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Influence of cation substitution on the magnetic properties of the FeCr2S4 ferrimagnet
Journal of Physics and Chemistry of Solids 66 (2005) 2040–2043 www.elsevier.com/locate/jpcs
Influence of cation substitution on the magnetic properties of the FeCr2S4 ferrimagnet V. Tsurkan a,*, J. Groza b, G. Bocelli c, D. Samusi a, P. Petrenco a, V. Zestrea a, M. Baran d, R. Szymczak d, H. Szymczak d, M. Mu¨cksch e, F. Haider e, R. Tidecks e b
a Institute of Applied Physics, Academy of Sciences of Moldova, MD 2028, Chisinau, R. Moldova Chemical Engineering/Materials Science Department, University of California at Davis, Davis, CA 95616, USA c IMEM, CNR, 43100 Parma, Italy d Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland e Institute of Physics, University of Augsburg, 86135, Augsburg, Germany
Abstract The influence of cation substitution on the magnetic properties of single and polycrystals of FeCr2S4 spinel has been studied. The tetrahedral A-site substitution of the Fe by Cu in Fe1KxCuxCr2S4 was found to increase significantly the value of temperature Tm of the spin-glass like magnetization anomaly, whereas the octahedral B-site substitution of the Cr by In decreases Tm. This effect is suggested to result from a structural transformation influenced by variation of internal (chemical) pressure due to lattice contraction (Cu) or expansion (In). The observed reduced values of the Curie temperature for Cu-substituted single crystals compared to that of the polycrystalline samples are attributed to presence of Cl ions in samples detected by electron-probe microanalysis. The observed reduced value of saturation magnetization in the polycrystals compared to the single crystals is ascribed to the effect of surface anisotropy. Based on the experimental data the superexchange is concluded to be the dominant exchange interaction for 0%x%0.5 in Fe1KxCuxCr2S4, whereas the indirect exchange through the charge carriers is considered of minor importance. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Magnetic materials; B. Crystal growth; D. Magnetic properties
1. Introduction Current interest in giant magnetoresistance effect for magnetoelectronic applications stimulated search for new materials possessing such an important property. Among them, the manganese perovskites received considerable attention [1]. Recently, Ramirez et al. [2] pointed out on giant magnetoresistance of another group of magnetic materials, e.g. ternary chromium chalcogenide spinels. One of them, FeCr2S4 ferrimagnet, was shown to exhibit the colossal magnetoresistance at a Curie temperature TCZ170 K comparable with that of the manganites. Substitution of Fe by Cu increases TC to values even above the room temperature, keeping at the same time the moderate magnetoresistive * Corresponding author. Address: Institut fur Physik, Experimentalphysik V, Universitat Augsburg, Universitatsstrasse 1, 86159 Augsburg, Germany. Tel.: C49 821 5983340; fax: C49 821 5983225. E-mail address: [email protected] (V. Tsurkan).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.040
properties. Although the magnetic and electrical properties of Fe1KxCuxCr2S4 system are known since 1967 [3,4], the data were obtained mainly on the polycrystalline samples which prevented an adequate description of the physical phenomena and characterization of the intrinsic properties of these compounds. For example, the reduced value of the magnetic moment of the polycrystalline samples was explained by different authors as due to deviation from the stoichiometry or by mixed valence and even by low spin state of chromium [3,4]. Recent magnetization studies of the FeCr2S4 single crystals [5] reveal anomalies at low temperatures that are masked in polycrystalline samples by the domain walls scattering on the grain boundaries because of strong anisotropy. Similarly, large differences were found in conductivity and specific heat behaviour of poly- and single crystals [6]. Here we present the results of detailed investigation of structural and magnetic properties on the Fe1KxCuxCr2S4 poly- and single crystals. Particular attention is paid to the low-temperature magnetization anomaly at Tmz60 K found earlier in the pure compound [5] and attributed to structural lattice transformation [7].
