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Magnetic phase evolution in Fe substituted GdBaCo2O5.5
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 152–157
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Magnetic phase evolution in Fe substituted GdBaCo2O5.5 N. Thirumurugan, A. Bharathi , C.S. Sundar Condensed Matter Physics Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 July 2009 Received in revised form 13 August 2009 Available online 6 September 2009
Magnetization and magneto-resistance experiments have been carried out on well characterized samples of the GdBaCo2 xFexO5.5 series. Zero field cooled magnetization measurements in the low concentration Fe samples suggest, that the low temperature anti-ferromagnetic phase transforms sequentially to several ferromagnetic phases, before transforming to a paramagnetic state with increase in temperature. The anti-ferromagnetic to the first ferromagnetic phase transition is associated with a large negative magneto-resistance for Fe fractions upto x= 0.075. Isothermal magnetization measurements in the ferromagnetic like region of the samples, suggests the presence of mixtures of two ferromagnetic phases. Similar measurements performed at low temperatures where anti-ferromagnetic-like phase is stabilised suggest the presence of a mixture of anti-ferromagnetic and ferromagnetic phases. Magnetization and magneto-resistance are seen to collapse for Fe fractions, x 40.1. Based on these studies a plausible scenario of the evolution of magnetism with Fe substitution in GdBaCo2O5.5, is suggested. & 2009 Elsevier B.V. All rights reserved.
Keywords: Cobaltates Phase separation Magnetization Magnetoresistance PACS: 75.47. m 75.60.Ej
1. Introduction Cobalt based oxygen deficient double perovskite LnBaCo2O5.5 that form for a large number of rare earths (Ln), with square planar Co–O networks has been under recent investigation [1–4]. In particular the Gd based system, GdBaCo2O5.5 7 d, has been intensely investigated. The electronic and magnetic properties of the GdBaCo2O5.5 compound are determined by the Co–O layers. These layers have two kinds of structural motives, viz., pyramidal CoO5 and octahedral CoO6. In the stoichiometric oxide, the Co ions in both the pyramids and octahedra are in the 3+ state. The variety of spin states possible for Co3 + ion are low spin Co3 + (LS, S= 0), intermediate spin Co3 + (IS, S= 1) and high spin Co3 + (HS, S= 2). The small energy differences between them make spin state transitions possible as a function of temperature and structural distortion. The stoichiometric compound shows several magnetic transitions as a function of temperature [2]. At low temperatures the system is in an anti-ferromagnetic state and with increase in temperature magnetization increases in narrow temperature range from 240 to 270 K, suggestive of a ferromagnetic like order. On further increasing the temperature the magnetization drops and system becomes paramagnetic. In this entire low temperature regime the sample remains insulating. At 340 K the paramagnetic insulating state undergoes a first order transition to a weakly metallic phase. The microscopic origin of the magnetic
Corresponding author.
E-mail address: [email protected] (A. Bharathi). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.09.007
transitions as well as the metal-to insulator transition is still widely debated [2,5–12]. The addition or depletion of oxygen has led to a shift of all the phase transitions in the GdBaCo2O5.5 compound leading to a complex phase diagram [2]. The transport property investigations indicated hole doping for O excess and electron doping for O deficiency [2]. Neutron scattering data in the HoBaCo2O5.5 system suggest the presence of O ordering leading to different superstructures and magnetic order for a specific O content [6]. Taking this experimental fact into account would imply that for intermediate O stoichiometries phase separation will occur. The effect of chemical substitution on the magnetic and transport properties in GdBaCo2O5.5 has only recently been attempted [13–15], where the associated changes in oxygen stoichiometries were also measured. Based on transport and magnetic studies a phase diagram [13] was determined for Ni substituted GdBaCo2O5.5. At large concentration of Ni the ground state becomes ferromagnetic, and for intermediate concentrations phase separation into anti-ferromagnetic and ferromagnetic regions were suggested. Fe substitution at Co site in GdBaCo2O5.5 was carried out for a limited fraction of Fe substitution [16], the presence of two ferromagnetic phases for small Fe substitutions was inferred based on magnetization measurements. These results are in striking contrast with conclusions arrived at in the TbBaCo2 xFexO5.5 system, where based on the temperature dependent magnetization behaviour and on Neutron powder diffraction studies it has been concluded that the high temperature magnetic phase present in the substituted sample is ferromagnetic with a large component of a G-type anti-ferromagnetic phase [17,18]. It is
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thus evident that magnetic phases induced by Fe substitution in LnBaCo2O5.5 is not well understood and more work is required. Here we present magnetization and magnetoresistance measurements in Fe substituted GdBaCo2 xFexO5.5 samples, over a wide range of Fe substitution, viz. 0ox o0.4, in which oxygen stoichiometry is determined by iodometric titration. The thermo-magnetic behaviour in the Fe substituted samples is obtained by zero field cooled (ZFC) and field cooled (FC) measurements in the 5–300 K temperature range. Magnetization hysteresis curves are measured in the ferromagnetic and anti-ferromagnetic regions in all samples of the series.
2. Experimental details The GdBaCo2 xFexO5.5 7 d (x= 0.0–0.4) samples were prepared by solid state reaction starting from high purity Gd2O3, Fe2O3, Co3O4 and BaCO3. After the initial decarbonation at 850 1C for 18 h, the samples were ground and pelletised and heat treated at 1050 1C for a period of 30 h in air [19]. The pelletisation and heat treatments were repeated three times to increase the homogeneity. X-ray diffraction measurements were carried out in a STOE diffractometer using Cu Ka radiation in the Bragg Brentano geometry, in the 2y range 10–801 at room temperature. The temperature dependent magnetization and isothermal magnetization as a function of applied field were carried out for all samples using a cryogenic make, liquid helium based vibrating sample magnetometer. ZFC and FC measurements in the 4–300 K temperature range were carried out under a measuring field of 100 Oe. Isothermal magnetization measurements were carried out in the ferromagnetic region (255 K) and at low temperatures where there was an indication of the presence of an AFM phase. Magneto-resistance (MR) was measured in the low concentration samples in an exchange cryostat in which the temperature could be varied from 6 to 300 K, in fields upto 12 T.
