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Magnetic properties of Cu Fe VO with 3.9 3.4 6 24 lyonsite structure 1 1 1 1,2 3 J. Typek , G. Zolnierkiewicz , M. Bobrowska , N. Guskos , A. Blonska-Tabero 1 Institute of Physics, West Pomeranian University of Technology, Szczecin, Al. Piastow 48, 70-311 Szczecin, Poland. 2 Department of Solid State Physics, Faculty of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece. 3 Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology, Szczecin, Al. Piastow 42, 71-065 Szczecin, Poland. 0 2 4 6 8 10 12 0 50 100 150 200 250 300 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Susceptibility [10 -4 em/ (g Oe)] 0.1 kOe 1 kOe 10 kOe 70 kOe Reciprocal susceptibility [10 4 g Oe/emu] Temperature [K] 0.000 0.002 0.004 0.006 0.008 0.010 0 50 100 150 200 250 300 0 5 10 15 20 25 30 0.000 0.002 0.004 0.006 c • T [ emu× K/g] Temperature [K] Temperature [K] 0.0 0.1 0.2 0.3 0.4 10 20 30 40 50 60 0 1 2 3 4 5 6 0 10 20 30 40 50 60 70 T=300 K Magnetization [m B /f.u.] T=2 K T=5 K Magnetic field [kOe] 0 2 4 6 8 10 -10 -5 0 5 10 -40 -30 -20 -10 0 Magnetic field [kG] T=70 K EPR absorption derivative [arb. units] T=4 K 0 50 100 150 200 250 300 2.0 2.5 3.0 3.5 4.0 4.5 50 100 150 200 250 300 1.998 2.000 2.002 2.004 2.006 g-factor Temperature [K] g-factor Temperature [K] 40 80 120 160 200 240 280 1.17 1.18 1.19 1.20 0 50 100 150 200 250 300 1 2 3 4 5 6 7 Linewidth [kG] Temperature [K] Linewidth [kG] Temperature [K] 0.00 0.05 0.10 0.15 0.20 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 Ln(D H[G]) Reciprocal temperature, T -1 [1/K] 7 K 30 K 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 0.5 1.0 1.5 2.0 2.5 3.0 3.5 3.370 3.375 3.380 3.385 0.6 0.7 0.8 Linewidth [kG] Resonance field [kG} 4 K 6 K 10 K 5 K 7 K 8 K Linewidth [kG] Resonance field [kG] 44 K 300 K 16 K 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 1 2 3 4 0 50 100 150 200 250 300 2 4 6 8 Integrated intensity I EPR [arb. units] Reciprocal integra- ted intensity 1/I EPR [arb. units] T• I EPR [ arb. units] Temperature [K] 0 20 40 60 80 100 120 140 160 0.0 0.5 1.0 1.5 2.0 T CW =-3.1 K Reciprocal integrated intensity [arb. units] Temperature [K] T CW =-73.3 K Magnetic field [kOe] Cooling mode Curie constant C [10 -4 emu K/g Oe] Effective magnetic moment [ì B /f.u.] Curie-W eiss constant T CW [K] Neel temperature [K] ZFC 5.97 0.01 FC 24 4.65 -134 - ZFC 2.75 0.1 FC 102 9.58 -42 - ZFC 2.50 1 FC 109 9.90 -42 - ZFC - 10 FC 106 9.76 -39 - ZFC - 70 FC 106 9.76 -40 - The synthesis of Cu Fe VO was performed by the standard solid-state 3.9 3.4 6 24 reaction method according to the reaction: 3.9 CuO + 1.7 Fe O + 3 V O = Cu Fe VO 2 3 2 5 3.9 3.4 6 24 Electron paramagnetic resonance study (EPR) were carried out on a conventional X–band (í = 9.4 GHz) Bruker E 500 spectrometer with the 100 kHz magnetic field modulation. EPR measurements were carried in the 4–300 K temperature range using an Oxford Instrument helium-flow cryostat. The dc susceptibility measurements were carried out in the 2-300 K temperature range using an MPMS-7 SQUID magnetometer and in magnetic fields up to 70 kOe in the zero-field-cooled (ZFC) and field-cooled (FC) modes. Experimental The compound Cu Fe VO is a new vanadate, which has been obtained recently in the 3.9 3.4 6 24 ternary oxide system CuO-V O -Fe O . It is known from literature that the components of this 2 5 2 3 system are active in catalytic processes such as, for example, oxidation reactions of: benzene to phenol, methanol to formaldehyde, isobutane to isobutene or toluene to benzaldehyde. As the compound contains magnetic iron ions it could display interesting magnetic characteristics. Very interesting magnetic phenomena were found in many similar vanadate