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(a) TEM image of a 100 nm-thick (In0.909,Fe0.091)As layer (sample B0 in Table I) grown on a GaAs substrate, taken from the GaAs direction. (b) In, Fe, and As atomic concentrations obtained by energy dispersive x-ray spectroscopy taken at 6 points marked by * in the above TEM image. (c)High-resolution TEM lattice image taken at an (In0.909,Fe0.091)As area close to the substrate (marked by the red rectangle in (a)). Inset shows the transmission electron diffraction of this (In,Fe)As layer. (d) Three-dimensional atom distribution of Fe, In, and As in a 100 nm-thick (In0.95,Fe0.05)As layer (sample A4), obtained by the laser assisted three-dimensional atom probe technique. One dot (red, green, blue) corresponds to one (Fe, In, As) atom.
Magnetic circular dichroism spectra of (a)-(d) (In0.95,Fe0.05)As samples (A1–A4 in Table I) with electron concentrations of 1.8 × 1018, 2.9 × 1018, 6.2 × 1018, and 1.8 × 1019 cm−3, respectively, and (e)-(h) (In0.92,Fe0.08)As samples (B1–B4 in Table I) with electron concentrations of 1.3 × 1018, 1.5 × 1018, 9.4 × 1018, and 2.8 × 1019 cm−3, respectively, measured at 10 K and under a magnetic field of 1 T applied perpendicular to the film plane. With increasing the electron and Fe concentrations, the MCD spectra show strong enhancement at optical critical point energies E 1 (2.61 eV), E 1 + Δ1 (2.88 eV), E 0′ (4.39 eV), and E 2 (4.74 eV) of InAs. (i) MCD spectrum of a 44 nm-thick Fe thin film grown on a GaAs substrate at 30 °C. The spectrum is clearly different from that of (In,Fe)As.
(a) and (b) Magnetization (M-T curves) of samples A4 and B4, measured under FC (1 Tesla) and ZFC conditions. The magnetic field (20 Oe) is applied in-plane along the GaAs[−110] direction. The insets show the magnetization hysteresis loops (M-H) of samples A4 and B4 measured at 10 K. The magnified M-H curves near the origin are shown in the bottom-right of the insets, which show the remanent magnetization and coercive forces. (c) Discrete multi-domain model (plan view of the (In,Fe)As film). Pink areas (indicated by dashed lines) are ferromagnetic with n > 1019 cm−3, while white areas with n < 1019 cm−3 are not. Magnetization directions of ferromagnetic domains are indicated by yellow arrows. Each domain has size much larger than the film thickness of 100 nm (see text). (d) Magneto-optical imaging of sample B4 under zero magnetic field at 4 K. The light source is a halogen lamp with a white light. Discrete ferromagnetic domains (shown by green dotted circles) with sizes of ∼10 μm are visible. Areas with small Faraday rotation (white area) between ferromagnetic domains correspond to paramagnetic areas.
(a) T C vs. electron concentration and (b) resistivity vs. temperature summarized for sample series A and B. An electron concentration threshold of about 1019 cm−3 is needed for ferromagnetism, which is also the boundary for metal-insulator transition.
List of (In1− x ,Fe x )As samples. All samples were grown at 236 °C.
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