- Email: [email protected]
Magnetic properties of the Mg2FeV3O11−x site-disordered vanadate
Journal of Non-Crystalline Solids 352 (2006) 4179–4182 www.elsevier.com/locate/jnoncrysol
Magnetic properties of the Mg2FeV3O11 N. Guskos
x
site-disordered vanadate
a,b,* ,
V. Likodimos c, J. Typek b, G. Zolnierkiewicz b, R. Szymczak d, A. Blonska-Tabero e
a
e
Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece b Institute of Physics, Szczecin University of Technology, Al.Piasto´w 17, 70-310 Szczecin, Poland c Physics Department, National Technical University of Athens, 157 80 Athens, Greece d Institute of Physics, Polish Academy of Sciences, 02-668 Warszawa, Poland Institute of Chemistry and Environmental Protection, Szczecin University of Technology, Al.Piasto´w 42, 71-065 Szczecin, Poland Available online 18 September 2006
Abstract The magnetic properties of the Mg2FeV3O11 x ternary vanadate, characterized by disorder between diamagnetic Mg2+ and high-spin Fe ions, are studied using dc magnetization and electron paramagnetic resonance (EPR). The dc susceptibility shows antiferromagnetic interactions between Fe3+ spins with a Curie–Weiss temperature of H = 50(1) K, followed by spin-glass-like freezing at Tf 2.8 K, suggesting significant spin frustration. Temperature-dependent EPR measurements confirm the antiferromagnetic coupling of Fe3+ spins at high temperatures, while a distinct divergence is observed at T 50 K. This behavior is associated with the formation of spin clusters providing two different energy scales for the magnetic interactions. The magnetic response of Mg2FeV3O11 x is similar to that of the Znanalogue compound, though the observed differences of the implicated energy scales indicate that magnetic inhomogeneity depends on the extent of cation disorder. Published by Elsevier B.V. 3+
PACS: 75.50.Lk; 76.30.Fc Keywords: Magnetic properties
1. Introduction Multicomponent vanadate oxides exhibit a rich variety of structural and magnetic properties mainly driven by the presence of cation disorder in their crystal structure [1–3]. Structural studies of the recently synthesized ternary vanadates, M2FeV3O11 x (M = Zn and Mg), have shown a disordered distribution of iron and diamagnetic M ions, which could be responsible for significant differences in their physical properties [1,2,4,5]. Site disorder and frustration, frequently observed in transition metal oxides, may hinder the magnetic long-range order, the most prominent * Corresponding author. Address: Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece. Tel.: +30 210 72 76 711; fax: +30 210 72 84 782. E-mail address: [email protected] (N. Guskos).
0022-3093/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2006.07.009
examples being spin glasses [6] and geometrically frustrated antiferromagnets, where all spin interactions cannot be simultaneously minimized due to lattice geometry constraints [7,8]. According to a recent study of Zn2FeV3O11 x [3], the dc magnetic susceptibility showed the presence of antiferromagnetic (AFM) interactions between Fe3+ spins with a Curie–Weiss temperature H = 58(1) K, whereas a transition to a frozen, spin-glass-like state was identified at Tf = 2.55 K, indicating frustrated magnetic interactions [3]. Electron paramagnetic resonance (EPR) measurements confirmed the AFM coupling of Fe3+ spins at high temperatures, while anomalous behavior was observed at T 55 K, coinciding with the mean-field energy scale provided by the Curie–Weiss temperature [3]. For the homologous Mg2FeV3O11 compound, structural studies have shown that the presence of Mg atoms promotes cation
4180
N. Guskos et al. / Journal of Non-Crystalline Solids 352 (2006) 4179–4182
disorder [1,4]. In particular, Fe was found to be distributed non-statistically with Mg atoms on all three available crystallographic sites, in contrast to the Zn analogue. In this work, the magnetic properties of the Mg2FeV3O11 x vanadate, where both the disordered distribution of Mg atoms and the possible presence of oxygen deficiency may vary the system’s magnetic heterogeneity, are studied using dc magnetization and electron paramagnetic resonance (EPR) measurements. 