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Spin-Glass Behavior, Magnetic, and IR Spectroscopy Analysis of Multimetallic Compound Ni0.25Mn1.25[Fe(CN)6]·6.1H2O
Multimetallic Prussian blue compound Ni0.25Mn1.25[Fe(CN)6]·6.1H2O has been prepared by coprecipitation. The temperature-dependent magnetic susceptibilities show the magnet transition for the compound at 8.5 K. According to DC variable-temperature magnetic susceptibility paramagnetic Curie temperature is −9.32 K. The observed value of coercive field (Hc) and the remanent magnetization (Mr) for the compound are 0.32 KOe and 0.36 . According to study of zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves and AC magnetization curves, there exists a spin-glass behaviour in the compound, which exhibits freezing temperature K, below magnetic transition K; that glass behavior is termed “reentrant” spin glass.
Recently, molecule-based magnets, which can be synthesized by chemical process and have a main property of supramolecular structure, exhibit magnetic properties due to magnetic exchange interaction between magnetic ions [1–3]. The design and synthesis of molecule-based magnets have become one of the research foci on the physics and chemistry. Ones have attracted extensive attention recently owe to the high molecular ferromagnets had been discovered and the molecule-based magnets show a kinds of fascinating magnetic phenomenon such as photomagnetic effect, thermal induced magnetic properties, and magnetic-pole reversal which have important potential application [4–6]. Among various types of molecule-based magnetism materials, Prussian blue analogues ·xH2O  (molecular structure of Prussian blue analogue compound as shown in Figure 1) play an important role due to their special structure and outstanding magnetic properties as molecule-based magnets. The multimetal Prussian blue compound Ni0.25Mn1.25[Fe(CN)6]·6.1H2O was synthesized and has been studied for its magnetic properties through elemental analysis, IR, Mossbauer spectrum, magnetic measurements, and so forth.
2.1. Materials and Physical Measurements
NiCl2·6H2O, Mn(SO4)2·6H2O, and K3Fe(CN)6 are of reagent grade and without further purification. Elemental analyses (C, H, and N) were performed on Perkin-Elmer 2400 II analyser. IR spectrum was recorded on a Perkin-Elmer FT-IR spectrophotometer as KBr pellet in the 4000~400 cm−1 range. Magnetization measurements were measured by a Quantum Design MPMS-7S superconducting quantum interference device (SQUID) magnetometer in the scope of 2–300 K.
2.2. Synthesis of Ni0.25Mn1.25[Fe(CN)6]·6.1H2O
Polycrystalline samples of Ni0.25Mn1.25[Fe(CN)6]·6.1H2O have been prepared in coprecipitation method. A mixture of aqueous solutions of Co(NO3)2 (25 mL, 0.25 mmol) and NiSO4 (125 mL, 1.25 mmol) was poured in aqueous solution of K3[Fe(CN)6] (100 mL, 1 mmol). Then the mixture solution was left to stand at room temperature for an appropriate period of time until those reactants were finished. A light brown precipitation was obtained, and precipitation then was filtered, washed many times with demineralized water, and finally dried under IR lamp for about 50 minutes. Elemental analysis to measure C, H, and N mass ratio: found: C, 17.44%; H, 3.09%; N, 21.23%; calculation: C, 17.79%; H, 3.03%; N, 20.74%.
3. Results and Discussion
3.1. IR Spectrum Analysis
IR spectrum of the compound has been recorded over the 400–4000 cm−1 range shown in Figure 2. It shows two obvious bands at 2075.01 and 2151.74 cm−1 indicating the existence of two types of cyanide groups in the crystal lattice of compound [7–9]. Compounds with CN− functional group are easily identified by their stretching frequencies in 2200–2000 cm−1 range, which are consistent with the formation of bridging cyanide groups, and there are two different coordination environments. Moreover, the broad peaks at 3432.90 cm−1 and 1615.02 cm−1 are assigned to the v (O–H) of the crystal water stretching vibrations.
