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Magnetic and resonance studies of the laves phase compounds GdT2 (T ≡ Rh, Ir, Pt, Ru, Mn)
Journal of the Less-Common
P7
Metals, 78 (1981) P7 - P19
MAGNETIC AND RESONANCE STUDIES OF THE LAVES COMPOUNDS GdTa (T = Rh, Ir, Pt, Ru, Mn)
PHASE
A. TAR1 Department
of Physics, University of Petroleum and Minemls, Dhahmn (Saudi Arabia)
C. LARICA The Blackett Labomtory, Physics Department, Imperial College of Science and Technology, Prince Consort Road, London SW7 2B.Z (Gt. Britain) J. POPPLEWELL School of Physical and Molecular Sciences, Gwynedd LL5 2UW (Gt. Britain)
University College of North Wales, Bangor,
(Received September 5, 1980)
Summary A detailed study was made of the Laves phase compounds GdTa (T = Rh, Ir, Pt, Ru, Mn). The study was carried out by observing the spinecho nuclear magnetic resonance (NMR) of 165H~, which was introduced as a dilute substitutional impurity, the electron spin resonance (ESR) of Gd3’ and magnetic measurements. The NMR of holmium reveals the presence of a strong crystalline electric field in these compounds, particularly in GdRh, and GdMns. Except for GdMn,, for which the extraction of ESR parameters was not possible, the investigation suggests that the gadolinium ESR is bottlenecked in all the compounds studied. Both the magnetic and the ESR studies of GdMna reveal magnetic and structural phase transitions at 283 K and 106 K respectively. The g shift is negative for gadolinium in GdRua for which a saturation moment of (6.31+ 0.15)~~ is found. In other alloys the g shift is positive and the saturation moment for gadolinium is found to be approximately 7~~ which is considerably higher than values reported earlier and in excellent agreement with values reported more recently.
1. Introduction A brief glance at the literature data reveals a considerable lack of information about the RTa compounds where R is a rare earth and T is a 4d or 5d transition metal. This is partly because of the costliness of the transition metals and partly because of their non-magnetic character. So far, there0022-5088/81/0000-00001$02.50
@ Elsevier Sequoia/Printed in The Netherlands
P8
fore, much greater effort has been directed towards the alloys of rare earths with the iron group metals. Early magnetic measurements on the intermetallic compounds of rare earths with 4d and 5d transition metals are incomplete; this is the case even for the gadolinium compounds which have received considerably more attention. In most cases the paramagnetic Curie temperatures 8, and the effective moment Pelf of the rare earth ions are not given (see, for example, ref. 1). Furthermore, recent studies of GdRhs by Hrubeck and Steinen [2] and Tari and Larica [ 31, and of GdPta by Dormann et at. [ 41 have revealed saturation moments for gadolinium in these compounds that are rather larger than those previously reported [5,6] . As in the magnetic measurements, electron spin resonance (ESR) studies of gadolinium in GdTa compounds are not complete and are not conclusive. For example, the g values reported for gadolinium in the same compound differ considerably from one author to another (see, for example, ref. 7). Moreover, recent ESR studies of GdRhs [ 31 and GdIra [ 81 compounds have shown that the gadolinium ESR in these compounds is bottlenecked, although neither bottlenecking nor positive g shifts are normally expected in compounds with strong d-band character. Studies of the hyperfine interaction by means of nuclear magnetic resonance (NMR) in GdTa inter-metallic compounds have so far been restricted mainly to the NMR of gadoli~um [4, 9 - 111, but since ~doli~um is an S-state ion such studies cannot give info~ation about the crystalline electric field in these compounds. It is of interest therefore to use hohnium as a dilute substitutional impurity for the study of the hyperfine interactions. Accordingly, we investigated the Laves phase compounds of gadolinium with rhodium, iridium, platinum, ruthenium and manganese, mostly with a small amount of holmium substituted for gadolinium, by means of NMR, ESR and magnetic measurements. The inclusion of results for GdRhs, which have been published previously, is for the purpose of comparison. The GdMn:, compound is of interest because of the uncertainty of the nature of the magnetic ordering of the RMna compounds [l] and the present work contributes to the info~a~on available, ~~0~~ some uncertainty still exists.
2. Samples and experimental techniques The compounds were prepared by arc melting of the constituents under 1 atm of argon gas. For manganese compounds approximately 3 at.% extra manganese was added prior to the melts in order to compensate for the manganese deficiency arising from weight loss. Also the pressure was considerably higher to reduce weight loss. In the remaining compounds the weight loss was negligible. A total of ten compounds were made and were investigated. With the exception of two compounds, a second sample of
P9
GdMna and GdRuz, they contained approximately 1 -3 at.% Ho. The alloys were melted and remelted a number of times and X-ray and metallographic investigations did not reveal any detectable second phases. Therefore heat treatments were carried out only on GdMns and GdRuz, on GdMn, to eliminate any effect which may be attributed to strain or metallurgical defects and on GdRus to ensure the stabilization of the Cl4 phase. On the holmium-free GdMnz sample a hardness test was also carried out and a search was made for unreacted gadolinium in the compound; this gave negative results. Magnetic measurements were made using a vibrating sample magnetometer in the temperature interval 4.2 - 300 K and ESR measurements were made by means of an X-band spectrometer on powder samples at temperatures in the range 77 K < T < 300 K. The NMR measurements were made in zero applied field on powder samples at 1.2 K by means of a phase coherent microwave spin-echo spectrometer operating between 6 and 7 GHz. For the magnetic and ESR measurements the temperature sensors used were Au-O.O3at.%Fe/chromel thermocouples. The saturation magnetizations were obtained at 4.2 K from a plot of the magnetization I against the inverse l/H of the applied field extrapolated to infinite applied field. The Curie temperatures of the samples were obtained from 12-H/I isotherms and were supplemented by the plots of I2 against Tc - T.
