Magnetic frustration in M-Fe-V-O system

Magnetic frustration

in M-Fe-V-O system

1

1

G. Zolnierkiewicz ,

N. Guskos ,

1,2

J. Typek

1

Institute of Physics, West Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland

2

Solid State Physics, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece

Vanadium compounds M-Fe-V-O (M (II) = Zn (II), Mg (II), Cu (II), Mn (II) and Co (II)) with magnetic and non-magnetic ions in cationic sublattices exhibit structural randomness (iron ions can occupy positions of ions M(II)). Deficiency of oxygen in these compounds may be responsible for the emergence of competing magnetic interactions which prevent the formation of the long-range magnetic order at high temperatures. Study of the temperature dependence of magnetic susceptibility and electron paramagnetic resonance (EPR)

3 +

spectra of compounds from the M-Fe-V-O system has revealed the presence of significant spin frustration. It may be due to the Fe ions located in cationic sublattices. Magnetic measurements of these systems showed the existence of strong antiferromagnetic interaction at high temperatures with a high value of the Curie-Weiss temperature. Competing

magnetic interactions allow to form a long range magnetic order only at low temperatures. In particular, the static magnetic susceptibility for compounds M FeV O (M(II)= 2 3 11

3 +

Zn(II) and Mg(II)) revealed the existence of antiferromagnetic interaction between the Fe spins with the Curie-Weiss temperature of è = -55 Ê and the phase transition to a spin

glass state at T = 2.5-2.8 K. Strong changes of EPR parameters were observed at about 50 K. Similarly, in M Fe V O compounds the high-temperature long-range magnetic order f 3 4 6 24

was not registered. For these compounds the Curie-Weiss temperature is high. Competition and frustration of magnetic processes may be responsible for the lack of the long-range order at high temperatures despite the presence of a strong coupled correlated spin system. The aim of this report is to present our work on magnetic properties of compounds from the M-Fe-V-O system studied by dc magnetization and magnetic resonance technique.

Introduction

Curie-Weiss linear fit Mg3Fe4V6O24 H /M [m o l* T *A -1 *m -2 ] Temperature T [K] m_eff=2.65(1)m_B/Fe q= -115.2 + -1.2 K H=100 Oe q= -112.9 + - 0.4 K H= 50 kOe 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 c = M /H [ A *m 2 *m o l -1 *T -1 ] Temperature [K] ZFC H=100 Oe FC H=100 Oe ZFC H=50 kOe FC H=50 kOe 0 1000 2000 3000 4000 5000 6000 7000 -50 -40 -30 -20 -10 0 10 20 30 40 50 d c " /d H [a rb . u n it s .] Magnetic field H [G] Co3Fe4V6O24 Cu3Fe4V6O24 Mg3Fe4V6O24 Mn3Fe4V6O24 Zn3Fe4V6O24 290 K 50 100 150 200 250 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 1 2 3 4 5 6 7 8 Mg₃Fe₄V₆O₂₄ g e ff Temperature T [K] L in e w id th D H ( k G )

For an M Fe V O compounds EPR line broadens with decreasing temperature, while decreases rapidly at low 3 4 6 24

temperatures (> 30 K). At low temperatures, after disappearance of the main resonance line many weak and narrow resonance lines appear, which are likely to come from other paramagnetic centres such as complexes of V (IV) ions or iron (III) at low-symmetry sites of the crystal field. Analysis of lineshapes showed that lines can be properly fitted by Lorentzian function. Resonance absorption signal includes also contribution from the negative fields due to linearly polarized microwave field. This contribution has a significant effect when the linewidth is comparable with the resonance field. Distortion of the shape of the resonance line and its broadening suggests the long-range magnetic order at high temperatures.

Figure 3b shows the temperature dependence of g (T) and the resonance linewidth ÄH (T) obtained by fitting of the eff pp

experimental spectrum by Lorentzian function. Significant change is observed for both parameters at temperatures

below 80 K, and above T ~ 8.5 K and T ~ 3 K (Fig. 2b). Broadening of the resonance linewidth in the temperature 1 2

range (3<T/T <10) observed often for AFM dielectrics is usually associated with the change of static magnetic N

susceptibility. Temperature dependence of the resonance linewidth ÄH (T) can be represented by the following pp

relationship: ÄH (T)=[÷o(T)/÷(T)]ÄHo, where ÷ (T)=C/T is a free single-ion magnetic susceptibility, C is a Curie pp o

