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Non-magnetic NiFe-M Alloy Underlayers for Spin-Valve and Tunnel-Valve Antiferromagnets

IP.com Disclosure Number: IPCOM000013843D
Original Publication Date: 2002-May-03
Included in the Prior Art Database: 2003-Jun-18

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This report describes a multilayer spin-valve or tunnel-valve magnetic thin-film structure with an underlayer film composed of a NixFeyMz alloy where the element M and the composition z are chosen to make the ferromagnetic alloy NixFez non-ferromagnetic or weakly-ferromagnetic at room temperature. NixFey with x approximately 0.8 and y=1-x is generally known as an underlayer beneficial to the growth of Mn-based antiferromagnets such as FeMn, NiMn, PtMn, IrMn, OsMn, PdMn for the purpose of exchange biasing of a thin magnetic layer. Here we describe a structure in which the NiFe underlayer is replaced by a NiFe-alloy so that the underlayer becomes non-ferromagnetic (Fig.1 below). By this method, the NiFe-based underlayer does not produce a magnetic field, thus eliminating any adverse effect on the magnetic performance and biasing condition of the spin-valve or tunnel-valve sensor. The composition of the NiFe-based alloy underlayer can be chosen to have no adverse effect on the exchange biasing characteristics of the antiferromagnetic layer (Fig.1 below). Aditionally, alloy underlayers have increased electrical resistance, thus improving the output signal of a spin-valve device based on this structure (Fig.2 below). Finally non-magnetic NiFe-based alloy underlayers are more corrosion resistant than other non-magnetic underlayers such as Cu. Thin-film underlayers (also known as buffer layers) are an integral part of spin-valve and tunnel-valve magnetoresistive sensors fabricated using sputter-deposition. These layers are generally thin films (<100 Angstroms thick) and serve a variety of roles: 1) They provide physical and chemical separation for the sensor from the underlying substrate. 2) They serve as a template for the growth of subsequent layers by virtue of their crystalline structure and texture. 3) They provide an electrically conducting path which affects the electrical and magnetotransport properties of the device. NixFe1-x alloys, in particular those with x>0.5, are known in the art as beneficial underlayers for the growth of Mn-based antiferromagnets which are an integral part of spin-valve and tunnel-valve structures. The benefit of using NiFe alloys for this purpose is generally a high value of the exchange biasing field between the antiferromagnet and the pinned ferromagnetic layer of the spin-valve or tunnel-valve. This benefit is believed to originate from the face-centered-cubic crystal structure of NixFe1-x alloys with x>0.5, and the (111)-oriented crystal structure and fine-grained microstructure of NiFe thin-films. However, the disadvantage of using NiFe alloys as underlayers resides in their ferromagnetic properties. The magnetic signal of the underlayer interferes with the magnetic characterization of the spin-valve or tunnel-valve and creates demagnetization fields which can influence and deteriorate the magnetic performance of the spin-valve or tunnel-valve device. It is therefore of interest to find an underlayer which retains the advantages of NiFe without the disadvantages inherent to the use of a ferromagnetic material for this purpose. In this context, we describe the use of NixFeyMz alloys as underlayers for spin-valves and tunnel-valve structures, where the alloying element M and the composition z are chosen so that the alloy film is non-ferromagnetic at room temperature. Elements such as Al, Ti, Cu, Cr, Ta, V, Pt, Pd, Zn, Zr, Nb, Rh, Ru, Ag, W and Si are well known in the art to cause a reduction in the ferromagnetic moment when alloyed to Ni, Fe or NiFe alloys. The composition z required to make a given alloy non-ferromagnetic at room temperature will depend on the specific alloying element and the specific NixFey composition. Ideally, M and z are also chosen so that the advantageous structural and microstructural properties of the underlayer are retained. Typically, the ratio of x to y is about 4, but can be adjusted to optimize the underlayer growth and crystalline texture characteristics. To illustrate the principle of this invention, Fig.1 shows the magnetic properties of a IrMn/CoFe exchange-biased stack grown on various underlayers. Such a bilayer represents the pinned layer of a spin-valve or tunnel-valve structure. In this case we compare 5 different 30 Angstrom-thick underlayers: NiFe (with 80 atomic% Ni), NiFeM alloys where M=25% Al, 25% Ti or 50% Cu, and pure Cu. The main result is that the use of these NiFe alloys instead of NiFe does not deteriorate the exchange bias properties of the antiferromagnet, as seen by the fact that the hysteresis loop of the pinned layer remains