CPP GMR and magnetostriction improvement by laminating Co₉₀Fe₁₀ free layer with thin Fe₅₀Co₅₀ layers

[0001] 1. Field of the Invention

[0002] This invention relates generally to giant magnetoresistive (GMR) magnetic field sensors having a spin valve structure and a “current-perpendicular-to-the-plane” (CPP) configuration. More particularly, it relates to such a sensor that has both an enhanced GMR ratio and low coefficient of magnetostriction.

[0003] 2. Description of the Related Art

[0004] Magnetic read sensors that utilize the giant magnetoresistive (GMR) effect for their operation are generally of the “current-in-the-plane” (CIP) configuration, wherein current is fed into the structure by leads that are laterally disposed to either side of an active sensor region and the current moves through the structure essentially within the planes of its magnetic and other conducting layers. Since the operation of GMR sensors depends on the detection of resistance variations in their active magnetic layers caused by changes in the relative directions of their magnetic moments, it is important that a substantial portion of the current passes through those layers so that their resistance variations can have a maximally detectable effect. Unfortunately, CIP GMR sensor configurations typically involve layer stacks comprising layers that are electrically conductive but not magnetically active and that play no role in providing resistance variations. As a result, portions of the current are shunted through regions that produce no detectable responses and, thereby, the overall sensitivity of the sensor is adversely affected. The CPP sensor configuration avoids this current shunting problem by disposing its conducting leads vertically above and below the active sensor stack, so that all of the current passes perpendicularly through all of the layers as it goes from the lower to the upper lead. The CPP configuration thereby holds the promise of being effective in reading magnetically recorded media having recording densities exceeding 100 Gbit/in².

[0005] The pertinent prior art cited below has offered no similar method for improving the sensitivity of the CPP design having a synthetic spin valve stack configuration. Lederman et al. (U.S. Pat. No. 5,627,704) discloses a CPP GMR stack structure formed within a gap located in one of two pole layers of a magnetic yoke structure which also has a transducing gap formed in an ABS plane. The two pole pieces of the yoke serve to guide magnetic flux to the GMR stack which has current leads above and below it and permanent magnet biasing layers horizontally disposed on either side of it.

[0006] Dykes et al. (U.S. Pat. No. 5,668,688) discloses a spin valve CPP configuration in which the active layers form a stack of uniform width disposed between upper and lower shield and conductor layers.

[0007] Smith et al. (U.S. Pat. No. 6,473,279) teaches a CPP-GMR sensor whose ferromagnetic free layer is maintained in a single domain state by a layer configuration in which the free layer is separated from a pinning layer (below the free layer) by a non-magnetic spacer layer and an additional ferromagnetic layer is formed above the free layer and separated from it by an additional non-magnetic spacer layer formed of Ru. The Ru layer induces an anti-ferromagnetic exchange coupling between the additional ferromagnetic layer and the free layer and there is also a direct magnetostatic coupling between the additional ferromagnetic layer and the free layer. This combined interaction stabilizes the domain state of the free layer.

[0008] Redon et al. (U.S. Pat. No. 6,344,954) teaches a magneto-resistive tunnel junction whose ferromagnetic free layer and pinned layers are made of various layers of spin polarizing materials.

[0009] Nishimura (U.S. Pat. No. 6,226,197) teaches a magnetic thin film memory using a variety of ferromagnetic layered materials While the prior art cited above does make use of ferromagnetic materials like those to be used in the novel formation of the present invention, they do not address the issue of improving the sensitivity of a CPP device by a the formation of a free layer having improved magnetic characteristics. In particular, a good free layer for read head operation should be magnetically soft (have low coercivity) so that it can easily respond to external magnetic field fluctuations, yet it must also exhibit a small positive magnetostriction between 10⁻⁶ and 10⁻⁷ to reduce stress-induced magnetic anisotropy common in a free layer that is typically under compressive stress. The method of the present invention will produce such an improved free layer. In addition, the method of the present invention can also be advantageously applied to the formation of a synthetic antiferromagnetic pinned layer with improved characteristics. The cited prior art does not make reference to the improvement of CPP device performance by either such free layer or pinned layer improvement.

[0010] Accordingly, it is a first object of this invention is to provide a novel current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) read-sensor stack structure of a synthetic spin valve configuration, having improved sensitivity and small positive magnetostriction.

[0011] It is a second object of this invention to provide a method to controllably optimize the magnetostriction of a free layer to fulfill requirements of specific applications.

[0012] It is a third object of this invention to provide a method of improving CPP GMR sensor performance by the formation of a synthetic antiferromagnetic pinned layer having improved characteristics.