2. Experimental Polycrystalline samples of Mg2FeV3O11 x were obtained by the solid state reaction method using a stoichiometric mixture of the MgO, V2O3 and Fe2O3 oxides [1,9]. The crystal structure of the compound was studied by neutron powder diffraction [4], which showed the presence of a major phase with triclinic symmetry and unit cell parameters of a = 0.64437(9) nm, b = 0.68146(9) nm, c = 1.0109(1) nm, a = 97.35(1)°, b = 103.45°, c = 101.51°, and Z = 2. Density was experimentally determined to be d = 3.48(3) g/cm3, in agreement with the calculated density of dcal = 3.479 g/cm3 [4]. Iron atoms could be distributed with magnesium atoms on three crystallographic sites, two octahedral and one bipyramidal. Site-independent population refinements resulted in the following distribution: M(1) = 0.28(3)Fe + 0.72(3)Mg, M(2) = 0.59(2)Fe + 0.41(2)Mg, M(3) = 0.12(2)Fe + 0.88(2)Mg [4]. Magnetization measurements were carried out on a MPMS-5 SQUID magnetometer in the temperature range of 2–300 K, in zero-field-cooled (ZFC) and field-cooled (FC) modes. The EPR spectra were recorded using a Bruker E 500 X-band spectrometer with magnetic field modulation of 100 kHz. The magnetic field was scaled with a usual NMR technique. The measurements were made in the temperature range from 4.2 K to 300 K using an Oxford Instrument flow cryostat and a standard hot air flow system. 3. Results and discussion 3.1. dc magnetization measurements Fig. 1 shows the temperature dependence of the inverse susceptibility, v 1(T), derived from ZFC magnetization measurements at H = 100 Oe as (MZFC/H) 1 for Mg2FeV3O11 x. Curie–Weiss behavior can be observed in the temperature range of 50–300 K, irrespective of the strength of the applied magnetic field, while a paramagnetic-like downturn of v 1 is detected at lower temperatures, a common behavior of most of frustrated antiferromagnets [7]. Fitting the experimental v 1(T) data to the Curie–Weiss law at T > 50 K yields an effective moment of 5.40(1)lB per formula unit, which is close to the spin-only value of 5.92lB for Fe3+ ions, and a negative Curie–Weiss temperature H = 50(1) K. These values are close to those reported for Zn2FeV3O11 x in the same tem-
Fig. 1. Temperature dependence of inverse dc susceptibility v 1 = (MZFC/ H) 1 for Mg2FeV3O11 x at H = 100 Oe. The solid line shows the Curie– Weiss fit at high temperatures (T > 50 K). The inset shows the field dependence of the dc magnetization at various temperatures.
perature range [1,3], confirming the presence of high-spin 3d5 iron. The value of the Curie–Weiss temperature shows significant AFM coupling, further evidenced by the field dependence of the isothermal dc magnetization, M(H), at various temperatures (shown in the inset of Fig. 1). The low magnetization (with respect to that of high-spin iron ions at 2 K) sustained even at high magnetic fields verifies the presence of AFM interactions causing compensation of a large fraction of Fe3+ moments. Fig. 2a presents the low temperature dependence of the ZFC and FC magnetization, M/H, for Mg2FeV3O11 x at various magnetic fields. A weak but distinct maximum is noticeable at 2.8 K in the ZFC magnetization at low fields, below which the MZFC and MFC branches diverge, signaling the onset of irreversibility. At higher magnetic fields (H > 100 Oe), the maximum becomes gradually smeared out, while the onset of irreversibility shifts towards lower temperatures, thus indicating a spin-glass-like transition. Plots of the temperature derivative d(MZFC/H)/dT are shown in Fig. 2b for various magnetic fields as a function of temperature. The freezing temperature, Tf, can be clearly identified from the center of the resonance-like curve at 2.8 K, where all plots of the d(MZFC/H)/dT vs T temperature derivative cross for different fields [10]. The value is slightly higher than that obtained for the Zn counterpart, where Tf = 2.55 K [3]. Furthermore, comparison of the Curie–Weiss temperature that determines the mean-field energy scale for the AFM interactions of Fe3+ spins and the freezing temperature, results in the ratio of f = H/ Tf 18 (23 for Zn2FeV3O11) that shows the presence of significant spin frustration for Mg2FeV3O11 x [7]. Two distinct energy scales can be thus identified for both M2FeV3O11 x vanadates (M = Zn and Mg), which may reflect the presence of two separate spin systems, one stemming from strongly correlated spins and the other due to a
N. Guskos et al. / Journal of Non-Crystalline Solids 352 (2006) 4179–4182
Fig. 2. (a) Low temperature dependence of the ZFC and FC magnetization for Mg2FeV3O11 x at various magnetic fields. (b) Plots of temperature derivative d(MZFC/H)/dT vs.T at low temperatures for various applied fields. The arrow indicates the freezing temperature, Tf 2.8 K.
