The intriguing behaviour of bilayer manganites

Transition metal oxides are an intriguing class of materials since it is possible to drastically modify their properties by changing their temperature, applying a magnetic field or simply by irradiating them with light. Of particular interest are the manganites that exhibit complex interactions bewteen the spin and and orbital degrees of freedom. To fully understand the nature of this delicate balance between the different phases of the same material it is crucial to exploit the versatility of high intensity X-rays sources. Scientists from the University of Durham have been using the Nanoscience beamline to study the atomic scale effects in manganites and have demonstrated a control of the electron orbital ordering through a thermal transition near room temperatures. Their results have been published in the journal Physical Review B.

In Detail

Exploitation and control of electron spin (known as spintronics) and orbital ordering (known as orbitronics) is fundamental in creating a new generation of electronics.. Common devices currently rely on the electron charge in order to form memory devices and switches for data storage. The switching of electron spins and orbitals, however, is many times faster than manipulating charge. This raises the possibility of creating ultra-fast memory devices that store information with the spin or orbital state of an electron.

Researchers from the University of Durham have used resonant soft X-ray diffraction to study a material in which the orbital order can be controlled thermally The bilayer manganite Pr(Sr0.1 Ca0.9)2Mn2O7 is an unusual material, as the orthorhombic crystal structure (see figure on the left) has been shown to confine the formation of the orbital order stripes along a single direction. Using soft X-ray diffraction, the Durham team were able to show that the orbital order occurs along the a axis below 350 K. Cooling the material below 300 K resulted in the orbital order stripes on the Mn sites rotating and aligning with the b axis. The angle dependent diffraction measurements carried out at Diamond provided the first direct observations of this change in the orbital order.

At lower temperatures a structural phase transition was also measured which resulted in a change of all three lattice parameters. However, energy dependent scans of the diffraction peak revealed that this did not have any effect on the orbital ordering.

Sarnjeet Dhesi, Principal Beamline Scientist for the Nanoscience beamline says, “These results are very interesting because they show the first direct evidence of orbital rotations that are insensitive to structural changes. In the future, synchrotrons and FELs will be used to understand the ultrafast dynamics of spin and orbital ordering. using, for instance, time resolved soft X-ray resonant diffraction. This is an area we are actively developing at Diamond ”

T. A. W. Beale, S. R. Bland, R. D. Johnson, P. D. Hatton, J. C. Cezar, S. S. Dhesi, M. v. Zimmermann, D. Prabhakaran, and A. T. Boothroyd, Thermally induced rotation of 3d orbital stripes in Pr(Sr0.1Ca0.9)2Mn2O7, Phys Rev B 79, Issue 5
DOI: 10.1103/PhysRevB.79.054433