Magnetic properties of Ruddlesden-Popper phases ${\mathbf{Sr}}_{3-x}{\mathbf{Y}}_x\left({\mathbf{Fe}}_{1.25}{\mathbf{Ni}}_{0.75}\right){\mathbf{O}}_{7-\delta }$: A combined experimental and theoretical investigation

We present a comprehensive study of the magnetic properties of ${\mathrm{Sr}}_{3-x}{\mathrm{Y}}_x\left({\mathrm{Fe}}_{1.25}{\mathrm{Ni}}_{0.75}\right){\mathrm{O}}_{7-\delta }\left(0\le x\le 0.75\right)$. Experimentally, the magnetic properties are investigated using superconducting quantum interference device (SQUID) magnetometry and neutron powder diffraction (NPD). This is complemented by a theoretical study based on density functional theory as well as the Heisenberg exchange parameters. Experimental results show an increase in the Néel temperature $\left(T_N\right)$ with an increase of Y concentrations and O occupancy. The NPD data reveal that all samples are antiferromagnetically ordered at low temperatures, which has been confirmed by our theoretical simulations for the selected samples. Our first-principles calculations suggest that the three-dimensional magnetic order is stabilized due to finite interlayer exchange couplings. The latter give rise to finite interlayer spin-spin correlations, which disappear above $T_N$.

Figure 1

Crystal structure of the ideal $A_3{B}_2{\mathrm{O}}_7$ RP structure. There are two different sites for the alkaline-earth or rare-earth elements (A1 and A2) and three different types of oxygen sites (O1, O2, and O3) in the crystal structure. The octahedra surrounding the transition-metal elements are shown to visualize the environment of the magnetic elements.

Figure 2

Top: Field-cooled magnetization $M_{\text{FC}}$ (left-hand scale) and $\partial M_{\text{FC}}/\partial T$ (right-hand scale) vs temperature $T$ for the $x=0.3$ sample. The applied magnetic field was $H=80\mathrm{kA}/\mathrm{m}$. Bottom: $T_N$ vs Y concentration.

Figure 3

Magnetization $M$ vs magnetic field $H$ for samples with different Y concentrations $x$. The inset shows the magnetization curve for the sample with $x=0.6$, showing a weak anomaly at small fields corresponding to a small amount $(\approx 0.25$ wt. %) of Ni impurity phase.

Figure 4

$T_N$ vs occupancy of the O3 site (lower scale; blue circles) and $c$-axis length (upper scale; red squares). The numbers in the plot correspond to the $x$ values in that point.

Figure 5

Neutron powder diffraction pattern at $T=10\mathrm{K}$ for the $x=0.75$ sample. Solid lines and symbols indicate measured and calculated patterns, respectively. The difference between observed and calculated patterns is shown at the bottom. The bars indicate the positions of the chemical (top) and magnetic (bottom) Bragg reflections.

Figure 6

Left: Positions of the different $3d$ atoms in the RP structure for ${\mathrm{Sr}}_{2.25}{\mathrm{Y}}_{0.75}\left({\mathrm{Fe}}_{1.25}{\mathrm{Ni}}_{0.75}\right){\mathrm{O}}_{7-\delta }$ are indicated as well as the intralayer $\left(J_1\right)$ and interlayer $(J_2$ and $J_3)$ exchange interactions. Right: Magnetic structure obtained by Rietveld refinement of neutron powder diffraction data.

Figure 7

Exchange parameters between an atom shown in the legend and all its magnetic neighbors as a function of distance for $x=0.50$ (left panel) and $x=0.75$ (right panel). For comparison, the atoms belonging to the same plane are shown together.

Figure 8

Temperature dependence of the normalized spin-spin correlation function as a function of distance per lattice constant $a\left(R/a\right)$ with the intralayer $\left(J_1\right)$ and interlayer $(J_2$ and $J_3)$ exchange couplings for the case of interactions between ${\mathrm{Fe}}_3$ and its neighbors. Simulations are performed for the alloy concentration with $x=0.5$.

Figure 9

Normalized spin-spin correlation function as a function of distance per lattice constant $a\left(R/a\right)$ for the case of interactions between ${\mathrm{Fe}}_3$ and its neighbors. The simulations are performed for the $x=0.5$ Y concentration for various strengths of the $J_3$ parameter.

Figure 10

Normalized specific heat as a function of temperature for various strengths of the $J_3$ parameter, for the $x=0.5$ Y concentration.