Magnetic field tunability of spin-polarized excitations in a high-temperature magnet

Abstract

We bring together magnetic circular dichroism, photoconductivity, and complementary first principles calculations in order to unravel spin-charge interactions in the high Curie temperature magnet NiFe2O4. Analysis uncovers a massive set of well-isolated spin down states, a metamagnetic transition involving spin on the Ni center that switches the electronic structure of this system, and photoconductivity that depends on magnetic field. These findings open the door for the creation and control of spin-polarized excitations from minority channel charge transfer in spinel ferrites.

Magnetic field control of charge excitations in CoFe2O4

“…In NiFe 2 O 4 , electronic structure calculations reveal that the Ni states reside in either the minority or majority channel depending on whether spins are in the ↓↓↑ or ↑↓↑ state. 40 A similar swap of the Co density of states is anticipated here as the magnetic field is swept across B c ,Co . Just as I MCD quantifies the number of states involved in Co → hybridized Fe( O h ) + Co excitations (with Co charge accessing a different set of states above the Fermi level depending on the field direction), Δ I MCD reveals the small fraction of excitations that are spin independent and insensitive to field reversal.…”

“…The MCD spectrum of CoFe 2 O 4 is similar in magnitude to that of NiFe 2 O 4 , 40 although in the Ni analog, the oscillator strength and the series of bandgaps are pushed to higher energies. The excitations in CoFe 2 O 4 thus have a much better overlap with the solar spectrum from both a bandgap and density of state perspective.…”

“…The Ni compound also has less structure in σ PC . 40 Exchange splitting is the origin of spin-dependent excitations in the ferromagnetic insulator Y 3 Fe 5 O 12 as well. 40,50 …”

Site-specific spectroscopic measurement of spin and charge in (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices

“…The on-site d -to- d excitations of Fe 3+ have much lower intensity (Table 1 ). Importantly, features in the dichroic spectra Δ α ( E ) M C D are directly proportional to net magnetization, and since we can analyze this effect at different energies, the response can be correlated with specific iron centers 34 – 36 .…”

“…The dichroic response can be modeled using the calculated matrix elements of the optical conductivity tensor as 36 , 43 : Here, is the refractive index of RCP or LCP light arising from the dielectric function ϵ , d is the film thickness, and c is the speed of light. The theoretical predictions for both the parent compounds and the (3, 1) superlattice were made based on this equation.…”