3. Results The room temperature lattice parameters generated from the XRD patterns of GdBaCo2 xFexO5.5 7 d using the PCW [20] program, indexed to primitive orthorhombic structure with Pmmm symmetry, are shown in Fig. 1 (the notation of a, b and c are as denoted in Ref. [2]). It is clear from the figure that the c lattice parameter increases with Fe substitution whereas a and b lattice parameters remain more or less unaltered. It is to be noted
Fig. 1. Variation of cell parameters a, b/2 and c/2 with Fe fraction x.
Fig. 2. Variation of Co4 + as obtained from iodometric titration with Fe fraction x.
the samples remain orthorhombic with increase in Fe concentration unlike that seen in TbBaCo2 xFexO5.5 system, wherein for x 40.1 the tetragonal structure is stabilized. In order to obtain the valence state and any preferred site ¨ occupation for Fe, room temperature Mossbauer measurements was carried out for the GdBaCo2 xFexO5.5 sample with Fe fraction of x= 0.4. A single quadrupolar split two finger pattern was observed, which on analysis revealed an isomer shift of 0.26 mm/s and the quadrupolar splitting of 0.99 mm/s. This combination of isomer shift and quadrupolar splitting corresponds to Fe occupying a single site, which is octahedrally co-ordinated with oxygen with the valency of Fe being 3+ [21,22]. The Co4 + fraction in the samples was determined by iodometric titration as prescribed in [23] and is shown for all Fe substitutions in Fig. 2. From the figure it is apparent that the Co4 + concentration increases in proportion to the Fe fraction substituted into the system. Since Co4 + fraction is proportional to the Fe fraction substituted and Fe is in 3+ state, charge balance requires that the oxygen content increase in proportion to Fe fraction x and therefore any typical sample in the series can be referred to as GdBaCo2 xFexO5.5 + x/2. Fig. 3(a) and b shows ZFC and FC magnetization curves on GdBaCo2 xFexO5.5 + x/2 series, where x varies from 0 to 0.1. As reported in the literature [1,2], the ZFC M(T) curve in the x= 0.0 sample shows the expected initial decrease due to the paramagnetic contribution of Gd. In a narrow temperature range 240–270 K an increase in magnetization is seen, which is associated with the formation of the ferromagnetic phase. Beyond 270 K there is sharp fall in magnetization due to the ferromagnetic to paramagnetic transition. A close look at the ZFC curves in samples with small Fe concentration 0.025ox o0.1 reveals several features. At low temperatures, apart from the paramagnetic contribution due to Gd, a systematic increase in the background magnetization with increase in Fe content is evident. With increase in temperature the increase in magnetization develops in stages, for Fe content upto x= 0.075. The magnetization steps show broadening and occur at lower temperatures with increase in Fe content. In the sample with x= 0.1 the low temperature magnetization is large in the whole low temperature range, as if steps are no more distinguishable. The overall magnitude of ZFC magnetization for each sample, in the ferromagnetic region, is seen to decrease with increase in Fe content. The FC M(T) data shown in Fig. 3(b) for x upto 0.1 indicate that with decrease in temperature a ferromagnetic order builds that is present over a wider temperature range with increasing Fe content upto x = 0.1. The ZFC and FC data for the x= 0.05 sample is
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Fig. 3. (a–d) ZFC and FC magnetization curves for GdBaCo2 xFexO5.5 + x/2 with x indicated in figure, at an applied field of 100 Oe. Inset of (c) shows ZFC and FC curves in the x= 0.05 sample to demonstrate the large thermo-magnetic hysteresis. Inset in (d) shows ZFC and FC data in the x= 0.4 sample carried out at 1000 Oe.
Fig. 4. M versus H curves for GdBaCo2
xFexO5.5 + x/2
at 255 K for (a) x= 0.0, (b) x = 0.05, (c) x =0.1 (d) x= 0.2 and (e) x = 0.3 sample (f) x = 0.4.
shown in inset of Fig. 3(d), where a large thermo-magnetic hysteresis is clearly seen. For samples with x = 0.2 and 0.3 the overall magnitude of both ZFC and FC magnetization fall drastically at all temperatures (Fig. 3(c) and (d)), although the overall temperature dependence is similar to that observed in the x= 0.1 sample. The magnitude of magnetization is observed to be small in the x = 0.4 sample and was measurable only at 1000 Oe field. The bifurcation of the ZFC and FC data is visible in x =0.4
sample, indicating the occurrence of a magnetically ordered clusters even for the highest Fe content (cf. inset Fig. 3(c)). To examine the nature of the ferromagnetic phase, isothermal magnetization measurements were carried out at 255 K in several samples (see Fig. 4). It is clear from the figure that the pristine sample shows a narrow hysteresis loop (see Fig. 4(a)). The linear increase in magnetization observed at larger fields is due to the paramagnetic contribution from the Gd moment. From Fig. 4(b)
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Fig. 5. M versus H curves for GdBaCo2 xFexO5.5 + x/2 at temperature below the AFM like transition for (a) x= 0.0, (b) x = 0.025, (c) x = 0.05 and (d) x= 0.1 at the temperatures indicated. Inset in 5(b) shows hysteresis curve close to zero field in expanded scale.
