Ian Terry

Synthesis of monodispersed fcc and fct FePt/FePd nanoparticles by microwave irradiation

A simple microwave heating method has been used for the stoichiometrically controlled synthesis of FePt and FePd nanoparticles using Na2Fe(CO)4 and Pt(acac)2/Pd(acac)2 as the main reactants. By varying the solvents and surfactants, the microwave assisted reactions have shown a significant advantage for the rapid production of monodisperse fcc FePt nanoparticle metal alloys which can be converted to the fct phase at low temperatures (364 °C). Microwave reactions at high pressure (closed system) have led to the direct formation of a mixture of fcc and fct phase FePt nanoparticles. Room temperature structural and magnetic properties of materials have been characterized by X-ray diffraction, HRTEM and magnetic measurements. The onset of ordering has been investigated by in situ high temperature X-ray diffraction studies.

Band tails and the insulator-metal transition in the persistent photoconductor Cd1-xMnxTe:In

Abstract Persistent photoconductivity has been demonstrated in the dilute magnetic semiconductor Cd 1-x Mn x Te:In and has been used to elucidate the shape of the band tail. It is further shown that the insulator-metal transition may be approached, in one sample, by controlling the persistent carrier density with illumination.

Study of the magnetic interactions in Ba2PrRu1−xCuxO6 using neutron powder diffraction

High-resolution neutron powder diffraction has been used to determine the crystal and magnetic structures of Ba2PrRuO6 and Ba2PrRu0.90Cu0.10O6. Both materials are 2116 distorted double perovskites and the magnetic structure, which develops below ∼110 K, consists of two interpenetrating Type I antiferromagnetic sublattices of the Ru5+ and Pr3+ ions. When saturated, the moments are ∼1.9 μB for Ru5+ and ∼1.1 μB for Pr3+ in the c-direction, though copper doping introduces an extra ab-component of ∼0.65 μB. From the study of variable temperature neutron diffraction data the principal magnetic interactions in the materials have been determined. Although an overall antiferromagnetic structure is adopted, the Ru–O–Pr ferromagnetic interaction is a factor of ∼4 times stronger than the Ru–O–O–Ru antiferromagnetic interaction.

Low temperature magnetoresistance of the persistent photoconductor Cd0.9Mn0.1Te:In

Abstract The magnetoresistance of the dilute magnetic persistent photoconductor Cd 0.9 Mn 0.1 Te:In is reported for temperatures between 1.3 and 17 K, and at magnetic fields of up to 8 T. The measurements were made at a stable persistent photocarrier concentration of 4.3 × 10 16 cm −3 produced after illuminating with sub-bandgap radiation. We demonstrate that the data may be interpreted with a model based upon the existence of bound magnetic polarons in the material. Here the magnetoresistance is a result of the difference between the magnetization of the magnetic moments of the polaron and the magnetization of the host material. We obtain quantitative fits to the magnetoresistance data using this model and assuming that the magnetization of both the polaron and the host material may be described by modified Brillouin functions.

Microwave-Assisted Synthesis, Microstructure, and Magnetic Properties of Rare-Earth Cobaltites

The series of perovskite rare-earth (RE) doped cobaltites (RE)CoO3 (RE = La-Dy) was prepared by microwave-assisted synthesis. The crystal structure undergoes a change of symmetry depending on the size of the RE cation. LaCoO3 is rhombohedral, S.G. R3̅c (No. 167), while, for the rest of the RE series (Pr-Dy), the symmetry is orthorhombic, S.G. Pnma (No. 62). The crystal structure obtained by X-ray diffraction was confirmed by high-resolution transmission electron microscopy, which yielded a good match between experimental and simulated images. It is further shown that the well-known magnetism in LaCoO3, which involves a thermally induced Co3+ (d6) low spin to intermediate or high spin state transition, is strongly modified by the RE cation, and a rich variety of magnetic order has been detected across the series.

