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A study of the magnetic properties of Gd3Pd4 in applied magnetic fields
Journal of Magnetism and Magnetic Materials 103 (1992) 129-138 North-Holland
A study of the magnetic properties of Gd3Pd 4 in applied magnetic fields S. T a n o u e Research and Development Division, Sumitomo Metal Industries, Ltd., Amagasaki 660, Japan and Ames Laboratory *, Iowa State University, Ames, 1.4 50011, USA
K.A. Gschneidner, Jr. and R.W. McCallum Ames Laboratory * and Department of Materials Science and Engineering, Iowa State-University, Ames, IA 50011, USA Received 11 March 1991; in revised form 11 July 1991
The low temperature heat capacity and magnetic susceptibility were measured on alloys of the Gd3Pd 4 and Gd4Pd 5 compositions. The GdaPd 5 sample was found to be a two phase alloy consisting of Gd3Pd 4 and Gd3Pd4-GdPd eutectic. The Gd3Pd 4 compound was confirmed to be antiferromagnetic with a N6el temperature of 18 K. Another magnetic transition was observed = 6 K which has characteristics of ferromagnetic or ferrimagnetic ordering. In applied magnetic fields up to 9.85 T, a heat capacity peak developed at --- 13 K while the 18 K peak shifted to lower temperatures in fields > 5.3 T. The entropy associated with the 13 K anomaly was estimated to be less than 2% of total magnetic entropy. The origin of this anomaly was thought to be due to some type of spin reorientation of some (few) of the antiferromagnetic moments in the applied field. The low temperature heat capacities of La3Pd 4 and Lu3Pd 4 were also measured. The electronic specific heat coefficients were found to be 10.5 and 5.72 mJ/mol K 2, respectively, and the Debye temperatures are 171 and 191 K, respectively.
1. Introduction Lanthanide compounds are thought to be suitable for magnetic refrigeration because the lanthanide atoms have a large magnetic moment compared to the transition metals. Gadolinium compounds are of special interest because gadolinium has no angular momentum, and therefore, crystal fields do not reduce the available magnetic entropy associated with magnetic ordering. * The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under contract no. W-7405-ENG-82. This work was supported by the Office of Basic Energy Sciences.
For the refrigeration materials, there are at least four requirements as follows: 1) the ordering temperature is 1 or 2 K below the desired operating temperature; 2) a suitable magnetic ordering of the lambdatype; 3) a large magnetic moment; and 4) a high thermal conductivity. Gadolinium gallium garnet (GGG) has been recommended as a refrigeration material by several authors [1,2]. But because of its low thermal conductivity, it may not be as useful as metallic gadolinium compounds which generally exhibit good thermal conductivities. GdPd, for example, is also a candidate for magnetic refrigeration for cooling down to about 40 K [3].
0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
130
S, Tanoue et al. / Magnetic properties ot'Gd3Pd 4
We were primarily interested in studying a ferromagnetic gadolinium compound which orders below 20 K, and according to the literature the compound Gd4Pd 5 appeared to be a good candidate. Loebich and Raub [4] measured the magnetic susceptibility from 140 to 720 K and reported a paramagnetic Curie temperature (O v) of 2 K and an effective moment (Peef) of 8.0/xu from its Curie-Weiss behavior. After preparing this alloy, metallographic analysis clearly revealed that the sample consisted of two phases - G d P d and Gd3Pd 4 - and that the reported phase diagram [5] is clearly wrong. Thus our efforts were concentrated on the compound Gd3Pd4, but the results reported in literature on its magnetic properties disagreed. Yakinthos et al. [6] reported Op = - 11 K and Peff = 8.09/-tB and no observed ordering down to 4.2 K; Gamari-Seale [7] (a co-author of the previous p a p e r [6]) reported the same Op and Pelf values but also noted small susceptibility maxima at 17 and 6 K, which shifted with applied fields; and Buschow and De Mooij [8] give Op = - 1 8 , Pelf = 8.8/X B and an observed N6el t e m p e r a t u r e of 18 K. Compounds of lower gadolinium concentrations would be expected to have lower ordering temperatures, but they were not studied because of an expected increase of the lattice entropy relative to the magnetic entropy, which is not favorable for magnetic refrigeration [9,10]. It is worthwhile, however, to study the Gd3Pd 4 compound because there is no data about its heat capacity and only limited results concerning its behavior under magnetic fields except for magnetic susceptibility m e a s u r e m e n t s [6-8].
2. Experimental procedures The rare earth metals used in this study were prepared at the Materials Preparation Center of the Ames Laboratory and the impurity levels in the lanthanum, gadolinium and lutetium metals are presented in table 1. The palladium was purchased from Westinghouse Materials of Ohio as 99.98% pure sponge which was electron b e a m melted before it was used in the preparation of the alloys for this study.
Table 1 C h e m i c a l analysis of s t a r t i n g rare e a r t h metals. Only impurities with 10 p p m a t o m i c or m o r e are listed. I m p u r i t y levels are ppm atomic Impurity
Lanthanum
Gadolinium
Lutetium
H C N O F Fe Ta W
1650 34 128 190 80 < 8 < 2 < 2
1090 170 67 766 < 25 19 l l) 20
346 189 37 130 < 27 78 < 1 < 6
The alloys were prepared by arc melting in an atmosphere of Ar gas. In the melting process, button shaped samples were turned over and remelted several times to ensure homogeneity. Then the buttons were remelted into 8 mm diameter cylindrical rods. The samples weighed about 10 g and the weight losses were less than 1%. Metallography results confirmed that the alloy of the Gd3Pd 4 composition was single phase, while the G d a P d 5 composition consisted of primary Gd3Pd 4 and G d 3 P d 4 - G d P d eutectic. At the same time X-ray diffraction confirmed the crystal structure of Gd3Pd4, which has a rhombohedral cell, with the space group R3, and is isostructural with Pu3Pd 4. The triply primative hexagonal lattice constants are a = 13.267(4) A and c = 5.716(5) A. These values were in good agreement with those obtained by other authors [8,11]. Heat capacity measurements were carried out at zero magnetic field from 1.6 to 80 K and in magnetic fields of 2.46, 5.32, 7.53, and 9.85 T from 1.6 to 22 K. Magnetic susceptibility measurements were made from 1.6 to 260 K using a Faraday m a g n e t o m e t e r at magnetic fields from 0.6 to 1.8 T. Magnetization measurements were performed in a Quantum Design squid magnetometer at fields from 0.1 to 5.4 T and temperatures from 2 to 70 K. For temperatures below 40 K the sample was warmed to 40 K and then cooled in zero field prior to measurement. For temperatures below 20 K, and at 24 and 28 K, measurements were made in both increasing and decreasing fields. At all other temperatures the samples were measured in an increasing field.
S. Tanoue et al. / Magnetic properties of Gd 3 Pd 4
The slopes of the magnetization curve at both high and low field were determined by a linear regression fit to the data using eleven data points between 4.4 and 5.4 T, and twenty data points between 0.1 and 2 T, respectively.