V. Tsurkan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2040–2043
2. Experimental Single crystals of Fe1KxCuxCr2S4 with 0%x%0.95 and FeCr2K2xIn2x with 0%x%0.15 were grown by chemical vapour transport from the polycrystals prepared by solid state reactions. The composition and phase homogeneity of samples was checked by electron-probe microanalysis (EPMA) and X-ray diffraction. Magnetic properties were studied using the vibrating sample (model PAR 4500) and SQUID (MPMS 7 Quantum Design) magnetometers in a temperature range of 2–400 K and magnetic fields up to 70 kOe. 3. Results and discussion We observed an opposite effect of tetrahedral A-site and octahedral B-site cation substitution on the Curie temperature and the magnetization anomaly at low temperatures. Fig. 1(a) presents the temperature dependence of the magnetization measured in low fields for several Fe1KxCuxCr2S4 single crystals with different substitution level x. When decreasing the temperature, the magnetization of the pure compound (xZ0) shows a cusp at a temperature Tm (w60 K) after the demagnetizing limited plateau that marks the onset of the long-range order at the Curie temperature TC(w167 K). Below the Tm, the magnetization exhibits a spin-glass like behaviour with a splitting of zero field cooled (ZFC) and field cooled (FC)
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magnetization and correspondent onset of coercivity and remanence [5]. The substitution of the Fe ions by Cu increases strongly the value of the temperature Tm for concentration x below 0.2. At the higher substitution of Cu the low-temperature anomaly at Tm is smeared out. The substitution of Cr ions by In was found to reduce the value of Tm. In addition, the low-field magnetization of the In doped samples shows a noticeable temperature hysteresis at around Tm (Fig. 1(b)) suggesting a first-order transformation. This result correlates with that of the ultrasonic study of the FeCr2S4 single crystals, which revealed a step-like decrease of the elastic modulus at Tm, ascribed to the Jahn-Teller active Fe2Cions and the resulting orbital ordering accompanied by a structural lattice transformation [7]. Fig. 2(a) presents the field dependence of the magnetization at 4.2 K for different substitution in the Fe1KxCuxCr2S4 system measured on the single crystalline samples along the axis of hard magnetization h111i. The increase of x is accompanied by the increase of the saturation magnetization Ms, the value of which is found to be larger for the single crystalline samples than for the polycrystals of the same nominal composition. To examine the effect of the non-stoichiometry on the Ms, different annealings were performed in vacuum and sulphur atmospheres that influence mainly the number of chalcogen defects. However, we observed that these heat treatments did not (a)
(a)
0.9 0.75 0.6 0.5 0.4 0.3 0.2 0.1
FC
0.1
x=0 0.05 0.1 ZFC
Tm
x=0
0.3 0.1 0.05 0.2
M (µβ / f.u.)
M (µB / f.u.)
4
2
x=0 0.05
H II <111>
Fe1-xCuxCr2S4
0 (b)
0.0 0.2
M (µB / f.u.)
100
FC
200
FeCr1.95In 0.05S4 M (µβ / f.u.)
(b)
Tm
0.1
1
ZFC FC warm FC cool
ZFC
0 0.0
0
FeCr2S4 single crystal, H II <100> single crystal powdered polycrystal polycrystal powdered
40
80
120
160
0
20
40
60
H (kOe)
T (K) Fig. 1. Temperature dependences of the ZFC and FC magnetization measured in a field of 100 Oe for the single crystals: (a) Fe1KxCuxCr2S4 (0%x%0.3); (b) FeCr1.95In0.05S4. Magnetization is in Bohr magnetons per formula unit.
Fig. 2. Field dependences of the magnetization at 4.2 K: (a) for Fe1KxCuxCr2S4 single crystals (SC) measured along the hard axis h111i; (b) for bulk and powdered FeCr2S4 poly- and single crystals (along the easy h100i axis in case of the bulk SC).
V. Tsurkan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2040–2043 Table 1 Results of the EPMA investigation of the Fe1KxCuxCr2S4 single crystals
(a) 400
Fe1-xCuxCr2S4 350
TC (K)
300 250 200 single crystal polycrystal
150 (b)
5
60
40 3
HA ( kOe )
Ms (µB/f.u.)
