95th CEMS Colloquium

Title

Renaissance of Ferromagnetic Semiconductors and Spintronics Applications

Abstract

   Ferromagnetic semiconductors (FMSs) with high Curie temperature (T_C) are strongly required for spintronics device applications. So far, the mainstream study of FMSs is Mn-doped III-V FMSs; however they are only p-type and their T_C  is much lower than 300 K. In this study, we present a new class of FMSs with high T_C , Fe-based narrow-gap III-V FMSs. Because Fe atoms are in the isoelectronic Fe3+ state in III-V, the carrier type can be controlled independently and thus both n-type and p-type FMSs are obtained. Using low-temperature molecular beam epitaxy, we have successfully grown both p-type FMS [(Ga,Fe)Sb [1], (Al,Fe)Sb [2]] and n-type FMSs [(In,Fe)As [3], (In,Fe)Sb [4]]. The most notable feature in these Fe-based FMSs is that the T_C value increases monotonically as the Fe content increases; and there is a tendency that T_C is higher as the bandgap is narrower, which contradicts the prediction of the mean-field Zener model. Intrinsic room-temperature ferromagnetism has been observed in (Ga_1-x,Fe_x)Sb with x > 23% [1] and (In_1-x,Fe_x)Sb with x > 16% [4], which are promising for practical spintronic devices operating at room temperature.

    We also present our findings on new magnetotransport phenomena in heterostructures containing these Fe-doped FMSs. In an Esaki diode composed of a 50 nm-thick n-type FMS (In,Fe)As (6% Fe) / 250 nm-thick p+ InAs:Be, we found that the magnetic-field-dependence of the current flowing through the pn junction (magnetoconductance, MC) can be largely controlled, both in sign and magnitude, with the bias voltages V [5,6]: The diode shows small positive MC (~0.5%) at V < 450 mV, but the MC changes its sign and magnitude at V > 450 mV, reaching -7.4% (at 1T) at V = 650 mV. This bias-controlled MC originates from the change in the band components of (In,Fe)As that participate in the spin-dependent transport.  Furthermore, we found that the current flowing in a nonmagnetic n-type InAs quantum well (QW) that is interfaced to an insulating p-type (Ga,Fe)Sb layer (20% Fe, TC > 300 K) exhibits a giant change of approximately 80% at high magnetic field and that its magnitude can be controlled by ten-fold using a gate. The mechanism for this large magnetoresistance is attributed to a strong magnetic proximity effect (MPE) via the s-d exchange coupling at the InAs/(Ga,Fe)Sb interface. It was found that a spin splitting in the InAs QW is induced by MPE, which can be varied between 0.17 meV and 3.8 meV by the gate voltage [7]. Other studies on ferromagnetic semiconductor heterostructures are underway and novel phenomena and properties are being investigated [7-11]; these new properties of the Fe-doped FMS-based materials and devices provide novel functionalities for the future spin-based electronics.

References
[1] Appl. Phys. Lett. 108, 192401 (2016) <Featured article>.
[2] Appl. Phys. Lett. 107, 232405 (2015) <Featured article>.
[3] Appl. Phys. Lett. 101, 182403 (2012); Appl. Phys. Rev. 1, 011102 (2014) <invited paper>.
[4] Appl. Phys. Express 11, 063005 (2018).
[5] Nature Commun. 7, 13810/1-8 (2016).
[6] Appl. Phys. Lett. 112, 102402 (2018).
[7] Nature Phys. 15, 1134 (2019).
[8] Phys. Rev. Lett. 122, 107001 (2019).
[9] Phys. Rev. B 99, 014431 (2019).
[10] Nature Commun. 12, 4201 (2021).
[11] Jpn. J. Appl. Phys. 60, 010101 (2021) <invited paper>.