Magnetotransport properties of SrFeO2.95 perovskite

PERGAMON

Solid State Communications 120 (2001) 283±287

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Magnetotransport properties of SrFeO2.95 perovskite Y.M. Zhao a,*, P.F. Zhou a, X.J. Yang b, G.M. Qiu a, L. Ping c a Department of Physics, Shantou University, Shantou 515063, Guangdong People's Republic of China Department of Scienti®c Research, Shantou University, Shantou 515063, Guangdong People's Republic of China c Department of Electrical Engineering, Shantou University, Shantou 515063, Guangdong People's Republic of China b

Received 9 August 2001; accepted 28 August 2001 by C.N.R. Rao

Abstract The resistivity, magnetoresistance, and MoÈssbauer effect of the metallic spiral antiferromagnet SrFeO2.95 were examined in the temperature range 4.5±300 K. Two peaks were found in the plot of the temperature derivative of resistivity (dr /dT ) versus temperature. The temperature of the upper peak, TN, corresponding to the onset of antiferromagnetic order, decreases monotonically from 105 to 98 K as the applied ®eld increases from 0 to 9 T. This is due to the reduced antiferromagnetic superexchange interaction, while the temperature of the second transition, Ta, which corresponds to a peak at low temperatures and coincides with the weak anomaly in x (T ) curve, remains constant upto 3.8 T and shows a rapid increase in the range from 40 to 65 K, as the magnetic ®eld increases further. This is caused by the enhancement of the disappearance of Fe 31d paramagnetic domains and also by the enhancement of the helical±conical spin transformation for the applied ®eld greater than 3.8 T. q 2001 Published by Elsevier Science Ltd. PACS: 75.30.2m; 72.00.oo; 71.30.1h Keywords: A. Magnetically ordered materials; D. Electronic transport; D. Electron±electron interactions

Colossal magnetoresistance (CMR) in some manganese oxides (such as the perovskite La12xSrxMnO3 and pyrochlore Tl2Mn2O7) has been found to accompany a transition from a metallic ferromagnetic low-temperature phase to a paramagnetic high-temperature phase [1,2]. The implication is that magnetic scattering of itinerant carriers from enhanced ¯uctuations near Tc has an important role. This prompts one to determine the principal differences in the behavior of more familiar metallic ferromagnets such as Ni and Fe, other perovskite ferromagnets (for example, La12xCaxCoO3 [3]), or doped magnetic semiconductor (for example, Ge12xMnxTe and Eu12xGdxTe [4]). The size of the magnetoresistance differs by at least an order of magnitude among these compounds, and each material might seem to demand a separate analysis, because the magnetic mechanisms can be very different. In the perovskite manganites and cobaltites as well as traditional ferromagnets such as Fe and Ni, the magnetic exchange arises from hopping of the conduction electrons (either the local `double exchange' or the long-range RKKY coupling), whereas in the Mn pyro* Corresponding author. E-mail address: [email protected] (Y.M. Zhao).

chlores and magnetic semiconductors such as EuSe [5] and CdCr2Se4 [6] the magnetic coupling is mediated at least partly by bound electrons (superexchange). Thus, the criteria for achieving (and hence, optimizing) CMR is not clear, and this presents a challenge for materials scientists. The question would arise whether these effects can be found in other Fe-based perovskite materials. The cubic perovskite SrFeO3, in which iron is present as Fe 41, is interesting due to its unusual magnetic and electronic properties. SrFeO3 maintains its cubic structure and a metallic conductivity of < 10 23 V cm down to 4 K [7], in contrast to trivalent LaFeO3, which is an antiferromagnetic insulator. According to ligand ®eld theory, four 3d electrons in an octahedral crystal ®eld can assume either a high-spin 3 4 t2g eg con®guration or a low-spin t2g con®guration, with the latter con®guration being stabilized when the crystal ®eld splitting, 10 Dq, is large. The magnitude of the magnetic moment was measured using neutron scattering by Takeda 41 et al. [8] as m Fe ˆ 3:1mB in the screw antiferromagnetic state below TN < 134 K [9], suggesting that SrFeO3 is high spin with three electrons ®lling the t2g band, and the remaining eg electron itinerant. It has been proposed that these itinerant d electrons are accommodated in a broad

0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00389-1

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Fig. 1. Temperature dependence of ac susceptibility, x 0 , of SrFeO2.95 measured in 3 G(on left-hand scale) with different frequency, and inverse ac susceptibility 1/x 0 (on right-hand scale).