[0013] The objects stated above will be achieved by a novel laminated free layer configuration within a CPP synthetic spin valve design and/or a novel laminated pinned layer configuration. The free layer comprises layers of CoFe or CoFe laminated on Cu (CoFe when used hereinafter will refer specifically to Co₉₀Fe₁₀ of approximately 5 angstroms thickness, interspersed with very thin layers (called lamina herein, because of their thinness) of FeCo (specifically Fe₅₀Co₅₀) of less than 3 angstroms but preferably approximately 0.5 angstroms thickness. While the examples discussed below will all use Fe₅₀Co₅₀, the same advantages of the present invention can be obtained by using layers of any of the Fe-rich ferromagnetic alloys Co₇₅Fe₂₅, Co₇₀Fe₃₀, Co₆₀Fe₄₀, Co₆₅Fe₃₅ or more generally Co_xFe_1−x with x between 0.25 and 0.75. Therefore, when the symbol FeCo is used hereinafter, it will refer to layers of the Fe-rich ferromagnetic alloys such as Co₇₅Fe₂₅, Co₇₀Fe₃₀, Co₆₀Fe₄₀, Co₆₅Fe₃₅ or, more generally Co_xFe_100−x with x between 25 and 75.

[0014] While FeCo, used alone or grown on a Cu layer (FeCo/Cu), has certain properties that are advantageous in a free layer, it has other properties that make it undesirable for that use. For example, FeCo has larger bulk and interface spin asymmetry parameters than a CoFe layer, which is advantageous. On the other hand, FeCo has a high coercivity and a large positive magnetostriction, both of which are distinctly disadvantageous. CoFe by itself or when grown on a Cu layer (CoFe/Cu), on the other hand, has a low coercivity, which is distinctly advantageous in a free layer, yet it also has a large negative magnetostriction, in the range between −10⁻⁶ to −10⁻⁷, which is disadvantageous.

[0015] The objects of the present invention will be realized by interspersing thin lamina of FeCo with thicker layers of CoFe and Cu spacer layers in repeated structures. These combinations will retain the low coercivity of the CoFe, introduce the advantageous spin asymmetry of the FeCo while bringing magnetostriction values within the acceptable positive limits between 10⁻⁷ and 10⁻⁶ by combining the large positive magnetostriction values of the FeCo with the large negative magnetostriction values of the CoFe. In addition, the bulk and interface properties of the FeCo enhance the overall bulk scattering coefficient of the laminated free layer, thereby enhancing the CPP GMR ratio. Moreover, the arrangement and multiplicity of the lamina make it possible to fine-tune the coercivity and magnetostriction of the free layer, which is distinctly advantageous for the fabrication of a variety of devices.

[0016] The novel laminated free layer will be formed within a typical CPP bottom synthetic type spin valve structure common in the prior art, such as that illustrated schematically in FIG. 1. The CPP structure may or may not also include the novel pinned layer to be described below. The layers of such a CPP structure include a seed layer (44), an antiferromagnetic pinning layer (46), a synthetic antiferromagnetic pinned layer (48) further comprising a second ferromagnetic layer (denoted AP2) (50), a coupling layer (52) and a first ferromagnetic layer (AP1) (54), a Cu spacer layer (25), a ferromagnetic free layer (20) and a capping layer (12). In such a prior art formation the AP1 and AP2 layers would typically be layers of CoFe in a thickness range of between approximately 20–40 angstroms and the free layer (20) would also be a layer of CoFe within the same thickness range or a layer of CoFe grown on a layer of Cu (not shown). Biasing layers and conductor layers are not shown. In the present invention, the novel laminated free layer will replace the single free layer of CoFe shown in the figure.

[0017] The novel laminated pinned layer configuration, which can be used alone or in conjunction with the novel free layer, refers specifically to a new formation of the structure of the first ferromagnetic layer (AP1), which is (54) in FIG. 1. Two different laminated AP1 layers were formed and tested in a CPP sensor stack that was otherwise identical. AP1 layer “A,” which may be considered a reference layer, was formed in a multiply laminated configuration: (CoFe 10/Cu 2)×7/CoFe 10, which means seven repeated layers of 10 angstrom thick CoFe on 2 angstrom thick Cu, formed on a 10 angstrom thick layer of CoFe. This configuration was compared to AP1 layer “B,” which was formed as (CoFe10/Cu2)×3/(FeCo10/Cu2)×3/FeCo10. Thus, in configuration B, part of AP1 is replaced by FeCo10/Cu2 laminations, keeping the total magnetic moment of AP1 constant.

