Bulk intergrowth of a topological insulator with a room-temperature ferromagnet

Huiwen Ji, J. M. Allred, Ni Ni, Jing Tao, M. Neupane, A. Wray, S. Xu, M. Z. Hasan, and R. J. Cava

Phys. Rev. B 85, 165313 – Published 19 April 2012

Abstract

We demonstrate that the layered room-temperature ferromagnet Fe $_{7}$ Se $_{8}$ and the topological insulator Bi $_{2}$ Se $_{3}$ form crystallographically oriented bulk composite intergrowth crystals. The morphology of the intergrowth in real space and reciprocal space is described. The basal planes of Bi $_{2}$ Se $_{3}$ and Fe $_{7}$ Se $_{8}$ are parallel in the micron-scale intergrowth, and hence the good cleavage inherent to the bulk phases is retained. Both phases in the intergrowth crystals display their intrinsic bulk properties: the ferromagnetism of the Fe $_{7}$ Se $_{8}$ is anisotropic, with the magnetization easy axis in the plane of the crystals, and angle-resolved photoemission spectroscopy characterization shows that the topological surface states remain present on the Bi $_{2}$ Se $_{3}$ and that a gap can be observed in the surface state dispersion. Crystals of nominal composition Bi $_{2 - x}$ Fe $_{x}$ Se $_{3}$ are shown to be bulk intergrowths of the two phases Bi $_{2}$ Se $_{3}$ and Fe $_{2}$ Se $_{3}$ . Significant solubility of Fe in Bi $_{2}$ Se $_{3}$ is not observed.

Received 7 March 2012

DOI:https://doi.org/10.1103/PhysRevB.85.165313

Authors & Affiliations

Huiwen Ji¹, J. M. Allred¹, Ni Ni¹, Jing Tao², M. Neupane³, A. Wray³, S. Xu³, M. Z. Hasan³, and R. J. Cava^1,*

¹Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
²Department of Physics, Brookhaven National Laboratory, Upton, New York 11973, USA
³Department of Physics, Princeton University, Princeton, New Jersey 08544, USA

^*rcava@princeton.edu

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Vol. 85, Iss. 16 — 15 April 2012

