High temperature EPR study of the
M Fe V O (M=Cu, Zn, Mg and Mn)
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4
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compounds
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1
1
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2
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N. Guskos , G. Zolnierkiewicz , J. Typek , M. Pilarska , C. Aidinis and A. Blonska- Tabero
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Department of Physics, West Pomeranian University of Technology, Szczecin, Al. Piastow 48, 70-311 Szczecin, Poland.
2
Department of Electronics-Computers-Telecommunications and Control, Faculty of Physics, University of Athens,
Panepistimioupolis, GR-157 84 Athens, Greece.
3
Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology,
Al. Piastow 17, 70-310 Szczecin, Poland.
Figure 1 shows the magnetic resonance spectra of four investigated M Fe V O (M(II)= Cu, Zn, Mg and Mn) samples registered at different temperatures in the high-temperature range. In the whole studied range of temperatures a 3 4 6 24
symmetrical and intense resonance line was observed for all four investigated compounds. The experimental spectra were successfully fitted with Lorentzian lineshape function.
Figures 2-4 present temperature dependence of the calculated EPR spectra parameters: effective g-factor (g ), peak-to-peak linewidth (Deff B ), and the integrated intensity (I ) as well as the inverse of integrated intensity (1/I ). The pp int int
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effective g-factor was calculated from the resonance field B and the resonance condition (g = h nr eff / mB B ), while the integrated intensity was assumed to be equal to I =A·Dr int B , where A is the amplitude of EPR signal in the first derivative mode. pp
As the resonance field increases with increasing temperature, the g-factor slightly decreases on heating samples above ~340 K. For compounds with magnetic ions at M (II) site, at a temperature of about 450 K, a reverse process is evident. In
340 K - 450 K range the shift of the resonance field dB (defined as dr B =B (T)-B (T+Dr r r T)) shows a nearly linear temperature dependence (see Fig. 2). The following values of dB /Dr T temperature gradients of that resonance field shift were
-3 -3 -3 -3
calculated: dB /? T=7·10 mT/K for Cu Fe V O , dr 3 4 6 24 B /? T=6·10 mT/K for Zn Fe V O , dr 3 4 6 24 B /? T=6·10 mT/K for Mg Fe V O and dr 3 4 6 24 B /? T=9·10 mT/K for Mn Fe V O . The occurrence of additional magnetic ions (Cu and Mn) in the crystal r 3 4 6 24
structure increases the value of that temperature gradient.
Figure 3 presents temperature dependence of the DB linewidth. For the two compounds with magnetic ions in M(II) position the linewidth decreases monotonically with increasing temperature, but for Zn Fe V O and Mg Fe V O at pp 3 4 6 24 3 4 6 24
about 400 K an increase of linewidth with temperature increase is observed.
In Figure 4 the temperature dependence of the EPR integrated intensity (I ) and the reciprocal of integrated intensity (1/I ) is presented. The inverse of magnetic susceptibility behaves according to Curie-Weiss law, I =C/(T- qint int int ), in the
temperature range 333 - 433 K, where the constant C is related to an effective magnetic moment and the Curie-Weiss temperature q is positive for ferromagnetic interaction and negative for antiferromagnetic interaction between the involved
spins. For our compounds the following values of q were calculated: 56.4(2) K for Cu(II), 47.8(2) K for Zn(II), 17.2(2) K for Mg(II) and -170.8(2) K for Mn(II) compounds. It is not unexpected that the strongest interactions are found in
Cu Fe V O and Mn Fe V O compounds where two different magnetic ions are present in the unit cell.3 4 6 24 3 4 6 24
In Fig. 5 the relation between the Curie constant C and Curie-Weiss temperature q is shown. Remarkably, an effective magnetic moment of Cu Fe V O and Mn Fe V O compounds is smaller than in Zn Fe V O and Mg Fe V O what 3 4 6 24 3 4 6 24 3 4 6 24 3 4 6 24
indicates on existence of antiferromagnetic interaction between Fe and Cu/Mn ions. This interaction must be especially strong in Mn Fe V O compound and it overcomes the weaker ferromagnetic interaction in Fe sublattice, causing an 3 4 6 24
overall effective interaction to be antiferromagnetic, as evidenced by negative sign of the Curie-Weiss temperature. The clustering of magnetic ions, leading to magnetic frustration, is not excluded in Cu Fe V O and Mn Fe V O compounds.3 4 6 24 3 4 6 24
It is well known that when the narrowing of the resonance line is produced by temperature dependent exchange interaction, the lineshape is Lorentzian. This is also the case for all our EPR spectra. As the exchange-narrowing spin-spin
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interaction among Fe ions is operating in all four of studied compounds, it explains the line narrowing observed there below 400 K. The exchanged-narrow linewidth is usually calculated from the following equation: DB=DBdip/B , where Dex B is dip
the dipolar linewidth determined by dipol-dipol interaction experienced by each spin from all its neighbouring spins, and B is the exchange field. As B is proportional to the Curie-Weiss temperature, B ~qex ex ex , it follows that the exchange
narrowed linewidth will be inversely proportional to q. This explains why the observed linewidths of our four samples at temperatures lower than 400 K order in that particular manner – in samples with two different magnetic ions we have
stronger magnetic interactions and thus narrower EPR lines than in compounds with only Fe magnetic ion.