3. Results and discussion 3.1. Magnetization and susceptibility measurements The reciprocal mass susceptibilities of the compounds with the exception of GdMn2 are shown in Fig. 1. The inverse susceptibilities vary linearly with temperature over the entire paramagnetic range and the ferromagnetic Curie temperatures of the samples coincide with their paramagnetic Curie temperatures as shown in Table 1. This is in accord with the molecular field model where Tc = OP, x-l = T - Tc and I2 = Tc - T. Also listed in Table 1 are the saturation moments per mole of the compounds and the effective moments of the rare earth ions in the paramagnetic region. The effective moments of gadolinium in GdRh2 and GdRu2 are close to the free ion value of Gd3’, gp, {S(S + 1)}1’2 = 7.9411, (formula unit)-‘, but are considerably higher in GdIr2 and GdPt2. The saturation moments per formula unit of all the compounds are rather larger than those previously reported by various authors [ 5,6] and are in excellent agreement with those obtained recently for gadolinium in GdRh2 [ 21 and GdPt2 [4]. The ordering temperatures of the compounds are in good agreement with those formerly reported except that for GdIr2. The value that we found for the Curie temperature of this compound (Tc = 78 f 1 K) is substantially lower than that previously reported [6,12,13] (Tc = 89 K) but falls on the graph of the Curie temperatures of RIr2 compounds against the de Gennes factor (Fig. 2) (see also ref. 14).
PlO
Fig, 1. The reciprocal mass su~eptibilities of four of the
Ho0+01Gdo.e9Rhe; a, Ho0,0sGdo.a&; A, HoomGdo.w%
compounds&died: o, X, Hoo.o3Gdo.w%-
With the exception of that of GdIvIna,the helium temperature magnetizations of ail the alloys are almost saturated at 9 kG, the highest field that was available to us. The compound GdMna has a very itching behaviour. On lowering the temperature it first undergoes a weak magnetic transition at about 283 K (Fig. 3). The magnetization rises faJrly rapidly around this temperature, suggesting either the setting in of a short-range order or correlated moments. The ESR measurements also indicate a transition at this temperature (see Section 3.3). This transition has not previously been reported. Eelow 283 K the slope of the inverse susceptib~ity decreases progressively with decreasing temperature, which suggests a gradual increase in the size of the correlated regions. At about 106 K the compound undergoes a structural phase change, indicated by a sudden discontinuity in the susceptibility. Simultaneously the inverse s~eptib~~ takes a hyperbolic form typical of a fe~ma~et (Fig, 4). AIthougb the transition at 283 K may suggest the presence of a small amount of a gadolinium-rich solid solution with a Curie temperature just below that of gadolinium, we failed to detect such a phase in the compound, as has already been stated in Section 2. Furthermore the magnetization against field graph at room ~mpemt~e is a straight line and that at 265 K does not indicate the presence of any foreign phase. The saturation moment of this compound is 3.16~ra mol-I. If we assign a value of 7~~ to gadolinium in the ordered state, then the moment
Pll
co F1
Fi-
P12
T, (Kl 100 -
.
90 80 70 -
60
I2
I
50
IO
i
8
.
‘I
-L
-2
Fig. 2. A comparison of the Curie temperatures of the RIrz compounds with the de Gennes factor G = (g - 1)2J(J + 1): X, de Gennes factor; 0, Curie temperatures; 0, the Curie temperature of Ho-GdIr2 obtained in this work.
180
200
220
240
260
i 280
TM
1
Fig. 3. The temperature variation of the magnetization of GdMn2: 0, H = 0.8 kG; 0, H = 3.21 kG.
on a manganese ion is 1.92clg. This is a relatively small moment for manganese, and is probably a result of both the filling of the manganese d band and the antiferromagnetic coupling between manganese moments which are coupled antiparallel to the gadolinium moment.
P13
l..._o...
.
50
0
.
.
.
100
.
.
150
.
..I....1
200
T(K)
Fig. 4. The variations with temperature of the magnetic susceptibility and its inverse. for GdMnz: 0, H = 0.4 kG; 0, H = 0.8 kG.
