Magnetic Properties of La0.81Sr0.19Mn0.9Fe0.1−xZnxO3 (x = 0, x = 0.05)

Magnetic properties of polycrystalline La0.81Sr0.19Mn0.9Fe0.1−xZnxO3 (x = 0, 0.05) have been investigated by means of electron spin resonance, magnetic susceptibility, and Mössbauer measurements. Both samples show a clear ferromagnetic transition. The Curie temperature TC decreases on increasing Fe content (x = 0.05—TC = 222 K; x = 0—TC = 148 K). Mössbauer studies indicate that Fe in these compounds is in the trivalent high-spin state. The temperature evolution of the Mössbauer spectra at low temperatures (T < TC) is typical for ferromagnetic clusters with a wide distribution in size and magnetic correlation length. The inverse susceptibility of all the samples deviates from the Curie–Weiss law above TC, indicating the presence of fluctuations on approaching magnetic order. An anomalous downturn of the inverse susceptibility for x = 0.05 significantly above TC and the concomitant observation of ferromagnetic resonance signals coexisting with the paramagnetic resonance up to approximately room temperature, is caused by a Griffiths-like behavior. This regime is characterized by the coexistence of ferromagnetic entities within the globally paramagnetic phase.


Introduction
La 1−X A X Mn 1−Y Fe Y O 3 (A = Ca, Sr) belongs to the family of hole-doped mixed valence manganite perovskites, exhibiting a colossal magnetoresistance (CMR) and magnetocaloric effect [1][2][3][4]. The hole doping of manganite perovskites, realized by substitution of a divalent element for La 3+ or by formation of cation vacancies, results in a fraction of Mn 4+ ions on the Mn 3+ sites [5]. The Mn 3+;4+ mixed valence leads to ferromagnetic (FM) Mn 3+ -Mn 4+ double-exchange (DE) interactions, competing with antiferromagnetic (AFM) Mn 3+ -Mn 3+ super-exchange (SE) interactions [5]. The case of Fe substitution is particularly interesting because of the large extent to which it can replace Mn due to similar ionic radii of Fe 3+ and Mn 3+ . The magnetic investigations of La 1−x Sr x Mn 1−y Fe y O 3 (with x = 0.3 and y = 0.15-0.25) and La 1−x Ca x Mn 1−y Fe y O 3 (with x = 0.37 and y = 0-0.18) show that in both systems the FM Curie temperature T C decreases with increasing y [2,4]. Such behavior of the transition temperature T C is connected with the weakening of the double-exchange FM interaction by Fe doping [2,4,6]. In this article, we present X-ray diffraction, magnetization, electron spin resonance (ESR), and Mössbauer experiments on polycrystalline samples of  [6] indicate the important role of magnetic cluster formation both in the FM ordered and in the paramagnetic (PM) phase. Especially in the latter one, the Griffiths phase can be tuned by the balance of disorder (Zn 2+ ) and AFM interactions (Fe 3+ ). with x = 0 and x = 0.05 were synthesized by traditional ceramic processing, including two grindings in a ball mill with addition of alcohol and preliminary burning at 1273 K for 4 h. The final sintering step was performed in air at 1473 K for 10 h, and the samples were cooled together with the furnace. To provide stoichiometric oxygen content, the samples were annealed at 1223 K and partial pressure of oxygen in the gas phase p O 2 = 10 -1 Pa. The choice of annealing conditions was based on the data of Ref. [7]. Annealing was carried out for 96 h. Phase composition and unit-cell parameters were determined by powder X-ray diffraction at room temperature (diffractometer Shimadzu XRD-7000, CuK α radiation). The X-ray diffraction analysis of the synthesized manganites x = 0 and x = 0.05 showed that both compounds belong to the proper space group R3c. The corresponding unit-cell parameters, presented in Table 1, were determined and refined using the .94 program package [6,8,9].

