In the present work, the dynamics of the spins and the structural parameters of thermal treated samples of Eu1−xFexCrO3 (x = 0, 0.1 and 0.2) were investigated. The ac-magnetic susceptibility (χac) was measured near TN for frequencies (f) in the range 10 - 104 Hz, magnitude of the ac magnetic field of 10 Oe and for 5 ≤ T ≤ 300 K. X-ray diffraction data were used for determining the lattice parameters and the bonding angle θB (Cr– O(2) – Cr) for 100 ≤ T ≤ 300 K. The maximum in χac was found to shift to higher values of T for increasing values of f. The Vogel-Fulcher law was used for analyzing χac yielding values for the characteristic relaxation time τ0, activation energy Ea/kB and glassy temperature TG, respectively, in the ranges 2.10 - 3.96 ps, 46.5 - 47.2 K and 169.9 -176.1 K. The super-exchange parameter , where d is the length of the bound Cr-O(2), was also obtained yielding a good correlation with the corresponding values of TN.
Multifunctional rare-earth (RE) orthochromites [RE]CrO3 materials gained a lot of attention lately due to their potential in developing new technologies.1–5 The orthochromites crystallize in a distorted perovskite structure with a space group Pbnm. The distortion of the ideal cubic perovskite structure actually occurs in the dodecahedral A-sites occupied by the RE-ions. The octahedron occupied by the transition metal ions (B-sites) is less distorted due to a slightly rotation.1,6 It is also known that the magnetic properties of [RE]CrO3 are mainly due to the antiferromagnetic (AF) interactions among the Cr3+ ions. The magnetic moments of the Cr3+ ions order in a canted antiferromagnetic (CAFM) phase showing a permanent ferromagnetic (FM) component below the Néel temperature (TN).7 The overall magnetic ordering is actually complex due to the presence of an isotropic super-exchange interaction and of an antisymmetric-exchange Dzyaloshinskii–Moriya (DM) coupling leading to the coexistence of CAFM with a collinear antiferromagnetic (AFM) phase.8–17 Moreover, the occurrence of time-dependent spin-glass-like phenomena has also been reported in some perovskites.2,3,18 Furthermore, the magnetic interactions in [RE]CrO3 can be tuned by modifying the bond angle (θB) and bond length (d) associated to the Cr-O(2)-Cr interaction by replacing chromium by other transition metals.2,3,18
Eu3+ is known for having no net magnetic moment in opposite to transition metals ions. On the other hand, the large difference among their ionic radii varies the length and the angle of the bonds in the chromite EuCrO3.1,2 Thus, the influence of the addition of Fe3+ ions in the structure, magnetic properties and the spin dynamics of nanopowders of Eu1-xFexCrO3, for x=0.0, 0.1 and 0.2, are investigated. The slopes of the temperature dependence of the lattice parameters were found to vary for temperatures below TN. Additionally, the spin-dynamics yielded spin-glass-like properties that were analyzed by using the Vogel-Fulcher law. The super-exchange parameter J obtained by measuring θB and d was found to have a good correlation with the experimental values of TN.
II. SAMPLES AND TECHNIQUES
Nanopowders of Eu1-xFexCrO3 (x=0.0, 0.1 and 0.2) were synthesized by using a combustion reaction method.2,19 The powders were then annealed for 24h at 1073K for reducing the amount of micro-strains. X-ray diffraction (XRD) was used for characterizing the structure from room temperature down to 100K by using a Rigaku Smartlab. The Maud software and the Rietveld method were employed for determining the lattice parameters and the length and the angle of the bounds. Magnetization (M) and ac-magnetic susceptibility (χac) measurements were performed by using an ACMS modulus of a Physical Property Measurement System, made by Quantum Design, for applied magnetic fields (H) and temperatures (T), respectively, in the ranges ±85kOe and 5 ≤ T ≤ 300K. The magnetization was also measured for a given value of H following two steps: (1) the sample was first cooled down to 5K under no applied magnetic field (ZFC) and (2) the sample was first cooled down to 5K with the H already applied (FC). After cooling the samples under each procedure the measurement was performed with the samples being warmed up to 300K. χac was measured for frequencies (f) in the range 10 ≤ f ≤ 104 Hz while keeping the ac magnetic field constant (hac=10Oe) for temperatures above and below TN.
III. EXPERIMENTAL RESULTS
XRD spectra measured for different values of T for a non-doped sample (x=0) are shown in Fig. 1(a). It is important to call the attention to a diffraction peak near 24.3° that it is only seen below TN. This peak is shown in details in the left-hand side inset of Fig. 1(a). Similar behavior has also been observed in other materials by either varying the sample composition or the temperature in both single-crystals and in nanoparticles, and it has been attributed to structural phase transitions induced by Jahn-Teller distortions.20–22 Due to the thermal contraction of the sample, the diffraction peaks shift to higher values of angles as the sample is cooled down as shown in the right-hand side inset of Fig. 1(a) for the peak near 33.2°. The T-dependence of the lattice parameters (a, b and c) for the three sample concentrations (x=0.0, 0.1 and 0.2) are shown in Fig. 1(b). One may notice that the slopes of the T-dependence for the lattice parameters varies when the samples are cooled down below TN. Interesting also that above TN, a, b and c present nearly the same T-dependence while below TN the lattice parameters for the sample with x=0.1 yielded a stronger variation with T. At this point it is important to recall that the overall structural parameters depend on the degree of distortion in the sample prior the structural transition takes place. The distortions, on the other hand, are strongly influenced by the sample concentration, by the ionic distribution and on amount of micro-strains present in the samples. Thus, no simple relation with x are expected for the lattice parameters below TN. The room temperature XRD data did also reveal that samples crystalizes into a single orthorhombic crystalline structure with space group Pbnm. These findings were used in conjunction with the VESTA software23 to draw schematic pictures for the position of the atoms for x=0 and 0.2 (Fig. 2). The mismatch in the ionic atomic radii of Eu+3 (=120.6pm) and Fe+3 (=92pm) increases the distortion in the CrO6 polyhedrons which, in turn, modifies the Cr-O(2)-Cr angle and the corresponding Cr-O(2) bounding length. The values of θB, d and the goodness-of-fitting parameter (G.O.F) are listed in Table I.
