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Magneto-caloric effect in antiferromagnetic superconductors
Solid State Communications, Printed in Great Britain.
Vo1.54,No.3,
pp.253-255,
1985.
0038-1098185 $3.00 + .OO Pergamon Press Ltd.
MAGNETO-CALORIC EFFECT IN ANTIFERROMAGNETIC SUPERCONOUCTORS K.
Institut
Rogacki*,
U.
Poppe,
and F.
Pobell**
filr Festkorperforschung, Kernforschungsanlage D-5170 JUlich, W.-Germany and G. Kozlowski Wagner College, Staten Island,
New York
(Received 4 February 1985 by O.V. Lounasmaa) We have investigated
adiabatic magnetization cooling in the antiferromagnetic superconductors GdMo6S8 and TbMo&8 in their antiferromagnetic states. At 0.5 K the temperature decreased by 0.04 K for GdMogS8 and by 0.13 K for TbMogSR when a magnetic field of 2.5 kOe was applied. In the superconducting state the magnetization cooling is partly masked by a heating effect, possibly resulting from flux flow. In the spin canted and in the paramagnetic states,increasing the field produced a temperature rise.
1.
Introduction
pends on the magnetic state of the samples, on whether they are superconducting or normal, and on the temperature distance from the temperatures of the phase transitions. Our results agree qualitatively with the predictions of Ref. 3.
Adiabatic magnetization of an antiferromagnet gives rise to cooling because an external field enhances fluctuations of the magnetic moments which are antiparallel to the field, and it decreases the degree of magnetic order; heat is then absorbed from the lattice. Eventually, the antiferromagnetic state becomes unstable and the sample enters a paremagnetic or spin canted state (our experiments can not distinguish between these two possibilities; in the following we use “paremagnetic” for both situations) in which magnetization results in heating because the field now enhances the order. A very large magnetization cooling effect in a rare earth compound was observed by Clark and Callen when they magnetized antiferrimagnetic ytterbium iron garnet1 whereas usually magnetization cooling is a rather weak effect.2 Recently, two of the present authors have considered the magnatocaloric effect in antiferrcxnagnetic superconductors.3 The new aspect is the coupling of the magnetic and superconducting states, and a possible influence of superconductivity on the magnetocaloric effect. In this paper we report the first experimental results for the magneto-caloric effect in antiferromagnetic superconductors. For this purpose we have investigated the properties of the Chevrel phases GdMogS8 and TbMogS8. In these compounds superconductivity and antiferromsgnetism coexist mainly because of s small value of the exchsnge constant of about 0.03 eV/atom.4 We will demonstrate that the degree of cooling or heating produced by increasing the field de-
2.
+ Permanent and present address:
Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 53-529 Wroclsw, Poland
+* Present address: Universitiit W.-Germany
Samples and Apparatus
For preparation of the samples we first heated GdS or TbS, MO, and S (of composition Gdl.07Mog.3S8 and Tbl.07Mog.3S8) at loooO C for 5 h in a quartz ampoule to obtain Chevrel phase powder. This powder was pressed at 12 bar to a cylinder of 5.4 mmdiameter and 11 mmlength, giving a weight of about 1.0 g. The pressed cylinders were heat treated in an evacuated molybdenum ampoule for 24 h at 1350~ C. X-ray analysis showed no impurity phases above a limit of 3 X, except for about 3 X MO. The GdMNS8 sample had a residual resistivity ratio of 24. Its inductively measured superconducting and antiferromagnetic trsnsition temperatures were T, = 1.4 K and TN = 0.90 K, respectively. In the calorimeter, the samples were supported by fine nylon strings. Adiabaticity of this support was adequate because the temperature of a sample stsyed constant to better than 1 mK/hour even when s AT of loo mK existed between the sample and its isothermal surrounding. A 1 mmthick Cu wire (1.5 g) wss wrapped around the samples and fixed there with a small amount of GE 7031 varnish. This wire could be thermally coupled to the mixing chamber of a dilution refrigerstor by closing a mechanical heat switch. The magnetic field wss provided by s superconducting solenoid (lo cm long, 2.5 cm diem.) thermally anchored at the mixing chamber. As thermometers we used ground-down Matsushita carbon resistors. One of them, with a mass of 3 mg was glued directly to the sample with a small amount of GE varnish. The second one of 14 mg mass was glued to s Cu-holder which was screwed to the upper end of the Cu wire outside of the magnetic field region. The magneto-resistance AR/R of the first thermometer at T < 2 K was smaller than lo-3 for H < 2.5 kOe; this cor-
Physikalisches Institut, Bayreuth, D-8580 Beyreuth,
253
MAGNATO-CALORIC EFFECTIN ANTIFERROMAGNETIC SURERCONDUCTORS
254
Vol.