4
20 2
0.0
0.2
0.4
0.6
0.8
1.0
0
x Fig. 3. Variation with the substitution x: (a) of the Curie temperature TC; (b) of the saturation magnetization Ms and of the anisotropy field HA for the poly- and single crystals in the Fe1KxCuxCr2S4 system.
change the value of Ms in the polycrystals and therefore, the sulphur non-stoichiometry cannot be responsible for the reduced value of Ms. Fig. 2(b) gives the evidence of the mechanism of the moment reduction. We found that powdering of the FeCr2S4 single crystalline sample reduces substantially the value of Ms that may be explained by the increased influence of surface anisotropy. Thus, we ascribed the reduced value of the saturation magnetization in the polycrystals to the effect of the surface anisotropy. Powdering of the FeCr2S4 polycrystal shows a similar reduction of Ms. Fig. 3 shows the variation of the basic magnetic parameters in the Fe1KxCuxCr2S4 system. The Curie temperature TC, determined by kink-point method, increases nearly linear with the substitution for the range 0%x%0.5 followed by some flattening for the higher values of x up to 0.95. The value of TC increases again only for the xZ1. We note that TC for the single crystals deviate substantially from that of the polycrystals. The difference in TC between them increases with x up to 0.5 and became nearly constant at higher x. This difference might be attributed to a combined influence of the cation vacancies (mainly Fe defects) and the substitution of sulphur ions by chlorine in single crystals. The chlorine presence was revealed by electron-probe microanalysis (Table 1). The Cl content was found to increase along with the increase of the x. The substitution of Fe by Cu reduces strongly the magnetocrystalline anisotropy. In the pure FeCr 2S4 the anisotropy field HA Z2K1/M s, as determined from
Nominal concentration (x)
Fe
Cu
Cr
S
Cl
0 0.05 0.05 0.1 0.2 0.3 0.4 0.4 0.5 0.5 0.5 0.75 0.9
1.01 0.94 0.93 0.85 0.77 0.63 0.56 0.56 0.48 0.49 0.48 0.21 0.14
– 0.04 0.06 0.11 0.23 0.31 0.40 0.37 0.48 0.49 0.50 0.79 0.83
2.01 2.03 2.02 2.04 2.02 2.03 2.02 2.04 2.03 2.0 2.04 1.99 2.03
3.96 3.96 3.94 3.93 3.91 3.88 3.78 3.88 3.82 3.83 3.83 3.78 3.68
0.04 0.06 0.06 0.06 0.06 0.16 0.24 0.14 0.20 0.19 0.17 0.24 0.32
the magnetization curves along three principal cubic axes, reaches the high value of 60 kOe at 4.2 K. Here K1 and Ms are the first anisotropy constant and the saturation magnetization, respectively. We note that only 10% of substitution reduces this value by factor of two. With the further increase of x the anisotropy field decreases, but at a lower rate (Fig. 3(b)). These data correlate well with those determined by electron-spin resonance [8,9]. In the FeCr2K2xIn2xS4 system in the range 0%x%0.15 the Curie temperature and the saturation magnetization decreases with x both for poly- and single crystals. In the polycrystalline samples with substitution x above 0.2 a phase separation was found to occur with a mixture of the phases close to FeIn2S4 and FeCr1.7In0.3S4. In Fig. 4 the lattice constant a0 vs Cu substitution concentration x is presented for the poly- and single crystalline Fe1KxCuxCr2S4 samples. A large difference in the a0 values for poly- and single crystals is detected both showing a0 decrease with the increase of x. This difference might be also attributed to Cl for S substitution. Considering the mechanisms of the Curie temperature variation in the Fe1Kx Cux Cr2 S4 system we note that 10.00
Fe1-xCuxCr2S4 9.95
a0(Å)
2042
9.90
9.85
9.80 0.0
single crystal polycrystal
0.2
0.4
0.6
0.8
1.0
x Fig. 4. Variation of the lattice constant a0 with the substitution x for the polyand single crystals of the Fe1KxCuxCr2S4 system.
