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Resistivity and thermopower of the antiferromagnetic system MnTexSe1−x
Solid State Communications, Vol. 104, No. 9, pp. 565-567, 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 003s1098/97 $17.00+.00
PII: s0038-1098(97)00341-4
RESISTIVITY AND THERMOPOWER OF THE ANTIFE~OMAG~~C
SYSTEM MnTe,Se I-x
R.B. Prabhu,” V.N. Kamat DalaLb J.B.C. Efrem Desaa and S.N. Bhatia’ “C)epartment of Physics, (~ep~me~t of Chemist), Goa University, Taleigao Plateau, Goa 403 206, India ‘Department of Physics, Indian Institute of Technology, Powai, Mumbai 400 076, India (Received 9 May 1997; accepted 30 June 1997 by C.N.R. Rao) Measurement of resistivity and thermopower on the antiferromagnetic system MnTe,Se I_n are reported for various values of x. Magnon drag contribution to the thermopower was also detected and measured in all the cases. Resistivity shows semiconductor behaviour with anomalous rise at low temperatures for non-zero values of x. Possible mechanisms responsible for the resistivity anomalies are pointed out. 0 1997 Elsevier Science Ltd
1. INTRODUCTION The antiferromagnetic systems belonging to a class called “Nickel Arsenide Type Compounds” are attracting a lot of interest because of the large variety of physical properties with new and exotic features exhibited by them [l]. These features include anomalies in susceptibility and thermopower in the vicinity of the Neel temperature which bring out some new features like spin quotations and magnon drag effects generally not observed in other ~tife~omagne~c systems. A compound belonging to this category which has been studied earlier in detail is MnTe. Another related compound is MnSe which has NaCl type structure and is antiferromagnetic. When Se is added to the parent compound MnTe in increasing proportion to form a solid solution a transition from NiAs to NaCl type structure takes place and the crystalline ionicity increases from 0.3g to. 0.67 [2]. An important observation was made by Zanmarchi and Hass [3] on the thermopower of MnTe for which the Neel tem~rat~e is 300 K. They found an anom~ous rise in ~e~opower starting at 200 K. This was ascribed to magnon drag effect. Neutron scattering experiments revealed the existence of magnon modes in the vicinity of Neel temperature. In this paper we present our measurements of resistivity and thermopower on the antiferromagnetic system MnTe,Se I._~for various values of x. We have detected a magnon drag contribution to thermopower in all concentration x. The resistivity shows a semiconductor like behavior in all the cases, however there is seen an anomalous rise at low temperatures for all values of x < 1. This is obviously
due to scattering by the magnetic Se atoms dissolved in the host MnTe. Similar results have been observed in the metallic systems like chromium alloys doped with manganese and vanadium. We believe that this is the first instance of observing the phenomenon in a magnetic semiconductor. In Section 2 we present the experimental methods used. Final results are presented in the two tables. Section 3 is devoted to a brief discussion of the results and suggestions for further work. 2. ~PER~E~~
METHODS
Samples of MnTe,Se I-Xwere prepared by mixing the powdered components sealed in evacuated silica tubes (- 10m6mm of Hg) and heated for three days at 650” to 750°C. The compounds were prepared from spectroscopically pure (>99.9%) Mn, Te and Se. They were then quenched to room temperature, reground to -200 mesh size and the above procedure repeated once more to ensure complete homogeneity. The lattice parameters were dete~ined by X-ray powder diffraction. The values are in agreement with the results of American Standard for Testing Materials Cards. The magnetic susceptibility was measured by the Gouy method using Hg(Co(CNS)h) as the reference standard. The sample was suspended from a silver wire which was connected to a balance that could read up to fifth decimal place. The stability of temperature was better than 0.5 K. Magnetic fields in the range of 1000 to 3000 Oe were employed. Since practically no field dependence was observed only the results obtained for 2200 Oe were used for analysis. The results for the crystal symmetry,
S66
THERMOPOWER OF THE ANTIFERROMAGNETIC SYSTEM MnTe,Se I_~ Vol. 104, No. 9
Table 1. Crystal symmetry, c/a ratios, types of magnetism and Neel temperature Compounds
Crystal symmetry
da ratio
Type of magnetism
Neel temperature
MnTe ~nTeo.9Seo.l MnTeo.sSea.~ MnTe0.65Se0.35 MnTea.sSeo.5 MnTea.&ea.75
NiAs NiAs NiAs NiAs NaCl NaCl
1.618 1.612 1.62’7 1.636
AF AF AF AF AF AF
318 312 280 266 250 216
AF - Antiferromagnetic. c/a ratio and Neel temperature for all samples are presented in Table 1. The resistivity and thermopower were measured using the standard four probe method and the differential method respectively. The measurements were taken from room tem~ratu~ to 30 K employing the Displex closed cycle refrigerator, Keithley models 181 digital nanovoltmeter and 224 constant current source. Readings were taken both while cooling and warming of the sample. The results of the resistivity measurements were given in Fig. l(a) to (e). The results of thermopower measurements are presented in Fig. 2(a) to (d) and the magnon drag contribution is summarized in Table 2.
