Patent Application
20150103431
1. Field of the Invention
Embodiments described herein generally relate to a magnetic recording device for recording/reproducing data using the magnetization state of a recording medium. More specifically, embodiments described herein relate to a spin-torque oscillator (STO) with an anti-ferromagnetic coupling interlayer.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
In recent years, the data recording density of magnetic recording devices has continued to increase and the size of 1 bit of a magnetic recording mark for recording to a magnetic medium continues to become smaller. When the magnetic recording density exceeds about 1 Tera bit per square inch (Tbpsi), there is a risk of data recorded to a magnetic recording medium being erased at room temperature due to the effects of heat fluctuation. In order to prevent data from being erased by the effect of heat fluctuation, it is generally necessary to raise the coercive force of the magnetic recording medium. However, there is a limit to the amount of magnetic flux released by a magnetic recording head from recording data by magnetization reversal of a magnetic recording medium.
Measures for solving the above referenced problem have recently focused on assisted recording systems for recording data in conjunction with other technology. One such measure that has been proposed to achieve a high recording density is a method in which a microwave assisted magnetic recording (MAMR) head is utilized. A high frequency magnetic field is applied to recording bits in a magnetic recording medium in order to weaken the coercive force of the recording bits. In this method, data may be recorded using a conventional magnetic recording head. A MAMR enabled magnetic recording head utilizes an STO for generating a microwave (high frequency AC magnetic field). Typically the STO may include a field generation layer (FGL) for generating an AC magnetic field, a spacer layer, and a spin polarization layer (SPL) for transmitting spin polarized torque.
High quality recording can be achieved because the coercive force of the recording medium is lowered when the AC magnetic field is applied to the recording medium. This phenomenon is known as the “assist effect.” Thus, it is important to develop an STO that generates an adequately large AC magnetic field in the MAMR. However, as the value of the applied current to the STO increases, reliability is reduced by a temperature increase of the STO.
Therefore, there is a need in the art for an STO structure where both the FGL and the SPL oscillate and obtain a high assist effect for a low conducting current.
Embodiments described herein generally relate to a magnetic recording device for recording/reproducing data using the magnetization state of a recording medium. More specifically, embodiments described herein provide an STO structure having an SPL and an FGL with an anti-ferromagnetic coupling interlayer disposed between the SPL and FGL. The anti-ferromagnetic coupling interlayer may enable the STO structure to obtain a high assist effect even when operated with a low conducting current.
In one embodiment, an MAMR head is provided. The MAMR head may comprise a main magnetic pole, a trailing shield, and an STO disposed between the main magnetic pole and the trailing shield. The STO may comprise a first magnetic layer, and anti-ferromagnetic coupling interlayer, and a second magnetic layer. The first magnetic layer, the anti-ferromagnetic coupling interlayer, and the second magnetic layer may be laminated in order from the main pole. An anti-ferromagnetic coupling energy of the first magnetic layer and the second magnetic layer may be between about −0.2 erg/cm2 and about −4.0 erg/cm2.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG. 3C1-3C6 depict the time dependence of the magnetization in the film plane of the FGL and the SPL when varying amounts of input current are applied in a conventional AF-mode STO.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention. Furthermore, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).
The STO structure, according to various embodiments described herein, may be disposed between a main magnetic pole of a recording head and a trailing shield. The STO may comprise a first perpendicular magnetic layer (SPL) having an axis of magnetic anisotropy in the direction perpendicular to a film plane, an anti-ferromagnetic coupling conduction layer, and a magnetic layer (FGL) effectively having a plan of easy magnetization in the film plane. The STO may exhibit AF-mode oscillations and conduct current from the FGL to the SPL. The SPL film thickness may be thinner than that of the FOL. Anti-ferromagnetic coupling between the FGL and the SPL may be achieved by the anti-ferromagnetic coupling interlayer.
The magnetic disks 110 may include circular tracks of data on both the top and bottom surfaces of the disk. A magnetic head 180 mounted on a slider may be positioned adjacent a track. As each disk spins, data may be written on and/or read from the data track. The magnetic head 180 may be coupled to the actuator arm 130. The actuator arm 130 may be configured to swivel around an actuator axis 131 to place the magnetic head 180 adjacent a particular data track.
The above description of a typical magnetic disk storage system and the accompanying illustration of
The magnetic read head 211 may be a magnetoresistive (MR) read head that includes an MR sensing element 204 located between MR shields S1 and S2. In other embodiments, the magnetic read head 211 may be a magnetic tunnel junction (MTJ) read head that includes an MTJ sensing device 204 located between MR shields S1 and S2. The magnetic fields of the adjacent magnetized regions in the magnetic disk 202 are detectable by the MR (or MTJ) sensing element 204 as the recorded bits.
The write head 210 may include a return pole 206, an STO 230 disposed between a main pole 220 and a trailing shield 240, and a coil 218 that excites the main pole 220. A recording magnetic field generated from the main pole 220 and the trailing shield 240 helps making the magnetic field gradient of the main pole 220 steep. The main pole 220 may be a magnetic material such as a CoFe alloy. In one embodiment, the main pole 220 may have a saturated magnetization (Ms) of 2.4 T, a torque width of about 60 nm, and a thickness of about 300 nanometers (nm). The trailing shield 240 may be a magnetic material such as a NiFe alloy. In one embodiment, the trailing shield 240 has an Ms of about 1.2 T.