compounds. It is probable that magnetic ions could play an important role in the determination of the compound's catalytic properties so the knowlegde of its magnetic state is a crucial factor. Two complimentary methods of magnetic characterization were employed: dc magnetization measurements as a function of temperature and an extenal magnetic field, and electron paramagnetic resonance (EPR) in the microwave frequency range. Introduction H C H J g kT kT H J g J g C H J g kT kT H J g J g C H M B B B B 3 2 2 2 2 2 2 2 1 1 1 1 1 1 1 coth coth ) ( + œ u u e º Ø ÷ ÷ o ö ç ç e a - ÷ o ö ç e a + œ u u e º Ø ÷ ÷ o ö ç ç e a - ÷ o ö ç e a = m m m m Modified Langevin equation: Table 2. Values of parameters in the modified Langevin equation obtained from fitting experimental points M(H) (see Fig. 4) 3+ As the temperature decreases, the Fe magnetic moments experience short range AFM correlations. The evidence for it is that, for T>2T , the EPR resonance shows signifant g-shift and a strong T- N dependence of the line broadening expected for a short range magnetic interaction in AFM materials above the Neel temperature T : N The solid line in Fig. 9 shows the fitting of the data (in 7<T<30 K range). 3 -1.64 The fitting parameters are: ÄH =548(39) G, C =19(2)·10 G/K , 0 1 EPR T =1.1(5) K, n=1.6(4). N In Fig. 12 the temperature dependence of the reiprocal integrated intensity is presented. The experimental values follow the Curie-Weiss law, I=C/(T-T ), in two different temperature ranges. In the low CW temperature range (7-30 K) T =-3.1 K, in the high-temperature range CW (40-120 K) T =-73.7 K. CW EPR Figure 1. Temperature dependence of the dc magnetic susceptibility ÷(T) (left axis) and reciprocal -1 susceptibility ÷ (T) (right axis) in ZFC mode measured at four different magnetic fields (H=0.1, 1, 10, 70 kOe). Figure 2. Temperature dependence of the dc magnetic susceptibility ÷(T) in ZFC and FC modes in the low temperature range registered at four different magnetic fields (H=0.01, 0.1, 1, 10 kOe) Figure 3. Temperature dependence of ÷?T product for the Cu Fe VO compound. The inset shows this dependence in 3.9 3.4 6 24 the low temperature range. Figure 4. Isothermal magnetization at three different temperatures: T= 2 K and T=5 K (upper panel), and T=300 K (lower panel). The solids lines are the best fits to Langevine function. Table 1. Values of magnetic parameters of the Cu Fe VO compound calculated from the 3.9 3.4 6 24 dc magnetization measurements. The Curie-Weiss law was applied in 70-250 K range. Figure 5. Selection of the registered EPR spectra of Cu Fe V O at different temperatures. 3.9 3.4 6 24 Figure 6. An example of fitting of the experimental EPR spectra of Cu Fe VO (black line) with 3.9 3.4 6 24 Lorentzian lineshape function (red line).Upper panel T= 4 K, bottom panel T=70 K Figure 7. Temperature dependence of the g-factor. The inset shows the same dependence in an extended g-factor scale. Figure 8. Temperature dependence of the peak-to- peak lnewidth. The inset shows the same dependence in an extended linewidth scale. Figure 9. Temperature dependence of linewidth in the log(Ä)-1/T frame Figure 10. Linewidth dependence on the resonance field at different temperatures. The insets shows this dependence in an extended scale. Figure 11. Temperature dependence of the EPR integrated intensity (upper panel), reciprocal of integrated intensity (middle panel) and the product of integrated intensity and temperature (bottom panel). () n EPR N T T C H T H - - + D = D 1 0 ) ( T [K] C 1 g 1 J 1 g 2 J 2 C 2 C 3 2 0.025 10.666 2.497 1.667 0.00001 5 1.538 3.471 12.212 0.127 0 300 1 9.303 0 0 0 Fe V O Cu Figure 12. Temperature depenence of the reciprocal integrated intensity. Cu Fe V O 3 4 6 24
