Patent Publication Number: US-8982514-B2

Title: Magnetic oscillator

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 13/430,074, filed Mar. 26, 2012, which is a Continuation Application of PCT Application No. PCT/JP2009/066970, filed Sep. 29, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a magnetic oscillator. 
     BACKGROUND 
     It is known that a microwave signal of a steady state, which responds to a direct current, can be generated by using spin transfer effect which occurs in a magnetic multilayer film of nanometer scale (for example, see S. I. Kiselev et al. “Microwave oscillations of a nanomagnet driven by a spin-polarized current” Nature 425, 380 (2003)). The origin of the microwave signal is magnetization oscillation of a magnetization oscillation part in a magnetic multilayer film. In an experiment, in a current-perpendicular-to-plane (CPP) giant-magnetoresistive (GMR) effect film and a magnetic tunnel junction (MTJ) film, when the current density exceeds the order of 10 7  A/cm 2 , steady magnetization oscillation of high frequency (GHz) is detected. 
     Microwave generators using spin transfer effect generated in a magnetic multilayer film are called spin transfer oscillators, magnetic oscillators, and spin transfer oscillators. By a remarkably-advanced fine processing technology, it has become possible to process a CPP-GMR film and a magnetic tunnel junction film in a submicron size of about 100 nm×100 nm. Magnetic oscillators are expected to be applied to minute microwave sources and resonators, and have been actively researched as a research of spintronics. The frequency of a microwave signal generated from a magnetic oscillator depends on a current, and a magnetic field which acts on magnetization of a magnetization oscillation part in a magnetization multilayer film. In particular, by using its magnetic field dependence that the magnetization oscillation frequency changes according to the magnetic field, it has been proposed to apply magnetic oscillators to magnetic sensors for an HDD which replace a GMR head and a TMR head (for example, see JP-A 2006-286855 (KOKAI)). When a magnetic oscillator is used as a magnetic sensor for an HDD, the magnetic field of the HDD medium is sensed by detecting change in frequency caused by the magnetic field. 
     Conventional magnetic oscillators have a structure in which a microwave signal caused by oscillation of magnetization in a magnetoresistive element having a ferromagnetic multilayer film is taken out. The magnetoresistive element has a three-layer structure including a magnetization free layer, a spacer layer, and a magnetization pinned layer, as a basic structure. When a direct current I flows through the magnetoresistive element by a power supply, the magnetization in the magnetization free layer is oscillated by a spin transfer effect between the magnetization free layer and the magnetization pinned layer, and an angle θ between the magnetization of the magnetization free layer and the magnetization of the magnetization pinned layer changes from moment to moment. With the change of the relative angle θ, the element resistance changes from moment to moment mainly by magnetoresistive effect, and therefore an alternating-current component of the voltage is produced. By extracting the alternating-current component of the voltage by a bias tee, a microwave signal is obtained. 
     A direct current I generated by a power source is not a desired value, but must be a current value which exceeds a threshold current value Ic that depends on the structure of the magnetoresistive element module including a ferromagnetic multilayer film and the surrounding magnetic field environment. Only when I&gt;Ic is satisfied, magnetization oscillation is induced in the magnetization free layer by the spin transfer effect. The value of the threshold current Ic is determined by a cross section of the magnetoresistive element and a threshold current density value. It is known that the threshold current density value is about 10 7  A/cm 2 . 
     In the meantime, there is a quality (Q) factor as a quantity which indicates a character of the oscillator. As an example of a Q-factor, there is mentioned an oscillating circuit which uses a crystal oscillator as a resonator. It is known that crystal oscillators have a high Q-factor of the order of 10 6 . An oscillating circuit which uses a crystal oscillator as a resonator achieves a Q-factor of the order of 10 3  to 10 4 , and obtains stable oscillation. The Q-factor is a dimensionless quantity which is defined as follows, and a large Q-factor means that oscillation is stable. 
     
       
         
           
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                 consumed 
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                     dissipated 
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     Oscillated state is often recognized by a frequency spectrum thereof, and in such a case the Q-factor is defined by Q=f0/Δf. The symbol f0 represents an oscillation frequency, and the symbol Δf represents a full width at half maximum of an oscillation peak of the frequency spectrum. 