54, No. 3
responds to a relative temperature change of 2 - 10-3. The rate of field change was 3 Oe/sec and data were only taken when thermal equilibrium was reached which took less than 1 min after the field was changed. 3.
Results
In Figs1 to 3 we show the change of temperature of GdMo&g when the magnetic field was adiabatically increased to 2.5 kOe and then reduced to zero. As Fig. 1 demonstrates, increasing the field results in a reversible temperature increase when the sample is in the parsmsgnetic state. The heating effect is reduced if the temperature approaches the Neel temperature where th specific heat of the sample rises steeply. s In Figs. 2 and 3 we show the msgneto-caloric effect in the antiferromagnetic and in the paramsgnetic states of normal and superconducting GdMo&. In the superconducting state of antiferromagnetic GdMo& we see only very weak cooling or even some warming. In addition, the temperature change becomes irreversible at the lowest temperatures. We believe that these observations arise from heat production due to flux flow in the mixed state. The effect is more
lu
nan
m --”
L
%2
2
2 ii ic
0
1
MAGNETIC
2 FIELD
I: kOe 1
Figure 2
‘-7-----7r
Temperature of GdMo&g in the antiferromagentic end in the paramagnetic (at Ii > Tip) state as a function of magnetic field. In the superconducting state at H < H,2 the magnetization cooling is reduced by a heating effect.
1A
O.Q(OQ 0 MAGNETIC
2
1
FIELD
l
1
0
MAGNETIC
[ kOe ‘1
2 FIELD
[ kOe ]
Figure 3 Figure 1 Temperature of psramagnetic GdMot;SRin the normal states as a function of ..-- and -.._ suoerconductinq ~, magnetic field. The transition from the superconducting to the normal state occurs at Hc2.
Temperature of antiferromagnetic GdMo& as s function of magnetic field. In the superconducting state for H < H,2 the magnetization cooling is reduced by a heating effect which also results in the shown irreversibilities.
Vol.
54, No. 3
MAGNETO-CALORIC EFFECTIN ANTIFERRONAGNETIC SUPERCONDUCTORS
pronounced at low temperatures where the magnetic specific heat has been decreased from its large peak the heating
near
TN. At H < 1 klle and T < 0.4
K,
effect is ten times larger when the field direction is changed, possibly due to flux annihilation. Due to this heating effect, we can observe a reversible magneto-caloric effect of antiferromagnetic GdMo6SRonly at H > Hc2. For temperatures T > 0.8 K, the transition to the paremagnetic state at H = HP occurs within the field range of our experiments. For H > Hp we see heating by increasing the field as expected. From data as shown in Fig. 2 we obtain the temperature dependence of the field at which GdMo6S0enters the paramagnetic state. These data are shown in Fig. 4. Their extrapolation agrees well with the inductively measured Neel temperature TN = 0.90 K. A considerable larger cooling effect has been observed TbMo6S0. At 0.5 K we observed that the temperature of the sample decreased by about
255
0.13 K when the field was increased to 2.5 kOe. For this compound the magnetic field range where antiferromagnetic order and supercondutitivity can coexist in a magnetic field is much larger than in GdMo6SR. Details on the results for TbMo6S.qwill be reported in a forthcoming paper. Qualitatively, our results agree with the theoretical results presented in Ref. 3. For example, keeping the strong increase of the specific heat for increasing T towards TN in mind,5 Fig. 3 demonstrates that the magnetization cooling effect increases for increasing temperature. A quantitative comparison between the measured and calculated3 magnetocaloric effect will be possible when the specific heat of our samples as a function of magnetic field and temperature has been measured.
Acknowledgement - We thank Dr. R.M. Mueller support of this work.
for
0.90
0.80 TEMPERATURE
T [K]
Figure 4 Magnetic field Hp at which GdMo& enters the paremagnetic state as a function of temperature.
References
1. A.E. Clark and E. Callen, Physical Review Letters 23, 307 (1969). 2. J.H. Schellenberg and S.A. Friedberg; Journal Applied Physics 34, 1087 (1963) 3. G. Kozlowski and K. Rogacki, Physica Status Solidi (b) 114, K 5 (1982). 4. M. Ishikawa and 0. Fischer, Solid State Communications 24, 747 (1977); M. Ishikawa and J. Muller, Solid State Communications 27,
761 (1978); M. Ishikawa, 0. Fischer, and J. Muller, Chap. 5 in “Superconductivity in Ternary Compounds, II” eds. M.B. Maple and 0. Fischer, Springer Verlag, 1982. 5. L.D. Woolf, M. Tovar, H.C. Hamaker, and M.B. Maple, Physics Letters, w, 363 (1979).