131
200 + La3Pd. - Lu3Pd.
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_
e~
--'~ lO0,
_ -
~
3. Experimental results d~÷~'~ - -
The results of heat capacity measurements made on Gd3Pd 4 and "Gd4Pd5" from 1.6 to 80 K are shown in fig. 1. These curves show that G d 3 P d 4 has one magnetic phase transition at about 18 K, whereas Gd4Pd 5 has two transitions corresponding to the magnetic ordering of Gd3Pd 4 (18 K) and GdPd (37 K) [3]. To determine the magnetic contribution to the heat capacity, the lattice and electronic contributions to the heat capacity must be subtracted from the total heat capacity. To estimate these two contributions the heat capacities of L a 3 P d 4 and L u 3 P d 4 w e r e measured (see fig. 2 and table 2), and the electronic plus lattice contributions of Gd3Pd 4 were assumed to be equal to the average heat capacity of the former two compounds, bec a u s e L a 3 P d 4 a n d L u 3 P d 4 have the same crystal structure as Gd3Pd4, have no magnetic moments, and because the atomic numbers of lanthanum and lutetium lie seven atomic numbers on either side of gadolinium. The magnetic contribution of G d 3 P d 4 t o the heat capacity is shown in fig. 3. A close examination of the heat capacity curve from
50
+ +**_ - -
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20
I
I
30
'
I
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40 50 T (K)
I
I
I
60
70
80
90
Fig. 2. The heat capacity of La3Pd 4 and Lu3Pd 4 from 1.5 to 80 K.
Table 2 Fit parameters to the low temperature heat capacities of La3Pd 4 and Lu3Pd 4 Compound La3Pd 4 Lu3Pd 4
•D
(mJ/mol K 2)
/3 (mJ/mol K 4)
(K)
10.5 +2.7 5.72+0.34
2.739+0.049 1.945+0.037
171+1 191+1
Y
2 to 18 K indicates that the curve has an unusual shape in that it bulges out towards the low temperature side, rather than the normal concave lambda-type shape. This suggests an additional magnetic ordering occurring between 5 and 7 K, which has been confirmed by magnetic suscepti-
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T (K)
Fig. 1. The heat capacities of Gd3Pd 4 and a two phase alloy of the overall composition GdaPd 5 as a function of temperature.
-I0
0
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2'0
3b
40
50
T (K)
Fig. 3. The magnetic contribution to the heat capacity of Gd3Pd 4.
S. Tanoue et al. / Magnetic properties of Gd3Pd ~
132
bility and magnetization measurements (to be discussed later). It is also noted that the magnetic heat capacity above T N (18 K) is excessive, more than might be expected from a typical lambda type antiferromagnetic heat capacity peak, especially above = 25 K. Normally one would expect the magnetic heat capacity to reach zero at = 2T N, 36 K in this case. Magnetic measurements do not indicate anything unusual for T > 30 K, so one can rule out long range magnetic order, although short range magnetic order might account for this excess heat capacity. According to the simple spin wave theory, the low temperature limit for a ferromagnetic contribution to heat capacity is proportional to T 3/2 and that of an antiferromagnetic contribution to heat capacity is proportion to T 3. The observed magnetic contribution, however, was approximately proportional to T 2, but since the lowest magnetic ordering temperature is = 6 K we may need to make measurements down to lower temperatures to determine the correct temperature dependence of the magnetic contribution to the heat capacity. The total magnetic entropy is shown in fig. 4. The calculated value up to 40 K is 16.41 J / m o l - G d K and is in good agreement with the theoretical value S = R ln(2J + 1 ) = 17.24 J / m o l - G d K, i.e. 95% of the theoretical value. The magnetic field dependence of heat capacity up to 22 K is shown in fig. 5. The heat capacity was measured at five
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Fig. 5. The magnetic field dependence of the magnetic contribution to the heat capacity of Gd3Pd 4 as a function of temperature.
fields (0, 2.54, 5.32, 7.53 and 9.85 T), and the electronic and lattice contributions to the heat capacity were subtracted, assuming that these contributions are independent of magnetic fields. Only the data taken at 0, 5.32 and 9.85 T are shown, because the zero field and 2.54 T results are essentially identical, and so are the 5.32 and 7.53 T data. The data shown in fig. 5, reveals that a new peak in the heat capacity emerges at = 13 K at intermediate fields (5.32 and 7.53 T) while the 18 K peak is slightly smaller in magnitude but remains at 18 K. At the highest field (9.85 T) the 13 K peak essentially disappears and the 18 K peaks shifts down to = t5 K. The magnetic susceptibility as a function of temperature is shown in fig. 6. The shape of the susceptibility curve is characteristic of antiferromagnetic ordering with a Ndel temperature of 19.9 K. This temperature is in good agreement with the corresponding peak observed in heat capacity measurement. Below about 10 K, there is evidence for additional ordering with minor peaks at 9, 5.5 and 2.5 K, however, none are very distinct and may be due to scatter in the data. Gamari-Seale [7] also found evidence for at least one additional minor peak at 6 K, but her data were taken at large temperature intervals (only 7 data points below 20 K) compared to our measurements ( = 35 points). In order to clarify the situation a series of magnetization measurements were taken.
S. Tanoue et al. / Magnetic properties of Gd3Pd 4 0.8
0.8 . . . . . ...
0.61
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,~ O.
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T (K) Fig. 6. The magnetic susceptibility of Od3Pd 4 as a function of temperature. The inset shows an enlargement of the low temperature region. These data were taken on a Faraday magnetic susceptibility apparatus at a field of 0.656 T.
The squid magnetization data for T = 2 to 50 K and H = 0.1 to 5.4 T are given in fig. 7a. A number of features are worth noting. The first is that while the M vs. H curves are linear at high temperature, at low temperature there is a distinct change in slope. However, at both the lowest and the highest fields the M vs. H slopes are essentially constant for a given temperature. The point at which the slope changes moves to higher fields with increasing temperature. For a paramagnetic material, such a change of slope comes from a ferromagnetic impurity phase which is saturated in low fields. If this were the case, the point at which the slope changes would move to lower fields with increasing temperature since the coercivity of the ferromagnet will decrease with increasing temperature, however, just the opposite is actually observed. The low field slopes H < 2 T, and the high field slopes H > 4.4 T are shown in fig. 7b. For T > 8 K the low field slope is in good agreement with the results from the Faraday magnetometer (fig. 6). For T < 8 K, the difference between the low field squid results and the Faraday magnetometer results is attributed to the presence of hysteresis in the M vs. H curve. The squid result reflects the initial magnetization curve while the procedure used in the Faraday magnetometer corresponds to a minor hysteresis loop. The high field slope increases with increasing temperature up to a temperature of 54 K followed by a decrease to the Curie law value at
133
60 K. The data suggest that there is a competition between a ferromagnetic interaction and an antiferromagnetic interaction. In high magnetic fields the ferromagnetic interaction coupled with the field energy results in a field dependent ferromagnetic alignment while in low fields magnetic order is suppressed to lower temperatures with the antiferromagnetic interaction predominating at the 18 K ordering temperature but below about 8 K the ferromagnetic interaction takes over. The ferromagnetic nature of the low temperature ordering is clearly demonstrated by the presence of hysteresis at 6 K, and perhaps at 8 K. Above 50 K the magnetic susceptibility follows a Curie-Weiss behavior, and a least squares fit of the data gave a paramagnetic Curie temperature of - 9 . 5 K and an effective magnetic moment of 8.19/XB/Gd. These values are slightly different from those reported by Buschow and Mooij [8] (Op = - 18 K and Peff = 8.8~B/Gd), but in good agreement with those reported by Yakinthos et al. [6] (Op = - 11 K and Peff = 8.08/za/Gd).