In Zn Fe V O and Mg Fe V O compounds yet another dynamic relaxation process operates above 400 K (see Fig. 3). In consequence, in both samples the linewidth starts to increase as temperature increases. In general, an increase 3 4 6 24 3 4 6 24
of linewidth (linear) with temperature increase can be the result of three mechanism: the direct spin-phonon process (modulation of the crystalline electric field by lattice vibration involving one phonon in the relaxation process), modulation of static Dzyaloshinsky-Moriya interaction between a pair of magnetic ions by a single phonon, and the bottleneck scenario involving relaxation of localised magnetic moments to the lattice via the highly mobile electron system in the metallic regime. It seems very probable that the first mechanism operates in our two Zn and Mg compounds as the second mechanism is usually realized in spin system S=1/2, while the third mechanism requires no significant shift of the g-factor,
contrary to what is observed in our samples (Fig. 2). This spin-lattice mechanism is either suppressed or moved to much higher temperatures in Mn Fe V O and Cu Fe V O compounds by the presence of additional magnetic ions that 3 4 6 24 3 4 6 24
magnify the exchange narrowing processes by increasing and frustrating exchange interactions between magnetic ions.
Results
• EPR spectra of four compounds M Fe V O (M(II) = Cu(II), Zn(II), Mg(II), Mn(II)) have been studied in the high temperature range.
3 4 6 24Temperature dependence of the resonance field, linewidth and integrated intensities of the observed single and intense Lorentzian line
has been determined.
At temperatures T<400 K the observed line is exchanged narrowed in all studied compounds - the effect is much larger in Mn Fe V O
3 4 6 24and Cu Fe V O compounds where an additional magnetic ion is placed at M(II) site.
3 4 6 24The presence of two different magnetic ions in M Fe V O structure can be invoked also from temperature dependence of the resonance
3 4 6 24field and the integrated intensity. In consequence, more complicated magnetic structure of Mn Fe V O and Cu Fe V O as compared to
3 4 6 24 3 4 6 24Mg Fe V O and Zn Fe V O compounds leads to the competition of ferromagnetic and antiferromagnetic interactions and associated
3 4 6 24 3 4 6 24with them magnetic frustration effects
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•
Conclusions
-4 -3 -2 -1 0 1 2 3 4 5 293 K 333 K 373 K 413 K 453 K 493 K 293 K 493 K Cu 3Fe4V6O24 -4 -3 -2 -1 0 1 2 3 4 5 293 K 493 K Zn 3Fe4V6O24 0 100 200 300 400 500 600 -4 -3 -2 -1 0 1 2 3 4 d c " /d H [a rb . u n it s ] Magnetic field [mT] 493 K 293 K Mg 3Fe4V6O24 0 100 200 300 400 500 600 700 -2 -1 0 1 2 3 493 K 293 K Mn 3Fe4V6O24 300 350 400 450 500 1.990 1.995 2.000 2.005 2.010 2.015 g -f a c to r Temperature [K] Cu3Fe4V6O24 Mn 3Fe4V6O24 Mg3Fe4V6O24 Zn3Fe4V6O24 300 350 400 450 500 40 50 60 70 80 90 100 110 120 130 140 150 160 L in e w id th [m T ] Temperature [K] Cu3Fe4V6O24 Mg3Fe4V6O24 Zn 3Fe4V6O24 Mn3Fe4V6O24 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 0.8 1.0 1.2 1.4 0.40 0.45 0.50 0.55 0.60 0.56 0.60 0.64 0.68 0.72 0.60 0.65 0.70 0.75 0.80 0.85 0.90 Cu3Fe4V6O24 R e c ip ro c a l in te g ra te d in te n s it y [a rb . u n it s ] 1.6 1.8 2.0 2.2 2.4 2.6 Mg3Fe4V6O24 300 350 400 450 500 1.3 1.4 1.5 1.6 1.7 1.8 Mn3Fe4V6O24 In te g ra te d in te n s it y [a rb . u n it s ] Temperature [K] 300 350 400 450 500 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Zn3Fe4V6O24 In te g ra te d in te n s it y [a rb . u n it s ] R e c ip ro c a l in te g ra te d in te n s it y [a rb . u n it s ] 0 20 40 60 80 100 120 140 160 180 0 10 20 30 40 50 60 70 80 E P R C u ri e c o n s ta n t [1 /K ]EPR Curie-Weiss temperature |qCW| [K]
Mg
Zn
Cu
Mn
Figure 1. EPR spectra of investigated M Fe V O compounds registered at 3 4 6 24
different temperatures. Figure 4. Temperature dependence of the integrated intensity (left axis) and reciprocal integrated intensity (right axis) in M Fe V O compounds.
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Figure 2. Temperature dependence of g -factor in M Fe V O eff 3 4 6 24
compounds. The dotted lines are a guide to the eyes.
Figure 3. Temperature dependence of the linewidth ÄB in pp
M Fe V O compounds. The dotted lines are a guide to the 3 4 6 24
eyes.
Figure 5. Dependence of the Curie constant C on the
Curie-Weiss temperature q for all investigated samples.
The dotted line is a guide to the eyes.
Structure of M Fe V O showing the arrangement of Fe1 and Fe2 octahedra.3 4 6 24
SEM image of Mg Fe V O . 3 4 6 24
Materials Research Bulletin 37 (2002) 849-858]
[M.Kurzawa, A. Blonska-Tabero/
SEM image of Zn Fe V O . [M.Kurzawa, A. Blonska-Tabero/ 3 4 6 24