3.2. NMR measurements The total hyperfine field (HF) of a rare earth nucleus in a metallic host may be written as
B=B,
+B, +Bt
(1)
where BI is the intraionic HF arising from the orbiting of the 4f electrons, B, is that arising from the conduction electrons polarized by the spin of the probe rare earth ion and Bt is the total transferred hyperfine field (THF) arising from both the nearest-neighbour and the distant rare earth ions. The quadrupole parameter p is similarly made up of contributions from 4f electrons, conduction electrons and the lattice. The magnetic dipole parameter a is related to the HF B through the relation
where yn is the nuclear gyromagnetic ratio of the probe ion (y,/2n = 8.77 MHz T-’ for holmium). ar, up and a, are related to BI, B, and Bt respectively in a similar way. The intraionic hyperfine parameters aI and pr are related to (J,) and (Jz2>through the relations
(J,, a1 = a0 J and
3(Jz2>- J(J + 1) PI =
J(2J-1)
PO
(4)
P14
where J = 8, a0 = 6497 f 8 MHz and p. = 62.7 + 3 MHz for holmium [15]. If the THF of a rare earth ion R is B(R) then that of a probe ion, in this case hohnium, in the same compound may be reasonably estimated using the hyperfine coupling parameters A listed by Campbell [ 161 through the relation
AWo)
B,(Ho) = -B,(R) A(R)
(5)
The contribution to the HF of the conduction electrons polarized by a rare earth ion is expected to be proportional to the z component of its spin S, since this polarization is principally caused by the exchange interaction. Therefore we may write
B* =KpCSz~=K*d~ where S is the spin of the rare earth ion and I&, is a constant. In this expression use is made of Gs,)/S = (J,)/J which applies to the heavy rare earth metals. Using the data obtained by Dormann and coworkers [4,10,11] for 165Gd, ‘“Gd and 13’La in Gd1_,LhT2 (T 1 Rh, Ir, Pt) we estimated B,(Ho) and B,(Ho). Using these values in eqns. (5) and (6) we obtained (J,) and hence Or>= gpg(Jz) for holmium in these compounds. These are listed in Table 2. TABLE 2 The hyperfine parameters for 165H~ in the compounds Ho-GdTz
Compound
a* (MHz)
p (MHz)
(T = Rh, Ir, Pt, Mn)
(Estimated moment holmium)lpB
(I,, (T)
at (T)
9 * 0.02 9.35 f 0.3 9.5 f 0.03 -
4.98 i 0.42 3.04 i 0.44 4.80 * 0.38
14.51 5 0.03 12.15 f 0.06 7.1 i: 0.1 -
for
_ Ho-GdRha Ho-GdIr2 Ho-GdPt2 Ho-GdMng
6000 (6205 (6250 6250
* * i *
1 2%) 2%) 2
51.8 f 0.5 73 f 3 60 f 1 53*1
The NMR samples contained 1 at.% Ho. a The uncertainty in a was usually less than 3 MHz; the uncertainties in the values in parentheses arise from very weak NMR signals as a result of which the lines could not be labelled with absolute certainty.
The hyperfine parameters p and a, and hence the bare moment, are lowest for holmium in GdRh2. The quadmpole parameter p of holmium in GdMn2 is also very low. If we expect 6p/p = nsa/a where n = 2, then a considerably lower value for a would be expected for holmium in this compound. Unfortunately sufficient data on GdMn2 are not available and therefore the individual contributions to the total HF cannot be estimated.
P15
Although the room temperature structure of this compound is cubic, it undergoes a s~~~~ phase change at about 106 K. The extraionic cont~bu~o~ to p, therefore, may be significant at low temperatures. In any case, the evidence from both a and p is that the moment of holmium in this compound is considerably reduced. This may be because of crystal field quenching or moment transfer from manganese to halmium as suggested by Chamberlain [17j _ The hyperfine parameters of GdPtz are not of great interest. The quadrupole parameter of holmium in Cdlkz, however, is quite large and remains a puzzle. 3.3. ESR meaurements The temperature variations of the g value and the breadth AN of the alloys, with the exception of GdMn2, are shown in Figs. 5 - 8. In the paramagnetic region the linewidths AN vary linearly with temperature and may conveniently be expressed as AH = 4 + bT. Table 3 lists the g shifts Ag, the thermal gradient b of the linewidth, the temperature ‘-r,, of the linewidth minimum and the estimated densities of s and d electrons at gadolinium sites where available [lo, 11 J . The saturation moments g, per mole of the samples are also included in this table to show the correlation between Ag and p,.
9 -0
z
2.06 2.02 l
1.92 W
i
ma
150
xx0
250
300
XKI
150
200 '
250
300
iti(>
1300 llOO9# 700 500 I
@I :,
loo
200
300
4K) '
(b)
%EGr--
Fig. 5. The temperature dependence (a) of the g value and (b) of the linewidth of HoGdRh,, Fig. 6. The temperature GdIr2.
dependence (a) of the g value and (b) of the linewidth of Ho-
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Fig. 7. The temperature GdPt2.
dependence
(a) of the g value and (b) of the linewidth
of Ho-
Fig. 8, The temperature
dependence
(a) of the g value and (b) of the linewidth
of GdRuz.
