E. Pollert

Structure and magnetic properties of Pr1−xSrxMnO3 perovskites

Abstract A comprehensive account of structural features in mixed Mn3+, Mn4+ perovskites, Pr1−xAxMnO3 (A = Ca, Sr, Ba) is given. The newly investigated Sr system was studied at various temperatures by X-ray and neutron diffraction, magnetic, and conductivity measurements. A low-temperature transition from the activated type to the degenerate type of conductivity was observed in the Sr system for x = 0.3–0.4. It occurs 20–60 K below the ferromagnetic Curie point.

Magnetic heating by cobalt ferrite nanoparticles

In the quest for suitable materials for hyperthermia we explored the preparation and properties of nanoparticles of Co ferrite. The material was produced by coprecipitation from water solution of Co and Fe chlorides and afterwards annealed at 400, 600 and 800 °C. The resulting particles were characterized by XRD, TEM, Mossbauer spectroscopy, and dc and ac magnetometry. The heating experiments in ac magnetic fields of various amplitudes were performed with diluted systems of particles suspended in agarose gel and the results were interpreted on the basis of the ac magnetic losses measured at various temperatures. The increase of magnetic losses and consequently of the heating efficiency with increasing temperature is explained by the strong dependence of the constant of magnetocrystalline anisotropy of Co ferrite on temperature.

Manganite perovskite nanoparticles for self-controlled magnetic fluid hyperthermia: about the suitability of an aqueous combustion synthesis route

Unaggregated La0.82Sr0.18MnO3 + δ perovskite nanoparticles with a mean crystallite size of 22 nm were successfully synthesized through an aqueous combustion process (Glycine Nitrate Process, GNP) which takes advantage of exothermic, fast and self-sustaining chemical reactions between metal nitrates and glycine as a suitable organic reducing agent. The influence of G/N molar ratio on the phase purity, crystallite size and manganese valency was screened. Fuel-rich conditions were selected to improve chelation of the cations in acidic pH and ensure an accurate control of the cationic composition. Fast calcination was optimized to enhance crystallinity of the nanoparticles and subsequent milling step was performed to favour their desaggregation. The manganite nanoparticles were thoroughly characterized by X-ray diffraction (XRD), elemental chemical analysis, Mohr salt titration and transmission electron microscopy (TEM). According to a process derived from the Stobers method, they were uniformly coated with a 5 nm thick silica shell, as evidenced by TEM, infrared spectroscopy, ζ potential measurements and dynamic light scattering experiments. Preliminary heating experiments in a ac magnetic field showed these core@shell nanoparticles fulfill the requirements for self-controlled magnetic fluid hyperthermia, considering their size (20–70 nm) and their maximum heating temperature (43 °C) which is controlled by the Curie temperature of the magnetic cores.

Neutron diffraction study of the modulated structure of Bi2(Sr, Ca)3Cu2O8+y

Abstract The modulated structure of the 80 K superconductor Bi 2 (Sr, Ca) 3 Cu 2 O 8+y is refined from the single-crystal neutron diffraction data on a monodomain block of dimensions 1.5 × 1.0 × 0.03 mm 3 . The modulation is recognized as commensurate with period 4.75 a , resulting in a supercell 19a × b × c (a = 5.397 (1), b = 5.401 (1), c = 30.716 (3) A ) and symmetry Pnaa. The essential feature of the structure is the periodic insertion of additional oxygen rows into BiO planes, which in turn causes large displacive modulation in all layers. The ideal oxygen stoichiometry is y = 4 19 ∽ 0.21 . Bonds in the BiO and SrO planes form characteristic chains running along the a -axis. The coordination of the Bi 3+ cations is uniform and consists of three mutually nearly perpendicular bonds to oxygens of 2.0–2.2 A lengths. The Sr 2+ cations are bonded only to eight oxygen atoms at the most. About 7% of the Sr sites, located in the vicinity of the inserted oxygen, are possibly vacant. The Ca site situated between the CuO 2 planes is substituted to 30% by strontium.

