EPR and Magnetometry of Mixed Phases in FeVO4–Co3V2O8 System
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Magnetic properties of five samples obtained in reactions between nFeVO4 and (1 − n)Co3V2O8 (with the composition index n = 0.54, 0.50, 0.40, 0.30 and 0.20) have been investigated by dc magnetometry and electron paramagnetic resonance (EPR). The investigated samples contained two phases: one having lyonsite-type structure and the other Co3V2O8. Temperature dependence of dc magnetic susceptibility showed the Curie–Weiss paramagnetic behaviour with a strong antiferromagnetic (AFM) interaction in the high-temperature range leading to magnetic transitions at 6 and 12 K. For all samples, the magnetic hysteresis loop in isothermal magnetisation was registered at T = 5 K indicating the existence of a ferromagnetic component. Samples n = 0.54 and 0.50 displayed a large coercive field and remanent magnetisation in contrast to the other three samples due to higher content of the lyonsite-type structure. EPR spectra of the studied samples showed a broad asymmetric line that was decomposed on lorentzian or gaussian components. Temperature dependence of the spectral parameters (resonance field, linewidth, integrated intensity) of these components was studied in high-temperature range (T > 90 K). The observed spectra were attributed to different types of magnetic spin clusters.
Phases of the lyonsite- or howardevansite-type (abbreviated to L-type and H-type, respectively) structures contain isolated VO4 tetrahedrons which build a lattice of orthovanadates (V) of divalent metals . These in turn are connected with strong catalytic activity in the reaction of the oxidizing dehydrogenation of the saturated to unsaturated compounds . From this point of view, the FeVO4–Co3V2O8 system is an interesting object of investigation because in samples from that system two phases of the L-type and the H-type structures have been detected [3, 4, 5]. The L-type structure can accommodate cationic mixing as well as cationic vacancies therefore a large number of chemical compounds can adopt this structure type. The H-type structure is found in the products of contemporary fumarole activity of few volcanoes .
The starting compounds, FeVO4 and Co3V2O8, have very interesting magnetic properties, especially in the low-temperature range. Triclinic iron vanadate FeVO4 large single crystals displayed two magnetic orderings at 20 and 13 K related to two different Fe ligand environments of octahedral FeO6 and trigonal bipyramidal FeO5 in a six-column doubly bent chain . The nanocrystalline FeVO4 prepared by co-precipitation method has shown similar transitions from paramagnetic to antiferromagnetic (AFM) state at 14 and 19.9 K . Magnetic interactions among three different Fe cation sites in FeVO4 are responsible for large magnetic frustration evidenced by a negative and large Curie–Weiss temperature TCW = − 125 K in paramagnetic phase . Co3V2O8 compound is an example of material in which the geometrically frustrated buckled kagome lattice is realised [10, 11, 12, 13, 14, 15, 16]. The kagome staircase lattice of Co3V2O8 contains Co2+ (S = 3/2) ions and complicated interplay of magnetic interactions yields a rich magnetic phase diagram. The material is paramagnetic at high temperature (above 11.3 K), and ferromagnetic (FM) at low temperature (below 6 K) or under a large applied field. Between these temperatures different AFM spin density wave phases (incommensurate and commensurate) have been detected [12, 13].
Magnetic properties of a number of phases from the FeVO4–Co3V2O8 system have been already studied using dc magnetometry and EPR/FMR (electron paramagnetic resonance/ferromagnetic resonance) techniques [17, 18, 19, 20]. In Ref. , the EPR spectra of twenty samples from FeVO4–Co3V2O8 system at room temperature (RT) were presented. The spectra displayed a strong dependence on initial concentrations of FeVO4 and Co3V2O8 and large differences in spectra intensity. In Refs. [18, 19, 20], more detailed magnetic investigations of samples from the nFeVO4/(1 − n)Co3V2O8 system (where n is the composition index, 0 < n < 1) were presented. In Ref. , magnetic study of samples with the composition indexes n = 0.81 and 0.83 was published, in Ref.  samples with n = 0.96, 0.86, 0.84 and 0.83 have been investigated, while in Ref.  samples with n = 0.82, 0.80, 0.78 and 0.76 were discussed. A very intricate nature of magnetism due to a mixture of phases, complicated crystal structure and competition of magnetic interactions was demonstrated. In the high temperature, a strong AFM interaction was registered which depended on the concentration of FeVO4 in initial mixture. At low temperature, the hysteresis loops were observed for these compounds and loop parameters were strongly dependent on initial mixture content.
