Canted ferromagnetic order in nonsuperconducting $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$

The magnetic order in a series of nonsuperconducting $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ single crystals is investigated in detail by polarized neutron diffraction. The Fe and Eu magnetic sublattices are found to be almost magnetically decoupled in this system. With the Ni doping, the Eu sublattice shows a dramatic change in its magnetic ground state, from the A-type antiferromagnetic order for $\mathit{x}\le 0.02$, in which the spins lie in the $\mathit{ab}$ plane, to a pure canted ferromagnetic order for $0.04\le \mathit{x}\le 0.20$, in which the spins are canted with a small angle off the $\mathit{c}$ axis. By comparison with the $\mathit{c}$-axis perfectly aligned ferromagnetic structure widely observed in other superconducting compounds, the lack of superconductivity in $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ might be associated with the formation of in-plane ferromagnetism. The doping-induced change of the lattice constants and accordingly the variation of the strength of indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments is speculated to be responsible for the dramatic change of the Eu spin structure. In addition, a spin reorientation of the ${\mathrm{Eu}}^{2+}$ moments in an intermediate temperature range is observed for $\mathit{x}=0.20$, in which some in-plane spin fluctuations persists above the long-range ordering temperature $T_{\mathrm{Eu}}$.

Figure 1

Temperature dependences of the normalized in-plane resistivity (a) and molar specific heat (b) of the $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ single crystals. The inset in (b) enlarges the low-temperature specific heat around $T_{\mathrm{Eu}}$. The vertical dashed lines and arrows mark the Fe-SDW order and the magnetic order of Eu, respectively. The specific-heat data of the parent compound ($x=0$) is taken from Ref. [33] for a comparison.

Figure 2

Temperature dependences of the peak intensities of the $(1,0,3)$ magnetic reflection associated with the Fe-SDW order in $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ crystals with $\mathit{x}=0.02$, 0.04, and 0.06, respectively. The vertical arrows mark $T_{\mathrm{SDW}}$, the onset temperature of the Fe-SDW order. The dashed lines represent the fittings using a power law for $\mathit{T}<{\mathit{T}}_{\mathrm{SDW}}$ and the background for $\mathit{T}>{\mathit{T}}_{\mathrm{SDW}}$.

Figure 3

Rocking-curve scans of the representative reflections in $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ crystals with $\mathit{x}=0.02$ (a)–(b), 0.04 (d)–(e), 0.06 (g)–(h), and 0.20 (j)–(k), collected in the ${\mathit{x}}_{\mathrm{SF}}$ channel (diamonds) and ${\mathit{x}}_{\mathrm{NSF}}$ channel (squares), at 3.6 K (solid symbols) and 25 K (open symbols), respectively. The total intensity (circles) as the sum of those in the two channels and the fittings using a Gaussian profile (solid lines) are also shown. (c), (f), (i), and (l) show the temperature dependences of the peak intensities recorded in the ${\mathit{x}}_{\mathrm{SF}}$ channel for representative magnetic reflections. The counting time is 300 s for $(0,0,3)$ in (c) ($\mathit{x}=0.02$), 120 s for $(2,0,0)$ and $(0,0,2)$ in (f) ($\mathit{x}=0.04$), 300 s for $(2,0,0)$ and 60 s for $(0,0,2)$ in (i) ($\mathit{x}=0.06$), and 300 s for $(2,0,0)$ and 60 s for $(0,0,2)$ in (l) ($\mathit{x}=0.20$). The vertical dashed lines mark $T_{\mathrm{Eu}}$, the magnetic ordering temperature of Eu.

Figure 4

The ground-state ($\mathit{T}$ = 3.6 K) magnetic structure of $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ for $\mathit{x}\le 0.02$ (a), $\mathit{x}=0.04$ (b), $\mathit{x}=0.06$ (c), and $\mathit{x}=0.20$ (d), as determined by polarized neutron diffraction. The ${\mathrm{Eu}}^{2+}$ spins in (b)–(d) are canted off the $\mathit{c}$ axis with angles of $7.7(\pm 0.6){}^{\circ }, 11.5(\pm 0.3){}^{\circ }$, and $13.2(\pm 1.2){}^{\circ }$, respectively.

Figure 5

The phase diagram of the $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ system for $\mathit{x}\le 0.20$, illustrating the evolution of the magnetic order with Ni doping. The $T_{\mathrm{SDW}}$ and $T_{\mathrm{Eu}}$ values for the parent compound ($\mathit{x}=0$) are obtained from Ref. [5]. The yellow, red, and blue regions represent the Fe-AFM state, A-type AFM state of Eu, and canted FM state of Eu, respectively.