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Control of Ferromagnetism by Manipulating the Carrier Wavefunction in N-Type Ferromagnetic Semicondu
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Control of Ferromagnetism by Manipulating the Carrier Wavefunction
in N-Type Ferromagnetic Semiconductor (In, Fe)As Quantum Wells

Ferromagnetic semiconductors (FMSs) attract much attention because they have the characteristics of both ferromagnets and semiconductors, and are thus important materials for fundamental research in the fields of solid-state physics and materials science, and for spintronics device applications including nonvolatile memory and logic devices such as spin-transistors. The common way to fabricate such FMSs is to dope a sizable amount of magnetic atoms (e.g., Mn, Fe, Cr) into nonmagnetic semiconductors (InAs, GaAs, Ge) while maintaining their crystal structure. Carriers mediate the ferromagnetic couplings between those magnetic atoms in FMSs, thus one can control their magnetism by controlling the carriers in the ferromagnetic thin films. Particularly, in a quantum well (QW) that contains a FMS thin film, it has been long proposed that manipulating the spatial overlap of the carrier wavefunctions and the thin FMS layer would yield pronounced effects on the magnetic properties of the FMS layer. However, such "wavefunction engineering" of ferromagnetism has not been realized so far. Recently, a research group at the University of Tokyo reported the first observation of such functionality in InAs/(In,Fe)As/InAs trilayer QWs [1].

Among many types of FMSs, (In,Fe)As is unique and promising because it is the only reliable n-type FMS in III-V based semiconductors. In this material, Fe atoms in the isovalent state (Fe3+) replace In atoms and thus play only the role of local magnetic moments. Electron carriers are independently supplied by donors such as Be (at interstitial sites, double donors) or Si. All the samples in this work were grown by molecular-beam epitaxy (MBE), with the structure InAs (5 nm)/(In0.95, Fe0.05)As:Be (t1 nm)/InAs (t2 nm) on GaAs substrates. The Be concentration was fixed at 5횞1019 cm-3. This trilayer structure forms a surface QW due to the vacuum potential at the surface and the conduction band offset at the InAs/GaAs interface (0.93 eV).

The magnetic circular dichroism (MCD) spectra of these QWs showed systematic blue shifts of the order of tens of meV from the bulk values (measured on a 100-nm-thick (In,Fe)As sample) when the thickness t1 of the (In,Fe)As layer was decreased from 10 to 2 nm. The blue shift energies were also increased when fixing t1 = 5 nm and decreasing t2 of the bottom InAs from 30 to 5 nm. The results are caused by the quantum size effect (QSE), and clearly indicate that the electron wavefunctions in (In,Fe)As extend into the InAs layers on both sides. The high coherency of the electron wavefunctions (coherence length >40 nm), which is the direct consequence of the fact that electron carriers in (In,Fe)As reside in the conduction band, makes it attractive for quantum devices such as resonant tunneling diodes.

The research group at the University of Tokyo also demonstrated the "wavefunction engineering" of ferromagnetism in a trilayer QW structure InAs(5 nm)/(In,Fe)As (5 nm)/InAs(5 nm). The thickness of the trilayer QW was gradually reduced by 1.2 nm each time using precise wet etching, as shown in Fig. 1. As the etching proceeded, the MCD spectrum gradually shifted to higher energy. At the first four times of etching, only the non-magnetic InAs top layer was etched, leaving the (In,Fe)As layer untouched. However, decreasing the QW thickness affects the electron wavefunctions i in the trilayer QW, shifting them toward the bottom InAs buffer, as simulated by the self-consistent calculation in Fig 1(b). This leads to a decrease in the overlap of i and the (In,Fe)As layer, and a modulation in TC from 22 K down to 10 K as shown in Fig. 1(c) (red circles). The mean-field theory approach for ferromagnetic QWs gives a qualitatively good description of TC (Fig. 1 (c), blue curve), as well as revealing an unexpectedly large s-d exchange interaction N0慣 of 4.5 eV in (In,Fe)As. The large N0慣 in (In,Fe)As, whose origin remains to be elucidated, suggests an approach toward high-TC FMS in which narrow-gap FMSs would play a more important role.


Fig. 1: (a) Blue shift in MCD spectra of (In,Fe)As with gradual wet etching. (b) Simulation of the QW potential and electron density distribution n with etching. (c) Corresponding change of TC.

References

[1] Anh, L. D., Hai, P. N., Tanaka, M., Appl. Phys. Lett. 104, 046404 (2014).

 

 
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