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Antiferromagnetic behaviour of germanium monoselenide connected with the negative magnetoresistance effect
Solid State Communications, Vol. 30, pp. 665-669. Pergamon Preas Ltd. 1979. PrInted In Great Britain. ANTIFERROMAGNETIC BEHAVIOUR OF GERMANIUM MONOSELENIDE CONNECTED WITH THE NEGATWE MAGNETORESISTANCE EFFECT D.S. Kyriakos, 3. Tsoukalas and N.A. Economou Department of Physics, University of Thessaloniki, Greece (Received 19 February 1979 byM. Balkanski) The transport properties of GeSe indicate an antiferromagnetic character up to the temperature of 376K. Above that temperature a metallic behaviour is materialized. Magnetic measurements confirm these findings indicating a Néel temperature at 381 K above which GeSe becomes paramagnetic. The magnetic moment is to be attributed to spin only angular momentum processes. THE PHENOMENON of negative magnetoresistance in germanium monoselenide has been reported recently [1J.An increase in the electrical conductivity in the presence of a magnetic field has been observed at ternperatures a little above the room temperature, increasing with temperature up to a certain point (376 K) at which a sharp drop is observed and the change in resistance becomes positive. The temperature at which the change from the negative to the positive behaviour occurs almost coincides with the temperature at which a reversal in the decrease of the “zero magnetic field electrical resistivity” is observed. It is also the temperature above which the number of carriers, deduced from the results of the Hall measurements, starts being lade-
pendent of temperature. These results led to the conclusion that a semiconductor to metal transition occurs in GeSe. From optical measurements it was found that GeSe has an Indirect gap with differentroom temperature values at the two principal directions in the cleavage plane, which is the (001) plane [2]. The values along the a and b axis are 1.22 and 1.18eV respectively. It was also found that these values for the indirect gaps decrease with increasing temperature linearly, with a different slope. From extrapolation of the low ternperature results it is expected that the gaps should become equal (but would not vanish) in the vicinity of the temperature at which the electrical anomaly appears.
. 60
~
~
La. 20
0
I
300
320
340
360
300
Fig. 1. The temperature dependence of(— ~o/p)”2for the two principle directions on the (001) plane. 665
666
ANTIFERROMAGNETIC BEHAVIOUR OF GERMANIUM MONOSELENIDE Vol. 30, No.10
1.8 1.6 E
1.4
1.2W 1.0 0.8
-
0.6 0.4
0.2 0.0
. I
0
2
4
6
8
10
12 )I.
I
14 81KG]
I
16
18
Fig. 2. The dependence of the magnetic moment of a sample of mass equal to 5.6 x 10-2 g upon the magnetic induction. As it is mentioned also in [2] an exothermic anomaly the temperature [5] by the following scheme (2) has been observed to occur at this temperature in DTA {s(T)}~”2 (T + T~) investigations. Astructural investigation performed with X-ray Debye measurements or transmission electron where T~ii a parameter with dimensions of temperature. microscopy did not reveal any detectable change in This effect should disappear at high defect concentrathe structure [31. tions [6]. The above facts point out that the negative magIn Fig. 1 we present experimental results obtained netoresistance observed in GeSe is associated with in this work plotting the quantity i~p/p)”2vs localized states rather than extended ones. Toyozawa temperature. The linear behaviour observed is in suggested [4] that the existence of localised spins may accordance with equation (2), while the value of T~ be responsible for the increase of the conductivity in which results from the intercept of the extrapolated the presence of the magnetic field. Thus the negative straight line with the temperature axis is equal to magnetoresistance effect may be described by the 365 K, while the proportionality factor is negative. following equation This result indicates that indeed the negative magneto(—
— —~
=
—
[p(H) p(O)]/p(0) —
=
S(T)H2
(I)
p where p(0), p(H) are the resistivities in the absence and in the presence of the magnetic field correspondingly and 5(T) is a magnetoresistance coefficient related to
resistance effect in GeSe seems to be connected directly with an antiferromagnetic alignement of the
spins up to its Ned temperature above which a paramagnetic behaviour is expected connected with the metallic state. To test the above conclusions we measured the
Vol. 30, No.10
ANTIFERROMAGNETIC BEHAVIOUR OF GERMANIUM MONOSELENIDE
667
3.8
u
8—18KG 3 d —5.54 DIiOOl’cr~
x
3.0
°
OIIOP
2~
2.2
1.8
1.4
ii!