s p band formed between the metal eg orbitals and the O 2p bands, giving rise to metallic conductivity [8,10]. This may explain the absence of any Jahn±Teller distortion down to 4.2 K, which would be expected for a localized d 4 system. MoÈssbauer spectroscopy studies, based on the behavior of doped La12xSrxFeO3 (0.1 , x , 0.6) [11] also indicate a high-spin state for SrFeO3. On the other hand, for oxygen de®cient compounds of SrFeO32d , the NeÁel temperature decreases from 134 K for d ˆ 0 to 80 K for d ˆ 0:16 and the electrical conductivity changes by increasing d from metallic to semiconducting, with activation energy increasing upto to 0.08 eV for d ˆ 0:16 [11]. Although this particular compound has been studied well by MoÈssbauer effect, magnetization, and neutron diffraction techniques [11,12], with unfortunately several contra-

Fig. 2. Resistivity, r…T†; as a function of temperature in applied ®eld of 0, 5, 7, and 9 T for SrFeO2.95. All curves shown are for the cooling cycle only. Inset: magnetization of SrFeO2.95 as a function of ®eld at several temperatures.

dictions still persisting and leading to confusion regarding the nature of the 3d electrons. These con¯icting results may be traced back to the great sensitivity of the magnetic properties and to the oxygen substoichiometry. In our previous paper [13], a large magnetoresistance effect has been reported in perovskite SrFeO2.95 that do not posses distortion-induced ion and manganese or cobalt. The realization of CMR in Fe-based compound having the perovskite structure should open up a vast range of materials for further exploration and exploitation of this effect. In this paper, the magnetotransport properties of SrFeO2.95 compound is reported in detail. Polycrystalline samples of SrFeO32d were synthesized by direct solid state reaction of SrCO3 and Fe2O3, mixed in stoichiometric molar proportions. The preparation is described in greater detail elsewhere [14]. Analysis for available oxygen using redox titration led to the composition SrFeO2.95. The resistivity of samples as a function of magnetic ®eld (perpendicular to the probing current) and temperature was measured by the four-probe method, using physical properties measurement system (PPMS) with a superconducting 9 T magnet. Magnetic properties were measured using dc and ac SQUID system with a maximum magnetic ®eld of 5 T. MoÈssbauer absorption powder spectra were recorded at various temperatures using a conventional spectrometer with a 57Co/Rh source. Isomer shift values were quoted relative to metallic a-Fe at 293 K. Powder X-ray diffraction analysis showed that the produced samples are single phase, and the crystal structure at room temperature exhibits cubic perovskite symmetry  Electron microscopy with lattice constant a ˆ 3:852 A: spectra showed that the underlying sample is absolutely homogeneous with a very weak oxygen de®cient ordering. Fig. 1 shows the temperature dependence of the susceptibility on the left-hand scale and inverse susceptibility (x 21) on the right-hand scale. The minimum in temperature dependence of the inverse susceptibility, x 21(T ), at T N ˆ 107 K is associated with the onset of helical antiferromagnetism [13]. The strong deviation of x 21(T ) from Curie±Weiss law in the temperature range between 107 and 200 K is caused by spin ¯uctuation effect [14]. A frequency independent weak feature in x 21 at 65 K was also observed from the MoÈssbauer result, due to the coexistence of antiferromagnetic and paramagnetic domain in the temperature region 50±80 K. M at low temperature shows a rapid increase at low ®elds (see 5 K data in the inset of Fig. 2) and increases without saturation upto 5 T. The low-®eld rise of M is also seen in helical antiferromagnets MnSi [15] and is caused by domain rotation. The domain rotation of our compound has been con®rmed by MoÈssbauer spectra. Table 1 summarizes our MoÈssbauer data. The slight ¯uctuation of hyper®ne ®eld (see the data in Table 1) between 50 and 80 K does not allow us to see any abrupt change of the angle between the magnetic moments of neighboring t2g spins clearly in this helical magnet with non-collinear spin con®guration around 65 K. The

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Table 1 Isomer shift (IS), quadrupole shift (2e ), quadrupole splitting (QS), hyper®ne ®eld (Hf), and relative intensity (%) of iron site observed in 57Fe MoÈssbauer spectra of SrFeO32d at 4.5, 50, 60, 65, 70, 80, and 293 K, respectively T (K)

Iron site

IS (^0.02 mm/s)

2e (^0.02 mm/s) or QS (^0.02 mm/s) for paramagnetic site

Hf (^0.1 T)

% (^5)

4.5

Fe 41 Fe 31 Fe 41 Fe 41 Fe 41 Fe 41 Fe 31d Fe 41 Fe 31d Fe 41 Fe 31d

0.16 0.39 0.16 0.16 0.16 0.15 0.13 0.14 0.13 0.06 0.13

0 2 0.2 0 0 0 0 0 0 0 0 0.57

32.6 43.5 31.1 30.8 30.2 29.6 Paramagnetic 28.3 Paramagnetic Paramagnetic Paramagnetic