|Fe-concentration .||0.0 .||0.1 .||0.2 .|
|Fe-concentration .||0.0 .||0.1 .||0.2 .|
The ZFC and FC magnetizations were measured in the T-range 5–300K by applying a dc magnetic field of 100Oe showing an irreversibility for T≤TN (Fig. 3(a)). The FC magnetization yielded a T-dependence that it is typical of ferromagnetic materials even though the net magnetization comes from the CAF induced by distortions in the CrO6 polyhedrons. Moreover, the FC magnetization shows a slightly decrease below about 50K for the Fe-doped samples. This result has also been observed in others compounds2,4,5,11 and it has been attributed to the AF-coupling of the magnetic moment of the RE3+ ions with the one of Cr3+ and to the decreasing in the average bond length RE-Cr. It was noticed that values of M were strongly influenced by the annealing that reduce the micro-strains present in the as-prepared samples.24–26 The ZFC magnetization is substantially smaller the corresponding FC data while a kink was observed near TN. This is somewhat expected specially in powder samples due to frozen of the magnetic moments by the local random-anisotropy.
Hysteresis loops recorded at T=5K are shown in Fig. 3(b). The magnetization data are typical of CAF with a somewhat strong FM component, e.g., non-vanishing coercivity and remanence. The FM result from the canting in the magnetic moment of the Cr3+ ions and from the coupling of the moments of Cr3+ ions with the ones of Fe3+ for the doped samples. The influence of the Fe-concentration in both coercivity and remanence is better seen in the inset of Fig. 3(b). Interesting, for both the coercivity and the remanence decrease for increasing values of x, a trend also found for TN.
The dynamics of the spins was investigated by measuring χac near TN by varying f in the range 10-104 Hz. The maximum of the in-phase component of χac was found to shift to higher values of T while the corresponding amplitude decreases for increasing values of f as shown for a non-doped sample in Fig. 4(a). This behavior is characteristic of spin-glass-like systems where the maximum shift in χac per decade of frequency is given by , where ΔT is the variation in the temperature (Tf) at the maximum of χac measured for the upper and lower values of f, and TG is the glassy-temperature.27–30 The shifting in Tf can be accounted by using the Vogel-Fulcher-law (VFL): , where τ=1/f is the measuring time, τ0 a characteristic relaxation time, Ea is the thermal activation energy and kB the Boltzmann constant. Fig. 4 shows a τ vs. Tf plot for x=0 and the corresponding fitting to the VFL (solid line). The VFL was also fitted to the data for the samples with x=0.1 and 0.2 and the fitting parameters are listed in Table II.
|x .||0.0 .||0.1 .||0.2 .|
|τ0 (10-12 s)||2.15||3.96||2.10|
|x .||0.0 .||0.1 .||0.2 .|
|τ0 (10-12 s)||2.15||3.96||2.10|
IV. FURTHER DISCUSSIONS AND CONCLUSIONS
The super-exchange interaction is expected to be influenced by the Fe-concentration which, in turn, influences TN (TN ∼ J). Along this line, J.-S. Zhou and J. B. Goodenough proposed a model for J taking into account its dependence with θB and d: .16,24,25 For instance, J is expected to increase by either increasing θB or decreasing d. The Zhou-Goodenough expression was used to estimate J by using the parameters measured for T (=180K), which is close but above TN. The dependence of J with x and the correspondence of J with TN are shown, respectively, in Fig. 5(a) and Fig. 5(b). It is important to notice the good correlation of TN with the corresponding values of J. The dependences of θB and d with x are shown, respectively, in the insets (a) and (b) of Fig. 5.
In summary, nanopowders of Fe-doped europium orthochromites were prepared and their structural and magnetic properties were investigated. The XRD data measured by varying T shown a new diffraction peak and a change in the slope in the T-dependence of the lattice parameters for values of T below TN. θB and d were used to estimate de super-exchange parameter yielding a good correlation with TN. Moreover, the addition of iron decreased the values of TN as well as the ferromagnetic component in the canted-antiferromagnetic phase. Thus, the iron-concentration can be used for tuning the structural and the magnetic properties of europium-chromite. Finally, the χac data showed the presence of a spin-glass-like property that was properly accounted for by the Vogel-Fulcher law.
This work was partially supported by CNPq, CAPES, FACEPE and FINEP (Brazilian Agencies).
All authors contributed equally to this work.
The data that support the findings of this study are available from the corresponding author upon reasonable request.