     A magnetic oscillator realized by a CPP-GMR film (hereinafter referred to as a “GMR oscillator”) is obtained when a spacer layer of the magnetoresistive element is formed of a non-magnetic metal layer such as Cu. It has been known from experiment that oscillation of Q≈(10 GHz/1 MHz)≈10 4  is obtained by a GMR oscillator (for example, see W. H. Rippard et al. “Current-driven microwave dynamics in magnetic point contacts as a function of applied field angle” Physical Review B 70, 100406 (R) (2004)). Specifically, GMR oscillators have performance which is greater than or equal to oscillating circuits which use a crystal oscillator as a resonator, with respect to the Q-factor. The reason why GMR oscillators can achieve a high Q-factor is that a large current can flow through GMR oscillators that are artificial metal lattices, all of which are formed of metal material. It is known that a full width at half maximum Δf of the frequency spectrum is generally in inverse proportion to the square of current I (that is, Δf∝1/I 2 ). The value of Δf becomes extremely small by flowing a large current, and thus a high Q-factor can be achieved. A high Q-factor is an advantage of GMR oscillators. GMR oscillators have, however, a disadvantage that a single GMR oscillator outputs a weak electric power of the order of nanowatts (nW) at most, which is far from a practical electric power level of microwatts (μW) and is not desirable for application. The reason why a GMR oscillator outputs a weak electric power of the order of nanowatt is that GMR oscillators have a small magnetoresistive (MR) ratio of several percent at most. A structure of increasing an output power by arranging GMR oscillators in an array has been proposed (for example, see S. Kaka et al. “Mutual phase-locking of microwave spin torque nano-oscillators” Nature 437, 389 (2005)). In the case of arranging GMR oscillators in an array, however, it is necessary to arrange at least dozens of GMR oscillators in an array and synchronize all the oscillators with each other, to increase the output power to a microwatt level. Therefore, it is difficult to manufacture the magnetic oscillator. 
     On the other hand, magnetic oscillators achieved by a magnetic tunnel junction film (hereinafter referred to as “TMR oscillators”) are obtained when a tunnel barrier is used as the spacer layer. In recent years, high-quality magnetic tunnel junction films which have low resistance and a high MR ratio have been developed, and expected to be applied to spin injection magnetic random access memories (spin-RAM). In particular, it has been known by experiments that the MR ratio in a TMR (MgO-TMR) film which has a magnesium oxide (MgO) barrier is several hundred percent or more. TMR oscillators can obtain large oscillation power P since they have a high MR ratio. The oscillation power generated by magnetic oscillators using an MgO-TMR film is actually coming near practical microwatt electric power level, and the maximum power level which has been reported at present is 0.16 μW. It is impossible, however, to cause a large current to flow through magnetic oscillators using a magnetic tunnel junction film such as an MgO-TMR film, unlike GMR oscillators, due to the problem of insulation breakage by a tunnel barrier, and thus it is difficult for the oscillators to realize a high Q-factor. 
     There are many cases where magnetization oscillation cannot be excited in the first place in TMR oscillators. This is also due to insulation breakdown of the tunnel barrier. This is because there are many cases where insulation breakdown is caused by a current which is smaller than the threshold current Ic, although magnetization oscillation is excited in the free layer by the spin transfer effect only when I&gt;Ic is satisfied as described above. 
     JP-A 2009-194070(KOKAI) discloses a complex magnetic oscillator which is obtained by magnetostatic-coupling an oscillation driving module formed of a GMR oscillator with an output module formed of a TMR oscillator, with good use of characters of GMR oscillators and TMR oscillators. It is necessary, however, to manufacture the two oscillators very close to each other, that is, 300 nm or less, to perform magnetostatic coupling, and thus the manufacturing process is difficult both in a planar structure and a layered structure. 
     As described above, each of GMR oscillators and TMR oscillators have merits and demerits. The merit of GMR oscillators is a high Q-factor, and the demerit thereof is small oscillation power. The merit of TMR oscillators is large oscillation power, that is, high output power, and the demerit thereof is a low Q-factor. 
     Therefore, it is required for magnetic oscillators to have the advantages of GMR oscillators and TMR oscillators, that is, a high Q-factor and high output power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a magnetic oscillator according to embodiments. 
         FIG. 2  is a cross-sectional view illustrating a magnetic oscillator according to a first embodiment. 