Magnetic field dependence of the dielectric constant in Cd1-x,MnxTe:In on approaching the insulator-to-metal transition

Abstract This study presents the frequency, temperature and magnetic field dependence of the capacitance in the dilute magnetic semiconductor Cd0.9Mn0.1Te: In near the insulator-to-metal transition. By exploiting the persistent photoconductivity effect or the large magnetic coupling between carriers and the localized (Mn2+) spins, it is possible to tune the transition in one sample either by optically changing the carrier concentration n or by applying a magnetic field H. The results indicate at least two contributions to the capacitance: a hopping term which is temperature and frequency dependent, and a term which varies only with n and H and is related to the polarizability of the bound charge carriers. This latter term is most evident at high frequencies and low temperatures and diverges as the transition is approached from the insulating side. The application of a magnetic field shifts the onset of the divergence to lower carrier concentrations, lending credence to earlier transport results which impl...

Controlling nickel nanoparticle size in an organic/metal-organic matrix through the use of different solvents.

Nickel nanoparticles have been created in an organic-based matrix by the reaction of Ni(COD)2 (COD = 1,5-bis-cyclooctadiene) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4). The size of the nickel nanoparticles can be controlled by the use of different solvents and inclusion of tetrahydrofuran (THF) within the reaction to stabilise the Ni(0) atoms from the Ni(COD)2. Materials are characterised with a combination of X-ray diffraction, electron microscopy and magnetometry and it is found that samples made using a halocarbon solvent resulted in clustered bulk Ni particles (size ≤ 10 nm) with anomalously high superparamagnetic blocking temperatures. Using an isocyanide solvent produces smaller (size ∼ 1 nm), well dispersed particles that show little evidence of superparamagnetic blocking in the range of temperatures investigated (>2 K). In all samples there is another component which dominates the magnetic response at low temperatures and shows an interesting temperature dependent scaling behaviour when plotted as M vs. B/T which we believe is related to the organo-metallic matrix that the particles are trapped within. We propose that the enhanced blocking temperature of particles synthesised using halocarbon solvents can be attributed to inter-particle dipolar interactions and nanoparticle-matrix exchange interactions.

Low temperature magnetic susceptometer based upon a dc superconducting quantum interference device

A low temperature magnetic susceptometer was constructed by combining an Oxford Instruments Heliox probe sorption pumped He3 cryostat with a Quantum Design model 5000 dc superconducting quantum interference device sensor, giving an effective temperature range from 300mKto4.2K. The smallest resolvable change in magnetic moment detected by this system was found to be Δ∼1×10−12JT−1 (1×10−9emu).

Magnetic ordering of defects in a molecular spin-Peierls system

With interest in charge transfer compounds growing steadily, it is important to understand all aspects of the underlying physics of these systems, including the properties of the defects and interfaces that are universally present in actual experimental systems. For the study of these defects and their interactions a spin-Peierls (SP) system provides a useful testing ground. This work presents an investigation within the SP phase of potassium TCNQF4 where anomalous features are observed in both the magnetic susceptibility and ESR spectra for temperatures between 60 K and 100 K. Muon spin spectroscopy measurements confirm the presence of these anomalous magnetic features, with low temperature zero-field data exhibiting the damped oscillatory form that is a characteristic signature of static magnetic order. This ordering is most likely due to the interaction between structurally correlated magnetic defects in the system. The critical behaviour of the temperature dependent muon spin rotation frequency indicates that a 2D Ising model is applicable to the magnetic ordering of these defects. We show that these observations can be explained by a simple model in which the magnetic defects are located at stacking faults, which provide them with a 2D structural framework to constrain their interactions.

Dipolar glass and magneto-electric coupling within a π-stacked organic system

There is much interest in the search for novel materials that show ferroelectric as well as magneto-electric coupling, such as that observed in multiferroics. Within organic based materials the electronic polarisation can originate from a charge distribution across a molecule or molecules, and so one must search for systems that have an electronic (and magnetic) dipole that is intrinsic. One such material is tetraethylammonium bis-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)2) which is a charge transfer system with a single electron that is delocalised across a TCNQ dimer. We show that dielectric measurements yield anomalies at the cation freezing structural distortion and when singlet–triplet excitations freeze. In both cases the electric response is glassy and at low temperature the corresponding magnetic measurements evidence the strong magneto-electric coupling within the material showing scaling behaviour similar to spin glass systems.