Magnetic Susceptibility and Magnetization
Magnetization measurements were performed using a superconducting quantum interference device (SQUID) magnetometer MPMS5 (Quantum Design) in magnetic fields up to H ≤ 50 kOe for the temperature regime 1.8 ≤ T ≤ 400 K.  Table 2). The temperature dependence of the inverse susceptibility χ −1 (T) measured in different magnetic fields for the compound x = 0.05 is shown in the lower frame of Fig. 1. For H = 10 kOe at high T, χ −1 (T) varies linearly with T indicating a Curie-Weiss (CW) behavior. However, for the measurements done in lower dc fields (100 and 1000 Oe), χ −1 (T) shows a downward deviation from linearity below a temperature T G ~ 364 K, significantly above T C , which turns out to be the stronger the smaller the external field and is a typical signature of a Griffiths phase [10][11][12][13][14][15][16][17], where FM clusters coexist in the PM matrix. As the applied external magnetic field increases, the extent of deviation in magnetic susceptibility systematically diminishes until a pure CW behavior is observed. This is related to the fact that the magnetization of the PM matrix increases linearly with increasing H [10,11,17], while the contribution of the FM clusters is saturated already in small fields and, hence, for a sufficiently high field, the PM susceptibility dominates over the weakly correlated FM clusters. The effective moment determined from the CW law as μ eff = 5.305μ B is strongly enhanced compared to the theoretical value of 4.6μ B calculated from the square root of the sum of squared ionic moments of Mn 3+ (S = 2), Mn 4+ (S = 3/2), and Fe 3+ (S = 5/2) weighted by the corresponding stoichiometric factors and using the spin-only g value g = 2 [17]. This observation corroborates the general tendency for FM cluster formation in the PM phase in this compound.

Magnetic Properties of…
Compared to x = 0.05, the χ −1 (T) curve of the Zn-free compound x = 0 ( Fig. 2) does not exhibit any obvious downturn deviation from the CW law which could be a signature of a Griffiths phase. Note that the deviation to higher values from linearity below a certain temperature above T C is due to fluctuations characteristic of usual ferromagnets. The effective moment determined from the CW law as μ eff = 4.749μ B approximately matches the theoretical value of 4.834μ B . Thus, in the PM phase, the tendency for FM cluster formation is significantly weaker for x = 0 than for x = 0.05. Figure 3 shows the magnetization of La 0.81 Sr 0.19 Mn 0.9 Fe 0.1−x Zn x O 3 (x = 0, 0.05) as a function of magnetic field at 2 K. The magnetization of the compound x = 0.05 reveals practically no hysteresis and immediately reaches the full saturation value at weak external fields. For x = 0, one observes a weak hysteresis and, above 15 kOe, the magnetization still slightly increases on increasing magnetic field. The saturation magnetization (M S ) in both compounds was obtained from a linear extrapolation of the high-field data [6,18] as function of the inverse magnetic field down to 1/H = 0 as illustrated in the left inset of Fig. 3 and the values of M S (T = 2 K) are deduced as 3.049μ B and 2.409μ B , to be compared to the theoretical values 3.11μ B and 2.91μ B , respectively, which are calculated assuming g = 2 for all magnetic ions and antiparallel alignment of the Fe ions with respect to the Mn ions. While the theoretical value for x = 0.05 is in fair agreement with the experimental value, it is overestimated for x = 0. This is probably due to the fact that the linear extrapolation of M vs 1/H is not applicable here, because of the non-zero curvature of the data.