60
‘ 3 -4
200
100
I
O-
: t$ \: :. y.
.?.*. :-;. ,. . . : . , - i:_*c-
,%&+’
.-.-
-.&_.-
while warming while cooling
24 D.
3. RESULTS C Results of resistivity me~~ern~nts on MnTe,Ser, are shown in Fig. 1 for x = 1,0.9,0.8, OS and 0.25. For pure MnTe (X = 1) the resistivity shows the characteristics of a semiconductor. The band gap calculated from the curve is found to be 46.62 meV. All other Bsamples show an overall semiconductor like behaviour however there are anomalies in the temperature range 0. ,.-54-8’7 K. The resistivity increases rapidly at a lower * 100 200 300 Itemperature reaches maximum at a higher temperature i and then falls again maintaining the original semicon‘Zt; ; : while warrnlng ‘%-._.:s- :R ductor behaviour. Since this is absent in pure MnTe, it 200 ,while cooling :. can be concluded that the strong scattering is due to the -a dissolved magnetic Se atoms. Similar results have been obtained earlier by Galkin 147and Fawcett and Galkin 151 when ~t~e~omagnetic chromium is doped with vanadium and manganese impurities. The authors have concluded that the anomalous rise in resistivity is due to * 6000 impurity magnetic resonance scattering as the Fermi while warming level is tuned by adding the impurity. In the present .. case we cannot at once jump to such a conclusion. 5%. \ There are two possible mechanisms that lead to strong -.. e. --A_____ scattering by Se atoms. One is Kondo scattering by the 100 200 300 magnetic Se atoms which can form a Kondo lattice. If the Temperature (k) intersite exchange interaction between Se atoms is small the single site scattering can lead to the Kondo scattering Fig. 1. Tem~rature de~ndence of resistivity of as is observed in many rare earth systems. Another MnTeSe 1-xforx = (a) 1, (b) 0.9, (c) 0.8, (d) 0.5, (e) 0.25. 4
;
.-.
5
. _...
..
‘.
Vol. 104, No. 9 THERMOPOWER OF THE ANTIFERROMAGNETJC SYSTEM MnTe,SeI, 40a
Table 2. Magnon drag contribution to tbermopower
300
Compounds
Magnon drag contribution AS (/LVK-‘)
MnTeo.9Seo.l MnTeo.65Seo.35 MnTea.sSe0.s MnTeo.&%7s
86 120 152 105
200
100
0
300
200
#@“” ..- *.=@-
22” ; 100 a
b.
0
100
200
3.90 . ..-
lw-•
1 J!F
567
I 200
/*** ./---
d. lb0
.
