Publications by Amalia Coldea


Synthesis and characterisation of Sr<inf>1.2</inf>Nd<inf>0.8</inf>Mn<inf>0.6</inf>Rh<inf>0.4</inf>O<inf>4</inf>

Journal of Materials Chemistry 13 (2003) 1166-1172

DJ Gallon, PD Battle, SJ Blundell, AI Coldea, MJ Rosseinsky, C Steer

A polycrystalline sample of Sr1.2Nd0.8Mn0.6Rh0.4O4 has been synthesised and characterised by neutron diffraction, dc magnetometry, muon spectroscopy and magnetotransport measurements. Ir is an n = 1 member of the Ruddlesden-Popper (RP) An + 1BnO3n + 1 family, having tetragonal (I4/mmm) symmetry [a = 3.86418(6), c = 12.4414(2) Å]. Rh and Mn cations are disordered over the six-coordinate sites within the perovskite layers of the RP structure and the A sites are occupied by a disordered distribution of Nd and Sr. In contrast to the analogous compound Sr1.2La 0.8Mn0.6Rh0.4O4, which showed long range ferromagnetic ordering in an applied field ar 2 K, a spin-glass transition was observed ar 16 K for the Nd-containing compound. No long range magnetic order was observed ar 2 K. A maximum magnetoresistance of ∼ 30% in a 140 kOe field was found at 125 K.


Millimetre-wave response of the novel organic superconductors beta ''-ET4[(H3O)M(C2O4)(3)]center dot S (M=Fe3+, S=C6H5CN and C6H5NO2)

SYNTHETIC MET 137 (2003) 1225-1226

A Narduzzo, A Ardavan, AI Coldea, J Singleton, A Akutsu-Sato, H Akutsu, SS Turner, R Day

The high frequency (f=70 GHz) magneto-optical response of beta"-ET4[(H3O)M(C2O4)(3)].S, where M=Fe3+, S=C6H5CN and C6H5NO2, has been investigated in the temperature range 1.4 K < T < 120 K. The angle dependence of the EPR spectra and the anisotropy of the g-factors for the two materials are presented.


Magnetotransport measurements on beta ''-(BEDT-TTF)(4)[(H3O)Ga3+(C2O4)(3)](center dot) C6N5NO2

SYNTHETIC MET 137 (2003) 1313-1314

AF Bangura, AI Coldea, J Singleton, A Ardavan, AK Klehe, A Akutsu-Sato, H Akutsu, SS Turner, P Day

Transport measurements have been carried out on single crystals of the charge transfer salt beta"-(BEDT-TTF)(4)[(H3O) Ga3+ (C2O4)(3)]. C6H5NO2, using magnetic fields of up to 33 T and temperatures down to 0.55 K. The material is found to be superconducting with T. 7.5 K and an in-plane critical field B-c2 greater than or equal to 33 T, and to exhibit Shubnikov-de Haas (SdH) oscillations. The temperature and angle dependence of the Shubnikov-de Haas oscillations and the upper critical field are reported, allowing information about the anisotropy of the upper critical field, effective mass m*, and the Fermi-surface (FS) of the material to be deduced.


Changes of the spin dynamics in perovskite and bilayer manganite

PHYSICA B 326 (2003) 500-504

AI Coldea, SJ Blundell, CA Steer, FL Pratt, D Prabhakaran, JF Mitchell

We report a series of muSR measurements performed on single crystals of La1-x,SrxMnO3, (0 less than or equal to x less than or equal to 0.125) and on a polycrystalline bilayer manganite La2-2x Sr1+2xMn2O7 (v = 0.52). We find that the temperature dependence of the spinlattice relaxation rate is strongl, dependent on the hole doping and the associated changes in the magnetic structure. Y The systems have competing, anisotropic interactions, leading to a complex interplay of charge, spin and orbital order. This has a significant influence on the spin dynamics, both for perovskite and layered manganites. (C) 2002 Elsevier Science B.V. All rights reserved.


