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Narrow gap semiconductor FeSb2 crystallizes into C18 marcasite type orthorhombic crystal structure where as MnSb2 does not exist [2,3]. Magnetic susceptibility of FeSb2 exhibits diamagnetic to paramagnetic crossover around 100K and it displays large magnetoresistance [ρ(70kOe) – ρ(0)] ⁄ ρ(0) ≈ 2200 % in the temperature range 40K < Tcr < 80K. The large magnetoresistance observed in FeSb2 is of comparable magnitude of the magnetoresistance observed in GMR materials (e.g. Lanthanum manganites [4,5]). Although Goodenough’s energy band scheme [6] for FeSb2 had been successful for qualitative explanation of electrical and magnetic properties, it does not account quantitatively for large magnetoresistance and a temperature induced spin state transition observed by Petrovic et al. [4,7]. Fesb2 and its compounds are of interest because of their unique TE transport properties below 100 K [8]. FeSb2 is known as low temperature thermoelectric (TE) material with narrow energy band gap (0.1 – 0.3 eV) [ 9, 10]. Neutron diffraction studies of FeSb2 showed no evidence of magnetic ordering down to 4.2K [11]. Gonçalves da silva [12] had theoretically predicted that FeSb2 is near a magnetic instability and can be pushed into stable magnetic state by the addition of acceptor impurities. In fact using strongly correlated valence and conduction band state model, he had proposed a magnetic phase diagram for Fe1-yMnySb2 system. We had therefore synthesized Fe1-yMnySb2 for 0 ≤ y ≤ 0.2 and checked the solid solubility of Mn for Fe into marcasite type structure of FeSb2 and the validity of Gonçalves da silva’s predictions [12] along with the 57Fe Mössbauer studies at 300K [13]. We report here 57Fe Mössbauer spectroscopic techniques at 80K and the magnetic measurements performed for all three compositions has been measured from 20K to 300K for exploring the magnetic properties of the series compositions of Fe1-yMnySb2 for 0 ≤ y ≤ 0.2

Experimental details

High purity (>99.999%) iron (Fe), Manganese (Mn) and antimony (Sb) were weighed in stoichiometric proportions to an accuracy of ± 0.0001 gram. The weighed quantity of each constituent element was filled in quartz ampoule which subsequently was evacuated (≈10^-2 torr) and sealed. Following the similar procedure as described by  R. K. Sharma et al. [13], we synthesized and characterized three compositions corresponding to y = 0.0, 0.1, 0.2.  For ⁵⁷Fe Mössbauer measurements at 80K liquid nitrogen crystostat was used. The temperature was controlled to an accuracy of ± 1K with the help of temperature controller. The absorber was filled in an annular copper ring of   ~ 1.6 mm diameter and 1 mm depth. This ring was kept at the centre of crystostat. Mössbauer spectra of all three specimens were folded and fitted using a standard data analysis program developed by Jernberg and Sundqvist [14]. Magnetization measurements were made on a PARC (Princeton Applied Research Co.) make vibrating sample magnetometer model 155. The measurements were made in the temperature range 20K to 300K. For temperature down to 20K a close cycle Helium refrigerator crystostat was used; temperature was controlled using Lake Shore temperature model 330 with an accuracy of ± 2.5K. The magnetic moments (M) versus temperature (T) were measured in the presence of external magnetic field of 3 kOe. All of the measurements have been made in field cooled (FC) mode. In this mode the sample is cooled from 300K to 20K, in presence of magnetic field and M versus T is recorded during warming cycle. The sample weight inside the Perspex cup was kept 200 mg for all three measurements.

⁵⁷Fe Mössbauer spectroscopy

Mössbauer spectra of all the specimens at 80K are shown in Fig.1. The experimental data points have been shown by dots whereas the solid line is the computed envelope of the single quadrupole split doublet. The percentage absorption ranges between ~ 5 % and ~ 13 %, the largest being for FeSb₂. Table 1 gives the values of Mössbauer parameters viz., isomer shift (IS), quadrupole splitting (QS), line width (full width at half maximum, LW) and χ², the goodness–of–fit–parameter. The line width of the specimens range between 0.366 mm/s to 0.425 mm/s (against 0.28 mm/s for natural iron) and also larger goodness–of–fit parameter, χ² ranging from 2.595 to 8.433. The large line width for the series specimens is suggestive of the magnetic broadening for these Mn substituted specimens at 80K, which is also supported by the magnetic measurements done for these specimens. 

The large line width for FeSb₂ at 80K is probably due to its competing tendency of crossover from diamagnetic to the paramagnetic character although we have observed the diamagnetic to paramagnetic total state crossover at T ~ 107K. This may also be understood in terms of highly correlated electron system FeSb₂ for which the top of the valence and bottom of the conduction bands are derived from the d-like atomic orbitals resulting into the existence of strong coulomb and exchange interactions between holes in the valence band and electrons in the conduction band and such effects become significant at a temperature where thermal energy kT of the electrons exceed the band gap. One can expect at such a temperature the Mössbauer line broadening due to the d electronic population fluctuation. One may expect the magnetic hyperfine field (B_hf) to be small from the spectral shape of Fe_0.9Mn_0.1Sb₂ but the QS value is large. The minimum resolvable magnetic hyperfine field at Fe site should be around 0.28 mm/s which is equivalent to the ~ 10 kOe with this instrument. For Fe_0.8Mn_0.2Sb₂ spectra the best computer fitting corresponding to one sextet revealed a very small hyperfine field at iron site (~ 40 kOe) but there is no significant change in line width. Due to larger quadrupole interaction energy than the magnetic dipole interaction energy, this programme can not fit unambiguously these spectra. So, the spectra should be fitted with the eight line patterns resulting from the diagonalization of the full Hamiltonian for the combined magnetic dipole and electric quadrupole interactions. 