[0023] The present invention is a GMR spin valve sensor in a CPP (current-perpendicular-to-plane) synthetic pinned layer spin-valve configuration, having a novel laminated free layer of low coercivity and low positive magnetostriction. This sensor may also include a novel laminated synthetic pinned layer. The novel free layer includes multiple thin lamina of FeCo interspersed with thicker layers of CoFe, which can be varied to provide an optimal combination of coercivity and magnetostriction. The novel pinned layer includes a ferromagnetic layer in which layers of FeCo replace layers of CoFe for an improvement in the GMR ratio of the sensor.

[0024] Referring to FIGS. 2a–e, there is shown in each figure a schematic cross-sectional view of a stack configuration (a configuration of layers) used to experimentally determine optimum free layer configurations for achieving desired coercivity and magnetostriction values. Within these stack formations, the various layers are formed advantageously by sputtering. FIG. 2a is a reference stack having a prior art free layer, while FIGS 2b–e are exemplary stacks comprising laminated free layers of the present invention in various configurations. Each stack is formed so that its particular free layer configuration is formed between a substrate layer, which is a [seed layer/pinning layer/spacer layer] and a capping layer, which are the same for each stack. More specifically, each of the five configurations has the form:
Ta50/Ru20/MnPt150/Cu30[Free Layer] Cu10/Ta50
where Ta50 is a layer of tantalum 50 angstroms thick, Ru20 is a layer of ruthenium 20 angstroms thick, MnPt is a layer of antiferromagnetic manganese-platinum 150 angstroms thick and Cu30 is a layer of copper 30 angstroms thick. This combination of five layers, the substrate layer, is referred to simply as (10) in each of the figures, FIGS. 2a–e. The capping layer is a layer of copper 10 angstroms thick over which is a layer of tantalum 50 angstroms thick. This capping layer is referred to as (100) in each of the figures FIGS. 2a–d. The free layer, denoted generically as (20) in each of the figures, FIGS. 2a–d, is configured differently in each figure, and will now be described in detail.

[0025] Referring first to FIG. 2a, there is shown the prior art reference configuration. There is seen the free layer (20) which is a single layer of CoFe (Co₉₀Fe₁₀) formed to a thickness of approximately 30 angstroms, which is typical of a prior art sensor such as illustrated in FIG. 1. Testing of this configuration indicates a coefficient of magnetostriction, λ=−6.90×10⁻⁶ and a coercivity, H_c=6 Oe.

[0026] Referring next to FIG. 2b, there is shown a first exemplary free layer (20) which is a laminated configuration formed in accord with the present invention and which will be formed between the substrate (10) and capping layer (100). As we will see in this and the succeeding figures of FIG. 2, the variation of laminas, layers and spacer layers will permit the formation of a free layer with magnetostriction coefficients ranging between positive and negative values.

[0027] The first layer of this free layer is a first layer of CoFe (22) formed to a thickness between approximately 5 and 15 angstroms, with approximately 10 angstroms being preferred. On this layer is formed an ultra- thin layer (referred to hereinafter as a lamina) of FeCo (24) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. In all the following examples, FeCo refers specifically to Fe₅₀Co₅₀, of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred; but as has been noted above, more generally Co_xFe_100−x with x between 25 and 75 can be used to fulfill the objects of the invention. On this lamina is formed a second layer of CoFe (26) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this layer is formed a Cu layer (28) of thickness between approximately 1 and 4 angstroms, with approximately 2 angstroms being preferred. The non-magnetic Cu layer acts as a spacer layer and has been experimentally shown to have beneficial effects on the magnetic performance parameters of ferromagnetic layers grown upon it and to allow the advantageous adjustment of magnetostriction values and GMR enhancement. On this Cu layer is formed a third layer of CoFe (30) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this CoFe layer there is formed a second lamina of FeCo (32) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. On this lamina is formed a second layer of Cu (33) of thickness between approximately 1 and 4 angstroms, with approximately 2 angstroms being preferred. On this Cu layer is formed a fourth layer of CoFe(34) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. Testing of this configuration indicates a coefficient of magnetostriction, λ=+9.00×10^{31 7} and a coercivity, H_c=13.