Vo1.54,No.3,
pp.253-255,
1985.
0038-1098185 $3.00 + .OO Pergamon Press Ltd.
MAGNETO-CALORIC EFFECT IN ANTIFERROMAGNETIC SUPERCONOUCTORS K.
Institut
Rogacki*,
U.
Poppe,
and F.
Pobell**
filr Festkorperforschung, Kernforschungsanlage D-5170 JUlich, W.-Germany and G. Kozlowski Wagner College, Staten Island,
New York
(Received 4 February 1985 by O.V. Lounasmaa) We have investigated
adiabatic magnetization cooling in the antiferromagnetic superconductors GdMo6S8 and TbMo&8 in their antiferromagnetic states. At 0.5 K the temperature decreased by 0.04 K for GdMogS8 and by 0.13 K for TbMogSR when a magnetic field of 2.5 kOe was applied. In the superconducting state the magnetization cooling is partly masked by a heating effect, possibly resulting from flux flow. In the spin canted and in the paramagnetic states,increasing the field produced a temperature rise.
1.
Introduction
pends on the magnetic state of the samples, on whether they are superconducting or normal, and on the temperature distance from the temperatures of the phase transitions. Our results agree qualitatively with the predictions of Ref. 3.
Adiabatic magnetization of an antiferromagnet gives rise to cooling because an external field enhances fluctuations of the magnetic moments which are antiparallel to the field, and it decreases the degree of magnetic order; heat is then absorbed from the lattice. Eventually, the antiferromagnetic state becomes unstable and the sample enters a paremagnetic or spin canted state (our experiments can not distinguish between these two possibilities; in the following we use “paremagnetic” for both situations) in which magnetization results in heating because the field now enhances the order. A very large magnetization cooling effect in a rare earth compound was observed by Clark and Callen when they magnetized antiferrimagnetic ytterbium iron garnet1 whereas usually magnetization cooling is a rather weak effect.2 Recently, two of the present authors have considered the magnatocaloric effect in antiferrcxnagnetic superconductors.3 The new aspect is the coupling of the magnetic and superconducting states, and a possible influence of superconductivity on the magnetocaloric effect. In this paper we report the first experimental results for the magneto-caloric effect in antiferromagnetic superconductors. For this purpose we have investigated the properties of the Chevrel phases GdMogS8 and TbMogS8. In these compounds superconductivity and antiferromsgnetism coexist mainly because of s small value of the exchsnge constant of about 0.03 eV/atom.4 We will demonstrate that the degree of cooling or heating produced by increasing the field de-
2.
+ Permanent and present address:
Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 53-529 Wroclsw, Poland
+* Present address: Universitiit W.-Germany
Samples and Apparatus
For preparation of the samples we first heated GdS or TbS, MO, and S (of composition Gdl.07Mog.3S8 and Tbl.07Mog.3S8) at loooO C for 5 h in a quartz ampoule to obtain Chevrel phase powder. This powder was pressed at 12 bar to a cylinder of 5.4 mmdiameter and 11 mmlength, giving a weight of about 1.0 g. The pressed cylinders were heat treated in an evacuated molybdenum ampoule for 24 h at 1350~ C. X-ray analysis showed no impurity phases above a limit of 3 X, except for about 3 X MO. The GdMNS8 sample had a residual resistivity ratio of 24. Its inductively measured superconducting and antiferromagnetic trsnsition temperatures were T, = 1.4 K and TN = 0.90 K, respectively. In the calorimeter, the samples were supported by fine nylon strings. Adiabaticity of this support was adequate because the temperature of a sample stsyed constant to better than 1 mK/hour even when s AT of loo mK existed between the sample and its isothermal surrounding. A 1 mmthick Cu wire (1.5 g) wss wrapped around the samples and fixed there with a small amount of GE 7031 varnish. This wire could be thermally coupled to the mixing chamber of a dilution refrigerstor by closing a mechanical heat switch. The magnetic field wss provided by s superconducting solenoid (lo cm long, 2.5 cm diem.) thermally anchored at the mixing chamber. As thermometers we used ground-down Matsushita carbon resistors. One of them, with a mass of 3 mg was glued directly to the sample with a small amount of GE varnish. The second one of 14 mg mass was glued to s Cu-holder which was screwed to the upper end of the Cu wire outside of the magnetic field region. The magneto-resistance AR/R of the first thermometer at T < 2 K was smaller than lo-3 for H < 2.5 kOe; this cor-
Physikalisches Institut, Bayreuth, D-8580 Beyreuth,
253
MAGNATO-CALORIC EFFECTIN ANTIFERROMAGNETIC SURERCONDUCTORS
254
Vol.