         FIG. 3  is a cross-sectional view illustrating a planar shape of the magnetic oscillator illustrated in  FIG. 1 . 
         FIG. 4  is a cross-sectional view illustrating a magnetic oscillator according to a second embodiment. 
         FIG. 5  is a cross-sectional view illustrating a magnetic oscillator according to a third embodiment. 
         FIG. 6  is a cross-sectional view illustrating a magnetic oscillator according to Example 1. 
         FIG. 7  is a schematic diagram illustrating a power spectrum measurement system in the magnetic oscillator illustrated in  FIG. 6 . 
         FIG. 8  is a perspective view of a magnetic recording and reproducing apparatus according to the embodiments. 
         FIG. 9  is a perspective view of a magnetic head assembly illustrated in  FIG. 8 , as viewed from the magnetic disk side. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a magnetic oscillator includes a layered film and a pair of electrodes. The layered film includes a first ferromagnetic layer, an insulating layer stacked on the first ferromagnetic layer, and a second ferromagnetic layer stacked on the insulating layer. The pair of electrodes is configured to apply a current to the layered film in a direction perpendicular to a film surface of the layered film. Regions having different resistance area products are provided between the first ferromagnetic layer and the second ferromagnetic layer. 
     The embodiment provides a magnetic oscillator of high Q-factor and high output power. The magnetic oscillator of the embodiment can be used for a microwave source, a resonator, and a magnetic sensor or the like. 
     Magnetic oscillators according to embodiments will be explained hereinafter with reference to the accompanying drawings. In the embodiments, like reference numbers denote like elements, and duplication of explanation will be avoided. Each drawing is a schematic diagram, and the illustrated shape, dimension and ratio include parts different from those of the actual oscillator. When the oscillator is actually manufactured, they can be properly changed in consideration of the following explanation and publicly known art. 
       FIG. 1  illustrates a schematic structure of a magnetic oscillator according to one embodiment. As illustrated in  FIG. 1 , the magnetic oscillator is formed to have a layered structure obtained by successively stacking a first ferromagnetic layer  1 , an insulating layer (also referred to as tunnel barrier layer)  2 , and a second ferromagnetic layer  3 . Although, in  FIG. 1 , the first ferromagnetic layer  1  is a magnetization pinned layer whose magnetization is fixed and the second ferromagnetic layer  3  is a magnetization free layer whose magnetization is not fixed, the structure is not limited to it. As one example, the first ferromagnetic layer  1  may be a magnetization free layer, and the second ferromagnetic layer  3  may be a magnetization pinned layer. Alternatively, both of the first and second ferromagnetic layers  1  and  3  may be a magnetization free layer. In a magnetization free layer, direction of magnetization is changed in accordance with the external magnetic field. In addition, in the example illustrated in  FIG. 1 , the magnetization of the second ferromagnetic layer  3  is maintained by the external magnetic field in a direction which is antiparallel with the direction of the magnetization of the first ferromagnetic layer  1 , to easily generate magnetization oscillation caused by the spin transfer effect. The following explanation mainly shows the example in which the first ferromagnetic layer  1  illustrated in  FIG. 1  is a magnetization pinned layer, and the second ferromagnetic layer  3  is a magnetization free layer. 
     The first and second ferromagnetic layers  1  and  3  are formed of Co, Ni, or Fe, or alloy which includes at least one of them. At both end parts of at least one of the first and second ferromagnetic layers  1  and  3 , a pair of bias magnetization films may be provided which apply a bias magnetic field. One of the first and second ferromagnetic layers  1  and  3  may be an exchange coupled film which is obtained by stacking a ferromagnetic layer which has in-plane magnetic anisotropy and an antiferromagnetic layer. Alternatively, one of the first and second ferromagnetic layers  1  and  3  may be an exchange coupled film which is obtained by stacking a ferromagnetic layer which has in-plane magnetic anisotropy, a nonmagnetic intermediate layer which controls the magnitude of the bias magnetic field, and an antiferromagnetic layer. Alternatively, one of the first and second ferromagnetic layers  1  and  3  may be an exchange coupled film which is obtained by stacking an artificial ferrimagnetic film which has in-plane magnetic anisotropy and an antiferromagnetic layer. 
