Spin dynamics and spin-glass state in Fe-doped cobaltites

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Department of Physics, Chungbuk National University, Cheongju, 361-763, ... Department of Physics, Sookmyung Women's University, Seoul 140-742, Korea.
JOURNAL OF APPLIED PHYSICS

VOLUME 95, NUMBER 11

1 JUNE 2004

Spin dynamics and spin-glass state in Fe-doped cobaltites Manh-Huong Phana) Department of Physics, Chungbuk National University, Cheongju, 361-763, Koreaa and Department of Aerospace Engineering Bristol University, Queens Building, University Walk, Bristol, BS8 1TR, United Kingdom

The-Long Phan, Thanh-Nhan Huynh, and Seong-Cho Yu Department of Physics, Chungbuk National University, Cheongju, 361-763, Korea

Jang Roh Rhee Department of Physics, Sookmyung Women’s University, Seoul 140-742, Korea

Nguyen Van Khiem and Nguyen Xuan Phuc Institute of Materials Science, NCST, Nghiado, Caugiay, Hanoi, Vietnam

共Presented on 9 January 2004兲 A thorough study of the magnetic and transport properties of La0.5Sr0.5Co1⫺x Fex O3 (0⭐x⭐0.6) compounds has been made. The Fe substitution destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Fe substitution. The charge localization due to Fe substitution is likely to have its origin in the electronic configuration rather than in its ionic size. The hole-poor regions, corresponding to the Fe-rich regions, would also dilute the magnetic lattice and thereby prevent the occurrence of long-range order. Spin-glass behavior was observed for x⭓0.5 compositions and is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co3⫹ and Co4⫹ , and the antiferromagnetic interactions like Co-O-Fe and Fe-O-Fe. A dynamic scaling analysis of ac susceptibility data using conventional critical slowing down indicates a finite spin-glass phase-transition temperature T g ⬇85 K and a dynamic exponent z v ⬇12.4, for x⫽0.5 composition. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1667442兴

Recently, Itoh et al.1 conducted a magnetization study on La1⫺x Srx CoO3 (0⭐x⭐0.5) cobalt oxides and drew up a phase diagram as an evident separation of a spin-glass phase (0⭐x⭐0.18) region from a cluster-glass (0.18⭐x⭐0.5) one. More recently, however, Nam et al.2 found a coexistence of ferromagnetic and glassy behaviors in the compound of La0.5Sr0.5CoO3 . It is assumed that the reason for this differential is that such a magnetic field 共⬃100 Oe兲 might be large enough to entirely suppress the aging effect,1 which is ascribed to the cluster growth slowed down by the presence of the frustration.2 To obtain more insight into the nature of spin dynamics and spin-glass-like phenomena in such a cobaltate system, we investigated the magnetic and transport properties of polycrystalline La0.5Sr0.5CoO3 systems by substituting Co sites with iron. La0.5Sr0.5Co1⫺x Fex O3 (0⭐x⭐0.6) samples were prepared using the standard ceramic method. X-ray diffraction data confirmed the quality of the samples. The temperature dependences of ac magnetic susceptibility ␹ ⬘ (T) and resistivity ␳ (T) were measured using a closed cycle helium refrigerator. The temperature dependences of zero-field-cooled 共ZFC兲 and field-cooled 共FC兲 magnetization. M ZFC(T) and M FC(T), respectively, were performed using a noncommercial vibrating sample magnetometer 共VSM兲 and a Quantum

Design MPMS5 one. Electron paramagnetic resonance 共EPR兲 measurements were performed at 9.2 GHz 共X band兲 with a Jeol JES-TE300 EPR spectrometer. In Fig. 1, we show the temperature dependence of resis-

a兲

FIG. 1. Resistivity 共␳兲 is plotted against temperature 共T兲 for representative La0.5Sr0.5Co1⫺x Fex O3 (x⫽0, 0.1, 0.4, and 0.6兲 samples.