and (c) it is seen that with increase in Fe fraction to x= 0.05 and 0.1 the hysteresis loop widens. A careful perusal of the shape of the hysteresis loops shown in Fig. 4(b) and (c) suggest that they could arise from a mixture of two ferromagnetic phases. Similar loops have been observed in mutilayers of SmCo and Fe [24] and nanoscopic mixtures of Fe3Pt and FePt [25]. The observance of these composite hysteresis loops point to the presence of a mixture of soft and hard ferromagnetic phases in the sample with x =0.05 and 0.1 at 255 K. Beyond an Fe fraction of x =0.1, there is a qualitative change in the shape of the high temperature hysteresis curves, as shown in Fig. 4(d–f). These hysteresis curves suggest that the permeability shows a marked decrease with increase in Fe content beyond x= 0.1. The curves in the x = 0.3 and 0.4 sample are nearly paramagnetic. The evolution of the hysteresis loops with increasing Fe content suggests that magnetic interactions become more isotropic and diminish altogether in the high concentration samples. It was shown earlier from the temperature dependent magnetization data that at low temperature the low concentration Fe substituted samples have a AFM like drop (cf. Fig. 3). To see if the magnetic field has any influence on the AFM nature in the Fe substituted samples, isothermal magnetization measurements have been carried out in the AFM like region in the low concentration samples. The measured magnetic hysteresis for several low concentration samples is shown in Fig. 5. In the pristine sample the isothermal hysteresis curve was traced at 185 K. It is clear from Fig. 5(a), that in the pristine sample there is a meta-magnetic transition [2] induced as a function of magnetic field and the field induced ferromagnet shows the characteristic hysteresis for fields 42.5 T, consistent with earlier reports [2]. For the lowest Fe substitution, viz., in the x =0.025 sample, the AFM to the ferromagnetic phase occurs at 200 K (see Fig. 2). The isothermal magnetization measurement carried out at 185 K is shown in Fig. 5(b). Although, according to ZFC magnetization curve, there should be an AFM phase at this temperature a soft ferromagnetic hysteresis loop is observed in Fig. 5(b). The
hysteresis curve magnified close to zero field is shown in inset of Fig. 5(b). A bulging feature is discernible for fields less than 1 T. This suggests that the hysteresis curve shown in main panel of Fig. 5(b), could arise from a superposition of a ferromagnetic loop and meta-magnetic loop similar to that shown in Fig. 5(a). This implies that the observation of field induced meta-magnetic transition of the AFM phase is masked by the ferromagnetic hysteresis occurring from ferromagnetic regions of the sample, indicating that both FM and AFM phases co-exist at this temperature for the x = 0.025 sample (cf. Fig. 5(b)). Similarly, an examination of the hysteresis loops at 110 and 100 K in x = 0.05 and 0.1 samples shown in Fig. 5(c) and (d) and the corresponding M(T) curves shown in Fig. 3 indicate that FM and AFM phases are present in samples with Fe fractions o0.1. The ferromagnetic component is seen to increase with Fe content. It is well known that at the AFM/FM transition in GdBaCo2O5.5 is associated with a magneto-resistance [2]. From Fig. 3(a) it was evident that AFM/FM transition occurs in stages in the low concentration Fe samples. To investigate the associated MR behaviour we measured resistivity in the 50 to 300 K range in zero field and in 6 T field. The MR was determined from the difference in the resistance at 6 T and 0 T, normalized to the zero field resistance, at a given temperature T by ((R(T,0)–R(T,6T))/ (R(T,0))), evaluated in percentage. The measured MR for the low concentration Fe samples is shown in the lower panel of Fig. 6. In the pristine sample 10% MR occurs in the temperature regime where the AFM phase goes into the FM phase similar to earlier reports [1,2]. From the MR and magnetization data shown in Fig. 6 it is apparent that even in Fe substituted samples the occurrence of MR is associated with the transition from the AFM to the first FM phase, as indicated by arrows in Fig. 6. The magnitude of MR shows a marked increase in the x =0.025 sample ( 50%), when compared to that in the pristine sample. Further it is seen to occur over a wider temperature range. Similarly large MR is observed in the x= 0.05 and 0.075 samples. Since the occurrence of MR in this system is associated with the conversion of the AFM phase to the
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Fig. 6. MR (upper panel) and ZFC M(T) compared for low Fe content samples (xo 0.1). Arrows indicate the correlated changes in magnetization and magnetoresistance.
FM phase, it appears that all samples with Fe fraction x o0.1 have a sizeable AFM phase. This taken in conjunction with magnetization measurements give clear evidence that an AFM phase co-exists with the FM phase at low temperatures in the Fe samples with x r0.1.