10
I
40
I
I
10
100
I
130
t 1°C] Fig. 3. The temperature dependence of the magnetic susceptibility for two sample orientations. —~‘
magnetic moment of GeSe at room temperature. In Fig. 2 a plot of the magnetic moment vs the magnetic induction is presented for a sample with a mass equal to 5.26 x 10-2g. The dependence of the magnetic
defmed. Now O/TN = 0.874. This value is to be cornpared with unity which is to be expected from the
moment on the field indicates a typical antiferromagnetic behaviour [7]. In another experiment the dependence of the magnetic susceptibility on tern-
The above results Indicate that the susceptibility may be expressed by the relation
perature was investigated, in the temperature range from room temperature up to 140°C.This dependence is presented Fig. 3 whereforthe magneticfield susceptibility is plotted vs In temperature, a constant ofB = 18Kg and for two orientations of the sample,B 1(001) and B 11(001). The measurements were performed using a VSM. The results presented in this last Fig. 3 indicate a typical antiferromagnetic behaviour which becomes paramagnetic above 381 K.
molecular field theory when the molecular field constant N 1, is equal to zero [81.
I
T—O
(3) C 3K’. Now the with the constant C equal to 1.026 x 10 constant C is expressed by the relation [8]
—
=
X
N C
=
2~~S(S + 1) 0g 3k
(4)
where N 3 and g, I-~B,50and is kthehave number their of usual magnetic meaning. dipoles Usingper thecmvalues susceptibility vs temperature we obtain the diagram g = 2, the gyromagnetic ratio of the carriers with spin presented in Fig. 4. By extrapolating the paramagnetic only angular momentum and S = 1/2, the number N is 3, so that the number0of branch, using a least fit, fromequal the Intercept with found equal to 1.65 1021 iscm the temperature axis square a temperature to 0 = 333 K is magnetic dipoles per xmole Replotting the data using the reciprocal molar
668
ANTIFERROMAGNETIC BEHAVIOUR OF GERMANIUM MONOSELENIDE Vol. 30, No.10 6 A
A
5. A
Co
~
A
4.
A A
=‘C
I— —Pc
43
A A
2
1•
0 290
I
I
320
350
I
300 410 ~ T[K] Fig. 4. Plot of the reciprocal molar susceptibility vs temperature. Extrapolating the linear paramagnetic branch the intercept with the temperature axis defines the temperature 333 K. 1 (5) 4.50 x 10~mor where Mm~is the molar mass and d the density, which is not an unreasonable number if we attribute them to vacancies. Using the value for N obtained from (5)111 the expression [7] N
=
N0~2i
itLeff =
=
I3MmoikXm(T ______________ 0) N —
(6)
with T 386K, 0 = 333K and y.,,, = 3.59 x 10~g~ cm3 we find p~equal to 1.755 Bohr magnetons. Now if we consider that =
gp~J[J(J + 1)]
(7)
using for p~the value found above and for g =2, we obtain.T = 0.5 whIch indicates that the assumption introduced previously, that there is no contribution of the orbital angular momentum to the magnetic moment,
is valid. Therefore we may to conclude the magnetic moment is due exclusively the spinthat magnetic moment. In conclusion we may say that the transport properties of GeSe indicate an antiferromagnetic character up to the temperature of 376 K. Above that temperature a “metallic” behaviour is materialized. Magnetic measurements confirm these findings indicating a Néel temperature 381 K above which magnetic. Theatmagnetic moment is CaSe to be becomes attributedparato spin only angular momentum processes. The high temperature behaviourshould be attributed to a screening of the localized levels rather than to a true metallic behaviour [9]. We especially thank Professor Sir N.F. Mott for his helpful suggestions which initIated this work. REFERENCES 1. D.S. Kyriakos, 0. Valasiades & N.A. Economou, Semiconductor Conference, EdInburgh (1978).
Acknowledgment
—
Vol. 30, No.10 2. 3. 4. 5.