The main pole 220 and the trailing shield 240 have ends 260, 270 defining part of the ABS, and the STO 230 may be disposed between the main pole 220 and the trailing shield 240. The STO 230 may be surrounded by an insulating material in a cross-track direction (into and out of the paper). During operation, the STO 230 generates an AC magnetic field that travels to the magnetic disk 202 to lower the coercivity of the region of the magnetic disk 202 adjacent to the STO 230. The STO 230 will be discussed in detail below. The write head 210 may also include a heater 250 for adjusting the distance between the read/write head 200 and the magnetic disk 202. The location of the heater 250 is not limited to above the return pole 206, as shown in
FIG. 3C1-3C6 depict the time dependence of the magnetization in the film plane of the FGL and the SPL when varying amounts of input current are applied in a conventional AF-mode STO. For example, when a small current, such as 2 mA is applied, the FGL may not oscillate because the applied spin torque is too small. Thus, the resulting AC magnetic field that was generated decreases and an assist effect is not obtained. If a larger current, such as 3 mA, is applied, the time averaged AC magnetic field generated by the FGL attenuates and a large assist effect may not be obtained. The unstable FGL oscillations at low current are a result of the magnitude of the spin torque applied to the FGL. The SPL is proportional to the conducted current value and inversely proportional to the film thickness and the saturated magnetization. The saturated magnetization and the film thickness of the SPL must be smaller than those of the FGL, therefore, the magnitude of the spin torque applied to the SPL is larger than the spin torque applied to the FGL. As result, when the relative bias current is relatively low, the SPL oscillates relatively stably but the FGL repeatedly oscillates and stops because the torque applied to the FGL is small.
If an even larger current is applied, such as 4 mA, the FGL and the SPL maintain a large in plane magnetization component and oscillate stably over time and a large assist effect is effectively obtained under these conditions. As previously described, a large application current may be necessary to obtain a large assist effect because the FGL does not oscillate stably when a small current is applied to the STO.
The FGL 408 may comprise a magnetic material or magnetic alloy, such as CoFe, and the FGL 408 may have a thickness of between about 5 nm and about 15 nm, such as about 10 nm. The perpendicular anisotropic magnetic field (Hk) of the FGL 408 may be from about −1 to about 1, such as about 0. The saturated magnetization (Ms) may be from about 1 T to about 3 T, such as about 2.3 T. In certain embodiments, it may be desirable to increase the in-plane component of the FGL 408 magnetization. As such, a material having a larger Ms and a zero or negative perpendicular anisotropic energy may be employed for the FGL 408.
The SPL 404 may also comprise a magnetic material or magnetic alloy, such as Co, Ni, or CoNi, and the SPL 404 may have a thickness of between about 2.5 nm and about 4.5 nm, such as about 3.5 nm. The perpendicular anisotropic magnetic field of the SPL 404 may be from about 10 kOe to about 16 kOe, such as about 143 kOe. The non-magnetic layer 406, or anti-ferromagnetic coupling interlayer, may be disposed between the SPL 404 and the FGL 408. As a result, the SPL 404 and the FGL 408 may be anti-ferromagnetically coupled.
The non-magnetic layer 406 may comprise a non-magnetic material, such as Cu, Cr, Ru, Rh or Ir. A thickness of the non-magnetic layer 406 may be between about 0.4 nm and about 1.5 nm, such as about 0.8 nm. The exchange coupling energy of the FGL 408 and the SPL 404, the perpendicular anisotropic film and the magnetic layer, respectively, may exhibit a plane of easy magnetization in the film plane from about −0.2 erg/cm2 to about −4.0 erg/cm2, such as about −1.6 erg/cm2. In a conventional AF-mode STO, the interlayer may be Cu, have a film thickness of about 3 nm, and exhibit an exchange coupling energy of about 0 erg/cm2. Thus, the MAMR head 400 with the STO 230 having the anti-ferromagnetic coupling interlayer 406 may provide for an improved signal to noise ratio (SNR) when compared to a conventional AF-mode STO.
In AF-mode operation, the current may flow from the FGL 408 in the direction of the SPL 404. As such, the current may flow from the trailing shield 240 to the main pole 220. In the STO 230 structure described above, the SPL 404 may easily increase the reversal speed of the SPL 404 because the SPL 404 is positioned on the main pole 220 side that has a strong magnetic field of a trailing gap. Further, the stability of the oscillations of the FGL 408 and the SPL 404 may be improved.
The STO structure of
The range of the exchange coupling energy of the SPL and FGL that may obtain an advanced assist effect utilizing the anti-ferromagnetic coupling interlayer 406 are explained below.
Because the exchange coupling energy dependence of the SNR corresponds well to the exchange coupling energy dependence of the AC magnetic field strength shown in
The range of the exchange coupling energy obtained by the high AC magnetic field and the SNR as described above may be realized by utilizing an appropriate material and film thickness in the anti-ferromagnetic coupling interlayer 406. Materials such as Ru, Cr, Cu, Rh, and Ir may be utilized as the anti-ferromagnetic coupling interlayer 406 and may be implemented with a film thickness of between about 0.4 nm to about 1.5 nm.
Table 1 depicts the relationship between the AC magnetic field and the SNR when the materials described with regard to
In sum, the STO, according to the embodiments described herein, may exhibit AF-mode oscillations and conduct current from the FGL to the SPL. The SPL film thickness may be thinner than that of the FGL. Anti-ferromagnetic coupling between the FGL and the SPL may be achieved by the anti-ferromagnetic coupling interlayer. Ultimately, the anti-ferromagnetic coupling interlayer may enable the FGL and the SPL to oscillate stably in the AF-mode and obtain a high assist effect for a low conducting current.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