Electron Spin Resonance
To further identify the presence of a Griffiths phase, we performed electron spin resonance (ESR) measurements on the La 0.81 Sr 0.19 Mn 0.9 Fe 0.05 Zn 0.05 O 3 and The signal of ESR is sensitive to the variation of localized environment of magnetic ions. Due to this feature, ESR has been also applied to identify the existence of Griffiths phases in some manganites previously [10][11][12][19][20][21][22][23]. The ESR measurements have been performed using a BRUKER EMXplus spectrometer working at X-band frequency (9.4 GHz) and equipped with a nitrogen gas-flow cryostat. Due to the lock-in technique with field modulation, the field derivative of the absorbed microwave power (dP/dH) is recorded as a function of the externally applied field. The observed ESR spectra of  Figure 4 depicts a series of ESR spectra in a temperature range from 215 to 320 K across the phase-transition regime (near and above T C ) for La 0.81 Sr 0.19 Mn 0.9 Fe 0.05 Zn 0.05 O 3 . At T ≥ 300 K only a single PM resonance line shows up corresponding to the paramagnetic state. But at lower temperatures, the spectra do not only consist of a PM signal due to the majority of Mn 3+ -Mn 4+ spins, but also exhibit an intriguing FMR signal at lower resonance fields. Compared to PM resonance lines, the extra peak shows a notable difference which is strongly dependent on temperature. As shown in the left and right part of Fig. 4, with the decrease of temperature, the intensity of this peak does not only increase but also the peak position gradually moves towards lower magnetic field. Therefore, from 300 K down to 220 K, two peaks coexist indicating that two different magnetic phases coexist in the system. At T = 215 K, both peaks finally merge into a single broad peak, which further shifts to lower field region. In Fig. 1, at T < 220 K, one recognizes that the sample has undergone the PM-FM phase transition and all PM phase completely turns into the FM phase. Consequently, at T < 220 K, there remains only a single FMR peak to be detected. Thus, the For La 0.81 Sr 0.19 Mn 0.9 Fe 0.1 O 3 , we observed a typical paramagnetic resonance line with a g-factor of approximately 2 in the temperature range T > 160 K, which is slightly higher than the onset temperature of the PM-FM transition in χ −1 (T) (cf. Figure 2). To lower temperature, distortions of the resonance line show up on crossing T C and the signal exhibits the characteristic shape of FMR in a polycrystalline sample. The FMR shifts to lower field with decreasing temperature. Thus, the results of both χ −1 (T) and ESR show the normal PM-FM transition for La 0.81 Sr 0.19 Mn 0.9 Fe 0.1 O 3 .
The full temperature dependences of the resonance fields H res and linewidth data ΔH are summarized in Fig. 6. For x = 0.05, the splitting of the resonance fields is another key parameter for characterization of the coexistence of magnetic phases. The open and solid circles depict the temperature dependence of the resonance fields for PM and FM peak, respectively. One can see that the position of the PM peak is almost constant at a magnetic field of H = 3386 Oe showing only a weak temperature dependence. The FM resonance field can be resolved at about 3 kOe at 300 K and gradually shifts to lower fields on decreasing temperature. That the FM peak cannot be resolved up to T G ~ 364 K, may be related to the fact that the precondition for the appearance of a well-defined FMR line is the formation of plate-like FM clusters, which is not necessarily fulfilled directly at the onset of the GP. Above the Curie temperature T C , the linewidth ΔH of the PM line exhibits a minimum value of 210 Oe at a temperature of 270 K, diverges to lower temperature and increases approximately with linear slope (characteristic for relaxation via spin-phonon coupling [24]) to higher temperature, whereas ΔH of the FMR slightly increases with temperature and then decreases on approaching T G . For x = 0, the temperature dependences of resonance field and linewidth of the PM peak show a similar behavior above T C like for x = 0.05.
Thus, the ESR results support the findings of the magnetization measurements, i.e. for both compounds, one observes the typical behavior of the PM resonance on approaching a FM transition with resonance shift and line broadening due to enhanced internal demagnetization fields and critical fluctuations. For x = 0.05, the additional FMR line corroborates the coexistence of a FM phase in the PM regime. Zn 2+ substitution enhances the number of Mn 4+ spins and in turn FM Mn 3+ -Mn 4+ DE, and at the same time, it increases disorder. Both effects favor the formation of a GP.

