Magnetic properties of the intercalated compound ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$

We have carried out a comprehensive investigation of the magnetic properties of the intercalated ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$ by the means of dc magnetization, polarized neutron scattering, and x-ray magnetic circular dichroism (XMCD) measurements. The results obtained by these techniques support a spin order in each of the Fe-O bilayers that is the same as observed in the nonintercalated $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$. In the Fe-O single layers, a magnetic moment appears to be induced by the application of a magnetic field, paramagnetic-like in the first approximation. Furthermore, a spin-charge coupling was revealed by the XMCD measurements.

Figure 1

Crystal structure of (a) rhombohedral $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ with three Fe-O bilayers in the unit cell, and (b) hexagonal ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$ with two Fe-O bilayers and one monolayer in the unit cell. The inset shows the projection of one bilayer in the $ab$ plane.

Figure 2

Temperature dependence of magnetization measured with a field of $100\mathrm{Oe}$ applied parallel to the $c$ direction during cooling (FC), warming (FW), and warming after zero-field cooling (ZFC) in a temperature range of 10–$300\mathrm{K}$ for different ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$ single crystals. SC1 and SC3 are scaled for better visibility. $T_{\mathrm{C}}$ is assigned by the steepest slope in the FC curve, $T_{\mathrm{f}}$ by the maximum of the ZFC curve.

Figure 3

(a) Temperature-dependent magnetization during field cooling and followed by field warming at different magnetic fields for SC3. FC, and FW perfectly overlap for all used fields. (b) Low-temperature magnetization obtained by cooling in different magnetic fields for ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$ (squares) and for $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ (circles). For $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ also shown (small diamonds) is $M\left(H\right)$ obtained in decreasing $H$ after cooling in $9\mathrm{T}$ (taken from the full $M\left(H\right)$ loop shown in Fig. 4.5(b) in [18]).

Figure 4

$M(H\parallel c$) loops measured on SC3 at different temperatures. Inset: Hysteresis loop including the virgin curve measured after cooling in zero field at $160\mathrm{K}$.

Figure 5

(a) Temperature dependence of thermoremanent magnetization of ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$ after cooling in two different external fields with $2\mathrm{K}$/min and the corresponding field-cooling curves. (b) Total moment at low temperature from Fig. 3 and linear fitting (red line) of FC-TRM (red circles). The subtraction of the modeled single-layer contribution from the total moment results in the tentative bilayer contribution (blue curve).

Figure 6

H-T “phase” diagram for SC3 of ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$ extracted from different DC measurements. Three magnetic “phases” are labeled PM, fM, CG; see text for labeling. The red circles were obtained from the FC measurements in different fields and the cyan squares from ZFC measurements in different fields. Arrows indicate the direction of the temperature change for which a feature occurs. The blurred blue area indicates that the transition CG-fM is arrested below $\sim 60$ K.

Figure 7

Neutron scattered intensity in the ($hh\ell$) plane at $4.2\mathrm{K}$ in spin-flip [(a), (c)] and non-spin-flip [(b), (d)] with neutron polarization (a), (b) parallel to $\vec{Q}$ direction and (c), (d) perpendicular to scattering plane for five detector positions and counting time 24 s each position (sample SC1). When the polarization of the neutrons is parallel to the scattering vector, both in-plane and out-of-plane components of the magnetic contribution are measured in the spin-flip (SF) channel and the full nuclear coherent contribution is measured in the non-spin-flip (NSF) channel, i.e., structural reflections ($00\ell$) with $\ell =2n$.

Figure 8

Diffuse magnetic scattering at the ($\frac{1}{3}\frac{1}{3}\ell$) line (intensity averaged along $hh0$ from 0.25 to 0.4) in the spin-flip channel for neutron polarization parallel to $\vec{z}$ direction at 4.2 K. The smoothed data (blue) and the intensity model for the random distribution of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ (green line, squares: with, without Debye-Waller factor) are also shown. Background is subtracted for the measured data.

Figure 9

(a) Diffuse magnetic scattering averaged along the $\ell$ direction in the range $\ell =-5$ to $\ell =5$ in the spin-flip channel measured with $\vec{P}\parallel {\vec{Q}}_{\mathrm{average}}$ vs $hh0$ for selected temperatures. The data were fitted with a Lorentzian (solid curves) and the background is subtracted for all curves. The inset shows the temperature dependence of both integrated and peak intensities. The peak intensities of [20, 22] are also shown for comparison. (b) Correlation length and full widths at half maxima extracted from the data fitting for various temperatures.

Figure 10

(a) XAS spectrum $[\mathrm{XAS}=({\mu }_{-}+{\mu }_{+})/2]$ of stoichiometric ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$ and $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ crystals in a field of 4 T normalized to the peak maximum. (b) The corresponding XMCD signals $[\mathrm{XMCD}=({\mu }_{+}-{\mu }_{-})]$. The $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ XMCD signal was taken from [18] and scaled through division by 1.75.