Modified field generation layer for microwave assisted magnetic recording

[0001] The invention relates to the general field of microwave assisted magnetic recording with particular reference to the design of the field generator portion of the spin torque oscillator.

[0002] Microwave assisted magnetic recording, MAMR, is one of several future technologies that are expected to extend perpendicular magnetic recording beyond 1 terabit per square inch. In this scheme, a field generator is placed in close proximity to the write element to produce a high frequency oscillating field in the media film plane. The frequency range of the oscillating field could be from 5 GHz to 50 GHz. Because of ferromagnetic resonance (FMR), it becomes possible to switch media grains at fields below their normal coercivity i.e. a lower write field may be used, but only in the immediate vicinity of the microwave assisted write element.

[0003] A microwave field generator is basically a spin torque oscillator (STO). It resembles a current-perpendicular-to-plane (CPP) GMR or TMR structure in that the current flows perpendicular to the film, passing through a spacer layer into a reference layer. The magnetization directions in the stack are, however, different from those of a CPP GMR/TMR sensor.

[0004] As schematically illustrated in the top section of FIG. 1, the simplest configuration for a STO is a tri-layer stack consisting of a spin injection layer (SIL) 12, non-magnetic spacer 16 (metallic or insulating), and field generating layer (FGL) 13. The SIL magnetization must be kept perpendicular to the plane of the film, either by an external magnetic field or through its intrinsic perpendicular magnetic anisotropy (PMA). As electrons in oscillating stack 14 transit SIL 12 their spins become polarized by the magnetization present in SIL 12. The degree of spin polarization is further enhanced by the greater probability of electrons, whose magnetization is in the same direction as reference layer 17, being able to transit spacer 16. Spin torque oscillation then occurs in the FGL 13, resulting in the generation of microwaves.

[0005] The lower section of FIG. 1 shows how the STO is positioned as part of a perpendicular magnetic write head for use in MAMR. The STO (rotated 90° relative to its orientation in the upper section of FIG. 1) is positioned with its top electrode 11 up against main pole 22 and it bottom electrode 18 up against trailing shield 21. When data is to be written, microwaves emerging from the STO illuminate recording medium layer 20 (seen above soft magnetic underlayer 19) just ahead of main pole 22.

[0006] A paper by C. Slonczewski [1] on spin-transfer torque (STT) magnetization switching has attracted considerable interest due to its potential application to spintronic devices such as STT-MRAM on a gigabit scale. Recently, J-G. Zhu et al. [2] described another spintronic device called a spin transfer oscillator where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. See FIG. 1 above.

[0007] In two recent patent applications by Headway [3] [4], it was shown that a large perpendicular magnetic anisotropy (PMA) can be established in both the [Co/Ni]xn and [CoFe/Ni]xn multilayer systems by using a seed layer of Ta/Ru/Cu in combination with a relatively low total thickness. Thus an effective MAMR device can be fabricated with these [Co/Ni] and/or [CoFe/Ni]xn multilayer PMA systems.

[0008] Furthermore, it was shown that the performance of FGLs such as FeCo, FeCoAl, etc can be improved by inserting a magnetic layer such as a [Co/Ni]xn multilayer or a [CoFe/Ni]xn multilayer having PMA. However, these patent applications do not cover all possible schemes for the top STO case. The present invention discloses particular STO structures that utilize [Co(Fe)/Ni] multilayer PMA coupling with a high saturation magnetization (Ms) material such as FeCo to form the FGL.

• [0009] [1] C. Slonczewski, “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996)
• [0010] [2] J. Zhu et al, “Microwave Assisted Magnetic Recording”, IEEE. Trans. Magn. 44, 125 (2008)
• [0011] [3] K. Zhang, et. al. Headway application Ser. No. 12/456,621
• [0012] [4] K. Zhang, et. al. Headway application Ser. No. 12/800,196

[0013] A routine search of the prior art was performed with the following references of interest being found:

[0014] U.S. Pat. No. 7,616,412 (Zhu et al) discloses a reference stack comprising both perpendicular and in-plane components for microwave assisted magnetic recording while U.S. Pat. No. 7,352,658 (Shimazaki et al) shows a first magnetic film having perpendicular anisotropy and a second magnetic film having either perpendicular or in-plane anisotropy.