4. Discussion
Gd3Pd 4 was confirmed to be antiferromagnetic with a N6el temperature of 18 K. But an additional anomaly was found in the magnetic contribution to heat capacity and also in the magnetic susceptibility and magnetization measurements at --- 6 K. These data indicate complex magnetic interactions occur in Gd3Pd4, which have sensitive field and temperature dependences. An examination of the crystal structure of Gd3Pd4, which is isotypic with Pu3Pd 4 [12], reveals that there is only one set of gadolinium positions in this structure - the 18 f (in the hexagonal system) set, which has no special symmetry (1). The bond distances for the Gd3Pd 4 should be essentially the same as for those reported by Cromer et al. [12] for Pu3Pd 4 because the a and c lattice parameters of Gd3Gd 4 are 0.58 and 0.49% smaller, respectively, than those of the Pu3Pd 4 compound. These authors report one short P u - P u ( G d - G d ) distance of 3.44 .& and two neighboring atoms at an intermediate distance of 3.75 A and two sets of two neighboring
134
S. Tanoue et al. / Magnetic properties of Gd ~Pd 4
1 40
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40K 50K
i
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6.0
Applied Field (T) 0.9
mm m
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T (K) Fig. 7. (a) Magnetization measurements of Gd3Pd 4 from 2 to 70 K at 2 K intervals. An offset of 500 e m u / m o l per 2 K interval has been added to the data except for 70 K curve. (b) Magnetic susceptibility (dM/dH) as a function of temperature from 2 to 70 K from linear regression fit for low (0.1-2 T) and high (4.4-5.4 T) fields. The error bars represent the standard error from the regression fit.
atoms each at 3.95 and 3.97 .~, which compares to a bond distance of 3.60 ~, in pure Gd metal. Thus there are several different local environments
around a gadolinium atom, just with respect to other gadolinium atoms, and this alone could account for a variety of magnetic ordering tern-
135
S. Tanoue et al. / Magnetic properties of Gd3Pd 4
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Applied Field (T) ÷
Fig. 9. The field dependence of the high temperature N~el ordering temperature.
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that the heat capacity sample had a preferred orientation in which the (001) directions were perpendicular to the cylinder axis.
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~, : 5.32 T o: 7.53 T o:9.85 T
4
Fig. 8. The magnetic contribution to the heat capacity of Gd3Pd4 as a function of temperature in magnetic fields parallel to the sample axis (top) and perpendicular to the sample axis (bottom). peratures, without considering the influence of the neighboring palladium atoms. In order to check if the crystal orientation had an affect on the anomalies observed in the high magnetic heat capacities, the heat capacity was also measured such that the magnetic field was in a direction perpendicular to the cylinder axis of the heat capacity sample (the initial measurements were made with the magnetic field parallel to the cylinder axis). The results of these measurements are shown in fig. 8. The change in the upper ordering temperature ( = 18 K) with applied magnetic field is essentially the same for either orientation of the sample in the field, see fig. 9. The general shapes of the 13 K peak did not change, however, some shifts in temperature and magnitudes are evident. This suggests that the magnetic interactions which cause the induced 13 K peak shown in fig. 8 have an orientation dependence. This could arise from a preferred orientation in the crystal which occurred during solidification. X-ray diffraction showed
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Fig. 10. The excess portion of the magnetic heat capacity of the 13 K (H = 5.3 T) peak as a function of temperature in the field parallel to the sample axis (top) and perpendicular to the sample axis (bottom).
S. Tanoue et al. / Magnetic properties of Gd3Pd 4
136
Table 3 Entropy associated with magnetic heat capacity anomaly at 13 K for fields greater than 5.32 T Field
5.32 7.53 9.85
Entropy (J/moI-Gd K) parallel direction
perpendicular direction
0.21 0.24 0.15
0.28 0.21 0.19
To evaluate the heat capacity associated with the 13 K anomaly, the fitting functions were made on either side of the peak and the excess portion of the magnetic heat capacity was obtained by subtraction of the fitting values from the observed C M values shown in fig. 8. The excess portions of the magnetic heat capacity are shown in fig. 10. The entropy associated with heat capacity anomalies are tabulated in table 3. The entropy associated with the anomalies is approximately 1.5% of total magnetic entropy. From the preferred orientation deduced from the X-ray measurements, the anomalous heat capacity results obtained when the applied magnetic field was oriented parallel to the sample axis corresponds to the influence of this field acting predominantly in the basal plane direction of the polycrystalline aggregate. Whereas when the magnetic field is applied perpendicular to the sample axis the field is predominantly parallel to c-axis of the crystallites. Unfortunately, because we are working with a polycrystalline material, which undoubtedly has complex magnetic interactions within a unit cell, it is difficult to come to any quantitative conclusions, except to say that there is a difference in the interaction of the magnetic moments of the gadolinium atoms with the direction of the applied magnetic field. The variation of the excess entropy of this 13 K anomaly with applied magnetic field could be a Fermi surface effect. That is, parts of the Fermi surface are destroyed (decreasing the entropy) or enlarged (increasing the entropy) by the field when it is increased from one value to another in the 0 to 9.85 T regime. A more likely possibility is that some sort of spin reorientation of one or more of the various antiferromagnetic alignments in Gd3Pd 4 is occurring as the field is increased.