Author to whom correspondence should be addressed; electronic mail: [email protected]

0021-8979/2004/95(11)/7531/3/$22.00

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© 2004 American Institute of Physics

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J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004

FIG. 2. Logarithm of resistivity (ln ␳) is plotted against temperature T 1/4.

tivity without magnetic field for representative values of x. At x⫽0 concentration the material is metallic, but it appears as a semiconductor to metal transition with a further increase of the Fe-doped content up to x⫽0.3, and finally becomes a semiconductor as x⭓0.4. Taking into account the logarithmic variation of resistivity with T 1/4, as shown in Fig. 2, we have found that for x⭓0.4 substitution, the resistivity can be described by the Mott’s variable range hopping model. The temperature dependences of magnetization of La0.5Sr0.5Co1⫺x Fex O3 (0⭐x⭐0.5) samples measured in an applied field of 100 Oe is shown in Fig. 3. As the Fe-doped content increases, the ferromagnetic ordering transition temperature T c decreases from ⬃250 (x⫽0) to ⬃166 K (x ⫽0.4). Moreover, at low temperatures below T c , a prominent separation of FC magnetization from ZFC magnetization in the compositions was observed, which is attributed to the magnetic frustration arising from the competitive coexistence of antiferromagnetic 共AFM兲 and ferromagnetic 共FM兲 interactions.1–3 In the present case, the low-field ZFC curves for x⫽0.5 and 0.6 compositions show a cusp at ⬃85 and ⬃90 K, respectively, at which the magnetization shows a

FIG. 3. Temperature-dependent magnetization taken both ZFC and FC at 100 Oe for La0.5Sr0.5Co1⫺x Fex O3 (0⭐x⭐0.5) samples. Inset: the real component of ac susceptibility ( ␹ ⬘ ) for the x⫽0.5 sample measured at 127 Hz, 1 Hz, and 10 kHz.

FIG. 4. EPR spectra of La0.5Sr0.5Co1⫺x Fex O3 with x⫽0.2 for various temperatures.

maximum. The assertion on the spin-glass behavior for x ⫽0.5 and 0.6 compositions becomes clearer as the shift of a peak/cusp 共or a maximum at T f ) seen in ␹ ⬘ (T) towards higher temperatures with higher frequencies was observed 共see the inset of Fig. 3兲. Below the maximum, the magnitude of ␹ ⬘ is frequency dependent, but it becomes independent of frequency at temperatures just above T f . This is a feature of conventional spin glasses.4 With decreasing temperature from the paramagnetic phase, the relaxation time of the spin glass slows down, leading to a divergence of the maximum relaxation time at T g , where the system enters the spin-glass state. The frequency-dependent maximum in ␹ ⬘ indicates the freezing temperature T f , where the maximum relaxation time ␶ of the system is equal to the characteristic time t ⫽1/␻ set by the frequency of the ac-susceptibility measurement. By measuring the variation of T f in a wide range of frequencies, we can judge if a spin-glass phase is approached by a fit of the data to conventional critical slowing down,5



␶ T f ⫺T g ⬀ ␶0 Tg



⫺z v

.

共1兲

At T g ⫽85 K (x⫽0.5), the best fit of 共1兲 brings about z v ⬇12.4 and ␶ 0 ⬇3.16⫻10⫺10 s. This magnitude of ␶ 0 is larger than in conventional spin glasses ( ␶ 0 ⬇10⫺13 s). This can be understood as the microscopic magnetic entities in the samples with x⭓0.5 are probably not single atomic spins, but nanosized clusters of ferromagnetically coupled spins. The value of zv is quite consistent with that of conventional three-dimensional 共3D兲 spin glasses.4 In order to further elucidate the internal dynamics of La0.5Sr0.5Co1⫺x Fex O3 (0⭐x⭐0.6) compounds, EPR spectra have been recorded above and below their Curie temperature. As expected, the low-temperature asymmetric signals become Lorentzian-like at temperature T⬎T C 共Fig. 4兲. This can be connected to when the resistivity was temperature dependent; a clear transition from an insulating state to a conducting one occurred as the temperature decreases. The effective g factor obtained for all compositions is ⬇1.98 共at T⬎T C ). At temperatures 共⬃300 K兲, the width of the EPR lines increases as the Fe content increases. This corresponds to a decrease of the exchange narrowing, indicating the low-

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Phan et al.

