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anomalous Hall effect controlled by charge carriers in a room-temperature ferromagnetic semiconductor would be strong evidence of intrinsic ferromagnetism and an important milestone towards the realization of semiconductor spintronics devices operable at room temperature

Anomalous Hall effect governed by electron doping in a room-temperature transparent ferromagnetic semiconductor.

NATURE MATERIALS, no. 4 (2004): 221-224

Cited by: 284|Views20
WOS NATURE

Abstract

Ferromagnetic semiconductors are believed to be suitable for future spintronics, because both charge and spin degrees of freedom(1,2) can be manipulated by external stimuli. One of the most important characteristics of ferromagnetic semiconductors is the anomalous Hall effect. This is because the ferromagnetically spin-polarized carrier c...More

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Introduction
  • Room-temperature ferromagnetic semiconductors are desired for application to practical devices.
  • The authors report on the anomalous Hall effect governed by electron doping in a room-temperature transparent ferromagnetic semiconductor,rutile Ti1–xCoxO2–δ.This result manifests the intrinsic nature of ferromagnetism in this compound, and represents the possible realization of transparent semiconductor spintronics devices operable at room temperature.
Highlights
  • Room-temperature ferromagnetic semiconductors are desired for application to practical devices
  • We report on the anomalous Hall effect governed by electron doping in a room-temperature transparent ferromagnetic semiconductor,rutile Ti1–xCoxO2–δ.This result manifests the intrinsic nature of ferromagnetism in this compound, and represents the possible realization of transparent semiconductor spintronics devices operable at room temperature
  • A well-known ferromagnetic response of charge carriers in ferromagnetic semiconductors is the anomalous Hall effect (AHE)2—the emergence of voltage transverse to both the applied current and external magnetic field proportional to the magnetization, that is attributed to asymmetric carrier scattering by magnetic impurities in the presence of spin-orbit interaction (Fig. 1b)
  • AHE controlled by charge carriers in a room-temperature ferromagnetic semiconductor would be strong evidence of intrinsic ferromagnetism and an important milestone towards the realization of semiconductor spintronics devices operable at room temperature
  • The gradual increase of the lattice constant with decreasing PO2 represents a systematic increase of δ, resulting in systematicdopingof electronstoincreaseσxx.Thevalueof ndetermined from ordinary Hall effect (OHE) ranges from 1018 cm–3 to 1022 cm–3 with a rather constant mobility of the order of 10–1 cm[2] V–1 s–1
  • As seen in Fig. 3a, ρxy increases rapidly up to 0.2 T with increasing magnetic field, and gradually decreases linearly with further increasing magnetic field. This behaviour shows that the anomalous part of ρxy, which is proportional to magnetization, is dominant for lower magnetic field, whereas the ordinary part of ρxy, which isproportionaltotheinverseof nyieldingthenegativelinearslopeof ρxy, overcomes ρAHE for higher magnetic field, as the magnetization saturates and ρOHE keeps increasing linearly with increasing magnetic field
Results
  • A well-known ferromagnetic response of charge carriers in ferromagnetic semiconductors is the anomalous Hall effect (AHE)2—the emergence of voltage transverse to both the applied current and external magnetic field proportional to the magnetization, that is attributed to asymmetric carrier scattering by magnetic impurities in the presence of spin-orbit interaction (Fig. 1b).
  • The contribution of ρAHE can be seen by subtracting ρOHE, wheras ρAHE cannot de detected for Ti0.97Co0.03O2–δ films grown under PO2 = 10–5 torr and 10–4 torr, having even lower n ∼1019 cm–3 and ∼1018 cm–3 at 300 K, respectively.
  • Ferromagnetic semiconductors has been proposed[13].In this theory,Hall conductivity (σxy ≡ ρxy/(ρxx2 + ρxy2)) is shown to be an essential measure of the strength of AHE,where the anomalous part of σxy depends onthecontentsof bothcarrierandspin,showinggoodcoincidencewith experimental results[14] for (Ga,Mn)As. The authors show the relationship between σAHE and σxx (∝ n) for a series of Ti1–xCoxO2–δ as shown, where ρAHE is evaluated by subtracting the linear background in ρxy versus magnetic-field curves.
  • The relationship shows a scaling behaviour being σAHE ∝ σxαx with α ∼1.5–1.7 for the data with different temperatures,x,and PO2.Sizable x dependence of σAHE is hardly seen, contrary to that of ρAHE, because the increase of ρxx with x cancels out the increase of ρAHE with x.
Conclusion
  • The scaling behaviour of AHE governed by σxx ensures that ferromagnetically spin-polarized charge carriers play a significant role in the emergence of ferromagnetism for this compound, detailed investigation will be needed both for experimental and theoretical studies.
  • The enhancement of σAHE over six orders of magnitude by increasing σxx may directly lead to the possible realization of semiconductor spintronics devices operable at room temperature in such as field-effect devices.
Funding
  • This work was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology in Japan, Grant-in-Aid for Creative Scientific Research (14GS0204 an 13NP0201), NEDO International Joint Research program (02BR3) and the Inamori Foundation
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