4. Discussion Since iodometric titration results indicate that the Co4 + concentration increases in proportion to the Fe substituted and ¨ room temperature Mossbauer studies indicate the Fe in 3+ state with an oxygen co-ordination of an octahedron, charge balance requires that excess O be introduced into the system. These O atoms will necessarily occupy the Gd–O layer, changing some of the Co–O pyramids to Co–O octahedra, which could result in the observed increase in the c lattice parameter. An increased c lattice parameter was observed with increase in Ln ion size in LnBaCo2O5.5, which results in a systematic decrease in the AFM to FM transition temperature [26,27]. The latter is intimately related to the magnitude of the crystal field splitting in the Co–O octahedra which depends on the size of the octahedra. The presence of AFM/FM transition that occurs in different temperature regimes in Fe substituted samples could imply the presence distorted Co–O octahedra. This can be elaborated as follows: in the GdBaCo2 xFexO5.5 + x/2 samples, for every two Fe3 + added, two Co4 + and one O atom is added to the structure, resulting in large disorder due to Fe substitution. The introduction of O leading to c
lattice expansion can result in expanded Co–O octahedra, locally. On the other hand the introduction of the smaller Co4 + ion would compress the Co–O octahedra in its neighbourhood. For low Fe concentrations there could also exist regions in the sample with undistorted Co–O octahedra. Thus there could arise a distribution of sizes of Co–O octahedra and corresponding crystal fields in the Fe substituted samples. According to spin state ordering model [5,6] or the pds hybridization model [26], temperature induced Co2 + –Co4 + formation in the Co3 + octahedra leads to the ferromagnetic coupling. The distribution of the crystal field splitting due to the distortion of the octahedrons in the Fe containing samples, could result in the Co2 + –Co4 + formation at different temperatures resulting in the ferromagnetic coupling occurring at lower/higher temperatures for larger/smaller sized Co–O octahedra. This could be the origin of the several FM like transitions observed in the ZFC M(T) curves for Fe fraction o0.1. Once a part of the sample transforms to the ferromagnetic phase at low temperature due to a small crystal field, it remains so until the ferromagnetic–paramagnetic transition temperature. Thus at higher temperature, when the remaining regions of the sample with higher crystal field transform into ferromagnetic phase, two or more ferromagnetic phases can co-exist. This can give rise to the composite hysteresis loops seen in the low concentration samples at high temperatures. The correlated behaviour of the ZFC magnetization and the magneto-resistance, suggests that in the low Fe concentration samples, at low temperature an AFM phase is present, which under application of magnetic field turns ferromagnetic, giving rise to MR. In addition to an AFM phase the low concentration samples show the presence of a ferromagnetic phase, possibly due to disorder induced uncompensated AFM state or Co4 + induced ferromagnetic coupling. Alternatively, the two ferromagnetic phases can occur due to meso-scopic phase separation into an O rich and O poor regions with different para– ferro and ferro–antiferro transition temperatures as suggested for GdBaCo2O5 + d samples for 0.5 o d o0.7 [2]. The double hysteresis loops were however not reported for this phase separated scenario [2]. The collapse of magnetic order in high concentration samples (x 40.1) can occur due to competing and frustrating magnetic interactions that can manifest at high Fe concentration. Similar observations of collapse of magnetism with increase in Fe content in the Fe substituted in the LaCoO3 system were understood to arise due to domination of frustrating magnetic interactions arising from nearest neighbour AFM Fe3 + –O–Fe3 + interaction and FM Co4 + (LS)–O–Fe3 + ; FM Co3 + (IS)–O–Co4 + (LS); FM Co3 + (IS)–O– Fe3 + , permitted by GKA rules [28]. These interactions can occur in the Fe doped GdBaCo2O5.5 sample in the pyramids, octahedra and across octahedra and pyramids. We now discuss the outcome of our results in the context of results on the Fe substituted LnBaCo2O5.5 samples carried out earlier [16–18]. Our heat treatment schedules, the preservation of orthorhombic structure with Fe substitution and magnetization behaviour are similar to that reported in Ref. [16]. Both our results and those of Ref. [16] point to the presence of two ferromagnetic phases in the Fe containing samples. Based on magnetization data, ¨ Mossbauer data [17] and neutron powder diffraction data [18], the magnetic phases in Fe substituted TbBaCo2 xFexO5.5 system is arrived at to be ‘ferromagnetic’ with a large component of G-type anti-ferromagnetic ordered phase at high temperature. The FC magnetization versus temperature for a Fe fraction of x= 0.1 in the GdBaCo2 xFexO5.5 system (see Fig. 3 and Ref. [16]) and that is seen in the TbBaCo2 xFexO5.5 [17] differ considerably. In particular FC M(T) shown in Ref. [17], show a clean drop in magnetization at 200 K corresponding to a AFM transition which is not evident in the present study and those of Ref. [16]. The observed difference in the magnetization behaviours seen in the TbBaCo2 xFexO5.5 7 d
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[17,18] and in GdBaCo2 xFexO5.5 7 d, for x= 0.1, may have its origin in the O stoichiometry values, which is 5.43 in the Tb sample [18], but 5.55 in the Gd sample, as shown in this study, possibly due to the difference in the heat treatment conditions. The O stoichiometry was not determined in Ref. [16]. This observation of variation in the magnetic transitions for different O contents is consistent with the magnetic phase diagram proposed as function of oxygen stoichiometry in GdBaCo2O5.5 7 d [2] which clearly shows different magnetic behaviours for oxygen deficiency and excess.
5. Summary A systematic investigation of the magnetic behaviour of Fe substituted GdBaCo2O5.5 samples has been carried out. Fe substitution is associated with an introduction of Co4 + and O substitution, which leads to local distortions in the structure and consequently in a distribution of crystal fields. Since ferromagnetic coupling in this system is associated with conversion of Co3 + in to Co4 + and Co2 + , their activation at a given temperature depends on the strength of the local crystal field. A lower crystal field can result in the occurrence of Co2 + and Co4 + at a lower temperature promoting ferromagnetic coupling. A distribution of crystal fields could therefore give rise to step-like increases in the M versus T curves at different temperatures. The possibility of coexisting magnetic phases at high temperatures can result in the observation of composite hysteresis curves. The low temperature hysteresis curves and magnetoresistance measurements clearly point to the presence of both AFM and FM regions in the Fe doped samples. These measurements indicate that the Fe substituted GdBaCo2O5.5 system is unstable to phase separation into two ‘ferromagnetic-like’ phases at high temperature and to AFM and ‘ferromagnetic-like’ phases at low temperature. References [1] I.O. Troyanchuk, N.V. Kasper, D.D. Khalyavin, H. Szymczak, R. Szymczak, M. Baran, Phys. Rev. Lett. 80 (1998) 3380. [2] A.A. Taskin, A.N. Lavrov, Yoichi Ando, Phys. Rev. B 71 (2005) 134414.