The entropy arises when the thermal energy is sufficient to overcome the magnetic energy (which caused the moment to align ferromagnetically at some applied field) and allows the spins to relax and to realign antiferromagnetically at the high temperature. As one increases the magnetic field a smaller fraction of the spins reorient, because the larger magnetic energy holds more of the spins in a ferromagnetic arrangement; and this is what is observed (see fig. 10 and table 3). This would also be consistent with the orientation dependence. Similar anomalies have found in ferromagnetic HoA12 [13], and the paramagnetic materials CeSi, (x -- 1.9) [14] and N i - P d alloys containing small quantities of Ni, ~ 1 at% [15]. In the paramagnetic materials the anomalies are attributed to the induced magnetic moments by the applied field. The entropy associated with the anomalies are order of 2 m J / g - a t o m K which is less than one hundredth of the entropy observed in our experiments. However, for these two materials, there is only one f or d electron per atom involved while for gadolinium there are seven f electrons. But even taking this into account the excess entropy in Gd3Pd 4 is still about 30 times larger than that observed in CeSi x and N i - P d alloys. Hill et al. [13] measured the heat capacity of TbAI 2 and HoAl 2 at zero applied field and found a heat capacity anomaly in HoA12. They concluded that the entropy associated with anomaly was only 3% of the total magnetic entropy. This value is of the same order of magnitude as our value, except that the anomaly in Gd3Pd 4 only emerges in a field more than 5 T. Hashimoto et al. [16] also found the heat capacity anomaly in HoA12 in both zero field and in applied fields. The authors thought that the anomaly might be attributed to complicated change of its magnetic structure. In summary there are three possible explanations for this anomaly as discussed above: 1) the Fermi surface effect; 2) the induced moment; and 3) spin reorientation. The Fermi surface effect is a possibility which cannot be ruled out altogether, but we think it is not as likely as the spin reorientation explanation, because it involves a change
S. Tanoue et al. / Magnetic properties of Gd3Pd4
6
+ : 5SM(9.85T) : 5SM(7.53T) o : 5SM(5.32T)
/ o
~
2
-r-
..+...÷---+---+""¢ fl .÷.: .~., _z~....t~...,x .ix , . a . a m-
v
•
~o -2
I 5
~ lO T (K)
i 15
I 20
,
Fig. 11. The temperature d e p e n d e n c e s of the magnetic entropy change ASM(T, H) of G d 3 P d 4 in various magnetic fields. [ASM(T, H ) = SM(T, 0)- SM(T, H)].
in the Fermi surface topology as a function of applied field and such changes are not commonly known. The second explanation is ruled out because the observed entropy associated with the anomaly is a factor of 30 times larger than that found in other materials which exhibit this kind of anomaly. We believe that a spin reorientation of antiferromagnetic moments is the most reasonable explanation. This has also been verified in the magnetic susceptibility (derived from the high field magnetization slopes) which exhibits a weak broad peak at = 13 K (fig. 7b). Both the heat capacity and the magnetic susceptibility essentially do not show appreciable change from the zero field data in fields less than 2.5-3 T, but above 3 T significant changes are observed. A measure of the effectiveness of a magnetic refrigerant is the change in entropy due to increasing the field from 0 to Hmax. The larger the change the better the refrigerant with all other things being equal. The variation of the entropy change upon applying a magnetic field to Gd3Pd 4 is shown in fig. 11 as a function of temperature from 0 to 21 K. The change in entropy is not particularly large which is typical for antiferromagnetic materials, i.e. 10-20% less than that of a ferromagnet. For example, = 1 K above the ordering temperatures of Gd0.o6Ero.94A112 the entropy change due to the magnetic field change from 0 to 5 T is 4 J / m o l K and 5 J / m o l K from 0 to 8 T [17]. The comparable values for GdaPd 4
137
are 0.5 J / m o l K from 0 to 5.32 T and 2.2 J / m o l K from 0 to 7.53 T, but since there are three Gd atoms in a mole these values need to be divided by three giving 0.17 and 0.73 J/mol-Gd K. It is noted that the magnetic field changes are not identically the same and that the magnetic atoms are different and so there is a larger entropy associated with erbium (23.1 J/mol-Er K) than Gd (17.3 J/mol-Gd K) if all the entropy is involved in the magnetic ordering process and none is due to crystal fields effects which is likely for Er, but not for Gd since it has a fully occupied 4f half-filled shell. These additional factors, however, cannot account for the large observed differences. These observations are consistent with molecular field theory calculations which show that the magnetic entropy changes due to increasing the magnetic field is significantly larger for a ferromagnet than for an antiferromagnet for the same magnetic atom.
5. Conclusion
Heat capacity and magnetic susceptibility measurements, X-ray diffraction and metallography on alloys of the Gd3Pd 4 and Gd4Pd 5 compositions confirmed that the latter is not a single phase alloy and consists of primary Gd3Pd 4 and Gd3Pd4-GdPd eutectic, and that the former is a single phase alloy which orders antiferromagnetically at 18 K with a second magnetic transition at = 6 K which is ferromagnetic or ferrimagnetic in origin. The effective magnetic moment Gd3Pd 4 is 8.19/z J G d atom. Heat capacity measurements on Gd3Pd 4 in applied magnetic fields revealed a magnetic anomaly at --- 13 K in an applied magnetic field greater than 5 T. The entropy associated with the anomaly is less than 2% of total magnetic entropy. The anomaly is thought to be due to a spin reorientation of some of the antiferromagnetic moments in an applied field. Finally, our data show that Gd3Pd 4 is not a good magnetic refrigerant, as would be expected for an antiferromagnetically ordering material.
138
S. Tanoue et al. / Magnetic properties of Gd ~Pd 4
Acknowledgements The authors wish to thank Dr. J. Tang and Dr. V. Pecharsky for their valuable comments and suggestions, and J.O. Moorman for his assistance in heat capacity measurements.
References [1] J.A. Barclay and W.A. Steyert, Cryogenics 22 (1982) 73. [2] B. Daudin, R. Lagnier and B. Salce, J. Magn. Magn. Mater. 27 (1982) 315. [3] J.A. Barclay, W.C. Overton, Jr. and C.B. Zimm, Low Temperature Physics, LT-17, Part 2 (1984) p. 157. [4] O. Loebich, Jr. and E. Raub, J. Less-Common Met. 31 (1973) 111. [5] O. Loebich, Jr. and E. Raub, J. Less-Common Met. 30 (1973) 47. [6] J.K. Yakinthos, H. Gamari-Seale and J. Laforest, Phys. Stat. Sol. (a) 40 (1977) KI05.
[7] H. Gamari-Seale, J. Magn. Magn. Mater. 22 (1980) 87. [8] K.H.J. Buschow and D.B. de Mooij, Phys. Stat. Sol. (a) 84 (1984) 207. [9] H. Oesterreicher and F.T. Parker, J. Appl. Phys. 55 (1984) 4334. [10] T. Hashimoto, T. Numasawa, M. Shino and T. Okada, Cryogenics 21 (1981) 647. [11] A. Palenzona and A. Iandelli, J. Less-Common Met. 34 (1974) 121. [12] D.T. Cromer, A.C. Larson and R.B. Roof, Jr., Acta Cryst. B29 (1973) 564. [13] T.W. Hill, W.E. Wallace, R.S. Craig and T. Inoue, J. Solid State Chem. 8 (1973) 364. [14] S.K. Dhar, K.A. Gschneidner, Jr., W.H. Lee, P. Klavins and R.N. Shelton, Phys. Rev. B 36 (1987) 341. [15] K. Ikeda, K.A. Gschneidner, Jr. and A.I. Schindler, Phys. Rev. B 28 (1983) 1457. [16] T. Hashimoto, K. Matsumoto, T. Kurihara, T. Numazawa, A. Tomokiyo, H. Yayama, T. Goto, S. Todo and M. Sahashi, Adv. Cryogen. Eng. Mater. 32 (1985) 279. [17] W. R. Johanson, G. Pierce, C.B. Zimm and J. A. Barclay, J. Appl. Phys. 64 (1988) 5892.