In any RTa compound where R can be replaced by gado~inium~a bottleneck is IikeIy to develop as the composition of pure GdTa is approached simply because the total gadolinium magnetization is becoming too large to be disposed of easily to the lattice. The dominance of the selectron density over the d-electron density is a further factor assisting the occurrence of a bottleneck since an s-electron band is blosefy coupled to the 4fv ion by a similar g value. The d electrons, however, are tightly bound to the lattice and hence are efficient in transferring the energy they receive from the relaxing probe ion to the lattice. The results in Table 3 therefore suggest that a deep bot~en~k~g should be expected for the gadolinium ESR in GdPt2. These results further suggest the presence of a shallow bottlenecking for the gadolinium resonances in GdRhs and GdIrs which has been confirmed by Tari and La&a f3] for GdRha and by Chiu et al, [S] for GdIra, Both the g shift Ag and the temperature gradient b of the linewidth for gadolinium in Ho-GdRha and Ho-GdPts are in agreement with those reported by Taylor and Coles [18] and Taylor et al. [ 19f for GdRhz and GdPta respectively. Gur ESR studies of Ho-GdRu2 and GdRua also give similar values for Ag and b (see Table 3). These results suggest that the presence of holmium does not affect the ESR parameters in any drastic way. There is, however, a sufficiently clear trend in the g values. The g value is ~onsis~utly higher in the alloys containing holmium, though the difference
7 f 0.05
+ 0.048 f 0.01 + 0.028 * 0.01
-0.01 f 0.008 -0.025 f 0.01
Ho-GdPt2
Ho-GdRu2 GdRu2
The g shifts are relative
G*2
7.16 * 0.15
+ 0.018 i 0.01 -0.002 ?r 0.007
to the gadolinium
10.5 f 0.5 7.8 f 1
5 f 0.5 6.2
5.5 f 3 9.5
2.6 i 0.2 3 f 0.5
d(AWldT (G K-l)
in the compounds
101% 3 100 f 5
67
10 80
es0
-65
105 f 3 @ 100 lOO*
Density of s electrons Ps (cfl,)
20
5550
=35
Pd @‘I
Density of d electrons
and GdTz (T = Rh, Ir, Pt, Ru)
Twin (K)
Ho-GdTz
free ionic value of g = 1.992.
6.31 f 0.15 -
f 0.05
Ho-GdIr2 GdJ.r2
6.96
+ 0.018 f 0.007 + 0.013 t 0.01
Psl~B
ESR parameters
Ho-GdRhz GdRh2
of the gadolinium
&
3
Compound
Summary
TABLE
19
8
18
Reference
P18
is very small. This is in accordance with the study of Re,&Zdo_ozPdess by Peter et al. [ 201, where R is a rare earth metal. They have found that the presence of a heavy rare earth impurity leads to a positive shift of the gadolinium g value from that for the binary PdGd alloy and that the presence of a light rare earth leads to a negative shift; the shift for 2% Ho is about 0.1. These shifts have been attributed to the strong response of the exchange-enhanced d band of the palladium. The shifts due to holmium in the compounds investigated here are much smaller than those in the palladium alloy. This is in keeping with the relatively weak enhancement of the 4d and 5d transition metals. Electron transfer from rare earths to the d bands of the transition metals in these compounds may also suppress the weak enhancement still further by filling the band. The saturation moment ps of the magnetic ion, in this case gadolinium, is related to the g shift through the relation ps =gp,Gs,)
= ELO + Agp,(Sz)
(71
where p. = gipk(Ss) is the bare moment of gadolinium and CS,> is the expectation value of its spin operator; gi is the ionic g value of gadolinium. The g shift therefore reflects the excess or deficiency in the saturation moment gs over the free ion value ,uo. Although it may be made somewhat less clear by the bottlenecking of the gadolinium ESR in these compounds, this correlation between g shift and saturation moment is sufficiency evident in Table 3. For gadolinium in GdRua a negative g shift and a value of 6.31,~~ is obtained for the saturation moment. In the remaining compounds the g shifts are positive and the saturation moment for gadolinium is approximatelY 7cLB. The g shift of gadolinium may be expressed as [ 211 &r = Ag,
+ Ag,‘d) = JRpR + J$d)~~(d)
(8)
where Aga is the g shift due to s and d electrons at the gadolinium site, AgTtd) is the shift due to d electrons at the T site, pR and prCd)are the densities of states and Ja and JTtd) are the exchange couplings of the gadolinium and conduction electron at the gadolinium site and the T site respectively. JR is positive since it is the intraionic exchange of gado~~ sites while Jr(“) is negative because of the non-o~ogon~~ of f and d electrons originating from different sites. The results in Table 3 are in satisfactory agreement with this statement. The g shift for gadolinium in LaRua is large, -0.17 [ 211, and much greater than our value for gadolinium in GdRus and Ho-GdRuz. Also the temperature gradient of the line has been found to be 24 + 5 G K-l for gadolinium in LaRua which is higher than the values we measured for our two ruthenium compounds. This implies that the gadolinium ESR in GdRua is bottlenecked as may be expected for an alloy so rich in gadolinium although the bottlenecking may not be as severe as in, say, GdAls. Indeed for gadolinium in YRua, where no bottlen~k~g is expected, Barberis et al. [21] have measured ag value of 1.96, i.e. a g shift of -0.032 f 0.02, a value not too different from ours.