V. Tsurkan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2040–2043
the observed concomitant decrease of the lattice constant with the substitution suggests a certain contribution of the superexchange known to be quite sensitive to inter-atomic spacing. Although the experimental results on the Cu and In substituted samples correlate qualitatively well with those of the influence of hydrostatic pressure on the Curie temperature and Tm of pure FeCr2S4 [10], the estimations of the TC and Tm changes with substitution x due to lattice contraction in case of Cu are lower by a factor of 7 and 4, respectively, compared to the experimentally observed ones. This may indicate that chemical pressure induced by cation substitution has a larger influence than the external pressure or that an additional mechanism contributes to the variation of these quantities. Indeed, the Cu doping increases the conductivity of samples compared to that of the pure compound (xZ0). This is probably an indication of the contribution of indirect exchange via charge carriers at least for x below 0.2. However, the non-monotonic variation of the conductivity with substitution [3,9] along with the monotonic increase of the Curie temperature with x in the range 0.2!x! 0.5 suggests the indirect exchange to be less important here. In addition, the insulating character of the samples for the whole substitution ranges, except xZ1, points out that the superexchange is the dominant magnetic exchange mechanism. Finally, in view of the unusual magnetization anomaly at Tm we must mention that a small amount of Cu (about 20%) is enough to destroy this anomaly, which for the pure FeCr2S4 compound we interpret in terms of an orbital liquid state at low temperatures [6]. 4. Summary We studied the effect of cation substitution on the magnetic properties of Fe1KxCuxCr2S4 and FeCr2K2xIn2xS4 crystals. A large difference in the Curie temperature and the saturation magnetization between poly-and single crystalline samples was observed, which we ascribe respectively to influence of Cl for S substitution and surface anisotropy. The change of the Curie temperature by the substitution is attributed mainly to
2043
variation of the superexchange interactions. The magnetization irreversibilities at low temperatures originating from the Jahn-Teller active Fe2C ions disappear at high Cu substitution (xO0.2). Acknowledgements The support of US CRDF-MRDA grant MP2-3047 (Chisinau) is gratefully acknowledged. References [1] Y. Tokura, Y. Tomioka, Colossal magnetoresistive manganites, J. Magn. Magn. Mater. 200 (1999) 1–23. [2] A.P. Ramirez, R.J. Cava, J. Krajewski, Colossal magnetoresistance in Crbased chalcogenide spinels, Nature 386 (1997) 156–159. [3] F.K. Lotgering, R.P. van Stapele, G.H.A.M. van der Steen, J.S. van Wieringen, Magnetic properties, conductivity and ionic ordering in Fe1KxCuxCr2S4, J. Phys. Chem. Solids 30 (1967) 799–804. [4] G. Haacke, L.C. Beegle, Magnetic properties of the spinel system Fe1KxCuxCr2S4, J. Phys. Chem. Solids 28 (1967) 1699–1704. [5] V. Tsurkan, M. Baran, R. Szymczak, H. Szymczak, R. Tidecks, Spin-glass like states in ferrimagnetic FeCr2S4, Physica B 296 (2001) 301–305. [6] V. Tsurkan, V. Fritsch, J. Hemberger, A. Krimmel, M. Mu¨cksch, N. Bu¨ttgen, H.-A. Krug von Nidda, D. Samusi, S. Ko¨rner, E.-W. Scheidt, M. Honal, S. Horn, R. Tidecks, A. Loidl, Orbital glass in FeCr2S4, Preprint cond-mat/0407026 (http:/xxx.lanl.gov), 2004. [7] D. Maurer, V. Tsurkan, S. Horn, R. Tidecks, Ultrasonic study of ferrimagnetic FeCr2S4: evidence for low temperature structural transformation, J. Appl. Phys. 93 (2003) 9173–9176. [8] V. Tsurkan, M. Lohmann, H.-A. Krug von Nidda, A. Loidl, S. Horn, R. Tidecks, Electron-spin-resonance studies of the ferrimagnetic semiconductor FeCr2S4, Phys. Rev. B 63 (2001) (125209/1–5). [9] V. Fritsch, J. Deisenhofer, R. Fichtl, J. Hemberger, H.-A. Krug von Nidda, M. Mu¨cksch, M. Nicklas, D. Samusi, J.D. Thompson, R. Tidecks, V. Tsurkan, A. Loidl, Anisotropic colossal magnetoresistance effects in Fe1KxCuxCr2S4, Phys. Rev. B 67 (2003) (144419/1–8). [10] V. Tsurkan, I. Fita, M. Baran, R. Puzniak, D. Samusi, R. Szymczak, H. Szymczak, S. Klimm, M. Klemm, S. Horn, R. Tidecks, Effect of pressure on the magnetic and transport properties of the ferrimagnetic semiconductor FeCr2S4, J. Appl. Phys. 90 (2001) 875–881.
