Room-Temperature Magneto-Optics of Ferromagnetic Transition-Metal-Doped ZnO Thin Films

Abstract

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...

Antiferromagnetic interaction between paramagnetic Co ions in the diluted magnetic semiconductorZn1−xCoxO

“…Reproducibly, exposure of Zn 1−x Co x O thin film to Zn vapor creates interstitial Zn defects and induces room-temperature ferromagnetism, and annealing the thin film in air quenches the ferromagnetism due to elimination of the interstitial Zn by oxidation. Neal et al 13 observed magnetic circular dichroism ͑MCD͒ signals with open hysteresis loop at the ZnO band edge of Co-doped ZnO thin films at room temperature, implying a splitting of the conduction band of ZnO through interaction with the magnetic moments of the Co ions. These observations rule out Co-cluster segregations or secondary phase formation as the origin of the ferromagnetism.…”

Exchange broadening of EPR line in ZnO:Co

“…The theoretical predictions based on the local spin density approximation (LSDA), triggered extensive studies of ZnO:TM alloys with a special focus on ZnO:Co as the most promising candidate for a room-temperature ferromagnetic (FM) semiconductor [3]. Many experiments have been reported on this material fabricated by a variety of methods [4][5][6]; however, the magnetic properties of ZnO:Co still remain a controversial issue since the observed magnetic behavior appears to be strongly dependent on the preparation methods and is poorly reproducible. Ferromagnetism was reported for thin films and bulk samples of ZnO:Co with a very large spread of spontaneous magnetic moment from 6.1 m B /Co to 0.01 m B /Co accompanied by a Curie temperature well above room temperature [4][5][6][7][8][9].…”

Structural and magnetic properties of (In1−xFex)2O3 (0.0⩽x⩽0.25) system: Prepared by gel combustion method

“…[2][3][4][5][6][7][8][9][10] For practical spintronic applications DMS with Curie temperature, Tc, above RT are required. Many promising oxides like TiO 2 , ZnO and SnO 2 and III-V semiconductor GaN doped with transition metal ions have been reported to exhibit ferromagnetic behavior near or above room temperature, [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] but the origin of ferromagnetism in these systems is poorly understood. Oxide systems based on host semiconductor with high solubility of magnetic ions are highly desirable to form thermodynamically stable magnetic semiconductors.…”