     The insulating layer  2  is formed of magnesium oxide (MgO) film, aluminum oxide (AlO) film, or the like. A magnetic oscillator using MgO film as the insulating layer  2  has a large magnetoresistive (MR) ratio, and thus can obtain high output power. 
     The magnetic oscillator illustrated in  FIG. 1  includes a pair of electrodes, that is, a lower electrode  42  and an upper electrode  43 , which apply a direct current I to a layered film  4  including the first ferromagnetic layer  1 , the insulating layer  2 , and the second ferromagnetic layer  3 , in a direction perpendicular to a film surface of the layered film  4 . The direction corresponds to a stacking direction in the layers  1 ,  2 , and  3  are stacked. The direct current I is supplied from a power supply  6 . The direct current I flows in a direction perpendicular to the film surface of the layered film  4 , that is, from the second ferromagnetic layer  3  to the first ferromagnetic layer  1  through the insulating layer  2 . The intensity of a tunnel current which flows through the insulating layer  2  depends on an angle between the magnetization of the first ferromagnetic layer  1  and the magnetization of the second ferromagnetic layer  3 . When the direct current I flows through the layered film  4 , the magnetization in the second ferromagnetic layer  3  is steadily oscillated by the spin transfer effect between the first and second ferromagnetic layers  1  and  3 . More specifically, when the direct current I flows from the second ferromagnetic layer  3  toward the first ferromagnetic layer  1 , electrons flow from the first ferromagnetic layer  1  toward the second ferromagnetic layer  3 , and electrons which are spin-polarized by the magnetization of the first ferromagnetic layer  1  are injected into the second ferromagnetic layer  3 . Then, the spin-polarized electrons act on the magnetization of the second ferromagnetic layer  3 , and precession of the magnetization of the second ferromagnetic layer  3  is induced. 
     In the case where both the first and second ferromagnetic layers  1  and  3  are magnetization free layers, when the direct current I flows through the layered film  4 , precession of the magnetizations of the first and second ferromagnetic layers  1  and  3  is induced with a fixed difference in phase between them. 
     The steady oscillation of the magnetization of the second ferromagnetic layer  3  becomes voltage oscillation by the magnetoresistive effect. More specifically, when the magnetization of the second ferromagnetic layer  3  is oscillated by supply of the direct current I, the relative angle between the magnetization of the first ferromagnetic layer  1  and the magnetization of the second ferromagnetic layer  3  changes from moment to moment. With change of the relative angle, the resistance of the oscillator changes from moment to moment mainly due to the magnetoresistive effect. Consequently, an alternating-current component is produced in a voltage between the lower electrode  42  and the upper electrode  43 . The alternating-current component of the voltage is extracted by a bias tee  7  which is formed of a capacitor and an inductance, and a microwave signal (also referred to as a high-frequency voltage) P is obtained as output. The number of vibrations of the high-frequency voltage is equivalent to the number of vibrations of the magnetization oscillation, and depends on the size and thickness of the magnetization free layer, the direct current, and the magnitude of the external magnetic field. The thickness is defined in the stacking direction. When the magnetic oscillator illustrated in  FIG. 1  is used for a magnetic recording and reproducing apparatus explained later, the magnetic oscillator can detect continuous change of the number of vibrations (that is, frequency) of the high-frequency voltage in accordance with the magnetic field from the recording bits of the rotated magnetic disk, and read out information of the recording bits. 
     To induce magnetization oscillation, it is necessary that the direct current I from the power supply  6  has a current value which exceeds a threshold current Ic (that is, I&gt;Ic). The threshold current Ic depends on the structure of the layered film  4  and the surrounding magnetic field environment. The threshold current Ic is determined by the threshold current density and a cross section of the layered film  4 , in a plane which is perpendicular to the stacking direction. Therefore, generally, to oscillate the magnetization, it is a necessary condition that a current density J in the layered film  4  exceeds a threshold current density Jc. 
