EFFECT OF Eu DOPING AND PARTIAL OXYGEN ISOTOPE SUBSTITUTION ON MAGNETIC PHASE TRANSITIONS
IN (Pr1_yEuy)0.7Ca0.3CoO3 COBALTITES
N. A. Babushkinaa* A. N. Taldenkova, S. V. Streltsovh>c, A. V. Kalinovd, T. G. Kuzmova', A. A. Kamenev*, A. R. KauV, D. I. Khomskii^, K. I. Kugel9
" National Research Center " Kurchatov Institute" 123182, Moscow, Russia
bInstitute of Metal Physics, Ural Branch, Russian Academy of Sciences 620990, Ekaterinburg, Russia
€ Ural Federal University 620002, Ekaterinburg, Russia
d All-Russian Electrical Engineering Institute 111250, Moscow, Russia
''Department of Chemistry, Moscow State University 119991, Moscow, Russia
1 II. Physikalisches Institut, Universität zu Köln 50937, Köln, Germany
g Institute for Theoretical and Applied Electrodynamics, Russian Academy of Sciences
125412, Moscow, Russia
Received May 22, 2013
We study experimentally and theoretically the effect of Eu doping and partial oxygen isotope substitution on the transport and magnetic characteristics and spin-state transitions in (Pri_BEu„)o.TCao.3Co03 cobaltites. The Eu doping level y is chosen in the range of the phase diagram near the crossover between the ferromagnetic and spin-state transitions (0.10 < y < 0.20). We prepared a series of samples with different degrees of enrichment by the heavy oxygen isotope 180, namely, with 90%, 67%, 43%, 17%, and 0% of 180. Based on the measurements of the ac magnetic susceptibility \{T) and electrical resistivity p{T), we analyze the evolution of the sample properties with the change of the Eu and 180 content. It is demonstrated that the effect of increasing the 180 content on the system is similar to that of increasing the Eu content. The band structure calculations of the energy gap between t>g and eg bands including the renormalization of this gap due to the electron-phonon interaction reveals the physical mechanisms underlying this similarity.
DOI: 10.7868/S0044451014020114
1. INTRODUCTION
Most magnetic oxides are characterized by a strong interplay of electron, lattice, and spin degrees of freedom giving rise to multiple phase transitions and different types of ordering. The phase transitions are often accompanied by the formation of different inhomoge-
* E-mail: babushkina-XA'fflnrcki.ru
noons states. In such a situation, the oxygen isotope substitution provides a unique tool for investigating in-homogeneous states in magnetic oxides, which allows studying the evolution of their properties in a wide range of the phase diagram. Sometimes, especially if a system is close to the crossover between different states (usually leading to phase separation), the isotope substitution can lead to significant changes in the ground state of the system [1].
A good example of such phenomena is provided by cobaltites. These perovskite cobalt oxides have attracted special interest owing to the possibility of the spin-state transitions (SST) for Co ions induced by temperature or doping [2 8] and the related phase separation phenomena [9 16]. The effect of 160 ^180 isotope substitution on the properties of (Pi'i-^Eut^o.TCao.sCoOs cobaltites (0.12 < y < 0.26) was studied previously in our paper [17]. It was found that with increasing the Eu content, the ground state of the compound changes from a "nearly metallic" fer-romagnct (ferromagnetic metallic clusters embedded into an insulating host) to a "weakly magnetic insulator" at y < ycr « 0.18, regardless of the isotope content. A pronounced SST was observed in the insulating phase (in samples with y > ycr), whereas in the nearly metallic phase (at y < ycr), the magnetic properties were quite different, without any indications of a temperature-induced SST. Using the magnetic, electrical, and thermal data, we constructed the phase diagram for this material. The characteristic feature of this phase diagram is a broad crossover range near ycr corresponding to a competition of the phases mentioned above. The 160 ^180 substitution gives rise to an increase in the SST temperature Tss and to a slight decrease in the ferromagnetic (FM) transition temperature Tfm-
However, a number of problems important for understanding the physics of systems with spin-state transitions have not been considered in the study reported in Ref. [17]. The most important question is the relation between the changes caused by varying the composition (increase of the concentration y of the smaller rare-earth ions Eu) and by isotope substitution, and the physical mechanism underlying these changes. From the phase diagram obtained in Ref. [17] and in this paper, we see that there exists some correlation between these changes, but the situation is not so simple: in the right part of the phase diagram, the SST temperature increases both with the increase in the Eu content y and with the increase in the isotope mass (in passing from 160 to 180). At the same time, in the left part of this phase diagram, the effect of increasing the Eu content and of increasing the oxygen mass on the phase transition (which is then the transition to a nearly ferromagnetic state) is just the opposite: an increase in the Eu content leads to a decrease in Tfm, but the increase in oxygen mass, to the increase in Tfm-
Another important open question concerns the behavior of separate phases in the regime of phase separation. There are many different correlated systems in which phase separation was detected in some range of
compositions, temperatures, external fields, etc. Typically, the measured transition temperatures in this case changes, e.g., with doping. But it often remains unclear whether this change is the effect occurring in separate regions of different phases or is just the result of averaging over the inhoniogcncous system. To answer these questions, we now carried out a detailed study of the behavior of (PrEuJCoOs using the possibility of fine tuning the properties of the system by partial isotope substitution. This partial substitution plays in effect the role similar to that of doping, external pressure, etc. The obtained results establish the possibility of "rescaling" the changes in the system with doping and with isotope substitution and allow us to clarify the questions formulated above.