Influence of cation substitution on the magnetic properties of the FeCr2S4 ferrimagnet V. Tsurkan a,*, J. Groza b, G. Bocelli c, D. Samusi a, P. Petrenco a, V. Zestrea a, M. Baran d, R. Szymczak d, H. Szymczak d, M. Mu¨cksch e, F. Haider e, R. Tidecks e b
a Institute of Applied Physics, Academy of Sciences of Moldova, MD 2028, Chisinau, R. Moldova Chemical Engineering/Materials Science Department, University of California at Davis, Davis, CA 95616, USA c IMEM, CNR, 43100 Parma, Italy d Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland e Institute of Physics, University of Augsburg, 86135, Augsburg, Germany
Abstract The influence of cation substitution on the magnetic properties of single and polycrystals of FeCr2S4 spinel has been studied. The tetrahedral A-site substitution of the Fe by Cu in Fe1KxCuxCr2S4 was found to increase significantly the value of temperature Tm of the spin-glass like magnetization anomaly, whereas the octahedral B-site substitution of the Cr by In decreases Tm. This effect is suggested to result from a structural transformation influenced by variation of internal (chemical) pressure due to lattice contraction (Cu) or expansion (In). The observed reduced values of the Curie temperature for Cu-substituted single crystals compared to that of the polycrystalline samples are attributed to presence of Cl ions in samples detected by electron-probe microanalysis. The observed reduced value of saturation magnetization in the polycrystals compared to the single crystals is ascribed to the effect of surface anisotropy. Based on the experimental data the superexchange is concluded to be the dominant exchange interaction for 0%x%0.5 in Fe1KxCuxCr2S4, whereas the indirect exchange through the charge carriers is considered of minor importance. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Magnetic materials; B. Crystal growth; D. Magnetic properties
1. Introduction Current interest in giant magnetoresistance effect for magnetoelectronic applications stimulated search for new materials possessing such an important property. Among them, the manganese perovskites received considerable attention [1]. Recently, Ramirez et al. [2] pointed out on giant magnetoresistance of another group of magnetic materials, e.g. ternary chromium chalcogenide spinels. One of them, FeCr2S4 ferrimagnet, was shown to exhibit the colossal magnetoresistance at a Curie temperature TCZ170 K comparable with that of the manganites. Substitution of Fe by Cu increases TC to values even above the room temperature, keeping at the same time the moderate magnetoresistive * Corresponding author. Address: Institut fur Physik, Experimentalphysik V, Universitat Augsburg, Universitatsstrasse 1, 86159 Augsburg, Germany. Tel.: C49 821 5983340; fax: C49 821 5983225. E-mail address: [email protected] (V. Tsurkan).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.040
properties. Although the magnetic and electrical properties of Fe1KxCuxCr2S4 system are known since 1967 [3,4], the data were obtained mainly on the polycrystalline samples which prevented an adequate description of the physical phenomena and characterization of the intrinsic properties of these compounds. For example, the reduced value of the magnetic moment of the polycrystalline samples was explained by different authors as due to deviation from the stoichiometry or by mixed valence and even by low spin state of chromium [3,4]. Recent magnetization studies of the FeCr2S4 single crystals [5] reveal anomalies at low temperatures that are masked in polycrystalline samples by the domain walls scattering on the grain boundaries because of strong anisotropy. Similarly, large differences were found in conductivity and specific heat behaviour of poly- and single crystals [6]. Here we present the results of detailed investigation of structural and magnetic properties on the Fe1KxCuxCr2S4 poly- and single crystals. Particular attention is paid to the low-temperature magnetization anomaly at Tmz60 K found earlier in the pure compound [5] and attributed to structural lattice transformation [7].