Magnetotransport measurements on β"-(BEDT-TTF)<inf>4</inf>[(H<inf>3</inf>O)Ga<sup>3+</sup>(C<inf>2 </inf>O<inf>4</inf>)<inf>3</inf>]. C<inf>6</inf>H<inf>5</inf>NO<inf>2</inf>

Synthetic Metals 137 (2003) 1313-1314

AF Bangura, AI Coldea, J Singleton, A Ardavan, AK Klehe, A Akutsu-Sato, H Akutsu, SS Turner, P Day

Transport measurements have been carried out on single crystals of the charge transfer salt β″-(BEDT-TTF)4[(H3O)Ga3+(C2 O4)3]. C6H5NO2, using magnetic fields of up to 33 T and temperatures down to 0.55 K. The material is found to be superconducting with Tc ≈ 7.5 K and an in-plane critical field Bc2 ≥ 33 T, and to exhibit Shubnikov-de Haas (SdH) oscillations. The temperature and angle dependence of the Shubnikov-de Haas oscillations and the upper critical field are reported, allowing information about the anisotropy of the upper critical field, effective mass m*, and the Fermi-surface (FS) of the material to be deduced.


Millimetre-wave response of the novel organic superconductors β″-ET<inf>4</inf>[(H<inf>3</inf>O)M(C<inf>2</inf>O<inf>4</inf>)<inf>3 </inf>].S (M=Fe<sup>3+</sup>, S= C<inf>6</inf>H<inf>5</inf>CN and C<inf>6</inf>H<inf>5</inf>NO<inf>2</inf>)

Synthetic Metals 137 (2003) 1225-1226

A Narduzzo, A Ardavan, AI Coldea, J Singleton, A Akutsu-Sato, H Akutsu, SS Turner, P Day

The millimeter-wave response of the novel organic superconductors was studied. The magneto-optical response of the two samples containing Fe3+ ions and the host solvents benzonitrile and nitrobenzene. The angle dependence of the electron paramagnetic resonance (EPR) spectra and the anisotropy of the g-factors for the two materials were presented. The angle of dependence of the absorption spectra at 1.4 K for the two samples were compared.


A mu SR study of the spin dynamics in Ir-diluted layered manganites

PHYSICA B 326 (2003) 513-517

CA Steer, SJ Blundell, AI Coldea, IM Marshall, T Lancaster, PD Battle, D Gallon, AJ Fargus, MJ Rosseinsky

Dilution on the B-site with Ir or Rh can lead to the enhancement of ferromagnetic couplings in layered manganites. We report a muSR study of the spin dynamics of the layered manganites La0.25Sr1.75MnpIr1-pO4 (p = 0.5, 0.75) as a function of the dilution. We have characterized the static magnetic behavior using SQUID magnetic measurements and used muon spin relaxation spectroscopy to provide evidence for short-range ferromagnetic correlations. Furthermore, the effect of dilution on the critical dynamics in this two-dimensional spin system is reported and related to the dynamics above and below the critical percolation threshold. (C) 2002 Elsevier Science B.V. All rights reserved.


Non-adiabatic small polaron hopping in the n = 3 ruddlesden-popper compound Ca<inf>4</inf>Mn<inf>3</inf>O<inf>10</inf>

Journal of Physics Condensed Matter 15 (2003) 6817-6833

J Lago, PD Battle, MJ Rosseinsky, AI Coldea, J Singleton

Magnetotransport properties of the compound Ca4Mn3 O10 are interpreted in terms of activated hopping of small magnetic polarons in the non-adiabatic regime. Polarons are most probably formed around Mn3+ sites created by oxygen sub-stoichiometry. The application of an external field reduces the size of the magnetic contribution to the hopping barrier and thus produces an increase in the conductivity. We argue that the change in the effective activation energy around TN is due to the crossover to variable range hopping conduction as antiferromagnetic order sets in.


A muon-spin relaxation (μSR) study of the geometrically frustrated magnets Gd<inf>3</inf>Ga<inf>5</inf>O<inf>12</inf> and ZnCr<inf>2</inf>O<inf>4</inf>

Journal of Physics Condensed Matter 14 (2002)

IM Marshall, SJ Blundell, FL Pratt, A Husmann, CA Steer, AI Coldea, W Hayes, RCC Ward

We present the results of muon-spin-relaxation experiments for two materials which show geometric frustration. ZnCr22O4 has a spinel structure with S = 3/2 spins on a lattice of corner-sharing tetrahedra. Our experiments show that a local magnetic field which is quasi-static on the muon timescale develops below Tc = 12.5 K, a transition which has been associated with a three-dimensional analogue of the spin-Peierls transition. In contrast, Gd3Ga5O12 has a garnet structure with S = 7/2 spins arranged on interpenetrating triangular sublattices. In this material the muon data exhibit a temperature-dependent spin-relaxation rate indicative of slow spin fluctuations. We discuss these differing behaviours and relate them to the underlying physics in the two materials.


