45 ° direction . it is assumed here that magnetic anisotropy after the pre - annealing process is expressed as ku0 , and ku1 + ku2 is equal to ku0 in the heat treatment at a temperature equal to or lower than the pre - annealing temperature . fig5 b shows activation energy obtained with respect to the composite film and the amorphous film , respectively , on the basis of the method shown in &# 34 ; journal of applied physics &# 34 ; vol . 53 , p . 2279 , 1982 . as for the composite film , even the pre - annealing temperature is increased to 300 ° c ., the increase in the activation energy is small such as 1 . 7 to 1 . 8 ev . on the other hand in the amorphous film , when the pre - annealing temperature is increased from 250 ° c . to 300 ° c ., the activation energy is increased from 2 . 2 to 2 . 7 ev . since the anisotropy rotation in the composite film is hardly influenced even by the pre - annealing at 300 ° c ., it is found that 1 . 8 ev or less of the activation energy is sufficient . although the activation energy of the amorphous film is small just after formation by sputtering , it increases to 2 ev or larger by heating after the head forming process . fig6 shows the change in anisotropy of the composite magnetic material by the heat treatment , which is used for the magnetoresistive head according to the invention . in the diagram , when the magnetization is oriented in the track width direction of the mr sensor as a final magnetizing direction of the shielding layer , the positive magnetic anisotropy is obtained and an anisotropic magnetic field hk is positive . in the case where the magnetizing direction perpendicularly crosses the magnetizing direction of the shielding layer ( magnetizing direction of the fixed magnetizing direction layer ), the negative magnetic anisotropy is obtained and the anisotropic magnetic field hk is turned to be negative . as shown in the diagram , in the positive magnetic anisotropy , there is no magnetic wall in the shielding layer having the air bearing surface and the magnetization of the mr sensor is not disturbed . in the negative magnetic anisotropy , however , magnetic walls at 90 ° c . occur in the shielding layer having the air bearing surface and the magnetic domain control layer of the mr sensor or the mr sensor is magnetically influenced . when performing heat treatment on a material having positive magnetic anisotropy by applying the magnetic field to the magnetizing direction of the fixed magnetizing direction layer , the higher a heat treatment temperature ta1 is or the smaller the anisotropic magnetic activation energy of the magnetic material is , the shorter the time tc1 required for the magnetic anisotropy to be turned from the track width direction to the magnetizing direction of the mr layer . a fine line shows the case of the amorphous material . in case of conbzr , hk = 120 e just after the formation of the film , the magnetic anisotropy was easily turned by the heat treatment at 250 ° c . for 30 minutes and hk became - 60 e . in order to return the direction to the original one , the heat treatment in the track width direction was given . more than three hours were required so that hk became smaller than 0 . hk was as small as 2 to 30 e and the magnetic domain structure was not stabilized . the above action is due to the stabilizing effect of the heat treatment on the amorphous material . in an amorphous structure having large gaps just after fabrication , magnetic atoms are easily rearranged and the magnetizing direction can be easily reversed . however , it indicates that it is difficult to again reverse the magnetizing direction after completion of the process using the hours - long heat treatment . it is important to determine at which stage the magnetic filed is applied in the magnetizing direction of the mr layer during the process . since the pattern of the shielding layer is large and the shape is isotropic , means for adjusting magnetic deformation and controlling the magnetic anisotropy by using magnetic elastic effect cannot be applied . on the other hand , the thick line shows a case of the magnetic head using the composite magnetic film according to the invention , in which 4 to 5 mol % of sio 2 is added to nife . time tc1 &# 39 ; required for the turn was 10 minutes . hk was stable after change from + 80 e to - 80 e . even the magnetizing direction was again reversed after three hours , hk was recovered to 80 e after 10 minutes . that is , it shows that hk is not stabilized by the heat treatment . it can be considered that the nife grain growth in the film is inhibitted by oxide and a structure having many grain boundaries is obtained , and that the number of grain boundaries is hardly decreased by the recovery in the heat treatment at 250 ° c . when such a material is used as the shielding material , the anisotropy can be recovered by the heat treatment regardless of the changes in the magnetic field during the process . with respect to the head according to the first embodiment in the invention and the head as the comparison example , the magnetic domains in the shielding layer are observed by mfm after processing the air bearing surface . in fig7 the percentage of presence or occurrence of the magnetic walls in the lower shielding layers of 100 heads according to the invention and 100 heads for comparison is shown . the percentage of occurrence of the magnetic walls in the lower shielding layer according to the first embodiment is 5 % and that in the upper shielding layer is 0 %. in the comparison example , however , the percentage of occurrence of the magnetic walls in the lower shielding layer is 70 % and that in the upper shielding layer is 2 %. in the second embodiment , the same shielding layer is used and an al 2 o 3 gap layer of 0 . 