Comparative study on the magnetic properties of Fe-substituted Co 2 Sn 1-x Fe x O 4 spinel oxides and its exchange bias effect

Herein, we report the Fe-substituted Co 2 Sn 1 − x Fe x O 4 (0 ≤ x ≤ 0.4) inverse spinel’s oxide using the solid-state reaction method. X-ray reveals the single-phase cubic structure with space group Fd3m. With increasing Fe in Co 2 Sn 1 − x Fe x O 4 spinel oxide, the transition temperature rise. The ac susceptibility at different frequencies also con�rms a spin-glassy state at lower temperatures. The strong exchange bias effect appears in the sample having Fe substitution (x = 0.2) under the presence of constant temperature ~ 10 K. The high-temperature susceptibility of Curie-Wise �tting shows that the system changes from antiferromagnetic exchange (x < 0.2) to ferromagnetic exchange (x > 0.2).


Introduction
Spinel compounds are considered as an emerging eld of research with abundant application, starting from biomedicine [1] to modern data storage devices [2].Its general formula is AB 2 O 4 , where the A-site is occupied by divalent cations and the B-site is lled with trivalent cations [3].The physical and chemical properties of these compounds are mainly in uenced by the distribution of cations on either A or B-sites within the voids [4].Most of the spinel compounds exhibit multiferroics properties such as magnetodielectric [5], magnetoelectric [6], and ferroelectric [7], which make them useful with a variety of applications in different elds including metallurgical [8], data-storage devices [9], magnetic sensors, and chemical industries [10].Furthermore, spinel compounds may also be helpful in various technologies for the fabrication of microwave sensors [11,12], optoelectronic [13], high-speed magneto-optical data recording, and reading devices [14].Amongst these spinel compounds, the magnetic spinel is particularly interesting because of their excellent supersonic conductivity, and electric and magnetic properties [15].Their exible elemental composition and large physiochemical characteristics make them excellent for multifunctional devices [16].Moreover, magnetic spinel compounds have also been observed to be promising results in biomedicine, such as in magnetic resonance imaging (MRI) [17,18] and as heating mediators in magnetic uid hyperthermia [19,20].Several studies on the preparation of magnetic spinel compounds and magnetic saturation points have already been reported.For instance, the magnetic spinel chromite of MCr 2 O 4 (M = Fe, Co, and Ni) was prepared via a sol-gel auto-combustion route with magnetic phase transition temperatures (Tc) 80, 83, and 90 K, respectively [21].Similarly, T C = 41 K is noted for Co 2 SnO 4 , which was synthesized by the solid-state reaction method [22].Moreover, Cobalt Chromite nanoparticles with T C = 97 K were synthesized via co-precipitation technique [23].Furthermore, SnCo 2 O 4 and MnCr 2 O 4 have been reported, showing T C of 42 K and 43 K, respectively [24,25].In addition, Ni 0.5 Fe 0.5 Cr 2 O 4 was synthesized using the sol-gel approach with T C = 90 K [26].
In this study, we synthesized Co 2 Sn 1 − x Fe x O 4 spinel oxide using a solid-state reaction method with different Fe substitutions.The crystalline structure and phase of the Fe-concentrated Co 2 Sn 1 − x Fe x O 4 compound were studied using an X-ray diffractometer.The VSM measurement indicates the phase transition from paramagnetic to ferromagnetic observed with gradual substitution of Fe-contents with Sn.Also, the Curie-Weiss tting of the magnetic susceptibility at high temperatures indicated that the substitution of Fe causes a magnetic transition exchange from antiferromagnetic to ferromagnetic.Further, the results of AC magnetic susceptibility measurements at several frequencies showed that the Fe-concentrated samples (x ≤ 0.3) at lower temperatures have a spin glass state.Moreover, it has been determined from the analysis that the strong exchange bias effect is signi cantly enhanced by Fesubstituent materials when kept at a lower temperature (10 K).Our experimental results and previous ndings indicate that the developed system may be applied in various applications including catalysis, sensors, batteries, degradation of dyes, and drug delivery.

Material synthesis
The polycrystals of Co 2 Sn 1 − x Fe x O 4 (0 ≤ x ≤ 0.4) samples were synthesized by the solid-state reaction method.All the precursors including Co 3 O 4 , SnO 4, and Fe 3 O 4 with a purity of 99.99% were selected as raw materials and proportioned according to the nominal stoichiometric ratio.These precursors were mixed and blended with the help of a mortar pestle for 6 hours.The evenly distributed powders were heated at 1100 o C for 12 h and cooled to room temperature.The samples were ground again for 6 hours and pressed into 13 mm diameter slices.These pallets were heat-treated at 1350 o C for 24 hours and then cooled to room temperature naturally with the furnace to achieve single phase formation.

