fDivision of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
gDepartment of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India
Received 29 November 2008;
accepted 9 January 2009.
Available online 19 January 2009.
Transport and magnetic properties of LaCoO3-based compounds, doped with 20% Sr and 2.5, 5, 10, 15, 20 and 30% Fe, were investigated by means of magnetization, resistivity and magnetoresistance measurements as well as by 57Fe Mössbauer spectroscopy. While the temperature dependence of the dc and ac magnetic susceptibilities reveals the presence of magnetic phase separation accompanied by spin-glass and cluster-glass behavior, the electrical resistivity and magnetoresistance characteristics indicate that the mesoscopic structure of the present compounds is rather well described as consisting of ferromagnetic, metallic grains embedded in an insulating matrix. The effect of the partial Co → Fe substitution on the bulk magnetic and transport properties, as well as on the local state of Co and Fe ions is discussed.
Keywords: Magnetic semiconductors; CMR; GMR; Spin glass; Mössbauer spectroscopy
Fig. 1. Temperature dependence of the in-phase susceptibility measured in H = 365 A/m auxiliary ac magnetizing field characterized by frequencies of 133 Hz, 1000 Hz and 10 kHz. The arrows point in the direction of increasing frequency. For x = 0.2 and 0.3 only the data recorded at 133 Hz are shown (lowest panel).
Fig. 2. Temperature dependence of the field-cooled (FC) and zero-field-cooled (ZFC) dc susceptibility and – for the case of x = 0.3 – their inverse measured in H = 4000 A/m auxiliary magnetizing field for the samples La0.8Sr0.2Co1−xFexO3 with x = 0.025, 0.1 and 0.3. Arrows in the inset indicate the direction of the recording process.
Fig. 3. Temperature dependence of the electrical resistivity of the compounds La0.8Sr0.2Co1−xFexO3 with x = 0.025, 0.15, 0.2 and 0.3. The inset shows the same data as a function of . Note the logarithmic scales.
Fig. 5. 57Fe Mössbauer spectra of La0.8Sr0.2Co1−xFexO3 – measured at or near 100 K – for various values of x. The notation x = 0 indicates that the corresponding spectrum was obtained via emission Mössbauer spectroscopy carried out with the help of a La0.8Sr0.2CoO3 sample doped with 100 ppm radioactive 57Co (see  and ).
Fig. 6. The probability Pn(x) of the event that in the La0.8Sr0.2Co1−xFexO3 perovskite a B-site (Co or Fe) ion has exactly n (n = 0…6) Fe ions as B-site nearest neighbor, as a function of the iron concentration x.
Fig. 7. The slope of ln ρ versus T−1/2 as a function of the iron concentration x in the compounds La0.8Sr0.2Co1−xFexO3, where ρ is the electrical resistivity as displayed in Fig. 3. The corresponding value of the constant C – calculated on the basis of Eq. (2) – is shown beside the points. The standard deviation of the slope values is less than the radius of the circles.
Fig. 8. The slope of −MR(1/T) – as displayed in Fig. 4 – as a function of the iron concentration x in the compounds La0.8Sr0.2Co1−xFexO3. For x = 0.025 and 0.15 the slope below 53 K (denoted by open circles) is different from that found for T > 53 K (denoted by full circles). The standard deviation of the slope values is less than the radius of the circles.