A study of the magnetic properties of Gd3Pd 4 in applied magnetic fields S. T a n o u e Research and Development Division, Sumitomo Metal Industries, Ltd., Amagasaki 660, Japan and Ames Laboratory *, Iowa State University, Ames, 1.4 50011, USA
K.A. Gschneidner, Jr. and R.W. McCallum Ames Laboratory * and Department of Materials Science and Engineering, Iowa State-University, Ames, IA 50011, USA Received 11 March 1991; in revised form 11 July 1991
The low temperature heat capacity and magnetic susceptibility were measured on alloys of the Gd3Pd 4 and Gd4Pd 5 compositions. The GdaPd 5 sample was found to be a two phase alloy consisting of Gd3Pd 4 and Gd3Pd4-GdPd eutectic. The Gd3Pd 4 compound was confirmed to be antiferromagnetic with a N6el temperature of 18 K. Another magnetic transition was observed = 6 K which has characteristics of ferromagnetic or ferrimagnetic ordering. In applied magnetic fields up to 9.85 T, a heat capacity peak developed at --- 13 K while the 18 K peak shifted to lower temperatures in fields > 5.3 T. The entropy associated with the 13 K anomaly was estimated to be less than 2% of total magnetic entropy. The origin of this anomaly was thought to be due to some type of spin reorientation of some (few) of the antiferromagnetic moments in the applied field. The low temperature heat capacities of La3Pd 4 and Lu3Pd 4 were also measured. The electronic specific heat coefficients were found to be 10.5 and 5.72 mJ/mol K 2, respectively, and the Debye temperatures are 171 and 191 K, respectively.
1. Introduction Lanthanide compounds are thought to be suitable for magnetic refrigeration because the lanthanide atoms have a large magnetic moment compared to the transition metals. Gadolinium compounds are of special interest because gadolinium has no angular momentum, and therefore, crystal fields do not reduce the available magnetic entropy associated with magnetic ordering. * The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under contract no. W-7405-ENG-82. This work was supported by the Office of Basic Energy Sciences.
For the refrigeration materials, there are at least four requirements as follows: 1) the ordering temperature is 1 or 2 K below the desired operating temperature; 2) a suitable magnetic ordering of the lambdatype; 3) a large magnetic moment; and 4) a high thermal conductivity. Gadolinium gallium garnet (GGG) has been recommended as a refrigeration material by several authors [1,2]. But because of its low thermal conductivity, it may not be as useful as metallic gadolinium compounds which generally exhibit good thermal conductivities. GdPd, for example, is also a candidate for magnetic refrigeration for cooling down to about 40 K [3].
0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
130
S, Tanoue et al. / Magnetic properties ot'Gd3Pd 4
We were primarily interested in studying a ferromagnetic gadolinium compound which orders below 20 K, and according to the literature the compound Gd4Pd 5 appeared to be a good candidate. Loebich and Raub [4] measured the magnetic susceptibility from 140 to 720 K and reported a paramagnetic Curie temperature (O v) of 2 K and an effective moment (Peef) of 8.0/xu from its Curie-Weiss behavior. After preparing this alloy, metallographic analysis clearly revealed that the sample consisted of two phases - G d P d and Gd3Pd 4 - and that the reported phase diagram [5] is clearly wrong. Thus our efforts were concentrated on the compound Gd3Pd4, but the results reported in literature on its magnetic properties disagreed. Yakinthos et al. [6] reported Op = - 11 K and Peff = 8.09/-tB and no observed ordering down to 4.2 K; Gamari-Seale [7] (a co-author of the previous p a p e r [6]) reported the same Op and Pelf values but also noted small susceptibility maxima at 17 and 6 K, which shifted with applied fields; and Buschow and De Mooij [8] give Op = - 1 8 , Pelf = 8.8/X B and an observed N6el t e m p e r a t u r e of 18 K. Compounds of lower gadolinium concentrations would be expected to have lower ordering temperatures, but they were not studied because of an expected increase of the lattice entropy relative to the magnetic entropy, which is not favorable for magnetic refrigeration [9,10]. It is worthwhile, however, to study the Gd3Pd 4 compound because there is no data about its heat capacity and only limited results concerning its behavior under magnetic fields except for magnetic susceptibility m e a s u r e m e n t s [6-8].
2. Experimental procedures The rare earth metals used in this study were prepared at the Materials Preparation Center of the Ames Laboratory and the impurity levels in the lanthanum, gadolinium and lutetium metals are presented in table 1. The palladium was purchased from Westinghouse Materials of Ohio as 99.98% pure sponge which was electron b e a m melted before it was used in the preparation of the alloys for this study.
Table 1 C h e m i c a l analysis of s t a r t i n g rare e a r t h metals. Only impurities with 10 p p m a t o m i c or m o r e are listed. I m p u r i t y levels are ppm atomic Impurity
Lanthanum
Gadolinium
Lutetium
H C N O F Fe Ta W
1650 34 128 190 80 < 8 < 2 < 2
1090 170 67 766 < 25 19 l l) 20
346 189 37 130 < 27 78 < 1 < 6
The alloys were prepared by arc melting in an atmosphere of Ar gas. In the melting process, button shaped samples were turned over and remelted several times to ensure homogeneity. Then the buttons were remelted into 8 mm diameter cylindrical rods. The samples weighed about 10 g and the weight losses were less than 1%. Metallography results confirmed that the alloy of the Gd3Pd 4 composition was single phase, while the G d a P d 5 composition consisted of primary Gd3Pd 4 and G d 3 P d 4 - G d P d eutectic. At the same time X-ray diffraction confirmed the crystal structure of Gd3Pd4, which has a rhombohedral cell, with the space group R3, and is isostructural with Pu3Pd 4. The triply primative hexagonal lattice constants are a = 13.267(4) A and c = 5.716(5) A. These values were in good agreement with those obtained by other authors [8,11]. Heat capacity measurements were carried out at zero magnetic field from 1.6 to 80 K and in magnetic fields of 2.46, 5.32, 7.53, and 9.85 T from 1.6 to 22 K. Magnetic susceptibility measurements were made from 1.6 to 260 K using a Faraday m a g n e t o m e t e r at magnetic fields from 0.6 to 1.8 T. Magnetization measurements were performed in a Quantum Design squid magnetometer at fields from 0.1 to 5.4 T and temperatures from 2 to 70 K. For temperatures below 40 K the sample was warmed to 40 K and then cooled in zero field prior to measurement. For temperatures below 20 K, and at 24 and 28 K, measurements were made in both increasing and decreasing fields. At all other temperatures the samples were measured in an increasing field.
