Zdenek Jirak

Neutron diffraction study of Pr1 − xCaxMnO3 perovskites

Abstract A thorough powder neutron diffraction study of Pr 1 − x Ca x - Mn 3+ 1 − x Mn 4+ x O 3 (0 x 3+ ions and their orbitals (0 ⩽ x ⪅ 0.3, 0.3 ⪅ x x ⪅ 0.9 and 0.9 ⪅ x ⩽ 1) was revealed. The corresponding prototypes are x = 0, 0.5, 0.75 and 1. The departures from these ideal compositions introduce substancial changes in magnetic interactions and, as a consequence, a variety of magnetic structures (including different noncollinear ones) are observed at low temperatures. The magnetic structures are interpreted by considering the role of superexchange and double exchange.

Structural anomalies associated with the electronic and spin transitions in LnCoO 3

Abstract.A powder X-ray diffraction study, combined with magnetic susceptibility and electricntransport measurements, was performed on a series of LnCoO3 perovskites (Lnxa0=xa0Y, Dy, Gd, Sm, Nd,nPr and La) over a temperature range 100–1000xa0K. A non-standard temperature dependence of thenobserved thermal expansion was modelled as a sum of three contributions: (1) weighted sum ofnlattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediaten(IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population ofnexcited (IS or HS) states of Co3+ ions over the course of the diamagnetic-paramagneticntransition. (3) An anomalous expansion due to excitations of Co3+ ions to anothernparamagnetic state accompanied by an insulator-metal transition.nThe anomalous expansion is governed by parameters that are found to vary linearly with the Lnnionic radius. In the case of the first magnetic transition it is the energy splitting Enbetween the ground low spin state and the excited state, presumably the intermediate spin state.nThe energy splitting E, determined by a fit to magnetic susceptibility, decreases withntemperature. The values of E determined for LaCoO3 and YCoO3 at T=0xa0K are 164xa0K andn2875xa0K respectively, which fall to zero at T=230xa0K for LaCoO3 and 860xa0K for YCoO3. The secondnanomalous expansion connected with a simultaneous magnetic and insulator-metal transition isncharacterized by its center at T=535xa0K for LaCoO3 and 800xa0K for YCoO3. The change of the unitncell volume during each transition is independent of the Ln cation and is about 1% in bothncases. n n n

Two C-type antiferromagnets with different magnetoresistive properties: Sm0.15Ca0.85MnO3 and Pr0.15Sr0.85MnO3

Abstract The investigation of Sm 0.15 Ca 0.85 MnO 3 and Pr 0.15 Sr 0.85 MnO 3 manganites shows that both oxides exhibit a metal to insulator transition around 110 and 260xa0K, respectively, but they differ by the existence of CMR properties for the former, in contrast to the latter. The neutron powder diffraction and electron microscopy studies show that the resistive transitions are linked to the structural and magnetic ones. Both oxides are C-type antiferromagnets at low temperature, with a similar d 3 z 2 − r 2 orbital polarization which is associated with ordering of the orbitals into chains, no charge ordering being detected. Although they exhibit a similar distortion of their MnO 6 octahedra, these two C-type antiferromagnets differ mainly by the tilt arrangement of the octahedra, leading to the P2 l /m and I4/mcm structures for Sm 0.15 Ca 0.85 MnO 3 and Pr 0.15 Sr 0.85 MnO 3 respectively. Moreover, few percents of a G-type antiferromagnetic phase has also been evidenced in Sm 0.15 Ca 0.85 MnO 3 . The existence of this secondary phase is discussed in connection with its CMR properties.

Influence of Mn-site doping upon orbital and charge ordering in the Pr0.5A0.5Mn1−xMxO3 manganites (A=Sr, Ca and M=Cr, Al)

Abstract The influence of Cr and Al-doping upon orbital and charge ordering in Pr0.5A0.5MnO3 manganites (A=Sr, Ca) has been investigated combining magnetic and transport measurements with neutron powder diffraction and electron microscropy. It is shown that Cr-doping induces or reinforces the ferromagnetic metallic state, and destroys the ordering and associated antiferromagnetism whatever is its nature, A or CE-type. On the contrary, the orbital and charge ordering and associated antiferromagnetic ground states are essentially retained after doping with diamagnetic aluminium. Moreover, the Al-doping in Pr0.5Sr0.5MnO3 expands the antiferromagnetic region and suppresses the ferromagnetism at intermediate temperatures.

Structure and magnetism in the Pr1−xNaxMnO3 perovskites (0 ⩽ x ⩽ 0.2)

Abstract The mixed-valence manganites Pr 1− x Na x MnO 3 have been investigated by neutron diffraction, electric transport and magnetic measurements. Similarly to related systems with divalent alkali earths, the increasing monovalent sodium substitution decreases the Jahn–Teller deformation of the MnO 6 octahedra, lowers the resistivity and changes gradually the magnetic ordering from the layered type antiferromagnetism ( x =0) through spin-canted arrangements ( x ∼0.05) to the pure ferromagnetism (0.10⩽ x ⩽0.15) with T C ∼125xa0K. The samples with ferromagnetic ground state are not metallic below T C but show appreciable magnetoresistive effects in a broad temperature region. The electronic localization at low temperatures is further enhanced in the sample with the maximum sodium content x ∼0.2. Electron and neutron diffraction evidences that Pr 0.8 Na 0.2 MnO 3 exhibits a commensurate charge and orbital ordering of the Mn 3+ /Mn 4+ (1:1) kind below T co =215xa0K, followed with a transition to the antiferromagnetic arrangement of pseudo-CE type at T N =175xa0K, analogous to that of previously studied Pr 0.65 Ca 0.35 MnO 3 . In addition, Pr 0.8 Na 0.2 MnO 3 undergoes below ∼50xa0K a spin reorientation and, simultaneously, ferromagnetic clusters in the charge-ordered matrix are formed. By application of external field of 2–5xa0T below T co , the insulating charge-ordered antiferromagnet is transformed to a metallic ferromagnetic state which is persistent below ∼60xa0K, i.e. temperature close to the spin reorientation transition.

