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Spin glass behaviour of Fe3−xTixO4
SPIN GLASS B E H A V I O U R OF Fe3_xTixO4
C. R A D H A K R I S H N A M U R T Y , S. D. L I K H I T E and R. N A G A R A J A N Tara Institute of Fundamental Research, Bombay-400 005, India
Susceptibility and hysteresis measurements indicate that the system Fe3_xTixO 4 forms only monodomains for 1.0 ) x ) 0.3 irrespective of the physical grain size. Sharp susceptibility maxima, resembling those of spin glasses, are obtained for 1.0 > x ~ 0.8, which could be attributed to the combined effect of decreasing anisotropy and the monodomains becoming superparamagnetic with increasing temperature.
The maxima in the susceptibility (X)-temperature curves and other properties of spin glasses have been explained by Murani [1] in terms of simple qualitative ideas based on relaxation times of superparamagnetic particles. Wohlfarth [2] pointed out the similarity between the x - T curves of spin glasses and basaltic rocks. Since the magnetic properties of basalts arise due to the titanomangetite system with the general formula Fea_xTixO4, the properties of this system have been re-examined to see how far they bear any resemblance to those of spin glasses. Micropowders of the system Fe3_xTixO4, with x from 0.0 to 1.0, were prepared by the dry method and their X - T behaviour and hysteresis properties were studied using methods described earlier [3, 4]. The titanomagnetites of compositions x = 0.0 to 1.0 are referred hereafter as TM0 to TM100, the number following T M representing the at% of Ti in the material
l 60
Fig. 1 gives the x - T curves for TMS0 to TM100. The susceptibility maxima of these materials do have a striking similarity to those of spin glasses. For TM80, the peak value of X is about 50 times that at 77 K. Fig. 2 shows the hysteresis loops obtained for TMS0 in a peak field of 5000 Oe at different temperatures. It is interesting to note that the coercive force (He) which is about 2800 Oe at 77 K, decreases to less than 100 Oe at 240 K. Moreover, the overall hysteresis behaviour indicates that magnetocrystalline anisotropy (KI) dominates at low temperatures, whereas the uniaxial shape anisotropy dominates around 160 K, as can be inferred from the relative remanance values deducible from the hysteresis loops. Also, these features suggest the presence of only monodomains in the sample. For TM85 and TM96, the
Fe2.2oTio.ao04(TM80)
--- Fez.15T
~
4O
77K
lOOK
\== 20 J
I0 v .:K
Fe2.o4Tio.96 04 (TM96}
2
Fe2TiO4 (TMIO0)
-
i
il
iI_i
160K i
I
IOO
200
30~
T(K) Fig. 1. Low-field ac susceptibility-temperature curves for some titanomagnetites.
ill
1
240K
Fig. 2. Hysteresis loops for Fe2.2Tio.sO4 (TMS0) at different temperatures. Scale (same for all loops): x-axis, one small division°250 Oe and for y-axis, arbitrary. (The slight curvature seen near the saturation regions of the loops is due to instrumental noise.)