[0028] Referring next to FIG. 2c, there is shown a second exemplary free layer (20) which is a laminated configuration formed in accord with the present invention. The first layer of this free layer is a first layer of CoFe (42) formed to a thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this layer is formed a layer of Cu (44) of thickness between approximately 1 and 4 angstroms, with approximately 2 angstroms being preferred. On this Cu layer is formed a second layer of CoFe (46) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this CoFe layer is formed a lamina of FeCo (48) (in all these examples, specifically Fe₅₀Co₅₀) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. On this lamina is formed a third layer of CoFe (50) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this layer is formed a second Cu layer (52) of thickness between approximately 1 and 4 angstroms, with approximately 2 angstroms being preferred. On this Cu layer is formed a fourth layer of CoFe (54) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this CoFe layer there is formed a second lamina of FeCo (56) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. On this lamina is formed a fifth layer of CoFe (58) of thickness between approximately 2.5 and 7.5 angstroms with approximately 5 angstroms being preferred. On this CoFe layer is then formed a third layer of Cu (60) of thickness between approximately 1 and 4 angstroms, with approximately 2 angstroms being preferred. On this Cu layer is formed a sixth layer of CoFe(62) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. Testing of this configuration indicates a coefficient of magnetostriction, λ=−8.90×10⁻⁷ and a coercivity, H_c=13.9.

[0029] Referring next to FIG. 2d, there is shown a third exemplary free layer (20) which is a laminated configuration formed in accord with the present invention. The first layer of this free layer is a first layer of CoFe (72) formed to a thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this layer is formed a layer of Cu (74) of thickness between approximately 1 and 4 angstroms, with approximately 1 angstrom being preferred. On this Cu layer is formed a second layer of CoFe (76) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this CoFe layer is formed a lamina of FeCo (78) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. On this lamina is formed a third layer of CoFe (80) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this layer is formed a second Cu layer (82) of thickness between approximately 1 and 4 angstroms, with approximately 1 angstrom being preferred. On this Cu layer is formed a fourth layer of CoFe (84) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this CoFe layer there is formed a second lamina of FeCo (86) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. On this lamina is formed a fifth layer of CoFe (88) of thickness between approximately 2.5 and 7.5 angstroms with approximately 5 angstroms being preferred. On this CoFe layer is then formed a third layer of Cu (90) of thickness between approximately 1 and 4 angstroms, with approximately 1 angstrom being preferred. On this Cu layer is formed a sixth layer of CoFe(91) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. Testing of this configuration indicates a coefficient of magnetostriction, λ=−8.00×10⁻⁷ and a coercivity, H_c=12.0.

[0030] Referring next to FIG. 2e, there is shown a fourth exemplary free layer (20) which is a laminated configuration formed in accord with the present invention. The first layer of this free layer is a first layer of CoFe (102) formed to a thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this layer is formed a lamina of FeCo (104) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. On this lamina is formed a second layer of CoFe (106) of thickness between approximately 2.5 and 7.5 angstroms with approximately 5 angstroms being preferred. On this layer is formed a first layer of Cu (108) of thickness between approximately 1 and 4 angstroms, with approximately 2 angstrom being preferred. On this Cu layer is formed a third layer of CoFe (110) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this CoFe layer is formed a second lamina of FeCo (112) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. On this lamina is formed a fourth layer of CoFe (114) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this layer is formed a second Cu layer (116) of thickness between approximately 1 and 4 angstroms, with approximately 2 angstrom being preferred. On this Cu layer is formed a fifth layer of CoFe (118) of thickness between approximately 2.5 and 7.5 angstroms, with approximately 5 angstroms being preferred. On this CoFe layer there is formed a third lamina of FeCo (120) of thickness less than 3 angstroms, with approximately 0.5 angstroms being preferred. On this lamina is formed a sixth layer of CoFe (122) of thickness between approximately 2.5 and 7.5 angstroms with approximately 5 angstroms being preferred. Testing of this configuration indicates a coefficient of magnetostriction, λ=−1.90×10⁻⁷ and a coercivity, H_c=8.0.

[0031] It is to be noted that one purpose of presenting these examples is to indicate the degree of control over magnetostriction and coercivity that is provided by the interspersal of the FeCo lamina and the Cu layers with the CoFe layers. Although the first example (FIG. 2a) provided a desirable positive magnetostriction, it should not be considered as the only configuration that would yield such a magnetostriction.

[0032] Referring next to FIGS 3a–c, there are shown three graphs, schematically indicating the relationship between CPP resistance (vertical axis) and applied magnetic field, Hy (Oe) (horizontal axis) for three different free layer configurations. The maximum height differential of the graphs (shown as ΔR) is indicative of the GMR ratio (ΔR/R) and the horizontal distance (arrows) between the two portions of the graph (forward and reverse swing of the magnetic field) is an indication of the coercivity of the free layer. Referring first to FIG. 3a, there is shown the performance of a prior art type free layer of the form (the numbers in parentheses being thicknesses in angstroms):
CoFe(10)/Cu(2)/CoFe(10)/Cu(2)/CoFe(10)

[0033] The graph is indicative of a low coercivity, which is measured to be approximately 6.2 Oe and a good GMR ratio of approximately 2.05%.

