Temperature evolution of the Mn electronic and local structure in Nd0.5Sr0.5MnO3

Abstract

Complementary x-emission (XES) and x-ray absorption (XAS) measurements of Nd0.5Sr0.5MnO3 as a function of temperature have been performed to investigate the electronic and local structure changes of the Mn atom. Specifically, variations of the Kα and Kβ emission lines positions and shapes depend on the oxidation and the spin state of the Mn sites as well as the degree of d covalency/itinerancy. Half-doped Nd0.5Sr0.5MnO3 is particularly representative since the three macroscopic phases, i.e. charge ordered insulator, ferromagnetic conductor and paramagnetic insulator; appear at different temperatures. Above T = 230 K is a paramagnetic insulator, in the range 170 K ≤ T ≤ 230 K is a magnetoresistive ferromagnetic metal and below T = 170 K is an antiferromagnet with CO. Our XES study, including both the Mn Kα and Kβ emission lines, has shown that only modest changes are observed with temperature accompanying the passage to the metallic-ferromagnetic phase, which are reversed when entering the charge-ordered one. In terms of the electronic and magnetic state, this finding indicates that the Mn spin and oxidation state are not significantly changed by the different magnetic and electrical behaviour of Nd0.5Sr0.5MnO3. The largest differences have been observed between Nd0.5Sr0.5MnO3 and the single valent compounds (NdMnO3, CaMnO3, SrMnO3), the electronic and spin state of Nd0.5Sr0.5MnO3 being intermediate but closer to the Mn3+ compound. The high resolution XAS data do not show large changes with temperature either. The main changes take place at the pre-edge region. While the single valent compounds exhibit a monotonic evolution with decreasing temperature, in Nd0.5Sr0.5MnO3 the structures in the difference signal show a change of sign coincident with the magnetic and electrical transitions. These thermal effects can be related to thermal variations in the Mn d covalency. Moreover, the high-resolution XANES spectrum of Nd0.5Sr0.5MnO3 strongly disagrees with the 1:1 linear superposition of NdMnO3 and SrMnO3 spectra at any temperature but it is nicely reproduced by the 1:1 linear superposition of the Nd0.5Sr0.5MnO3 spectrum shifted by ±ΔE=0.3 eV correspondent to a charge segregation δ< 0.2 e-. This result confirms that the charge segregation of the charge ordered phase is very small and the two different Mn sites are in intermediate valence states Mn+3.5±δ/2. These XAS and XES results indicate that the charge and spin state of the Mn atom in the half-doped Nd0.5Sr0.5MnO3 hardly changes among the three macroscopic phases supporting the strong competition between the ferromagnetic conductor and the charge ordered insulator behaviours in the manganites.