arXiv subjects
Publications and source records attributed to B. Dabrowski.
Structural and magnetic properties have been studied for polycrystalline Zn_1-xMn_xO (x=0.02, 0.03, 0.05). Low-temperature (~500 oC) synthesis leaves unreacted starting ZnO and manganese oxides. Contrary to a recent report, no bulk ferromagnetism was observed for single-phase materials synthesized in air at temperatures above 900 oC. Single-phase samples show paramagnetic Curie-Weiss behavior.
In order to investigate the coexistence of magnetism and superconductivity on a microscopic scale, local studies using zero-field spin-echo ^{99,101}Ru NMR and ^{63,65}Cu NQR have been carried out on both superconducting and non-superconducting samples of magnetically ordered RuSr_{2}EuCu_{2}O_{8} and RuSr_{2}GdCu_{2}O_{8}. ^{99,101}Ru NMR signals were observed at 1.3 K over two distinct frequency ranges. The high frequency spectra (110 to 150 MHz), which are essentially the same for all samples, are characterized by five sharp peaks for each isotope corresponding to a hyperfine field of approximately 590 kOe with the appropriate quadrupole splitting. The low frequency spectra (40 to 90 MHz), which show significant variation from sample to sample, correspond to a much lower hyperfine field with considerable broadening and quadrupole features that are barely resolvable. As reported previously, the low frequency spectra are attributed to the Ru^{4+} valence state while the high frequency spectra are attributed to the Ru^{5+} valence state. ^{63,65}Cu NQR features were observed (26 to 34 MHz), although with considerable broadening. The results are discussed in terms of the microscopic magnetic structure, mixed valence state for Ru, occupancy of the Ru sites, and the existence and role of impurity phases.
Structural and magnetic properties have been studied for polycrystalline Zn_{1-x}TM_xO, where TM (transition metal ions) = Mn, Fe, and Co. No bulk ferromagnetism was observed for single-phase materials, contrary to the existing theories. Single-phase samples demonstrate paramagnetic Curie-Weiss behavior with antiferromagnetic interactions, similar to other diluted magnetic semiconductors. Non-optimal synthesis conditions lead to formation of second phases that are responsible for spin-glass behavior (ZnMnO_3 impurity for Zn_{1-x}Mn_xO (S. Kolesnik et al., J. Supercond.: Incorp. Novel Magn. 15, 251 (2002)) or high-temperature ferromagnetic ordering (Co metal for Zn_{1-x}Co_xO with the Curie temperature T_C > 800 K or (Zn,Fe)_3O_4 for Zn_{1-x}Fe_xO with T_C = 440 K).
We combine the results of magnetic and transport measurements with Mossbauer spectroscopy and room-temperature diffraction data to construct the magnetic phase diagram of the new family of cubic perovskite manganites SrMn_1-xFe_xO_3. We have found antiferromagnetic ordering for lightly and heavily Fe-substituted material, while intermediate substitution leads to spin-glass behavior. Near the SrMn_0.5Fe_0.5O_3 composition these two types of ordering are found to coexist and affect one another. The spin glass behavior may be caused by competing ferro- and antiferromagnetic interactions among Mn^4+ and observed Fe^3+ and Fe^5+ ions.
We discuss the properties of superconducting derivatives of the RuSr2GdCu2O8 (1212-type) ruthenocuprate, for which heterovalent doping has been achieved through partial substitution of Cu ions into the RuO2 planes (Ru1-xSr2GdCu2+xO8-d, 0 Ru doping achieved in these phases is found to decrease the temperature for magnetic ordering as well the volume fraction of the magnetic phase.
The synthesis of SrRuO3 under high-pressure oxygen produces a nonstoichiometric form with randomly distributed vacancies on the Ru-sites, along with a significantly reduced ferromagnetic ordering temperature. In order to gain additional insight into the suppression of the ferromagnetism, local studies utilizing 99,101 Ru zero-field spin-echo NMR, and Ru K-edge XAFS, along with complimentary magnetization and x-ray diffraction measurements, have been carried out on samples of SrRuO3 annealed at both ("ambient") atmospheric pressure and "high-pressure" oxygen (600 atm). Consistent with previous work, the NMR spectrum for "ambient" SrRuO3 consists of two well-defined peaks at 64.4 MHz and 72.2 MHz corresponding to the 99Ru and 101Ru isotopes, respectively, and a hyperfine field of 329 kG. Although the magnetization measurements show a lower ferromagnetic ordering temperature for the "high-pressure" oxygen sample (90 K compared to 160 K for the "ambient" sample), the NMR spectrum shows no significant shift in the two peak frequencies. However, the two peaks exhibit considerable broadening, along with structure on both the low and high frequency sides which is believed to be quadrupolar in origin. Analysis of the Ru K-edge XAFS reveals more disorder in the Ru-O bond for the "high-pressure" oxygen sample compared to the "ambient" sample. Furthermore, XANES of Ru K-edge analysis indicates no difference in the valence of Ru between the two samples. The magnetic behavior indicates the existence of some vacancies on the Ru sites for the "high-pressure" oxygen sample.
Alternating current susceptibility and direct current magnetization have been studied for polycrystalline Ca$_{1-x}$Mn$_x$O. On increasing the Mn content, magnetic ordering changes from spin-glass behavior for $0.25 \leqslant x \leqslant 0.4$ to antiferromagnetic order. The paramagnetic/antiferromagnetic transition is of second order for $0.5 \leqslant x \leqslant 0.65$ and of first order for $x \geqslant 0.7$. For low Mn concentrations, the high-temperature alternating current susceptibility can be described by a diluted Heisenberg magnet model developed for diluted magnetic semiconductors.
