${L1}_{0}\text{ }\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Pd}$ Synthetic Antiferromagnet through an fcc Ru Spacer Utilized for Perpendicular Magnetic Tunnel Junctions

${L 1}_{0} Fe − Pd$ Synthetic Antiferromagnet through an fcc Ru Spacer Utilized for Perpendicular Magnetic Tunnel Junctions

De-Lin Zhang, Congli Sun, Yang Lv, Karl B. Schliep, Zhengyang Zhao, Jun-Yang Chen, Paul M. Voyles, and Jian-Ping Wang

Phys. Rev. Applied 9, 044028 – Published 19 April 2018

Abstract

Magnetic materials that possess large bulk perpendicular magnetic anisotropy (PMA) are essential for the development of magnetic tunnel junctions (MTJs) used in future spintronic memory and logic devices. The addition of an antiferromagnetic layer to these MTJs was recently predicted to facilitate ultrafast magnetization switching. Here, we report a demonstration of a bulk perpendicular synthetic antiferromagnetic (PSAFM) structure comprised of a (001) textured $Fe − Pd / Ru / Fe − Pd$ trilayer with a face-centered-cubic (fcc) phase Ru spacer. The ${L 1}_{0} Fe − Pd$ PSAFM structure shows a large bulk PMA ( $K_{u} \sim 10.2 Merg / {cm}^{3}$ ) and strong antiferromagnetic coupling ( $- J_{IEC} \sim 2.60 erg / {cm}^{2}$ ). Full perpendicular magnetic tunnel junctions (PMTJs) with a ${L 1}_{0} Fe − Pd$ PSAFM layer are then fabricated. Tunneling magnetoresistance ratios of up to approximately 25% (approximately 60%) are observed at room temperature (5 K) after postannealing at $350 ° C$ . Exhibiting high thermal stabilities and large $K_{u}$ , the bulk PMTJs with an ${L 1}_{0} Fe − Pd$ PSAFM layer could pave a way for next-generation ultrahigh-density and ultralow-energy spintronic applications.

Received 25 October 2017
Revised 15 February 2018

DOI:https://doi.org/10.1103/PhysRevApplied.9.044028

Physics Subject Headings (PhySH)

Antiferromagnetism Magnetic anisotropy Spintronics

Ferromagnets Magnetic multilayers Magnetic tunnel junctions Transition metal alloys

Magnetization measurements

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

De-Lin Zhang¹, Congli Sun³, Yang Lv¹, Karl B. Schliep², Zhengyang Zhao¹, Jun-Yang Chen¹, Paul M. Voyles³, and Jian-Ping Wang^1,*

¹Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, USA
²Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA
³Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA

^*To whom correspondence should be addressed. jpwang@umn.edu

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Vol. 9, Iss. 4 — April 2018

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Figure 1
(a) The $M − H$ loops of the $Fe − Pd (3 nm) / Ru (1.1 nm) / Fe − Pd (3 nm)$ PSAFM structure measured at room temperature. The inset of (a) shows the schematic diagram of the ${L 1}_{0} Fe − Pd$ PSAFM structure. The Cr and Pt buffer layers are used to induce the (001) texture, and the thin Ru layer is used as a spacer. (b) XRD pattern with out-of-plane $θ - 2 θ$ scans for the ${L 1}_{0} Fe − Pd$ PSAFM structure; the $Cr / Pt$ buffer layer is used as a reference.
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Figure 2
(a) HAADF STEM images of the ${L 1}_{0} Fe − Pd$ PSAFM structure; the clearly epitaxial layered structure is observed. (b) A magnified STEM image of the $Fe − Pd / Ru / Fe − Pd$ trilayer, which shows the smooth interface between the Fe-Pd and Ru layers. Meanwhile, the lattice spacing of the Fe-Pd and Ru layers is calculated from TEM; labeling in TEM image. (c) The crystalline structure of the Fe-Pd material with the tetragonal AuCu-type phase. The blue (small) ball denotes the Fe atom, and the red (big) ball represents the Pd atom. (d) The crystalline structure of the Ru material with fcc phase. The fcc phase Ru thin films are not experimentally realized; they are just theoretically predicted by first-principles calculation. Its theoretical lattice constant is $a = b = c = 0.3826 nm$ .
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Figure 3
(a) A schematic illustration of the ${L 1}_{0} Fe − Pd$ SAFM PMTJ stack in which the ${L 1}_{0} Fe − Pd$ PSAFM layer couples with the Co-Fe-B layer through a thin Ta layer to form the bottom free layer. The ${[Co / Pd]}_{n}$ and Co-Fe-B layers form the top reference layer by a thin Ta layer. The thin Ta layer plays a very important role as the coupling spacer and diffusion barrier, which can be used to block the Pd diffusion when the MTJ devices are annealed at high temperature. (b) The ABF STEM image of the $Co − Fe − B / MgO / Co − Fe − B$ region of the ${L 1}_{0} Fe − Pd$ SAFM PMTJ stack, which is used to determine the quality of the MgO tunneling barrier. (c),(d) The room-temperature out-of-plane $M − H$ loops of the Fe-Pd free layer with a stack of $Fe − Pd / Ru / Fe − Pd / Ta / Co − Fe − B$ and the ${L 1}_{0} Fe − Pd$ SAFM PMTJ stack. These stacks are postannealed at $350 ° C$ by the RTA process.
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Figure 4
The tunneling magnetoresistance versus external magnetic field (TMR- $H$ ) loops of the ${L 1}_{0} Fe − Pd$ SAFM PMTJ devices postannealed by RTA at (a) $300 ° C$ , (b) $350 ° C$ , (c) $375 ° C$ , and (d) $400 ° C$ . The testing is carried out at room temperature. The external magnetic field is swapped from $- 1500$ to $+ 1500 Oe$ along the perpendicular plane of the devices. (e) The TMR ratio as a function of the postannealing temperatures of the ${L 1}_{0} Fe − Pd$ SAFM PMTJ devices. The inset is the optical microscopy image of the real ${L 1}_{0} Fe − Pd$ SAFM PMTJ device. (f),(g) TMR ratio as a function of the annealing temperatures for the ${L 1}_{0} Fe − Pd$ SAFM PMTJ devices annealed at 350 and $400 ° C$ , respectively. (h),(i) The temperature dependence of the resistance of the parallel state (open squares) and the antiparallel state (open circles) in the ${L 1}_{0} Fe − Pd$ SAFM PMTJ devices annealed at $350 ° C$ and $400 ° C$ , respectively.
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L10 Fe−Pd Synthetic Antiferromagnet through an fcc Ru Spacer Utilized for Perpendicular Magnetic Tunnel Junctions

De-Lin Zhang, Congli Sun, Yang Lv, Karl B. Schliep, Zhengyang Zhao, Jun-Yang Chen, Paul M. Voyles, and Jian-Ping Wang

Phys. Rev. Applied 9, 044028 – Published 19 April 2018

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${L 1}_{0} Fe − Pd$ Synthetic Antiferromagnet through an fcc Ru Spacer Utilized for Perpendicular Magnetic Tunnel Junctions