The exchange biasing of a ferromagnetic (F) film by an adjacent antiferromagnetic (AF) film is a phenomenon that has proven to have many useful applications in magnetic devices, and was first reported by W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 (1959). Whereas the magnetic hysteresis loop of a ferromagnetic single-layer film is centered about zero field, a F/AF exchange-coupled structure exhibits an asymmetric magnetic hysteresis loop which is shifted from zero magnetic field by an exchange-bias field. In addition to an offset of the magnetic hysteresis loop of the F film, the F film in a F/AF exchange-coupled structure typically shows an increased coercivity below the blocking temperature of the AF film. The blocking temperature is typically close to but below the Neel or magnetic ordering temperature of the AF film. The detailed mechanism that determines the magnitude of the exchange bias field and the increased coercive field arises from an interfacial interaction between the F and AF films.
The most common CIP magnetoresistive device that uses an exchange-coupled structure is a spin-valve (SV) type of giant magnetoresistive (GMR) sensor used as read heads in magnetic recording disk drives. The SV GMR head has two ferromagnetic layers separated by a very thin nonmagnetic conductive spacer layer, typically copper, wherein the electrical resistivity for the sensing current in the plane of the layers depends upon the relative orientation of the magnetizations in the two ferromagnetic layers. The direction of magnetization or magnetic moment of one of the ferromagnetic layers (the “free” layer) is free to rotate in the presence of the magnetic fields from the recorded data, while the other ferromagnetic layer (the “fixed” or “pinned” layer) has its magnetization fixed by being exchange-coupled with an adjacent antiferromagnetic layer. The pinned ferromagnetic layer and the adjacent antiferromagnetic layer form the exchange-coupled structure.
One type of proposed CPP magnetoresistive device that uses an exchange-coupled structure is a magnetic tunnel junction (MTJ) device that has two ferromagnetic layers separated by a very thin nonmagnetic insulating tunnel barrier spacer layer, typically alumina, wherein the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. The MTJ has been proposed for use in magnetoresistive sensors, such as magnetic recording disk drive read heads, and in non-volatile memory elements or cells for magnetic random access memory (MRAM). In an MTJ device, like a CIP SV GMR sensor, one of the ferromagnetic layers has its magnetization fixed by being exchange-coupled with an adjacent antiferromagnetic layer, resulting in the exchange-coupled structure.
Another type of CPP magnetoresistive device that uses an exchange-coupled structure is a SV GMR sensor proposed for use as magnetic recording read heads. The proposed CPP SV read head is structurally similar to the widely used CIP SV read head, with the primary difference being that the sense current is directed perpendicularly through the two ferromagnetic layers and the nonmagnetic spacer layer. 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.
In these types of magnetoresistive devices, high spin polarization of the ferromagnetic materials adjacent the nonmagnetic spacer layer is essential for high magnetoresistance. The most common type of materials used for both the free and pinned ferromagnetic layers are the conventional alloys of Co, Fe and Ni, but these alloys have only relatively low spin-polarization of approximately 40%. More recently, certain half-metallic ferromagnetic Heusler alloys with near 100% spin polarization have been proposed. One such alloy is the recently reported alloy Co2Cr0.6Fe0.4Al (T. Block, C. Felser and J. Windeln, “Spin Polarized Tunneling at Room Temperature in a Huesler Compound—A non-oxide Material with a Large Negative Magnetoresistance Effect in Low magnetic Fields”, IEEE International Magnetics Conference, April 28-May 2, Amsterdam, The Netherlands). Other half-metallic ferromagnetic Heusler alloys are NiMnSb and PtMnSb that have been proposed as “specular reflection” layers located within the ferromagnetic layers in CIP SV read heads, as described in published patent application U.S. Ser. No. 2002/0012812 A1. With respect to the half-metallic ferromagnetic Heusler alloy NiMnSb, no exchange bias was observed when it was deposited on a layer of FeMn antiferromagnetic material, as reported by J. A. Caballero et al., “Magnetoresistance of NiMnSb-based multilayers and spin-valves”, J. Vac. Sci. Technol. A16, 1801-1805 (1998). In an undated article made available on the internet, exchange biasing of certain multilayers of half-metallic ferromagnetic Heusler alloys was supposedly observed without the need for exchange-coupling with an antiferromagnetic layer, as reported by K. Westerholt et al, “Exchange Bias in [Co2MnGe/Au]n, [Co2MnGe/Cr]n, and [Co2MnGe/Cu2MnAl]n Multilayers.”
What is needed is a magnetoresistive device with an exchange-coupled structure that includes a half-metallic ferromagnetic Heusler alloy.