Characterizations
The XRD pattern of the samples was examined by Philips Panalytical X' pert diffractometer (XRD, Philips PW-1830) with Cu-Kα radiation (λ = 1.5406Å).The structural analysis was done using GSAS software, followed by Rietveld re nement of the X-ray patterns through GSAS software.A physical property measurement system (PPMS, Quantum Design) was employed for the investigation of the magnetic properties.

Structure and crystallinity measurement
The crystal structure and phase analysis of Co 2 Sn 1 − x Fe x O 4 was measured using an X-ray diffraction measurement.Figure 1 (a) shows the single-phase spinel cubic structure of the Co 2 Sn 1 − x Fe x O 4 compound with an Fd3m space group.All the chosen substitution range indicates that each diffraction peak of the Co 2 Sn 1 − x Fe x O 4 sample is well matched the diffraction peaks in the standard card (JCPDS No. 00-029-0514) [27,28].The increase in Fe-substituent is associated with a shifting of the main peak at the Miller index of {311} towards a higher angle which shows that Fe-substitution tended to decrease lattice parameters in the sample [29].By carefully re ning their respective XRD spectra, we obtained precise values of the lattice parameters of the Fe-substituted samples, as shown in Fig. 1 (b-f).
The re ned lattice parameters a, unit cell volume V, and re ned reliability parameters R p , R wp, and χ 2 are shown in Table 1.As the amount of Fe-concentration increases, the lattice parameters 'a' and cell volume 'V' of the Fe-substituted samples decreases as shown in Table 1; owing to the larger ionic radius specie (Sn 4+ ion, 0.89 Å) being partially replaced by the smaller ionic radius specie (Fe 3+ ion 0.785 Å) [30].It is observed that there was no Co 2 FeO 4 phase emerged at the selected Fe-substitution range and there is no effect of substitution on the crystal structure of the Co 2 Sn 1 − x Fe x O 4 sample.As a result of it, Fesubstitution only reduced the lattice parameter of the Co 2 Sn 1 − x Fe x O 4 as con rmed in Table 1.
Table 1 Lattice parameter a, unit cell volume V, and re ned reliability parameters R p , R wp, and

Magnetic hysteresis measurements
The PPMS analysis of Co 2 Sn 1-x Fe x O 4 samples was performed to check the magnetic response of all the samples with and without Fe-substituted compound.Furthermore, the coercivity also increases gradually with the increase of Fe-amount at a xed temperature of 10 K [31].From the graphs, the eld-cooled (FC) hysteresis loops move towards the negative horizontal axis, indicating exchange bias (EB) [32].Overall, the FC and ZFC have similar tendencies except for the EB effect and magnetization intensity at the highest magnetic eld.The saturation magnetization is also seen with the increase of Fe-amount resembles the literature [33].The hysteresis loops of low-substituted materials, as illustrated in Fig. 2 (b, c), do not saturate with an increase in magnetic eld.This is because of the antiferromagnetic exchange that predominates in these samples.Thus, ferromagnetic exchange dominates in the high-concentration sample which can be seen in Fig. 2 (d).

Magnetic susceptibility and exchange bias effect
The temperature dependence of magnetization (M) in ZFC and FC processes with H = 100 Oe is shown in Fig. 3 (a-b), and all of the Fe-substituent samples demonstrated the transition from paramagnetic to ferromagnetic [34].The paramagnetic-ferrimagnetic transition temperature, determined by the temperature at which the magnetic susceptibility begins to increase signi cantly, increases from ~ 40.5 K to ~ 154 K for the substitution amount, corresponding to the maximum value of the magnetic susceptibility peak From ~ 37.1 K at H to ~ 64.7 K at H, the above results show that Fe-concentration in Co 2 SnO 4 system can not only cause a great increase of its ferrimagnetic transition temperature but also can cause a signi cant enhancement of its ferromagnetism [35].The temperature increases sharply, and after reaching the maximum value, the curve begins to coincide with the curve.If the temperature at which the curve begins to overlap with the curve begins to overlap is recorded as, with the increase of doping amount, it increases from ~ 40 K at to ~ 155K at.These results imply that Fe-substituted Co 2 Sn 1- x Fe x O 4 greatly increases ferromagnetic characteristics, including the strength of magnetic moments and transition temperature [36].From the determined temperature range, the curves of the Fe-concentrated sample lies entirely above the temperature axis, indicating that the high Fe-substitution level prevents the formation of the low-temperature spin glassy state.The spin-glass transition happens at very low temperatures.ZFC-FC hysteresis and spin-glassy-like [37] behavior are observed at higher temperatures which can be seen in Fig. 3 (b).The exchange bias eld of each sample obtained at 10 K temperature is shown in Fig. 3 (b).It can be seen that at low doping amount, it gradually increases with the increase of doping amount, reaches a maximum, and then gradually decreases with further increase.The following is a qualitative explanation of this variable behavior with respect to doping amount combined with the experimental magnetic susceptibility of DC and AC.The DC and AC magnetic susceptibilities measurements, demonstrate ferromagnetic exchange with higher Fe-concentration.The effect of exchange bias is negligible in high Fe-substituted samples.Thus, a shift in the spin state may play a role in magnetization reversal [38].
Magnetic exchange occurs between the ferromagnetic and antiferromagnetic phases in samples with intermediate substitution.Resultantly, the exchange bias is greater in the substituted sample because of the frozen spin napping on the surface spin of the ferromagnetic phase [39].