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Magnetic phase evolution in Fe substituted GdBaCo2O5.5 N. Thirumurugan, A. Bharathi , C.S. Sundar Condensed Matter Physics Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 July 2009 Received in revised form 13 August 2009 Available online 6 September 2009
Magnetization and magneto-resistance experiments have been carried out on well characterized samples of the GdBaCo2 xFexO5.5 series. Zero field cooled magnetization measurements in the low concentration Fe samples suggest, that the low temperature anti-ferromagnetic phase transforms sequentially to several ferromagnetic phases, before transforming to a paramagnetic state with increase in temperature. The anti-ferromagnetic to the first ferromagnetic phase transition is associated with a large negative magneto-resistance for Fe fractions upto x= 0.075. Isothermal magnetization measurements in the ferromagnetic like region of the samples, suggests the presence of mixtures of two ferromagnetic phases. Similar measurements performed at low temperatures where anti-ferromagnetic-like phase is stabilised suggest the presence of a mixture of anti-ferromagnetic and ferromagnetic phases. Magnetization and magneto-resistance are seen to collapse for Fe fractions, x 40.1. Based on these studies a plausible scenario of the evolution of magnetism with Fe substitution in GdBaCo2O5.5, is suggested. & 2009 Elsevier B.V. All rights reserved.
Keywords: Cobaltates Phase separation Magnetization Magnetoresistance PACS: 75.47. m 75.60.Ej
1. Introduction Cobalt based oxygen deficient double perovskite LnBaCo2O5.5 that form for a large number of rare earths (Ln), with square planar Co–O networks has been under recent investigation [1–4]. In particular the Gd based system, GdBaCo2O5.5 7 d, has been intensely investigated. The electronic and magnetic properties of the GdBaCo2O5.5 compound are determined by the Co–O layers. These layers have two kinds of structural motives, viz., pyramidal CoO5 and octahedral CoO6. In the stoichiometric oxide, the Co ions in both the pyramids and octahedra are in the 3+ state. The variety of spin states possible for Co3 + ion are low spin Co3 + (LS, S= 0), intermediate spin Co3 + (IS, S= 1) and high spin Co3 + (HS, S= 2). The small energy differences between them make spin state transitions possible as a function of temperature and structural distortion. The stoichiometric compound shows several magnetic transitions as a function of temperature [2]. At low temperatures the system is in an anti-ferromagnetic state and with increase in temperature magnetization increases in narrow temperature range from 240 to 270 K, suggestive of a ferromagnetic like order. On further increasing the temperature the magnetization drops and system becomes paramagnetic. In this entire low temperature regime the sample remains insulating. At 340 K the paramagnetic insulating state undergoes a first order transition to a weakly metallic phase. The microscopic origin of the magnetic
Corresponding author.
E-mail address: [email protected] (A. Bharathi). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.09.007
transitions as well as the metal-to insulator transition is still widely debated [2,5–12]. The addition or depletion of oxygen has led to a shift of all the phase transitions in the GdBaCo2O5.5 compound leading to a complex phase diagram [2]. The transport property investigations indicated hole doping for O excess and electron doping for O deficiency [2]. Neutron scattering data in the HoBaCo2O5.5 system suggest the presence of O ordering leading to different superstructures and magnetic order for a specific O content [6]. Taking this experimental fact into account would imply that for intermediate O stoichiometries phase separation will occur. The effect of chemical substitution on the magnetic and transport properties in GdBaCo2O5.5 has only recently been attempted [13–15], where the associated changes in oxygen stoichiometries were also measured. Based on transport and magnetic studies a phase diagram [13] was determined for Ni substituted GdBaCo2O5.5. At large concentration of Ni the ground state becomes ferromagnetic, and for intermediate concentrations phase separation into anti-ferromagnetic and ferromagnetic regions were suggested. Fe substitution at Co site in GdBaCo2O5.5 was carried out for a limited fraction of Fe substitution [16], the presence of two ferromagnetic phases for small Fe substitutions was inferred based on magnetization measurements. These results are in striking contrast with conclusions arrived at in the TbBaCo2 xFexO5.5 system, where based on the temperature dependent magnetization behaviour and on Neutron powder diffraction studies it has been concluded that the high temperature magnetic phase present in the substituted sample is ferromagnetic with a large component of a G-type anti-ferromagnetic phase [17,18]. It is
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thus evident that magnetic phases induced by Fe substitution in LnBaCo2O5.5 is not well understood and more work is required. Here we present magnetization and magnetoresistance measurements in Fe substituted GdBaCo2 xFexO5.5 samples, over a wide range of Fe substitution, viz. 0ox o0.4, in which oxygen stoichiometry is determined by iodometric titration. The thermo-magnetic behaviour in the Fe substituted samples is obtained by zero field cooled (ZFC) and field cooled (FC) measurements in the 5–300 K temperature range. Magnetization hysteresis curves are measured in the ferromagnetic and anti-ferromagnetic regions in all samples of the series.
2. Experimental details The GdBaCo2 xFexO5.5 7 d (x= 0.0–0.4) samples were prepared by solid state reaction starting from high purity Gd2O3, Fe2O3, Co3O4 and BaCO3. After the initial decarbonation at 850 1C for 18 h, the samples were ground and pelletised and heat treated at 1050 1C for a period of 30 h in air [19]. The pelletisation and heat treatments were repeated three times to increase the homogeneity. X-ray diffraction measurements were carried out in a STOE diffractometer using Cu Ka radiation in the Bragg Brentano geometry, in the 2y range 10–801 at room temperature. The temperature dependent magnetization and isothermal magnetization as a function of applied field were carried out for all samples using a cryogenic make, liquid helium based vibrating sample magnetometer. ZFC and FC measurements in the 4–300 K temperature range were carried out under a measuring field of 100 Oe. Isothermal magnetization measurements were carried out in the ferromagnetic region (255 K) and at low temperatures where there was an indication of the presence of an AFM phase. Magneto-resistance (MR) was measured in the low concentration samples in an exchange cryostat in which the temperature could be varied from 6 to 300 K, in fields upto 12 T.