small fraction of weakly interacting spins. The variation observed between the two compounds can be attributed to the higher degree of structural disorder in the Fe/Mg distribution. 3.2. EPR measurements Representative EPR spectra at different temperatures are shown in Fig. 3 for Mg2FeV3O11 x. A single, broad EPR line is observed, slightly asymmetric with an almost Lorentzian shape typical of exchange-narrowed systems. The resonance’s intensity and the corresponding spin susceptibility can be well accounted for by assuming that almost all Fe3+ ions are in the high-spin state (S = 5/2) and contribute to the EPR spectra at high temperatures, as in Zn2FeV3O11 x [3]. A weak, narrow line is superimposed on the broad EPR line at g 2, which may arise from a low concentration of paramagnetic centers, such as vanadium ions with valence lower than 5+. The derivative EPR spectra have been fitted to a full Lorentz line
4181
Fig. 3. EPR spectra of Mg2FeV3O11 x at various temperatures (m 9.45 GHz). Solid lines show the best-fit curves to a single Lorentzian line shape.
comprising the tail of resonance absorption in the negative field, a consequence of the linearly polarized rf field that gains importance when the line width becomes comparable to the resonance field. Fig. 4 summarizes the temperature variation of the EPR parameters, namely the resonance field, Hr, the line width, DH (half-width at half-height) and the resonance intensity, IEPR(T), derived by double integration of the EPR line. A sharp drop of all the EPR parameters is observed below T 49 K, very close to the Curie–Weiss temperature, H = 50(1) K, determined from the high-T dc susceptibility. At lower temperatures, a weak but well-defined peak appears in the resonance intensity, IEPR(T), at T 12 K, accompanied by a smaller variation in Hr and DH. Similar results have been reported recently for Zn2FeV3O11 x, although they occur at T 55 K and are less pronounced than in the present case; the low temperature variation is even weaker [3]. This behavior resembles to some extent the temperature variation of the EPR spectra expected near magnetic phase transitions in ordinary antiferromagnets [11] or spin-glasses [12]. At the same time, this is in contrast with the static magnetization and neutron diffraction results, where no evidence of any distinct magnetic
4182
N. Guskos et al. / Journal of Non-Crystalline Solids 352 (2006) 4179–4182
mogeneity through formation of a small fraction of magnetic vanadium spins (e.g. V4+ with spin S = 1/2 that may disrupt or enhance the exchange coupling between Fe3+ moments. 4. Conclusions
Fig. 4. Temperature dependence of the resonance field (Hr), line width (DH, upper panel) and resonance intensity (IEPR, lower panel) of Mg2FeV3O11 x.
