MAGNETIC INVESTIGATIONS
OF Nb Sb VO
6
3
25
1 1 1 2 2
Janusz Typek , Grzegorz Zolnierkiewicz , Marta Bobrowska , M. Piz , Elzbieta Filipek
1
Institute of Physics, West Pomeranian University of Technology, Szczecin, Al. Piastow 48, 70-311 Szczecin, Poland
2
Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology, Szczecin, Al. Piastow 42,
71-065 Szczecin, Poland
Conclusions
Experimental
(T) dependence indicate on the presence of a few different magnetic enti-ties. The susceptibility is very small near RT, and becomes negative in big-ger external magnetic fields. The sign of ·T curve slope, which indicate on FM (negative) or AFM (positive) interaction depends on the strength of an
0 50 100 150 200 250 300 138 140 142 144 1.9676 1.9680 1.9684 1.9688 1.9692 1.9696 Linewidth [G] Temperature [K] g-factor 2.75 3.00 3.25 3.50 3.75 4.00 4.25 -8 -6 -4 -2 0 2 4 6 1.40 1.45 1.50 1.55 1.60 1.65 1.70 -0.10 -0.05 0.00 0.05 290 K 20 K 10 K 6 K 4 K 50 K 20 K 10 K 6 K EPR ampli tud e [arb.uni ts] Magnetic field [kG] 4 K 0 1 2 3 4 5 6 7 0.00 0.05 0.10 0.15 0.20 0.25 2 K 10 K 60 K 290 K Magne tisa tio n [B /f .u.] Magnetic field [T] 2 4 6 8 10 12 14 16 0.03 0.06 0.09 0.12 0.15 0.18 150 180 210 240 270 300 -0.45 -0.30 -0.15 0.00 0.15 70 kOe 1 kOe 100 Oe T emu K/ ( mol Oe ) ] Temperature [K] 10 Oe 70 kOe 1 kOe 100 Oe 10 Oe ] 0 50 100 150 200 250 300 0 10 20 30 40 50 60 70 80 5 10 15 20 40 60 80 100 120 140 160 Temperature [K] FC10 Oe FC1000 Oe FC100 Oe FC10 Oe Temperature [K] [1 0 -3 em u/m ol Oe ] dc susceptibility [ m ol Oe /em u ] [ mo lOe /e mu ] 140 160 180 200 220 240 260 280 300 0.4 0.5 0.6 0.7 0.8 0.9 1000 1200 1400 1600 1800 2000 2200 140 160 180 200 220 240 260 280 300 -1.01 -1.00 -0.99 -0.98 -0.97 -0.96 -0.95 -0.94 -0.93 -1070 -1060 -1050 -1040 -1030 -1020 -1010 -1000 -990 140 160 180 200 220 240 260 280 300 -1.16 -1.14 -1.12 -1.10 -1.08 -1.06 -1.04 -1.02 -1.00 -0.98 -1000 -980 -960 -940 -920 -900 -880 -860 140 160 180 200 220 240 260 280 300 -1.60 -1.55 -1.50 -1.45 -1.40 -1.35 -1.30 -760 -740 -720 -700 -680 -660 -640 -620 Temperature [K] 10 Oe Temperature [K] 100 Oe Temperature [K] 1 kOe Temperature [K] 70 kOe [ mo lOe /e mu ] [ 1 0 -3emu /mol Oe ] [ 1 0 -3e m u /mo l Oe ] 0 50 100 150 200 250 300 0.0 0.4 0.8 1.2 1.6 0 10 20 30 0 10 20 30 40 50 60 0 4 8 12 16 0.0 0.2 0.4 0.6 0.8 1.0 Temperature [K]
Reciprocal Integrated intensity [arb. units]
EPR integrated intensity [arb. units]
EPR measurements
The main feature of the EPR spectrum is a single, symmetric and rather strong resonance line in a magnetic field near g-factor close to 2. The line can be easily fitted with a Lorentzian lineshape indicating that its origin is from an exchange coupled system of S= spins. On cooling from RT the amplitude of the line increases significantly, but its g-factor and linewidth show only small temperature variation in temperature range down to 60 K . An interesting thermal behavior of g-factor and linewidth is observed only in the low temperature range (T<60 K). On further cool-ing the sample g-factor initially steeply increase to reach a maximum value at ~15 K and then equally significantly decreases on approach to 4 K. This strange behavior might be explained by an effect of emerging and competition of local magnetic field of FM and AFM type during cooling the sample. Similarly, a steep increase of linewidth on cooling the sample below 8 K might be the effect of freezing of the local magnetic field around paramagnetic spins.
The vanadium ions in the Nb VSb O compound could be only in 4+ (EPR active) or 5+ (EPR inactive) valence 6 3 25
states, it was calculated that 28% of the vanadium ions were magnetic and thus in the 4+ valence state. Another
Dc magnetisation measurements
(T)= + =C/(T-T )+CW 0 CW 0 ( )=m M H 1 2 2coth 2µ H B kT -coth µ H B kT + m2S+1 (2S+1) coth (2S+1)µ H B kT -coth µ H B kT +P T H( ) • From: 1 2 +4m =0.23(1) follows that 23% of vanadium ions are Vf.u.B
m10.(2)=0.0026(5) follows that > 0.3% of vanadium ions are in 2S+1 state
B
f.u.
EPR line was observed only in the low temperature range. Its amplitude
has also increased with temperature decrease, and g factor (close to 4.3)
and linewidth didn't show any temperature variation. This EPR signal at
g~4.3 could be due to a forbidden (ΔM =±2) transition in a pair V4+-V4+ S
(S=1). The integrated intensity was calculated as the product of line amplitude and squared linewidth. In the high temperature range (100 K<T<250 K), when the Curie-Weiss law is applied, the Curie-Weiss
con-stant T = -122 K is obtained, what indicates on a strong AFM interaction CW
between spins in that temperature range.
external magnetic field. In the low temperature range – effective interaction changes sign, from FM at higher temperatures to AFM at lower temperatures.
The presence of negative magnetisation is most probably not related to the diamagnetic Meissner effect- the origin is due to AFM inter-action of two magnetic sublattices
The new compound of the formula Nb VSb O in the Nb-V-Sb-O system investigated in this study was obtained by solid 6 3 25
state reaction in air: V O + 6 T-Nb O + 3 α-Sb O +3/2 O = 2 Nb VSb O2 5 2 5 2 4 2 6 3 25
Two investigation methods were used: dc SQUID magnetometry and electron paramagnetic resonance (EPR). The mag-netization study was carried out on the Quantum Design Magnetic Property Measurements System MPMS XL-7 with a superconducting quantum interference device magnetometer in magnetic fields up to 70 kOe and in the 2–300 K temper-ature range. The EPR spectra were obtained on a conventional X-band Bruker ELEXSYS E 500 spectrometer in the 4-290 K temperature range by using an Oxford Instruments ESP helium-flow cryostat.
• Three magnetic subsystem have been identify: 2 paramagnetic (S= and S~4.5) and AFM clusters • There is a large number of magnetic ions (~ 25% of all vanadium ions)
• There is magnetic competition of FM and AFM interactions
• Magnetic ions congregate on a grain boundaries and structural imperfections
• Charge transfer from oxygen to metal ions is responsible for magnetic properties of Nb Sb VO6 3 2 5
1 2
1 2