3. Results The room temperature lattice parameters generated from the XRD patterns of GdBaCo2 xFexO5.5 7 d using the PCW [20] program, indexed to primitive orthorhombic structure with Pmmm symmetry, are shown in Fig. 1 (the notation of a, b and c are as denoted in Ref. [2]). It is clear from the figure that the c lattice parameter increases with Fe substitution whereas a and b lattice parameters remain more or less unaltered. It is to be noted
Fig. 1. Variation of cell parameters a, b/2 and c/2 with Fe fraction x.
Fig. 2. Variation of Co4 + as obtained from iodometric titration with Fe fraction x.
the samples remain orthorhombic with increase in Fe concentration unlike that seen in TbBaCo2 xFexO5.5 system, wherein for x 40.1 the tetragonal structure is stabilized. In order to obtain the valence state and any preferred site ¨ occupation for Fe, room temperature Mossbauer measurements was carried out for the GdBaCo2 xFexO5.5 sample with Fe fraction of x= 0.4. A single quadrupolar split two finger pattern was observed, which on analysis revealed an isomer shift of 0.26 mm/s and the quadrupolar splitting of 0.99 mm/s. This combination of isomer shift and quadrupolar splitting corresponds to Fe occupying a single site, which is octahedrally co-ordinated with oxygen with the valency of Fe being 3+ [21,22]. The Co4 + fraction in the samples was determined by iodometric titration as prescribed in [23] and is shown for all Fe substitutions in Fig. 2. From the figure it is apparent that the Co4 + concentration increases in proportion to the Fe fraction substituted into the system. Since Co4 + fraction is proportional to the Fe fraction substituted and Fe is in 3+ state, charge balance requires that the oxygen content increase in proportion to Fe fraction x and therefore any typical sample in the series can be referred to as GdBaCo2 xFexO5.5 + x/2. Fig. 3(a) and b shows ZFC and FC magnetization curves on GdBaCo2 xFexO5.5 + x/2 series, where x varies from 0 to 0.1. As reported in the literature [1,2], the ZFC M(T) curve in the x= 0.0 sample shows the expected initial decrease due to the paramagnetic contribution of Gd. In a narrow temperature range 240–270 K an increase in magnetization is seen, which is associated with the formation of the ferromagnetic phase. Beyond 270 K there is sharp fall in magnetization due to the ferromagnetic to paramagnetic transition. A close look at the ZFC curves in samples with small Fe concentration 0.025ox o0.1 reveals several features. At low temperatures, apart from the paramagnetic contribution due to Gd, a systematic increase in the background magnetization with increase in Fe content is evident. With increase in temperature the increase in magnetization develops in stages, for Fe content upto x= 0.075. The magnetization steps show broadening and occur at lower temperatures with increase in Fe content. In the sample with x= 0.1 the low temperature magnetization is large in the whole low temperature range, as if steps are no more distinguishable. The overall magnitude of ZFC magnetization for each sample, in the ferromagnetic region, is seen to decrease with increase in Fe content. The FC M(T) data shown in Fig. 3(b) for x upto 0.1 indicate that with decrease in temperature a ferromagnetic order builds that is present over a wider temperature range with increasing Fe content upto x = 0.1. The ZFC and FC data for the x= 0.05 sample is
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Fig. 3. (a–d) ZFC and FC magnetization curves for GdBaCo2 xFexO5.5 + x/2 with x indicated in figure, at an applied field of 100 Oe. Inset of (c) shows ZFC and FC curves in the x= 0.05 sample to demonstrate the large thermo-magnetic hysteresis. Inset in (d) shows ZFC and FC data in the x= 0.4 sample carried out at 1000 Oe.
Fig. 4. M versus H curves for GdBaCo2
xFexO5.5 + x/2
at 255 K for (a) x= 0.0, (b) x = 0.05, (c) x =0.1 (d) x= 0.2 and (e) x = 0.3 sample (f) x = 0.4.
shown in inset of Fig. 3(d), where a large thermo-magnetic hysteresis is clearly seen. For samples with x = 0.2 and 0.3 the overall magnitude of both ZFC and FC magnetization fall drastically at all temperatures (Fig. 3(c) and (d)), although the overall temperature dependence is similar to that observed in the x= 0.1 sample. The magnitude of magnetization is observed to be small in the x = 0.4 sample and was measurable only at 1000 Oe field. The bifurcation of the ZFC and FC data is visible in x =0.4
sample, indicating the occurrence of a magnetically ordered clusters even for the highest Fe content (cf. inset Fig. 3(c)). To examine the nature of the ferromagnetic phase, isothermal magnetization measurements were carried out at 255 K in several samples (see Fig. 4). It is clear from the figure that the pristine sample shows a narrow hysteresis loop (see Fig. 4(a)). The linear increase in magnetization observed at larger fields is due to the paramagnetic contribution from the Gd moment. From Fig. 4(b)
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Fig. 5. M versus H curves for GdBaCo2 xFexO5.5 + x/2 at temperature below the AFM like transition for (a) x= 0.0, (b) x = 0.025, (c) x = 0.05 and (d) x= 0.1 at the temperatures indicated. Inset in 5(b) shows hysteresis curve close to zero field in expanded scale.