     The magnetic oscillator illustrated in  FIG. 1  includes a region  51  which has a high resistance area product (RA) (hereinafter referred to as a “high RA region”), and a region  52  which has low resistance area product (hereinafter referred to as a “low RA region”), between the first and second ferromagnetic layers  1  and  3 . The term “resistance area product” indicates a resistance per unit area (for example, 1 μm 2 ) in a plane perpendicular to the direction of flow of the current. The resistance area product of the high RA region  51  is denoted by RA 1 , and the resistance area product of the low RA region  52  is denoted by RA 2 . RA 1  is higher than RA 2 . When a constant current I is supplied to the magnetic oscillator which includes the high RA region  51  and the low RA region  52 , a voltage between the first and second ferromagnetic layers  1  and  3  is fixed, and thus the ratio of the current density of the current which flows through the low RA region  52  to the current density of the current which flows through the high RA region  51  is determined by the ratio of RA of the region  52  to RA of the region  51 . Specifically, a current density J 2  in the low RA region  52  is (RA 1 /RA 2 ) times as large as a current density J 1  in the high RA region  51 . In such a case, a current of high current density flows through the low RA region  52 , and the magnetization of the second ferromagnetic layer  3  is strongly excited. Since the second ferromagnetic layer  3  extends over the low RA region  52  and the high RA region  51 , the magnetization of the high RA region  51  is excited by spin waves, and magnetization oscillation which is larger than that caused by excitation of the magnetization oscillation by spin torque in the current density J 1  is excited in the high RA region  51 . Specifically, in the second ferromagnetic layer  3 , the magnetization of the low RA region  52  is strongly excited, oscillation of the magnetization of the low RA region  52  is transmitted to the magnetization of the high RA region  51  by exchange interaction, and the magnetization of the high RA region  51  is strongly excited. As a result, large change in resistance is generated, and high output power is obtained. In an extreme case, even when the current density does not exceed the threshold in the high RA region  51 , the magnetic oscillator according to the present embodiment can oscillate the magnetization of the second ferromagnetic layer  3  and obtain high output power, as long as a current density of the threshold value or more is achieved in the low RA region  52 . 
     A resistance R 1  of the high RA region  51  is represented by the expression R 1 =RA 1 /S 1 , by using an area S 1  of the high RA region  51  and the resistance area product RA 1  of the high RA region  51 . In the same manner, a resistance R 2  of the low RA region  52  is represented by the expression R 2 =RA 2 /S 2 , by using an area S 2  of the low RA region  52  and the resistance area product RA 2  of the low RA region  52 . A resistance R between the first and second ferromagnetic layers  1  and  3  is represented by the expression R=(R 1 ×R 2 )/(R 1 +R 2 ), and a voltage V is represented by the expression V=I×(R 1 ×R 2 )/(R 1 +R 2 ). In addition, as described above, the current density J 2  in the low RA region  52  is (RA 1 /RA 2 ) times as large as the current density J 1  in the high RA region  51 . Therefore, as the difference between the resistance area products RA 1  and RA 2  increase and the area of the low RA region  52  decreases, the current density of the current which flows through the low RA region  52  increases, and the magnetization of the second ferromagnetic layer  3  is oscillated more strongly. As described above, the magnetic oscillator illustrated in  FIG. 1  can realize high output power and a high Q-factor, by achieving a partly large current density in the layered film  4  having a high MR ratio. 
     First Embodiment 
     A magnetic oscillator according to a first embodiment will be explained hereinafter with reference to  FIG. 2  and  FIG. 3 . 
       FIG. 2  illustrates a schematic structure of the magnetic oscillator according to the first embodiment, and  FIG. 3  illustrates a planar shape of a layered film  4  illustrated in  FIG. 2 , as viewed from the stacking direction. The magnetic oscillator illustrated in  FIG. 2  includes a layered film  4  including a first ferromagnetic layer  1 , a second ferromagnetic layer  3 , and an insulating layer  2  that is interposed between the ferromagnetic layers  1  and  3 , like the magnetic oscillator illustrated in  FIG. 1 . The first and second ferromagnetic layers  1  and  3  and the insulating layer  2  have the same planar shape, and form the layered film  4 . In the first embodiment, magnetization of the second ferromagnetic layer  3  is controlled by applying an external magnetic field H, such that the magnetization of the second ferromagnetic layer  3  is antiparallel with a direction of magnetization of the first ferromagnetic layer  1 . As illustrated in  FIG. 3 , a cross sectional shape (or a planar shape) of the layered film  4  in a plane perpendicular to a stacking direction, is formed such that the cross sectional shape has at least one edge having a large curvature or the cross sectional shape is tapered as it goes toward a direction opposite to the direction of the magnetization of the first ferromagnetic layer  1 . For example, the cross sectional shape of the layered film  4  is formed in a shape obtained by replacing sides and vertices of an acute-angled isosceles triangle with curves. When the external magnetic field H is applied such that the magnetization of the second ferromagnetic layer  3  is directed to the edge having a large curvature, the magnetization around the vertex is more stabilized in a direction running along the edge than in a direction of the external magnetic field, and thus the magnetization is warped by the effect of the shape.  FIG. 3  schematically illustrates spatial distribution of the magnetization in the second ferromagnetic layer  3  by arrows. 
