The heart of a computer is a non-volatile storage device that is referred to as a magnetic disk drive. The magnetic disk drive includes a magnetic disk, and write and read heads. The write and read heads are supported by a slider that is mounted on a suspension arm. When the magnetic disk rotates, an actuator swings the suspension arm to place the write and read heads over selected circular tracks on the surface of the rotating magnetic disk. An airflow generated by the rotation of the magnetic disk causes an air-bearing surface (ABS) of the slider to fly at a very low elevation (fly height) over the surface of the rotating magnetic disk. The write and read heads write magnetic transitions to and read magnetic transitions from the rotating magnetic disk, respectively. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions.
The write head includes a coil embedded in an insulation stack that is sandwiched between main and auxiliary poles. The main and auxiliary poles are magnetically coupled at a back gap and are coated with an overcoat. A write current conducted through the coil induces a magnetic flux in the main pole that causes a magnetic field to write the aforementioned magnetic transitions to the rotating magnetic disk.
The read head has traditionally included a current-in-plane (CIP) giant magnetoresistive (GMR) sensor. The GMR sensor includes a magnetically pinned layer and a magnetically free layer separated by an electrically conductive nonmagnetic spacer layer. The relative orientations of the magnetizations of the pinned and free layers change the electrical resistance of the GMR sensor based on the spin-dependent scattering of conduction electrons in the GMR sensor.
Recently, in order to improve the performance of read heads at very small track widths, researchers have focused on the development of current-perpendicular-to-plane (CPP) GMR and tunneling magnetoresistive (TMR) sensors. The TMR sensor also includes a magnetically pinned layer and a magnetically free layer, but both are separated by an electrically insulating nonmagnetic barrier layer. The relative orientations of the magnetizations of the pinned and free layers change the electrical resistance of the TMR sensor based on the spin-dependent tunneling of conduction electrons through the barrier layer.
In order for the TMR sensor to operate stably at very small track widths, it is desired that the free layer has a negative, or at least zero, saturation magnetostriction, λS. After receiving compressive stresses induced by mechanical lapping in the fabrication process of the write and read heads, a free layer with a negative λS longitudinally biases its own magnetization in a longitudinal direction parallel to the ABS in the absence of an external magnetic field. Thus, a free layer only needs low longitudinal bias fields provided by neighboring hard-magnetic films for stable read performance, thereby causing high read sensitivity. In contrast, a free layer with a positive λS transversely biases its own magnetization in a transverse direction perpendicular to the ABS in the absence of an external magnetic field. Thus, a free layer with a positive λS requires high longitudinal bias fields for stable read performance, thereby causing low read sensitivity.
Thus, it is important to use a free layer with a desired negative λS to ensure stable read performance. The most extensively explored TMR sensor with pinned and free layers made of ferromagnetic 60% Co-20% Fe-20% B (in atomic percent) alloys separated by an barrier made of a thin MgOX film, exhibits superior TMR properties. However, its free layer exhibits very highly positive λS mainly due to the high Fe content. For example, in prior art, this TMR sensor exhibits a TMR coefficient, ΔRT/RJ, (where RJ is a minimum junction resistance measured when the magnetizations of the pinned and free layers are parallel to each other, and RJ+ΔRT is a maximum junction resistance measured when the magnetizations of the pinned and free layers are antiparallel to each other) of as high as 138% at a junction resistance-area product, RJ AJ, (where AJ is a junction area) of as low as 2.4 Ω-μm2, after annealing for 2 hour at 360° C. in 8,000 Oe in a high vacuum oven.
It is believed that the high Fe and B contents cause two microstructural effects. First, during depositions on a wafer, the Co—Fe—B pinned layer grows with an amorphous phase, so that the Mg—O barrier layer can grow freely with its {001} crystalline planes in parallel to the wafer surface (or with a <001> crystalline texture). Subsequently, the Co—Fe—B free layer also grows with an amorphous phase. Second, during annealing, Co—Fe—B polycrystalline grains with a body-center-cubic (bcc) <001> crystalline texture nucleate at two Mg—O interfaces, and then grow in the entire Co—Fe—B pinned and free layers. This crystallization results in an epitaxial relationship among the pinned, barrier and free layers, which facilitates coherent spin polarization through the two Mg—O interfaces. As a result, this TMR sensor exhibits superior TMR properties. However, this TMR sensor exhibits a λS of more than 6×10−6, not only due to the high Fe content, but also due to the impractically high temperature which causes unwanted interfacial mixing and inevitably deteriorates overall ferromagnetic properties. Such an impractical high temperature is considered to be crucial for the desired transformation from the amorphous phase (formed after deposition due to the high B content) to the polycrystalline phase.
Therefore, there is a need for the free layer to exhibit a negative λS, while still facilitating the TMR sensor to exhibit superior TMR properties. In addition, to ensure manufacturability, this TMR sensor with such a free layer must be fabricated without using an impractical high temperature.