Probing Magnetic Exchange Interactions with Helium

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Probing Magnetic Exchange Interactions with Helium

C. Trainer, C. M. Yim, C. Heil, L. S. Farrar, V. Tsurkan, A. Loidl, and P. Wahl

Phys. Rev. Lett. 127, 166803 – Published 12 October 2021

Abstract

Controlling and sensing spin polarization of electrons forms the basis of spintronics. Here, we report a study of the effect of helium on the spin polarization of the tunneling current and magnetic contrast in spin-polarized scanning tunneling microscopy (SP STM). We show that the magnetic contrast in SP STM images recorded in the presence of helium depends sensitively on the tunneling conditions. From tunneling spectra and their variation across the atomic lattice we establish that the helium can be reversibly ejected from the tunneling junction by the tunneling electrons. The energy of the tunneling electrons required to eject the helium depends on the relative spin polarization of the tip and sample, making the microscope sensitive to the magnetic exchange interactions. We show that the time-averaged spin polarization of the tunneling current is suppressed in the presence of helium and thereby demonstrate voltage control of the spin polarization of the tunneling current across the tip-sample junction.

Received 17 November 2020
Revised 3 September 2021
Accepted 8 September 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.166803

Physics Subject Headings (PhySH)

Antiferromagnetism Magnetoresistance

Magnetic tunnel junctions

Scanning tunneling microscopy Scanning tunneling spectroscopy Spin-polarized scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

C. Trainer ¹, C. M. Yim ^1,2, C. Heil ³, L. S. Farrar ¹, V. Tsurkan ^4,5, A. Loidl ⁴, and P. Wahl ¹

¹SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, United Kingdom
²Tsung Dao Lee Institute & School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
³Institute of Theoretical and Computational Physics, Graz University of Technology, NAWI Graz, 8010 Graz, Austria
⁴Center for Electronic Correlations and Magnetism, Experimental Physics V, University of Augsburg, D-86159 Augsburg, Germany
⁵Institute of Applied Physics, MD 2028 Chisinau, Republic of Moldova

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Issue

Vol. 127, Iss. 16 — 15 October 2021

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Images

Figure 1
(a) Schematic of the geometry of the experiment with a He atom as PP in the tunneling junction. $d$ denotes the tip-sample distance (Sec. S2A of the Supplemental Material [9]). Magnetic moments in ${Fe}_{1 + x} Te$ are drawn vertical for clarity, but point into the plane of the figure in reality [17]. (b) Topographic SP STM image $z (r)$ of ${Fe}_{1 + x} Te$ measured in vacuum ( $V = 50 mV$ , $I = 100 pA$ ). The bicollinear AFM order of ${Fe}_{1 + x} Te$ is seen as a stripelike modulation. Inset: Fourier transformation $| \tilde{z} (q) |$ . (c) $z (r)$ image recorded with the same tip and in the same location as (b) after admission of He into the vacuum chamber at a partial pressure $p_{He} = 0.1 mbar$ . The magnetic contrast is hardly visible. Inset: corresponding $| \tilde{z} (q) |$ . (d) Amplitude $| \tilde{z} (q_{AFM}) |$ of the Fourier peak associated with the AFM order at different tunneling currents for a bias voltage of $V = 50 mV$ in vacuum, and with $p_{He} = 0.05 mbar$ and 0.1 mbar.
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Figure 2
Spectroscopy in He at $p_{He} = 0.1 mbar$ . (a) Differential conductance spectra $g (V)$ ( $V = 50 mV$ , $I = 80 pA$ , $V_{mod} = 5 mV$ ) recorded at different $d$ in the presence of He. A $g (V)$ spectrum taken in vacuum (yellow) is also shown ( $V = 100 mV$ , $I = 300 pA$ , $V_{mod} = 1 mV$ ). Spectra are vertically offset for clarity and normalized at $V = 250 mV$ . (b) DFT calculation of the trapping potential for the He atom in the junction for different adsorption sites. (c) The gap size $Δ (d)$ , plotted as $- Δ (d)$ , extracted from $g (V)$ spectra (defined as the inflection point at the gap edge) as a function of $d$ . $Δ (d)$ exhibits a similar functional form as the potential of the He atom found in the DFT calculation in (b). (d) $g (V)$ spectra recorded with the tip positioned above the hollow (blue) and on-top site (red) showing a larger gap for the hollow site ( $V = 20 mV$ , $I = 1 nA$ , $V_{mod} = 3.5 mV$ ).
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Figure 3
(a) $z (r)$ image for $d = 4.1 Å$ ( $V = 50 mV$ , $I = 30 pA$ ) and the simulated topography for $d_{DFT} = 7.6 Å$ for comparison. (b) calculated SP at the same $d_{DFT}$ for vacuum tunneling and for a layer of He in the hollow and the on-top sites. The largest magnetic contrast is observed for He in the hollow site. (c) Amplitude $| \tilde{z} (q_{AFM}) |$ of the magnetic contrast as a function of $d$ and for a range of voltages $V$ . The green dashed line represents the value for vacuum tunneling. (d) The SP $P$ extracted from the amplitude of the magnetic contrast in the simulated images. Red points are extracted from our model accounting for the He atom probing different sites during the measurement (for details see Sec. S1B of the Supplemental Material [9]). The blue curve is the amplitude of the magnetic contrast for a vacuum junction.
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Figure 4
(a) Topography $z (r)$ ( $V = 200 mV$ , $I = 1 nA$ ) and spatial map of binding energy $Δ (r)$ of the He atom extracted from a spectroscopic map $g (r, V)$ ( $V = 20 mV$ , $I = 1 nA$ , $V_{mod} = 3.5 mV$ ) acquired with a setpoint at which the magnetic contrast is not visible in the topography. The $Δ (r)$ map shows clear variations between different atomic sites. (b) Average profile from $z (r)$ (blue curve) and along the dashed line from $Δ (r)$ (red curve) in (a). Besides the difference in $Δ$ between the hollow and on-top sites, a $\sim 3 mV$ modulation is observed between hollow sites with different relative magnetization of tip and sample. (c) The apparent SP of the tunnel junction extracted from the $g (r, V)$ map. Blue points show true spin polarization, red points show enhancement due to the exchange interaction, leading to a giant apparent SP. There are three regimes: $A$ , He is ejected from the junction; $B$ , He is trapped only at some sites and for one relative magnetic configuration of tip and sample; and $C$ , He remains always in the junction.
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