P19
The ESR of gadolinium in GdMnz proved to be very complicated and the extraction of any meaningful values for the ESR parameters proved to be difficult, if not impossible. However, it confirms the transitions at 233 and 106 K found by magnetic measurements. In conclusion this investigation suggests that in the compounds under consideration the gadolinium ESR is bottlenecked. The saturation moment for gadolinium is approximately ‘7~~ and its g shift is positive except in GdRus. In this compound the g shift is negative and the saturation moment is 6.31~~. The ESR of gadolinium in GdMnz is difficult to analyse but it confirms the transitions revealed by magnetization and susceptibility measurements.
Acknowledgment We are grateful to Professor B. R. Coles for his critical reading of the manuscript.
References
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
W. E. Wallace, Rare-Earth Zntermetallics, Academic Press, New York, 1973, Chap. 12. J. Hrubeck and W. Steinen, J. Phys. (Paris), CoNoq. C5, 40 (1979) 198. A. Tari and C. Larica, J. Phys. (Paris), 41 (1980) 35. E. Dormann, M. Huck and K. H. J. Buschow, 2. Phys. B, 27 (1977) 141. J. Crangle and J. W. Ross, Proc. Znt. Conf. on Magnetism, Nottingham, 1964, Institute of Physics and The Physical Society, London, 1964, p. 240. R. M. Bozorth, B. T. Matthias, H. Suhl, E. Corenzwit and D. D. Davis, Phys. Rev., 115 (1959) 1595. R. H. Taylor, Adu. Phys., 24 (1975) 681. L. B. Chiu, P. R. Elliston, A. M. Stewart and K. N. R. Taylor, J. Phys. F, 9 (1979) 373. R. E. Gegenwarth, J. I. Budnick, S. Skaiski and J. H. Wemick, Phys. Rev. Lett., 18 (1967) 9. E. Dormann, M. Huck and K. H. J. Buschow, J. Magn. Magn. Mater., 4 (1977) 47. E. Dormann, L. Schaafhausen and K. H. J. Buschow, J. Magn. Magn. Mater., 2 (1976) 177. L. B. Chiu, P. R. Eiiiston, A. M. Stewart and K. N. R. Taylor, J. Phys. F, 9 (1979) 955. V. B. Compton and B. T. Matthias, Acta Crystallogr., 12 (1959) 651. B. T. Matthias, Z. Fisk and J. L. Smith, Phys. Lett. A, 72 (1979) 257. B. Bleaney, in R. J. Elliot (ed.), Magnetic Properties of Rare-Earth Metals, Plenum, New York, 1972, Chap. 8. I. A. Campbell, J. Phys. C, 2 (1969) 1338. J. R. Chamberlain, Physica B, 86 - 88 (1977) 138. R. H. Taylor and B. R. Coles, J. Phys. F, 5 (1975) 121. R. H. Taylor, I. R. Harris and W. E. Garden, J. Phys. F, 6 (1976) 1125. M. Peter, D. Shaitiel, J. H. Wernick, H. J. Williams, J. B. Mock and R. C. Sherwood, Phys. Rev. Lett., 9 (1962) 50. G. E. Barberis, J. P. Donoso, C. Rettori and D. Davidov, J. Less-Common Met., 70 (1980) P69.
P7
Metals, 78 (1981) P7 - P19
MAGNETIC AND RESONANCE STUDIES OF THE LAVES COMPOUNDS GdTa (T = Rh, Ir, Pt, Ru, Mn)
PHASE
A. TAR1 Department
of Physics, University of Petroleum and Minemls, Dhahmn (Saudi Arabia)
C. LARICA The Blackett Labomtory, Physics Department, Imperial College of Science and Technology, Prince Consort Road, London SW7 2B.Z (Gt. Britain) J. POPPLEWELL School of Physical and Molecular Sciences, Gwynedd LL5 2UW (Gt. Britain)
University College of North Wales, Bangor,
(Received September 5, 1980)
Summary A detailed study was made of the Laves phase compounds GdTa (T = Rh, Ir, Pt, Ru, Mn). The study was carried out by observing the spinecho nuclear magnetic resonance (NMR) of 165H~, which was introduced as a dilute substitutional impurity, the electron spin resonance (ESR) of Gd3’ and magnetic measurements. The NMR of holmium reveals the presence of a strong crystalline electric field in these compounds, particularly in GdRh, and GdMns. Except for GdMn,, for which the extraction of ESR parameters was not possible, the investigation suggests that the gadolinium ESR is bottlenecked in all the compounds studied. Both the magnetic and the ESR studies of GdMna reveal magnetic and structural phase transitions at 283 K and 106 K respectively. The g shift is negative for gadolinium in GdRua for which a saturation moment of (6.31+ 0.15)~~ is found. In other alloys the g shift is positive and the saturation moment for gadolinium is found to be approximately 7~~ which is considerably higher than values reported earlier and in excellent agreement with values reported more recently.
