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

Abstract

The structural and magnetic properties of two mixed-valence cobaltites with formal population of 0.30 Co 4+ ions per f.u., (Pr1−yYy)0.7Ca0.3CoO3 (y = 0 and 0.15), have been studied down to very low temperatures by means of the high-resolution neutron diffraction, SQUID magnetometry and heat capacity measurements. The results are interpreted within the scenario of the spin-state crossover from a room-temperature mixture of the intermediate spin Co3+ and low spin Co 4+ (IS/LS) at the to the LS/LS mixture in the sample ground states. In contrast to the yttrium free y = 0 that retains the metallic-like character and exhibits ferromagnetic ordering below 55 K, the doped system y = 0.15 undergoes a first-order metal-insulator transition at 132 K, during which not only the crossover to low spin states but also a partial electron transfer from Pr 3+ 4f to cobalt 3d states take place simultaneously. Taking into account the non-magnetic character of LS Co 3+ , such valence shift electronic transition causes a magnetic dilution, formally to 0.12 LS Co 4+ or 0.12 t2g hole spins per f.u., which is the reason for an insulating, highly non-uniform magnetic ground state without long-range order. Nevertheless, even in that case there exists a relatively strong molecular field distributed over all the crystal lattice. It is argued that the spontaneous FM order in y = 0 and the existence of strong FM correlations in y = 0.15 apparently contradict the single t2g band character of LS/LS phase. The explanation we suggest relies on a model of the defect induced, itinerant hole mediated magnetism, where the defects are identified with the magnetic high-spin Co 3+ species stabilized near oxygen vacancies.

Electrical resistivity anomaly, valence shift of Pr ion, and magnetic behavior in epitaxial (Pr1-yYy)1-xCaxCoO3 thin films under compressive strain

“…In the bulk (Pr 1-x RE y ) 1-x Ca x CoO 3 systems under ambient pressure, which show the simultaneous MI-SS transition, a volume contraction of $1% took place below T MI $ 100 K. 44 The entropy change makes DS $ 5 JÁK À1 Ámol À1 , 45,46 which means that the internal energy of the low-temperature phase is about DSÁT MI $ 500 JÁmol À1 lower, compared with the high-temperature phase. The present PYCCO films are fixed on the substrate and the lattice parameters and the unit cell volume are difficult to change.…”

Phase diagram of the (Pr1−ySmy)0.7Ca0.3CoBréard1, Veillon2, Hervé3 et al. 2022Phys. Rev. Materials1000View full textAdd to dashboardCite

“…Investigation of the closely related (Pr 1−y Eu y ) 0.7 Ca 3 CoO 3 showed the same qualitative behavior, with y crit ∼ 0.19 [11]. The most studied system in VSST literature is (Pr 1−y Y y ) 0.7 Ca 3 CoO 3 , owing to the fact that Y 3+ is nonmagnetic, which facilitates the analysis [10,14,27,30,32,[38][39][40]. However, the size difference with Pr 3+ is quite strong [41], which makes it difficult to follow the emergence of the VSST.…”

“…It also deserves to be noted that, for small y values, the low-T part of the curves appears as a smooth extrapolation of the high-T regime (following a Debye-like shape), whereas the high-y curves exhibit a downward shift below T * . This can be ascribed to the fact that the VSST is accompanied by a decrease in the unit cell volume upon cooling [1,2,7,27]. This induces an increase in Debye temperature when cooling below T * and thus a negative step in the lattice contribution (which is the greatest part of the baseline onto which the VSST peak is superimposed).…”

Acoustic phonon dispersion and diffuse scattering across the valence transition of (Pr0.85Y0.15)0.7Phelan1, Krogstad2, Schreiber3 et al. 2019Phys. Rev. B4020View full textAdd to dashboardCite

“…At the 1 st order MIT temperature (T MIT ), these materials undergo a distinct change in unit cell volume, with the lattice parameters dropping to significantly smaller values in the low temperature phase. This is thought to occur isomorphically, such that although the change in cell volume is anomalously large at T MIT , the crystallographic space group (Pnma does not change [7,22,23]. Recent single crystal diffraction experiments are indeed consistent with this picture [15].…”

“…The crystal possesses an orthorhombic symmetry (see Ref. [23] for structural refinements of a similar composition), but was twinned as is usual for a perovskite with lowerthan-cubic symmetry. Because of this, we are limited to a pseudocubic approximation of the…”