     When the magnetization of the first ferromagnetic layer  1  is fixed to be antiparallel with the magnetization of the second ferromagnetic layer  3 , the resistance area product RA is reduced due to the MR effect, in the region where the magnetization of the second ferromagnetic layer  3  is locally warped. When a direct current I flows through the layered film  4 , a low RA region  52  which has a low resistance area product has a locally high current density. When the magnetic oscillator is manufactured with an insulating layer  2  having a very small thickness of 1 nm or less and a large current is supplied to the magnetic oscillator for a long time with a voltage which does not exceed an insulation breakdown voltage, a very small leak path  60  is formed in the low RA region  52  by electromigration and soft breakdown. Although a resistance area product RA in the leak path  60  is at least a digit smaller than the resistance area product in the insulating layer  2 , the leak path  60  has a very small area of several square nanometers and has high resistance. Therefore, most of the current flows through a high RA region  51  other than the leak path  60 , and the MR ratio of the whole magnetic oscillator is reduced. However, the MR ratio of the whole magnetic oscillator is maintained at few score percent, which is a very high value in comparison with that of GMR elements. Since a current of a current density which is at least a digit higher than that in other regions flows through the leak path  60 , the magnetization of the second ferromagnetic layer  3  is very strongly excited and oscillated around the leak path  60 . Therefore, a high Q-factor is achieved in the magnetic oscillator. In addition, the magnetic oscillator can obtain high output power, since the region other than the leak path  60  generates large resistance change. 
     Second Embodiment 
     A magnetic oscillator according to a second embodiment will be explained hereinafter with reference to  FIG. 4 . 
       FIG. 4  illustrates a schematic structure of the magnetic oscillator according to the second embodiment. In the magnetic oscillator illustrated in  FIG. 4 , an insulating layer  2  which is interposed between first and second ferromagnetic layers  1  and  3  has different in-plane thicknesses. Since resistance (tunnel resistance) of an insulating layer  4  changes according to the thickness of the insulating layer  2 , the magnetic oscillator has regions which have different resistance area products in a plane between the first and second ferromagnetic layers  1  and  3 , in the magnetic oscillator having the above structure. When a current of high current density flows through a low RA region  52 , and magnetization of the second ferromagnetic layer  3  in the low RA region  52  is strongly oscillated. Oscillation of the magnetization in the low RA region  52  is transmitted to a high RA region  51  by exchange interaction, magnetization of the high RA region  51  is also strongly oscillated, and consequently large resistance change is generated. Therefore, the magnetic oscillator according to the second embodiment can obtain a high Q-factor and high output power. 
     Third Embodiment 
     A magnetic oscillator according to a third embodiment will be explained hereinafter with reference to  FIG. 5 . 
       FIG. 5  illustrates a schematic structure of the magnetic oscillator according to the third embodiment. As illustrated in  FIG. 5 , the magnetic oscillator includes a metal path  70 , which electrically connects first and second ferromagnetic layers  1  and  3 , on a sidewall of a layered film. Fine processing of the magnetic film is generally performed by ion milling. It is possible to adhere removed atoms again to the processed cross section, by changing processing conditions such as an ion acceleration voltage and an ion incident angle. After the oscillator is isolated by ion milling under conditions in which atoms are not adhered, ion milling is performed from a specific direction with respect to the oscillator under conditions in which atoms are adhered again, and thereby a minute metal path  70  which electrically connects the first and second ferromagnetic layers  1  and  3  can be formed on part of the sidewall of the oscillator. Specifically, the first and second ferromagnetic layers  1  and  3  are connected to each other via an insulating layer  2  which has high resistance area product and the metal path  70  which has low resistance area product. Therefore, it is possible to obtain a high Q-factor and high output power in the third embodiment, like the first and second embodiments. 