constant of the coupled paramagnetic system, while ÷ (T) is the static magnetic susceptibility and ÄHo is temperature independent linewidth (at high temperatures) combined with a contribution from the anisotropic

spin-spin interaction. In this case, for [ÄH (T)÷ (T)/ÄH ÷(T)] a minimum at 90 K is observed, and there is correlation pp o o

with a shift towards lower fields of g (T) factor. eff

Measurements of dc magnetization and EPR in the M Fe V O compounds revealed the existence of significant 3 4 6 24

magnetic frustration due to loss of oxygen and non-equivalence of two magnetic sublattices. Dc magnetic

susceptibility showed that the spin freezing at low temperatures can be produced in two magnetic sublattices with Tf

= 3K and 6K, and there is a strong correlation between the AFM at high temperatures. EPR studies showed the presence of AFM interactions and revealed the contribution of the magnetic ordering processes at around 230 K, that the static magnetization measurements did not confirmed. High frequency EPR (HF-EPR) measurements allowed to observe changes in the temperature dependence of g-factor and to compare it with the measurements at low frequency (X-band) EPR. Moreover, the use of HF-EPR can isolate the dominant type of magnetic interactions due to the application of very large magnetic fields.

Figure 3c shows temperature dependence of integrated intensity

I(T) and product I·T (right axis) for Mg Fe V O . Solid lines present the fitting for Fe(III)-Fe(III) dimer model.3 4 6 24

Results and discussion

Applayed magnetic field Co3Fe4V6O24 Cu3Fe4V6O24 Mg3Fe4V6O24 Mn3Fe4V6O24 Zn3Fe4V6O24 è – Curie-Weiss temperature [K] 10 Oe 100 – 200 K - - - -140.9±1.6 - 10 Oe 200 – 300 K - - - -29.9±2.9 - 50 Oe -119.0±0.5 -82.0±0.5 - - -101.4±0.9 100 Oe -119.4±0.7 -81.2±0.5 -115.2±1.2 -159.3±1.3 - 6000 Oe -128.5±1.3 -77.6±0.5 - -160.2±1.0 -100.7±0.9 50 000 Oe -111.4±0.9 -76.8±0.6 -112.9±0.4 -152.3±1.3 - an effective number of Bohr magnetons (ìeff) per one M3Fe4V6O24 molecule

10 Oe 100 – 200 K - - - 12.75±0.04 - 10 Oe 200 – 300 K - - - 10.54±0.05 - 50 Oe 13.12±0.01 11.07±0.01 - - 11.01±0.02 100 Oe 13.23±0.01 11.16±0.01 10.60±0.02 13.26±0.03 - 6000 Oe 13.36±0.03 11.07±0.01 - 13.17±0.01 10.89±0.02 50 000 Oe 12.98±0.02 11.06±0.01 10.59±0.01 12.96±0.03 -

Fig. 1a Alignment of Fe (1) and Fe (2) dimers, and their surrounding by vanadium tetrahedra.

Fig. 1b Spatial arrangement of Fe and Mn units in Mn Fe V O3 4 6 24

-1

Fig. 2a Temperature dependence of inverse magnetic susceptibility, ÷ =

-1

(MZFC / H) , for Mg Fe V O at H = 100 Oe and 50 kOe. The solid lines 3 4 6 24

are Curie-Weiss fits for high temperatures (T> 60 K)

Fig. 2b Temperature dependence of ZFC and FC magnetization for two different values of magnetic fields for the Mg Fe V O compound 3 4 6 24

at low temperatures.

Fig. 3a EPR spectra of M Fe V O (M(II)=Co(II), Cu(II), Mg(II), 3 4 6 24

Mn(II) and Zn(II) compounds.

Fig. 3b Temperature dependence of g (left axis) and linewidth ÄH eff pp

(right axis) for Mg Fe V O .3 4 6 24

Fig. 3c Temperature dependence of integrated intensity I(T) and product I·T (right axis) for Mg Fe V O . Solid lines present the fit for Fe dimer 3 4 6 24

model.

Conclusions

Structure and magnetic properties of v

M-Fe-V-O have been investigated. The magnetic

competition processes are responsible for lack of long range

magnetic order at high temperatures. At low temperatures

spin frustration was observed. The disorder of magnetic ions

in their sublattices and oxygen deficiency could play a

dominant role in the magnetic competition phenomena.

anadium compounds

Table 1. Curie-Weiss temperature and effective number of Bohr magnetons per one molecule.