1. Introduction A brief glance at the literature data reveals a considerable lack of information about the RTa compounds where R is a rare earth and T is a 4d or 5d transition metal. This is partly because of the costliness of the transition metals and partly because of their non-magnetic character. So far, there0022-5088/81/0000-00001$02.50
@ Elsevier Sequoia/Printed in The Netherlands
P8
fore, much greater effort has been directed towards the alloys of rare earths with the iron group metals. Early magnetic measurements on the intermetallic compounds of rare earths with 4d and 5d transition metals are incomplete; this is the case even for the gadolinium compounds which have received considerably more attention. In most cases the paramagnetic Curie temperatures 8, and the effective moment Pelf of the rare earth ions are not given (see, for example, ref. 1). Furthermore, recent studies of GdRhs by Hrubeck and Steinen [2] and Tari and Larica [ 31, and of GdPta by Dormann et at. [ 41 have revealed saturation moments for gadolinium in these compounds that are rather larger than those previously reported [5,6] . As in the magnetic measurements, electron spin resonance (ESR) studies of gadolinium in GdTa compounds are not complete and are not conclusive. For example, the g values reported for gadolinium in the same compound differ considerably from one author to another (see, for example, ref. 7). Moreover, recent ESR studies of GdRhs [ 31 and GdIra [ 81 compounds have shown that the gadolinium ESR in these compounds is bottlenecked, although neither bottlenecking nor positive g shifts are normally expected in compounds with strong d-band character. Studies of the hyperfine interaction by means of nuclear magnetic resonance (NMR) in GdTa inter-metallic compounds have so far been restricted mainly to the NMR of gadoli~um [4, 9 - 111, but since ~doli~um is an S-state ion such studies cannot give info~ation about the crystalline electric field in these compounds. It is of interest therefore to use hohnium as a dilute substitutional impurity for the study of the hyperfine interactions. Accordingly, we investigated the Laves phase compounds of gadolinium with rhodium, iridium, platinum, ruthenium and manganese, mostly with a small amount of holmium substituted for gadolinium, by means of NMR, ESR and magnetic measurements. The inclusion of results for GdRhs, which have been published previously, is for the purpose of comparison. The GdMn:, compound is of interest because of the uncertainty of the nature of the magnetic ordering of the RMna compounds [l] and the present work contributes to the info~a~on available, ~~0~~ some uncertainty still exists.
2. Samples and experimental techniques The compounds were prepared by arc melting of the constituents under 1 atm of argon gas. For manganese compounds approximately 3 at.% extra manganese was added prior to the melts in order to compensate for the manganese deficiency arising from weight loss. Also the pressure was considerably higher to reduce weight loss. In the remaining compounds the weight loss was negligible. A total of ten compounds were made and were investigated. With the exception of two compounds, a second sample of
P9
GdMna and GdRuz, they contained approximately 1 -3 at.% Ho. The alloys were melted and remelted a number of times and X-ray and metallographic investigations did not reveal any detectable second phases. Therefore heat treatments were carried out only on GdMns and GdRuz, on GdMn, to eliminate any effect which may be attributed to strain or metallurgical defects and on GdRus to ensure the stabilization of the Cl4 phase. On the holmium-free GdMnz sample a hardness test was also carried out and a search was made for unreacted gadolinium in the compound; this gave negative results. Magnetic measurements were made using a vibrating sample magnetometer in the temperature interval 4.2 - 300 K and ESR measurements were made by means of an X-band spectrometer on powder samples at temperatures in the range 77 K < T < 300 K. The NMR measurements were made in zero applied field on powder samples at 1.2 K by means of a phase coherent microwave spin-echo spectrometer operating between 6 and 7 GHz. For the magnetic and ESR measurements the temperature sensors used were Au-O.O3at.%Fe/chromel thermocouples. The saturation magnetizations were obtained at 4.2 K from a plot of the magnetization I against the inverse l/H of the applied field extrapolated to infinite applied field. The Curie temperatures of the samples were obtained from 12-H/I isotherms and were supplemented by the plots of I2 against Tc - T.
3. Results and discussion 3.1. Magnetization and susceptibility measurements The reciprocal mass susceptibilities of the compounds with the exception of GdMn2 are shown in Fig. 1. The inverse susceptibilities vary linearly with temperature over the entire paramagnetic range and the ferromagnetic Curie temperatures of the samples coincide with their paramagnetic Curie temperatures as shown in Table 1. This is in accord with the molecular field model where Tc = OP, x-l = T - Tc and I2 = Tc - T. Also listed in Table 1 are the saturation moments per mole of the compounds and the effective moments of the rare earth ions in the paramagnetic region. The effective moments of gadolinium in GdRh2 and GdRu2 are close to the free ion value of Gd3’, gp, {S(S + 1)}1’2 = 7.9411, (formula unit)-‘, but are considerably higher in GdIr2 and GdPt2. The saturation moments per formula unit of all the compounds are rather larger than those previously reported by various authors [ 5,6] and are in excellent agreement with those obtained recently for gadolinium in GdRh2 [ 21 and GdPt2 [4]. The ordering temperatures of the compounds are in good agreement with those formerly reported except that for GdIr2. The value that we found for the Curie temperature of this compound (Tc = 78 f 1 K) is substantially lower than that previously reported [6,12,13] (Tc = 89 K) but falls on the graph of the Curie temperatures of RIr2 compounds against the de Gennes factor (Fig. 2) (see also ref. 14).
