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© Cornelia E. Hintze

Magnet­ism in Perovskite Mangan­ites and Cobaltites at the Nano Scale

Cornelia E. Hintze – Hector Fellow Hilbert von Löhneysen

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.

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Decreas­ing nanopar­ti­cle size i.e. increas­ing surface-to-volume ratio leads to an elonga­tion of the Mn/Co‑O bond length and a reduc­tion of the Mn/Co‑O bond angle (not to scale). These changes result in a size-depen­dent increase of the magnetic moment in LaCoO3 and a size-depen­dent decrease of the ferro­mag­netic transi­tion temper­a­ture in La1-xSrxMnO3

Cornelia E. Hintze

Karlsruhe Insti­tute of Technology

Super­vised by

Prof. Dr.

Hilbert von Löhneysen


Hector Fellow since 2011Disziplinen Hilbert von Löhneysen