A New Ferromagnetic Semiconductor: MnxGe1-x
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Materials Science and Technology Division
2Present address: Seoul National University
What is a Ferromagnetic Semiconductor: Ferromagnetic semiconductors (FMSs) are materials that simultaneously exhibit semiconducting properties and spontaneous long-range ferromagnetic (FM) order. Classic examples, studied decades ago, include the europium chalcogenides and the chalcogenide spinels. The coexistence of these properties in a single material provides fertile ground for fundamental studies as well as a host of new applications. Development of novel devices languished for some time due to the inability to incorporate these materials with mainstream semiconductor device materials.
Interest in FMSs was rekindled with the discovery of spontaneous FM order in In1-xMnxAs in 1989 and Ga1-xMnxAs in 1996,1 when FM properties were realized in semiconductor hosts already widely recognized for semiconductor device applications. Although these new FMS materials have magnetic ordering, or Curie, temperatures (Tc) below room temperature, they have been closely studied for their potential in future spin-dependent semiconductor device technologies, with the expectation that further research will increase Tc. Ga1-xMnxAs, for example, has been used as a source of spin-polarized carriers in both light-emitting diodes and resonant tunneling diode heterostructures.
A New Material: We have prepared the first Group-IV ferromagnetic semiconductor, MnxGe1-x.2 While most recent experimental work on FMSs has focused on III-V and II-VI compounds, there is broad interest in the Group IV semiconductors, namely C, Si, Ge, and Si1-xGex. Ge is of particular interest because it is closely lattice matched to the technologically important AlyGa1-yAs family and has higher intrinsic hole mobilities than either GaAs or Si. A high hole concentration is essential to mediate the necessary FM exchange.1
We used nonequilibrium growth techniques such as molecular beam epitaxy and low substrate growth temperatures to minimize phase separation and the formation of unwanted compounds because of the low solubility of Mn in Ge. MnxGe1-x single-crystal films can be grown on both Ge and GaAs substrates, and we have discovered that the electronic and magnetic properties are very promising for FMS device applications. In particular, electric field control of the FM order should be achievable at gate voltages compatible with present CMOS technology (±0.5 V). Implementation of such a device is described below.
Magnetic and Electric Properties: Figure 7 shows magnetization as a function of temperature for a typical sample; Curie temperatures are derived from this type of data. Our films have Tc's in the range of 25 to 116 K, with a linear dependence on Mn concentration for 0.006 ≤ x ≤ 0.035. Magnetization loops (Fig. 7 inset) exhibit hysteretic behavior and have significant remanence—both clear signs of FM order. Saturation magnetizations up to 30 emu/cm3 indicate that only 45-60% of all the Mn atoms are magnetically active, and theory provides some insight into this behavior.
Hole densities at room temperature can be obtained from Hall measurements. The hole density increases with Mn concentration and ranges from 1019 to 1020 cm-3. Below the magnetic ordering temperature, transport measurements clearly show a large extraordinary Hall effect (EHE), another clear signature of FM order. Finally, the resistivity of our MnxGe1-x films decreases with temperature, indicating that samples are semiconducting rather than metallic in nature.
Insights from Theory: To investigate the microscopic origins of ferromagnetism in MnxGe1-x, we used electronic-structure calculations based on density-functional theory (DFT) with the aim of providing a first-principles foundation for future model descriptions. Mn preferentially occupies substitutional sites and creates only negligible distortion of the host lattice—Ge atoms are perturbed by less than 0.05 Å. Figure 8 illustrates how a Mn atom affects the Ge lattice; electron density (Fig. 8(a)) is shown as green, and spin density (Fig. 8(b)) is shown as blue. Note that the bonding density between a Mn and Ge atom near the Ge atom is nearly the same as between neighboring Ge.
The calculated magnetic moment of Mn in Ge is 3mB, in agreement with other DFT studies but contrary to Hund's rules (as applied to defects). By mapping DFT results for the spin interactions onto a Heisenberg model, we extract spin couplings whose sign and magnitude depend on both distance and crystallographic direction, with ferromagnetic interactions ultimately dominating. The interactions are hole mediated, and are reduced when the material is strongly compensated. Nearest-neighbor Mn pairs are strongly antiferro-magnetically coupled and hence do not participate in the FM ordering. This is consistent with our experimental data.
A Unique Application: The nonmetallic character of our samples permits control of the carrier density in simple gated structures (Fig. 9(a)) via application of a small gate voltage. Since the FM exchange is mediated by the holes, we should be able to control the FM order, an effect demonstrated with In1-xMnxAs, but with very high gate voltages (±125 V).3 The effect of gate voltage on FM order is shown in Fig. 9(b). The extraordinary component of the Hall voltage is clearly enhanced (suppressed) as the hole density is enhanced (suppressed) by a ±0.5 V gate voltage, confirming that FM exchange is hole mediated in MnxGe1-x.
Summary and Implications: Epitaxially grown MnxGe1-x is FM with semiconducting character. Electric field control of the FM order, a unique property of FMS compounds, has been demonstrated with conventional low voltage circuitry. This suggests a variety of applications including low power control of magnetic fields, voltage-tunable dichroic devices, and gated optical isolators.
[Sponsored by ONR and DARPA]