J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004

ering of the ferromagnetic interactions between the ions. On the other hand, the dependence of the peak-to-peak linewidth ⌬H pp with temperature shows minima that are usually interpreted as due to spin correlation. This fact can be attributed to either the existence of short-range magnetic order (x ⭓0.5) or perhaps to the formation of ferromagnetic clusters (x⫽0). Unlike the Fe-doped compounds, La0.5Sr0.5CoO3 revealed a large temperature dependence of the EPR intensity above T C , reflecting the existence of spin clusters over a wide range of temperatures.6 We now discuss remarkable influences of the Fe doping on the magnetic and transport properties of La0.5Sr0.5Co1⫺x Fex O3 (0⭐x⭐0.6). In general, the possible distortions in manganese and cobalt perovskites originate from the substitution of metal ions by other metal ions with different ionic radii. The ionic radii of Fe3⫹ and Co3⫹ /Co4⫹ have almost identical values.7 Thereby, the replacement of Co by Fe in La0.5Sr0.5Co1⫺x Fex O3 (0⭐x⭐0.6) did not produce significant lattice distortion, which had also been confirmed by the neutron diffraction studies.8 The considerable change in transport properties has been found and it has been suggested that the electronic configuration of Fe is more important than its ionic radius. T C decreases slightly as the Fe-doped content increases up to x⫽0.2 关that is, due to the development of a rather weak AFM Co-O-Fe superexchange compared to the preexisting FM Co-O-Co double-exchange 共DE兲兴, while it drops drastically upon higher Fe-doping contents, due to an additional appearance of AFM Fe-O-Fe superexchange interactions that suppress strongly the ferromagnetism and the conductivity. Below ⬃120 K, we propose that an upturn in the resistivity of the samples with x⭓0.1 might possibly be due to the spin-state transition of Co ions 6 0 e g ) to the thermally from the low-spin state 共LS, S⫽0, t 2g 4 2 exited high-spin state 共HS, S⫽2, t 2g e g ), because the electrical conduction in cobaltate systems strongly depends on the spin state of Co ions.9 The crystal field splitting between t 2g and e g states and Hund coupling energy are comparable, which causes temperature dependent spin-state transition from the LS state to the HS one. Due to the semiconducting behavior, the thermal activation of high-spin states also contributes to the electrical conductivity.10 Another note is that substitution of Sr2⫹ for La3⫹ in La1⫺x Srx CoO3 introduces

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3 2 high spin (t 2g e g ) Co4⫹ into this system.9 Meanwhile, the replacement of Co with iron also produces Co4⫹ ions, due to the 3d electronic configuration of Co4⫹ which is isoelectronic to the very stable Fe3⫹ ions. At low temperatures, the concentration of thermally excited Co3⫹ ions is small and the charge transport occurs by hopping between Co4⫹ sites. Meanwhile, the hopping of electrons between Fe and Co is not possible due to the lack of available states in the Fe e g↑ level. Therefore, the Fe substitution in La0.5Sr0.5Co1⫺x Fex O3 destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Fe substitution. Furthermore, spin-glass behavior observed for x⭓0.5 compositions is ascribed to the frustration of random competing exchange interactions, namely the FM DE interaction between Co3⫹ and Co4⫹ and the AFM interactions like Co-O-Fe and Fe-O-Fe. However, the persistence of ferromagnetism up to x⫽0.4, which is evidently confirmed by the Arrott plots of M 2 vs H/M as a clear signature of spontaneous magnetization below T c , would imply that the FeO-Fe interactions in the system are somehow weak, or that part of them is superexchange ferromagnetic.

Research at Chungbuk National University was supported by the Korean Research Foundation Grant No. KRF2003-005-C00018. 1

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