1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a dual CPP sensor with a Heusler alloy free layer and low current-induced noise.
2. Background of the Invention
One type of conventional MR sensor used as the read head in magnetic recording disk drives is a “spin-valve” (SV) sensor. A SV MR sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance (ΔR), with the measure of the output of the sensor, or its magnetoresistance, being ΔR/R.
In a magnetic recording disk drive SV read sensor or head, the stack of layers are located in the read “gap” between magnetic shields. The magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the SV is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor.
CPP-SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January 2002. Another type of CPP sensor is a magnetic tunnel junction (MTJ) sensor, also called a tunneling MR (TMR) sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a TMR read head the nonmagnetic spacer layer is an electrically insulating material, such as TiO2, MgO or Al2O3, while in a CPP-SV MR read head the nonmagnetic spacer layer is formed of an electrically conductive material such as Cu.
Conventional ferromagnetic materials, such as NiFe and CoFe alloys, have typically been used for the free and pinned layers in both CIP and CPP sensors. However, as the density and bandwidth of recording devices increase, there is a need to increase the magnetoresistance of the sensor. Ferromagnetic Heusler alloys have been investigated for use in the ferromagnetic free and pinned layers of CPP sensors because they are known to have high spin-polarization. A Heusler alloy is a metal alloy based on a Heusler phase. Heusler phases are intermetallics with particular composition and crystal structure. A perfect ferromagnetic Heusler alloy with 100% spin-polarization will result in large magnetoresistance when incorporated into a CPP spin-valve sensor.
However, the high spin polarization of Heusler alloys in the free and pinned layers increases the susceptibility of CPP sensors to current-induced noise and instability. CPP sensors in general are susceptible to current-induced noise and instability because the spin-polarized current flows perpendicularly through the ferromagnetic layers and produces a spin transfer torque on the local magnetization. This can produce continuous gyrations of the magnetization, resulting in substantial low-frequency magnetic noise if the sense current is above a certain level. This effect is described by J.-G. Zhu et al., “Spin transfer induced noise in CPP read heads,” IEEE TRANSACTIONS ONMAGNETICS, Vol. 40, pp. 182-188, January 2004. This undesirable effect generally increases with ferromagnetic materials that have high spin-polarization, like Heusler alloys.
It has been demonstrated that dual CPP-SV sensors may reduce the sensitivity of the free layer to spin-torque-induced instability. (J. R. Childress et al., “Dual current-perpendicular-to-plane giant magnetoresistive sensors for magnetic recording heads with reduced sensitivity to spin-torque-induced noise” J. Appl. Phys. Vol. 99, 08S305, 2006). Dual CPP sensors are well-known. In a dual CPP sensor a second spacer layer is located on the other side of the free layer and a second pinned layer is located on the second spacer layer. U.S. Pat. No. 5,668,688 describes a dual CPP-SV sensor.
US2005/0073778 A1 describes a dual CPP-SV sensor with a Heusler alloy in both the free layer and both pinned layers. However, such a sensor still exhibits undesirable current-induced noise. In particular, noise caused by spin-torque instability of the pinned layers can be a problem.
What is needed is a dual CPP sensor that takes advantage of the high spin-polarization of Heusler alloys but produces minimal current-induced noise without loss of magnetoresistance or sensor resolution.
The invention is a dual CPP sensor wherein the free ferromagnetic layer is formed of a Heusler alloy and each of the pinned ferromagnetic layers is required to be formed of a ferromagnetic material other than a Heusler alloy, like a conventional CoFe or NiFe material. The Heusler alloy material in the free layer may be a known ferromagnetic Heusler alloy material or an alloy with a composition substantially the same as that of a known Heusler alloy, and which results in high magnetoresistance due to enhanced spin polarization and/or enhanced spin-dependent scattering compared to conventional ferromagnetic materials. Each of the two pinned ferromagnetic layers may be an antiparallel (AP) pinned structure wherein first (AP1) and second (AP2) ferromagnetic layers are separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions AP1 and AP2 layers oriented substantially antiparallel. Each AP2 layer is adjacent to one of the two nonmagnetic spacer layers in the dual CPP sensor. The AP2 layers are required to be formed of a ferromagnetic material other than a Heusler alloy. The dual CPP sensor has a higher ΔRA (product of the change in resistance times the cross-sectional area) and lower susceptibility to spin-torque induced noise at a given current density than a dual CPP sensor with a Heusler alloy material in both the free and pinned layers, and thus achieves a higher signal-to-noise ratio (SNR).