S. Tanoue et al. / Magnetic properties of Gd 3 Pd 4
The slopes of the magnetization curve at both high and low field were determined by a linear regression fit to the data using eleven data points between 4.4 and 5.4 T, and twenty data points between 0.1 and 2 T, respectively.
131
200 + La3Pd. - Lu3Pd.
150. ÷ ÷÷'~
_
e~
--'~ lO0,
_ -
~
3. Experimental results d~÷~'~ - -
The results of heat capacity measurements made on Gd3Pd 4 and "Gd4Pd5" from 1.6 to 80 K are shown in fig. 1. These curves show that G d 3 P d 4 has one magnetic phase transition at about 18 K, whereas Gd4Pd 5 has two transitions corresponding to the magnetic ordering of Gd3Pd 4 (18 K) and GdPd (37 K) [3]. To determine the magnetic contribution to the heat capacity, the lattice and electronic contributions to the heat capacity must be subtracted from the total heat capacity. To estimate these two contributions the heat capacities of L a 3 P d 4 and L u 3 P d 4 w e r e measured (see fig. 2 and table 2), and the electronic plus lattice contributions of Gd3Pd 4 were assumed to be equal to the average heat capacity of the former two compounds, bec a u s e L a 3 P d 4 a n d L u 3 P d 4 have the same crystal structure as Gd3Pd4, have no magnetic moments, and because the atomic numbers of lanthanum and lutetium lie seven atomic numbers on either side of gadolinium. The magnetic contribution of G d 3 P d 4 t o the heat capacity is shown in fig. 3. A close examination of the heat capacity curve from
50
+ +**_ - -
0
I
'
l0
I
'
20
I
I
30
'
I
'
40 50 T (K)
I
I
I
60
70
80
90
Fig. 2. The heat capacity of La3Pd 4 and Lu3Pd 4 from 1.5 to 80 K.
Table 2 Fit parameters to the low temperature heat capacities of La3Pd 4 and Lu3Pd 4 Compound La3Pd 4 Lu3Pd 4
•D
(mJ/mol K 2)
/3 (mJ/mol K 4)
(K)
10.5 +2.7 5.72+0.34
2.739+0.049 1.945+0.037
171+1 191+1
Y
2 to 18 K indicates that the curve has an unusual shape in that it bulges out towards the low temperature side, rather than the normal concave lambda-type shape. This suggests an additional magnetic ordering occurring between 5 and 7 K, which has been confirmed by magnetic suscepti-
200
5O
- Gd3Pd. " + Gd.Pds 1 5O o
o
I00-
E
.~* R-
•
50o
......... "
,,--"
~'''*"-"
30
2o10-
j ~ , ' ' ..... ""/"
/ 0 'Ib ' 2'0' dO' 4'0'5b ' ~0'7b' go' 90
T (K)
Fig. 1. The heat capacities of Gd3Pd 4 and a two phase alloy of the overall composition GdaPd 5 as a function of temperature.
-I0
0
Ib
2'0
3b
40
50
T (K)
Fig. 3. The magnetic contribution to the heat capacity of Gd3Pd 4.
S. Tanoue et al. / Magnetic properties of Gd3Pd ~
132
bility and magnetization measurements (to be discussed later). It is also noted that the magnetic heat capacity above T N (18 K) is excessive, more than might be expected from a typical lambda type antiferromagnetic heat capacity peak, especially above = 25 K. Normally one would expect the magnetic heat capacity to reach zero at = 2T N, 36 K in this case. Magnetic measurements do not indicate anything unusual for T > 30 K, so one can rule out long range magnetic order, although short range magnetic order might account for this excess heat capacity. According to the simple spin wave theory, the low temperature limit for a ferromagnetic contribution to heat capacity is proportional to T 3/2 and that of an antiferromagnetic contribution to heat capacity is proportion to T 3. The observed magnetic contribution, however, was approximately proportional to T 2, but since the lowest magnetic ordering temperature is = 6 K we may need to make measurements down to lower temperatures to determine the correct temperature dependence of the magnetic contribution to the heat capacity. The total magnetic entropy is shown in fig. 4. The calculated value up to 40 K is 16.41 J / m o l - G d K and is in good agreement with the theoretical value S = R ln(2J + 1 ) = 17.24 J / m o l - G d K, i.e. 95% of the theoretical value. The magnetic field dependence of heat capacity up to 22 K is shown in fig. 5. The heat capacity was measured at five
"O ¢.D I
20-
Theoretical
÷
2 o g
++ ÷
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-
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-/
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value
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o
'
Y
i
1o
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T (K) Fig. 4. The magnetic entropy per Od atom as a function of temperature.
50 * 0.00 T
5.32 T o 9.85 T
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2
,~*' ~
20.
100
/
,
,
0
~
°
1
5
,
i
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'
i
15 T (K)
'
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Fig. 5. The magnetic field dependence of the magnetic contribution to the heat capacity of Gd3Pd 4 as a function of temperature.
fields (0, 2.54, 5.32, 7.53 and 9.85 T), and the electronic and lattice contributions to the heat capacity were subtracted, assuming that these contributions are independent of magnetic fields. Only the data taken at 0, 5.32 and 9.85 T are shown, because the zero field and 2.54 T results are essentially identical, and so are the 5.32 and 7.53 T data. The data shown in fig. 5, reveals that a new peak in the heat capacity emerges at = 13 K at intermediate fields (5.32 and 7.53 T) while the 18 K peak is slightly smaller in magnitude but remains at 18 K. At the highest field (9.85 T) the 13 K peak essentially disappears and the 18 K peaks shifts down to = t5 K. The magnetic susceptibility as a function of temperature is shown in fig. 6. The shape of the susceptibility curve is characteristic of antiferromagnetic ordering with a Ndel temperature of 19.9 K. This temperature is in good agreement with the corresponding peak observed in heat capacity measurement. Below about 10 K, there is evidence for additional ordering with minor peaks at 9, 5.5 and 2.5 K, however, none are very distinct and may be due to scatter in the data. Gamari-Seale [7] also found evidence for at least one additional minor peak at 6 K, but her data were taken at large temperature intervals (only 7 data points below 20 K) compared to our measurements ( = 35 points). In order to clarify the situation a series of magnetization measurements were taken.
S. Tanoue et al. / Magnetic properties of Gd3Pd 4 0.8
0.8 . . . . . ...
0.61
•
o.s: i 0.3-
,~ O.
".
o.j
"1
*+., 0.41 . . . . . . . . .
x 0.2 ~
÷÷÷÷÷0
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~'÷÷'<...,.~
O. 1 o
.. 20 30 40
T ( K)
"~"'~'~
0
160
zd0
50
".-.+
3o0
T (K) Fig. 6. The magnetic susceptibility of Od3Pd 4 as a function of temperature. The inset shows an enlargement of the low temperature region. These data were taken on a Faraday magnetic susceptibility apparatus at a field of 0.656 T.
