Papers by Stanislaw Kolesnik
Physical Review B, 2000
The nuclear and magnetic structures of Sr 2 FeMoO 6 have been investigated using neutron powder d... more The nuclear and magnetic structures of Sr 2 FeMoO 6 have been investigated using neutron powder diffraction and Mössbauer spectroscopy at temperatures between 10 and 460 K. Fe and Mo atoms are found to order on alternate sites, as expected, giving rise to a double-...
Physical Review B, 2001
The crystal and magnetic structures of newly synthesized Sr 1Ϫx Ca x MnO 3 (0рxр1) and Sr 1Ϫy Ba ... more The crystal and magnetic structures of newly synthesized Sr 1Ϫx Ca x MnO 3 (0рxр1) and Sr 1Ϫy Ba y MnO 3 (yр0.2) perovskite materials ͑of the ABO 3 type͒ were investigated using neutron and synchrotron x-ray powder diffraction at temperatures between 10 and 530 K. Upon decreasing the size of the A-site ion ͑or the unit-cell size͒ a series of structural transitions appears at room temperature from cubic Pm3 m to tetragonal I4/mcm ͑at xϳ0.3͒ to orthorhombic Pbnm at xϳ0.4. In agreement with neutron-diffraction data, resistive and magnetic measurements show that the samples are antiferromagnetic with Néel temperatures T N , varying from 233 to ϳ125 K and from 233 to 212 K by increasing the Ca and Ba contents, respectively. The observed variation of T N cannot be solely explained by the changes of the unit-cell size or the average Mn-O-Mn bond angle . The behavior of T N can be satisfactorily described as a function of ͗cos 2 ͘ related to the superexchange interaction integral, and 2 , the A-site ionic size variance ͓L. M. Rodriguez-Martinez and J. P. Attfield, Phys. Rev. B 54, R15 622 ͑1996͔͒ regardless of the structural symmetry of the perovskite material.
Predictions of room temperature ferromagnetism in transition-metal-doped Zn_1-xTM_xO (TM = Mn, Fe... more Predictions of room temperature ferromagnetism in transition-metal-doped Zn_1-xTM_xO (TM = Mn, Fe, Co, Ni) suggest possible application of this material in semiconductor spintronics. Pulsed-laser-deposited Zn_1-xCo_xO thin films have shown controversial results. In some of those films ferromagnetism with TC near room temperature has been observed. We present results of ac susceptibility measurements on bulk Zn_1-xTM_xO (TM = Mn, Fe, Co, Cu). Single-phase wurtzite samples were synthesized in air or in the 1%H/Ar atmosphere up to a solubility limit x ˜ 0.1-0.2. Zn_1-xTM_xO samples reveal antiferromagnetic TM^2+-TM^2+ interactions, similar to other diluted magnetic semiconductors. For selected samples high-temperature ferromagnetism in Co- and Fe- doped ZnO is observed after synthesis at conditions that lead to the presence of Co-metal and ferromagnetic iron-oxide impurities, respectively. While the magnetic moment of Mn^2+ ions in ZnO can be described as spin-only with the gyromagnetic factor g = 2, our results show that the g-factor values for Co^2+ and Fe^2+ in ZnO are significantly higher and exceed the values observed in other diluted magnetic semiconductors. Supported by NSF (DMR-0105398) and the State of Illinois under HECA.
Journal of Superconductivity and Novel Magnetism
We describe results of several macroscopic and local measurements of magnetic and superconducting... more We describe results of several macroscopic and local measurements of magnetic and superconducting properties for pure and Ce and Cu substituted RuSr 2 GdCu 2 O 8 . From various experiments the physical properties phase diagram is derived as a function of the hole doping. We show that the large contribution of polarized Gd 3+ magnetic moments to magnetization causes apparent absence of the Meissner state and augments the weak Ru ferromagnetism. This weak ferromagnetism in the superconducting state can be significantly enhanced by the external magnetic field. These results indicate that the low-temperature behavior can be qualitatively explained assuming a quasi-two-dimensional character of the superconducting regions.
Physical Review B, 1999
The magnetic structure and its correlation with the Jahn-Teller (JT) distortion in lightly doped ... more The magnetic structure and its correlation with the Jahn-Teller (JT) distortion in lightly doped La{sub 1-x}SrâMnOâ (0.11â¤xâ¤0.185) have been studied as a function of temperature from 10â300 K using neutron powder diffraction. A correlation between the ferromagnetic spin orientation and the coherent JT distortion was observed, indicating a coupling between the structural and the magnetic properties. The arrangement of the spins varies from ferromagnetic ordered mainly along the {ital b} axis (x=0.11) to ferromagnetic almost along the {ital c} axis (x=0.185) in the {ital Pbnm} symmetry. {copyright} {ital 1999} {ital The American Physical Society}
Alternating current susceptibility and direct current magnetization have been studied for polycry... more Alternating current susceptibility and direct current magnetization have been studied for polycrystalline Ca1−xMnxO. On increasing the Mn content, magnetic ordering changes from spin-glass behavior for 0.25 x 0.4 to antiferromagnetic order. The paramagnetic/antiferromagnetic transition is of second order for 0.5 x 0.65 and of first order for x 0.7. For low Mn concentrations, the high-temperature alternating current susceptibility can be described by a diluted Heisenberg magnet model developed for diluted magnetic semiconductors.