and (c) it is seen that with increase in Fe fraction to x= 0.05 and 0.1 the hysteresis loop widens. A careful perusal of the shape of the hysteresis loops shown in Fig. 4(b) and (c) suggest that they could arise from a mixture of two ferromagnetic phases. Similar loops have been observed in mutilayers of SmCo and Fe [24] and nanoscopic mixtures of Fe3Pt and FePt [25]. The observance of these composite hysteresis loops point to the presence of a mixture of soft and hard ferromagnetic phases in the sample with x =0.05 and 0.1 at 255 K. Beyond an Fe fraction of x =0.1, there is a qualitative change in the shape of the high temperature hysteresis curves, as shown in Fig. 4(d–f). These hysteresis curves suggest that the permeability shows a marked decrease with increase in Fe content beyond x= 0.1. The curves in the x = 0.3 and 0.4 sample are nearly paramagnetic. The evolution of the hysteresis loops with increasing Fe content suggests that magnetic interactions become more isotropic and diminish altogether in the high concentration samples. It was shown earlier from the temperature dependent magnetization data that at low temperature the low concentration Fe substituted samples have a AFM like drop (cf. Fig. 3). To see if the magnetic field has any influence on the AFM nature in the Fe substituted samples, isothermal magnetization measurements have been carried out in the AFM like region in the low concentration samples. The measured magnetic hysteresis for several low concentration samples is shown in Fig. 5. In the pristine sample the isothermal hysteresis curve was traced at 185 K. It is clear from Fig. 5(a), that in the pristine sample there is a meta-magnetic transition [2] induced as a function of magnetic field and the field induced ferromagnet shows the characteristic hysteresis for fields 42.5 T, consistent with earlier reports [2]. For the lowest Fe substitution, viz., in the x =0.025 sample, the AFM to the ferromagnetic phase occurs at 200 K (see Fig. 2). The isothermal magnetization measurement carried out at 185 K is shown in Fig. 5(b). Although, according to ZFC magnetization curve, there should be an AFM phase at this temperature a soft ferromagnetic hysteresis loop is observed in Fig. 5(b). The
hysteresis curve magnified close to zero field is shown in inset of Fig. 5(b). A bulging feature is discernible for fields less than 1 T. This suggests that the hysteresis curve shown in main panel of Fig. 5(b), could arise from a superposition of a ferromagnetic loop and meta-magnetic loop similar to that shown in Fig. 5(a). This implies that the observation of field induced meta-magnetic transition of the AFM phase is masked by the ferromagnetic hysteresis occurring from ferromagnetic regions of the sample, indicating that both FM and AFM phases co-exist at this temperature for the x = 0.025 sample (cf. Fig. 5(b)). Similarly, an examination of the hysteresis loops at 110 and 100 K in x = 0.05 and 0.1 samples shown in Fig. 5(c) and (d) and the corresponding M(T) curves shown in Fig. 3 indicate that FM and AFM phases are present in samples with Fe fractions o0.1. The ferromagnetic component is seen to increase with Fe content. It is well known that at the AFM/FM transition in GdBaCo2O5.5 is associated with a magneto-resistance [2]. From Fig. 3(a) it was evident that AFM/FM transition occurs in stages in the low concentration Fe samples. To investigate the associated MR behaviour we measured resistivity in the 50 to 300 K range in zero field and in 6 T field. The MR was determined from the difference in the resistance at 6 T and 0 T, normalized to the zero field resistance, at a given temperature T by ((R(T,0)–R(T,6T))/ (R(T,0))), evaluated in percentage. The measured MR for the low concentration Fe samples is shown in the lower panel of Fig. 6. In the pristine sample 10% MR occurs in the temperature regime where the AFM phase goes into the FM phase similar to earlier reports [1,2]. From the MR and magnetization data shown in Fig. 6 it is apparent that even in Fe substituted samples the occurrence of MR is associated with the transition from the AFM to the first FM phase, as indicated by arrows in Fig. 6. The magnitude of MR shows a marked increase in the x =0.025 sample ( 50%), when compared to that in the pristine sample. Further it is seen to occur over a wider temperature range. Similarly large MR is observed in the x= 0.05 and 0.075 samples. Since the occurrence of MR in this system is associated with the conversion of the AFM phase to the
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Fig. 6. MR (upper panel) and ZFC M(T) compared for low Fe content samples (xo 0.1). Arrows indicate the correlated changes in magnetization and magnetoresistance.
FM phase, it appears that all samples with Fe fraction x o0.1 have a sizeable AFM phase. This taken in conjunction with magnetization measurements give clear evidence that an AFM phase co-exists with the FM phase at low temperatures in the Fe samples with x r0.1.