54, No. 3
responds to a relative temperature change of 2 - 10-3. The rate of field change was 3 Oe/sec and data were only taken when thermal equilibrium was reached which took less than 1 min after the field was changed. 3.
Results
In Figs1 to 3 we show the change of temperature of GdMo&g when the magnetic field was adiabatically increased to 2.5 kOe and then reduced to zero. As Fig. 1 demonstrates, increasing the field results in a reversible temperature increase when the sample is in the parsmsgnetic state. The heating effect is reduced if the temperature approaches the Neel temperature where th specific heat of the sample rises steeply. s In Figs. 2 and 3 we show the msgneto-caloric effect in the antiferromagnetic and in the paramsgnetic states of normal and superconducting GdMo&. In the superconducting state of antiferromagnetic GdMo& we see only very weak cooling or even some warming. In addition, the temperature change becomes irreversible at the lowest temperatures. We believe that these observations arise from heat production due to flux flow in the mixed state. The effect is more
lu
nan
m --”
L
%2
2
2 ii ic
0
1
MAGNETIC
2 FIELD
I: kOe 1
Figure 2
‘-7-----7r
Temperature of GdMo&g in the antiferromagentic end in the paramagnetic (at Ii > Tip) state as a function of magnetic field. In the superconducting state at H < H,2 the magnetization cooling is reduced by a heating effect.
1A
O.Q(OQ 0 MAGNETIC
2
1
FIELD
l
1
0
MAGNETIC
[ kOe ‘1
2 FIELD
[ kOe ]
Figure 3 Figure 1 Temperature of psramagnetic GdMot;SRin the normal states as a function of ..-- and -.._ suoerconductinq ~, magnetic field. The transition from the superconducting to the normal state occurs at Hc2.
Temperature of antiferromagnetic GdMo& as s function of magnetic field. In the superconducting state for H < H,2 the magnetization cooling is reduced by a heating effect which also results in the shown irreversibilities.
Vol.
54, No. 3
MAGNETO-CALORIC EFFECTIN ANTIFERRONAGNETIC SUPERCONDUCTORS
pronounced at low temperatures where the magnetic specific heat has been decreased from its large peak the heating
near
TN. At H < 1 klle and T < 0.4
K,
effect is ten times larger when the field direction is changed, possibly due to flux annihilation. Due to this heating effect, we can observe a reversible magneto-caloric effect of antiferromagnetic GdMo6SRonly at H > Hc2. For temperatures T > 0.8 K, the transition to the paremagnetic state at H = HP occurs within the field range of our experiments. For H > Hp we see heating by increasing the field as expected. From data as shown in Fig. 2 we obtain the temperature dependence of the field at which GdMo6S0enters the paramagnetic state. These data are shown in Fig. 4. Their extrapolation agrees well with the inductively measured Neel temperature TN = 0.90 K. A considerable larger cooling effect has been observed TbMo6S0. At 0.5 K we observed that the temperature of the sample decreased by about
255
0.13 K when the field was increased to 2.5 kOe. For this compound the magnetic field range where antiferromagnetic order and supercondutitivity can coexist in a magnetic field is much larger than in GdMo6SR. Details on the results for TbMo6S.qwill be reported in a forthcoming paper. Qualitatively, our results agree with the theoretical results presented in Ref. 3. For example, keeping the strong increase of the specific heat for increasing T towards TN in mind,5 Fig. 3 demonstrates that the magnetization cooling effect increases for increasing temperature. A quantitative comparison between the measured and calculated3 magnetocaloric effect will be possible when the specific heat of our samples as a function of magnetic field and temperature has been measured.
Acknowledgement - We thank Dr. R.M. Mueller support of this work.
for
0.90
0.80 TEMPERATURE
T [K]
Figure 4 Magnetic field Hp at which GdMo& enters the paremagnetic state as a function of temperature.
References
1. A.E. Clark and E. Callen, Physical Review Letters 23, 307 (1969). 2. J.H. Schellenberg and S.A. Friedberg; Journal Applied Physics 34, 1087 (1963) 3. G. Kozlowski and K. Rogacki, Physica Status Solidi (b) 114, K 5 (1982). 4. M. Ishikawa and 0. Fischer, Solid State Communications 24, 747 (1977); M. Ishikawa and J. Muller, Solid State Communications 27,
761 (1978); M. Ishikawa, 0. Fischer, and J. Muller, Chap. 5 in “Superconductivity in Ternary Compounds, II” eds. M.B. Maple and 0. Fischer, Springer Verlag, 1982. 5. L.D. Woolf, M. Tovar, H.C. Hamaker, and M.B. Maple, Physics Letters, w, 363 (1979).