95th CEMS Colloquium


Prof. Masaaki Tanaka (The University of Tokyo)


17:30 - 18:30, November 24, 2021 (Wednesday)


Administrative Headquarters 2F, RIKEN


Renaissance of Ferromagnetic Semiconductors and Spintronics Applications


   Ferromagnetic semiconductors (FMSs) with high Curie temperature (TC) 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 TC  is much lower than 300 K. In this study, we present a new class of FMSs with high TC , 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 TC value increases monotonically as the Fe content increases; and there is a tendency that TC 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 (Ga1-x,Fex)Sb with x > 23% [1] and (In1-x,Fex)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.

[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>.