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.
The dual CPP read head according to this invention has application for use in a magnetic recording disk drive, the operation of which will be briefly described with reference to
Each of the two pinned ferromagnetic layers in a dual CPP-SV sensor may be a single pinned layer or an antiparallel (AP) pinned structure. An AP-pinned structure has first (AP1) and second (AP2) ferromagnetic layers separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. The AP2 layer, which is in contact with the nonmagnetic APC layer on one side and the sensor's electrically conducting spacer layer on the other side, is typically referred to as the reference layer. The AP1 layer, which is typically in contact with an antiferromagnetic or hard magnet pinning layer on one side and the nonmagnetic APC layer on the other side, is typically referred to as the pinned layer. Instead of being in contact with a hard magnetic layer, AP1 by itself can be comprised of hard magnetic material so that AP1 is in contact with an underlayer on one side and the nonmagnetic APC layer on the other side. The AP-pinned structure minimizes the net magnetostatic coupling between the reference/pinned layers and the free ferromagnetic layer. The AP-pinned structure, also called a “laminated” pinned layer, and sometimes called a synthetic antiferromagnet (SAF), is described in U.S. Pat. No. 5,465,185.
Each of the pinned layers in the dual CPP-SV sensor in
Located between the lower shield layer S1 and the bottom AP-pinned structure are the bottom electrical lead 126 and a seed layer 125. The seed layer 125 may be a single layer or multiple layers of different materials. Located between the top AF layer 164 and the upper shield layer S2 are a capping layer 112 and the top electrical lead 113. The capping layer 112 may be a single layer or multiple layers of different materials, such as a Cu/Ru/Ta trilayer.
In the presence of an external magnetic field in the range of interest, i.e., magnetic fields from recorded data on the disk 12, the magnetization direction 111 of free layer 110 will rotate while the magnetization directions 121, 161 of reference layers 120, 160, respectively, will remain substantially fixed and not rotate. The rotation of the free-layer magnetization 111 relative to the reference-layer magnetizations 121, 161 results in a change in electrical resistance. Hence, when a sense current IS is applied between top lead 113 and bottom lead 126, the resistance change is detected as a voltage signal proportional to the strength of the magnetic signal fields from the recorded data on the disk.
The leads 126, 113 are typically Ta or Rh. However, any low resistance material may also be used. They are optional and used to adjust the shield-to-shield spacing. If the leads 126 and 113 are not present, the bottom and top shields S1 and S2 are used as leads. The seed layer 125 is typically one or more layers of NiFeCr, NiFe, Ta, Cu or Ru. The AF layers 124, 164 are typically a Mn alloy, e.g., PtMn, NiMn, FeMn, IrMn, IrMnCr, PdMn, PtPdMn or RhMn. If a hard magnetic layer is used instead of an AF layer it is typically a CoPt or FePt alloy, for example CoPtCr, in which case a Cr or Cr-alloy seed layer is often used below the hard magnetic layer. The capping layer 112 provides corrosion protection and is typically formed of Ru or Ta.
In the conventional dual CPP-SV sensor the ferromagnetic layers 122 (AP1), 162 (AP1), 120 (AP2), 160 (AP2) and 110 (free layer) are typically formed of crystalline CoFe or NiFe alloys, or a multilayer of these materials, such as a CoFe/NiFe bilayer. The AP2 layers can also be laminated structures to obtain a high degree of spin-dependent interface scattering. For example the AP2 layers can be a FM/XX/FM/ . . . /XX/FM laminate, where the ferromagnetic (FM) layers are formed of Co, Fe or Ni, one of their alloys, or a multilayer of these materials, such as a CoFe—NiFe—CoFe trilayer; and the XX layers are nonmagnetic layers, typically Cu, Ag, or Au or their alloys, and are thin enough that the adjacent FM layers are strongly ferromagnetically coupled.
For example, the AP2 layers may be a CoFe alloy, typically 10 to 40 Å thick, and the free ferromagnetic layer 110 may be a trilayer of a CoFe alloy/NiFe alloy/CoFe alloy where the CoFe alloys are typically 5-15 Å thick, and the NiFe alloy is typically 10-30 Å thick. The APC layers in the AP-pinned structures are typically Ru or Ir with a thickness between about 4-10 Å.