Inverse magnetic susceptibility
The inverse susceptibility versus temperature curve indicates a deviation from the Curie-Weiss law with the deviation of temperatures above the Curie temperature shown in Fig. 4 (a-d), while the inset of Fig. 4 (a) shows the susceptibility with temperature for pure Co 2 SnO 4 [40].It can be seen that except for the sample , the other four samples (x = 0, 0.1, 0.3, 0.4) showed good linear behavior in the hightemperature range which indicates that these samples showed paramagnetic behavior at high temperatures.At the low Fe-substituted sample, the curie wise temperature is negative as shown in Fig. 4 (a, b), and for the high concentration sample, become positive as seen by Fig. 4 (c, d) is associated with the literature [41].Generally, the  CW > 0, the system shows ferromagnetic interaction and at  CW < 0 it displays antiferromagnetic interaction as well [42].Likewise, substitution has the potential to greatly enhance the ferromagnetic properties of a system.Through careful adjustment of the substitution levels, the magnetic interaction within the system can be shifted from an initial anti-ferromagnetic state at low substitution levels to a highly ferromagnetic state at higher substitution levels that is observed in Fig. 4 (a-d) as having similarity with the literature [43].The ZFC susceptibility of Fe-substituent samples exhibits negative magnetization behavior similar to those of the unsubstituted samples at lower temperatures, while the negative magnetization of unsubstituted samples at lower temperatures is related to the formation of spin glass states, suggesting that the Fe-substitution samples have glassy behavior similar to that of the unsubstituted Co 2 SnO 4 samples at low temperature [44].
The obtained Curie-Weiss constant is given in Table 2.It can be seen that the Curie-Weiss constant increases gradually with the increase of substitution amount as illustrated in Table 2.The experimental value of the tted effective magnetic moment is in good agreement with the theoretical value, indicating that the magnetism in the sample comes from the joint contribution of Co 2+ , Co 3+, and Fe 3+ .

Magnetization loop measured at different temperature
The magnetization loop measured at constant temperature in ZFC mode for the Fe-substituted sample is shown in Fig. 5 (a-d).ZFC hysteresis loops show centrosymmetric about the origin and obvious hysteresis.The coercivity of the sample increases rst and reaches a maximum by decreasing temperature to 10 K, whereas the H C (T) of the un-substituent sample is essentially zero at 10 K as seen from the inset of Fig. 5 (a-b).For low Fe-concentration samples observed from the inset of Fig. 5 (a, b), the remanent magnetization increases gradually to a maximum and then decreases slowly with lowering temperature [45].However, the high substituted amount sample shows a monotonically increasing behavior with the temperature decrease Fig. 5 (c-d), as stated in the literature [46].The maximum magnetization measured at the highest magnetic eld (60 kOe) gradually decreases with the increase in temperature as drawn in Fig. 5 (a-d).ZFC hysteresis loops at each given temperature exhibit a attened "S" shape for low substitution that can be seen in Fig. 5 (a-b) while showing an elongated "S" shape for high substitution [47] as observed in Fig. 5 (c-d).Based on the results obtained from the measurements of the hysteresis loop and magnetic susceptibility, it can be inferred that the antiferromagnetic exchange interaction is the main contributing factor to the magnetic properties in samples with low concentrations.On the other hand, in samples with high Fe-amount as maintained in Fig. 5 (a-b), the ferromagnetic exchange interaction takes over as the dominant factor, governing the magnetic behavior.The interaction between A and B magneton lattices will also change [48].The introduction or alteration of these complex exchanges may be the reason for the dominant ferromagnetic exchange as shown in Fig. 5 (c-d) in the samples with higher Fe-substitution content.

