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

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 ${\mathrm{NiFe}}_2{\mathrm{O}}_4$. 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 the magnetic field. These findings open the door for the creation and control of spin-polarized excitations from minority channel charge transfer in spinel ferrites.

Figure 1

(a, b) Crystal structure of ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ showing the spin configuration at 0 T and above $B_{c\left(\mathrm{Ni}\right)}$ where the Ni spin is flipped to align with the field. (c, d) Projected density of states (DOS) from hybrid functional calculations [12] depicting Ni $\left(O_h\right)$ $\to$ Fe $(O_h$ and $T_d)$ charge transfer excitations in the minority and majority channels for the two spin configurations of interest.

Figure 2

(a) MCD spectra of ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ at $\pm 10$ T along with the linear absorption. The points on the energy axis define the band gaps [12], and the shaded regions emphasize the excitation character in each energy window. (b) Derivative of $I_{\mathrm{MCD}}$, along with inset emphasizing the spectral asymmetry near 1.57 eV and 100 meV splitting. (c) Comparison of experimental and theoretical MCD spectra (with a rigid shift of $-0.6$ eV). (d) MCD intensity at constant energies vs field. The dashed lines guide the eye. Magnetization (in orange) is included for comparison [9]. (e) Residual MCD signal obtained from $\mathrm{\Delta }{I}_{\mathrm{MCD}}$ in the positive and negative field directions along with the corresponding theoretical difference between the calculated MCD response when Ni spin is parallel to Fe $\left(O_h\right)$ vs Fe $\left(T_d\right)$ moments.

Figure 3

(a) Photoconductance of ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ measured at a series of illumination energies compared with the absorption spectrum. (b) Example $I\text{−}V$ curves taken using a broadband xenon lamp. (c) Example $I\text{−}V$ curves using a combination of light (2.0 eV) and magnetic field $(\approx 1.5$ T) as indicated. Magnetoresistance measurements for the light off state are included for completeness. (d) Field-induced changes in photoconductivity are displayed as magnetoresistance. The blue line guides the eye. The teal dots on the energy axis indicate band-gap positions, the shaded regions emphasize the character of the excitations in each energy window, and the dashed horizontal dark green line denotes the intrinsic magnetoresistance [7]. The schematic shows the measurement geometry.