G. A. Gehring

Two magnetic regimes in doped ZnO corresponding to a dilute magnetic semiconductor and a dilute magnetic insulator.

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 behavior 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 localized spins with static localized states. The magnetism is quenched when carriers in the localized 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 polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.

Room-temperature magneto-optics of ferromagnetic transition-metal-doped ZnO thin films

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 150 K, 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 behavior.

The domain structure in ultrathin magnetic films

Abstract We discuss the domain structure in ultrathin magnetic films (of a few monolayer thickness), where the easy direction of magnetization is perpendicular to the film. We calculate the dependence of the width D of domains on film thickness L within the continuum approximation. Provided that the thickness of the film is large compared with the range of the exchange interactions, re, we find that D is proportional to L eπD0/2L, where D0 is the ‘dipolar length’. However, for L

Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature

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 doped ZnO 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 5?B and 3?B respectively. Furthermore a clear correlation between the magnetization 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 magnetization on carrier 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. We observe a positive magnetoresistance but no anomalous Hall effect or anisotropic magnetoresistance in the ferromagnetic samples.

Room temperature ferromagnetism in pristine MgO thin films

Robust ferromagnetic ordering at, and well above room temperature is observed in pure transparent MgO thin films (<170 nm thick) deposited by three different techniques. Careful study of the wid ...

Complex Multicolor Tilings and Critical Phenomena in Tetraphilic Liquid Crystals

X-shaped molecules undergo reversible thermal transitions between phase-separated and mixed states. 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.

Numerical and approximate analytical results for the frustrated spin-1/2 quantum spin chain

We study the T=0 frustrated phase of the 1D quantum spin- 1/2 system with nearest-neighbour and next-nearest-neighbour isotropic exchange known as the Majumdar-Ghosh Hamiltonian. We first apply the coupled-cluster method of quantum many-body theory based on a spiral model state to obtain the ground-state energy and the pitch angle. These results are compared with accurate numerical results using the density matrix renormalization group method, which also gives the correlation functions. We also investigate the periodicity of the phase using the Marshall sign criterion. We discuss particularly the behaviour close to the phase transitions at each end of the frustrated phase.

Investigation of magnetite thin films produced by pulsed laser deposition

Abstract Thin films of magnetite (Fe 3 O 4 ) with thicknesses in the range 10–1000 nm have been produced by the XeCl excimer laser ( λ = 308 nm) ablation of both Fe 3 O 4 and metallic 57 Fe targets. Deposition conditions have been investigated in an attempt to fabricate films reproducibly at low laser fluences so as to minimize ablation bouldering. The ablation of metallic Fe has been shown to produce a smaller concentration of boulders than the ablation of bulk magnetite. It is believed that this is mainly due to the difference in primary ablation mechanisms between the two target materials. The use of single-crystal, lattice-matching substrates was found to be necessary at lower fluences (1–3 J cm −2 ) for the ablation of metallic Fe, whereas polycrystalline, non-lattice-matching substrates were adequate for deposition with moderate laser fluences (5–6 J cm −2 ). Film quality has been assessed by a range of techniques including XRD, SEM, AFM/MFM and CEMS. Using CEMS it has been established that the films produced from a metallic target contained a significant amount of metallic Fe, together with secondary oxide phases. The influence of film thickness on the Verwey transition has been investigated via electrical conductivity and SQUID magnetometry. It is found that there is a systematic reduction of Verwey temperature with decreasing film thickness; this is attributed to the effect of strain.

Role of donor defects in enhancing ferromagnetism of Cu-doped ZnO films

Zn0.97Cu0.03O films were prepared by pulsed-laser deposition on c-cut sapphire substrates under various conditions in order to investigate the growth-dependent properties of the films. All the films exhibit room temperature ferromagnetism. Samples deposited at low temperature and low pressure show large saturation magnetization (Ms). Moreover, the enhancement of Ms was observed in films annealed both in vacuum and in Zn vapor. However, postannealing in air led to the remarkable reduction of ferromagnetism. The results show that the itinerant electrons introduced by oxygen-deficient or Zn-rich atmospheres may play a significant role in room temperature ferromagnetism observed in this ZnCuO dilute magnetic semiconductor. It is consistent with carrier-induced ferromagnetism. The magnetization strongly depends on the appearance of free carriers and is relatively insensitive to whether they arose from VO or/and Zni.

Magnetic and transport properties of n-type Fe-doped In2O3 ferromagnetic thin films

Room temperature ferromagnetism was observed in n-type Fe-doped In2O3 thin films deposited on c-cut sapphire substrates by pulsed laser deposition. Structure, magnetism, composition, and transport studies indicated that Fe occupied the In sites of the In2O3 lattice rather than formed any metallic Fe or other magnetic impurity phases. Magnetic moments of films were proved to be intrinsic and showed to have a strong dependence on the carrier densities, which depended on the Fe concentration and its valence state as well as oxygen pressure.