Spin glass behavior
The temperature dependency of AC susceptibility was measured at a frequency ranging from 10 to 10 KHz probes, further exploring the synthesized system's spin glass (SG) behavior.The inset of Fig. 6 (d) demonstrates that the peak height drops and move towards the high temperature TP (f) as the frequency of the alternating magnetic eld increases.By displaying with a straight line can be obtained, as shown in the inset of Fig. 6 (d).
The functional relation of con rms that the system has a glass dynamic characteristic near the freezing temperature (T B ) as indicated in insets of Fig. 6 (a-c), resembling the literature [49].We can also see from the graphs Fig. 6 (a-d) that the freezing temperature increases gradually with the increase of substitution amount, while the two kinetic parameters, including the relaxation time ( o ) and the critical index (Zv), decrease step-by-step [50] as shown in Table 3.By increasing the alternating magnetic eld frequency as shown in Fig. 6 (a-d), the peak height decreases gradually but corresponding to the peak value remains at 154 K, independent of the frequency.The frequencyindependent freezing temperature in the Fe-substituted samples indicates that the spin glass states disappear because of higher concentration dominated by ferromagnetic exchange, a similar result observed in Cobalt-Chromite spinel [25].When the ferromagnetic exchange is much stronger than the antiferromagnetic exchange, the spin glass behavior disappears and this may be the plausible reason for the basic disappearance of the spin glass state in the samples.

Conclusions
The samples of Co 2 Sn 1 − x Fe x O 4 (0 ≤ x ≤ 0.4) were successfully prepared using the solid-state reaction method.XRD pattern showed that Co 2 Sn 1 − x Fe x O 4 is a single-phase cubic spinel structure.The magnetic behavior and exchange bias effects of all the Fe-substituted Co 2 Sn 1 − x Fe x O 4 (0 ≤ x ≤ 0.4) samples were studied in detail.Moreover, Curie-Weiss's tting of high-temperature magnetic susceptibility indicated that the system's magnetic exchange shifted from antiferromagnetic (at x < 2) to ferromagnetic (at x > 2) as Fe-substitution increased.The spin glassy state that existed in the low-temperature Fe-doped sample was con rmed by AC magnetic susceptibility.Our synthesized magnetic spinel oxides are attractive, showing the exible elemental composition and physiochemical characteristics that make them ideal for biological imaging, and therapy applications.

Declarations
Acknowledgement: This work was supported by the National Natural Science Foundation of China (Grant No.11474111).We would like to thank the staff of the Analysis Center at HUST for their assistance in various measurements.

Funding
This work was supported by the National Natural Science Foundation of China (Grant Nos.11474111).
Con icts of interest/Competing interests Figure 7 (a) shows the FC hysteresis loops of Co 2 Sn 0.8 Fe 0.2 O 4 samples measured at different cooling elds at 10 K temperature.The cooling elds selected in the experiment are 2, 5, 8, 10, 15, 20, and 30 kOe from small to large.The FC hysteresis loop and the lower half branch close to the magnetic eld axis under different cooling elds were locally enlarged in the upper left and lower right corners of the gure to better show the cooling eld's effect.The illustration Fig. 7 (a) shows that as the cooling eld increases, the FC hysteresis loop moves to the left along the magnetic eld axis, indicating that sample exchange bias behavior depends on cooling eld size and can be enhanced by increasing the cooling eld. Figure 7 (b) shows the exchange bias eld of each sample under each cooling eld at a xed temperature of 10 K.It can be observed that when the cooling eld (H FC ) is within the limit of 2 kOe to 15 kOe, the exchange bias eld increases rapidly with the cooling eld [51].As the cooling eld reaches 15 kOe, its size keeps growing while the growth rate of the sample exchange bias eld signi cantly slows down, suggesting the saturation level.The cooling eld provides a uniaxial anisotropy for the sample.
The exchange bias effect is enhanced when the cooling eld size is low.When the cooling eld reaches 15 kOe and continues to increase, the uncompensated spins that can rotate in the direction of the cooling eld are nearly saturated.Therefore, if the cooling eld continues to increase its size, the sample exchange bias eld also gets saturated [52].

Figure 2 (
a) shows the relationship between magnetization with an externally applied magnetic eld for Co 2 SnO 4 sample taken eld cooling (FC) and zero elds cooled (ZFC) conditions.It indicates the paramagnetic behavior of the sample without the Fesubstitution but Fig.2 (b-d) shows that with the increase of Fe-substituent in Co 2 Sn 1-x Fe x O 4 , the phase change from paramagnetic to ferromagnetic observed that can be seen from M-H hysteresis loops.

Figure 2 M
Figure 2

Table 2
Curie-Weiss tting parameters of Fe-substituted Co 2 Sn 1 − x Fe x O 4 samples.

Table 3
Kinetic tting parameters of Co 2 Sn 1-x Fe x O 4 doped samples under alternating magnetic eld