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Images

Figure 1
Characterization of the Bi $_{2}$ Se $_{3}$ :Fe $_{7}$ Se $_{8}$ intergrowth crystals by large-scale diffraction methods. (a) Characteristic region of powder x-ray diffraction patterns for ground crystals grown for different Fe $_{7}$ Se $_{8}$ concentrations (1− $x$ )Bi $_{2}$ Se $_{3}$ : $x$ Fe $_{7}$ Se $_{8}$ . The primary pattern is from Bi $_{2}$ Se $_{3}$ , whose expected Bragg peak positions are marked with ticks. The second phase Fe $_{7}$ Se $_{8}$ peaks grow in intensity with increasing $x$ . (b) The diffraction pattern from the cleaved basal-plane crystal surface of a 3 × 3 mm 0.9Bi $_{2}$ Se $_{3}$ :0.1Fe $_{7}$ Se $_{8}$ intergrowth crystal. Clearly observed are the 00 $l$ reflections from Bi $_{2}$ Se $_{3}$ , which obey the rhombohedral diffraction condition $l$ $=$ 3 $n$ , and the 00 $l$ reflections from Fe $_{7}$ Se $_{8}$ . This shows that the basal planes of the two phases in the intergrowth crystals are aligned.Reuse & Permissions
Figure 2
Real-space and reciprocal space characterization of the intergrowth crystals. (a) Backscattered electron image on the cleaved-basal-plane surface of an 0.95Bi $_{2}$ Se $_{3}$ :0.05Fe $_{7}$ Se $_{8}$ intergrowth crystal showing the distribution of the Fe $_{7}$ Se $_{8}$ phase (dark areas) and the Bi $_{2}$ Se $_{3}$ phase (light area). The intergrowth of basal-plane regions occurs at the tens of microns length scale. (b) The associated EDS data that identifies the two regions in (a) as Fe $_{7}$ Se $_{8}$ (dark regions) and Bi $_{2}$ Se $_{3}$ (light regions). No detectable Fe can be found in the Bi $_{2}$ Se $_{3}$ regions. (c) A side-on view of an intergrowth crystal showing that the intergrowth occurs on the micron scale in the direction perpendicular to the cleavage plane. (d) A much rarer but also occasionally encountered shallow angle growth of Bi $_{2}$ Se $_{3}$ on the basal plane of Fe $_{7}$ Se $_{8}$ . Reciprocal space planes of intergrowth crystals obtained by single-crystal x-ray diffraction are shown in panels (e) and (f). These reciprocal space planes are in the planes of the physical crystal plates. The reciprocal lattices for Bi $_{2}$ Se $_{3}$ are marked by white lines and of Fe $_{7}$ Se $_{8}$ are marked by green lines.Reuse & Permissions
Figure 3
Magnetic and ARPES characterization of 0.95Bi $_{2}$ Se $_{3}$ :0.05Fe $_{7}$ Se $_{8}$ intergrowth crystals. The insets to panel (a) show that the crystal is ferromagnetic at room temperature and that the saturation magnetization grows with decreasing temperature. The main panel in (a) shows the development of the magnetization with applied field at 100 K for the crystal aligned with the magnetic field perpendicular to the plate ( $H$ // $c$ ) and the magnetic field in the plane of the plate ( $H$ // $a b$ ). The easy axis of the magnetization is in the basal plane. (b) and (c) High-resolution ARPES spectra along the high symmetry $M$ -Γ- $M$ direction showing that the topological surface states are clearly present on the intergrowth crystals at 25 K. The photon energy employed in the measurement is noted; the Fermi energy is at $E$ $=$ 0; the bulk conduction band (BCB) is seen for $E$ > ∼−0.2 eV and the bulk valence band (BVB) for $E$ < ∼−0.5 eV. The surface state (SS) bands are the fine features between the top of the BVB and the bottom of the BCB.Reuse & Permissions
Figure 4
Real space, magnetic, and ARPES characterization of bulk single crystals of Bi $_{1.85}$ Fe $_{0.15}$ Se $_{3}$ . As in the (1− $x$ )Bi $_{2}$ Se $_{3}$ : $x$ Fe $_{7}$ Se $_{8}$ case, these crystals are again two-phase intergrowths, actually described as the intergrowth composite 0.925Bi $_{2}$ Se $_{3}$ :0.075Fe $_{2}$ Se $_{3}$ . (a) The real-space intergrowth of the two phases on a basal-plane crystal cleavage surface. The Fe $_{1 - x}$ Se phase has the formula Fe $_{2}$ Se $_{3}$ , determined by EDS, and is shown as the dark phase in the secondary electron image. Energy dispersive x-ray spectroscopy analysis shows that there is no Fe present in the Bi $_{2}$ Se $_{3}$ (the lighter phase in the image). (b) The magnetic characterization of a 0.925Bi $_{2}$ Se $_{3}$ :0.075Fe $_{2}$ Se $_{3}$ (i.e. Bi $_{1.85}$ Fe $_{0.15}$ Se $_{3}$ ) crystal, showing weaker ferromagnetism than in the Bi $_{2}$ Se $_{3}$ :Fe $_{7}$ Se $_{8}$ intergrowth crystals, developing at lower temperature. At high fields, the intrinsic diamagnetism of the Bi $_{2}$ Se $_{3}$ is clearly seen in both field directions. The easy axis of the magnetization is in the basal plane. (c) High-resolution ARPES characterization of one of the Bi $_{1.85}$ Fe $_{0.15}$ Se $_{3}$ composite intergrowth crystals along the high-symmetry $M$ -Γ- $M$ direction showing that the topological surface states are clearly present on the intergrowth crystals at 25 K. The photon energy employed in the measurement is noted; the Fermi energy is at $E$ $=$ 0; the bulk conduction band (BCB) is seen for $E$ > ∼−0.05 eV and the bulk valence band (BVB) for $E$ < ∼−0.4 eV. The surface state (SS) bands are the fine features between the top of the BVB and the bottom of the BCB.Reuse & Permissions

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