Magnetic properties of Fe–Co ferromagnetic layers and Fe–Mn/Fe–Co bilayers obtained by thermo-ionic vacuum arc

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Abstract

Simple Fe–Co ferromagnetic films and Fe–Mn antiferromagnetic films of different compositions were grown on epitaxial Si substrates with Ta buffer layers via thermo-ionic vacuum arc method. Fe–Co/Fe–Mn bilayer structures were obtained by following similar growing conditions as for the simple films. The magnetic behavior concerning easy axis distribution, anisotropy energy and coercive field of the Fe–Co films exchange coupled to the antiferromagnetic layers were discussed with respect to the magnetic behavior of the simple, uncoupled, ferromagnetic films. The composition of the antiferromagnetic films has a sensible influence on the magnetic parameters of the exchange coupled ferromagnetic films. The largest coupling and the narrowest easy axis distribution are induced in the bilayer system with equatomic composition of the Fe–Mn antiferromagnetic layer.

Introduction

Since the discovery of the exchange bias effect more than 50 years ago, by Meiklejohn and Bean [1], a new and exciting branch of modern magnetism has been rapidly grown up. The origin of the exchange bias effect is related to the interfacial exchange coupling of a thin ferromagnetic (F) layer sharing a common interface with an appropriate antiferromagnetic (AF) layer. The exchange coupling leads either to a shift of the hysteresis loop of the F layer with respect to the direction of the applied field, when an unidirectional anisotropy energy is involved at the interface (the shift is called exchange bias field) or to an increased coercivity, when only a common uniaxial anisotropy energy is involved. Clearly, both features lead to a different magnetization reversal of the F layer interfacially pinned to the AF layer, as compared with the case of a free F layer. Therefore, new spin valve devices were developed on basis of giant magneto-resistance (GMR)/tunneling magneto-resistance (TMR) effects generated in conductive/insulator thin films sandwiched between free and pinned F layers [2], [3], [4]. The equatomic composition of Fe–Mn was used for long time as a convenient AF layer and Fe20Ni80 (permalloy) as a suitable low coercive F layer in exchange biased spin valves [3], [5]. In the state of art spin valve devices, Ir–Mn has replaced Fe–Mn due to enhanced intrinsic magnetic properties (e.g. higher Néel temperature as well as exchange anisotropy energy) and Co–Fe has replaced Ni–Fe due to a decreased atomic inter-diffusion at the F/Cu(conductive layer) interface. However, to optimize Fe–Mn as a valuable AF layer in AF/F bilayers for particular applications could become more convenient with respect to lower fabrication costs. This paper promotes a detailed study of the interfacial coupling in Fe–Mn/Fe–Co bilayer systems of different Fe concentrations in the Fe–Mn coupling layer, taking the advantage of the combinatorial processing procedure of the thermo-ionic vacuum arc method [6], [7]. The orientation dependence of the anisotropy energy and coercive fields in the bilayer systems were compared and discussed with respect to the equivalent parameters of the free Fe–Co thin films.

Section snippets

Experimental

Simple ferromagnetic Fe–Co and antiferromagnetic Fe–Mn thin films as well as Fe–Mn/Fe–Co bilayers were deposited by thermo-ionic vacuum arc method on Ta buffer layers grown on (1 0 0) Si plackets (Ta thin films were also top deposited as protective cap layers). The currently used experimental arrangement of the thermo-ionic vacuum arc method, involves two electron beams generated by heated cathodes and accelerated by high anodic voltages. The materials, placed in special crucibles at the anodes,

Results and discussions

The relative content of Fe and Co in the 2 extremities samples F_1 and F_4, as determined by EDX, was 50–50 at.% in sample F_1 and 52–48 at.% in sample F_4, respectively. Within the error limits of 1–2 at.%, we may consider that all the F_n films present the same composition, close to the equatomic ratio. This consideration is also in agreement with the preparation procedure based on the evaporation of an Fe–Co alloy from only one evaporation source. However, the ratio between the relative wt.% of

Conclusions

Three layered systems consisting of simple F (Fe50Co50) and AF (FexMn100−x) films, as well as of exchange coupled AF/F layers have been successfully prepared by thermo-ionic vacuum arc method. The Fe–Mn relative content in the AF films was varied from 75–25 at.% to 45–55 at.% by positioning the epitaxial Si substrates at different distances from the Fe and respectively Mn sources. Buffer and cap layers of Ta were used in all situations. Simple F films present a magnetic texture with an average

Acknowledgement

The financial support through the Romanian National project PNII-71-032 is highly acknowledged.

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