Spin-glass behavior in mixed metai oxides with a rulUe . . type structure

We report here on the magnetic properties of compounds of composition Fe1−xCrxSbO4 and Fe1−xGaxSbO4. The introduction of paramagnetic Cr3+ and diamagnetic Ga3+ into the rutile‐related iron antimonate lattice does not destroy the antisite atomic ordering which exists in iron antimonate of composition FeSbO4. The initial slope of the Curie temperature dependence on x is similar in both series, indicating that Fe3+‐Cr3+ interactions are very small. The magnetic susceptibility measurements recorded from the compounds of composition Fe1−xCrxSbO4, x 0.4, and Fe1−xGaxSbO4, x>0.1 to undergo a spin‐glass transition above 4.2 K is associated with a dilution effect.


INTRODUCTION
We have recently reported!on the observation of spinglass behavior in the rutile-related iron an timon ate of composition FeSb0 4 , which contains a superlattice,2 and have associated the spin-glass transition with antisite atomic ordering.lWe are currently accumulating experimental data to increase the general understanding of the mechanisms involved in spin-glass behavior, in this paper we describe the changes in the magnetic properties of this rutile-related material which result from the substitution of the Fe3+ species by paramagnetic Cr 3 + and diamagnetic Ga 3 -;.• EXPERIMENT ThecompouncisofcompositionFe1 __ xCrxSb0 4 (x = 1, 0.7,0.6,0.4,0.1) and Fe j __ x Ga x Sb0 4 (x = 0.7,0.5,0.3,0.1) were prepared by coprecipitation techniques.57Fe and I2lSb Mossbauer spectra were recorded at various temperatures between 77 and 298 K with a microprocessor controlled Mossbauer spectrometer using 57Co/Rh and Ca I2lSnO"j sources.All the spectra were computer fitted.The magnetic susceptibility measurements were performed in the temperature range between 1.7 and 80 K with a SHE SQUID magnetometer with applied fIelds up to 100 Oe, and with a Faraday balance in the temperature range between 80 and 1000 K with applied fields up to 5000 Oe.

RESULTS AND DISCUSSION
The magnetic susceptibility data recorded between L 7 and 1000 K for the compounds of composition Fe O .9 Gall.! Sb0 4 and Fe O .6 er OA Sb0 4 are shown in Figs. 1   and 2, respectively.The maximum in the zero-field-cooled  ..
Fe l _. x Cr x Sb0 4 (x = 0.1,0.4) and Fe O .9 Gao.1 Sb0 4 at low temperatures, together with the irreversible ZFC and FC magnetic susceptibilities at 1000 Oe, suggest that these three compounds behave as spin glasses.Compounds in which the Cr H concentration exceeds 0.4, or those in which the concentration of Ga 3 + is larger than 0.1 show no minimum in the low-temperature susceptibility data which suggests the disappearance of the spin-glass behavior or a decrease of the spin-freezing temperature below 4.2 K (Fig. 3).
The 57Fe Mossbauer spectra recorded at 298 and 77 K from the compounds in the series Fe 1 _ x Ga x Sb0 4 and Fe \_ x Cr x SbO 4 were all similar (Fig. 4) and show the presence of high-spin Fe H cations.The similarities between the chemical isomer shift and quadrupole splitting data recorded from the two series of mixed oxides are consistent with the antisite atomic ordering which exists in iron antimonate being conserved when Cr H and Ga H enter the lattice.The 12ISb Mossbauer spectra recorded at 298 K from ali the samples were characteristics of Sb5+.The observation of broadened lines in the spectra recorded at 77 K is indicative of the presence of a supertransferred magnetic hyperfine field at the 121~% nuclei. 2 The magnetic susceptibility data recorded from all the compounds in the high-temperature regime, Le., T> 300 K, obey Curie-Weiss laws which allow an evaluation of both the Curie temperature (j and the effective dipolar magnetic moment /-lew ' The results are collected in Table t The () value increase with decreasing values of x, and the effective dipolar magnetic moment varies within each series according to the concentration of paramagnetic Fe3+ and Cr H ions or dia- . " magnetic Ga 3 + ions.The initial slopes ofthe 8(x) curves are similar for the two series showing that the magnetic interactions between Cr 3 + and Fe~"+ and between Cr 3 -t-and Cr3+ are very smalL Although the results obtained in this hightemperature regime are characteristic of pure paramagnetic behavior, the large values of e indicate that strong antiferromagnetic interactions exist between the spins.The paramagnetic behavior in both series is conserved until a temperature of 80 K is attained for compounds in which the iron concentration is less than 0.6, Le., x > 0.4.The magnetic susceptibility data for compounds in which x < 0.4 begins to deviate from the Curie-Weiss behavior at temperatures below 200 K.The decrease of the effective paramagnetic moment in this regime is indicative of onset of clustering between the individual Fe3+ species at these temperatures.However, after a regime in which the data fonow a Curie-type dependence on temperature a new straight line relationship with smaller magnetic moments is observed which corresponds to the formation of clusters with antiferromagnetic correlations.The positive () values of this new Curie-Weiss behav- ior may be related with ferromagnetic interactions between clusters.In the final temperature regime, i.e., T < 20 K, a minimum is observed in the ZFC susceptibility which corresponds to the freezing of the spins.The freezing temperature decreases when x increases, and no minimum in the ZFC susceptibility is observed above 4.2 K in compounds in which x> 0.4.
Despite evidence of the cluster formation below 200 K, the Mossbauer data above 77 K do not show any slow relaxation phenomena.The results indicate that the relaxation times of the short-range magnetically ordered clusters are significantly faster than the Mossbauer time scale 7 M' 3 Further Mossbauer spectroscopic experiments below 77 K are currently in progress to examine the variation of the relaxation times with temperature.In this respect it is pertinent to note that the broad-lined unresolved 57Fe Mc)ssbauer spectrum recorded l at 77 K from iron antimonate, FeSb0 4 , shows that at this lower temperature some ofthe short-range magnetically ordered clusters have a relaxation time 7 comparable to the M6ssbauer time scale.It therefore appears that although the introduction of Cr 3 ~ and Ga H into the rutile-type structure does not affect the occurrence of cluster formation, the relaxation times at given temperatures of the clusters in the Fe l _' X Cr x Sb0 4 and Fe 1 _ X Ga x Sb0 4 systems are faster than both 7 and 1'M as a consequence of the decrease in the cluster-cluster magnetic correlation.
FIG. L Temperature dependence of the inverse of the magnetic susceptibility of the compound Feo.9Gllo.tShO •.
FIG. 2. Temperature dependence of the inverse of the magnetic susceptibility of the compound Feo.6CrO .•SbO •.The inset shows the irreversible zerofield-cooled and field-cooled susceptibilities.
FIG. 3. Temperature dependence of the inverse of the magnetic susceptibility of the compound Feo.1 Gao.) SbO •.

TABLE I .
Effective magnetic moments per formula unit and Curie temperatures for compounds of composition Fe'._xCrxSbO.. and  Fe'._xGaxSbO.deduced from the high-temperature regime (T> 300 K).