In diesem Projekt wurden die Verän­de­run­gen von magne­ti­schen Eigen­schaf­ten bei Materia­lien auf der Nanome­ter­ebene unter­sucht. Mittels Mikro­emul­si­ons­syn­these konnten Nanopar­ti­kel herge­stellt werden. Es konnte ein direk­ter Zusam­men­hang zwischen den magne­ti­schen Eigen­schaf­ten und der Größe des Materi­als nachge­wie­sen werden.

The exact position of atoms in the crystal struc­ture of lantha­num mangani­tes and cobal­ta­tes (both anorga­nic ionic compounds) signi­fi­cantly affects their magne­tic proper­ties. The crystal struc­ture of these materi­als can be altered by pressure, substi­tu­tion of elements, or crystal­lite size: Since nanopar­tic­les have a large surface-to-volume ratio, their surface has a dominant effect on the crystal struc­ture, leading to changes compared to bulk materi­als. The varia­tion in size can thus be used to induce varia­ti­ons in magne­tic transi­tion tempe­ra­ture and magne­tic ordering.

Having produ­ced nanopar­tic­les of La1-xSrxM­nO3 and LaCoO3 by micro­emul­sion synthe­sis, it has been shown for La1-xSrxM­nO3, that decre­asing the nanopar­ticle size leads to an expan­sion of the unit cell, proba­bly due to surface adsor­ba­tes. The elonga­tion of the bond length results in a decrease of the magne­tic transi­tion tempe­ra­ture, whereby the mecha­nism is due to both reduc­tion of double-exchange coupling and intrin­sic size effect (see Figure). In LaCoO3, on the other hand, the elonga­tion of the Co‑O bond length induces a change in the Co3+ state from low-spin to high-spin, in contrast to bulk LaCoO3 (low spin at low tempe­ra­tures). In contrast to previously used prepa­ra­tion techni­ques for transi­tion-metal-oxide nanopar­tic­les, the micro­emul­sion method allows control of the nanopar­ticle size via a simple mixing ratio. Hence, crystal quality, oxygen stoichiome-try, and defect density are constant throug­hout the sample and changes in the magne­tic proper­ties can be directly related to size.