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,...

Magneto-optic studies of ZnO doped with transition metals Co, Mn, V and Ti indicate a significant magnetic circular dichroism (MCD) at the ZnO band edge at room temperature, together with an associated dispersive Faraday rotation. Similar spectra occur for each dopant which implies that the ferromagnetism is an intrinsic property of the bulk ZnO lattice. At 10 K additional paramagnetic contributions to the MCD are observed, but above about 150K, the magnitude of the MCD signal is dominated by the ferromagnetism and is almost temperature independent. The MCD at the ZnO band edge shows room temperature hysteretic behaviour. DOI: PACS numbers: 75.50 Pp, 75.50 Pp, The search for spintronic materials that combine semiconducting and ferromagnetic properties, dilute magnetic semiconductors (DMS), is currently the most topical field in magnetism. The compounds based on the wide-gap (3.4 eV) semiconductor ZnO are especially exciting because they exhibit ferromagnetism at room temperature [1][2][3][4][5][6], in contrast to the GaMnAs-based materials for which the highest reported Curie temperatures, T c , are still well below 300 K.Despite the growing body of evidence in favour of room temperature magnetic hysteresis in doped ZnO, the nature of the ferromagnetism is hotly contested. The original interest in ZnO was prompted by the prediction that the hole exchange mechanism found in GaMnAs would produce Curie temperatures above 300 K [7]. However, it is now known that the doping is normally n-type, for which the exchange is smaller [8]. Furthermore, the measured moments are often much smaller than the theoretical values, which suggests that the magnetism might be due to an impurity phase [9,10]. It is therefore highly important to carry out careful experimental studies that can elucidate the microscopic origin of the magnetism.In this Letter we present a detailed study of the magnetooptical (M-O) properties of ferromagnetic ZnO, together with other experimental details. A measurement of the magnetic circular dichroism (MCD) at photon energy E gives the difference in absorption for left and right circularly polarised light at that same energy. Hence it provides a clear indication of the extent to which the states involved in the transition at that particular energy are influenced by the magnetism. We concentrate here on the spectral region close to the band edge at ~3.4 eV, since this characterizes the intrinsic behaviour of the ZnO lattice. The results show that the ferromagnetism at 300K is intimately connected with the band electrons of ZnO and that the carriers are polarised. Furthermore, since ZnO is transparent in the blue/UV, the large Faraday rotations that we observe around 3 eV are potentially useful for applications in M-O processing.The Zn 1−x M x O samples studied here (M = transition metal) were grown as thin films by pulsed laser deposition (PLD) on sapphire (0001) substrates. Co, Mn, V and Ti were used as the dopants with concentrations up to 5%. An undoped ZnO film was also grown for reference. SQ...

The realization of semiconductors that are ferromagnetic above room temperature will potentially lead to a new generation of spintronic devices with revolutionary electrical and optical properties. Transition temperatures in dopedZnO are high but, particularly for Mn doping, the reported moments have been small. We show that by careful control of both oxygen deficiency and aluminium doping the ferromagnetic moments measured at room temperature in n-type ZnMnO and ZnCoO are close to the ideal values of 5µ B and 3µ B respectively. Furthermore a clear correlation between the magnetisation per transition metal ion and the ratio of the number of carriers to the number of transition metal donors was established as is expected for carrier induced ferromagnetism for both the Mn and Co doped films. The dependence of the magnetisation oncarrier density is similar to that predicted for the transition temperature for a dilute magnetic semiconductor in which the exchange between the transition metal ions is through the free carriers.

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Magnetoresistance measurements have been made at 5 K on doped ZnO thin films grown by pulsed laser deposition. ZnCoO, ZnCoAlO and ZnMnAlO samples have been investigated and compared to similar films containing no transition metal dopants. It is found that the Co-doped samples with a high carrier concentration have a small negative magnetoresistance, irrespective of their magnetic moment. On decreasing the carrier concentration, a positive contribution to the magnetoresistance appears and a further negative contribution. This second, negative contribution, which occurs at very low carrier densities, correlates with the onset of ferromagnetism due to bound magnetic polarons suggesting that the negative magnetoresistance results from the destruction of polarons by a magnetic field. An investigation of the anisotropic magnetoresistance showed that the orientation of the applied magnetic field, relative to the sample, had a large effect. The results for the ZnMnAlO samples showed less consistent trends.

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We use point contact Andreev reflection measurements to determine the spin polarization of the transport current in pulse laser deposited thin films of ZnO with 1% Al and with and without 2%Mn. Only films with Mn are ferromagnetic and show spin polarization of the transport current of up to 55 ± 0.5% at 4.2 K, in sharp contrast to measurements of the nonmagnetic films without Mn where the polarization is consistent with zero. Our results imply strongly that ferromagnetism in these Al doped ZnO films requires the presence of Mn. 74.45+c, 72.25.Dc, 75.50.Pp