ANTIFERROMAGNETIC BEHAVIOUROF GERMANIUM MONOSELENIDE
S.V. Viachos, A2. Laznbros, A. Thanailakis & N.A. Economou,Phys. Status SolIdI(b) 76,727 (1976). TX. Karakostas &N.A.Economou (to be published). Y. Toyozawa,J. Thys. Soc. Japan 17,986 (1962). Y. K&tayama& S. Tanaka, Phys. Rev. 153,873 (1967).
6. 7. 8. 9.
669
M. Balkanaki & A. Geismar, Solid State Commun. 4, 111 (1966). B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, US.A. (1972). A.H. Mornsh, The PhysicalPrinciples ofMagnetism, Wiley, New York (1965). N.F. Mott, (personal communication).
pendent of temperature. These results led to the conclusion that a semiconductor to metal transition occurs in GeSe. From optical measurements it was found that GeSe has an Indirect gap with differentroom temperature values at the two principal directions in the cleavage plane, which is the (001) plane [2]. The values along the a and b axis are 1.22 and 1.18eV respectively. It was also found that these values for the indirect gaps decrease with increasing temperature linearly, with a different slope. From extrapolation of the low ternperature results it is expected that the gaps should become equal (but would not vanish) in the vicinity of the temperature at which the electrical anomaly appears.
. 60
~
~
La. 20
0
I
300
320
340
360
300
Fig. 1. The temperature dependence of(— ~o/p)”2for the two principle directions on the (001) plane. 665
666
ANTIFERROMAGNETIC BEHAVIOUR OF GERMANIUM MONOSELENIDE Vol. 30, No.10
1.8 1.6 E
1.4
1.2W 1.0 0.8
-
0.6 0.4
0.2 0.0
. I
0
2
4
6
8
10
12 )I.
I
14 81KG]
I
16
18
Fig. 2. The dependence of the magnetic moment of a sample of mass equal to 5.6 x 10-2 g upon the magnetic induction. As it is mentioned also in [2] an exothermic anomaly the temperature [5] by the following scheme (2) has been observed to occur at this temperature in DTA {s(T)}~”2 (T + T~) investigations. Astructural investigation performed with X-ray Debye measurements or transmission electron where T~ii a parameter with dimensions of temperature. microscopy did not reveal any detectable change in This effect should disappear at high defect concentrathe structure [31. tions [6]. The above facts point out that the negative magIn Fig. 1 we present experimental results obtained netoresistance observed in GeSe is associated with in this work plotting the quantity i~p/p)”2vs localized states rather than extended ones. Toyozawa temperature. The linear behaviour observed is in suggested [4] that the existence of localised spins may accordance with equation (2), while the value of T~ be responsible for the increase of the conductivity in which results from the intercept of the extrapolated the presence of the magnetic field. Thus the negative straight line with the temperature axis is equal to magnetoresistance effect may be described by the 365 K, while the proportionality factor is negative. following equation This result indicates that indeed the negative magneto(—
— —~
=
—
[p(H) p(O)]/p(0) —
=
S(T)H2
(I)
p where p(0), p(H) are the resistivities in the absence and in the presence of the magnetic field correspondingly and 5(T) is a magnetoresistance coefficient related to
resistance effect in GeSe seems to be connected directly with an antiferromagnetic alignement of the
spins up to its Ned temperature above which a paramagnetic behaviour is expected connected with the metallic state. To test the above conclusions we measured the
Vol. 30, No.10
ANTIFERROMAGNETIC BEHAVIOUR OF GERMANIUM MONOSELENIDE
667
3.8
u
8—18KG 3 d —5.54 DIiOOl’cr~
x
3.0
°
OIIOP
2~
2.2
1.8
1.4
ii!
