Proximity-Driven Enhanced Magnetic Order at Ferromagnetic-Insulator--Magnetic-Topological-Insulator Interface

Proximity-Driven Enhanced Magnetic Order at Ferromagnetic-Insulator–Magnetic-Topological-Insulator Interface

Mingda Li, Cui-Zu Chang, Brian. J. Kirby, Michelle E. Jamer, Wenping Cui, Lijun Wu, Peng Wei, Yimei Zhu, Don Heiman, Ju Li, and Jagadeesh S. Moodera

Phys. Rev. Lett. 115, 087201 – Published 17 August 2015

Abstract

Magnetic exchange driven proximity effect at a magnetic-insulator–topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. Here we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI $EuS / {Sb}_{2 - x} V_{x} {Te}_{3}$ hybrid heterostructure, where V doping is used to drive the TI ( ${Sb}_{2} {Te}_{3}$ ) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

Received 8 May 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.087201

Authors & Affiliations

Mingda Li^1,2,3,*, Cui-Zu Chang^2,†, Brian. J. Kirby⁴, Michelle E. Jamer⁵, Wenping Cui⁶, Lijun Wu³, Peng Wei², Yimei Zhu³, Don Heiman⁵, Ju Li^1,7, and Jagadeesh S. Moodera^2,8,‡

¹Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
²Fracsis Bitter Magnet Lab, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
³Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
⁴Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
⁵Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA
⁶Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA
⁷Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
⁸Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

^*mingda@mit.edu
^†czchang@mit.edu
^‡moodera@mit.edu

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Vol. 115, Iss. 8 — 21 August 2015

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Images

Figure 1
(a) MI EuS–V-doped TI ${Sb}_{2} {Te}_{3}$ hybrid heterostructure. The arrows denote the spin direction. The V-doped TI layer has a perpendicular magnetic anisotropy, while EuS has in-plane anisotropy. Such a heterostructure may create an exotic magnetic environment near the interface, as illustrated in (b). For a given Eu ion (red arrow), it interacts with neighborhood intraplane Eu (orange arrow) through Heisenberg interaction, interplane Eu ions (green arrow) through superexchange interaction, spin-polarized states at the TI surface, and localized moments in the TI (blue arrow).
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Figure 2
(a) The configuration of PNR. $k_{i}$ , $k_{f}$ , and $Q$ denote the incident, reflected, and transferred wave vectors, respectively. (b) The spin $+$ and spin $-$ PNR data $R^{+}$ and $R^{-}$ for the $EuS / {Sb}_{1.9} V_{0.1} {Te}_{3}$ and $EuS / {Sb}_{2} {Te}_{3}$ samples, at low (5 mT) and high (0.7 T) in-plane guide fields. The fitting results are represented by solid lines and shifted for clarity. (c) The spin asymmetry for the reflectivity in (b). (d) The same spin asymmetry, but assuming control sample $EuS / {Sb}_{2} {Te}_{3}$ have exactly the same thickness as $EuS / {Sb}_{1.9} V_{0.1} {Te}_{3}$ . In this way, the spin asymmetry difference is dominated by magnetic structure only. At $\sim 0.4 {nm}^{- 1}$ , the difference comes from the effect of external magnetic field, while at $\sim 1.0 {nm}^{- 1}$ the difference mainly comes from V-dopants.
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Figure 3
PNR fitting profiles of doping-only sample ${Sb}_{1.9} V_{0.1} {Te}_{3}$ (a), proximity-only sample $EuS / {Sb}_{2} {Te}_{3}$ (b), and hybrid heterostructure $EuS / {Sb}_{1.9} V_{0.1} {Te}_{3}$ (c). NSLD, MSLD, and ASLD denote the nuclear, magnetic, and absorption scattering length density, which are measures of chemical contrast, magnetization, and neutron absorption, respectively. The proximity effect is identified directly as the finite magnetization signal (blue curves) in the region of a TI near the TI-EuS interface ( $\sim 15 nm$ ). The absorption-free feature in this region excludes the possible contribution which solely comes from interdiffused Eu ions. We see clearly that with V doping, the proximity magnetism is enhanced as a bump in (c), accompanied with a further suppression of magnetism of interfacial EuS ( $15 - 18 nm$ ). (d) HAADF TEM image of the $EuS / {Sb}_{1.9} V_{0.1} {Te}_{3}$ hybrid heterostructure. A sharp interface between the TI-MI is developed, indicating an epitaxial growth of EuS. This independent TEM result is quite consistent with (c) for uniformly distributed ASLD of EuS. The islandlike crystalline facets between the EuS and ${Al}_{2} O_{3}$ cap is also in very good agreement with the roughness in (c).
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Figure 4
Magnetic measurements of a 2 nm $EuS / 10 QL$ ${Bi}_{1} . 9 V_{0} . 1 {Te}_{3}$ hybrid heterostructure using a SQUID magnetometer. (a),(c) In-plane hysteresis at 5 K(a) and 7 K(c), respectively, showing a negative EB following a set field of $- 1 T$ which can be switched to positive bias by applying a positive resetting field. Inset of (a) is the out-of-plane magnetic hysteresis of the same sample, showing a finite remanent moment. (b) Schematic interfacial magnetic structure, where the interfacial EuS moments (horizontal arrows, the long arrow on the top of EuS means a saturated magnetization) are pinned by the exchange-coupled moments in the presence of a V-doped TI (vertical arrows). (d) EB and coercive field as a function of the in-plane resetting field at 5 K and 7 K, respectively.
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