Temperature
260
300
(k)
Fig. 2. Tem~rature dependence of ~ermopower of MnTe,Se,, for x = (a) 0.9, (b) 0.65, (c) 0.5, (d) 0.25. possible mechanism is the magnetic resonance scattering by the Se atoms which form impurity levels in the gap. The anomalous rise in resistivity is then due to magnetic resonance scattering as the Fermi level is tuned by adding tbe impurity. A sure way to decide between these two mechanisms is to measure the resistivity in magnetic field. A strong magnetic field suppresses the spin llip scattering in Kondo systems which results in negative magnetorcsistivity at tem~ratures below TM, where TM is the ~rn~rat~e at which resistivity is ~nimum. Thus ‘if the magnetoresistivity remains positive even at very low temperatures Kondo scattering can be ruled out. Such experiments are now planned for fields upto 15 T
and will be reported elsewhere along with the results of magnetothermopower. Thermopower results on MnTe,Sel_X am presented for x = 0.9, 0.65, 0.5, 0.25 in Fig. 2, Magnon drag effects are quite evident. Results for MnTe which are not presented here agree with the earlier work of Zanmarchi and Hass [3]. In that S starts rising at 200 K when the Neel tem~ra~re is 318 K. In all the other cases the situation is similar. In the last case (X = 0.25) the rise is rather slow in the vicinity of TN = 216 K. The magnon drag con~butions is seen to increase when Se concentration increases but decreases at x = 0.25. A possible explanation is that the spin wave spectrum alters with the addition of Se in a way not determined so far. It may be mentioned here that the magnon drag is caused because the momentum transferred from the electrons to the spins is not lost immediately but remains for a finite time ts in the spin system and during this time has a chance to return to the electrons. A complete analysis of the magnon drag con~ibution involves three relaxation times tr, f2 and t3 which can be derived via the optical conductivity. This requires it&a-red absorption studies. Work in this direction is in progress and will reported elsewhere. Acknowledgements--The
authors wish to thank the referee for his valuable and constructive comments. Financial assistance by the Council for Scientific and Industrial Research is gratefully acknowledged. RE~RENCES 1. Motizuki, K., J. Magn. Magn, Muter., 70, 1987, 1. 2. Kamat Dalal, V.N. and Prabhu, R.B., ~~ysica, B112,1982,42. G. and Hass, C., Philips Research 3. Zmarchi, Reports, 23, 1968, 389.
4. Galkin, V., J. Magn. Magn. Mater., 79, 1989, 327. 5. Fawcett, E. and Galkin, V., Preprint, 1991.
PII: s0038-1098(97)00341-4
RESISTIVITY AND THERMOPOWER OF THE ANTIFE~OMAG~~C
SYSTEM MnTe,Se I-x
R.B. Prabhu,” V.N. Kamat DalaLb J.B.C. Efrem Desaa and S.N. Bhatia’ “C)epartment of Physics, (~ep~me~t of Chemist), Goa University, Taleigao Plateau, Goa 403 206, India ‘Department of Physics, Indian Institute of Technology, Powai, Mumbai 400 076, India (Received 9 May 1997; accepted 30 June 1997 by C.N.R. Rao) Measurement of resistivity and thermopower on the antiferromagnetic system MnTe,Se I_n are reported for various values of x. Magnon drag contribution to the thermopower was also detected and measured in all the cases. Resistivity shows semiconductor behaviour with anomalous rise at low temperatures for non-zero values of x. Possible mechanisms responsible for the resistivity anomalies are pointed out. 0 1997 Elsevier Science Ltd
