Evidence for a universal relationship between magnetization and changes in the local structure in La _{1– x} Ca _x MnO ₃

Abstract

Ca dopants introduce holes in the Mn eg band which allows electronic transport; such samples exhibit a colossal magnetoresistance (CMR) when the sample becomes ferromagnetic at low T, for x in the range 0.2 < x < 0.5. Previous EXAFS studies on the perovskite manganites La 1−xCaxMnO3 indicated a correlation between changes in the local structure associated with polaron formation, and the sample magnetization for these samples. We have extended the EXAFS measurements to a wider range of Ca concentrations and to very high magnetic fields. Applying a magnetic field reduces the local distortion of each sample for temperatures near T c. These measurements provide clear evidence for a universal relationship between the local structure and the sample magnetization. For at least one sample (x = 0.3), there is still a significant change of σ 2 with T below Tc when the sample is fully magnetized, i.e. distortions are still present and continuing to be removed as T is lowered well below Tc.When LaMnO 3 is doped with Ca it introduces holes into the Mn e g band which leads to novel transport and magnetic properties such as colossal magnetoresistance (CMR) and ferromagnetism, particularly for Ca concentrations in the range 20-50% [1]. At high T, the system is a paramagnetic insulator (semiconductor); in this regime, the electrical conductivity takes place via hoping polarons which produce a broadened distribution of Mn-O bond lengths. At low T, the system is a ferromagnetic metal; in this case the electrons become delocalized and most (in some cases all) of the local distortions disappear.For LaMnO 3 there are four 3d electrons on each Mn atom (Mn +3 ). Three are tightly bound (in a t 2g state) and form a spin 3/2 moment. The forth electron is in the half-metallic e g band and is Hund-rule coupled parallel to the core Mn spin. Holes in this band (from doping with a divalent atom) provide the electrical transport (See Fig. 1). In the CMR regime, the ferromagnetic coupling is mediated through the double exchange interaction in which electrons (or holes) hop rapidly from one Mn atom to the next via an intervening O atom [2]. If the Mn spins are parallel, the electrons can hop rapidly with no spin flip; if the Mn spins are not parallel, it costs additional energy as the spin must also flip (See Fig. 1). As T approaches T c , the hopping slows down -the lattice has time to partially respond and a Jahn-Teller distortion begins to appear (with two longer and four shorter Mn-O bonds). This enhances the metal to insulator transition [3].Much work has been done on these systems -yet the way in which the magnetization develops is still not clear. Our earlier work [4,5] shows that there is a connection between changes in the local distortion (which we associate with a decrease in the polaron distortion) and the magnetization as T is lowered through the ferromagnetic transition temperature T c ; the hopping increases as the spins become aligned and the distortion decreases. Although the concentration of holes is considerably less than 50%, ...