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Magnetic properties of ferromagnetic GdN
PHYSICA ELSEVIER
Physica B 199&200 11994)631-633
Magnetic properties of ferromagnetic GdN D.X. Li*, Y. Haga, H. Shida, T. Suzuki Department of Physics, Faculty of Science. Tohoku University. Sendal 980, Japan
Abstract A polycrystalline GdN sample was prepared by the direct reaction of gadolinium metal and nitrogen gas at 1600°C and at a pressure of 1300 atm. We measured magnetization and the magnetic field dependence of the susceptibility and specific heat of GdN. Susceptibility and magnetization show ferromagnetic behavior for GdN with the saturation value reached at about 20 kOe. Specific heat measurements revealed a ferromagnetic transition in GdN at Tc = 58 K with an additional broad peak around 20 K. Experimental results were explained with an exchange interaction mechanism.
!. Introduction The magnetic structure of GdN has given rise to much controversy. Many authors [1-4] have reported for GdN a ferromagnetic transition at 65-75 K and a paramagnetic Curie temperature of 70-90 K. Based on magnetization and initial susceptibility measurements, Waehter et al. [5, 6] claimed GdN at low fields to be antiferromagnetic with a N6el temperature TN = 40 K. Band structure calculations [7] suggest a semiconducting character for GdN. However, experiments [6] indicate GdN to be semimetallic. Until now the ordering temperature of GdN was determined only from susceptibility measurements. There is no information about the magnetic transition of G d N from specific heat measurement. To determine the magnetic structure of G d N a further study of various physical properties is necessary. It is difficult to obtain stoichiometric GdN. We applied a high pressure method to prepare G d N for a first time by using a hot isostatic pressing furnace. Flakes of Gd metal of 99.9% purity in an open tungsten crucible were directly reacted with nitrogen at 1600°C and a pressure of *Corresponding attthor.
1300arm for 3 h. Then the sample was hydrostatically pressed into a cylindrical shape at 720°C and 1300 atm using a glass capsule method. A polycrystalline GdN sample with a high nearly theoretical, density, was obtained. X-ray diffraction showed a single phase of NaC1type GdN with a room temperature lattice constant a = 4.981(1),~, which is nearly the same as the value of the best stoichiometric single crystal G d N reported by Wachter et al. [6]. Using this sample, we measured magnetization and magnetic field dependence of susceptibility and specific heat. The experimental results are explained in terms of an exchange mechanism.
2. Results and discussion The magnetization of GdN is shown in the inset of Fig. 1. The measurements at 1.6 and 4.2 K reveal only a small ,;lfference in the field dependence of the Gd a + magnetic momepts with saturation values being reached at about 20kOe. The saturation moment:, of 6.84/~B/Gd a + and 6.88 ~tB/Gd3 + at 4.2 and 1.6 K, respectivel), are approximately 2% below the theoretical value of 7.0 PB per Gd 3 ~. Similar behavior has been reported
0921-4526/94/$07.00 ~ 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0290-W
632
D.X. Li etal./Physiea B 199&200 (1994) 631--673 I
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Fig. !. Temperature dependence of the magnetization of GdN. The inset shows the magnetization versus applied magnetic field for GdN at 4.2 and 1.6 K.
Fig. 2. Magnetic part of the specific heat of GdN in magnetic fields of 0, 1, 5, and 10 kOe. The inset shows the behavior of Cp/T versus T2.
by Gambino etal. [3]. At low fields, the magnetic moment of G d N increases rapidly and is nonlinear as in a ferromagnet. This behavior is very similar to the results on a pure G d N sample reported by Wachter et al. [6]. The magnetization curve, however, did not show any hysteresis effect for measurements carried out in increasing and decreasing magnetic fields. The temperature dependence of magnetization of GdN is plotted in Fig. ! for different magnetic fields. For T > 2 3 0 K , I/Z curves show Curie Weiss behavior. From the data for 230 K < T < 350 K, we determined the paramagnetic Curie temperature 0p = 81 K and effective magnetic moment/4rf = 7.92 #a at H = 500 Oe. At low temperatures, the susceptibility shows a strong field dependence. Wachter etal. [6-1 found a sharp antiferromagnetic transition in their susceptibility measurements in fields of 10 Oe. However, our experiments give no such clear evidence for antiferromagnetism in GdN, because only in a field of 2 Oe does a very small and broad peak appear at around 40 K. The results on our GdN sample shown in Fig. 1 correspond to ferromagnetic behavior in spite of the absence of hysteresis effect. The Curie temperature Tc was determined to be 58 K from Fig. I and from specific heat measurements explained in the following. The specific heat of GdN was measured at i.6 K < T < 80 K under magnetic fields er 0, 1, 5, and 10 kOe. Figure 2 shows the magnetic part of the specific heat obtained by subtracting the data of LuN from those of GdN. At zero field, we observe a clear anomalous peak centered at 58 K with a long tail on the high temperature side. This peak is clearly corresponding to a magnetic
phase transition. The transition temperature is much different from the N6el temperature of 40 K obtained by Wachter et al. [6]. The shape of this peak remains unchanged in magnetic fields up to 10 kOe, where the magnetic moment of the sample has reached 90% of the saturation value at 4,2 K (s~e Fig. I). We consider this peak corresponds to a ferromagnetic transition. Another interesting feature is a broad peak observed around 20 K. This can be seen more clearly in the plot C / T versus T z (inset of Fig. 2). Summarizing the above results, we consider our GdN sample to be a ferromagnet even though it is not very pure and shows no hysteresis effect. Because GdN crystallizes in the simple NaCl-typc structure and has a SST,2 ground state, no crystal-field effect is present. Thus in GdN the magnetic exchange interactions may be the most important interactions. Narita and Kasuya [8] serarate the magnetic exchange interaction between rare-e, rth atoms into mainly three different mechanisms. For '3dN, the 4f level is very low (7 10eV below the Fermi energy Ev [6]), thus the first mechanism, i.e. the RKKY type of interaction should be the dominant mechanism. The RKKY oscillation is very sensitive to the carrier concentration. Depending on the carrier concentration the RKKY oscillation can lie in the ferromagnetic or antiferromagnetic lobes. Different preparation methods lead to GdN samples with different degrees of stoichiometry and therefore with different carrier concentrations. This may be why G d N can exist as a ferromagnet as well ,s an antiferromagnet at low fields as shown by many authors.