In order to gain insight into suppression of the ferromagnetic ordering temperature, local studies utilizing 99,101Ru zero-field spin-echo NMR, along with complimentary magnetization, high-angle x-ray diffraction, and Ru K-edge XAFS measurements, have been carried out on samples of SrRuO3 annealed at both "ambient" (atmospheric) pressure and "high-pressure" oxygen (600 atm). For the "high pressure" sample, the results indicate structural disorder and the existence of some vacancies on the Ru sites, along with a reduced magnetic ordering temperature.
We discuss the properties of new superconducting compositions of Ru(1-x)Sr2RECu(2+x)O(8-d) (RE=Gd, Eu) ruthenocuprates that were synthesized at 600 atm. of oxygen at 1080 C. By changing ratio between the Ru and Cu, the temperature of superconducting transition (Tc) raises up to Tc max=72 K for x=0.3, 0.4. The hole doping achieved along the series increases with Cu->Ru substitution. For x different than x=0, Tc can be subsequently tuned between Tc max and 0 K by changing oxygen content in the compounds. The magnetic characteristics of the RE=Gd and Eu based compounds are interpreted as indicative of constrained dimensionality of the superconducting phase. Muon spin rotation experiments reveal the presence of the magnetic transitions at low temperatures (Tm=14-2 K for x=0.1-0.4) that can originate in the response of Ru/Cu sublattices. RuSr2Gd(1-y)Ce(1-y)Cu2O8 (0 Gd substitution. Properties of these two series allow us to propose phase diagram for 1212-type ruthenocuprates that links their properties to the hole doping achieved in the systems. Non-superconducting single-phase RuSr2GdCu2O8 and RuSr2EuCu2O8 are reported and discussed in the context of the properties of substituted compounds.
ac susceptibility has been studied for polycrystalline Zn$_{1-x}$Mn$_x$O. Stoichiometric samples demonstrate Curie-Weiss behavior, which indicates mostly antiferromagnetic interactions. Magnetic susceptibility can be described by a diluted Heisenberg magnet model developed for semimagnetic semiconductors. High-pressure oxygen annealing induces spin-glass like behavior in Zn$_{1-x}$Mn$_x$O by precipitation of ZnMnO$_3$ in the paramagnetic matrix.
Synthesis in 1% oxygen at 930C leads to non-superconducting RuSr2GdCu2O8 in which superconductivity subsequently can be induced with oxygen annealings without observable stoichiometric changes. Detailed ac susceptibility and zero-field muon spin rotation experiments reveal that the superconducting compounds always exhibit a lower temperature of the magnetic transition and larger values of the internal magnetic field in the ordered state of the Ru sublattice. The same annealings in oxygen, however, lead to non-superconducting RuSr2EuCu2O8. The magnetic properties of a newly discovered series of Ru1-xSr2GdCu2+xO8-y superconductors (maximum Tc=72 K for x=0.4) are reported. Low temperature behavior of the magnetization shows a significant contribution of the paramagnetic system of Gd ions to the magnetic response below the superconducting transition. Muon-spin rotation experiments reveal the presence of bulk magnetic transitions at low temperature for compounds with x<=0.4. XANES Cu-K absorption edge data show an increase in Cu valence with x.
We report on the properties of new ruthenocuprates Ru(1-x)Sr2GdCu(2+x)O(8-y) (x=0, 0.1, 0.2, 0.3, 0.4, 0.75) that extend the superconductivity found previously in RuSr2GdCu2O8 (Tc=45 K) to the solid solution with varied Ru/Cu ratios. The compounds have been synthesized in high-pressure oxygen atmosphere. The maximum temperature of the superconducting transition is 72 K for the x=0.3 and 0.4 compositions. The reported behavior of magnetization at low temperatures can be qualitatively explained assuming a quasi-two-dimensional character of the superconducting regions in compounds studied.
In previous studies on a number of under- and overdoped high temperature superconductors, including YBa_{2}Cu_{3}O_{7-y} and Tl_{2}Ba_{2}CuO_{6+δ}, the transition temperature T_c has been found to change with time in a manner which depends on the sample's detailed temperature and pressure history. This relaxation behavior in T_c is believed to originate from rearrangements within the oxygen sublattice. In the present high-pressure studies on HgBa_{2}CuO_{4+δ} to 0.8 GPa we find clear evidence for weak relaxation effects in strongly under- and overdoped samples ($T_c\simeq 40 - 50 K$) with an activation energy $E_{A}(1 bar) \simeq 0.8 - 0.9 eV$. For overdoped HgBa_{2}CuO_{4+δ} E_{A} increases under pressure more rapidly than previously observed for YBa_{2}Cu_{3}O_{6.41}, yielding an activation volume of +11 \pm 5 cm^{3}; the dependence of T_c on pressure is markedly nonlinear, an anomalous result for high-T_c superconductors in the present pressure range, giving evidence for a change in the electronic and/or structural properties near 0.4 GPa.
The vortex response to various ac-drives has been studied in high quality $YBa_2Cu_4O_8$ single crystals. Well within the vortex solid phase a re-entrant resistive state has been observed that is characterized by long relaxation times. Pulse current response and IV characteristics reveal that in this state the driven vortex system relaxes in a way to increase t he effective pinning force. A phenomenological model accounts for the observed features, in particular the clock-wise hysteresis of the IV-curves and the increase of the apparent critical current with applied current amplitude.