V. Tsurkan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2040–2043
2. Experimental Single crystals of Fe1KxCuxCr2S4 with 0%x%0.95 and FeCr2K2xIn2x with 0%x%0.15 were grown by chemical vapour transport from the polycrystals prepared by solid state reactions. The composition and phase homogeneity of samples was checked by electron-probe microanalysis (EPMA) and X-ray diffraction. Magnetic properties were studied using the vibrating sample (model PAR 4500) and SQUID (MPMS 7 Quantum Design) magnetometers in a temperature range of 2–400 K and magnetic fields up to 70 kOe. 3. Results and discussion We observed an opposite effect of tetrahedral A-site and octahedral B-site cation substitution on the Curie temperature and the magnetization anomaly at low temperatures. Fig. 1(a) presents the temperature dependence of the magnetization measured in low fields for several Fe1KxCuxCr2S4 single crystals with different substitution level x. When decreasing the temperature, the magnetization of the pure compound (xZ0) shows a cusp at a temperature Tm (w60 K) after the demagnetizing limited plateau that marks the onset of the long-range order at the Curie temperature TC(w167 K). Below the Tm, the magnetization exhibits a spin-glass like behaviour with a splitting of zero field cooled (ZFC) and field cooled (FC)
2041
magnetization and correspondent onset of coercivity and remanence [5]. The substitution of the Fe ions by Cu increases strongly the value of the temperature Tm for concentration x below 0.2. At the higher substitution of Cu the low-temperature anomaly at Tm is smeared out. The substitution of Cr ions by In was found to reduce the value of Tm. In addition, the low-field magnetization of the In doped samples shows a noticeable temperature hysteresis at around Tm (Fig. 1(b)) suggesting a first-order transformation. This result correlates with that of the ultrasonic study of the FeCr2S4 single crystals, which revealed a step-like decrease of the elastic modulus at Tm, ascribed to the Jahn-Teller active Fe2Cions and the resulting orbital ordering accompanied by a structural lattice transformation [7]. Fig. 2(a) presents the field dependence of the magnetization at 4.2 K for different substitution in the Fe1KxCuxCr2S4 system measured on the single crystalline samples along the axis of hard magnetization h111i. The increase of x is accompanied by the increase of the saturation magnetization Ms, the value of which is found to be larger for the single crystalline samples than for the polycrystals of the same nominal composition. To examine the effect of the non-stoichiometry on the Ms, different annealings were performed in vacuum and sulphur atmospheres that influence mainly the number of chalcogen defects. However, we observed that these heat treatments did not (a)
(a)
0.9 0.75 0.6 0.5 0.4 0.3 0.2 0.1
FC
0.1
x=0 0.05 0.1 ZFC
Tm
x=0
0.3 0.1 0.05 0.2
M (µβ / f.u.)
M (µB / f.u.)
4
2
x=0 0.05
H II <111>
Fe1-xCuxCr2S4
0 (b)
0.0 0.2
M (µB / f.u.)
100
FC
200
FeCr1.95In 0.05S4 M (µβ / f.u.)
(b)
Tm
0.1
1
ZFC FC warm FC cool
ZFC
0 0.0
0
FeCr2S4 single crystal, H II <100> single crystal powdered polycrystal polycrystal powdered
40
80
120
160
0
20
40
60
H (kOe)
T (K) Fig. 1. Temperature dependences of the ZFC and FC magnetization measured in a field of 100 Oe for the single crystals: (a) Fe1KxCuxCr2S4 (0%x%0.3); (b) FeCr1.95In0.05S4. Magnetization is in Bohr magnetons per formula unit.
Fig. 2. Field dependences of the magnetization at 4.2 K: (a) for Fe1KxCuxCr2S4 single crystals (SC) measured along the hard axis h111i; (b) for bulk and powdered FeCr2S4 poly- and single crystals (along the easy h100i axis in case of the bulk SC).
V. Tsurkan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2040–2043 Table 1 Results of the EPMA investigation of the Fe1KxCuxCr2S4 single crystals
(a) 400
Fe1-xCuxCr2S4 350
TC (K)
300 250 200 single crystal polycrystal
150 (b)
5
60
40 3
HA ( kOe )
Ms (µB/f.u.)