92 8 100 100 100 88 12 72 28 76 24

50 60 65 70 80 293

MoÈssbauer analysis results and the oxygen content O2.95 lead to d ˆ 0:6: The resistivity (r ) for H ˆ 0 and 9 T shows metal-like behavior (dr /dT . 0) in the temperature range 300±107 K, a rapid decrease just below TN and ®nally an upturn occurring around 25 K during cooling. Hysteresis in r in the temperature range over 25±80 K were found between cooling and warming. A large decrease in resistivity under 9 T occurs below 50 K. As T increases, magnetoresistance (MR) decreases to almost zero between 90 and 100 K, and shows a small peak around TN ˆ 105 K …,TN ˆ 107 K†: Small negative MR is also found up to 300 K. Fig. 2 shows the temperature dependence of resistivity (r ) at several applied ®eld. All curves are shown only for the cooling cycle. At 20 K MR remains a constant …MR ˆ 0† upto 3.8 T and shows smooth increase in the value as H increase further. The value of MR at 9 T is 12%, slightly less than the value obtained from iso®eld scan. The second transition appears at

Fig. 3. Temperature derivative of resistivity …dr=dT† versus temperature of SrFeO2.95 at several applied ®elds.

low temperatures, during which a decrease of resistivity is distinctly reduced. Thus, a clear resistivity minimum is exhibited and coincides with the weak anomaly in x (T ) curve. A low temperature resistivity minimum is found in several metallic oxides [16] whose origins vary from Kondo effect, electron±electron (e±e) interaction, and weak localization effects [17]. The resistivity data at low temperature, below the resistivity minimum can be ®tted well to r ˆ r0 2 mT 1=2 for different values of applied magnetic ®eld. The square-root temperature dependence of the electrical transport elucidates that the low temperature correction to the conductivity is due to electron±electron interactions in the presence of disorder. The ®rst term r0 ˆ rimp 1 rmag contains contribution from impurity scattering (r imp) and magnetic scattering (r mag). The ®t suggests that while e±e interaction is not affected by magnetic ®eld, a large decrease in r 0 occurs. Such a change in r 0 is caused by a decrease in r mag with increasing H. In a helical magnet, spin con®gurations can in¯uence r mag by changing from helical to conical, conical to fan, and ®nally to ferromagnetic alignment as the ®eld increases [15]. The magnetization data (in the inset of Fig. 2) at 5 K suggest that ferromagnetism is not achieved even at 5 T. The linear increase of M is possibly connected with the transition from helical to conical. The change of spin structure with H reduces the scattering of eg-electron, causing the observed negative magnetoresistance. Similar behavior of MR is also observed in metallic helical antiferromagnet MnSi [15]. The temperature derivative of the resistivities of our sample at different magnetic ®elds is given in Fig. 3. Two peaks were found in the temperature dependence of the temperature derivative of resistivity (dr /dT ) for every applied ®eld. The temperatures of upper peaks TN correspond to the antiferromagneic transition, while the second peak Ta appearing at low temperatures, during which a decrease of resistivity is distinctly reduced, coincides with the weak anomaly in x (T ) curve. Fig. 4 shows the dependence of Ta as well as TN on applied ®eld. At low ®eld Ta

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Y.M. Zhao et al. / Solid State Communications 120 (2001) 283±287

Fig. 4. Magnetic ®eld dependence of two transition temperatures Ta and TN.

remains constant upto 3.8 T, and shows a rapid increase in the values from 40 to 65 K as H increases further. On the other hand, TN, compared to the observed ®eld-induced change in Ta, decreases monotonously from 105 to 98 K as the applied ®eld increases. These observations could be understood from our MoÈssbauer result. The observed thermal evolution of r (0 T) is closely connected with the subtle variation in Fe 41/Fe 31 ratio and magnetic nature of these ions as found in MoÈssbauer study. The null quadrupolar electric interactions of Fe 41 ions from 300 K down to 4 K suggests that (Fe 41O6) 82 octahedra are not statically distorted. However, paramagnetic domains consisting mainly of Fe 31d ions with isomer shift IS ˆ 0:13 mm=s and relative intensity of about 25% remains down to 70 K and disappear as temperatures decrease. This Fe 31d electronic con®guration is due to the electron delocalization between Fe 31 and Fe 41. The MoÈssbauer data (see the data in Table 1) between 80 and 50 K clearly show that these paramagnetic domains vanish around 65 K, and thus the resistivity minimum around this temperature is connected with the disappearance of Fe 31d paramagnetic domains when the temperature decreases (T # 65 K). It is reasonable to assume that for temperatures below TN both the disappearance of Fe 31d paramagnetic domains and the ®eld induced helical±conical spin transformation are enhanced by the application of the magnetic ®eld higher than 3.8 T, and hence Ta increases with applied ®eld. This agrees well with our MR measurement results, where MR remains constant …MR ˆ 0† up to 3.8 T and shows a smooth increase as H increases further. The ®eld dependence in the isothermal magnetization measurement at 5, 60, 15 K, respectively, indicates that no hysteresis is apparent but a positive remnant magnetization is observed at zero applied ®eld. It has also been suggested that this residual ferromagnetic moment persists to higher temperatures [18,19]. Therefore, it is reasonable, that the magnetism of SrFeO2.95 is determined by a competition between ferromagnetic and antiferromagnetic exchange interaction, which is the reason for the helical spin structure.

