The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute (<4 at.%) Mn-doped ZnO. The Mn is found to carry an average magnetic moment of 0.16 mu(B) per ion. Our ab initio calculations find a valance state of Mn(2+) and that the magnetic moments are ordered ferromagnetically, consistent with the experimental findings. We have obtained room-temperature ferromagnetic ordering in bulk pellets, in transparent films 2-3 microm thick, and in the powder form of the same material. The unique feature of our sample preparation was the low-temperature processing. When standard high-temperature (T > 700 degrees C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.

Films of ZnO doped with magnetic ions, Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behaviour is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localised spins with static localised states. The magnetism is quenched when carriers in the localised states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarisation of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor. PACS numbers 75.50.Pp, 78.20.Ls, 73.61 The search for spintronic materials that combine both semiconducting and ferromagnetic properties is currently one of the most active research fields in magnetism. Compounds based on ZnO are especially exciting in this context since, in contrast to GaMnAs and InMnAs, they exhibit ferromagnetism at room temperature [1][2][3][4][5][6][7]. Despite the progress in developing ZnO as a spintronic material, there has been much controversy concerning the mechanism that causes the magnetism [8][9][10]. It has been found that not all doped films exhibit ferromagnetism, and that the mobile carrier density, n c , can be very different in those compounds that do. This implies that the established theory of carrier-mediated magnetism, which works well for ptype GaMnAs, is not generally applicable to this n-type material. For example, it has been found that the addition of Zn interstitials, which affects both the number of neutral and ionised donors, leads to an increase in the ferromagnetism [11], and a recent study of Al-doped ZnCoO has reported a variation of magnetisation with Al content rather than carrier density [12]. In fact, most of the work on doped ZnO has concentrated on the insulating phase [9,[13][14][15], to such an extent that some authors now refer to ZnO as a dilute magnetic insulator (DMI) [13].However, we have recently reported the observation of ferromagnetism in Al-doped films where n c is very high [16], which highlights the importance of exploring the full range of carrier densities from the insulating to the metallic phases.In this Letter we report a systematic study of the relationship between the magnetism and conductivity in transition metal (TM) doped ZnO. By studying a large number of films with a wide range of carrier densities, we have identified three distinct conductivity regimes:1. The insulating phase at low carrier densities, in which the conductivity at low temperatures arises from a variable range hopping (VRH) process [17]. In this regime, labelled VRH, the least conducting films are the most magnetic, as has been observed previously [13,14]. 2. The intermediate regime, labelled I,...

We report the experimental observation of strong exciton-photon coupling in a planar microcavity composed of an organic semiconductor positioned between two metallic ͑silver͒ mirrors. Via transmission and reflectivity measurements, we observe a very large, room temperature Rabi splitting in excess of 300 meV. We show that the Rabi-splitting is enhanced in all-metal microcavities by a factor of more than 2 compared to an organic film positioned between a silver mirror and a dielectric mirror. This enhancement results from the significantly larger optical fields that are confined within all-metal microcavities. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1517714͔Placing emitters of light such as excitons within microcavities is attractive for device applications and allows the study of new optical phenomena. The effect of the microcavity on the emission of light can be divided into two regimes. In the weak-coupling regime, the spatial and temporal distribution of the emitted radiation can be altered. This regime is employed in applications such as vertical cavity surface emitting lasers and resonant cavity light emitting diodes. 1,2In the strong-coupling regime, a mixing between optical and electronic ͑excitonic͒ states within the cavity occurs, leading to the appearance of new states termed cavity-polaritons. 3This effect is an intensive area of research due in part to the interest in coherent, stimulated effects in such systems that may lead to new optical devices. 4 The field has recently expanded to include Frenkel excitons supported by organic materials. [5][6][7] This has been important in that it has led to the observation of significantly larger, room temperature, strongcoupling effects, and opens the possibility of easily fabricated nonlinear optical devices.A key phenomenon associated with strong-coupling 8 is the anticrossing of the exciton and photon mode where, in the absence of a strong interaction, they would have crossed. Until now, investigations into strong-coupling in organic and inorganic materials have been conducted either using two Bragg reflectors ͑DBR͒ as microcavity mirrors or one Bragg reflector together with one optically thick metal mirror. In this letter we show that microcavities fabricated using just two metal mirrors can operate in the strong-coupling regime. All-metal cavities are characterized by relatively low Q-factors; 9 however we find that strong-coupling can still be achieved because the effective optical path length in an allmetal cavity is significantly shorter than that in microcavities based on one or more dielectric mirrors, providing a significant enhancement of the optical field within the organic semiconductor region of the cavity. Because of this enhancement we observe very large Rabi-splittings of over 300 meV.J-aggregates of organic dye molecules possess many features that make them particularly suitable to undergo strongcoupling in microcavities. 6 The narrow, inhomogeneous linewidths ͑40-50 meV͒ and very large oscillator strengths of molecular J-aggre...

T-shaped molecules with a rod-like aromatic core and a flexible side chain form liquid crystal honeycombs with aromatic cell walls and a cell interior filled with the side chains. Here, we show how the addition of a second chain, incompatible with the first (X-shaped molecules), can form honeycombs with highly complex tiling patterns, with cells of up to five different compositions ("colors") and polygonal shapes. The complexity is caused by the inability of the side chains to separate cleanly because of geometric frustration. Furthermore, a thermoreversible transition was observed between a multicolor (phase-separated) and a single-color (mixed) honeycomb phase. This is analogous to the Curie transition in simple and frustrated ferro- and antiferromagnets; here spin flips are replaced by 180° reorientations of the molecules.

Electrical manipulation of lattice, charge, and spin has been realized respectively by the piezoelectric effect, field-effect transistor, and electric field control of ferromagnetism, bringing about dramatic promotions both in fundamental research and industrial production.