Detection of uncompensated magnetization at the interface of an epitaxial antiferromagnetic insulator

Pavel N. Lapa, Min-Han Lee, Igor V. Roshchin, Kirill D. Belashchenko, and Ivan K. Schuller

Phys. Rev. B 102, 174406 – Published 5 November 2020

Abstract

We have probed directly the temperature and magnetic field dependence of pinned uncompensated magnetization at the interface of antiferromagnetic ${FeF}_{2}$ with Cu, using ${FeF}_{2} − Cu − Co$ spin valves. Electrons polarized by the Co layer are scattered by the pinned uncompensated moments at the ${FeF}_{2} − Cu$ interface giving rise to giant magnetoresistance. We determined the direction and magnitude of the pinned uncompensated magnetization at different magnetic fields and temperatures using the angular dependencies of resistance. The strong ${FeF}_{2}$ anisotropy pins the uncompensated magnetization along the easy axis independent of the cooling field orientation. Most interestingly, magnetic fields as high as 90 kOe cannot break the pinning at the ${FeF}_{2} − Cu$ interface. This proves that the pinned interfacial magnetization is strongly coupled to the antiferromagnetic order inside the bulk ${FeF}_{2}$ layer. Studies as a function of ${FeF}_{2}$ crystalline orientation show that uncompensated spins are only detected in a spin valve with (110) crystal orientation, but not in valves containing ${FeF}_{2} (100)$ and ${FeF}_{2} (001)$ . This observation is in agreement with symmetry-related considerations which predict the equilibrium boundary magnetization for the ${FeF}_{2} (110)$ layer.

Received 25 May 2020
Revised 21 October 2020
Accepted 23 October 2020
Corrected 20 November 2020

DOI:https://doi.org/10.1103/PhysRevB.102.174406

Physics Subject Headings (PhySH)

Antiferromagnetism Giant magnetoresistance Magnetotransport Spintronics

Interfaces Spin valves

T-symmetry

Condensed Matter, Materials & Applied Physics

Corrections

20 November 2020

Correction: A grant number was missing in the Acknowledgment section and has been inserted.

Authors & Affiliations

Pavel N. Lapa¹, Min-Han Lee ^1,2, Igor V. Roshchin³, Kirill D. Belashchenko ⁴, and Ivan K. Schuller ¹

¹Department of Physics, University of California, San Diego, La Jolla, California 92093, USA
²Materials Science and Engineering Program, University of California San Diego, La Jolla, California 92093, USA
³Department of Material Science and Engineering, Texas A&M University, College Station, Texas 77843-4242, USA
⁴Department of Physics and Astronomy and Nebraska Center of Materials and Nanoscience, University of Nebraska–Lincoln, Lincoln, Nebraska 68588-0299, USA

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Vol. 102, Iss. 17 — 1 November 2020

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Images

Figure 1
(a) Schematics illustrates scattering of electrons polarized by the Co layer on the pinned uncompensated magnetization at the ${FeF}_{2} − Cu$ interface. (b) Schematics shows rotation of the ${FeF}_{2} (110) / Cu / Co$ valve in an external magnetic field; easy axis (e.a.) is perpendicular to the long edge of the sample; the sample orientation is defined by an angle θ between the direction of magnetic field and the easy axis. Angular dependencies of resistance R for the ${FeF}_{2} (110) / Cu / Co$ AFSV measured at 10 K in a 90-kOe magnetic field after cooling the valve from 300 K (c) in the 90-kOe magnetic field applied along the easy axis of ${FeF}_{2}$ ( $θ_{cool} = 0^{\circ}$ ), (d) after cooling in zero magnetic field (ZFC). Circles show experimental data, grey lines are the fits to Eq. (1). Blue and red arrows show the orientation of the pinned uncompensated magnetization ( $M_{pin}$ ) at the ${FeF}_{2} − Cu$ interface (assuming the normal GMR) and the Co layer ( $M_{C o}$ ), respectively. The schematics in the boxes at (c) and (d) illustrate the mutual orientation of cooling magnetic field ( $H_{cool}$ ), $M_{pin}$ , and the ${FeF}_{2}$ easy axis for the particular field-cooling procedures.
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Figure 2
Temperature dependence of GMR for the ${FeF}_{2} (110) / Cu / Co$ AFSV measured in a 90-kOe magnetic field after cooling the valve in a 90-kOe magnetic field. The dashed vertical line shows the Néel temperature of bulk ${FeF}_{2}$ . The black curve is a guide to the eye.
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Figure 3
(a) Magnetic-field dependencies of GMR measured at 10 K (rectangles), 65 K (circles), and 70 K (stars). The data were obtained by cooling the valve to the corresponding temperature in a 90-kOe magnetic field and sequentially measuring the angular dependence incrementally ramping up $H_{meas}$ (red curves) and then down (blue curves). The error bars in (a) are shown only for a 5-kOe data point. (b) Angular dependencies of resistance measured at 70 K in a 10-kOe magnetic field after cooling the ${FeF}_{2} (110) / Cu / Co$ AFSV in the 90-kOe magnetic field applied along the easy axis of ${FeF}_{2}$ ( $θ_{cool} = 0^{\circ}$ ); the same dependencies measured in the 10-kOe magnetic field after applying (c) 50-kOe and (d) 70-kOe magnetic field ( $H_{a p p l}$ ) in the direction opposite to the cooling field ( $H_{cool}$ ). In (b)–(d), circles are experimental data, grey lines are the fits to Eq. (1).
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Figure 4
Angular dependencies of resistance for the ${FeF}_{2} (110) / Cu / Co$ AFSV measured at 10 K in a 90-kOe magnetic field after cooling the valve from 300 K in the 90-kOe magnetic field while sample orientations are (a) $θ_{cool} = 90^{\circ}$ , (b) $θ_{cool} = 70^{\circ}$ , (c) $θ_{cool} = 110^{\circ}$ . Circles are experimental data, grey lines are the fits to Eq. (1). The schematics at top of each plot illustrate the mutual orientation of the easy axis (e.a.) and the pinned magnetization ( $M_{pin}$ ) (assuming the normal GMR) with respect to the cooling field ( $H_{cool}$ ) direction. Schematic at top corner at (a) explicitly illustrates that the easy axis is not completely perpendicular to the long edge of the sample, consequently, a projection of $H_{cool}$ on the easy axis defines the direction of the pinned magnetization.
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Figure 5
Schematics illustrates the crystallographic and spin structures of the ${FeF}_{2} (110)$ layer with an atomic step. The green and red circles depict the Fe spins pointed in and out of the page, respectively, along the easy axis of ${FeF}_{2}$ .
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