Details of structural and magnetic transitions in Pr0.5Ca0.5−xSrxMnO3 perovskites

Abstract Magnetic measurements supported in some cases by neutron diffraction and resistivity measurements were used to investigate the changes of crystal and magnetic structures between 2 and 400xa0K across the system Pr 0.5 Ca 0.5− x Sr x MnO 3 , 0⩽ x ⩽0.5. While for x =0, the low-temperature antiferromagnetic (AF) state is reached through mediating incommensurate charge ordered (CO) structures, a ferromagnetic (F) charge-disordered phase is intercalated for x ⩾0.3 between paramagnetic (P) and AF states. A two-phase region CO+F was found in between, for 0.1⩽ x ⩽0.2. The transition to the AF state is always accompanied by large changes of lattice parameters that are compatible with cooperative Jahn–Teller (JT) effect and a corresponding polarization of e g electron orbitals to definite planes. The elementary cell volume shrinks more than 0.5% during the transition and a local minimum close to T N was found to exist in x =0.15 and 0.3. It was also discovered that the actual low-temperature AF state often contains some residua of F phase as well as some weakly coupled or disordered spins exhibiting spin glass behaviour. The broad spectrum of results including the observed deviations from the Curie–Weiss law in the P region has been consistently interpreted on the basis of an interplay of double exchange and the J–T effect, in combination with steric effects influencing the tolerance factor t and the e g bandwidth.

Ferromagnetic–antiferromagnetic transition in tetragonal La0.50Sr0.50MnO3

Abstract Tetragonal modification of the ferromagnetic perovskite La0.50Sr0.50MnO3 with TC=310xa0K has been investigated by neutron powder diffraction and by magnetic and transport measurements. The compound was found to be isostructural with the recently reported phase Pr0.50Sr0.50MnO3, space group I4/mcm. At low temperatures, it shows a structural transition which is accompanied with onset of the A-type antiferromagnetism. Ceramic material based on La0.50Sr0.50MnO3 exhibits good conductivity down to the lowest temperatures and shows appreciable magnetoresistive effects which are characteristic for granular perovskites.

Neutron diffraction study of crystal and magnetic structures of BaTi0.6Mg0.6Fe10.8O19 and BaTi2Co2Fe8O19

Abstract Two single-crystal samples of the barium hexaferrite with partial substitution of Fe 3+ ions by Ti 4+ + Mg 2+ and Ti 4+ + Co 2+ , respectively, have been investigated at ambient and cryogenic temperatures by the neutron diffraction. A break of the extinction law of group P6 3 /mmc was observed irrespective the temperature. The structure was refined within the trigonal P 3 m 1 symmetry. At the room temperature, the samples display a slightly canted modification of the Gorter-type ferrimagnetism with the easy-axis anisotropy for BaTi 0.6 Mg 0.6 Fe 10.8 O 19 and the easy-cone anisotropy for BaTi 2 Co 2 Fe 8 O 19 . The later type is related to the one-ion contribution of Co 2+ ions in octahedral sites. Conical-spiral arrangements of the block-type are established at low temperature.

Phase transition in Pr 0.5 Ca 0.5 CoO 3 and related cobaltites

We present an extensive investigation (magnetic, electric and thermal measurements and X-ray absorption spectroscopy) of the Pr0.5Ca0.5CoO3 and (Pr1−yYy)0.7Ca0.3CoO3 (yxa0=xa00.0625xa0−xa00.15) perovskites, in which a peculiar metal-insulator (M-I) transition, accompanied with pronounced structural and magnetic anomalies, occurs at 76xa0K and 40xa0−xa0132xa0K, respectively. The inspection of the M-I transition using the XANES data of Pr-L3-edge and Co-K-edge proofs the presence of Pr4+ ions at low temperatures and indicates simultaneously the intermediate spin to low spin crossover of Co species on lowering the temperature. The study thus definitively confirms the synchronicity of the electron transfer between Pr3+ ions and Co3+/4+O3 subsystem and the transition to the low-spin, less electrically conducting phase. The large extent of the transfer is evidenced by the good quantitative agreement of the determined amount of the Pr4+ species, obtained either from the temperature dependence of the XANES spectra or via integration of the magnetic entropy change over the Pr4+ related Schottky peak in the low-temperature specific heat. These results show that the average valence of Pr3+/Pr4+ ions increases (in concomitance with the decrease of the formal Co valence) below TMI for (Pr0.925Y0.075)0.7Ca0.3CoO3 up to 3.16+xa0 (the doping level of the CoO3 subsystem decreases from 3.30+xa0 to 3.20+), for (Pr0.85Y0.15)0.7Ca0.3CoO3 up to 3.28+xa0(the decrease of doping level from 3.30+ to 3.13+) and for Pr0.5Ca0.5CoO3 up to 3.46+ (the decrease of doping level from 3.50+ to 3.27+).

Spin-state crossover and low-temperature magnetic state in yttrium-doped Pr0.7Ca0.3CoO3

The structural and magnetic properties of two mixed-valence cobaltites with formal population of 0.30 Co