     Example 1 
       FIG. 6  is a cross-sectional view of a magnetic oscillator according to Example 1, which corresponds to the first embodiment. As illustrated in  FIG. 6 , the magnetic oscillator was obtained by forming layers on a glass substrate  41  by using a sputtering device, forming an upper electrode  43  and a lower electrode  42  by photolithography and ion milling, and processing a layered film  4  by electron-beam lithography and ion milling. 
     The first ferromagnetic layer  1  was formed of an exchange bias film which is obtained by stacking an antiferromagnetic layer  11  formed of IrMn and an artificial ferri-structure in which an intermediate layer  13  formed of Ru is interposed between a ferromagnetic layer  12  formed of CoFe and a ferromagnetic layer  14  formed of CoFeB, and has fixed magnetization. An insulating layer  2  was formed of MgO, and a second ferromagnetic layer  3  was formed of CoFeB. A lower electrode  42  was formed of Ta/Cu/Ta, an upper electrode  43  was formed of Au/Cu, and an insulator  44  was formed of SiO 2 . When a current flows through the magnetic oscillator having the above structure, precession of magnetization of CoFeB being the second ferromagnetic layer  3  is induced. 
     In the magnetic oscillator illustrated in  FIG. 6 , a layered film  4  was processed to have a cross section shape which is tapered toward an edge as illustrated in  FIG. 3 . An external magnetic field 500 Oe was applied to the layered film  4  in a direction toward the tapered edge (magnetization of the first ferromagnetic layer was fixed to be antiparallel), a current was supplied to the layered film, and thereby a leak path as illustrated in  FIG. 2  was formed in the insulating layer  2 . By forming the leak path in the insulating film  2 , the resistance of the oscillator was reduced, and the MR ratio thereof was reduced to 63%. 
     A lead of the upper electrode  43  and a lead of the lower electrode  42  of the magnetic oscillator were designed to serve as coplanar guide (waveguide) having a characteristic impedance of 50 Ω. 
       FIG. 7  illustrates a measurement system for oscillation power spectrum in the magnetic oscillator. As illustrated in  FIG. 7 , in the measurement system, a bias tee  103  is connected to a waveguide  101 , which transmits high-frequency oscillation from a magnetic oscillator  10 , through a high-frequency probe  102 , an input end of an amplifier  104  is connected to an output end of the bias tee  103 , and a spectrum analyzer  105  is connected to an output end of the amplifier  104 . In addition, a direct-current power supply  106  is connected to the bias tee  102 . 
     Before explanation of a measurement result of oscillation characteristics of the magnetic oscillator in the above Example 1, oscillation characteristics of a TMR oscillator in prior art will be explained hereinafter as Comparative Example 1. Although the TMR oscillator according to the Comparative Example 1 is manufactured with the same structure as that of the layered film illustrated in  FIG. 6 , the oscillator is processed to have an ellipsoidal planar shape (with a size of about 60 nm×120 nm). The TMR oscillator has a resistance area product RA of 12 Ωμm 2 , and an MR ratio of 130%. In measurement of oscillation characteristics of the TMR oscillator of Comparative Example 1, an external magnetic field of 300 Oe was applied to the TMR oscillator in a direction which is inclined by 10° from a direction in which magnetization of the first ferromagnetic layer and magnetization of the second ferromagnetic layer are antiparallel with each other, and direct current was made flow from the second ferromagnetic layer to the first ferromagnetic layer. When the current was gradually increased under the above condition, spin torque increased together with increase in current, and thus the full width at half maximum of the oscillation peak decreased. When a current having a current density of 4.0×10 6  A/cm 2  was supplied, however, the full width at half maximum was only reduced to about 300 MHz. On the other hand, when a current was made flow through the magnetic oscillator according to Example 1, the full width at half maximum was 84 MHz with a current density of 1×10 6  A/cm 2 , and oscillation peak having a full width at half maximum that does not exceed 100 MHz was recognized. The peak frequency f was about 3 GHz in both of the oscillators. In the magnetic oscillator in which a leak path is formed in the insulating layer  2  locally has high current density, and has small full width at half maximum Δf, although it has small average current density. Therefore, the magnetic oscillator has a high Q-factor which is defined by Q=f0/Δf. 