1 μm is applied as a sensor part , the spin valve part is formed by a fixed magnetizing layer of ni of 30 nm , nife of 0 . 5 nm , and co of 5 nm , a rotatable magnetizing direction layer of cu of 5 nm and nife of 10 nm , and ta of 5 nm , and an al 2 o 3 gap layer of 0 . 14 μm is formed . after a terminal is formed and the heat treatment is given at 200 ° c . for 30 minutes by the magnetic field applied in the height direction of the mr sensor , the heat treatment is given by the magnetic field applied in the track width direction at 150 ° c . for three hours . in this case , according to the second embodiment , the percentage of occurrence of the magnetic walls in the lower shielding layer is 10 % and that in the upper shielding layer is 0 %. with respect to the conventional amorphous magnetic film , the percentage of occurrence of the magnetic walls in the lower shielding layer is 60 % and that in the upper shielding layer is 3 %. in addition to sio 2 , an oxide such as zro 2 , tao 2 , al 2 o 3 or the like , or a compound such as bn , wc or the like can be also used as the material of the composite magnetic film . it is preferable that any additional amount be 2 to 10 mol % with respect to any case . when the additional amount is too small , the crystal particle size is enlarged and the surface roughness is increased . when the additional amount exceeds 10 mol %, heat conductivity of the shielding layer deteriorates and a temperature rise in the mr sensor is caused by the current supply . although the composite magnetic shielding layer is used in both of the upper and lower layers in the head according to the embodiment of the invention , it is also possible that the composite magnetic film is applied only to the lower shielding layer and the amorphous film or a crystalline film such as ni or fe is used as the upper shielding layer . the method of turning the magnetic anisotropy of the write head after formation is useful especially when a material having a highly saturated magnetic flux density such as conife in which the magnetic anisotropy is large and the magnetic permeability is small is used . fig8 shows a magnetic disk apparatus using the magnetoresistive head according to the invention . the apparatus basically comprises : a rotatable magnetic disk 203 ; a head 204 having magnetoresistive effect for recording and reproducing signals ; an actuator 205 ; and a signal processor . the magnetic disk is rotated 4000 to 8000 times per minute by a rotary shaft 202 and the magnetoresistive head 204 is floated above the magnetic disk at an interval of tens to hundreds angstroms . when an inductive part of the head 204 executes a writing operation , a recording binary digit string is formed concentrically by the rotation of the disk . further , the head is moved in the radial direction by the actuator 205 , thereby recording data on the whole disk . a predetermined current is flowed to the magnetoresistive head of the head 204 and the head 204 is moved by the actuator 205 to a predetermined position . after that , a change in resistance of the sensor part is obtained as a change in voltage of the head terminal . a reproduction signal is shaped and converted to data by an integrated circuit arranged on a base 201 and is transferred to an external apparatus such as a computer . in the magnetic disk apparatus using the magnetic head according to the invention shown in fig8 the quality of a reproduction waveform is improved as compared with a case of using the conventional head and a signal read error ratio of the apparatus is improved from 10 - 9 to 10 - 11 , that is , by more than two digits . there is consequently an advantage that the recording density of the apparatus can be increased by 50 %. in fig9 a dual spin valve head having a construction similar to that of fig3 is fabricated by forming a lower fixed magnetizing direction layer 51 and a spacer ta layer 58 on an nio layer 59 . as the lower shielding layer , a material in which 4 mol % of sio 2 is added to nife is used . the heat treatment to the fixed magnetizing direction layer and the nimn layer is given in step 3 in fig2 . the magnetic field is applied in the height direction of the mr sensor after completion of the head and the heat treatment is given at 200 ° c . for 30 minutes . after that , the magnetic field is applied in the track width direction and the heat treatment is given at 160 ° c . for three hours , thereby magnetizing direction of the shielding layer is also oriented in the track width direction . according to the head , an output higher than that of a typical spin valve head by 60 % is obtained . in the magnetic disk apparatus using the head , the recording density is increased by 30 % as compared with that of an apparatus using the typical spin valve head . according to the above embodiment , a permanent magnetic film such as cocrpt is used as the magnetic domain control layer . when the track width of the sensor part is 1 μm or narrower , the magnetic domain control layer may be omitted . in this case , it is necessary to reduce the thickness of a spacer film and to enhance a coupling magnetic field from the fixed magnetizing direction layer . although reproduction sensitivity is reduced , reliability of electrical connection at the electrode end is improved .