In order to investigate spin-charge coupling in multiferroic oxides, we measured the optical properties of BiFeO 3 . Although the direct 300 K charge gap is observed at 2.67 eV, absorption onset actually occurs at much lower energy with Fe 3+ excitations at 1.41 and 1.90 eV. Temperature and magnetic-field-induced spectral changes reveal complex interactions between on-site crystal-field and magnetic excitations in the form of magnon sidebands. We employ the sensitivity of these magnon sidebands to map out the magnetic-fieldtemperature phase diagram which demonstrates optical evidence for spin spiral quenching above 20 T and suggests a spin domain reorientation near 10 T.

We investigated the series of temperature and field-driven transitions in LuFe2O4 by optical and Mössbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the "order by fluctuation" mechanism for the development of charge order superstructure. Bragg splitting and large magneto-optical contrast suggest a low-temperature monoclinic distortion that can be driven by both temperature and magnetic field.

Particles of formula Rb0.24Co[Fe(CN)6]0.74@K0.10Co[Cr(CN)6]0.70·nH2O with a light-responsive rubidium cobalt hexacyanoferrate (RbCoFe) core and a magnetic potassium cobalt hexacyanochromate (KCoCr) shell have been prepared and exhibit light-induced changes in the magnetization of the normally light-insensitive KCoCr shell, a new property resulting from the synergy between the core and shell of a coordination polymer heterostructure. A single batch of 135 ± 12 nm RbCoFe particles are used as seeds to generate three different core@shell samples, with KCoCr shell thicknesses of approximately 11, 23 and 37 nm, to probe the influence of the shell thickness over the particles' morphology and structural and magnetic properties. Synchrotron powder X-ray diffraction reveals that structural changes in the shell accompany the charge transfer induced spin transition (CTIST) of the core, giving direct evidence that the photomagnetic response of the shell is magnetomechanical in origin. The depth to which the KCoCr shell contributes to changes in magnetization is estimated to be approximately 24 nm when using a model that assumes a constant magnetic response of the core within the series of particles. In turn, the presence of the shell changes the nature of the CTIST of the core. As opposed to the usually observed first order transition exhibiting hysteresis, the CTIST becomes continuous in the core@shell particles.

We investigated magnetoelastic coupling through the field-driven transition to the fully polarized magnetic state in quasi-two-dimensional [Cu(HF2)(pyz)2]BF4 by magnetoinfrared spectroscopy. This transition modifies out-of-plane ring distortion and bending vibrational modes of the pyrazine ligand. The extent of these distortions increases with the field, systematically tracking the low-temperature magnetization. These distortions weaken the antiferromagnetic spin exchange, a finding that provides important insight into magnetic transitions in other copper halides.