The formation of the `3d' bands in SrFeO32d system may favor ferromagnetism and, for this reason, ferromagnetism appears in the system SrFe12xCoxO3 even with a small amount of cobalt content [9,10]. On the other hand, the `3d' band of SrFeO3 is considered to be narrow from the fact that the Fe 41 magnetic moment in SrFeO3 is expected to reach the value close to that of high spin state and the internal magnetic ®eld is 330 kOe at 4.2 K [9]. It is likely that similar band formation are present in the SrFeO2.95 sample as shown in Table 1. Therefore, the superexchange interactions that are applicable to insulator may also be present. This antiferromagnetic interaction may disturb ferromagnetic interaction due to the band formation, giving rise to a helical spin structure. On the other hand, electrical conduction in SrFeO3 mainly occurs in the s p narrow band 3 of eg-parentage while t2g electrons in p p band are localized. At 300 K, eg electrons in s p band are itinerant as suggested by the low value of resistivity …r 300 ˆ 6:7 £ 1023 Vcm† and MoÈssbauer data. As T lowers to T N ˆ 107 K; interatomic Hund's coupling polarizes the s p conduction band. Ferromagnetic spin alignment of eg electrons in the s p band enhances carrier mobility and hence, r decreases rapidly just below T N ˆ 107 K: Hund's coupling can also ferromag3 netically align the localized t2g spins at each Fe sites, but 3 antiferromagnetic t2g -O : 2Pp-t32g superexchange interaction competes with it. The result is the helical spin structure with near-neighbor ferromagnetic coupling and next nearest-neighbor antiferromagnetic coupling [13,18,19]. Oda et al. [20] has shown that the angle between the magnetic moments of nearest-neighboring Fe ions increase from 428 at TN to 478 at 4.2 K in SrFeO2.90. The propagation vector Q was also found to show smooth increase from 0.118a p at TN to a constant 0.130a p …ap ˆ 2p=a0 † below 50 K [20]. It is likely that such changes in Q, equivalent to bond angle between the nearest Fe moments also occur in our sample. It is reasonable, therefore, that antiferromagnetic interactions can be suppressed by a magnetic ®eld, leading to a monotonical decrease of TN from 105 K for H ˆ 0 T to 98 K for H ˆ 9 T:The fact that antiferromagneic coupling is suppressed by the application of the magnetic ®eld is also evident in our iso®eld magnetization measurement where the antiferromagnetic ordering temperatures also decrease with increase in magnetic ®eld. In conclusion, we have studied the resistivity, magnetoresistance and MoÈssbauer effect in metallic spinel antiferromagnetic SrFeO2.95 in the temperature range from 4.5 to 300 K. When cooling from high temperatures, two transitions superimposed on a metallic behavior of the zero ®eld r curve can be observed. The upper transition appears as an abrupt decrease of resistivity on cooling and corresponds to the antiferromagnetic transition. The transition temperature decreases monotonously from 105 to 98 K as the applied ®eld increases, which can be attributed to the decrease of antiferromagnetic superexchange interaction. The second transition appears at low temperatures, during which a decrease of resistivity is distinctly reduced. Thus, a clear

Y.M. Zhao et al. / Solid State Communications 120 (2001) 283±287

resistivity minimum is exhibited and coincides with the weak anomaly in x (T ) curve. The temperature of second transition Ta remains constant up to 3.8 T and shows a rapid increase in the values from 40 to 65 K as H increases further. This is caused by the enhancement of disappearance of Fe 31d paramagnetic domains and of ®eld induced instability of the helical antiferromagnetic con®guration as the applied magnetic ®eld is higher than 3.8 T. Acknowledgements This project was funded in part by Y.D. HUO and NSFC Grant supported through M.E. China and Guangdong Province, China, respectively. References [1] A.P. Ramirez, J. Phys: Condens. Mater. 9 (1997) 8171. [2] M.F. Hundley, J.H. Nickel, R. Ramesh, Y. Tokura (Eds.), Science and Technology of Magnetic Oxides MRS Symposia Proceedings No. 494Materials Research Society, Pittsburgh, 1998. [3] S. Yamaguchi, H. Taniguchi, H. Takagi, T. Arima, Y. Tokura, J. Phys. Soc. Jpn 64 (1995) 1885. [4] S. von Molnar, S. Methfessel, J. Appl. Phys. 38 (1967) 959. [5] B. Batlogg, Phys. Rev. B 23 (1981) 650.

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