D.X. Li et al./ Physica B 199&200 (1994) 631-633
References [I] G. Busch, J. Appl. Phys. 38 (1967) 1386. [2] T.R. McGuir¢, R.J. Gambino, S.J. Pickart and H.A. Alperin, J. Appl. Phys. 40 (1969) 1009. [3] R.J. Gambino, T.R. McGuire, H.A. Alperin and S.J. Pickart, J. Appl. Phys. 41 (1970) 933.
633
[4] W. Stutius, Phys. Kondens. Mat. 10 (1969) 152. [5] R.A.Cutler and A.W. Lawson, J. Appl. Phys. 46 I1975t 2739. [6] P. Wachter and E. Kaldis, Solid State Commun. 34 I19801 241. [7] A. Hasegawa and A. Yanase, J. Phys. Soc. Japan 42 (1977)492. [8] A. Narita and T. Kasuya, J. Magn. Magn. Mater. 52 (1985} 373.
Physica B 199&200 11994)631-633
Magnetic properties of ferromagnetic GdN D.X. Li*, Y. Haga, H. Shida, T. Suzuki Department of Physics, Faculty of Science. Tohoku University. Sendal 980, Japan
Abstract A polycrystalline GdN sample was prepared by the direct reaction of gadolinium metal and nitrogen gas at 1600°C and at a pressure of 1300 atm. We measured magnetization and the magnetic field dependence of the susceptibility and specific heat of GdN. Susceptibility and magnetization show ferromagnetic behavior for GdN with the saturation value reached at about 20 kOe. Specific heat measurements revealed a ferromagnetic transition in GdN at Tc = 58 K with an additional broad peak around 20 K. Experimental results were explained with an exchange interaction mechanism.
!. Introduction The magnetic structure of GdN has given rise to much controversy. Many authors [1-4] have reported for GdN a ferromagnetic transition at 65-75 K and a paramagnetic Curie temperature of 70-90 K. Based on magnetization and initial susceptibility measurements, Waehter et al. [5, 6] claimed GdN at low fields to be antiferromagnetic with a N6el temperature TN = 40 K. Band structure calculations [7] suggest a semiconducting character for GdN. However, experiments [6] indicate GdN to be semimetallic. Until now the ordering temperature of GdN was determined only from susceptibility measurements. There is no information about the magnetic transition of G d N from specific heat measurement. To determine the magnetic structure of G d N a further study of various physical properties is necessary. It is difficult to obtain stoichiometric GdN. We applied a high pressure method to prepare G d N for a first time by using a hot isostatic pressing furnace. Flakes of Gd metal of 99.9% purity in an open tungsten crucible were directly reacted with nitrogen at 1600°C and a pressure of *Corresponding attthor.
1300arm for 3 h. Then the sample was hydrostatically pressed into a cylindrical shape at 720°C and 1300 atm using a glass capsule method. A polycrystalline GdN sample with a high nearly theoretical, density, was obtained. X-ray diffraction showed a single phase of NaC1type GdN with a room temperature lattice constant a = 4.981(1),~, which is nearly the same as the value of the best stoichiometric single crystal G d N reported by Wachter et al. [6]. Using this sample, we measured magnetization and magnetic field dependence of susceptibility and specific heat. The experimental results are explained in terms of an exchange mechanism.
2. Results and discussion The magnetization of GdN is shown in the inset of Fig. 1. The measurements at 1.6 and 4.2 K reveal only a small ,;lfference in the field dependence of the Gd a + magnetic momepts with saturation values being reached at about 20kOe. The saturation moment:, of 6.84/~B/Gd a + and 6.88 ~tB/Gd3 + at 4.2 and 1.6 K, respectivel), are approximately 2% below the theoretical value of 7.0 PB per Gd 3 ~. Similar behavior has been reported
0921-4526/94/$07.00 ~ 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0290-W
632
D.X. Li etal./Physiea B 199&200 (1994) 631--673 I
I
I
t
I
t
l
l
l
l
l
l
l
|
l
l 30
" .~ "-'1 " O ' q l"'i - " " ' "
4 ~'~OOOG
\
÷
;
' '
I ' " " '4
~
"
I
'
'
'
e %
m =I'2
I
I •
l_
~
• T=1.6K o T=4.2K
2
0
. . . .