The squid magnetization data for T = 2 to 50 K and H = 0.1 to 5.4 T are given in fig. 7a. A number of features are worth noting. The first is that while the M vs. H curves are linear at high temperature, at low temperature there is a distinct change in slope. However, at both the lowest and the highest fields the M vs. H slopes are essentially constant for a given temperature. The point at which the slope changes moves to higher fields with increasing temperature. For a paramagnetic material, such a change of slope comes from a ferromagnetic impurity phase which is saturated in low fields. If this were the case, the point at which the slope changes would move to lower fields with increasing temperature since the coercivity of the ferromagnet will decrease with increasing temperature, however, just the opposite is actually observed. The low field slopes H < 2 T, and the high field slopes H > 4.4 T are shown in fig. 7b. For T > 8 K the low field slope is in good agreement with the results from the Faraday magnetometer (fig. 6). For T < 8 K, the difference between the low field squid results and the Faraday magnetometer results is attributed to the presence of hysteresis in the M vs. H curve. The squid result reflects the initial magnetization curve while the procedure used in the Faraday magnetometer corresponds to a minor hysteresis loop. The high field slope increases with increasing temperature up to a temperature of 54 K followed by a decrease to the Curie law value at
133
60 K. The data suggest that there is a competition between a ferromagnetic interaction and an antiferromagnetic interaction. In high magnetic fields the ferromagnetic interaction coupled with the field energy results in a field dependent ferromagnetic alignment while in low fields magnetic order is suppressed to lower temperatures with the antiferromagnetic interaction predominating at the 18 K ordering temperature but below about 8 K the ferromagnetic interaction takes over. The ferromagnetic nature of the low temperature ordering is clearly demonstrated by the presence of hysteresis at 6 K, and perhaps at 8 K. Above 50 K the magnetic susceptibility follows a Curie-Weiss behavior, and a least squares fit of the data gave a paramagnetic Curie temperature of - 9 . 5 K and an effective magnetic moment of 8.19/XB/Gd. These values are slightly different from those reported by Buschow and Mooij [8] (Op = - 18 K and Peff = 8.8~B/Gd), but in good agreement with those reported by Yakinthos et al. [6] (Op = - 11 K and Peff = 8.08/za/Gd).
4. Discussion
Gd3Pd 4 was confirmed to be antiferromagnetic with a N6el temperature of 18 K. But an additional anomaly was found in the magnetic contribution to heat capacity and also in the magnetic susceptibility and magnetization measurements at --- 6 K. These data indicate complex magnetic interactions occur in Gd3Pd4, which have sensitive field and temperature dependences. An examination of the crystal structure of Gd3Pd4, which is isotypic with Pu3Pd 4 [12], reveals that there is only one set of gadolinium positions in this structure - the 18 f (in the hexagonal system) set, which has no special symmetry (1). The bond distances for the Gd3Pd 4 should be essentially the same as for those reported by Cromer et al. [12] for Pu3Pd 4 because the a and c lattice parameters of Gd3Gd 4 are 0.58 and 0.49% smaller, respectively, than those of the Pu3Pd 4 compound. These authors report one short P u - P u ( G d - G d ) distance of 3.44 .& and two neighboring atoms at an intermediate distance of 3.75 A and two sets of two neighboring
134
S. Tanoue et al. / Magnetic properties of Gd ~Pd 4
1 40
'
~
40K 50K
i
2.0
0
4.0
6.0
Applied Field (T) 0.9
mm m
m Low Field + High Field
m
m
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m m m m
111 [] m
¢~
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[] []
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,
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0.2
0
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'
,
60
T (K) Fig. 7. (a) Magnetization measurements of Gd3Pd 4 from 2 to 70 K at 2 K intervals. An offset of 500 e m u / m o l per 2 K interval has been added to the data except for 70 K curve. (b) Magnetic susceptibility (dM/dH) as a function of temperature from 2 to 70 K from linear regression fit for low (0.1-2 T) and high (4.4-5.4 T) fields. The error bars represent the standard error from the regression fit.
atoms each at 3.95 and 3.97 .~, which compares to a bond distance of 3.60 ~, in pure Gd metal. Thus there are several different local environments
around a gadolinium atom, just with respect to other gadolinium atoms, and this alone could account for a variety of magnetic ordering tern-
135
S. Tanoue et al. / Magnetic properties of Gd3Pd 4
50 45
÷: 0.00 T -:2.54 T ,~:5.32
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°:7.53 T o : 9.85 T -"
40
,, :,
~
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i
I
2
4
6
8
10
Applied Field (T) ÷
Fig. 9. The field dependence of the high temperature N~el ordering temperature.
*f"~
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230
.... ""
25 20
that the heat capacity sample had a preferred orientation in which the (001) directions were perpendicular to the cylinder axis.
'\\ h,,,o~ ~'~
.
,
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.
,
.
,
.
,
,
,
,
,
.
,
.
,
.
"'~oo , .
12 ]3 14 15 16 17 18 19 20 T (K)
~, : 5.32 T o: 7.53 T o:9.85 T
4
Fig. 8. The magnetic contribution to the heat capacity of Gd3Pd4 as a function of temperature in magnetic fields parallel to the sample axis (top) and perpendicular to the sample axis (bottom). peratures, without considering the influence of the neighboring palladium atoms. In order to check if the crystal orientation had an affect on the anomalies observed in the high magnetic heat capacities, the heat capacity was also measured such that the magnetic field was in a direction perpendicular to the cylinder axis of the heat capacity sample (the initial measurements were made with the magnetic field parallel to the cylinder axis). The results of these measurements are shown in fig. 8. The change in the upper ordering temperature ( = 18 K) with applied magnetic field is essentially the same for either orientation of the sample in the field, see fig. 9. The general shapes of the 13 K peak did not change, however, some shifts in temperature and magnitudes are evident. This suggests that the magnetic interactions which cause the induced 13 K peak shown in fig. 8 have an orientation dependence. This could arise from a preferred orientation in the crystal which occurred during solidification. X-ray diffraction showed
o2x /i\
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'-6 E
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o:7.53
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o: 9.85 T
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,
8
9
,
,
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,
,
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,
,
,
12 13 14 15 16 17 18 T (K)
Fig. 10. The excess portion of the magnetic heat capacity of the 13 K (H = 5.3 T) peak as a function of temperature in the field parallel to the sample axis (top) and perpendicular to the sample axis (bottom).