4
20 2
0.0
0.2
0.4
0.6
0.8
1.0
0
x Fig. 3. Variation with the substitution x: (a) of the Curie temperature TC; (b) of the saturation magnetization Ms and of the anisotropy field HA for the poly- and single crystals in the Fe1KxCuxCr2S4 system.
change the value of Ms in the polycrystals and therefore, the sulphur non-stoichiometry cannot be responsible for the reduced value of Ms. Fig. 2(b) gives the evidence of the mechanism of the moment reduction. We found that powdering of the FeCr2S4 single crystalline sample reduces substantially the value of Ms that may be explained by the increased influence of surface anisotropy. Thus, we ascribed the reduced value of the saturation magnetization in the polycrystals to the effect of the surface anisotropy. Powdering of the FeCr2S4 polycrystal shows a similar reduction of Ms. Fig. 3 shows the variation of the basic magnetic parameters in the Fe1KxCuxCr2S4 system. The Curie temperature TC, determined by kink-point method, increases nearly linear with the substitution for the range 0%x%0.5 followed by some flattening for the higher values of x up to 0.95. The value of TC increases again only for the xZ1. We note that TC for the single crystals deviate substantially from that of the polycrystals. The difference in TC between them increases with x up to 0.5 and became nearly constant at higher x. This difference might be attributed to a combined influence of the cation vacancies (mainly Fe defects) and the substitution of sulphur ions by chlorine in single crystals. The chlorine presence was revealed by electron-probe microanalysis (Table 1). The Cl content was found to increase along with the increase of the x. The substitution of Fe by Cu reduces strongly the magnetocrystalline anisotropy. In the pure FeCr 2S4 the anisotropy field HA Z2K1/M s, as determined from
Nominal concentration (x)
Fe
Cu
Cr
S
Cl
0 0.05 0.05 0.1 0.2 0.3 0.4 0.4 0.5 0.5 0.5 0.75 0.9
1.01 0.94 0.93 0.85 0.77 0.63 0.56 0.56 0.48 0.49 0.48 0.21 0.14
– 0.04 0.06 0.11 0.23 0.31 0.40 0.37 0.48 0.49 0.50 0.79 0.83
2.01 2.03 2.02 2.04 2.02 2.03 2.02 2.04 2.03 2.0 2.04 1.99 2.03
3.96 3.96 3.94 3.93 3.91 3.88 3.78 3.88 3.82 3.83 3.83 3.78 3.68
0.04 0.06 0.06 0.06 0.06 0.16 0.24 0.14 0.20 0.19 0.17 0.24 0.32
the magnetization curves along three principal cubic axes, reaches the high value of 60 kOe at 4.2 K. Here K1 and Ms are the first anisotropy constant and the saturation magnetization, respectively. We note that only 10% of substitution reduces this value by factor of two. With the further increase of x the anisotropy field decreases, but at a lower rate (Fig. 3(b)). These data correlate well with those determined by electron-spin resonance [8,9]. In the FeCr2K2xIn2xS4 system in the range 0%x%0.15 the Curie temperature and the saturation magnetization decreases with x both for poly- and single crystals. In the polycrystalline samples with substitution x above 0.2 a phase separation was found to occur with a mixture of the phases close to FeIn2S4 and FeCr1.7In0.3S4. In Fig. 4 the lattice constant a0 vs Cu substitution concentration x is presented for the poly- and single crystalline Fe1KxCuxCr2S4 samples. A large difference in the a0 values for poly- and single crystals is detected both showing a0 decrease with the increase of x. This difference might be also attributed to Cl for S substitution. Considering the mechanisms of the Curie temperature variation in the Fe1Kx Cux Cr2 S4 system we note that 10.00
Fe1-xCuxCr2S4 9.95
a0(Å)
2042
9.90
9.85
9.80 0.0
single crystal polycrystal
0.2
0.4
0.6
0.8
1.0
x Fig. 4. Variation of the lattice constant a0 with the substitution x for the polyand single crystals of the Fe1KxCuxCr2S4 system.