A hard magnetic bias layer (not shown), such as a CoPt or CoCrPt layer, may also be included outside the sensor stack near the side edges of the free ferromagnetic layer 110 or in the stack for magnetic stabilization or longitudinal biasing of the free ferromagnetic layer magnetization 111.
In this invention, the magnetoresistance of the conventional dual CPP-SV as described above was sought to be improved by substituting the conventional ferromagnetic materials used in the AP2 and free layers with a Heusler alloy, which is known to have high spin-polarization in its bulk form. A Heusler alloy is a ferromagnetic metal alloy based on a Heusler phase. Heusler phases are intermetallics with particular composition and crystal structure. Examples of Heusler alloys include but are not limited to the full Heusler alloys CO2MnX (where X is Al, Sb, Si, Sn, Ga, or Ge). Examples also include but are not limited to the half Heusler alloys NiMnSb, PtMnSb, and CO2FexCr(1-x)Al (where x is between 0 and 1). A perfect Heusler alloy with 100% spin-polarization will result in large magnetoresistance when incorporated into a CPP spin-valve sensor. However it is possible that in a thin-film form and at finite temperatures, the crystal structure of the Heusler alloy may deviate from its optimal structure and that the spin polarization will decrease. Nevertheless, a high magnetoresistance can still be obtained as long as the spin polarization exceeds that of conventional ferromagnetic alloys, or if spin-dependent scattering in the Heusler alloy is high. Therefore in this invention and as used herein a “Heusler alloy” shall mean an alloy with a composition substantially the same as that of a known Heusler alloy, and which results in high magnetoresistance due to enhanced spin polarization and/or enhanced spin-dependent scattering compared to conventional ferromagnetic materials such as NiFe and CoFe alloys.
A test structure of a single CPP-SV was fabricated with both the free layer and the single AP2 layer being formed of the Heusler alloy CO2MnGe. This device was patterned into a stack with a 50 nm diameter cross section. The ΔRA (product of the change in resistance times the cross-sectional area) of this test structure was measured as a function of current density and is shown as curve 300 in
Equally important is the noise generated by the sensor. The rapid drop in ΔRA observed for this sensor is due to the onset of spin-torque oscillations of the magnetization of the free layer. These oscillations result in noise in the output ΔV. Thus, the signal-to-noise ratio (SNR) will be even more degraded than expected by the lower ΔRA alone. This has been confirmed by resistance-field high frequency noise measurements taken at increasing bias voltage. For a test structure of a single CPP-SV with a CO2MnGe Heusler alloy in both the free and pinned layer, noise appears at a current density about one-tenth the current density at which it appears for a test structure of a single CPP-SV test structure with conventional CoFe alloy free and pinned layers.
A test structure of a single CPP-SV was then fabricated with the free layer material being primarily CO2MnGe and the single AP2 layer material being other than a Heusler alloy, i.e., a conventional ferromagnetic CO50Fe50 alloy. The ΔRA of this test structure was measured as a function of current density and is shown as curve 400 in
The free layer 210 is preferably a laminated free layer comprising a Heusler alloy layer 210a between ferromagnetic “nanolayers” or sublayers 210b, 210c. Layer 210a has a thickness in the range of about 20 Å to 80 Å and each of the sublayers 210b, 210c has a thickness less than about 15 Å. The sublayers may be formed of a conventional ferromagnetic material, for example CO50Fe50. This type of laminated free layer for a single CPP-SV sensor is described by Hoshiya et al., “Current-perpendicular-to-the-plane giant magnetoresistance in structures with half-metal materials laminated between CoFe layers”, J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 Jun. 2004, pp. 6774-6776.
The laminated free layer 210 may also include nonmagnetic spacer layers between 210a and 210b and/or 210a and 210c to enhance interface scattering. These nonmagnetic spacer layers should be thin enough to ferromagnetically couple 210a and 210b, and 210a and 210c, respectively. For example a 3-5 Å Cu layer may be used for the nonmagnetic spacer layers.
While the invention has been described with respect to a dual CPP-SV read head, the invention is applicable to other types of dual CPP sensors, such as a dual TMR read head.
While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.