Spin freezing and magnetic inhomogeneities in bilayer manganites

Physical Review Letters 89 (2001) 277601 4pp-

SJ Blundell, A I Coldea, C A Steer, J F Mitchell


Growth of large La1-xSrxMnO3 single crystals under argon pressure by the floating-zone technique

J CRYST GROWTH 237 (2002) 806-809

D Prabhakaran, AI Coldea, AT Boothroyd, S Blundell

We report the growth of single crystals of La1-xSrxMnO3 (x = 0.0-0.3) by the floating-zone technique in an image furnace under high argon pressure (6-8 atm). Ail excess of Mn in the polycrystalline starting material was used to compensate for the evaporation during growth. single crystals of diameter 8 mm and length 70 mm have been obtained. The phase purity, composition and quality of the crystals were analysed by X-ray diffraction and electron probe microanalysis. We discuss the effect on the crystal growth of varying the amount of Mn excess and of the composition of the growth atmosphere. We also present magnetization and resistivity data for a few of the samples. (C) 2002 Elsevier Science B.V. All rights reserved.


Crystal structure and magnetic properties of SrCaMnGaO<inf>5+δ</inf>

Journal of Solid State Chemistry 167 (2002) 188-195

PD Battle, AMT Bell, SJ Blundell, AI Coldea, DJ Gallon, FL Pratt, MJ Rosseinsky, CA Steer

The room-temperature crystal structure of the brownmillerite SrCaMnGaO5+δ (δ = 0.035) has been refined from neutron powder diffraction data; space group Ima2, a = 15.7817(6), b = 5.4925(2), c = 5.3196(2) Å. Mn and Ga occupy 99.0(2)% of the 6- and 4-coordinate sites, respectively. A combination of magnetometry, neutron diffraction and μSR spectroscopy has shown that the compound orders magnetically at 180 K, and that the low-temperature phase has a G-type antiferromagnetic structure, with an ordered magnetic moment of 3.30(2) μB per Mn at 2 K. Displaced hysteresis loops provide evidence that the atomic moment has an additional, glassy component. Magnetometry shows that significant short-range magnetic interactions persist above 180 K, and μSR that the spin fluctuations are thermally activated in this temperature region. The compound is an electrical insulator which at 159 K shows an unusually large magnetoresistance of 85% in 6T, increasing to 90% in 13 T. © 2002 Elsevier Science (USA).


Spin, charge, and orbital ordering in the B-site diluted manganates La<inf>2</inf>-<inf>x</inf>Sr<inf>x</inf>GaMnO<inf>6</inf>

Chemistry of Materials 14 (2002) 425-434

PD Battle, SJ Blundell, JB Claridge, AI Coldea, EJ Cussen, LD Noailles, MJ Rosseinsky, J Singleton, J Sloan

Oxidation of the ferromagnetic B-site diluted manganese(III) perovskite La2GaMnO6 is investigated by synthesis of the La2-xSrxGaMnO6 series. At the x = 0.3 composition, which corresponds to diamagnetic element substitution at the B-site of metallic LaO.7SrO.3MnO3, the MnIII and MnIV valences order in real space at low temperature with an unusual lamellar motif. Orbital ordering at the MnIII centers in this array maintains ferromagnetism and enhances the static coherent Jahn-Teller distortion over that found for the pure MnIII end-member, facilitating eg electron hopping in the insulating state. Further oxidation to x = 0.5 completely suppresses the charge and orbital ordering, leading to glassy rather than long-range ordered magnetism.


Evolution of the magnetic and magnetotransport properties of the Ga-substituted manganite compounds La<inf>2-x</inf>Sr<inf>x</inf>MnGaO<inf>6</inf>

Physical Review B - Condensed Matter and Materials Physics 65 (2002) 544021-5440210

AI Coldea, SJ Blundell, IM Marshall, CA Steer, J Singleton

We report a series of magnetic and magnetotransport studies performed on Ga-substituted manganite compounds La2-xSrxMnGaO6 (x≤0.7). We replace half of the Mn ions in LaMnO3 with diamagnetic ions Ga3+ and find that the system transforms into a ferromagnet as the result of local coherent and incoherent distortions. Following additional doping with Sr ions we find that the La2-xSrxMnGaO6 compounds remain ferromagnetic up to x∼0.3 but for higher concentrations the competition between the ferromagnetic and antiferromagnetic superexchange interactions, together with the randomness in distribution of the magnetic ions, lead to magnetic frustration, cluster-glass and spin-glass behavior. All compounds are insulating and the activation energy decreases almost linearly with x. μSR measurements suggest the existence of short-range magnetic correlations in the high-temperature region. We measure a significant increase in the muon-spin relaxation rate at low temperatures due to fluctuations of random internal magnetic fields. The observed magnetoresistance in the high-temperature region is mainly a consequence of the reduction of the spin disorder on a local scale, and is as high as 50% in 14 T and T>100 K for samples with x=0.3 and x=0.5.


