Effect of Spin Dilution on the Magnetic State of Delafossite CuFeO2 with an S = 5/2 Antiferromagnetic Triangular Sublattice
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This work describes the scandium doping effect on the structural and magnetic properties of delafossite-type oxides CuCr1−x Sc x O2. The lattice parameters were found to vary according to Vegard’s low. A reflection broadening is observed that is ascribed to local lattice distortion due to the ionic radius difference between Fe3+ and Sc3+. Magnetic susceptibility measurements show that the dominant interactions are antiferromagnetic (AFM) but that doping induces significant changes. A rather monotonous T N2 decrease as x increases, from T N2 = 9.1 K down to T N2 = 5.9 K for x = 0.00 to 0.25, respectively, but T N1 remains almost unchanged. The Sc substitution (S = 0) at the Fe sites (S = 5/2) tend to suppress the low ferromagnetic interactions in the magnetic structure; however, the magnetic exchange coupling J BB decreases under 5.0 meV with increasing x. The electric polarization decreases with x up to 63 μC/m2 for 5 %-Sc to 22 μC/m2 for 25 %-Sc.
KeywordsDelafossite Powder diffraction Raman spectroscopy Magnetic properties Electric polarization
Geometrically frustrated magnetic systems have received considerable attention in recent years due to the presence of extraordinary magnetic properties [1, 2]. Geometric frustration is a broad phenomenon that results from an intrinsic incompatibility between some fundamental interactions and the underlying lattice geometry usually based on triangles or tetrahedra. The delafossite CuFeO2 is of particular interest because of the discovery of multiferroic phenomena with either application of a magnetic field or the substitution of Fe3+ with nonmagnetic Al3+ ions [3, 4]. As a model material of triangular lattice antiferromagnet (TLA), CuFeO2 forms an Ising-like 4-sublattice antiferromagnetic order at low temperature, with spin moment pointing along the c-axis . This TLA is one of typical models to test the resonating valence bond idea of a 2D spin liquid state which was first proposed by Anderson  and was applied to the theory of high- T c superconductivity [7, 8]. The so-called 120∘ spin structure is realized in the ATL Heisenberg model with the nearest neighbor bilinear coupling , while the actually realized phase is still controversial if the quantum fluctuation is enhanced by low dimensionality, quantum spin nature, and geometrical frustration [10, 11]. From experimental view points, no material has been confirmed to show a spin liquid state till now, but some ATL and related lattice compounds [12, 13] are proposed to demonstrate the quantum state.
In order to clarify the ground state and explore a novel phenomenon in such quasi-2D ATL compounds, it is useful to investigate impurity effects on the electronic and magnetic ground states, because, in strongly correlated electron or frustrated systems, exotic phenomena are often induced by changes of filling, bandwidth , and quenched random field [15, 16], with a substitution of impurities. In such a context, we have investigated various substitution effects on the electronic, magnetic, and thermal properties of delafossite oxide CuFeO2 .
CuFeO2 is one of the quasi-2D ATL compounds. It forms a rhombohedral lattice with the space group of R-3m, viewed as the alternating stacking of edge-shared FeO6 octahedral (FeO2) layers and Cu layers . The Fe3+? ions (3 d 5, S = 5/2) form antiferromagnetic (AF) triangular sublattices and the compound shows a noncollinear 120° indicating they exist of the antiferromagnetic transitions in CuFeO2, from paramagnetic to collinear incommensurate T N1 = 14 K and then collinear commensurate below T N2 = 11 K [18, 19].
In this work, we report the preparation, structural characterizations and physical properties, and magnetic and electric polarization of several polycrystalline compounds belonging to the delafossite CuFe1−x Sc x O2 series. It is found that this introducing of the Sc into the Fe lattice sites in alternating ab layers has a significant effect on the structure. The occupied Fe 3d states interact covalently with the neighboring O atoms and hence indirectly modify the Cu 3d states, an effect which the 3 p 6 Sc atoms are unable to produce. This decreases the density of states at the top of the valence band, which are precisely those states expected to determine the mobility of p-type charge carriers formed on doping.
