Solid state materials are of fundamental importance due to the plethora of exciting physical properties that have been detected and can be used in technological applications. Two important properties which have been observed in solid state materials are high temperature superconductivity in cuprates such as YBa2Cu3O7 and colossal magnetoresistance (CMR) in manganites such as La1-xSrxMnO3. A superconductor exhibits zero electrical resistance below a critical temperature, Tc, and technological applications include high field superconducting magnets for magnetic resonance imaging and nuclear magnetic resonance. A CMR material exhibits a large increase in electronic conductivity upon application of a magnetic field so that the magnetoresistance, MR, is defined as MR = ((rH-r0)/r0). Magnetoresistant materials are of technological importance and are applied in magnetoresistive sensors and spintronic devices in which electron spins are used to process information. Our research is focussed on using chemical substitutions to tune magnetoresistant and superconducting properties as well as the synthesis of new materials with fascinating electronic and magnetic properties.
Rietveld refinement of X-ray and Neutron powder diffraction data (at facilities such as ISIS, Rutherford Appleton Laboratory, Didcot, UK and the ESRF and ILL, Grenoble, France) are used in order to correlate the structure-property relationships in the synthesised compounds.
Solid state synthesis
Rietveld refinement of X-ray and neutron powder diffraction data
Electronic and magnetic characterisation of novel oxides and pnictides
Magnetic structure determination
A selection of high temperature tube and box furnaces (up to 1600 oC) for solid state synthesis
Bruker D8 advance powder diffractometer
Stanton Redcroft TG/DTA
Novel Transition Metal Oxypnictides
The recent report of high temperature superconductivity in oxypnictides such as LnFeAsO1-xFx with superconducting transition temperatures (Tcs) up to 55 K has led to a rapid expansion in the research of oxypnictide materials. We have recently discovered the counterpart, colossal magnetoresistance (CMR) in Mn2+ based oxypnictides, so that both manifestations of correlated electronic behaviour are now found in these new materials. A reduction in resistivity of up to 95% is observed in NdMnAsO0.95F0.05 upon applying a magnetic field at low temperature. The CMR arises as a result of competition between an antiferromagnetic insulating phase and a paramagnetic semiconductor with field and is a new mechanism of CMR. Future spintronic devices will capitalise on the CMR mechanism for smaller, faster, cheaper and more efficient “green” computing applications. Our current research focuses on optimising the CMR observed in NdMnAsO0.95F0.05 to room temperature by tailored chemical substitutions. We are also interested in synthesising novel transition metal oxypnictides which could display a range of interesting electronic and magnetic properties.
Synthesis of New Mo5+ Materials
We are also interested in the synthesis of novel Mo5+ oxides which have been relatively unexplored until recently. We have studied the double perovskite series Ba2LnMoO6 (Ln = lanthanide, Y3+, Sc3+) and observed a plethora of fascinating magnetic and structural properties. Ba2YMoO6 is the first example of a valence bond glass which is a new magnetic state and occurs as a result of magnetic frustration between Mo5+ s = ½ spins which are arranged on the corners of a lattice of edge sharing tetrahedra. Consequently, all antiferromagnetic interactions cannot be satisfied simultaneously, leading to a gradual freezing of spins into a disordered pattern of spin singlets upon cooling. Ba2GdMoO6 is ferroelastic; ferroelastic materials exhibit spontaneous strain and can exhibit a phase change when stress is applied. Ba2SmMoO6 exhibits a simultaneous Jahn-Teller distortion and antiferromagnetic order. The Neel temperature of this material is anomalously high (TN = 130 K) which suggests that the orbital order precipitates the antiferromagnetic order. This is the first time that a Jahn-Teller distortion has been observed in a Mo5+ oxide and suggests that the synthesis of new Mo5+ oxides with the same spin orbital and lattice coupling could result in the observation of exotic magnetic and electronic properties. We are currently attempting the synthesis of novel Mo5+ oxides with a range of crystal structures to further explore the magnetic, structural and electronic properties of Mo5+ oxides.
Superconductivity and CMR in Metallocuprates
We have recently synthesised a new ruthenocuprate RuSr2Nd1.8-xCexY0.2Cu2O10-d. These new materials are extraordinary as they exhibit superconductivity for x < 0.7 (Tc =35 K) and large negative magnetoresistance (MR) for x = 0.7 below ~ 140 K. The MR appears to originate from the strong antiferromagnetism in the copper oxide layers and at 4 K negative magnetoresistance equal to -49 % is observed comparable to those (at higher temperatures) in spin-polarised conductors such as colossal magnetoresistant manganese oxide perovskites. This is the first time that large bulk negative magnetoresistance has been observed in a cuprate which also becomes superconducting upon oxidation of the copper ion. Further interesting results are obtained from chemical doping studies of RuSr2Nd2-x-yCexYyCu2O10-d. MR7T(5K) shows a robust correlation with the copper oxidation state and falls rapidly above a copper oxidation state of 2.04, which may signify the onset of d-wave pairing correlations between the Cu holes, eventually leading to superconductivity for a copper oxidation state > 2.06. The magnetotransport is also very sensitive to lattice effects. In a series of RuSr2R1.1Ce0.9Cu2O10-d samples where the doping level is constant, the high field MR does not correlate with the paramagnetic moment of the R cations, but shows an unprecedented crossover from –MR to +MR as A>, the mean A site (R1.1Ce0.9) cation radius decreases. This size effect is further evidenced from studies of Ru1-xTaxSr2Nd0.95Y0.15Ce0.9Cu2O10 materials; MR at 9 Tesla and 4 K increases from -28% to -49% as x increases from 0 – 0.2 which further expands the unit cell. We have also recently synthesised the novel Iridocuprate IrSr2Sm1.15Ce0.85Cu2O10 which is a rare example of a reentrant spin glass.
Current sources of funding include EPSRC (Colossal magnetoresistance in cuprates?) and the Royal Society (Novel colossal magnetoresistant manganese oxypnictides).
Dr Mclaughlin teaches the following courses:
- CM1022 Elements of Chemistry
- CM2516 Solids
- CM2012 Electrical Materials
- CM3037 Transition Metal Chemistry
- CM4026 Lanthanides and actidnides
- CM5003 Magnets, metals and superconductors
- Further Info
Professor Mclaughlin is currently director of research for the Department of Chemistry.