anomaly has been observed at 50–55 K for both M2FeV3O11 x (M = Zn and Mg) vanadates [2–4]. A magnetically inhomogeneous state can be suggested instead, assuming that at high temperatures exchange-coupled Fe3+ spins contribute to the EPR signal, while at lower temperatures the short-range AFM order, confined to distinct spatial regions, causes the sharp variation of the EPR parameters and inhibits long-range magnetic ordering. In particular, formation of AFM clusters with an energy gap of the order of 50 K may explain the steep reduction in the EPR intensity, the resonance field shift and the divergent behavior of DH in the relatively narrow temperature range of 40–60 K, by wiping out the corresponding spectral weight. The remaining Fe3+ spins, characterized by weaker exchange interactions, should prevail down to low temperatures, eventually leading to spin freezing at Tf. Magnetic inhomogeneity appears to be promoted in Mg2FeV3O11 x further than it is in the Zn compound. In this case, the distribution of Fe atoms on the bipyramidal sites, in addition to the octahedral ones, may enhance the AFM correlations between Fe3+ ions and promote the formation of larger clusters, causing the steeper variation of the EPR parameters at T 50 K for the Mg-compound. Variations in the oxygen deficiency, which has to be further assessed for the M2FeV3O11 (M = Zn and Mg) compounds, may also contribute to the system’s magnetic inho-
Magnetization and EPR measurements of the Mg2FeV3O11 x ternary vanadate have shown the presence of significant spin frustration due to the inherent site-disorder of Fe3+ with diamagnetic Mg2+ ions. In particular, the dc susceptibility has revealed AFM interactions between Fe3+ spins with a Curie–Weiss temperature of H = 50(1) K, followed by a transition to a frozen, spin-glasslike state at Tf = 2.8 K. The temperature variation of the EPR spectra has confirmed the AFM coupling of Fe3+ spins at high temperatures, while a sharp divergence of all the EPR parameters has been observed at T 50 K, followed by a minor one at T 11 K. This behavior is attributable to the presence of magnetic inhomogeneity due to the formation of AFM spin clusters, which prevent the system from reaching a long-range magnetic order and provide two different energy scales for the magnetic interactions reflected in the dc magnetization and EPR measurements. Comparison with the magnetic properties of the Zn-homologous compound has shown that the magnetic response of both systems is qualitatively similar, with minor differences of the implicated energy scales attributable to the variation of cation disorder. Acknowledgement This work was partially supported by grant PBZ-KBN1311/TO9/2005/29. References [1] X. Wang, D.A. Vander Griend, C.L. Stern, K.P. Poeppelmeir, J. Alloys Compd. 298 (2000) 119. [2] N. Guskos, J. Typek, A. Bezkrovnyy, M. Wabia, M. Kurzawa, E.A. Anagnostakis, G. Gasiorek, J. Alloys Compd. 337 (2004) 47. [3] V. Likodimos, N. Guskos, S. Glenis, R. Szymczak, A. Bezkrovnyy, M. Wabia, J. Typek, G. Gasiorek, M. Kurzawa, I. RychlowskaHimmel, A. Blonska-Tabero, Eur. Phys. J. B 38 (2004) 13. [4] N. Guskos, M. Wabia, M. Kurzawa, A. Bezkrovnyy, V. Likodimos, J. Typek, I. Rychlowska-Himmel, A. Blonska-Tabero, Radiat. Eff. Defects Solids 158 (2003) 369. [5] N. Guskos, A. Bezkrovnyy, J. Typek, N.Yu. Ryabova, A. BlonskaTabero, M. Kurzawa, M. Maryniak, J. Alloys Compd. 391 (2005) 20. [6] K. Binder, A.P. Young, Rev. Mod. Phys. 58 (1986) 801. [7] A.P. Ramirez, Annu. Rev. Mater. Sci. 24 (1994) 453. [8] J.E. Greedan, J. Mater. Chem. 11 (2001) 37. [9] I. Rychlowska-Himmel, A. Blonska-Tabero, J. Therm. Anal. Cal. 56 (1999) 205. [10] R.V. Chamberlin, M. Hardiman, L.A. Turkevich, R. Orbach, Phys. Rev. B 25 (1982) 6720. [11] D.L. Huber, Phys. Rev. B 6 (1972) 3180. [12] C.Y. Huang, J. Magn. Magn. Mater. 51 (1985) 1.
