Metal treatment – Stock – Magnetic
Reexamination Certificate
2001-04-30
2002-04-23
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C148S120000, C148S121000, C148S122000, C148S334000, C148S113000
Reexamination Certificate
active
06375761
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to materials for use in magnetic recording sensors and the like, and more particularly relates to a magnetoresistive material with two metallic magnetic phases.
2. Brief Description of the Prior Art
Materials which exhibit a change in resistance when exposed to a magnetic field are of use in preparing magnetic recording sensors, such as those used, for example, in computer disk drives. At the present time, state-of-the-art computer disk drives employ sensor materials which exhibit the anisotropic magnetoresistance effect (AMR). Materials which exhibit the AMR have a magnetoresistance which depends on how the magnetic field is applied with respect to the direction of current flow.
In other types of materials, which exhibit the giant magnetoresistance effect (GMR), a non-magnetic metallic conductor, such as copper, is necessary to create a disordered state of electron spins in a ferromagnet. However, the nonmagnetic metallic conductor does not contribute to desired scattering of the conduction electrons, and in fact, may act as a low resistance shunt path which decreases the magnetoresistance. These prior GMR materials include a magnetic metal and a non-magnetic metal.
Macroscopic ferrimagnets are a new class of phase separated magnetic materials which have been recently discovered, and are described, for example, in R. J. Gambino et al., 75
J. Appl. Phys.
1871 (1994). The macroscopic ferrimagnets include two magnetic phases with a negative magnetic exchange at the phase boundary. A prototypical example is the Co—EuS system which has 100 Å particles of EuS in a cobalt matrix. The EuS is exchange coupled antiferromagnetically to the cobalt at least at the Co/EuS interface. In the Co—EuS system, the small size of the EuS particles results in a large fraction of the EuS being in close proximity to the interface which is influenced by the strong Co/EuS exchange. It has been found that these materials display unusual magneto-optical properties, as described in R. J. Gambino and P. Fumagalli, 30
IEEE Trans. Magn.
4461 (1994), and magneto-transport properties, as described in R. J. Gambino and J. Wang, 33
Scr. Metall. Mater.
1877 (1995) and R. J. Gambino et al., 31
IEEE Trans. Magn.
3915 (1995). Magnetization and Kerr hysteresis loops have confirmed the macroscopic ferrimagnetic model for these systems. In measurements of the optical and magneto-optical properties of Co—EuS thin films, polar Kerr rotations of up to 2° have been observed in Co-rich films at photon energies of 4.5 eV, as described in P. Fumagalli et al, 31
IEEE Trans. Magn.
3319 (1995). Transport measurements show that the magnetoresistance of Co—EuS behaves like that of the widely studied granular giant magnetoresistance effect (GMR) materials, as described in S. Zhang, 61
Appl. Phys. Lett.
1855 (1992), which include particles of a ferromagnetic metal in a conductive, nonmagnetic matrix. In contrast, Co—EuS includes semiconducting, ferromagnetic particles in a conductive, ferromagnetic matrix of cobalt. As a consequence, the temperature dependence of the magnetoresistance is very different in the Co—EuS system as compared to the ordinary granular GMR materials. With respect to the magnitude of the effect, the magnetoresistivity change (&dgr;&rgr;) of the Co—EuS system is 8×10
−5
&OHgr;-cm at room temperature in a field of 1T, which is larger than other magnetoresistive materials. Even though the magnetoresistivity change of this system is large, the magnetoresistance defined as &dgr;&rgr;/&rgr; is small, typically 2~3%, because of the high resistivity of the material caused by a large volume fraction of the semiconducting EuS phase.
While materials exhibiting the AMR effect have enhanced the performance of computer disk drives, and while the aforementioned Co—EuS systems are promising, it would be desirable to develop materials having a larger change in resistance as a function of applied magnetic field strength, that is, a larger magnetoresistance effect. Such materials could permit the development of more sensitive magnetic recording sensors. It would be desirable to develop such materials which would exhibit the GMR as opposed to the AMR. In materials exhibiting the GMR, the resistivity decreases with the applied magnetic field independent of the direction of the applied field with respect to the direction of current flow. In addition to this desirable isotropy, the GMR is usually stronger than the AMR.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved material suitable for manufacturing more sensitive magnetic recording sensors.
It is another object of the present invention to provide such a material which exhibits the GMR.
It is a further object of the present invention to provide such a material which includes two ferromagnetic phases which are exchange coupled antiferromagnetically.
It is yet another object of the present invention to provide a method of manufacturing such a material.
It is a further object of the present invention to provide a method of sensing magnetic fields using such a material.
It is still another object of the present invention to provide a digital magnetic recording system which utilizes such a material in a read head.
In accordance with one form of the material of the present invention, a magnetoresistive material exhibits the GMR and has two phases. The first phase includes a matrix of an electrically conductive ferromagnetic transition metal or an alloy thereof. The second phase is a precipitate phase of an electrically conductive rare earth pnictide which exhibits ferromagnetic behavior when precipitated out of the matrix. The second phase is antiferromagnetically exchange coupled to the first phase. In a preferred form of the first embodiment, the matrix comprises cobalt and the precipitate phase comprises terbium nitride.
Thus, the present invention provides a new macroscopic ferrimagnet, in the system Co—TbN, which exhibits the GMR. The Co—TbN system demonstrates typical macroscopic ferrimagnet properties: a magnetic compensation point and negative GMR. The Co—TbN system with 32% TbN by volume composition shows 0.72% GMR under an applied field of 8 kOe at room temperature and 9% GMR at 250° K under an applied field of 40 kOe. In the Co—TbN system, the temperature dependence of the GMR is quite different from that of ordinary GMR materials, where the negative magnetoresistance decreases with increasing temperature. The GMR in the Co—TbN system increases with increasing temperature, which is due to the increase of ferromagnetic alignment of the Co and TbN with an applied field caused by the decrease of exchange coupling by temperature.
In an alternative form of material according to the present invention, the second precipitate phase comprises an electrically conductive Heusler alloy such as Co
2
MnSn or Co
2
TiSn.
The present invention also provides a method of manufacturing a magnetoresistive material of the types described above. The method includes the steps of providing a target (for example, a sheet metal or thin film target) of an electrically conductive ferromagnetic transition metal or an alloy thereof; locating a plurality of pellets of an electrically conductive rare earth element (or constituents of a Heusler alloy) on a surface of the target; sputtering the target and the pellets with ions in a suitable plasma, such as an argon plasma, to cause the film and the pellets to form an amorphous alloy of the electrically conductive ferromagnetic transition metal or alloy thereof, and the electrically conductive rare earth element (or constituents of a Heusler alloy); and subsequently annealing the amorphous alloy to cause formation of the precipitate phase within the matrix of the ferromagnetic transition metal or alloy thereof. Techniques other than sputtering can also be employed.
The present invention further provides a method of detecting magnetic field strength of a magnetic field associated with a magnetization pattern
Gambino Richard J.
Kim Tae-wan
Baker & Botts LLP
Sheehan John
The Research Foundation of State University of New York
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