3.2. DC Magnetic Susceptibility
The magnetic susceptibility of the compound was measured from 2 K to 300 K in 250 Oe field. Figure 3 shows the field-cooled magnetization versus temperature curve and a sharp increase in is observed around 21 K. Magnetic transition temperature was estimated from minima of versus curve, which corresponds to the steepest increase of magnetization with decreasing temperature (as shown in Figure 4). The phase transition the compound undergoes from a paramagnetic to ferro/ferrimagnetic type is about 8.5 K, which is lower than that for the parent compound Ni1.5[Fe(CN)6]·xH2O ( K) .
The inverse susceptibility as a function of temperature in the paramagnetic state is shown in Figure 5. The curve rises slowly with decrease of temperature from 300 to 25 K and then rises sharply as temperature continues to decrease. The shows a sharp maximum at 2 K. This kind of behaviour is a characteristic of a ferromagnet. The magnetic order results from the combination of ferromagnetic and neighboring antiferromagnetic interactions. Furthermore, high temperature DC susceptibility is found to obey the Curie-Weiss law.
Figure 6 shows the temperature dependence of in the temperature range of 20–280 K. The Curie constant () and the Curie-Weiss temperature are estimated by a linear fitting of at the linear region [8–10]. Fitting yielded that the Curie constant cm3·K·mol−1 and paramagnetic Curie temperature K. The values of , , and are different from those values for ferrimagnet Ni1.5[Fe(CN)6]·xH2O  and Mn3[Fe (CN)6]2·15H2O ( K) .
A curve of versus is shown in Figure 7, and the value at room temperature is 5.7 cm3·K·mol−1. Upon lowering the temperature, value sharply increases after 15 K with a further decrease of the temperature. The shows a sharp maximum value of 35.3 cm3·K·mol−1 at 9 K and then finally decreases more rapidly on further cooling. For a ferromagnetic compound, versus curve reaches a minimum before rising around magnetic ordering temperature [12–14].
A curve of versus is shown in Figure 8. The effective moment first slowly decreases to reach a minimum of 6.47 at 23 K and then sharply increases to reach maximum of 16.83 at 9 K and final decrease at lower temperature, indicating antiferromagnetic interaction between paramagnetic centers [15–17]. Magnetic transition temperature was estimated from minimum of versus curve, which corresponds to the steepest increase of magnetization with decreasing temperature (as shown in Figure 6; insert: plot of versus ). The compound undergoes a paramagnetic to ferro/ferrimagnetic type phase transition at 9.5 K, which could be attributed to an intermolecular antiferromagnetic interaction and/or a zero-field splitting (ZFS) effect. This kind of behaviour is a characteristic of a ferromagnet [18, 19].
3.3. Zero-Field-Cooled (ZFC) and Field-Cooled (FC) Magnetization
Figure 9 shows the curves of zero-field-cooled (ZFC) and field-cooled (FC) magnetization of the compound at different field , 100, 250, and 500 Oe. The values of increase and exhibit weak irreversibility in the field-cooled (FC) magnetization curves below K. There is a clear bifurcation phenomenon of the field-cooled () and zero-field-cooled () magnetization curves. is a bifurcation temperature point of which FC and ZFC magnetization curves separate out. In addition, the behavior that exhibits a maximum below is attributed to the cooperative freezing of spin glass (as shown in Figure 10). The irreversible behavior of and shift of bifurcation point to lower temperature with increasing are characteristic for spin glasses [10, 11]. It may be reasonable that these metal ions , , and through cyanide-bridged ligand have the coexistence of different valence states or spin states, the presence of inhomogeneity, and inherent structural disorder, which propagate possibly the ferromagnetic and antiferromagnetic exchange interaction via bridging cyanide, and there exist a structural disorder and a certain content of crystallization water which lead to the weak spin-orbit coupling. The spin-glass property is due to magnetic domain kinetics under different cooling conditions and the presence of available vacant sites in the lattice for the water molecules.