Magnetic properties of the Mg2FeV3O11 N. Guskos
x
site-disordered vanadate
a,b,* ,
V. Likodimos c, J. Typek b, G. Zolnierkiewicz b, R. Szymczak d, A. Blonska-Tabero e
a
e
Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece b Institute of Physics, Szczecin University of Technology, Al.Piasto´w 17, 70-310 Szczecin, Poland c Physics Department, National Technical University of Athens, 157 80 Athens, Greece d Institute of Physics, Polish Academy of Sciences, 02-668 Warszawa, Poland Institute of Chemistry and Environmental Protection, Szczecin University of Technology, Al.Piasto´w 42, 71-065 Szczecin, Poland Available online 18 September 2006
Abstract The magnetic properties of the Mg2FeV3O11 x ternary vanadate, characterized by disorder between diamagnetic Mg2+ and high-spin Fe ions, are studied using dc magnetization and electron paramagnetic resonance (EPR). The dc susceptibility shows antiferromagnetic interactions between Fe3+ spins with a Curie–Weiss temperature of H = 50(1) K, followed by spin-glass-like freezing at Tf 2.8 K, suggesting significant spin frustration. Temperature-dependent EPR measurements confirm the antiferromagnetic coupling of Fe3+ spins at high temperatures, while a distinct divergence is observed at T 50 K. This behavior is associated with the formation of spin clusters providing two different energy scales for the magnetic interactions. The magnetic response of Mg2FeV3O11 x is similar to that of the Znanalogue compound, though the observed differences of the implicated energy scales indicate that magnetic inhomogeneity depends on the extent of cation disorder. Published by Elsevier B.V. 3+
PACS: 75.50.Lk; 76.30.Fc Keywords: Magnetic properties
1. Introduction Multicomponent vanadate oxides exhibit a rich variety of structural and magnetic properties mainly driven by the presence of cation disorder in their crystal structure [1–3]. Structural studies of the recently synthesized ternary vanadates, M2FeV3O11 x (M = Zn and Mg), have shown a disordered distribution of iron and diamagnetic M ions, which could be responsible for significant differences in their physical properties [1,2,4,5]. Site disorder and frustration, frequently observed in transition metal oxides, may hinder the magnetic long-range order, the most prominent * Corresponding author. Address: Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece. Tel.: +30 210 72 76 711; fax: +30 210 72 84 782. E-mail address: [email protected] (N. Guskos).
0022-3093/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2006.07.009
examples being spin glasses [6] and geometrically frustrated antiferromagnets, where all spin interactions cannot be simultaneously minimized due to lattice geometry constraints [7,8]. According to a recent study of Zn2FeV3O11 x [3], the dc magnetic susceptibility showed the presence of antiferromagnetic (AFM) interactions between Fe3+ spins with a Curie–Weiss temperature H = 58(1) K, whereas a transition to a frozen, spin-glass-like state was identified at Tf = 2.55 K, indicating frustrated magnetic interactions [3]. Electron paramagnetic resonance (EPR) measurements confirmed the AFM coupling of Fe3+ spins at high temperatures, while anomalous behavior was observed at T 55 K, coinciding with the mean-field energy scale provided by the Curie–Weiss temperature [3]. For the homologous Mg2FeV3O11 compound, structural studies have shown that the presence of Mg atoms promotes cation
4180
N. Guskos et al. / Journal of Non-Crystalline Solids 352 (2006) 4179–4182
disorder [1,4]. In particular, Fe was found to be distributed non-statistically with Mg atoms on all three available crystallographic sites, in contrast to the Zn analogue. In this work, the magnetic properties of the Mg2FeV3O11 x vanadate, where both the disordered distribution of Mg atoms and the possible presence of oxygen deficiency may vary the system’s magnetic heterogeneity, are studied using dc magnetization and electron paramagnetic resonance (EPR) measurements. 