Mössbauer Effect Study
The Mössbauer measurements were carried out in a wide temperature range from room temperature to liquid nitrogen and liquid helium temperatures. The Mössbauer spectra of La 0.81 Sr 0.19 Mn 0.9 Fe 0.05 Zn 0.05 O 3 at different temperatures are shown in Fig. 7. At room temperature, the Mössbauer spectrum consists of a slightly asymmetric doublet with a distribution of isomer (IS) and quadrupole shifts (QS). In the insets of Fig. 7, the distribution functions of the Mössbauer parameters are given. They were restored by processing the experimental spectra with the SpectrRelax software [25]. At 300 K, the distribution function of Mössbauer parameters shows the presence of two well-defined centers of iron ions (with and without another iron ion as nearest neighbor) with an area ratio close to 1:5 at room temperature.
As it is known, the isomer shift reflects the density of s-electrons in the iron nucleus, and it is sensitive to the oxidation state of iron and the coordination number. The average values of the isomer and quadrupole shift of Fe nuclei in the studied sample are found as 0.35 mm/s and 0.17 mm/s, respectively. These values of the hyperfine parameters are typical for Fe 3+ ions in the high-spin state, (t 3 2g e 2 g ) [26,27]. Also, hyperfine parameters determined at low temperature are in good agreement with the data reported in the literature for similar compounds [28,29].
The substitution of La by a divalent cation, Sr, results partly in a change of the Mn valence from 3+ to 4+ [28], so that a part of Mn 3+ is transformed into Mn 4+ . In this case, it is reasonable to expect the manifestation of the presence of Fe 4+ ions in the room-temperature Mössbauer spectra in the form of absorption features in the region of low and negative velocities. As can be seen from the distribution function for the isomer shift in Fig. 7, in this velocity region, the features due to Fe 4+ ions are not observed. Probably, the preferable substitution of Fe 3+ ions for Mn 3+ ions is due to the identical ion size of these elements in the high-spin state. When the temperature decreases, the Mössbauer spectra remain practically the same above T C , but change dramatically in the magnetically ordered phase, first acquiring the form of relaxation spectra with a wide distribution of hyperfine magnetic fields up to 300 kOe, and then, in the temperature range of liquid helium, the Mössbauer spectra are transformed into magnetic sextets (Fig. 7, bottom frame). At a temperature of 4.2 K, a noticeable peak is observed in the distribution of hyperfine fields at a value of 520 kOe and an extended tail towards low fields (bottom inset in Fig. 7). Note that the Griffiths phase cannot be distinguished from a purely paramagnetic phase by means of Moessbauer spectroscopy, due to the fact that the volume fraction of the ferromagnetic clusters is less than 1% of the paramagnetic fraction, as has been shown earlier by Deisenhofer et al. [10], and hence cannot be resolved by Moessbauer spectroscopy.
The dependence of the mean hyperfine magnetic field on the temperature is given in Fig. 8a. According to the Mössbauer measurements, magnetic ordering of the iron ions is achieved at a temperature of 200 K for La 0.81 Sr 0.19 Mn 0.9 Fe 0.05 Zn 0.05 O 3 . The changes observed in the dependence of the mean quadrupole shift, ε, on temperature, also indicate the magnetic ordering of the iron ions (cf. Fig. 8b). When an ion is in a paramagnetic state, the splitting ΔE Q between the lines of the quadrupole doublet is determined by the product of the electric field gradient (EFG) q at the nucleus and the quadrupole moment Q of the nucleus, i.e. ΔE Q ≈ 2ε = e 2 qQ/2. In the case of magnetic ordering of resonant atoms and the appearance of a hyperfine magnetic field on the nucleus oriented with respect to the direction of the main axis of the quadrupole interaction tensor at some angle θ, the quadrupole shift ε is determined by the expression: ε = e 2 qQ(3cos 2 θ − 1)/8. The sharp jump of the quadrupole shift observed in the region of the magnetic ordering temperature is probably due The average quadrupole shift, ε, c the average isomer shift, δ, of the iron nuclei to the appearance of a hyperfine magnetic field on the nucleus at some angle to the direction of the EFG. The temperature dependence of the average isomer shift of iron ions presented in Fig. 8c, is mainly determined by the second-order Doppler effect. Its value decreases monotonically with increasing temperature. The solid line shows a curve that approximates, in the Debye approximation, the experimental values of the average isomer shift.
It should be noted that the relaxation-like shape of the Mössbauer spectra and the specific temperature dependence of the average hyperfine field on the iron nuclei, which are unusual for conventional magnetically ordered materials, suggest that at low temperatures, a complex magnetic correlation is formed in the samples under study.

Conclusions
In summary, we have investigated the magnetic behavior of polycrystalline samples of La 0.81 Sr 0.19 Mn 0.9 Fe 0.1−x Zn x O 3 (x = 0, 0.05). All samples show a clear FM-PM transition at T C . The downward deviation of PM magnetization data from the CW law together with the FMR line coexisting with the PM ESR signal suggests that the GP model is appropriate to describe the short-range ordered state for x = 0.05, while the upward deviation observed for x = 0 is caused by conventional fluctuations on approaching magnetic order. Mössbauer measurements show that in the studied perovskite-like manganites, doping with iron ions leads to the replacement of Mn 3+ ions by Fe 3+ ions in the high-spin state with the same ionic radius. Consequently, Fe ions occupy Mn sites without significant lattice distortions. However, this tendency can lead to phase separation and the formation of various magnetic clusters embedded in the paramagnetic matrix. The observed temperature dependence of the Mössbauer spectra of the compounds under study is typical of ferromagnetic clusters with a wide size distribution and magnetic correlation length. The temperature evolution of the spectra is probably determined by fluctuations of the iron magnetic moments due to the competition between the thermal energy and the energy of magnetic correlation.