[0015] In regard to U.S. Pat. No. 7,616,412 (Zhu et al.), this invention proposes to cover a reference layer system with a weaker perpendicular magnetic anisotropy (PMA). An example would be a system with both in-plane and out-of-plane anisotropies, but with the out-of-plane anisotropy being greater than the in-plane anisotropy, thereby tilting the magnetization partially out of the plane. As will become apparent below, this teaches away from the present invention where the in-plane and out-of-plane anisotropies are in different layers. Zhu et al. speculate that, even when the anisotropy is partially tilted in-plane, they can still have adequate signal readout without a sensor because of the in-plane component of the anisotropy. This suggests that without tilting, the readout would be zero because both the oscillating and the reference layers are parallel to each other (CPP GMR=0).

[0016] It has been an object of at least one embodiment of the present invention to provide a spin torque oscillator that generates microwaves at low current densities

[0017] Another object of at least one embodiment of the present invention to provide a method to enable a spin torque oscillator (STO) to generate microwaves at low current densities

[0018] Still another object of at least one embodiment of the present invention has been for said spin torque oscillator to be suitable for use in Microwave Assisted Magnetic Recording

[0019] A further object of at least one embodiment of the present invention has been for manufacture of said spin torque oscillator to be fully compatible with current methods for manufacturing such devices.

[0020] These objects have been achieved by replacing the conventional Field Generation Layer (FGL) with a bilayer—FGL1 and FGL2. FGL1 is characterized by having an easy axis that is normal to the spacer (i.e. it exhibits PMA) while FGL2 has its easy axis parallel to the spacer layer. (i.e. it exhibits conventional in-plane anisotropy).

[0021] Since FGL1 and FGL2 are strongly exchange coupled to each other, they will be excited simultaneously to oscillate as though they were a single layer.

[0022] A critical requirement of the invention is that, regardless of whether the device's spin injection layer (SIL) is above or below the spacer layer, FGL1 (which has the PMA) must be closer to the spacer than FGL2 (which has the in-plane anisotropy).

[0028] In order for a MAMR device to work, the FGL layer is required to generate a large high frequency magnetic field (Hac) typically in association with the highest saturation magnetization (Bs) of the FGL material. However, once Bs gets too high, the critical current density also become very large, thereby raising serious reliability concerns.

[0029] The Slonczewski model discussed earlier suggests that once the FGL layer has a PMA or partial PMA, the critical current density for spin transfer could be greatly reduced. The present invention discloses how the PMA of the [CoFe/Ni] multilayer system can be utilized to induce partial PMA in a high Bs FGL material such as FeCo through exchange coupling. Doing so, i.e. having a high moment FGL material with partial PMA, greatly facilitates the onset of FGL oscillation.

[0030] Referring now to FIGS. 2a and 2b, shown there are two prior art structures in which SIL layers 12 are located at the bottom and at the top, respectively. Also seen are spacer 16 and FGL 13.

[0031] The invented structures shown in FIGS. 3a and 3b also have their SIL layers at the bottom and top, respectively, but FGL 13 in each case can be seen to now be made up two sub-layers, 331 and 332, rather than being a single uniform layer.

[0032] For both FIGS. 3a and 3b, FGL sub layer 331 is the closest to spacer 16 and is characterized by having an easy axis that is normal to the spacer's two opposing surfaces (i.e. it exhibits conventional PMA) while FGL sub-layer 332 has its easy axis parallel to spacer 16 (i.e. it exhibits in-plane anisotropy). Since FGL sub-layers 331 and 332 are strongly exchange coupled to each other, they will be excited simultaneously so composite FGL layer 33 will oscillate as a single layer.

[0033] The benefit of having high PMA FGL 331 (such as a [Co(Fe)/Ni]xn or [Co(Fe)/Pt]xn) in direct contact with the high Bs FGL 332 material (such as FeCo or FeCoAl etc.) will be discussed further below. For the invented STO to work, the FGL layer has to be excited through spin torque exerted by reflected spin polarized electrons from the SIL. So, if FGL 331 is given a high PMA, the reflected polarized electrons from the SIL can easily excite it since SIL 12 and FGL 331 are now magnetized in the same direction, thereby greatly reducing the energy barrier between them.

