Fabrication of a single-crystal giant magnetoresistive device on a polycrystalline film

(Nanowerk News) NIMS and AIST have jointly succeeded in fabricating a giant magnetoresistive (GMR) device comprising single-crystal Heusler alloys on an practical silicon substrate (Acta Materialia, "Fully Epitaxial Giant Magnetoresistive Devices with Half-metallic Heusler Alloy Fabricated on Poly-Crystalline Electrode Using Three-Dimensional Integration Technology").
The team demonstrated for the first time that a single-crystal magnetoresistive device can be bonded onto the surface of a polycrystalline electrode using a wafer bonding technique. The fabrication of high-performance single-crystal devices has proven challenging, and these results may provide new guidance in the development of larger capacity hard disk drives (HDDs).
Cross-sectional transmission electron micrographs of the bonded interface between the single-crystal GMR film device and the polycrystalline electrode wafer (two photos at left) and magnetoresistance measured in the device post-bonding (right)
Cross-sectional transmission electron micrographs of the bonded interface between the single-crystal GMR film device and the polycrystalline electrode wafer (two photos at left) and magnetoresistance measured in the device post-bonding (right). (Image: NIMS) (click on image to enlarge)
Some Heusler alloys (e.g., Co2MnSi) are known to possess half metallic property-substances that act as conductors to electrons of only one spin orientation when their atoms are in an orderly arrangement. In previous research, a high-quality GMR device was fabricated using half-metallic Heusler alloys.
This device was reported to exhibit an extremely large magnetoresistance ratio, a characteristic vital to the development of hard disk read heads compatible with HDDs with nearly five times the recording density of current HDDs.
However, the fabrication of this GMR device required the use of a heat-resistant single-crystal magnesium oxide (MgO) substrate with an appropriate lattice structure, which is considerably more costly than using the industrially established small MgO substrate.
In addition, HDD read heads need to be fabricated on a polycrystalline magnetic shield, despite the fact that it is impossible to grow a single-crystal device directly on the surface of a polycrystalline film with crystal grains having various lattice orientations.
Moreover, Heusler alloys have to be annealed at a temperature of at least 300 °C to form an orderly atomic ordering. However, this annealing process also damages the magnetic shield layer.
These issues have prevented practical applications of GMR devices comprising single-crystal Heusler alloys.
This research team deposited a NiAl/CoFe buffer layer on the surface of a conventional, inexpensive silicon substrate and discovered that the substrate acquired significantly increased heat resistance and a smoother surface.
The team then fabricated a GMR device comprising single-crystal Heusler alloys on the silicon substrate and confirmed that the device performs comparably to the GMR device grown on the MgO substrate. The team also grew a polycrystalline electrode film on the surface of another substrate and fabricated a GMR film device comprising single-crystal Heusler alloys on it.
The team subsequently bonded the fabricated GMR film device onto a wafer using the latest three-dimensional integration technology. Through these processes, the team formed an extremely smooth, defect-free bonded interface between the polycrystalline and single crystal films by optimizing wafer bonding conditions.
The team also confirmed that bonding the fabricated GMR film device onto a wafer did not negatively affect the device's magnetoresistive performance.
The techniques the team developed are expected to eliminate the need to grow a single-crystal magnetoresistive device directly on a polycrystalline electrode film and completely resolve the damage issue caused by high-temperature treatment.
These techniques are applicable not only to the fabrication of GMR devices comprising single-crystal Heusler alloys but also for various other purposes, such as the integration of single-crystal tunneling magnetoresistance devices--another potentially promising high-performance technology--onto a heat-susceptible circuit board. The use of these techniques may promote practical use of high-performance single-crystal spintronic devices, thereby potentially contributing to the development of larger capacity HDDs and magnetoresistive random access memory.
Source: National Institute for Materials Science, Japan
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