Magneto-optics of complex oxides at terahertz frequencies

Thesis

This thesis presents experimental results on two complex oxide systems: Cu_1-xZn_xO and La_0.7Sr_0.3MnO₃:ZnO. The dynamic magnetoelectric response of these materials is obtained using terahertz time-domain spectroscopy, supported by Fourier-transform infrared spectroscopy, Raman spectroscopy and X-ray diffraction.

Evidence for an electromagnon in the multiferroic phase of CuO is presented for the first time. This high temperature (213-230K) electromagnon is linked to intersublattice exchange between two Cu sublattices. The temperature dependence of a magnon in the collinear antiferromagnetic phase is indicative of biquadratic exchange.

Broadening of the multiferroic phase on substitution of copper with zinc is reported along with a 25% depression of the Néel temperature due to spin dilution. Phonons and magnons broaden and shift in energy on alloying. However, the electromagnon is relatively insensitive, increasing in energy without widening. This indicates that electromagnons and dynamic magnetoelectric coupling can be mantained even in disordered spin systems. Strong spin-phonon coupling is present in both magnetically ordered phases as shown by the anomalous behavior of the A³_u phonon at T_N1 and a Raman-active mode associated with a magnetic modulation of a zone-folded acoustic phonon.

Dynamic 1THz magnetoresistance is found to be significantly larger than static magnetoresistance in La_0.7Sr_0.3MnO₃:ZnO vertically-aligned nanocolumns on LaAlO₃ substrates. The metal-insulator transition temperature is determined to be 300 K. Temperature dependent static and dynamic resistivity and magnetoresistance are discussed in terms of strain and grain boundary effects. Negative photoconductivity is observed and the dynamic response analysed.

Authors: Jones, S; Samuel Peter Philip Jones

• Publication date: 2014
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: multiferroic; terahertz spectroscopy; magnon; electromagnon
• Subjects: Condensed Matter Physics