news & views DILUTE MAGNETIC SEMICONDUCTORS Hidden order revealed Dilute magnetic semiconductors such as gallium manganese arsenide could be key to the development of spintronics. But the relationship between electronic transport and magnetic properties has been hotly debated. Data indicating the preservation of the non-magnetic character of the host material provide startling new insight. Michael E. Flatté magine striking a golf ball from a tee states may become high enough to form Ordinarily, electric current moves in the middle of a dense forest. !e ball extended-like states, contributing su&cient through ferromagnetic (Ga,Mn)As like a Iwould hit leaves, branches and trunks carriers to screen the disorder and allow golf ball that’s being continually hit through in rapid progression and not travel far. coherent charge transport similar to that a forest — slowly and incoherently. To But if the ball were hit from a tee on a associated with valence-band conduction. study the transport characteristics and platform suspended above the trees, it !e nature of the transport and electronic structure of the states in this would be free to "y considerably further. electronic structure in the prototypical material above the Fermi energy, Ohya A similar demonstration has now been dilute magnetic semiconductor (Ga,Mn) and co-workers1 constructed a series performed by Ohya et al. within the As has been the subject of lengthy debate. of resonant tunnelling diodes in which dilute magnetic semiconductor gallium At high levels of manganese doping, the carriers are injected from a non-magnetic manganese arsenide, (Ga,Mn)As, as material exhibits some extended-state semiconductor into a (Ga,Mn)As quantum reported in Nature Physics1. In this work, transport characteristics of a metallic well and subsequently collected through the golf ball is a carrier of electrical conductor4. And yet, the mean free path a Schottky barrier into a gold contact. For current (speci#cally, a hole), the forest is for charge carriers at the Fermi energy is hopping-mediated (‘forest-like’) transport, a distribution of magnetic dopant states less than one nanometre and their states are one would expect the conduction through near the Fermi energy, and the clear blue highly correlated5 — behaviour that is more such a device to decrease monotonically sky is the unperturbed valence band of the typical of the hopping-mediated transport as the thickness of the (Ga,Mn)As non-magnetic host semiconductor, gallium of an amorphous or highly disordered quantum well was increased. In contrast, arsenide (Fig. 1). !e results suggest that semiconductor. !is also has important the researchers found that the current long-standing questions over the electronic rami#cations for the mechanism by which oscillates periodically with thickness — a structure and origin of ferromagnetism ferromagnetism occurs in this material — signature of quantum interference e%ects in this and related dilute magnetic metallic systems may become magnetic that could only arise if the transport were semiconductors will require di%erent through the spin susceptibility of the coherent — even for thicknesses of up to solutions than currently advocated. valence band6, whereas a hopping system 20 nanometres, an order of magnitude Dilute magnetic semiconductors have would require another method such as by greater than the mean free path at the tantalized researchers for decades, as a double-exchange interactions7,8. Fermi level. Or in other words, the charge semiconductor is extremely sensitive to external perturbations such as doping and electric #elds, and thus the magnetic properties of such semiconductors should Holes move freely with little scattering be similarly responsive. !is tunability and little dependence on magnetic field with external #elds makes semiconductors central to fundamental tests of solid- state theories, and also ubiquitous in + + modern optoelectronics; a similarly h GaAs valence band h tunable magnetic semiconductor could unify optoelectronics and spintronics technologies. Preparation of a dilute Manganese dopant impurity band magnetic semiconductor typically begins Hole energy h+ h+ with a clean non-magnetic host, such Fermi level as gallium arsenide, into which a large number of magnetic atoms are added — typically 1–10% of the atoms in the crystal Holes hop between localized impurity states, to produce ferromagnetism2, much more are strongly scattered and magnetic than the part-per-billion level used in conventional electronic doping. At low Position (x) concentrations (below around 1%), these atoms create localized acceptor states around them3 that trap and scatter charge Figure 1 | Electronic transport near the Fermi level and at higher energies, as determined by the carriers (holes) that move through the measurements of Ohya and colleagues1. Near the Fermi level the transport is incoherent and strongly disordered solid by hopping. At higher magnetic. At higher energies the transport is coherent and nearly non-magnetic, indicating the presence levels, however, the density of these doping of valence band states that seem unrelated to the ferromagnetism in this material. NATURE PHYSICS | VOL 7 | APRIL 2011 | www.nature.com/naturephysics 285 © 2011 Macmillan Publishers Limited. All rights reserved news & views carriers pass through the (Ga,Mn)As like diode. But even the lowest-energy state localized and extended states, in these golf balls "ying free in clear blue sky. of undoped gallium arsenide, the lowest- materials. Improved understanding of !e simple fact of the researchers’ energy heavy-hole state, was visible in the relationship between the electronic demonstration of highly coherent the measured tunnelling spectra, and at a structure and the mechanism of transport through (Ga,Mn)As, even at voltage more than 100 mV higher than the ferromagnetism in these materials may also these higher carrier energies, suggests Fermi level. !ese measurements suggest guide the #eld towards higher-temperature new possibilities for the development of that all the holes reside in states generated ferromagnetic semiconductors or practical magnetic semiconductor devices such as by the magnetic dopants and not in the semiconductor spintronic devices. ❐ spin transistors. Yet their results provide valence band of the host. And the lack of a further surprise. !e coherent transport any magnetic signature from the valence Michael E. Flatté is in the Optical Science and they observe seems to be minimally band suggests its role in the mechanism of Technology Center and the Department of Physics a%ected by the magnetic character of ferromagnetism may be peripheral, unlike and Astronomy, "e University of Iowa, Iowa City, the material — they found no visible the picture in ref. 6. Iowa 52242, USA. di%erence in the energy of spin-up and !e dramatic variation in the character e-mail: michael_#[email protected] spin-down carriers travelling through the of the states in (Ga,Mn)As — from the coherent transport states. Consequently, incoherent, highly magnetic states near the References 1. Ohya, S., Takata, K. & Tanaka, M. Nature Phys. 7, 342–347 (2011). these states seem to be indistinguishable Fermi energy, to states a mere 50–80 meV 2. MacDonald, A. H., Schi%er, P. & Samarth, N. Nature Mater. from those of undoped gallium arsenide. away in energy that are coherent and 4, 195–202 (2005). Moreover, they seem to be una%ected by nearly non-magnetic — suggests there is 3. Yakunin, A. M. et al. Phys. Rev. Lett. 92, 216806 (2004). the level of electronic doping. For a highly still much to learn about the fundamental 4. Matsukura, F., Ohno, H., Shen, A. & Sugawara, Y. Phys. Rev. B 57, R2037–R2040 (1998). doped material, such as one with 15% of electronic properties and mechanism of 5. Richardella, A. et al. Science 327, 665–669 (2010). the cation sites occupied by manganese, ferromagnetism in this and related dilute 6. Dietl, T., Ohno, H., Matsukura, F., Cibert, J. & Ferrand, D. Science many of the valence band states should magnetic semiconductors. It may even 287, 1019–1022 (2000). have been #lled with holes and thus be the case that these measurements 7. Akai, H. Phys. Rev. Lett. 81, 3002–3005 (1998). 8. Sheu, B. L. et al. Phys. Rev. Lett. 99, 227205 (2007). 9 unavailable for hole transport through identify the presence of the mobility edge , 9. Lee, P. A. & Ramakrishnan, T. V. Rev. Mod. Phys. the well region of the resonant tunnelling characterizing the separation between 57, 287–337 (1985). 286 NATURE PHYSICS | VOL 7 | APRIL 2011 | www.nature.com/naturephysics © 2011 Macmillan Publishers Limited. All rights reserved.