Electrochemical approach to cross magnetic phase boundaries

The basic idea of this project is to electrochemically (slightly) change the composition of a material, which is close to a compositional magnetic phase transition. In the ideal case, this way one can reversibly switch between states with and without net magnetisation. The total set-up and operation scheme resembles a battery, where one electrode is the material of interest.

Our model system is La_xSr_yMnO₃ because of its room temperature FM/PM transition for x = y = 0.5. The essential mechanism for this transition is the ordering of Mn³⁺ and Mn⁴⁺ ions at. Doping the system with Li shifts the ratio of this two ions and  thus the condition for ordering. Figure 1 shows this concept in an artistic way.

We succeeded to synthesize thin films of La_xSr_yLi_zMnO₃, either with z=0, x+y=1 on the PM side of the transition, or with x+y+z=1 on the FM side. These were used as electrodes battery-like set ups, both with liquid and solid electrolytes. Magnetometry and x-ray absorption spectroscopy both showed a partial reversibility of the lithiation state, and polarised neutron reflectometry revealed a change in the mean magnetic moment of Mn of up to 1 μ_B. It is not clear jet, if this is associated with complete switching.

Further neutron reflectometry (NR) studies are planned. NR is ideally suited for this problem since it can probe the net magnetisation and the compositional changes with a high spatial resolution during operation.