Spin reorientation in ${\text{NdFe}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_3$: Neutron scattering and ab initio study

The structural, magnetic, and electronic properties of ${\mathrm{NdFe}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_3$ have been studied in a comprehensive manner using various experimental and theoretical methods. Mn/Fe sublattice of the compound orders into a G-type antiferromagnetic phase close to 250 K where the magnetic structure belongs to ${\mathrm{\Gamma }}_1$ irreducible representation ($G_y$) unlike ${\mathrm{\Gamma }}_4$ ($G_x$) representation observed in most of the orthoferrites and orthochromites. The magnetic structure undergoes a complete spin reorientation transition with temperature between 75 and 25 K where the magnetic structure exists as a sum of two irreducible representations (${\mathrm{\Gamma }}_1+{\mathrm{\Gamma }}_2$). At 1.5 K, the antiferromagnetic structure belongs entirely to ${\mathrm{\Gamma }}_2$ representation ($G_z$) with a small ferromagnetic component ($F_x$) due to Nd. The unusual spin reorientation and correlation between magnetic ground state and electronic structure have been investigated using phenomenological theory and first-principles calculations within GGA+$U$ and GGA+$U$+SO formalisms.

Figure 1

Observed x-ray diffraction pattern of NFMO at room temperature, refined using orthorhombic $Pbnm$ space group.

Figure 2

Observed and refined neutron diffraction patterns of NFMO at room temperature confirm formation of the orthorhombic structure with $Pbnm$ space group. The measurement was performed at PD-II using neutron wavelength of $1.2443\AA$.

Figure 3

ZFC-FC plots of NFMO from 2 to 350 K showing Néel temperature $\left(T_{\mathrm{N}}\right)$ at 250 K and weak ferromagnetism below 70 K.

Figure 4

$M\text{−}H$ plots of NFMO at (a) 5, (b) 10, (c) 25, (d) 45, (e) 70, (f) 100, (g) 200, and (h) 300 K.

Figure 5

Variation of retentivity with temperature in NFMO.

Figure 6

Evolution of magnetic peak in neutron diffraction data as a function of temperature. These temperature dependent measurements were performed at PD-I ($\lambda =1.094\AA$).

Figure 7

Experimental and refined neutron diffraction patterns of NFMO for various temperatures from 1.5 to 90 K measured at D1B(CRG). Inset shows the changes in the magnetic Bragg peak as a function of temperature. Inset in (e) Weak magnetic signal at ${38}^{\circ }$ due to Nd magnetism ($F_{\mathrm{x}}$).

Figure 8

Magnetic structure of NFMO represented by (a) ${\mathrm{\Gamma }}_1$ space group at $\gtrsim 70$ K and (b) ${\mathrm{\Gamma }}_2$ space group at 4 K.

Figure 9

Temperature variation of flipping ratio ($R$) under an applied magnetic guide field of 50 Oe for NFMO. Polarized neutron spectrometer was used for the measurement.

Figure 10

Three possible structural configurations of Fe and Mn ions considered in our calculations: (001) (left), (110) (middle), and (111) (right), respectively.

Figure 11

Spin-polarized DOS of Mn and Fe for the three cationic orderings within GGA+$U$ ($U=6.95$ eV and $J=0.95$ eV).

Figure 12

Comparision of total energies for Fe/Mn spins pointing along $a$, $b$, and $c$ directions in case of NFMO and ${\mathrm{NdFeO}}_3$ for (a) high-temperature and (b) low-temperature phases, respectively.