3.4. AC Magnetic Susceptibility
It was also confirmed that there exists a spin-glass behavior in the compound through AC magnetization curves. The AC magnetic susceptibility of the compound was measured at 4 Oe AC amplitude with zero-applied DC field when varying the frequencies () from 10 to 1000 Hz, as shown in Figure 11. The temperature dependence of zero-static field AC magnetic susceptibilities shows that the in-phase component () has a maximum at about 7.7 K for frequencies of 10, 32, 100, 320, and 997 Hz and that a significant out-of-phase component () appears, confirming the long-range ferromagnetic ordering. On decreasing temperature, the in-phase signals increase abruptly at around 12 K, reach the maximum at about 7.7 K, and then decrease slowly. The out-of-phase signals increase steadily to the maximum around 8 K and then decrease slowly, as shown in Figure 11. The fact that the () clearly shows a frequency dependence is typically assigned to spin glasses [12, 13]. The freezing temperature (), K, is defined by the maximum in the plot at low frequency. It undergoes a paramagnetic to ferromagnetic transition at around 7.7 K. In fact, the temperature value of the maximum of χ at a given frequency () corresponds to the blocking temperature (), whereby it is assumed that the switching of the oscillating AC field matches the relaxation rate of the magnetization.
Proportional relationship between freezing temperature and logarithm of frequency in spin-glass system can be described by quantifying the frequency dependence through the ratio , which can be written as . versus is shown in Figure 12. The value of freezing temperature of zero frequency is K by extrapolation method and the value of obtained for the compound is 0.0011, which fall within the range typical for the conventional spin-glass system (10−2-10−3). Surprisingly, both the in-phase and out-of-phase signals ( and ) go through a maximum with strong frequency dependence. Both in the real and in the imaginary components the peaks shift to lower temperatures with decreasing frequencies. However, the intensities of the peaks behave differently. While the intensity of the peaks for the real component increases with decreasing frequencies, in the imaginary component the intensity of the peaks decreases with decreasing frequencies. This behaviour of and is typical of a spin-glass state [16–19].
3.5. Field-Dependent Magnetization and Hysteresis Behavior
In order to further understand the nature of magnetic ordering, the ferromagnetism behavior is characterized by the measurements of field-dependent magnetization, as shown in Figures 11 and 13. The observed value is 3.85 at 50 kOe, but the compound does not reach full saturation and this behavior is likely to be related to the spin-glass behavior as well as the amount of spin and type of coupling in the compounds Ni1.5[Fe(CN)6]·xH2O  and Mn3[Fe(CN)6]2·15H2O .
Ferromagnetism in the compound Ni0.25Mn1.25[Fe(CN)6]·6.1H2O is also supported by hysteresis loop curves measured at 4 K as shown in Figure 14. The coercive field () was 0.32 kOe, which was smaller than that of the compound Ni1.5[Fe(CN)6]·xH2O ( KOe,4.4 K) . The remanent magnetization () with 0.32 for the compound. It is also obtained by the hysteresis loop curves.
Therefore, synthesis ideas of molecular alloy magnet can be regarded as a synthesis method to expand a new type of magnetic functional materials, whose magnetic properties can be tuned and controlled by changing the composition of different transition metal cations [15, 17–19].
We have reported a detailed investigation of magnetic properties of multimetallic Prussian blue compounds Ni0.25Mn1.25[Fe(CN)6]·6.1H2O. The temperature-dependent magnetic susceptibilities show the magnetism transition for the compound at 8.5 K. The Curie constant ( cm3·K·mol−1) and the Curie-Weiss temperature ( K) are obtained through a linear fitting of at the linear region [8–10].
The observed values of coercive field () and remanent magnetization () for the compound are 0.32 KOe and 0.36 . Moreover, there exists a spin-glass behaviour in the compound according to study of zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves and AC magnetization curves, which exhibits freezing temperature , below magnetic transition K. Such a glass behavior is termed “reentrant” spin glass. It was also confirmed by the behaviour of and , which go through a maximum with strong frequency dependence.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the National Natural Science Foundation of China (nos. 11164002 and 11364004) and Innovation Project of Guangxi Graduate Education under Grant no. 0991092.
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