Landau theory of compressible magnets near a quantum critical point

Landau theory is used to investigate the behaviour of a metallic magnet driven towards a quantum critical point by the application of pressure. The observed dependence of the transition temperature with pressure is used to show that the coupling of the magnetic order to the lattice diverges as the quantum critical point is approached. This means that a first order transition will occur in magnets (both ferromagnets and antiferromagnets) because of the coupling to the lattice. The Landau equations are solved numerically without further approximations. There are other mechanisms that can cause a first order transition so the significance of this work is that it will enable us to determine the extent to which any particular first order transition is driven by coupling to the lattice or if other causes are responsible.Comment: 12 pages including 5 figures, to be presented at MMM-Intermag conference and accepted for publication in Journal of Applied Physic

Grain boundary ferromagnetism in vanadium-doped In2_2O3_3 thin films

Room temperature ferromagnetism was observed in In$_2$O$_3 thin films doped with 5 at.temperatures ranging from 300 to 600$\,^{\circ}{\rm C}$. X-ray absorption fine structure measurement indicated that vanadium was substitutionally dissolved in the In$_2$O$_3$ host lattice, thus excluding the existence of secondary phases of vanadium compounds. Magnetic measurements based on SQUID magnetometry and magnetic circular dichroism confirm that the magnetism is at grain boundaries and also in the grains. The overall magnetization originates from the competing effects between grains and grain boundaries.Comment: 12 pages, 7 figures, 1 table, accepted by Europhysics Letter

On Using Magnetic and optical methods to determine the size and characteristics of nanoparticles embedded in oxide semiconductors

Films of oxides doped with transition metals are frequently believed to have magnetic inclusions. Magnetic methods to determine the amount of nanophases and their magnetic characteristics are described. The amount of the sample that is paramagnetic may also be measured. Optical methods are described and shown to be very powerful to determine which defects are also magnetic.Comment: Manuscript of poster to be presented at MMM-Intermag 2010. Accepted for publication in Magnetic Trans of IEE

Spin polarized transport current in n-type co-doped ZnO thin films measured by Andreev spectroscopy

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 $\pm$ 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.Comment: Published versio

Magneto-optical properties of Co/ZnO multilayer films

Multilayer films of ZnO with Co were deposited on glass substrates then annealed in a vacuum. The magnetisation of the films increased with annealing but not the magnitude of the magneto-optical signals. The dielectric functions for the films were calculated using the MCD spectra. A Maxwell Garnett theory of a metallic Co/ZnO mixture is presented. The extent to which this explains the MCD spectra taken on the films is discussed.Comment: This paper was presented at ICM (2009) and is accepted in this form for the proceeding

Growth of high quality yttrium iron garnet films using standard pulsed laser deposition technique

Thin films with properties comparable to bulk single crystals were grown by pulsed laser deposition using a substrate temperature of only 500 °C. This was achieved by a careful choice of both the oxygen pressure in the deposition chamber and the temperature of the air anneal. The best films were grown on gadolinium gallium garnet substrates but we also report data for films grown on the diamagnetic substrate yttrium aluminium garnet. The films were analysed using X-ray diffraction, near edge X-ray absorption and magnetometry. Our best films had a magnetisation of 143 emu/cm 3 and a coercive field of ~1 Oe

Magnetic and structural studies of some mixed metal oxides

Powder neutron diffraction and magnetic susceptibility measurements of the antiferromagnetic phases of MnxNi1-xO,MnxCo1-xO, (MnxFe1-x)zO and (CoxFe1-x)zO reveal that the magnetic moments of unlike ions are always effectively collinear, despite the presence of competing anisotropies. The magnetic moments of MnxNi1-xO (x = 0.24,0.48 and 0.77) at 5K are confined to (111) planes by dipole-dipole forces, and the small trigonal exchangestrictions are the products of opposed antiferromagnetic Mn2+-Mn2+ and ferromagnetic Ni2+-Mn2+ nearest neighbour interactions. In MnxCo1-xO (x = 0.05, 0.10, 0.25,0.36) at 5K, the orbital degener- acy of Co2+ is removed by both Jahn-Teller (J.T) and spin-orbit coupling (S.O) mechanisms, leading to orthorhombic or triclinic symmetries and partially quenched Co2+ moments. Monoclinic symmetry is observed for 0.512+ moments are evident. The cobalt concentration is too small to support a cooperative J.T stabilisation in Mn0.83Co0.17O, where a magnetostriction (c/a Previous results for CoxNi1-xO, and those for MnxCo1-xO, (MnxFe1-x)zO (x = 0.05, 0.1, 0.12, 0.23, 0.36,0.56,0.66,0.89) and (CoxFe1-x)zO (x = 0.04,0.12,0.50,0.63,0.81) at 5K indicate that the anisotropy order for the iron group monoxides is CoO>MnO»FezO≈NiO. The weak trigonal anisotropy of Fe2+ correlates with the near-cubic symmetries of (CoxFe1-x)z( (x = 0.04,0.12) and (MnxFe1-x)zO (0.11) magnetostriction of Fe2+ is observed in (CoxFe1-x)zO with x > 0.5. Measurements of vacancy-ferric interstitial ratios for (MnxFe1-x)zO and (CoxFe1-x)zO suggest that non-stoichiometry is accommodated by 6:2 or 8:3 defect clusters in the former, and by larger units in the latter. The observed magnetic moments of defective samples are normally larger than those calculated according to a previous model for FezO, and require the postulation of partial antiferromagnetic order around the clusters. The room temperature Mossbauer effect parameters of (MnxFe1-x)zO (02+ and Mn2+ ions are randomly distributed over octahedral sites, ferric ions are localised around defect clusters.</p