Synthesis and characterization of Ru-doped n = 1 and n = 2 Ruddlesden-Popper manganates

Chemistry of Materials 14 (2002) 3976-3983

DJ Gallon, PD Battle, SJ Blundell, JC Burley, AI Coldea, EJ Cussen, MJ Rosseinsky, C Steer

Polycrystalline samples of Sr3MnRuO7 and Sr2Mn0.5Ru0.5O4 have been synthesized and characterized by neutron diffraction, dc magnetometry, and magnetotransport measurements. They are, respectively, n = 2 and n = 1 members of the Ruddlesden-Popper (RP) An+1BnO3n+1 family; both have tetragonal (I4/mmm) symmetry and a disordered distribution of Ru and Mn over the six-coordinate sites within the perovskite layers of the RP structure. Neither compound shows long-range magnetic order at 2 K, but a spin-glass transition is observed at 16 K (n = 1) or 25 K (n = 2). In the case of the n = 2 compound only, the magnetic transition is accompanied by a reduction in the zero-field electrical conductivity. A maximum magnetoresistance of ∼8% in 14 T is found in both compounds.


Colossal magnetoresistance in the layered chromium sulfide Cr<inf>2</inf>S<inf>3-x</inf> (x=0.08)

Physical Review B - Condensed Matter and Materials Physics 64 (2001) 1324021-1324024

P Vaqueiro, AV Powell, AI Coldea, CA Steer, IM Marshall, SJ Blundell, J Singleton, T Ohtani

We report structural, magnetic, and magnetotransport properties of the two-dimensional chromium sulfide Cr2S2.92, which contains both Cr3+ and Cr2+ ions. Below its magnetic-ordering temperature (TN=118 K), Cr2S2.92 exhibits a weak spontaneous magnetization with a maximum around 90 K and a value of 0.013(4) μB per Cr ion at 5 K. The resistivity and magnetoresistance exhibit pronounced local maxima around TN (MR=48% in a field of 14 T). The magnetotransport properties are discussed in terms of magnetic-polaron formation.


Crystal structure and electronic properties of Ca4Mn2TiO9.93, an n=3 Ruddlesden-Popper compound

J MATER CHEM 11 (2001) 160-167

PD Battle, SJ Blundell, AI Coldea, EJ Cussen, MJ Rosseinsky, J Singleton, LE Spring, JF Vente

Traditional solid state synthetic methods were used to prepare a polycrystalline sample of the n = 3 Ruddlesden-Popper phase Ca4Mn2TiO9.93. The crystal structure (space group Pbca, a = 5.31411(5), b = 5.31148(5), c = 26.9138(2)Angstrom) was determined by the simultaneous analysis of neutron and X-ray diffraction data, with near-edge anomalous scattering being used to provide contrast between Mn and Ti cations. The latter show a small preference for the octahedral sites at the centre of the three-layer perovskite blocks within the structure. Neutron diffraction data collected at 5 K show no evidence for long-range magnetic ordering, although an enhanced magnetisation with a weak remanence is observed at low temperature; this is ascribed to the presence of antisymmetric exchange interactions. Ca4Mn2TiO9.93 is a semiconductor with a temperature-dependent activation energy of similar to 100 meV. Only weak (rho (B)/rho (0) > 0.9 in 14 T at 75 K) magnetoresistance was observed.


Control of magnetic ordering by Jahn--Teller distortions in Nd(2)GaMnO(6) and La(2)GaMnO(6).