PlO
Fig, 1. The reciprocal mass su~eptibilities of four of the
Ho0+01Gdo.e9Rhe; a, Ho0,0sGdo.a&; A, HoomGdo.w%
compounds&died: o, X, Hoo.o3Gdo.w%-
With the exception of that of GdIvIna,the helium temperature magnetizations of ail the alloys are almost saturated at 9 kG, the highest field that was available to us. The compound GdMna has a very itching behaviour. On lowering the temperature it first undergoes a weak magnetic transition at about 283 K (Fig. 3). The magnetization rises faJrly rapidly around this temperature, suggesting either the setting in of a short-range order or correlated moments. The ESR measurements also indicate a transition at this temperature (see Section 3.3). This transition has not previously been reported. Eelow 283 K the slope of the inverse susceptib~ity decreases progressively with decreasing temperature, which suggests a gradual increase in the size of the correlated regions. At about 106 K the compound undergoes a structural phase change, indicated by a sudden discontinuity in the susceptibility. Simultaneously the inverse s~eptib~~ takes a hyperbolic form typical of a fe~ma~et (Fig, 4). AIthougb the transition at 283 K may suggest the presence of a small amount of a gadolinium-rich solid solution with a Curie temperature just below that of gadolinium, we failed to detect such a phase in the compound, as has already been stated in Section 2. Furthermore the magnetization against field graph at room ~mpemt~e is a straight line and that at 265 K does not indicate the presence of any foreign phase. The saturation moment of this compound is 3.16~ra mol-I. If we assign a value of 7~~ to gadolinium in the ordered state, then the moment
Pll
co F1
Fi-
P12
T, (Kl 100 -
.
90 80 70 -
60
I2
I
50
IO
i
8
.
‘I
-L
-2
Fig. 2. A comparison of the Curie temperatures of the RIrz compounds with the de Gennes factor G = (g - 1)2J(J + 1): X, de Gennes factor; 0, Curie temperatures; 0, the Curie temperature of Ho-GdIr2 obtained in this work.
180
200
220
240
260
i 280
TM
1
Fig. 3. The temperature variation of the magnetization of GdMn2: 0, H = 0.8 kG; 0, H = 3.21 kG.
on a manganese ion is 1.92clg. This is a relatively small moment for manganese, and is probably a result of both the filling of the manganese d band and the antiferromagnetic coupling between manganese moments which are coupled antiparallel to the gadolinium moment.
P13
l..._o...
.
50
0
.
.
.
100
.
.
150
.
..I....1
200
T(K)
Fig. 4. The variations with temperature of the magnetic susceptibility and its inverse. for GdMnz: 0, H = 0.4 kG; 0, H = 0.8 kG.
3.2. NMR measurements The total hyperfine field (HF) of a rare earth nucleus in a metallic host may be written as
B=B,
+B, +Bt
(1)
where BI is the intraionic HF arising from the orbiting of the 4f electrons, B, is that arising from the conduction electrons polarized by the spin of the probe rare earth ion and Bt is the total transferred hyperfine field (THF) arising from both the nearest-neighbour and the distant rare earth ions. The quadrupole parameter p is similarly made up of contributions from 4f electrons, conduction electrons and the lattice. The magnetic dipole parameter a is related to the HF B through the relation
where yn is the nuclear gyromagnetic ratio of the probe ion (y,/2n = 8.77 MHz T-’ for holmium). ar, up and a, are related to BI, B, and Bt respectively in a similar way. The intraionic hyperfine parameters aI and pr are related to (J,) and (Jz2>through the relations
(J,, a1 = a0 J and
3(Jz2>- J(J + 1) PI =
J(2J-1)
PO
(4)
P14
where J = 8, a0 = 6497 f 8 MHz and p. = 62.7 + 3 MHz for holmium [15]. If the THF of a rare earth ion R is B(R) then that of a probe ion, in this case hohnium, in the same compound may be reasonably estimated using the hyperfine coupling parameters A listed by Campbell [ 161 through the relation
AWo)
B,(Ho) = -B,(R) A(R)
(5)
The contribution to the HF of the conduction electrons polarized by a rare earth ion is expected to be proportional to the z component of its spin S, since this polarization is principally caused by the exchange interaction. Therefore we may write
B* =KpCSz~=K*d~ where S is the spin of the rare earth ion and I&, is a constant. In this expression use is made of Gs,)/S = (J,)/J which applies to the heavy rare earth metals. Using the data obtained by Dormann and coworkers [4,10,11] for 165Gd, ‘“Gd and 13’La in Gd1_,LhT2 (T 1 Rh, Ir, Pt) we estimated B,(Ho) and B,(Ho). Using these values in eqns. (5) and (6) we obtained (J,) and hence Or>= gpg(Jz) for holmium in these compounds. These are listed in Table 2. TABLE 2 The hyperfine parameters for 165H~ in the compounds Ho-GdTz
Compound
a* (MHz)
p (MHz)
(T = Rh, Ir, Pt, Mn)
(Estimated moment holmium)lpB
(I,, (T)
at (T)
9 * 0.02 9.35 f 0.3 9.5 f 0.03 -
4.98 i 0.42 3.04 i 0.44 4.80 * 0.38
14.51 5 0.03 12.15 f 0.06 7.1 i: 0.1 -
for
_ Ho-GdRha Ho-GdIr2 Ho-GdPt2 Ho-GdMng
6000 (6205 (6250 6250
* * i *
1 2%) 2%) 2
51.8 f 0.5 73 f 3 60 f 1 53*1
The NMR samples contained 1 at.% Ho. a The uncertainty in a was usually less than 3 MHz; the uncertainties in the values in parentheses arise from very weak NMR signals as a result of which the lines could not be labelled with absolute certainty.
