Orbital ordering and the cooperative Jahn-Teller effect in single crystals of the magnetic Perovskite La7/8Sr1/8MnO3

Abstract

In the low-doped magnetic perovskite La7/8Sr1/8MnO3, a strong coupling exists between crystal structure and magnetic and electronic properties. At room temperature, the material is in a paramagnetic-semiconducting state, showing a dynamic Jahn–Teller (DJT) ordering of the Mn3+ ions. Below TJT=269 K however, the crystal goes into a cooperative Jahn–Teller (CJT) state, exhibiting antiferrodistorsive orbital ordering of the MnO6 octahedra. This leads to an increase in resistivity by a factor of 2 and to a drop of the paramagnetic susceptibility by about 20% [Physica B 155 (1989) 362; Phys. Rev. Lett. 71 (1993) 2331]. Around TC=188 K, a ferromagnetic transition occurs while the CJT effect becomes gradually weaker. Below TC the sample is metallic-like and shows a considerable colossal negative magnetoresistance (CMR) effect. Finally, the material goes into a charge-ordered, insulating state below TCO=147 K where the CJT effect completely vanishes. When crossing the face boundary from the DJT to the CJT state, the resistivity shows a jump-like increase by a factor of roughly 2.15. This results from a frustration of the charge transport at the onset of orbital ordering By assuming that each Mn4+ ion is surrounded only by Mn3+ nearest-neighbors and by taking into account only charge transport along the principal axes, the square-diagonals and the cube-diagonals, we are able to calculate a jump-like increase by a factor of roughly 2.15. This results from a frustration of the charge transport at the onset of orbital ordering. By assuming that each Mn4+ ion is surrounded only by Mn3+ nearest-neighbors and by taking into account only charge transport along the principal axes, the square-diagonals and the cube-diagonals, we are able to calculate a jump ratio of 2.26, corresponding well to the experimental value. The CJT transition also causes a sudden decrease in the magnetic moment, which has been studied by performing field-dependent SQUID measurements around TJT. Since the crystal is in a paramagnetic state around TJT, the magnetization M scales with the Brillouin function β(J), where J is the spin of small ferromagnetic clusters. Fitting the magnetization data with the Brillouin function results in a CJT-induced shrinking of magnetic clusters from roughly four Mn ions to three Mn ions. These cluster sizes can also be understood in terms of the orbital structure of the material in both phases.