[0034] Once FGL 331 is excited, the spin torque acting thereon will rapidly propagate into FGL 332 causing it to oscillate. In other words, FGL 331 can be thought of as acting as a spin torque ignition layer. The spin torque that originates in the SIL and the FGL will add up, since they are in the same direction, and will thereby enhance the FGL 332 oscillations. Additionally, electrons from FGL 332 will also help FGL 331 to oscillate in plane, thereby causing it to oscillate even more vigorously.

[0035] Similar mechanisms apply to FIG. 3b, the top STO case where SIL 21 is located on top of the FGL and where FGL 331 has a high PMA. In this case, the PMA layer has been placed on top of the in-plane magnetic anisotropy layer.

[0036] It is important to note that if the order of the FGL sub-layers is switched, for example if FGL 331 is inserted on top of FGL 332 (through modification of FIG. 3a) or FGL 331 is inserted below FGL 332 layer (through modification of FIG. 3b), the STO oscillator will not operate efficiently because the spin torque of the reflected electrons from the SIL and the spin torque of the electrons in FGL 331 now act in opposite directions, resulting in their mutual cancellation. Another example of incorrect insertion of a partial anisotropy FGL can also be seen in prior art FIG. 1

[0037] In summary, for both the bottom SIL and top SIL cases, the FGL sub-layer that has a PMA structure needs to be the one in immediate contact with the spacer layer. When that is the case, STO oscillations can be generated at current densities as low as 1×10⁸ A/cm².

[0038] For evaluation as a preferred embodiment, a bottom SIL structure was fabricated in the following configurations. Each value next to an individual layer indicates that film's thickness in Angstroms.

[0039]
Ta10/Ru20/Cu20/[CoFe(x %)2/Ni6]xn/spacer/[CoFe(x %)2/Ni6]xm/FeCo100/Ru10/Ta4 0/Ru30

[0040] In the above structure, Ta10/Ru20/Cu20 is used as a composite seed layer, [CoFe(x %)2/Ni5]xn multilayer structure as a reference layer, with x % ranging from 0% (pure Co/Ni case) to 90% for CoFe(x %) compositions and n ranging from 5 to 50 with a preferred range from 10 to 30 laminations; the spacer material can be Cu for CPP-GMR or MgO, AlOx, TiOx, ZnO for TMR. FeCo layer of 50 Å to 300 Å is served as the FGL layer. Finally, Ru10/Ta40/Ru30 is used as the capping layer.

[0041] The annealing temperature for such devices can be from 150 to 300° C. with a preferred range of 180 to 250° C. and annealing can be from 0.5 hours to 5 hours. In particular, [CoFe(x %)2/Ni5]xm (with m ranging from 5 to 30 with a preferred range of from 5 to 20) is inserted underneath the FeCo100 layer; since the [CoFe/Ni]xm layer is strongly magnetically coupled with the FeCo layer, the strong PMA of the [CoFe(x %)2/Ni5]xm will force the anisotropy of the FeCo to tilt partially toward the perpendicular direction thereby enabling the full FGL to oscillate at a current density as low as 1×10⁸ A/cm².

[0042] Note that in U.S. Pat. No. 7,616,412 (Zhu et al.) the PMA layer has the opposite sign of the PMA. This is a fundamental difference as it implies a totally different spin torque contribution that is of no value in the present context (even if it were manufacturable).

[0043] Shown in FIGS. 4a-4d are experimental results for a device built with the following specific dimensions:
Ta10/Ru20/Cu20/[Co(50%)Fe2/Ni5]x15/Cu20/[Co(50%)Fe2/Ni5]x10/Fe(70%)Co100/Ru10/Ta40/Ru30

[0044] FIG. 4a shows the desired strong oscillation features, including large dR rise, starting from a bias voltage as low as 30 my. FIGS. 4b and 4c confirm that high Q Ferromagnetic Resonance (FMR) at frequencies as high as 35 GHz has been achieved. FIG. 4d shows the peak FMR frequencies vs applied field for different bias voltages. It can be seen that, with the assistance of the PMA (from [Fe(50%)Co2/Ni6]-FGL1), high quality total FGL oscillations were obtained.