2. Experimental Polycrystalline samples of Mg2FeV3O11 x were obtained by the solid state reaction method using a stoichiometric mixture of the MgO, V2O3 and Fe2O3 oxides [1,9]. The crystal structure of the compound was studied by neutron powder diffraction [4], which showed the presence of a major phase with triclinic symmetry and unit cell parameters of a = 0.64437(9) nm, b = 0.68146(9) nm, c = 1.0109(1) nm, a = 97.35(1)°, b = 103.45°, c = 101.51°, and Z = 2. Density was experimentally determined to be d = 3.48(3) g/cm3, in agreement with the calculated density of dcal = 3.479 g/cm3 [4]. Iron atoms could be distributed with magnesium atoms on three crystallographic sites, two octahedral and one bipyramidal. Site-independent population refinements resulted in the following distribution: M(1) = 0.28(3)Fe + 0.72(3)Mg, M(2) = 0.59(2)Fe + 0.41(2)Mg, M(3) = 0.12(2)Fe + 0.88(2)Mg [4]. Magnetization measurements were carried out on a MPMS-5 SQUID magnetometer in the temperature range of 2–300 K, in zero-field-cooled (ZFC) and field-cooled (FC) modes. The EPR spectra were recorded using a Bruker E 500 X-band spectrometer with magnetic field modulation of 100 kHz. The magnetic field was scaled with a usual NMR technique. The measurements were made in the temperature range from 4.2 K to 300 K using an Oxford Instrument flow cryostat and a standard hot air flow system. 3. Results and discussion 3.1. dc magnetization measurements Fig. 1 shows the temperature dependence of the inverse susceptibility, v 1(T), derived from ZFC magnetization measurements at H = 100 Oe as (MZFC/H) 1 for Mg2FeV3O11 x. Curie–Weiss behavior can be observed in the temperature range of 50–300 K, irrespective of the strength of the applied magnetic field, while a paramagnetic-like downturn of v 1 is detected at lower temperatures, a common behavior of most of frustrated antiferromagnets [7]. Fitting the experimental v 1(T) data to the Curie–Weiss law at T > 50 K yields an effective moment of 5.40(1)lB per formula unit, which is close to the spin-only value of 5.92lB for Fe3+ ions, and a negative Curie–Weiss temperature H = 50(1) K. These values are close to those reported for Zn2FeV3O11 x in the same tem-
Fig. 1. Temperature dependence of inverse dc susceptibility v 1 = (MZFC/ H) 1 for Mg2FeV3O11 x at H = 100 Oe. The solid line shows the Curie– Weiss fit at high temperatures (T > 50 K). The inset shows the field dependence of the dc magnetization at various temperatures.
perature range [1,3], confirming the presence of high-spin 3d5 iron. The value of the Curie–Weiss temperature shows significant AFM coupling, further evidenced by the field dependence of the isothermal dc magnetization, M(H), at various temperatures (shown in the inset of Fig. 1). The low magnetization (with respect to that of high-spin iron ions at 2 K) sustained even at high magnetic fields verifies the presence of AFM interactions causing compensation of a large fraction of Fe3+ moments. Fig. 2a presents the low temperature dependence of the ZFC and FC magnetization, M/H, for Mg2FeV3O11 x at various magnetic fields. A weak but distinct maximum is noticeable at 2.8 K in the ZFC magnetization at low fields, below which the MZFC and MFC branches diverge, signaling the onset of irreversibility. At higher magnetic fields (H > 100 Oe), the maximum becomes gradually smeared out, while the onset of irreversibility shifts towards lower temperatures, thus indicating a spin-glass-like transition. Plots of the temperature derivative d(MZFC/H)/dT are shown in Fig. 2b for various magnetic fields as a function of temperature. The freezing temperature, Tf, can be clearly identified from the center of the resonance-like curve at 2.8 K, where all plots of the d(MZFC/H)/dT vs T temperature derivative cross for different fields [10]. The value is slightly higher than that obtained for the Zn counterpart, where Tf = 2.55 K [3]. Furthermore, comparison of the Curie–Weiss temperature that determines the mean-field energy scale for the AFM interactions of Fe3+ spins and the freezing temperature, results in the ratio of f = H/ Tf 18 (23 for Zn2FeV3O11) that shows the presence of significant spin frustration for Mg2FeV3O11 x [7]. Two distinct energy scales can be thus identified for both M2FeV3O11 x vanadates (M = Zn and Mg), which may reflect the presence of two separate spin systems, one stemming from strongly correlated spins and the other due to a
N. Guskos et al. / Journal of Non-Crystalline Solids 352 (2006) 4179–4182
Fig. 2. (a) Low temperature dependence of the ZFC and FC magnetization for Mg2FeV3O11 x at various magnetic fields. (b) Plots of temperature derivative d(MZFC/H)/dT vs.T at low temperatures for various applied fields. The arrow indicates the freezing temperature, Tf 2.8 K.