As regards the second question formulated above, just the possibility of fine tuning the properties of the system inside the region of phase separation, provided by partial isotope substitution, allows studying the behavior of different phases within this phase-separated regime individually which would be very difficult to achieve by other means. Our results obtained in this way demonstrate that not only the average critical temperatures change with doping and with isotope substitution but also "individual" transition temperatures (the ferromagnetic transition temperature in more metallic regions and the SST temperature in more insulating parts of the sample) do change with the chemical and isotope composition.
As regards the main, first question formulated above, about the mechanisms governing the change of properties of the system with chemical and isotope composition, the experimental findings reported in this paper allow us to formulate a realistic theoretical model clarifying the mechanisms underlying the pronounced isotope effects in cobaltites exhibiting SSTs. The theoretical analysis demonstrates that the main factor is the change of the effective bandwidth with the change of both chemical and isotope composition. The opposite trends in two parts of the phase diagram mentioned above find a natural explanation in this picture.
To analyze the effects of partial oxygen isotope substitution for doped cobaltites in the crossover region of the phase diagram, we have prepared a series of oxide materials with a nearly continuous tuning of their characteristics. This allows tracing the evolution of the relative content of different phases as a function of the ratio 180/160 of the contents of oxygen isotopes. We note that there were only a few investigations of this kind, one of which we undertook earlier for (Lai-yPr^o.TCao.sMiiOs nianganitos [18]. Here, the pronounced isotope effect manifesting itself in
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(Pri_yEuy)0.7Ca0.3CoO3 cobaltites indeed provides us with a unique possibility to address the problems discussed above through the use of partial oxygen isotope substitution.
2. EXPERIMENTAL
Polycrystalline (Pri_yEuy)0.7Ca0.3CoO3 samples were prepared by the chemical homogenization ("paper synthesis") method [19] through the use of the following operations. At first, nonconcentrated water solutions of metal nitrates Pr(NOs)3, Eu(NOs)3, Ca(N03)2, and Co(N03)2 of 99.95% purity were prepared. The exact concentration of dissolved chemicals was established by gravimetric titration and, in the case of Co-based solution, by means of potentiometric titration. The weighted amounts of metal nitrate solutions were mixed in the stoichiometric ratio and the calculated mixture of nitrates was dropped onto ash-free paper filters. The filters were dried out at about 80 °C and the procedure of the solution dropping was performed repeatedly. Then, the filters were burned out and the remaining ash was thoroughly ground. It was annealed at 800 °C for 2 h to remove carbon. The powder obtained was pressed into the pellets and sintered at 1000 °C in the oxygen atmosphere for 100 h. Finally, the samples were slowly cooled to room temperature by switching off the furnace.
Samples were analyzed at room temperature by the powder X-ray diffraction using Cu Ka radiation. All detectable peaks were indexed by the Prima space group. According to the X-ray diffraction patterns, all (Pri_yEuy)0.7Ca0.3CoO3 samples were obtained as single-phase polycrystalline materials.
We prepared a series of ceramic cobaltite samples with the degrees of enrichment by 18 O equal to 90%, 67%, 43%, 17%, and 0%. These values were determined by the changes in the sample mass in the course of the isotope exchange and by the m
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