Claire V. Colin

The Role of Order-Disorder Transitions in the Quest for Molecular Multiferroics: Structural and Magnetic Neutron Studies of a Mixed Valence Iron (II)-Iron (III) Formate Framework

Neutron diffraction studies have been carried out to shed light on the unprecedented order-disorder phase transition (ca. 155 K) observed in the mixed-valence iron(II)-iron(III) formate framework compound [NH(2)(CH(3))(2)](n)[Fe(III)Fe(II)(HCOO)(6)](n). The crystal structure at 220 K was first determined from Laue diffraction data, then a second refinement at 175 K and the crystal structure determination in the low temperature phase at 45 K were done with data from the monochromatic high resolution single crystal diffractometer D19. The 45 K nuclear structure reveals that the phase transition is associated with the order-disorder of the dimethylammonium counterion that is weakly anchored in the cavities of the [Fe(III)Fe(II)(HCOO)(6)](n) framework. In the low-temperature phase, a change in space group from P31c to R3c occurs, involving a tripling of the c-axis due to the ordering of the dimethylammonium counterion. The occurrence of this nuclear phase transition is associated with an electric transition, from paraelectric to antiferroelectric. A combination of powder and single crystal neutron diffraction measurements below the magnetic order transition (ca. 37 K) has been used to determine unequivocally the magnetic structure of this Néel N-Type ferrimagnet, proving that the ferrimagnetic behavior is due to a noncompensation of the different Fe(II) and Fe(III) magnetic moments.

Revealing the properties of Mn2Au for antiferromagnetic spintronics

The continuous reduction in size of spintronic devices requires the development of structures, which are insensitive to parasitic external magnetic fields, while preserving the magnetoresistive signals of existing systems based on giant or tunnel magnetoresistance. This could be obtained in tunnel anisotropic magnetoresistance structures incorporating an antiferromagnetic, instead of a ferromagnetic, material. To turn this promising concept into real devices, new magnetic materials with large spin-orbit effects must be identified. Here we demonstrate that Mn2Au is not a Pauli paramagnet as hitherto believed but an antiferromagnet with Mn moments of ~4 μB. The particularly large strength of the exchange interactions leads to an extrapolated Néel temperature well above 1,000 K, so that ground-state magnetic properties are essentially preserved up to room temperature and above. Combined with the existence of a significant in-plane anisotropy, this makes Mn2Au the most promising material for antiferromagnetic spintronics identified so far.

Magnetoelectric coupling in the cubic ferrimagnet Cu(2)OSeO(3)

We have investigated the magnetoelectric coupling in the lone pair containing piezoelectric ferrimagnet Cu(2)OSeO(3). Significant magnetocapacitance develops in the magnetically ordered state (T(c) = 60 K). We find critical behavior near T(c) and a divergence near the metamagnetic transition at 500 Oe. High-resolution x-ray and neutron powder diffraction measurements show that Cu(2)OSeO(3) is metrically cubic down to 10 K but that the ferrimagnetic ordering reduces the symmetry to rhombohedral R3. The metric cubic lattice dimensions exclude a magnetoelectric coupling mechanism involving spontaneous lattice strain, and this is unique among magnetoelectric and multiferroic materials.

Phase transitions and interface phenomena in the cryogenic temperature domain of a niobate nanostructured ceramic

Powder neutron diffraction and dielectric spectroscopy were used to investigate both crystallographic and dielectric permittivity properties of a Sr2KNb5O15 single phase ferroelectric oxide with nanosized grains ranging from 35 nm to 90 nm. Measurements were carried out in the temperature range from 10 K (cryogenic) to 550 K. All neutron diffraction data were indexed on the basis of a tetragonal double unit cell. From 10 K to room temperature the space group of the Sr2KNb5O15 ferroelectric phase was considered to be P4bm. The refinement of the paraelectric phase (at 550 K) was determined in the centrosymmetric space group P4/mbm. Dielectric spectroscopy measurements were performed in a thermal cycle. A set of four phase transitions non-related to symmetry changing was detected from Rietveld analysis of neutron powder diffraction data. During a thermal cycle, in the cryogenic temperature domain, strong thermal hysteresis is developed. Both phase transition and thermal hysteresis were correlated. These phenomena are associated with Nb-cation atomic displacements in the NbO6 octahedra along the c-axis direction and of the domain with different frequencies involving grains as well as an excess of interfaces ascribed to the grain boundary. The bulk/grain boundary interfaces in nanostructured ceramics are correlated with the thermal stability phenomenon.

New investigation of the magnetic structure of CoNb2O6 columbite

The structural and magnetic properties of the CoNb2O6 compound have been investigated with a particular interest in their low-dimensional magnetic behavior which is characterized by the presence of weakly interacting ferromagnetic chains. We investigate this cobalt niobate by combining magnetic measurements; x-ray and neutron diffraction (ND). The ND was carried out on powder samples at different temperatures above (20 K) and below the ordering temperature (2.4, 1.8, and 1.4 K). The compound exhibits an orthorhombic crystal structure of Pbcn symmetry, typical of a columbite structure. Magnetic ordering at 2.5 K is found with the propagation vector (0, 0.4, 0), in agreement with earlier studies. However, at lower temperatures the present investigation shows the coexistence of two different magnetic phases: the propagation vector (0.5, 0.5, 0) is found to be necessary to refine the ND measurements at both 1.8 and 1.4 K, in addition to (0, 0.5, 0) which was the only one reported in earlier works.

Thermistor behaviour and electric conduction analysis of Ni-doped niobate ferroelectric: the role of multiple β parameters

A new engineered non-stoichiometric niobate ceramic with a tetragonal tungsten bronze (TTB)-type structure is prepared by non-isovalent substitution at the niobium backbone. The compounds formula is KSr2(Ni0.75Nb4.25)O15−δ, with space group equal to P4bm (#100), a polar group. Electrical properties of this new ferroelectric-semiconductor ceramic are investigated by impedance spectroscopy in the temperature range from 453 up to 953 K. The curve of the resistance as a function of temperature presents further characteristics of a temperature sensor with a negative temperature coefficient (NTC). At high temperatures, the analysis of electrical parameters shows an NTC behaviour ceramic with three distinct thermistor characteristic parameters, β. The values of the β parameter change from 9.82 × 103 to 1.29 × 104 K, while the temperature coefficient of resistance α changes from −0.047 to −0.023 K−1. The NTC behaviour of KSr2(Ni0.75Nb4.25)O15−δ is compared with the behaviour of some dielectric and other ferroelectric semiconductors. β dependence with dc or the ac conductivity mechanism and its correlation with β quality are discussed.

Magnetic properties of the honeycomb oxide Na2Co2TeO6

We have studied the magnetic properties of

Spin-glass-like behavior and negative thermal expansion in antiperovskite Mn3Ni1−xCuxN compounds

{\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}

Effect of the M/Co substitution on magnetocrystalline anisotropy and magnetization in SmCo5−xMx compounds (M=Ga;Al)

, which features a honeycomb lattice of magnetic

Structural Study of a Doubly Ordered Perovskite Family NaLnCoWO6 (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb): Hybrid Improper Ferroelectricity in Nine New Members

{\mathrm{Co}}^{2+}