The aim of the present study is to examine the magnetic properties of five samples from the FeVO4–Co3V2O8 system using dc magnetometry and EPR techniques. In the current samples, the content of FeVO4 in initial mixtures is smaller (and Co3V2O8 bigger) than in previously published papers. The present study is thus a further step towards a full magnetic characterisation of all phases synthesized in the FeVO4–Co3V2O8 system. Despite belonging to the same system they show a great diversity of their physical and chemical properties, therefore, a thorough investigation of all phases is needed for comprehensive understanding of mechanism responsible for that multifaceted behaviour. Due to a complicated crystal structure and disorder introduced by randomness in metallic sites occupation, the magnetic frustration is expected to appear in these compounds as a result of competition between localised spins on a lattice interacting through various exchange pathways that cannot be simultaneously satisfied. This is an intensively studied subject in magnetism and the presented in this paper EPR and magnetometric studies of few compounds from the FeVO4–Co3V2O8 system could provide a fertile ground for observation of these exciting phenomena.
Chemical composition of initial mixtures and phases detected in samples after synthesis (data from )
The composition of initial mixtures (mol%)
Phases detected by XRD
L-type phase, Co3V2O8
DC magnetization measurements were carried out using an MPMS-7 SQUID magnetometer in 2–300 K temperature range and in magnetic fields up to 70 kOe in the zero-field cooling (ZFC) and field cooling (FC) modes. Standard X-band Bruker E 500 spectrometer (ν = 9.4 GHz) with magnetic field modulation of 100 kHz was used to record EPR spectra. Temperature EPR measurements were performed in the 90–290 K range using an Oxford nitrogen-flow cryostat.
3 Results and Discussion
The main components in all studied samples might be attributed to two spin subsystems—one encompassing AFM spin pairs/cluster with a non-magnetic ground level (S = 0) that is responsible for intensity increase at high temperatures, and the other subsystem of paramagnetic spin cluster that accounts for EPR signal increase at lower temperatures. On the other hand, the additional EPR components (S20b and S23b) might be due to another AFM spin pair/cluster, with different internal fields and different relaxation times in comparison to the previous one. An overall weak temperature dependence of EPR spectrum parameters indicates that the spin–spin interaction (dipole or exchange) plays an important role in the formation of EPR response in our samples.
The studied five samples from FeVO4–Co3V2O8 system were diphase, containing L-type phase and Co3V2O8. The content of L-type phase decreased with the increased content of Co in initial mixtures. The magnetic ions Co2+ and Fe3+ were in the high-spin state S = 3/2 and 5/2, respectively. The observed effective magnetic moment inferred from the Curie constant was higher than theoretical one, indicating an additional contribution from the defective V4+ (S = ½) ions. AFM interaction between paramagnetic ions was determined from the negative Curie–Weiss temperature. The strength of this AFM interaction diminished going from sample S19 to S21, but increased on further Co content increase in samples S22 and S23. At low temperatures, two magnetic transitions are found (at T = 6 K and 12 K) that might be related to transitions in Co3V2O8 phase. Magnetic anisotropy deduced from (χFC − χZFC) difference was small and similar in value in all samples with exception of sample S20. This might be associated with bigger sizes of nanometric spin clusters in that sample. A FM component was identified from hysteresis loop in all samples at T = 5 K. In samples S21, S22, and S23, Hc < 100 Oe and Mr < 1.3 emu/g, while in samples S19 and S20 Hc ~ 1000 Oe, Mr > 3 emu/g. These differences might be correlated with a higher content of Co3V2O8 phase in the former samples evidenced by XRD method. EPR spectra were either weak (samples S20, S23) or very weak (samples S19, S21, S22), indicating they were formed by structural and magnetic defects. Very broad lines in EPR spectra in all samples were attributed to nanometric magnetic spin pairs/clusters. In general, samples showed inhomogeneous magnetism due to the presence of two strong magnetic ions randomly distributed between different crystallographic sites and a mixture of two magnetic phases.
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