Journal of Magnetism and Magnetic Materials 15-18 (1980) 195-196 ©North Holland
19t3
196
c. Radhakrishnamurty et al./ Spin glass behaviour of Fe3_xTixO4
H c values are much larger than that of TM80 but the hysteresis behaviour of all of them is quite similar. TM100 was found to be too weak magnetically to obtain hysteresis loops by the method used in this study. The general magnetic properties of this system for 1.0 ~ x ~ 0.3 are similar to those of TM80 except that the K l value, inferred from H c, decreases and the saturation intensity increases with decreasing x. The susceptibility maxima become broader and less prominant with decreasing x, which could be partly due to the fact that the transition temperature shifts higher and higher finally approaching the magnetite (TM0) Curie point of 855 K. TM0 itself readily forms multidomain grains upon synthesis under identical conditions and for 0.3 > x > 0.0, the magnetic properties are not simple enough to interpret their domain state unambiguously from the present studies. The detailed results on the studies of the system for 0.7 >/ x ~ 0.0 will be reported elsewhere. The features described above are unaffected even after prolonged annealing of these materials at their formation temperature of 1350 K. Ishikawa [5] studied the hysteresis properties of a single crystal with a composition of TM95. The hysteresis loop of the single crystal at 78 K obtained by him, indicates that the crystal behaved as a single monodomain or possibly consisted of many monodomains only. Since we have obtained identical hysteresis loops for polycrystalline powder samples as well, it seems logical to presume that these materials form only m o n o d o m a i n s or ordered spin clusters in them and in that case the physical size of the grains will become quite immaterial in determining the hysteresis behaviour. Although, it is thus possible to infer from magnetic studies that
the substitution of Ti in magnetite inhibits the domain wall formation and hence gives rise to monodomains or spin clusters, an exact physical mechanism for this phenomenon is difficult to comprehend. Recently, a somewhat similar case of spin cluster formation was reported [6] for the hexaferrite BaFeaTi2Olr The superparamagnetic behaviour of the chemically and crystallographically homogeneous hexaferrite was attributed to magnetic inhomogeneity i.e. due to statistical fluctuations in composition. Hence, it appears that the presence of Ti in a ferrite could cause magnetic inhomogeneity resulting in the formation of monodomains or spin clusters. Cluster formation m a y then bring in additional anisotropies as was suggested by Wohlfarth [2]. In explaining the magnetic properties of such materials, a cautious approach may be warranted rather than treating them on a par with magnetite or other ferrites which can form multidomain state easily. Thus, the above discussion suggests that the system Fe3_xTixO4, with x from 0.3 to 1.0, seems to be capable of containing only monodomains and that the x - T curves of this material for 1.0 /> x ~ 0.8 closely resemble those of spin glasses.
References [1] A. P. Murani, J. Magn. Magn. Mat. 5 (1977) 95. [2] E. P. Wohffarth, Physica 86-88 B (1977) 852. [3] C. Radhakrishnamurty and S. D. Likhite, Earth and Planetary Sci. Lett. 7 (1970) 389. [4] C. Radhakrishnamurty, S. D. Likhite and N. P. Sastry, Phil. Mag. 23 (1971) 503. 15] Y. Ishikawa, Phys. Len. 24 A (1967) 725. 16] E. Kneller, M. Velicescu and F. Habery, J. Magn. Magn. Mat. 7 (1978) 49.
C. R A D H A K R I S H N A M U R T Y , S. D. L I K H I T E and R. N A G A R A J A N Tara Institute of Fundamental Research, Bombay-400 005, India
Susceptibility and hysteresis measurements indicate that the system Fe3_xTixO 4 forms only monodomains for 1.0 ) x ) 0.3 irrespective of the physical grain size. Sharp susceptibility maxima, resembling those of spin glasses, are obtained for 1.0 > x ~ 0.8, which could be attributed to the combined effect of decreasing anisotropy and the monodomains becoming superparamagnetic with increasing temperature.
The maxima in the susceptibility (X)-temperature curves and other properties of spin glasses have been explained by Murani [1] in terms of simple qualitative ideas based on relaxation times of superparamagnetic particles. Wohlfarth [2] pointed out the similarity between the x - T curves of spin glasses and basaltic rocks. Since the magnetic properties of basalts arise due to the titanomangetite system with the general formula Fea_xTixO4, the properties of this system have been re-examined to see how far they bear any resemblance to those of spin glasses. Micropowders of the system Fe3_xTixO4, with x from 0.0 to 1.0, were prepared by the dry method and their X - T behaviour and hysteresis properties were studied using methods described earlier [3, 4]. The titanomagnetites of compositions x = 0.0 to 1.0 are referred hereafter as TM0 to TM100, the number following T M representing the at% of Ti in the material
l 60
Fig. 1 gives the x - T curves for TMS0 to TM100. The susceptibility maxima of these materials do have a striking similarity to those of spin glasses. For TM80, the peak value of X is about 50 times that at 77 K. Fig. 2 shows the hysteresis loops obtained for TMS0 in a peak field of 5000 Oe at different temperatures. It is interesting to note that the coercive force (He) which is about 2800 Oe at 77 K, decreases to less than 100 Oe at 240 K. Moreover, the overall hysteresis behaviour indicates that magnetocrystalline anisotropy (KI) dominates at low temperatures, whereas the uniaxial shape anisotropy dominates around 160 K, as can be inferred from the relative remanance values deducible from the hysteresis loops. Also, these features suggest the presence of only monodomains in the sample. For TM85 and TM96, the
Fe2.2oTio.ao04(TM80)
--- Fez.15T
~
4O
77K
lOOK
\== 20 J
I0 v .:K
Fe2.o4Tio.96 04 (TM96}
2
Fe2TiO4 (TMIO0)
-
i
il
iI_i
160K i
I
IOO
200
30~
T(K) Fig. 1. Low-field ac susceptibility-temperature curves for some titanomagnetites.
