a preferred embodiment of the present invention is hereafter described with reference to the attached drawings . fig1 shows a schematic view of a major portion of a sputtering apparatus constructed in accordance with the invention . the apparatus has a vacuum vessel 10 which can be degassed to a desired pressure through a gas outlet port 11 by a vacuum pump or the like ( not shown ). an appropriate gas can be introduced into the vacuum vessel 10 through a gas inlet port 12 . two targets 1a and 1b are provided on the cathode side in the vacuum vessel 10 . a holder 7 , to which a substrate 6 is removably attached facing the targets 1a and 1b , is provided on the anode side in the vacuum vessel 10 . a power supply 8a is connected to the target 1a . a magnetic field generator including a permanent magnet 3a , which is a main magnet , and an electromagnet 5a , which is an auxiliary magnet , is provided behind the target 1a opposite the anode side . the permanent magnet 3a generates a closed magnetic field xa near the front side ( the upper side in fig1 ) of the target 1a the closed magnetic field xa substantially perpendicularly intersects the electric field generated between the anode and cathode of the sputtering apparatus . the intensity of the magnetic field xa can be changed by regulating the drive current to the electromagnet 5 . the permanent magnet 3a has a peripheral magnetic pole extending along substantially the entire periphery of the target 1a , and a central magnetic pole located substantially at the center of the magnet , so that the closed magnetic field xa is generated between the magnetic poles . the peripheral and the central magnetic poles are magnetically coupled to each other by a yoke 4a made of a soft magnetic material . the coil of the electromagnet 5a surrounds a portion of the yoke 4a , which projects and supports the central magnetic pole of the permanent magnet 3a . a power supply 9 for the electromagnet 5a can be controlled to alternate the intensity of the magnetic field xa . a magnetic field generator of the same construction and operation as the above - described magnetic field generator is provided behind the other target 1b . the latter magnetic field generator includes a permanent magnet 3b , which is a main magnet , an electromagnet 5b , which is an auxiliary magnet , and a yoke 4b . the targets 1a and 1b are made of mutually different metals , namely , a transition element and a rare earth element . for example , the target 1a may be made of an iron - cobalt alloy , and the other target 1b of a neodymium - dysprosium alloy . each of the targets 1a and 1b may be shaped as a disk , although the shape is not confined thereto . the peripheral portion of each of the permanent magnets 3a and 3b does not need to monolithically extend along the periphery of the corresponding target , and the peripheral portion may be made of a plurality of columnar or prismatic magnets juxtaposed along the periphery . since the temperature of the targets 1a and 1b rises due to sputtering discharge , which lowers the efficiency of sputtering , a conventional arrangement for causing cooling water to flow under the targets is provided to reduce the rise in the temperature . a conventional arrangement may be provided for causing cooling water to flow at the rear side of the substrate 6 , on which a thin film is deposited by sputtering , or a conventional heater may be provided to cool or heat the substrate depending on the properties of the thin film . the intensity of the magnetic fields generated by the permanent magnets 3a and 3b is high enough to magnetically saturate the targets 1a and 1b when no electrical current flows through the electromagnets 5a and 5b . it is important for the control of the sputtering discharge ( magnetron discharge ) that the density of the magnetic flux on a horizontal plane extending at a distance of 1 mm up from the front side ( the upper side in fig1 ) of each target be in a range of 80 to 800 gauss . if the density of the magnetic flux in the horizontal plane is less than 80 gauss , the sputtering discharge will not be maintained under a pressure of 20 m torr or less . if the magnetic flux density in the horizontal plane is more than 800 gauss , the radius of cycloidal movement of electrons caused by the magnetic field will be too small to efficiently promote ionization to maintain a desirable rate of sputtering . although the above - mentioned desirable range of the density of the magnetic flux on the horizontal plane varies depending on conditions such as the pressure and type of the gas of the atmosphere , the amount of the variation is small under ordinary sputtering conditions . sputtering can be properly performed when the density of the magnetic flux in the horizontal plane is 80 to 800 gauss . it is preferable that the density of the magnetic flux in the plane be 100 to 600 gauss . the direction of the electrical current which flows through each of the electromagnets 5a and 5b is set so as to reduce the intensity of the magnetic field generated by the corresponding permanent magnet 3a or 3b . the minimum intensity of the magnetic field , which is determined by both the permanent magnet and the electromagnet , is set so as to magnetically saturate the target and produce a leakage magnetic field xb whose flux density in the horizontal plane is 50 to 100 gauss . if the intensity of the total magnetic field in the horizontal plane is 50 to 100 gauss , the sputtering discharge cannot be sustained when the pressure of the atmosphere in the vacuum vessel 10 is 20 m torr or less . however , the sputtering discharge can be easily be sustained by increasing the intensity of the total magnetic field from the range of 50 to 100 gauss by 50 to 100 gauss , as shown at xa in fig1 . it is important that the waveform of the electrical current which is caused to flow through each electromagnet 5a and 5b be sinusoidal and the current have an offset value i 0 , as shown in fig2 . if the waveform is rectangular , the counterelectromotive force produced in the coil electromagnet will be so high as to likely damage the components of the power supply 9 for the electromagnet due to the resulting transient current . in a portion a ( fig2 ) of the period of the sine curve of the electrical current which flows through the electromagnet , the resultant magnetomotive force is sufficiently low that the sputtering discharge is turned off . in the other portion b of the period , the resultant magnetomotive force increases so that the sputtering discharge is turned on . therefore , the turn - on and turn - off of the sputtering discharge can be controlled by optionally setting the period of the sine curve . since the target to be subjected to sputtering is already magnetically saturated , even when the sputtering discharge remains turned off in the portion b , the electrical current for the electromagnet is still small until the sputtering discharge is turned on , and hence the load on the power supply for the electromagnet is made low . for the same reason , the sputtering discharge can be very easily switched between the on and the off states even if the period of turn - on and turn - off is short . the electrical currents which flow through the electromagnets 5a and 5b are controlled through the power supply 9 . the waveforms of the currents are determined so that the current i b for the electromagnet 5b flows through it at an appropriate timing , that is , at the time point immediately before the end of the sputtering discharge with the current i a for the electromagnet 5a , as shown in fig3 . as a result , the turn - on of the sputtering discharge is continually repeated at a very short period and stabilized . the discharge is performed above a boundary line y shown by a two - dot chain line in fig2 and 3 . for these reasons , a thin film such as a film of stratified layers of gadolinium , dysprosium , iron and cobalt , the thickness of each layer of which is 10 å to scores of 10 å and corresponds to the very short period of the sputtering discharge , can be deposited on the substrate 6 by the inventive sputtering apparatus , although it is very difficult to deposit such a thin film in a conventional art although permanent magnets are employed as the main magnets of the magnetic field generator in the above - described embodiment , the present invention is not confined thereto and may be otherwise embodied so that electromagnets are provided as the main magnets . moreover , the positions of the electromagnets forming the auxiliary magnets of the magnetic field generator are not confined to those of the above - described embodiment , and may be appropriately altered . in the sputtering apparatus provided in accordance with the present invention , permanent magnets and electromagnets are combined with each other as main and auxiliary magnets , and the electrical currents for exciting the electromagnets are controlled so that magnetic fields for causing sputtering discharge and other magnetic fields which will not cause a discharge are stably generated at a very short period . at a result , a thin film , which consists of stratified layers differing from each other in material and which has a smaller thickness than the thinnest film that can be deposited by a conventional sputtering apparatus , can be stably deposited without putting an undue load on a power supply as in the conventional sputtering apparatus .