[0045] For comparison purposes, we also built an STO device having the prior art structure in which FGL1-[Co(50%)Fe2/Ni6]x10 was placed on top of FGL2-FeCo100. As follows:
Ta10/Ru20/Cu20/[Co(50%)Fe2/Ni5]x15/Cu20/Fe(70%)Co100/[Co(50%)Fe2/Ni5]x10/Ru10/Ta40/Ru30

[0046] In this case, as discussed above, the spin torque from the SIL layer-[Co(50%)Fe2/Ni5]x15 is largely canceled by the FGL1 layer-[Co(50%)Fe2/Ni5]x10, so the total spin torque acting on the FGL of Fe(70%)Co100/[Co(50%)Fe2/Ni5]x10 is very small. As a result, the oscillation properties are very poor, as shown in FIGS. 5a-5d.

[0047] As a preferred embodiment, a top type (SIL layer on the top) structure with the following configuration was fabricated. Each value next to the individual layer indicates the film thickness in angstroms.

[0048] Ta10/Ru20/Cu20/FeCo100/[CoFe(x %)2/Ni5]xm/spacer/[CoFe(x %)2/Ni5]xn/Ru10/Ta4 0/Ru30

[0049] In the above structure, [CoFe(x %)2/Ni5]xm (with m ranging from 5 to 30 with a preferred range of 5 to 20) is inserted on top of the FeCo100 layer, since the [CoFe/Ni]xm layer is magnetic coupled with FeCo layer strongly, the strong PMA of the [CoFe(x %)2/Ni5]xm will force the anisotropy of the FeCo to tilt partially toward perpendicular direction whereby the full FGL was able to oscillate at current densities as low as 1×10⁸ A/cm².

[0050] Similar data for the bottom STO case was also achieved.

[0051] In summary, when the FGL layer next to the spacer has high PMA, better device performance results due to easier FGL oscillations at low current density.

[0052] General Comments on Dimensions, Possible Material Substitutions, Etc:

[0053] Seed layer thickness should fall within the following ranges (in Angstroms): Ta from 5 to 50; Ru from 5 to 100; Cu from 0 to 100 (in some cases Cu can be completely removed with only Ta/Ru as seed layers).

[0054] For [CoFe(x %)(t1)/Ni(t2)]xn multilayer structures, t1 of the Co layer can vary from 0.5 to 5 Å with a preferred range of 1.5 to 3 Å; t2 of Ni layer can range from 2 to 10 Å a with a preferred range of 3.5 to 8 Å; x in CoFe(x %) may range from 0 (pure Co case) to 90; for n in [CoFe(x) t1/Ni(t2)]xn, n may range 5 to 50 with a preferred range of 10 to 30. The spacer material may be Cu for CPP-GMR or MgO, AlOx, TiOx, or ZnO for TMR. A FeCo layer of 50 to 300 Å is suitable for as the FGL layer. Ru10/Ta40/Ru30 is used as the capping layer and an annealing temperature for such devices may range from 150 to 300° C. with a preferred range of 180 to 250° C.; annealing may be from 0.5 hours to 5 hours.

[0055] Inside the composite seed layer material: (Ta/Ru/Cu), Cu can be replaced by Ti, Pd, W, Rh, Au, Ag etc having thicknesses in a range of from 1 to 100 Å. The Ta/Ru/Cu composite seed layer can also be replaced by Ta/Ru with Ta thickness ranging from 5 to 50 Å and Ru ranging from 20 to 100 Å; Ta/Ru/Cu can also be replaced with Ta/Cu or Ta/NiCr with Ta ranging from 5 to 50 Å and Cu ranging from 20 to 50 Å or with NiCr ranging from 40 to 100 Å.

[0056] The [CoFe(x %)(t1)/Ni(t2)]xn systems can also be replaced by [CoFe(x %)(t1)/Pt(t2)]xn or [CoFe(x %)(t1)/Pd(t2)]xn or [CoFe(x %)(t1)/Ir(t2)]xn or other PMA systems such as CoPt, FePt, CoPd, FePd, TbFe(Co) etc.