1. INTRODUCTION The antiferromagnetic systems belonging to a class called “Nickel Arsenide Type Compounds” are attracting a lot of interest because of the large variety of physical properties with new and exotic features exhibited by them [l]. These features include anomalies in susceptibility and thermopower in the vicinity of the Neel temperature which bring out some new features like spin quotations and magnon drag effects generally not observed in other ~tife~omagne~c systems. A compound belonging to this category which has been studied earlier in detail is MnTe. Another related compound is MnSe which has NaCl type structure and is antiferromagnetic. When Se is added to the parent compound MnTe in increasing proportion to form a solid solution a transition from NiAs to NaCl type structure takes place and the crystalline ionicity increases from 0.3g to. 0.67 [2]. An important observation was made by Zanmarchi and Hass [3] on the thermopower of MnTe for which the Neel tem~rat~e is 300 K. They found an anom~ous rise in ~e~opower starting at 200 K. This was ascribed to magnon drag effect. Neutron scattering experiments revealed the existence of magnon modes in the vicinity of Neel temperature. In this paper we present our measurements of resistivity and thermopower on the antiferromagnetic system MnTe,Se I._~for various values of x. We have detected a magnon drag contribution to thermopower in all concentration x. The resistivity shows a semiconductor like behavior in all the cases, however there is seen an anomalous rise at low temperatures for all values of x < 1. This is obviously
due to scattering by the magnetic Se atoms dissolved in the host MnTe. Similar results have been observed in the metallic systems like chromium alloys doped with manganese and vanadium. We believe that this is the first instance of observing the phenomenon in a magnetic semiconductor. In Section 2 we present the experimental methods used. Final results are presented in the two tables. Section 3 is devoted to a brief discussion of the results and suggestions for further work. 2. ~PER~E~~
METHODS
Samples of MnTe,Se I-Xwere prepared by mixing the powdered components sealed in evacuated silica tubes (- 10m6mm of Hg) and heated for three days at 650” to 750°C. The compounds were prepared from spectroscopically pure (>99.9%) Mn, Te and Se. They were then quenched to room temperature, reground to -200 mesh size and the above procedure repeated once more to ensure complete homogeneity. The lattice parameters were dete~ined by X-ray powder diffraction. The values are in agreement with the results of American Standard for Testing Materials Cards. The magnetic susceptibility was measured by the Gouy method using Hg(Co(CNS)h) as the reference standard. The sample was suspended from a silver wire which was connected to a balance that could read up to fifth decimal place. The stability of temperature was better than 0.5 K. Magnetic fields in the range of 1000 to 3000 Oe were employed. Since practically no field dependence was observed only the results obtained for 2200 Oe were used for analysis. The results for the crystal symmetry,
S66
THERMOPOWER OF THE ANTIFERROMAGNETIC SYSTEM MnTe,Se I_~ Vol. 104, No. 9
Table 1. Crystal symmetry, c/a ratios, types of magnetism and Neel temperature Compounds
Crystal symmetry
da ratio
Type of magnetism
Neel temperature
MnTe ~nTeo.9Seo.l MnTeo.sSea.~ MnTe0.65Se0.35 MnTea.sSeo.5 MnTea.&ea.75
NiAs NiAs NiAs NiAs NaCl NaCl
1.618 1.612 1.62’7 1.636
AF AF AF AF AF AF
318 312 280 266 250 216
AF - Antiferromagnetic. c/a ratio and Neel temperature for all samples are presented in Table 1. The resistivity and thermopower were measured using the standard four probe method and the differential method respectively. The measurements were taken from room tem~ratu~ to 30 K employing the Displex closed cycle refrigerator, Keithley models 181 digital nanovoltmeter and 224 constant current source. Readings were taken both while cooling and warming of the sample. The results of the resistivity measurements were given in Fig. l(a) to (e). The results of thermopower measurements are presented in Fig. 2(a) to (d) and the magnon drag contribution is summarized in Table 2.
60
‘ 3 -4
200
100
I
O-
: t$ \: :. y.
.?.*. :-;. ,. . . : . , - i:_*c-
,%&+’
