arXiv subjects

V. Ovidiu Garlea

Publications and source records attributed to V. Ovidiu Garlea.

42 records · Page 3Linked to original sources

arXiv2013

Morphotropic Phase Boundaries in Ferromagnets: Tb_{1-x}Dy_xFe_2 Alloys

The structure and properties of the ferromagnet Tb_{1-x}Dy_xFe_2 (Terfenol-D) are explored through the morphotropic phase boundary (MPB) separating ferroic phases of differing symmetry. Our synchrotron data support a first order structural transition, with a broadening MPB width at higher temperatures. The optimal point for magnetomechanical applications is not centered on the MPB but lies on the rhombohedral side, where the high striction of the rhombohedral majority phase combines with the softened anisotropy of the MPB. We compare our findings with single ion crystal field theory and with ferroelectric MPBs, where the controlling energies are different.

cond-mat.mtrl-sci↗

arXiv2013

Crystal, magnetic, and electronic structures, and properties of new BaMnPnF (Pn = As, Sb, Bi)

New fluoropnictides BaMnPnF with Pn = As, Sb, Bi, are synthesized by stoichiometric reaction of elements with BaF\_2. The compounds crystallize in the tetragonal P4/nmm (No. 129, Z = 2) space group, with the ZrCuSiAs-type structure, as indicated by single crystal and powder X-ray diffraction results. Electrical resistivity results indicate that Pn = As, Sb, and Bi are semiconductors with band gaps of E\_g = 0.73 eV, E\_g = 0.48 eV and E\_g = 0.003 eV, respectively. Powder neutron diffraction reveals a G-type antiferromagnetic order below T\_N = 338(1) K for Pn = As, and below T\_N = 272(1) K for Pn = Sb. Magnetic susceptibility increases with temperature above 100 K for all the materials. Density functional calculations also find semiconducting antiferromagnetic compounds with strong in-plane and weaker out-of-plane exchange coupling that may result in non-Curie Weiss behavior above T\_N. There is strong covalency between Mn and pnictogen elements. The ordered magnetic moments are 3.65(5) μB/Mn for Pn = As, and 3.66(3) μB/Mn for Pn = Sb at 4 K, as refined from neutron diffraction experiments.

cond-mat.str-el↗

arXiv2012

Neutron Diffraction Study of anomalous negative thermal expansion in non-superconducting PrFe1-xRuxAsO

Neutron powder diffraction has been used to investigate the structural and magnetic behavior of the isoelectronically doped Fe pnictide material PrFe1-xRuxAsO. Substitution of Ru for Fe suppresses the structural and magnetic phase transitions that occur in the undoped compound PrFeAsO. Contrary to the behavior usually observed in 1111 pnictide materials, the suppression of both the structural and magnetic transitions does not result in the emergence of superconductivity or any other new ground state. Interestingly, PrFeAsO itself shows an unusual negative thermal expansion (NTE) along the c-axis, from 60K down to at least 4K; this does not occur in superconducting samples such as those formed by doping with fluorine on the oxygen site. We find that NTE is present for all concentrations of PrFe1-xRuxAsO with x ranging from 0.05 to 0.75. These results suggest that the absence of superconductivity in these materials could be related to the presence of NTE.

cond-mat.supr-con↗

arXiv2011

Tuning the magnetic ground state of a triangular lattice system

The anisotropic triangular lattice of the crednerite system Cu(Mn1-xCux)O2 is used as a basic model for studying the influence of spin disorder on the ground state properties of a two-dimensional frustrated antiferromagnet. Neutron diffraction measurements show that the undoped phase (x=0) undergoes a transition to antiferromagnetic long-range order that is stabilized by a frustration-relieving structural distortion. Small deviation from the stoichiometric composition alters the magnetoelastic characteristics and reduces the effective dimensionality of the magnetic lattice. Upon increasing the doping level, the interlayer coupling changes from antiferromagnetic to ferromagnetic. As the structural distortion is suppressed, the long-range magnetic order is gradually transformed into a two-dimensional order.

cond-mat.str-el↗

arXiv2010

Control of Correlations in Sr4V2O6Fe2As2 by Chemical Stoichiometry

We show using a combination of powder X-ray and neutron diffraction, first principles calculations, temperature- and field-dependent magnetization, heat capacity and resistivity data that the superconducting behavior of `Sr$_4$V$_2$O$_6$Fe$_2$As$_2$' is dependent on synthesis conditions, particularly, heating profiles result in unintentional chemical doping. This compound can be tuned from a state in which the vanadium electrons are itinerant with a high electronic density of states, to a state where the vanadium-oxide layers are insulating and presumably antiferromagnetic.

cond-mat.supr-con↗

arXiv2010

Magnetic Phase Transitions in NdCoAsO

Magnetization measurements reveal that NdCoAsO undergoes three magnetic phase transitions below room temperature. The crystal and magnetic structures of NdCoAsO have been determined by powder neutron diffraction, and the effects of the phase transitions on physical properties are reported. Near 69 K a ferromagnetic state emerges with a small saturation moment of about 0.2 Bohr magnetons, likely on Co atoms. At 14 K the material enters an antiferromagnetic state with propagation vector (0 0 1/2) and small ordered moments (~0.4 Bohr magnetons) on Co and Nd. Near 3.5 K a third transition is observed, and corresponds to the antiferromagnetic ordering, with the same propagation vector, of larger moments on Nd reaching 1.30(2) Bohr magnetons at 1.4 K. In addition, transport properties and heat capacity results are presented, and show anomalies at all three phase transitions.

cond-mat.supr-con↗