[0034] Referring next to FIG. 3b, there is shown the performance of a free layer containing only FeCo, specifically:
FeCo(10)/Cu(2)/FeCo(10)/Cu(2)/FeCo(10).

[0035] As has been previously noted, one of the disadvantages of FeCo is its high coercivity and this can be clearly seen in the graph. The measured coercivity of this configuration is 151 Oe and the GMR ratio is a good 2.25%. The good GMR ratio produced by FeCo has also been noted above.

[0036] Referring next to FIG. 3c, there is shown the performance of a free layer formed in accord with the present invention:
CoFe(10)/FeCo(0.5)/Cu(2)/FeCo(0.5)/CoFe(10)/FeCo(0.5)/Cu(2)/FeCo(0.5)/CoFe(10).
As can be seen in the graph, the GMR ratio is quite similar to that displayed by the pure FeCo free layer (FIG. 3b), while the coercivity is comparable to that of the CoFe/Cu free layer of FIG. 3a. Specifically, the GMR ratio is 2.22% and the coercivity is 5.9 Oe. Referring next to FIG. 4a, there is shown a schematic cross-sectional view of a synthetic antiferromagnetic pinned layer such as is illustrated in prior art FIG. 1 as layer (48), but having one layer (the AP1 layer) formed in accord with the present invention. Referring again to FIG. 1, it is seen that the two ferromagnetic layers forming the pinning layer are designated AP1 (54) and AP2 (50), with AP1 being closest to the free layer. The present invention provides a laminated structure for the AP1 layer, utilizing FeCo layers in a thickness range between approximately 5 and 15 angstroms, with 10 angstroms being preferred, in place of the prior art CoFe layers. Note that the FeCo layers could also be layers of the Fe-rich ferromagnetic alloys Co75Fe25, Co70Fe30, Co60Fe40, Co65Fe35 or more generally Co_xFe_100−x with x between 25 and 75. To this structure there is added thin layers of Cu, in a thickness range between 1 and 4 angstroms with approximately 2 angstroms being preferred. This new structure produces greatly improved sensor performance. Note, we are using the more generic term “layer” to describe the thicker FeCo layers in the pinned layer as opposed to our use of “lamina” as a distinguishing term to describe the ultra-thin layers of FeCo used in forming the free layer of the sensor.

[0037] Referring back to FIG. 4a, there is seen an enlarged cross-sectional schematic view of an AP1 layer (54), which is formed as a 7-layer structure, of which the first 3 layers (540), (541) and (542) are identical bilayers of (CoFe10/Cu2), the second 3 layers (643), (644) and (645) are identical bilayers of (FeCo10/Cu2) and the 7 th layer (746) is a single layer of FeCo 10. All the numbers refer to thicknesses in angstroms.

[0038] Referring next to FIG. 4b, there is shown a reference AP1 layer (54) of similar total magnetic moment, but formed of six identical (CoFe10/Cu2) bilayers (540), (541), (542), (543), (544) and (545) on a single layer of CoFe 10 (546) and therefore lacking any FeCo lamina. When the AP1 layers of FIGS. 4a and 4b were incorporated within an identical CPP sensor configuration, the layer of FIG. 4a, with FeCo lamina as indicated, demonstrated a marked improvement in sensor performance compared with the reference layer.

[0039] Referring next to FIGS 5a and b, there is shown graphical evidence of the improved performance of the FIG. 4a layered sensor element of the present invention as compared with the FIG. 4b layered sensor element. The two graphs plot variations in sensor resistance (ΔR) as an external magnetic field sweeps in two directions (the GMR ratio being ΔR/R), which is an indication of the sensor's sensitivity to the magnetic field variations in magnetically encoded media. The sensor element of FIG. 4a, as indicated in the graph of FIG. 5a, shows a GMR ratio of 2.25% that is enhanced by approximately 20% to the GMR ratio of 2.64% of the element of FIG. 4b as shown graphically in FIG. 5b.

[0040] As is understood by a person skilled in the art, the preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. Revisions and modifications may be made to methods, materials, structures and dimensions employed in forming a CPP GMR sensor of the synthetic spin valve type whose laminated free layer has good coercivity, a high GMR ratio and low coefficient of magnetostriction, while still providing the CPP GMR sensor of the synthetic spin valve type whose laminated free layer has good coercivity, a high GMR ratio and low coefficient of magnetostriction so formed, in accord with the spirit and scope of the present invention as defined by the appended claims.