Polycrystalline samples of CuFe1−x Sc x O20 ≤ x ≤ 0.25 were prepared by using the standard solid-state reaction. Stoichiometric mixtures (0.5 g) of Cu2O, Fe2O3, and Sc2O3 were ground and pressed in pellet, which were set in alumina crucible. The samples were fired several times at 1050 °C for 12 h. The X-ray powder diffraction patterns of the reacted pellets were collected with a PANalytical diffractometer equipped with a Cu K α source (K α1 and K α2) in the 2θ range from 10° to 90° at room temperature.
Magnetization dependence on temperature was measured in a superconducting quantum interference device (SQUID) magnetometer while heating from 4.0 to 300 K in 0.1 T.
The Raman spectra were recorded at room temperature with the 514.5 nm line of an Ar+ laser, excitation from a Spectra-Physics krypton ion laser. The compounds were studied with a low laser power (102 mW). One scanning of 60 s has been used for each spectral range. No damage of the material by the laser has been observed. The beam was focused onto the samples using the macroscopic configuration of the apparatus.
3 Results and Discussion
3.1 Structural Properties
Finally, we note that the oxygen stoichiometry cannot be reliably obtained by X-ray diffraction data, and the presence of three different mixed valences precludes a reliable use of chemical redox titration. However, all the samples being prepared in the same conditions (initial oxygen stoichiometry, amount of powder) and their oxygen content are assumed to close to 2 in all cases. This assumption is supported by a previous study of CuFeO2 showing that this compound does not accommodate large oxygen of stoichiometry . This could be confirmed by neutron diffraction, which was not available to us during this study.
3.2 Raman Spectroscopy
The delafossite structure belongs to point group C 3v and space group \(R\overline 3 m\). The four atoms in the primitive cell of its rhombohedral (\(R\overline 3 m\)) structure give rise to 12 optical phonon modes in the zone center (k∼0), among them three are acoustic and nine are optical. Γopt.R3c = A 1g + E g + 3A 2u + 3E u, out of which two phonons with A 1g and E g symmetry are Raman active. Γopt.R−3m = A 1g + E g . A 1g modes represent the vibration of Cu–O bonds along the c-axis whereas the doubly degenerate E-modes describe the vibration along the a-axis. Since there is only one mode of each symmetry, the exact eigenvector is determined without any lattice dynamical model required. Pellicer-Porres et al.  had discussed the phonon dispersion at the zone center for CuGaO2 delafossite. They proposed that the inversion center is lost along the direction of Γ(T) direction and the symmetry is reduced from D 3d to C 3v . According to compatibility relations, A 1g and A 2u modes transform to A 1 modes and E g and E u modes become E modes.
3.3 Magnetic Properties
Estimated Curie constant, Curie temperature, collinear commensurate T N2, and effective moments from the high-temperature paramagnetic region for CuFe1−x Sc x O2
C (emu mol−1 K−1)
μ eff(exp) (μ B)
μ eff(th) (μ B)
The spin dynamics of the geometrically frustrated triangular antiferromagnet multiferroic CuFeO2 has been mapped out using inelastic neutron scattering . They determined the relevant spin Hamiltonian parameters, showing that the sinusoidal model with a strong planar anisotropy correctly describes the spin dynamics. The weakly dispersive excitation along c reflects the 2D character of the magnetic interactions, but the spin dynamics in CuFeO2 clearly point out the relevance of the next-nearest-neighbor interaction to stabilize the magnetic order. As xincreases in CuFe1−x Sc x O2, in addition to strain effects, the number of magnetic nearest neighbor’s decreases and the magnetic ordering affect the coupling between in-plane next-nearest-neighbors interpreted as the signature of an important deformation of the perfect triangular lattice.