.-.-
-.&_.-
while warming while cooling
24 D.
3. RESULTS C Results of resistivity me~~ern~nts on MnTe,Ser, are shown in Fig. 1 for x = 1,0.9,0.8, OS and 0.25. For pure MnTe (X = 1) the resistivity shows the characteristics of a semiconductor. The band gap calculated from the curve is found to be 46.62 meV. All other Bsamples show an overall semiconductor like behaviour however there are anomalies in the temperature range 0. ,.-54-8’7 K. The resistivity increases rapidly at a lower * 100 200 300 Itemperature reaches maximum at a higher temperature i and then falls again maintaining the original semicon‘Zt; ; : while warrnlng ‘%-._.:s- :R ductor behaviour. Since this is absent in pure MnTe, it 200 ,while cooling :. can be concluded that the strong scattering is due to the -a dissolved magnetic Se atoms. Similar results have been obtained earlier by Galkin 147and Fawcett and Galkin 151 when ~t~e~omagnetic chromium is doped with vanadium and manganese impurities. The authors have concluded that the anomalous rise in resistivity is due to * 6000 impurity magnetic resonance scattering as the Fermi while warming level is tuned by adding the impurity. In the present .. case we cannot at once jump to such a conclusion. 5%. \ There are two possible mechanisms that lead to strong -.. e. --A_____ scattering by Se atoms. One is Kondo scattering by the 100 200 300 magnetic Se atoms which can form a Kondo lattice. If the Temperature (k) intersite exchange interaction between Se atoms is small the single site scattering can lead to the Kondo scattering Fig. 1. Tem~rature de~ndence of resistivity of as is observed in many rare earth systems. Another MnTeSe 1-xforx = (a) 1, (b) 0.9, (c) 0.8, (d) 0.5, (e) 0.25. 4
;
.-.
5
. _...
..
‘.
Vol. 104, No. 9 THERMOPOWER OF THE ANTIFERROMAGNETJC SYSTEM MnTe,SeI, 40a
Table 2. Magnon drag contribution to tbermopower
300
Compounds
Magnon drag contribution AS (/LVK-‘)
MnTeo.9Seo.l MnTeo.65Seo.35 MnTea.sSe0.s MnTeo.&%7s
86 120 152 105
200
100
0
300
200
#@“” ..- *.=@-
22” ; 100 a
b.
0
100
200
3.90 . ..-
lw-•
1 J!F
567
I 200
/*** ./---
d. lb0
.
Temperature
260
300
(k)
Fig. 2. Tem~rature dependence of ~ermopower of MnTe,Se,, for x = (a) 0.9, (b) 0.65, (c) 0.5, (d) 0.25. possible mechanism is the magnetic resonance scattering by the Se atoms which form impurity levels in the gap. The anomalous rise in resistivity is then due to magnetic resonance scattering as the Fermi level is tuned by adding tbe impurity. A sure way to decide between these two mechanisms is to measure the resistivity in magnetic field. A strong magnetic field suppresses the spin llip scattering in Kondo systems which results in negative magnetorcsistivity at tem~ratures below TM, where TM is the ~rn~rat~e at which resistivity is ~nimum. Thus ‘if the magnetoresistivity remains positive even at very low temperatures Kondo scattering can be ruled out. Such experiments are now planned for fields upto 15 T
and will be reported elsewhere along with the results of magnetothermopower. Thermopower results on MnTe,Sel_X am presented for x = 0.9, 0.65, 0.5, 0.25 in Fig. 2, Magnon drag effects are quite evident. Results for MnTe which are not presented here agree with the earlier work of Zanmarchi and Hass [3]. In that S starts rising at 200 K when the Neel tem~ra~re is 318 K. In all the other cases the situation is similar. In the last case (X = 0.25) the rise is rather slow in the vicinity of TN = 216 K. The magnon drag con~butions is seen to increase when Se concentration increases but decreases at x = 0.25. A possible explanation is that the spin wave spectrum alters with the addition of Se in a way not determined so far. It may be mentioned here that the magnon drag is caused because the momentum transferred from the electrons to the spins is not lost immediately but remains for a finite time ts in the spin system and during this time has a chance to return to the electrons. A complete analysis of the magnon drag con~ibution involves three relaxation times tr, f2 and t3 which can be derived via the optical conductivity. This requires it&a-red absorption studies. Work in this direction is in progress and will reported elsewhere. Acknowledgements--The
authors wish to thank the referee for his valuable and constructive comments. Financial assistance by the Council for Scientific and Industrial Research is gratefully acknowledged. RE~RENCES 1. Motizuki, K., J. Magn. Magn, Muter., 70, 1987, 1. 2. Kamat Dalal, V.N. and Prabhu, R.B., ~~ysica, B112,1982,42. G. and Hass, C., Philips Research 3. Zmarchi, Reports, 23, 1968, 389.
4. Galkin, V., J. Magn. Magn. Mater., 79, 1989, 327. 5. Fawcett, E. and Galkin, V., Preprint, 1991.