The increase in IS values at 80K should be correlated with the additional functional dependence of isomer shift with temperature due to lattice contraction [15] at lower temperatures. The unit cell volume of FeSb₂ at 80K (120.249 Å at 100K) is smaller than the volume at 300K (121.698 Å) [7]. On the other hand one may assume that at low temperatures the average number of d-electrons in valence band are more which could increase the shielding of s-electrons and reduce the s- electron density at the nucleus. This reduced s-electron density at the nucleus could increase the IS value at 80K for FeSb₂. 

There is an increase in quadrupole splitting at 80K compared to QS values at 300K for these specimens. In general, the quadrupole splitting is a function of temperature; both the valence electron contribution and the lattice contribution to the quadrupole splitting are temperature dependent [15]. The valence electron contribution to the quadrupole splitting is significant for FeSb₂ because it is a narrow gap semi conductor so that one may expect the number of average valence electrons at 80K to be much higher than that of 300K. For many compounds the lattice contribution to the quadrupole splitting is assumed to be constant with temperature provided that the lattice changes isotropically with temperature. However, if there is an anisotropic lattice expansion or contraction as is the case for FeSb₂, a temperature dependent lattice contribution must be included [15]. At 80K the lattice contribution to the quadrupole splitting is increased because the lattice distortion is increased as revealed by Petrovic et al. [7]. For FeSb₂ the c/a and c/b axial ratios they observed the decrease with temperature. At 100K the c/a and c/b values for FeSb₂ are 0.5444 and 0.4867 respectively while at 300K these are 0.5485 and 0.4892 respectively. This decreased c/a and c/b ratio with the decrease in temperature increases the distortion of the iron octahedra. This can lead to increase the lattice contribution to QS.

Thus, both valence electron contribution and lattice contribution to the quadrupole splitting increase leading to increase the net QS values at 80K for FeSb₂. One may extend these arguments for the similar variation of Mössbauer IS and QS parameters as temperature is lowered to 80K for y = 0.1 and y = 0.2 compositions as well. However, one should notice that IS and QS values at 80 K as one goes on substituting Mn for Fe in this series specimens increase and decrease respectively. But more definitive interpretation has to await the detailed full Hamiltonian analysis which will do later in near future. 

Table 1-   ⁵⁷Fe Mössbauer parameters at 80K for Fe_1-yMn_ySb₂ system for  0 ≤ y ≤ 0.2,Isomer shift δ is given with respect to α – Fe. 

Fe1-yMnySb2	I.S. (δ) (mm/sec)	Q.S. ( Δ) (mm/sec)	L.W. (Г) (mm/sec)	χ 2
y = 0.0	0.539 (± 0.0003)	1.454 (± 0.0005)	0.366 (± 0.0008)	8.433
y = 0.1	0.549 (± 0.0005)	1.424 (± 0.0001)	0.413 (± 0.0002)	5.981
y = 0.2	0.548 (± 0.0008)	1.338 (± 0.002)	0.425 (± 0.002)	2.812

Fig. 1- ⁵⁷Fe Mössbauer spectra observed at 80K for Fe_1-yMn_ySb₂ system for 0 ≤ y ≤ 0.2.

Magnetization measurements

The magnetic measurements (M versus T) have been recorded from 20K to 300K for all three samples of the series Fe_1-yMn_ySb₂. Fig. 2 depicts the variation of magnetic moment (M) as a function of temperature (T). Table 2 gives the diamagnetic to paramagnetic crossover temperature T_cr and the maximum magnetic moment observe at a particular temperature mentioned in parenthesis. An increase (~ 20 times) in the magnetic moment for Fe_0.8Mn_0.2Sb₂ has been observed at all temperatures as compared to FeSb₂. The above described behaviour of the Fe_1-yMn_ySb₂ for 0 ≤ y ≤ 0.2 may be understood in terms of strongly correlated electron systems as FeSb₂ and also for Fe_0.9Mn_0.1Sb₂ & Fe_0.8Mn_0.2Sb₂, the top of the valence and bottom of the conduction bands are derived from the d- like atomic orbitals which results in the existence of strong coloumb and exchange interactions between holes in the valence band and electrons in the conduction band and such effects become significant when temperature is varied in such a way that it exceeds the band gap in each of series composition.

Table 2   T_Cr (lower) and T_Cr (higher) values for Fe_1-yMn_ySb₂ system for  0 ≤ y ≤ 0.2.

Y	TCr (lower)	TCr (higher)	Maximum moment (emu/g) (temperature)
0.0	24K	107.53K	64 X 10-3 (166K)
0.1	24K	83K	74 X 10 -3 (186K)
0.2	not observed down to 20K	not observed down to 20K	1344 X 10-3 (124K)

Fig.2- Magnetization as a function of temperature, in the presence of an External field of 3 kOe, for the composition y = 0, 0.1 and 0.2 respectively, of the series Fe_1-yMn_ySb₂.

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