Silica encapsulated manganese perovskite nanoparticles for magnetically induced hyperthermia without the risk of overheating

Nanoparticles of manganese perovskite of the composition La(0.75)Sr(0.25)MnO(3) uniformly coated with silica were prepared by encapsulation of the magnetic cores (mean crystallite size 24 nm) using tetraethoxysilane followed by fractionation. The resulting hybrid particles form a stable suspension in an aqueous environment at physiological pH and possess a narrow hydrodynamic size distribution. Both calorimetric heating experiments and direct measurements of hysteresis loops in the alternating field revealed high specific power losses, further enhanced by the encapsulation procedure in the case of the coated particles. The corresponding results are discussed on the basis of complex characterization of the particles and especially detailed magnetic measurements. Moreover, the Curie temperature (335 K) of the selected magnetic cores resolves the risk of local overheating during hyperthermia treatment.

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.

Study of Pr1−xMn1+xO3 perovskites

Abstract The structural and magnetic properties of the Pr1−xMn1+xO3 perovskites were studied. The increase of x (i.e., Pr Mn ) leads to the decrease of the orthorhombic deformation and of the Neel temperature and, simultaneously, to an increase of the ferromagnetic contribution. The latter effect is explained from the suggested distribution of the cations (Pr3+1−xMn2+x)A(Mn3+1−xMn4+x)O2−3 by the double exchange of Mn3+ue5f8Mn4+ pairs at the B—sublattice.

Single-phase region of the 2212-BiSrCaCuO superconductor

Abstract The phase relationships and solid-solution region of the Bi 2 Sr 2 CaCu 2 O 8+δ (2212) superconductor were investigated at 850°C under air atmosphere in a cross-section of the system Biue5f8Srue5f8Caue5f8Cuue5f8(O) defined by the formula Bi 2+ y Sr 3− y − x Ca x Cu 2 O 8+δ . The system was studied by means of XRD and EMA and an approximate position of the 2212 single-phase region is delimited by the values of y =0.1 and 0.8≤ x ≤1.3. The single-phase region determined may be divided into two subregions, Sr-rich in the range 0.8≤ x ≤1.0 and Ca-rich in the range 1.1≤ x ≤1.3, these differing in the number and character of intergrowths. The highest superconducting transition temperature ( T c ) was found for x =0.8.

Distribution of cations in nanosize and bulk Co–Zn ferrites

The structural and magnetic properties of Co(1-x)Zn(x)Fe2O4 ferrites (Co-Zn ferrites) are investigated in a narrow compositional range around x = 0.6, which is of interest because of applications in magnetic fluid hyperthermia. The study by x-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements is done on nanoparticles prepared by the coprecipitation method and bulk samples sintered at high temperatures. In spite of the known preference of Zn2+ for tetrahedral (A) sites and Co2+ for octahedral [B] sites, the cations are distributed nearly evenly over the two sites of spinel structure and there is also a variable number of [B] site vacancies (see text), making cobalt ions trivalent. In particular for x = 0.6, the cationic distribution is refined to [Formula: see text] and [Formula: see text] for the 13 nm particles (T(C) = 335 K) and bulk sample (T(C) = 351 K), respectively.

Synthesis of HgBa2CuO4+δ by sol–gel method under controlled oxygen pressure; electron and thermal transport properties

Abstract Samples of mercury superconductor HgBa2CuO4+δ were prepared employing highly homogeneous and reactive precursor Ba2CuO3+x obtained by the sol–gel method. Mixture of Mn3O4/Mn2O3 was used to adjust p(O2) during the synthesis. This approach allows to achieve the appropriate p(O2) at the reaction temperature 720°C and multizone furnace is not required. The enhancement of the thermal conductivity below Tc, measured for the first time in Hg-type superconductor, indicates unusually strong phonon–electron coupling. The enhancement is accompanied by a sharp resistivity and thermopower decrease.