small fraction of weakly interacting spins. The variation observed between the two compounds can be attributed to the higher degree of structural disorder in the Fe/Mg distribution. 3.2. EPR measurements Representative EPR spectra at different temperatures are shown in Fig. 3 for Mg2FeV3O11 x. A single, broad EPR line is observed, slightly asymmetric with an almost Lorentzian shape typical of exchange-narrowed systems. The resonance’s intensity and the corresponding spin susceptibility can be well accounted for by assuming that almost all Fe3+ ions are in the high-spin state (S = 5/2) and contribute to the EPR spectra at high temperatures, as in Zn2FeV3O11 x [3]. A weak, narrow line is superimposed on the broad EPR line at g 2, which may arise from a low concentration of paramagnetic centers, such as vanadium ions with valence lower than 5+. The derivative EPR spectra have been fitted to a full Lorentz line
4181
Fig. 3. EPR spectra of Mg2FeV3O11 x at various temperatures (m 9.45 GHz). Solid lines show the best-fit curves to a single Lorentzian line shape.
comprising the tail of resonance absorption in the negative field, a consequence of the linearly polarized rf field that gains importance when the line width becomes comparable to the resonance field. Fig. 4 summarizes the temperature variation of the EPR parameters, namely the resonance field, Hr, the line width, DH (half-width at half-height) and the resonance intensity, IEPR(T), derived by double integration of the EPR line. A sharp drop of all the EPR parameters is observed below T 49 K, very close to the Curie–Weiss temperature, H = 50(1) K, determined from the high-T dc susceptibility. At lower temperatures, a weak but well-defined peak appears in the resonance intensity, IEPR(T), at T 12 K, accompanied by a smaller variation in Hr and DH. Similar results have been reported recently for Zn2FeV3O11 x, although they occur at T 55 K and are less pronounced than in the present case; the low temperature variation is even weaker [3]. This behavior resembles to some extent the temperature variation of the EPR spectra expected near magnetic phase transitions in ordinary antiferromagnets [11] or spin-glasses [12]. At the same time, this is in contrast with the static magnetization and neutron diffraction results, where no evidence of any distinct magnetic
4182
N. Guskos et al. / Journal of Non-Crystalline Solids 352 (2006) 4179–4182
mogeneity through formation of a small fraction of magnetic vanadium spins (e.g. V4+ with spin S = 1/2 that may disrupt or enhance the exchange coupling between Fe3+ moments. 4. Conclusions
Fig. 4. Temperature dependence of the resonance field (Hr), line width (DH, upper panel) and resonance intensity (IEPR, lower panel) of Mg2FeV3O11 x.