J Am Chem Soc 123 (2001) 1111-1122

EJ Cussen, MJ Rosseinsky, PD Battle, JC Burley, LE Spring, JF Vente, SJ Blundell, AI Coldea, J Singleton

The substitution of Ga(3+) into the Jahn--Teller distorted, antiferromagnetic perovskites LaMnO(3) and NdMnO(3) strongly affects both the crystal structures and resulting magnetic ordering. In both compounds the Ga(3+) and Mn(3+) cations are disordered over the six coordinate sites. La(2)GaMnO(6) is a ferromagnetic insulator (T(c) = 70 K); a moment per Mn cation of 2.08(5) mu(B) has been determined by neutron powder diffraction at 5 K. Bond length and displacement parameter data suggest Jahn--Teller distortions which are both coherent and incoherent with the Pnma space group symmetry of the perovskite structure (a = 5.51122(4) A, b = 7.80515(6) A, c = 5.52947(4) A) at room temperature. The coherent distortion is strongly suppressed in comparison with the parent LaMnO(3) phase, but the displacement ellipsoids suggest that incoherent distortions are significant and arise from local Jahn--Teller distortions. The preparation of the new phase Nd(2)GaMnO(6) has been found to depend on sample cooling rates, with detailed characterization necessary to ensure phase separation has been avoided. This compound also adopts the GdFeO(3)-type orthorhombically distorted perovskite structure (space group Pnma, a = 5.64876(1) A, b = 7.65212(2) A, c = 5.41943(1) A at room temperature). However, the B site substitution has a totally different effect on the Jahn--Teller distortion at the Mn(3+) centers. This phase exhibits a Q(2) mode Jahn--Teller distortion similar to that observed in LaMnO(3), although reduced in magnitude as a result of the introduction of Ga(3+) onto the B site. There is no evidence of a dynamic Jahn-Teller distortion. At 5 K a ferromagnetically ordered Nd(3+) moment of 1.06(6) mu(B) is aligned along the y-axis and a moment of 2.8(1) mu(B) per Mn(3+) is ordered in the xy plane making an angle of 29(2) degrees with the y-axis. The Mn(3+) moments couple ferromagnetically in the xz plane. However, along the y-axis the moments couple ferromagnetically while the x components are coupled antiferromagnetically. This results in a canted antiferromagnetic arrangement in which the dominant exchange is ferromagnetic. Nd(2)GaMnO(6) is paramagnetic above 40(5) K, with a paramagnetic moment and Weiss constant of 6.70(2) mu(B) and 45.9(4) K, respectively. An ordered moment of 6.08(3) mu(B) per Nd(2)GaMnO(6) formula unit was measured by magnetometry at 5 K in an applied magnetic field of 5 T.


Synthesis, structural chemistry and magnetic properties of La<inf>1+x</inf>A<inf>1-x</inf>InMnO<inf>6-δ</inf>: A = Ba, Sr; x = 0, 0.2

Journal of Materials Chemistry 11 (2001) 1656-1661

PD Battle, AI Coldea, NA Jordan, MJ Rosseinsky, J Singleton

A series of novel double perovskites La1+xA1-xInMnO6-δ (A = Sr, Ba) has been prepared and the room temperature crystal structures have been refined using X-ray and neutron powder diffraction. In all cases the B-site is occupied by a disordered arrangement of In3+ and Mn3+/4+ cations. However, whereas La1+xBa1-xInMnO6-δ exhibits two octahedral tilts and exists in a body-centred cell, space group Imma, La1+xSr1-xInMnO6-δ adopts the three-tilt GdFeO3 structure and has a primitive orthorhombic structure, space group Pnma. Magnetic susceptibility data show that all the compositions synthesised exhibit a spin glass freezing transition at ∼8 K. A negative magnetoresistance Δρ/ρ0 = 10.5% was measured at 164 K in a field of 14 T.


Chemically induced magnetism and magnetoresistance in La(0.8)Sr(1.2)Mn(0.6)Rh(0.4)O(4).

J Am Chem Soc 123 (2001) 7610-7615

PD Battle, AM Bell, SJ Blundell, AI Coldea, EJ Cussen, GC Hardy, IM Marshall, MJ Rosseinsky, CA Steer

It is shown by magnetometry and microSR spectroscopy that short-range magnetic interactions between the Mn cations in the nonmetallic K(2)NiF(4)-like phase La(0.8)Sr(1.2)Mn(0.6)Rh(0.4)O(4) become significant below approximately 200 K. Negative magnetoresistance (rho/rho(0) approximately 0.5 in 14 T at 108 K) is apparent below this temperature. Neutron diffraction has shown that an applied magnetic field of 5 T is sufficient to induce saturated (3.38(7)mu(B) per Mn) long-range ferromagnetic ordering of the atomic moments at 2 K, and that the induced ordering persists up to a temperature of 50 K in 5 T. Spin glass behavior is observed below 20 K in the absence of an applied field. The induced magnetic ordering is attributed to the subtle changes in band structure brought about by the external field, and to the controlling influence of Rh(3+) over the relative strength of competing magnetic exchange interactions.

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