S. Tanoue et al. / Magnetic properties of Gd3Pd 4
136
Table 3 Entropy associated with magnetic heat capacity anomaly at 13 K for fields greater than 5.32 T Field
5.32 7.53 9.85
Entropy (J/moI-Gd K) parallel direction
perpendicular direction
0.21 0.24 0.15
0.28 0.21 0.19
To evaluate the heat capacity associated with the 13 K anomaly, the fitting functions were made on either side of the peak and the excess portion of the magnetic heat capacity was obtained by subtraction of the fitting values from the observed C M values shown in fig. 8. The excess portions of the magnetic heat capacity are shown in fig. 10. The entropy associated with heat capacity anomalies are tabulated in table 3. The entropy associated with the anomalies is approximately 1.5% of total magnetic entropy. From the preferred orientation deduced from the X-ray measurements, the anomalous heat capacity results obtained when the applied magnetic field was oriented parallel to the sample axis corresponds to the influence of this field acting predominantly in the basal plane direction of the polycrystalline aggregate. Whereas when the magnetic field is applied perpendicular to the sample axis the field is predominantly parallel to c-axis of the crystallites. Unfortunately, because we are working with a polycrystalline material, which undoubtedly has complex magnetic interactions within a unit cell, it is difficult to come to any quantitative conclusions, except to say that there is a difference in the interaction of the magnetic moments of the gadolinium atoms with the direction of the applied magnetic field. The variation of the excess entropy of this 13 K anomaly with applied magnetic field could be a Fermi surface effect. That is, parts of the Fermi surface are destroyed (decreasing the entropy) or enlarged (increasing the entropy) by the field when it is increased from one value to another in the 0 to 9.85 T regime. A more likely possibility is that some sort of spin reorientation of one or more of the various antiferromagnetic alignments in Gd3Pd 4 is occurring as the field is increased.
The entropy arises when the thermal energy is sufficient to overcome the magnetic energy (which caused the moment to align ferromagnetically at some applied field) and allows the spins to relax and to realign antiferromagnetically at the high temperature. As one increases the magnetic field a smaller fraction of the spins reorient, because the larger magnetic energy holds more of the spins in a ferromagnetic arrangement; and this is what is observed (see fig. 10 and table 3). This would also be consistent with the orientation dependence. Similar anomalies have found in ferromagnetic HoA12 [13], and the paramagnetic materials CeSi, (x -- 1.9) [14] and N i - P d alloys containing small quantities of Ni, ~ 1 at% [15]. In the paramagnetic materials the anomalies are attributed to the induced magnetic moments by the applied field. The entropy associated with the anomalies are order of 2 m J / g - a t o m K which is less than one hundredth of the entropy observed in our experiments. However, for these two materials, there is only one f or d electron per atom involved while for gadolinium there are seven f electrons. But even taking this into account the excess entropy in Gd3Pd 4 is still about 30 times larger than that observed in CeSi x and N i - P d alloys. Hill et al. [13] measured the heat capacity of TbAI 2 and HoAl 2 at zero applied field and found a heat capacity anomaly in HoA12. They concluded that the entropy associated with anomaly was only 3% of the total magnetic entropy. This value is of the same order of magnitude as our value, except that the anomaly in Gd3Pd 4 only emerges in a field more than 5 T. Hashimoto et al. [16] also found the heat capacity anomaly in HoA12 in both zero field and in applied fields. The authors thought that the anomaly might be attributed to complicated change of its magnetic structure. In summary there are three possible explanations for this anomaly as discussed above: 1) the Fermi surface effect; 2) the induced moment; and 3) spin reorientation. The Fermi surface effect is a possibility which cannot be ruled out altogether, but we think it is not as likely as the spin reorientation explanation, because it involves a change
S. Tanoue et al. / Magnetic properties of Gd3Pd4
6
+ : 5SM(9.85T) : 5SM(7.53T) o : 5SM(5.32T)
/ o
~
2
-r-
..+...÷---+---+""¢ fl .÷.: .~., _z~....t~...,x .ix , . a . a m-
v
•
~o -2
I 5
~ lO T (K)
i 15
I 20
,
Fig. 11. The temperature d e p e n d e n c e s of the magnetic entropy change ASM(T, H) of G d 3 P d 4 in various magnetic fields. [ASM(T, H ) = SM(T, 0)- SM(T, H)].
in the Fermi surface topology as a function of applied field and such changes are not commonly known. The second explanation is ruled out because the observed entropy associated with the anomaly is a factor of 30 times larger than that found in other materials which exhibit this kind of anomaly. We believe that a spin reorientation of antiferromagnetic moments is the most reasonable explanation. This has also been verified in the magnetic susceptibility (derived from the high field magnetization slopes) which exhibits a weak broad peak at = 13 K (fig. 7b). Both the heat capacity and the magnetic susceptibility essentially do not show appreciable change from the zero field data in fields less than 2.5-3 T, but above 3 T significant changes are observed. A measure of the effectiveness of a magnetic refrigerant is the change in entropy due to increasing the field from 0 to Hmax. The larger the change the better the refrigerant with all other things being equal. The variation of the entropy change upon applying a magnetic field to Gd3Pd 4 is shown in fig. 11 as a function of temperature from 0 to 21 K. The change in entropy is not particularly large which is typical for antiferromagnetic materials, i.e. 10-20% less than that of a ferromagnet. For example, = 1 K above the ordering temperatures of Gd0.o6Ero.94A112 the entropy change due to the magnetic field change from 0 to 5 T is 4 J / m o l K and 5 J / m o l K from 0 to 8 T [17]. The comparable values for GdaPd 4
137
are 0.5 J / m o l K from 0 to 5.32 T and 2.2 J / m o l K from 0 to 7.53 T, but since there are three Gd atoms in a mole these values need to be divided by three giving 0.17 and 0.73 J/mol-Gd K. It is noted that the magnetic field changes are not identically the same and that the magnetic atoms are different and so there is a larger entropy associated with erbium (23.1 J/mol-Er K) than Gd (17.3 J/mol-Gd K) if all the entropy is involved in the magnetic ordering process and none is due to crystal fields effects which is likely for Er, but not for Gd since it has a fully occupied 4f half-filled shell. These additional factors, however, cannot account for the large observed differences. These observations are consistent with molecular field theory calculations which show that the magnetic entropy changes due to increasing the magnetic field is significantly larger for a ferromagnet than for an antiferromagnet for the same magnetic atom.
5. Conclusion
Heat capacity and magnetic susceptibility measurements, X-ray diffraction and metallography on alloys of the Gd3Pd 4 and Gd4Pd 5 compositions confirmed that the latter is not a single phase alloy and consists of primary Gd3Pd 4 and Gd3Pd4-GdPd eutectic, and that the former is a single phase alloy which orders antiferromagnetically at 18 K with a second magnetic transition at = 6 K which is ferromagnetic or ferrimagnetic in origin. The effective magnetic moment Gd3Pd 4 is 8.19/z J G d atom. Heat capacity measurements on Gd3Pd 4 in applied magnetic fields revealed a magnetic anomaly at --- 13 K in an applied magnetic field greater than 5 T. The entropy associated with the anomaly is less than 2% of total magnetic entropy. The anomaly is thought to be due to a spin reorientation of some of the antiferromagnetic moments in an applied field. Finally, our data show that Gd3Pd 4 is not a good magnetic refrigerant, as would be expected for an antiferromagnetically ordering material.
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Acknowledgements The authors wish to thank Dr. J. Tang and Dr. V. Pecharsky for their valuable comments and suggestions, and J.O. Moorman for his assistance in heat capacity measurements.
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