Exchange-biased spin valves with perpendicular magnetic anisotropy based on (Co/Pt) multilayers

Abstract

We have prepared spin valves exhibiting perpendicular magnetic anisotropy [perpendicular spin valves (PSVs)] by sputtering. These PSVs associate a “free” (Co/Pt) multilayer with a “pinned” (Co/Pt)/FeMn multilayer separated by various spacer materials (Pt, Cu, Al2O3). We carried out a comprehensive study of the magnetic and magnetotransport properties of the biased multilayers and of the complete spin valves. When the number of repeats in the (Co/Pt) exchange-biased multilayer is larger than 3, the samples present 100% remnant magnetization in the perpendicular configuration. The major hysteresis cycles exhibit two well-separated loops associated with the free and the exchange-biased (Pt/Co) multilayers. When optimized, the exchange-bias field can be larger than the coercivity of the pinned layer. Metallic PSVs with Cu spacers exhibit giant magnetoresistance but the amplitude is only of the order of 1% due to significant current shunting. In contrast, perpendicularly magnetized tunnel junctions are very promising.

Correlation between perpendicular exchange bias and magnetic anisotropy in IrMn∕[Co∕Pt]n and [Pt∕Co]n∕IrMn multilayers

“…Among them are Co/ Pt multilayers with CoO ͑Refs. 4-7͒ and NiO, 8 Co/ Pt, CoFe/ Pt, Co/ Pd, and FeNi/ FeMn multilayers with FeMn, [9][10][11][12][13][14][15] and NiFe/ CoO multilayers. 16 Although these systems are interesting for fundamental studies of the exchange bias phenomenon, the low blocking temperature of CoO and NiO and the low corrosion resistance of FeMn limit their application potential.…”

IrMn as exchange-biasing material in systems with perpendicular magnetic anisotropy

“…[2][3][4][5][6][7] Although it has been intensively studied for systems with in-plane magnetization, it has hardly been used in recent studies on perpendicular exchange bias. [8][9][10][11][12][13][14][15][16][17][18] In a recent paper, Sort et al showed that Co/ Pt multilayers with out-ofplane magnetization can be biased by growing an IrMn layer on top. 19 In this paper we present an overview on the exchange-biasing properties of IrMn in systems with perpendicular magnetic anisotropy.…”

Large anomalous enhancement of perpendicular exchange bias by introduction of a nonmagnetic spacer between the ferromagnetic and antiferromagnetic layers

“…For instance, the FM uniaxial anisotropy in these systems ͑and also H C and the saturation magnetization, M S ) can easily be tuned by simply varying the number of ML repeats or the relative thickness of the Pt and Co layers. 11,12 Perpendicular exchange bias also offers the possibility to prepare spin valves or tunnel junctions with perpendicular-to-plane magnetization 14,15 Different series of multilayers with the composition ͓Pt (20 Å)/Co (4 Å)͔ n /Pt (t)/FeMn (130 Å), with nϭ2, 3, and 5 and t between 0 and 20 Å, were deposited onto thermally oxidized Si wafers by dc magnetron sputtering and capped with a 20 Å Pt layer. Some other samples ͑described in the following paragraphs͒ were also prepared.…”

“…The loops correspond to tϭ0, 2, 4, and 20 Å. The (Pt/Co) n /FeMn system exhibits a crossover from in-plane to perpendicular-to-plane easy axis when increasing the number of ͑Pt/Co͒ repeats, 11,15 due to the interplay between the perpendicular anisotropy induced by the Co/Pt interfaces and the large in-plane anisotropy induced at the Co/FeMn interface together with the shape anisotropy. A similar effect is observed for low number of repeats as the Pt spacer thickness increases.…”