4. Discussion Since iodometric titration results indicate that the Co4 + concentration increases in proportion to the Fe substituted and ¨ room temperature Mossbauer studies indicate the Fe in 3+ state with an oxygen co-ordination of an octahedron, charge balance requires that excess O be introduced into the system. These O atoms will necessarily occupy the Gd–O layer, changing some of the Co–O pyramids to Co–O octahedra, which could result in the observed increase in the c lattice parameter. An increased c lattice parameter was observed with increase in Ln ion size in LnBaCo2O5.5, which results in a systematic decrease in the AFM to FM transition temperature [26,27]. The latter is intimately related to the magnitude of the crystal field splitting in the Co–O octahedra which depends on the size of the octahedra. The presence of AFM/FM transition that occurs in different temperature regimes in Fe substituted samples could imply the presence distorted Co–O octahedra. This can be elaborated as follows: in the GdBaCo2 xFexO5.5 + x/2 samples, for every two Fe3 + added, two Co4 + and one O atom is added to the structure, resulting in large disorder due to Fe substitution. The introduction of O leading to c
lattice expansion can result in expanded Co–O octahedra, locally. On the other hand the introduction of the smaller Co4 + ion would compress the Co–O octahedra in its neighbourhood. For low Fe concentrations there could also exist regions in the sample with undistorted Co–O octahedra. Thus there could arise a distribution of sizes of Co–O octahedra and corresponding crystal fields in the Fe substituted samples. According to spin state ordering model [5,6] or the pds hybridization model [26], temperature induced Co2 + –Co4 + formation in the Co3 + octahedra leads to the ferromagnetic coupling. The distribution of the crystal field splitting due to the distortion of the octahedrons in the Fe containing samples, could result in the Co2 + –Co4 + formation at different temperatures resulting in the ferromagnetic coupling occurring at lower/higher temperatures for larger/smaller sized Co–O octahedra. This could be the origin of the several FM like transitions observed in the ZFC M(T) curves for Fe fraction o0.1. Once a part of the sample transforms to the ferromagnetic phase at low temperature due to a small crystal field, it remains so until the ferromagnetic–paramagnetic transition temperature. Thus at higher temperature, when the remaining regions of the sample with higher crystal field transform into ferromagnetic phase, two or more ferromagnetic phases can co-exist. This can give rise to the composite hysteresis loops seen in the low concentration samples at high temperatures. The correlated behaviour of the ZFC magnetization and the magneto-resistance, suggests that in the low Fe concentration samples, at low temperature an AFM phase is present, which under application of magnetic field turns ferromagnetic, giving rise to MR. In addition to an AFM phase the low concentration samples show the presence of a ferromagnetic phase, possibly due to disorder induced uncompensated AFM state or Co4 + induced ferromagnetic coupling. Alternatively, the two ferromagnetic phases can occur due to meso-scopic phase separation into an O rich and O poor regions with different para– ferro and ferro–antiferro transition temperatures as suggested for GdBaCo2O5 + d samples for 0.5 o d o0.7 [2]. The double hysteresis loops were however not reported for this phase separated scenario [2]. The collapse of magnetic order in high concentration samples (x 40.1) can occur due to competing and frustrating magnetic interactions that can manifest at high Fe concentration. Similar observations of collapse of magnetism with increase in Fe content in the Fe substituted in the LaCoO3 system were understood to arise due to domination of frustrating magnetic interactions arising from nearest neighbour AFM Fe3 + –O–Fe3 + interaction and FM Co4 + (LS)–O–Fe3 + ; FM Co3 + (IS)–O–Co4 + (LS); FM Co3 + (IS)–O– Fe3 + , permitted by GKA rules [28]. These interactions can occur in the Fe doped GdBaCo2O5.5 sample in the pyramids, octahedra and across octahedra and pyramids. We now discuss the outcome of our results in the context of results on the Fe substituted LnBaCo2O5.5 samples carried out earlier [16–18]. Our heat treatment schedules, the preservation of orthorhombic structure with Fe substitution and magnetization behaviour are similar to that reported in Ref. [16]. Both our results and those of Ref. [16] point to the presence of two ferromagnetic phases in the Fe containing samples. Based on magnetization data, ¨ Mossbauer data [17] and neutron powder diffraction data [18], the magnetic phases in Fe substituted TbBaCo2 xFexO5.5 system is arrived at to be ‘ferromagnetic’ with a large component of G-type anti-ferromagnetic ordered phase at high temperature. The FC magnetization versus temperature for a Fe fraction of x= 0.1 in the GdBaCo2 xFexO5.5 system (see Fig. 3 and Ref. [16]) and that is seen in the TbBaCo2 xFexO5.5 [17] differ considerably. In particular FC M(T) shown in Ref. [17], show a clean drop in magnetization at 200 K corresponding to a AFM transition which is not evident in the present study and those of Ref. [16]. The observed difference in the magnetization behaviours seen in the TbBaCo2 xFexO5.5 7 d
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[17,18] and in GdBaCo2 xFexO5.5 7 d, for x= 0.1, may have its origin in the O stoichiometry values, which is 5.43 in the Tb sample [18], but 5.55 in the Gd sample, as shown in this study, possibly due to the difference in the heat treatment conditions. The O stoichiometry was not determined in Ref. [16]. This observation of variation in the magnetic transitions for different O contents is consistent with the magnetic phase diagram proposed as function of oxygen stoichiometry in GdBaCo2O5.5 7 d [2] which clearly shows different magnetic behaviours for oxygen deficiency and excess.
5. Summary A systematic investigation of the magnetic behaviour of Fe substituted GdBaCo2O5.5 samples has been carried out. Fe substitution is associated with an introduction of Co4 + and O substitution, which leads to local distortions in the structure and consequently in a distribution of crystal fields. Since ferromagnetic coupling in this system is associated with conversion of Co3 + in to Co4 + and Co2 + , their activation at a given temperature depends on the strength of the local crystal field. A lower crystal field can result in the occurrence of Co2 + and Co4 + at a lower temperature promoting ferromagnetic coupling. A distribution of crystal fields could therefore give rise to step-like increases in the M versus T curves at different temperatures. The possibility of coexisting magnetic phases at high temperatures can result in the observation of composite hysteresis curves. The low temperature hysteresis curves and magnetoresistance measurements clearly point to the presence of both AFM and FM regions in the Fe doped samples. These measurements indicate that the Fe substituted GdBaCo2O5.5 system is unstable to phase separation into two ‘ferromagnetic-like’ phases at high temperature and to AFM and ‘ferromagnetic-like’ phases at low temperature. References [1] I.O. Troyanchuk, N.V. Kasper, D.D. Khalyavin, H. Szymczak, R. Szymczak, M. Baran, Phys. Rev. Lett. 80 (1998) 3380. [2] A.A. Taskin, A.N. Lavrov, Yoichi Ando, Phys. Rev. B 71 (2005) 134414.
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