Correlating the increases in lattice parameters a in CuFe1−x Sc x O2 system to the nearest-neighbor magnetic exchange coupling J BB taken from magnetic structure (inset of Fig. 7) which is close to 5.0 meV for the mother CuFeO2 . The Sc substitution (S = 0) at the Fe sites (S = 5/2) tend to suppress the low ferromagnetic interactions in the magnetic structure J 1(1) and J 1(2) ; however, the magnetic exchange coupling J BB decreases under 5.0 meV with increasing x. In the other hand, the increases in lattice parameters a which is due to the substitution at the Fe sites (r (Fe3+) = 0.64 Å) with larger atoms of Sc3+ (r (Sc3+) = 0.72 Å) tend to decrease the unstable AFM interactions through the incommensurate modulated structure (J 1(3), J 2, and J 3)  and will becomes increasingly stable. This decrease for the low FM and AFM interactions is a result of the mean field approximation breaking down as Fe composition decreases and next-nearest-neighbor interactions become less, and tend to suppress the magnetic order in CuFe1−x Sc x O2 system with increasing x content. The magnitude of J BB is similar to previous studies of Fe–O–Fe exchange in structurally similar spinels with edge-sharing octahedra of Fe [3, 37, 38]. A ZFC splitting occurs in the x = 0.25 sample below 4 K, which is consistent with splittings seen in low Al content samples studied by Okuda et al. . Such behavior is attributed to chemical disorder that results in spin-glass behavior.
3.4 Electric Polarization
The polarization of the CuFe1−x Sc x O2 samples (Fig. 8b) has been also measured using the same electric field cooling procedure as was applied for undoped CuFeO2. It also reveals a small electric polarization for pure all compounds, whose maximum at ∼ T N2 is correlated with anomalies in magnetic susceptibility. This provides new data for the physics of CuFeO2, since the electric polarization was reported in single crystal in the same temperature region but only under magnetic field application . The existence of electric polarization in the polycrystalline pristine samples in zero external magnetic fields indicates that spontaneous electric polarization may exist along other crystallographic direction than those already tested in single crystals. Therefore this suggests that an electric polarization flop between different directions may be induced by magnetic field application in the single crystal of CuFeO2. The existence of electric polarization in all samples related to magnetic transition at T N2 implies the existence of intrinsic magnetoelectric coupling in the present polycrystalline samples. This is in accordance with a previous paper reporting on Al dopinginduced electric polarization in monocrystalline sample of CuFeO2 .
This effect can be compared with the induced change of the background magnetic state that has been reported for Al-doped crystals. Finally, the similarity of the effects induced by Al3+? or Sc3+? demonstrates that electronic configuration (S = 0) is highly favorable for doping-induced electric polarization effect. More importantly, the fact that polarization decreases in zero magnetic field indicates the possible advantage of the Sc3+ doping over the Fe3+ one which is reported to induce incommensurate magnetic structure , only in the doping range of 0.05 ≤ x ≤ 0.25.
In summary, we investigated an effect of spin dilution on the structural and magnetic properties of delafossite CuFeO2 having an S = 5/2 ATL by the substitution of Sc3+ for Fe3+. The spin dilution suppresses the long-range 120∘ Néel state and induces a dimensional crossover of low-energy magnetic excitation from that of the anisotropic 3D AF magnon to that of the 2D AF excitation which may be a mixture of contributions of a major short range 2D AF correlation manifested by the Warren-type magnetic peak profile and a minor conventional long-range 2D AF. Such a crossover is due to the increases of interlayer magnetic interaction (J BB) in the residual Fe ATL due to the disorder introduced by the Sc substitution, which is consistent with the gradual decrease of T N2 with increasing x content.
Electric polarization decreases with x up to 63 μC/m2 for 5 %-Sc to 22 μC/m2 for 25 %-Sc around T N2. This decrease reports the existence of intrinsic magnetoelectric coupling and induces incommensurate magnetic structure in the present polycrystalline samples.
This work was financially supported by Université Joseph Fourier (UJF Chimie), Grenoble, France.
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