V. Tsurkan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2040–2043
the observed concomitant decrease of the lattice constant with the substitution suggests a certain contribution of the superexchange known to be quite sensitive to inter-atomic spacing. Although the experimental results on the Cu and In substituted samples correlate qualitatively well with those of the influence of hydrostatic pressure on the Curie temperature and Tm of pure FeCr2S4 [10], the estimations of the TC and Tm changes with substitution x due to lattice contraction in case of Cu are lower by a factor of 7 and 4, respectively, compared to the experimentally observed ones. This may indicate that chemical pressure induced by cation substitution has a larger influence than the external pressure or that an additional mechanism contributes to the variation of these quantities. Indeed, the Cu doping increases the conductivity of samples compared to that of the pure compound (xZ0). This is probably an indication of the contribution of indirect exchange via charge carriers at least for x below 0.2. However, the non-monotonic variation of the conductivity with substitution [3,9] along with the monotonic increase of the Curie temperature with x in the range 0.2!x! 0.5 suggests the indirect exchange to be less important here. In addition, the insulating character of the samples for the whole substitution ranges, except xZ1, points out that the superexchange is the dominant magnetic exchange mechanism. Finally, in view of the unusual magnetization anomaly at Tm we must mention that a small amount of Cu (about 20%) is enough to destroy this anomaly, which for the pure FeCr2S4 compound we interpret in terms of an orbital liquid state at low temperatures [6]. 4. Summary We studied the effect of cation substitution on the magnetic properties of Fe1KxCuxCr2S4 and FeCr2K2xIn2xS4 crystals. A large difference in the Curie temperature and the saturation magnetization between poly-and single crystalline samples was observed, which we ascribe respectively to influence of Cl for S substitution and surface anisotropy. The change of the Curie temperature by the substitution is attributed mainly to
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variation of the superexchange interactions. The magnetization irreversibilities at low temperatures originating from the Jahn-Teller active Fe2C ions disappear at high Cu substitution (xO0.2). Acknowledgements The support of US CRDF-MRDA grant MP2-3047 (Chisinau) is gratefully acknowledged. References [1] Y. Tokura, Y. Tomioka, Colossal magnetoresistive manganites, J. Magn. Magn. Mater. 200 (1999) 1–23. [2] A.P. Ramirez, R.J. Cava, J. Krajewski, Colossal magnetoresistance in Crbased chalcogenide spinels, Nature 386 (1997) 156–159. [3] F.K. Lotgering, R.P. van Stapele, G.H.A.M. van der Steen, J.S. van Wieringen, Magnetic properties, conductivity and ionic ordering in Fe1KxCuxCr2S4, J. Phys. Chem. Solids 30 (1967) 799–804. [4] G. Haacke, L.C. Beegle, Magnetic properties of the spinel system Fe1KxCuxCr2S4, J. Phys. Chem. Solids 28 (1967) 1699–1704. [5] V. Tsurkan, M. Baran, R. Szymczak, H. Szymczak, R. Tidecks, Spin-glass like states in ferrimagnetic FeCr2S4, Physica B 296 (2001) 301–305. [6] V. Tsurkan, V. Fritsch, J. Hemberger, A. Krimmel, M. Mu¨cksch, N. Bu¨ttgen, H.-A. Krug von Nidda, D. Samusi, S. Ko¨rner, E.-W. Scheidt, M. Honal, S. Horn, R. Tidecks, A. Loidl, Orbital glass in FeCr2S4, Preprint cond-mat/0407026 (http:/xxx.lanl.gov), 2004. [7] D. Maurer, V. Tsurkan, S. Horn, R. Tidecks, Ultrasonic study of ferrimagnetic FeCr2S4: evidence for low temperature structural transformation, J. Appl. Phys. 93 (2003) 9173–9176. [8] V. Tsurkan, M. Lohmann, H.-A. Krug von Nidda, A. Loidl, S. Horn, R. Tidecks, Electron-spin-resonance studies of the ferrimagnetic semiconductor FeCr2S4, Phys. Rev. B 63 (2001) (125209/1–5). [9] V. Fritsch, J. Deisenhofer, R. Fichtl, J. Hemberger, H.-A. Krug von Nidda, M. Mu¨cksch, M. Nicklas, D. Samusi, J.D. Thompson, R. Tidecks, V. Tsurkan, A. Loidl, Anisotropic colossal magnetoresistance effects in Fe1KxCuxCr2S4, Phys. Rev. B 67 (2003) (144419/1–8). [10] V. Tsurkan, I. Fita, M. Baran, R. Puzniak, D. Samusi, R. Szymczak, H. Szymczak, S. Klimm, M. Klemm, S. Horn, R. Tidecks, Effect of pressure on the magnetic and transport properties of the ferrimagnetic semiconductor FeCr2S4, J. Appl. Phys. 90 (2001) 875–881.