ill
1
240K
Fig. 2. Hysteresis loops for Fe2.2Tio.sO4 (TMS0) at different temperatures. Scale (same for all loops): x-axis, one small division°250 Oe and for y-axis, arbitrary. (The slight curvature seen near the saturation regions of the loops is due to instrumental noise.)
Journal of Magnetism and Magnetic Materials 15-18 (1980) 195-196 ©North Holland
19t3
196
c. Radhakrishnamurty et al./ Spin glass behaviour of Fe3_xTixO4
H c values are much larger than that of TM80 but the hysteresis behaviour of all of them is quite similar. TM100 was found to be too weak magnetically to obtain hysteresis loops by the method used in this study. The general magnetic properties of this system for 1.0 ~ x ~ 0.3 are similar to those of TM80 except that the K l value, inferred from H c, decreases and the saturation intensity increases with decreasing x. The susceptibility maxima become broader and less prominant with decreasing x, which could be partly due to the fact that the transition temperature shifts higher and higher finally approaching the magnetite (TM0) Curie point of 855 K. TM0 itself readily forms multidomain grains upon synthesis under identical conditions and for 0.3 > x > 0.0, the magnetic properties are not simple enough to interpret their domain state unambiguously from the present studies. The detailed results on the studies of the system for 0.7 >/ x ~ 0.0 will be reported elsewhere. The features described above are unaffected even after prolonged annealing of these materials at their formation temperature of 1350 K. Ishikawa [5] studied the hysteresis properties of a single crystal with a composition of TM95. The hysteresis loop of the single crystal at 78 K obtained by him, indicates that the crystal behaved as a single monodomain or possibly consisted of many monodomains only. Since we have obtained identical hysteresis loops for polycrystalline powder samples as well, it seems logical to presume that these materials form only m o n o d o m a i n s or ordered spin clusters in them and in that case the physical size of the grains will become quite immaterial in determining the hysteresis behaviour. Although, it is thus possible to infer from magnetic studies that
the substitution of Ti in magnetite inhibits the domain wall formation and hence gives rise to monodomains or spin clusters, an exact physical mechanism for this phenomenon is difficult to comprehend. Recently, a somewhat similar case of spin cluster formation was reported [6] for the hexaferrite BaFeaTi2Olr The superparamagnetic behaviour of the chemically and crystallographically homogeneous hexaferrite was attributed to magnetic inhomogeneity i.e. due to statistical fluctuations in composition. Hence, it appears that the presence of Ti in a ferrite could cause magnetic inhomogeneity resulting in the formation of monodomains or spin clusters. Cluster formation m a y then bring in additional anisotropies as was suggested by Wohlfarth [2]. In explaining the magnetic properties of such materials, a cautious approach may be warranted rather than treating them on a par with magnetite or other ferrites which can form multidomain state easily. Thus, the above discussion suggests that the system Fe3_xTixO4, with x from 0.3 to 1.0, seems to be capable of containing only monodomains and that the x - T curves of this material for 1.0 /> x ~ 0.8 closely resemble those of spin glasses.
References [1] A. P. Murani, J. Magn. Magn. Mat. 5 (1977) 95. [2] E. P. Wohffarth, Physica 86-88 B (1977) 852. [3] C. Radhakrishnamurty and S. D. Likhite, Earth and Planetary Sci. Lett. 7 (1970) 389. [4] C. Radhakrishnamurty, S. D. Likhite and N. P. Sastry, Phil. Mag. 23 (1971) 503. 15] Y. Ishikawa, Phys. Len. 24 A (1967) 725. 16] E. Kneller, M. Velicescu and F. Habery, J. Magn. Magn. Mat. 7 (1978) 49.