     Example 2 
     In a magnetic oscillator according to Example 2, which corresponds to the third embodiment, a TMR film was manufactured by the same process as that explained in Example 1, and the film was processed such that the oscillator had an ellipsoidal cross section shape (or an ellipsoidal planar shape) of about 110 nm×150 nm. The TMR film had an RA of 14 Ωμm 2 , and an MR ratio of 110%. Next, the TMR film was overreached by ion milling, and thereby an oscillator including a metal path was manufactured by re-adhesion of metal on a sidewall. The manufactured oscillator had a resistance area product RA of about 5.2 Ωμm 2 , and an MR ratio of 10%. When an external magnetic field of 330 Oe was applied to the oscillator in a direction of an easy axis (which is almost antiparallel with the first ferromagnetic layer  1 ) of the second ferromagnetic layer  3  to supply a current having a current density of 4.2×10 6  A/cm 2 , an oscillation peak was recognized around a frequency of 4.4 GHz, and an output power of 310 pW was obtained with a line width of 24 MHz. As the magnetic oscillator according to Comparative Example 2, a non-shorted oval oscillator which has a size of about 60 nm×120 nm and manufactured by using a TMR film having an RA of 12 Ωμm 2  and an MR ratio of 130%. When an external magnetic field of 300 Oe was applied to the magnetic oscillator of Comparative Example 2 in a direction that is inclined by 10° from the easy axis to supply a current having a current density of 4.0×10 6  A/cm 2 , the full width at half maximum in the frequency peak was about 300 MHz, the output power was 120 pW. An oscillator which includes a short path (metal path) has smaller line width, that is, a higher Q-factor, since it is strongly oscillated, although it has a lower MR ratio. Therefore, the oscillator with a short path can obtain higher output power. 
     Next, a magnetic recording and reproducing apparatus according to an embodiment will be explained hereinafter with reference to  FIG. 8  and  FIG. 9 . 
       FIG. 8  illustrates a schematic structure of a magnetic recording and reproducing apparatus  150  according to the embodiment. As illustrated in  FIG. 8 , the magnetic recording and reproducing apparatus  150  comprises a magnetic disk (magnetic recording medium)  151 . The magnetic disk  151  is rotated in a direction of arrow A by a spindle motor that is attached to a spindle  152 . An actuator arm  154  is held by a pivot  153  which is provided near the magnetic disk  151 . A suspension  155  is attached to a distal end of the actuator arm  154 . A head slider  156  is supported on a lower surface of the suspension  155 . The head slider  153  is provided with a magnetic head which includes one of the magnetic oscillators that are explained with reference to  FIG. 1  to  FIG. 5 . A voice coil motor  157  is formed in a proximal end part of the actuator arm  154 . 
     When the magnetic disk  151  is rotated, the actuator arm  154  is rotated by the voice coil motor  157  and the head slider  156  is loaded onto the magnetic disk  151 , an air bearing surface (ABS) of the head slider  156  provided with the magnetic head is held with a predetermined floating quantity from the surface of the magnetic disk  151 . In this state, information recorded on the magnetic disk  151  can be read out. 
     The head slider  156  may be of a contact motion type in which the slider contacts the magnetic disk  151 . 
       FIG. 9  is an enlarged perspective view of a magnetic head assembly which includes the actuator arm  154  and a distal end part, as viewed from the magnetic disk side. A magnetic head assembly  160  includes an actuator arm  155 , and the suspension  154  is connected to one end of the actuator arm  155 . A head slider  153  which has the magnetic head including one of the magnetic oscillators explained with reference to  FIG. 1  to  FIG. 5  is attached to a distal end of the suspension  154 . A lead line for writing and reading signals is drawn in the suspension  154 , and the lead line  164  is electrically connected to each electrode of the magnetic head incorporated in the head slider  153 . The lead line  164  is connected to an electrode pad  165  of the magnetic head assembly  160 . 
     According to the magnetic reproducing apparatus of the embodiment, it is possible to read information that is magnetically recorded on the magnetic disk  151  with a high recording density, by the magnetic head including one of the magnetic oscillators explained with reference to  FIG. 1  to  FIG. 5 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.