Journal of Applied Physics
We present magnetization and resistivity data on polycrystalline samples of Pr 1Ϫx Sr x MnO 3 (0.... more We present magnetization and resistivity data on polycrystalline samples of Pr 1Ϫx Sr x MnO 3 (0.58 рxр1.00). All the samples are antiferromagnetic insulators at low temperatures. For 0.78Ͻx Ͻ0.90, and for xϭ0.58, both resistivity and magnetization show a clear hysteresis which indicates the first-order nature of the antiferromagnetic transition. Neutron diffraction data show that the paramagnetic-to-antiferromagnetic transition is accompanied by a cubic-to-tetragonal structural transition for 0.78ϽxϽ0.90, and tetragonal-to-orthorombic transition for xϭ0.58. The other samples exhibit continuous transitions to the antiferromagnetic state. Our results show that the structural transitions are the origin of the discontinuous magnetic and resistive transitions. A moderate magnetoresistance effect can be observed for a lower Sr content possibly due to ferromagnetic fluctuations.
Acta Physica Polonica Series a
Influence of pressure on the phase transition temperature TC and on the spontaneous magnetisation... more Influence of pressure on the phase transition temperature TC and on the spontaneous magnetisation M 0 of highly Sr substituted and oxygen nonstoichiometric La1−xSrxCoO 3−δ samples was investigated. An increase in TC and M 0 with pressure in investigated samples was found, indicating strengthening of ferromagnetic double exchange interactions, and the expansion of volume of ferromagnetic phase as a result of enhancement in ferromagnetic interactions in both cluster-glass and long range ferromagnetic states. We did not observe clear correlation between changes in pressure coefficient dT C /dP and changes in the formal oxidation state of the Co ion.
We report the synthesis of SrMn_1-xFe_xO3 and SrMn_1-xGa_xO_3-delta perovskite compounds and desc... more We report the synthesis of SrMn_1-xFe_xO3 and SrMn_1-xGa_xO_3-delta perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Antiferromagnetic ordering is found for lightly and heavily Fe-substituted material, while intermediate substitution leads to spin-glass behavior caused by competing ferro- and antiferromagnetic interactions among Mn^4+ and observed Fe^3+ and Fe^5+. Maximally oxygenated Ga-solid solution compounds are found to order antiferromagnetically, with the transition temperature linearly decreasing as Ga content increases to x=0.4. Increasing Ga content to x=0.5 or reducing the oxygen content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.5 and SrMn_0.7Ga_0.3O_2.5 below 30 K. Complex crystallographic and magnetic structures are found for ordered Sr_2MnGaO_6-delta as a function of oxygen content. Supported by NSF (DMR-0302617), the State of Illinois under HECA, and U.S. Department of Energy, BES ' Materials Sciences (W-31-109-ENG-38)
Ruthenates display exotic mix of magnetic and superconducting properties that are very sensitive ... more Ruthenates display exotic mix of magnetic and superconducting properties that are very sensitive to dimensionality of the system and chemical composition. We have discovered that synthesis at high-pressure oxygen decreases Tc from 160 to 85 K for extensively studied SrRuO_3. Samples with reduced Tc 's show the same orthorhombic Pbnm structure as common 160 K phase. Neutron diffraction data reveal slightly decreased Ru-O bond-lengths and minute increased Ru-O-Ru bond angles. Data indicate also formation of vacancies on the Ru-sites (1-v) while the oxygen content remains stoichiometric. These results are in agreement with thermogravimetric measurements and confirm that the reason for decreased Tc is an increased oxidation state of Ru 4/(1-v).
We report the synthesis of Sr 1-x Ca x MnO 3 and La 0.5 Ba 0.5 MnO 3 perovskites over extended ca... more We report the synthesis of Sr 1-x Ca x MnO 3 and La 0.5 Ba 0.5 MnO 3 perovskites over extended cation and oxygen composition ranges and describe the dependence of their phase stability on the tolerance factor t = t(x,T,δ) that is a function of composition, temperature, and oxygen content.
Acta Physica Polonica Series a
An influence of hydrostatic pressure, P , on phase transition temperature, T C , and spontaneous ... more An influence of hydrostatic pressure, P , on phase transition temperature, T C , and spontaneous magnetisation, M 0 , of selected perovskite ruthenates (SrRuO3, La0.2Sr0.8RuO3, SrRu0.9Mn0.1O3, and SrRu0.9Cr0.1O3) was determined to 12 kbar. A decrease in T C with pressure was found for all of the studied samples. The M0(P ) remains unchanged for most of the samples, except for the La 0.2 Sr 0.8 RuO 3 sample. The weakening of ferromagnetic interactions with increasing pressure is consistent with complex band structure effects related to the modulation of the Ru-O hybridisation by the change of structural distortion.
Physical Review B
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Papers by Stanislaw Kolesnik