-,~ ~ OFT-,
0
|
,-,
. . . .
100
i
[
'i' i'
200
L
~[js"
/
H(T)
, ;"+t
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,
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T(K)
F
/
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~',
0
I ,
/
I
20
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o
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•
..,,o.
:,:.,,. • ..,o,o.
.
.
,
I
40 T(K)
.
t
.
A
"~1
:! I
60
,
,
~ l
80
Fig. !. Temperature dependence of the magnetization of GdN. The inset shows the magnetization versus applied magnetic field for GdN at 4.2 and 1.6 K.
Fig. 2. Magnetic part of the specific heat of GdN in magnetic fields of 0, 1, 5, and 10 kOe. The inset shows the behavior of Cp/T versus T2.
by Gambino etal. [3]. At low fields, the magnetic moment of G d N increases rapidly and is nonlinear as in a ferromagnet. This behavior is very similar to the results on a pure G d N sample reported by Wachter et al. [6]. The magnetization curve, however, did not show any hysteresis effect for measurements carried out in increasing and decreasing magnetic fields. The temperature dependence of magnetization of GdN is plotted in Fig. ! for different magnetic fields. For T > 2 3 0 K , I/Z curves show Curie Weiss behavior. From the data for 230 K < T < 350 K, we determined the paramagnetic Curie temperature 0p = 81 K and effective magnetic moment/4rf = 7.92 #a at H = 500 Oe. At low temperatures, the susceptibility shows a strong field dependence. Wachter etal. [6-1 found a sharp antiferromagnetic transition in their susceptibility measurements in fields of 10 Oe. However, our experiments give no such clear evidence for antiferromagnetism in GdN, because only in a field of 2 Oe does a very small and broad peak appear at around 40 K. The results on our GdN sample shown in Fig. 1 correspond to ferromagnetic behavior in spite of the absence of hysteresis effect. The Curie temperature Tc was determined to be 58 K from Fig. I and from specific heat measurements explained in the following. The specific heat of GdN was measured at i.6 K < T < 80 K under magnetic fields er 0, 1, 5, and 10 kOe. Figure 2 shows the magnetic part of the specific heat obtained by subtracting the data of LuN from those of GdN. At zero field, we observe a clear anomalous peak centered at 58 K with a long tail on the high temperature side. This peak is clearly corresponding to a magnetic
phase transition. The transition temperature is much different from the N6el temperature of 40 K obtained by Wachter et al. [6]. The shape of this peak remains unchanged in magnetic fields up to 10 kOe, where the magnetic moment of the sample has reached 90% of the saturation value at 4,2 K (s~e Fig. I). We consider this peak corresponds to a ferromagnetic transition. Another interesting feature is a broad peak observed around 20 K. This can be seen more clearly in the plot C / T versus T z (inset of Fig. 2). Summarizing the above results, we consider our GdN sample to be a ferromagnet even though it is not very pure and shows no hysteresis effect. Because GdN crystallizes in the simple NaCl-typc structure and has a SST,2 ground state, no crystal-field effect is present. Thus in GdN the magnetic exchange interactions may be the most important interactions. Narita and Kasuya [8] serarate the magnetic exchange interaction between rare-e, rth atoms into mainly three different mechanisms. For '3dN, the 4f level is very low (7 10eV below the Fermi energy Ev [6]), thus the first mechanism, i.e. the RKKY type of interaction should be the dominant mechanism. The RKKY oscillation is very sensitive to the carrier concentration. Depending on the carrier concentration the RKKY oscillation can lie in the ferromagnetic or antiferromagnetic lobes. Different preparation methods lead to GdN samples with different degrees of stoichiometry and therefore with different carrier concentrations. This may be why G d N can exist as a ferromagnet as well ,s an antiferromagnet at low fields as shown by many authors.
D.X. Li et al./ Physica B 199&200 (1994) 631-633
References [I] G. Busch, J. Appl. Phys. 38 (1967) 1386. [2] T.R. McGuir¢, R.J. Gambino, S.J. Pickart and H.A. Alperin, J. Appl. Phys. 40 (1969) 1009. [3] R.J. Gambino, T.R. McGuire, H.A. Alperin and S.J. Pickart, J. Appl. Phys. 41 (1970) 933.
633
[4] W. Stutius, Phys. Kondens. Mat. 10 (1969) 152. [5] R.A.Cutler and A.W. Lawson, J. Appl. Phys. 46 I1975t 2739. [6] P. Wachter and E. Kaldis, Solid State Commun. 34 I19801 241. [7] A. Hasegawa and A. Yanase, J. Phys. Soc. Japan 42 (1977)492. [8] A. Narita and T. Kasuya, J. Magn. Magn. Mater. 52 (1985} 373.