10
I
40
I
I
10
100
I
130
t 1°C] Fig. 3. The temperature dependence of the magnetic susceptibility for two sample orientations. —~‘
magnetic moment of GeSe at room temperature. In Fig. 2 a plot of the magnetic moment vs the magnetic induction is presented for a sample with a mass equal to 5.26 x 10-2g. The dependence of the magnetic
defmed. Now O/TN = 0.874. This value is to be cornpared with unity which is to be expected from the
moment on the field indicates a typical antiferromagnetic behaviour [7]. In another experiment the dependence of the magnetic susceptibility on tern-
The above results Indicate that the susceptibility may be expressed by the relation
perature was investigated, in the temperature range from room temperature up to 140°C.This dependence is presented Fig. 3 whereforthe magneticfield susceptibility is plotted vs In temperature, a constant ofB = 18Kg and for two orientations of the sample,B 1(001) and B 11(001). The measurements were performed using a VSM. The results presented in this last Fig. 3 indicate a typical antiferromagnetic behaviour which becomes paramagnetic above 381 K.
molecular field theory when the molecular field constant N 1, is equal to zero [81.
I
T—O
(3) C 3K’. Now the with the constant C equal to 1.026 x 10 constant C is expressed by the relation [8]
—
=
X
N C
=
2~~S(S + 1) 0g 3k
(4)
where N 3 and g, I-~B,50and is kthehave number their of usual magnetic meaning. dipoles Usingper thecmvalues susceptibility vs temperature we obtain the diagram g = 2, the gyromagnetic ratio of the carriers with spin presented in Fig. 4. By extrapolating the paramagnetic only angular momentum and S = 1/2, the number N is 3, so that the number0of branch, using a least fit, fromequal the Intercept with found equal to 1.65 1021 iscm the temperature axis square a temperature to 0 = 333 K is magnetic dipoles per xmole Replotting the data using the reciprocal molar
668
ANTIFERROMAGNETIC BEHAVIOUR OF GERMANIUM MONOSELENIDE Vol. 30, No.10 6 A
A
5. A
Co
~
A
4.
A A
=‘C
I— —Pc
43
A A
2
1•
0 290
I
I
320
350
I
300 410 ~ T[K] Fig. 4. Plot of the reciprocal molar susceptibility vs temperature. Extrapolating the linear paramagnetic branch the intercept with the temperature axis defines the temperature 333 K. 1 (5) 4.50 x 10~mor where Mm~is the molar mass and d the density, which is not an unreasonable number if we attribute them to vacancies. Using the value for N obtained from (5)111 the expression [7] N
=
N0~2i
itLeff =
=
I3MmoikXm(T ______________ 0) N —
(6)
with T 386K, 0 = 333K and y.,,, = 3.59 x 10~g~ cm3 we find p~equal to 1.755 Bohr magnetons. Now if we consider that =
gp~J[J(J + 1)]
(7)
using for p~the value found above and for g =2, we obtain.T = 0.5 whIch indicates that the assumption introduced previously, that there is no contribution of the orbital angular momentum to the magnetic moment,
is valid. Therefore we may to conclude the magnetic moment is due exclusively the spinthat magnetic moment. In conclusion we may say that the transport properties of GeSe indicate an antiferromagnetic character up to the temperature of 376 K. Above that temperature a “metallic” behaviour is materialized. Magnetic measurements confirm these findings indicating a Néel temperature 381 K above which magnetic. Theatmagnetic moment is CaSe to be becomes attributedparato spin only angular momentum processes. The high temperature behaviourshould be attributed to a screening of the localized levels rather than to a true metallic behaviour [9]. We especially thank Professor Sir N.F. Mott for his helpful suggestions which initIated this work. REFERENCES 1. D.S. Kyriakos, 0. Valasiades & N.A. Economou, Semiconductor Conference, EdInburgh (1978).
Acknowledgment
—
Vol. 30, No.10 2. 3. 4. 5.
ANTIFERROMAGNETIC BEHAVIOUROF GERMANIUM MONOSELENIDE
S.V. Viachos, A2. Laznbros, A. Thanailakis & N.A. Economou,Phys. Status SolIdI(b) 76,727 (1976). TX. Karakostas &N.A.Economou (to be published). Y. Toyozawa,J. Thys. Soc. Japan 17,986 (1962). Y. K&tayama& S. Tanaka, Phys. Rev. 153,873 (1967).
6. 7. 8. 9.
669
M. Balkanaki & A. Geismar, Solid State Commun. 4, 111 (1966). B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, US.A. (1972). A.H. Mornsh, The PhysicalPrinciples ofMagnetism, Wiley, New York (1965). N.F. Mott, (personal communication).