anomaly has been observed at 50–55 K for both M2FeV3O11 x (M = Zn and Mg) vanadates [2–4]. A magnetically inhomogeneous state can be suggested instead, assuming that at high temperatures exchange-coupled Fe3+ spins contribute to the EPR signal, while at lower temperatures the short-range AFM order, confined to distinct spatial regions, causes the sharp variation of the EPR parameters and inhibits long-range magnetic ordering. In particular, formation of AFM clusters with an energy gap of the order of 50 K may explain the steep reduction in the EPR intensity, the resonance field shift and the divergent behavior of DH in the relatively narrow temperature range of 40–60 K, by wiping out the corresponding spectral weight. The remaining Fe3+ spins, characterized by weaker exchange interactions, should prevail down to low temperatures, eventually leading to spin freezing at Tf. Magnetic inhomogeneity appears to be promoted in Mg2FeV3O11 x further than it is in the Zn compound. In this case, the distribution of Fe atoms on the bipyramidal sites, in addition to the octahedral ones, may enhance the AFM correlations between Fe3+ ions and promote the formation of larger clusters, causing the steeper variation of the EPR parameters at T 50 K for the Mg-compound. Variations in the oxygen deficiency, which has to be further assessed for the M2FeV3O11 (M = Zn and Mg) compounds, may also contribute to the system’s magnetic inho-
Magnetization and EPR measurements of the Mg2FeV3O11 x ternary vanadate have shown the presence of significant spin frustration due to the inherent site-disorder of Fe3+ with diamagnetic Mg2+ ions. In particular, the dc susceptibility has revealed AFM interactions between Fe3+ spins with a Curie–Weiss temperature of H = 50(1) K, followed by a transition to a frozen, spin-glasslike state at Tf = 2.8 K. The temperature variation of the EPR spectra has confirmed the AFM coupling of Fe3+ spins at high temperatures, while a sharp divergence of all the EPR parameters has been observed at T 50 K, followed by a minor one at T 11 K. This behavior is attributable to the presence of magnetic inhomogeneity due to the formation of AFM spin clusters, which prevent the system from reaching a long-range magnetic order and provide two different energy scales for the magnetic interactions reflected in the dc magnetization and EPR measurements. Comparison with the magnetic properties of the Zn-homologous compound has shown that the magnetic response of both systems is qualitatively similar, with minor differences of the implicated energy scales attributable to the variation of cation disorder. Acknowledgement This work was partially supported by grant PBZ-KBN1311/TO9/2005/29. References [1] X. Wang, D.A. Vander Griend, C.L. Stern, K.P. Poeppelmeir, J. Alloys Compd. 298 (2000) 119. [2] N. Guskos, J. Typek, A. Bezkrovnyy, M. Wabia, M. Kurzawa, E.A. Anagnostakis, G. Gasiorek, J. Alloys Compd. 337 (2004) 47. [3] V. Likodimos, N. Guskos, S. Glenis, R. Szymczak, A. Bezkrovnyy, M. Wabia, J. Typek, G. Gasiorek, M. Kurzawa, I. RychlowskaHimmel, A. Blonska-Tabero, Eur. Phys. J. B 38 (2004) 13. [4] N. Guskos, M. Wabia, M. Kurzawa, A. Bezkrovnyy, V. Likodimos, J. Typek, I. Rychlowska-Himmel, A. Blonska-Tabero, Radiat. Eff. Defects Solids 158 (2003) 369. [5] N. Guskos, A. Bezkrovnyy, J. Typek, N.Yu. Ryabova, A. BlonskaTabero, M. Kurzawa, M. Maryniak, J. Alloys Compd. 391 (2005) 20. [6] K. Binder, A.P. Young, Rev. Mod. Phys. 58 (1986) 801. [7] A.P. Ramirez, Annu. Rev. Mater. Sci. 24 (1994) 453. [8] J.E. Greedan, J. Mater. Chem. 11 (2001) 37. [9] I. Rychlowska-Himmel, A. Blonska-Tabero, J. Therm. Anal. Cal. 56 (1999) 205. [10] R.V. Chamberlin, M. Hardiman, L.A. Turkevich, R. Orbach, Phys. Rev. B 25 (1982) 6720. [11] D.L. Huber, Phys. Rev. B 6 (1972) 3180. [12] C.Y. Huang, J. Magn. Magn. Mater. 51 (1985) 1.