The hyperfine parameters p and a, and hence the bare moment, are lowest for holmium in GdRh2. The quadmpole parameter p of holmium in GdMn2 is also very low. If we expect 6p/p = nsa/a where n = 2, then a considerably lower value for a would be expected for holmium in this compound. Unfortunately sufficient data on GdMn2 are not available and therefore the individual contributions to the total HF cannot be estimated.
P15
Although the room temperature structure of this compound is cubic, it undergoes a s~~~~ phase change at about 106 K. The extraionic cont~bu~o~ to p, therefore, may be significant at low temperatures. In any case, the evidence from both a and p is that the moment of holmium in this compound is considerably reduced. This may be because of crystal field quenching or moment transfer from manganese to halmium as suggested by Chamberlain [17j _ The hyperfine parameters of GdPtz are not of great interest. The quadrupole parameter of holmium in Cdlkz, however, is quite large and remains a puzzle. 3.3. ESR meaurements The temperature variations of the g value and the breadth AN of the alloys, with the exception of GdMn2, are shown in Figs. 5 - 8. In the paramagnetic region the linewidths AN vary linearly with temperature and may conveniently be expressed as AH = 4 + bT. Table 3 lists the g shifts Ag, the thermal gradient b of the linewidth, the temperature ‘-r,, of the linewidth minimum and the estimated densities of s and d electrons at gadolinium sites where available [lo, 11 J . The saturation moments g, per mole of the samples are also included in this table to show the correlation between Ag and p,.
9 -0
z
2.06 2.02 l
1.92 W
i
ma
150
xx0
250
300
XKI
150
200 '
250
300
iti(>
1300 llOO9# 700 500 I
@I :,
loo
200
300
4K) '
(b)
%EGr--
Fig. 5. The temperature dependence (a) of the g value and (b) of the linewidth of HoGdRh,, Fig. 6. The temperature GdIr2.
dependence (a) of the g value and (b) of the linewidth of Ho-
P16
Fig. 7. The temperature GdPt2.
dependence
(a) of the g value and (b) of the linewidth
of Ho-
Fig. 8, The temperature
dependence
(a) of the g value and (b) of the linewidth
of GdRuz.
In any RTa compound where R can be replaced by gado~inium~a bottleneck is IikeIy to develop as the composition of pure GdTa is approached simply because the total gadolinium magnetization is becoming too large to be disposed of easily to the lattice. The dominance of the selectron density over the d-electron density is a further factor assisting the occurrence of a bottleneck since an s-electron band is blosefy coupled to the 4fv ion by a similar g value. The d electrons, however, are tightly bound to the lattice and hence are efficient in transferring the energy they receive from the relaxing probe ion to the lattice. The results in Table 3 therefore suggest that a deep bot~en~k~g should be expected for the gadolinium ESR in GdPt2. These results further suggest the presence of a shallow bottlenecking for the gadolinium resonances in GdRhs and GdIrs which has been confirmed by Tari and La&a f3] for GdRha and by Chiu et al, [S] for GdIra, Both the g shift Ag and the temperature gradient b of the linewidth for gadolinium in Ho-GdRha and Ho-GdPts are in agreement with those reported by Taylor and Coles [18] and Taylor et al. [ 19f for GdRhz and GdPta respectively. Gur ESR studies of Ho-GdRu2 and GdRua also give similar values for Ag and b (see Table 3). These results suggest that the presence of holmium does not affect the ESR parameters in any drastic way. There is, however, a sufficiently clear trend in the g values. The g value is ~onsis~utly higher in the alloys containing holmium, though the difference
7 f 0.05
+ 0.048 f 0.01 + 0.028 * 0.01
-0.01 f 0.008 -0.025 f 0.01
Ho-GdPt2
Ho-GdRu2 GdRu2
The g shifts are relative
G*2
7.16 * 0.15
+ 0.018 i 0.01 -0.002 ?r 0.007
to the gadolinium
10.5 f 0.5 7.8 f 1
5 f 0.5 6.2
5.5 f 3 9.5
2.6 i 0.2 3 f 0.5
d(AWldT (G K-l)
in the compounds
101% 3 100 f 5
67
10 80
es0
-65
105 f 3 @ 100 lOO*
Density of s electrons Ps (cfl,)
20
5550
=35
Pd @‘I
Density of d electrons
and GdTz (T = Rh, Ir, Pt, Ru)
Twin (K)
Ho-GdTz
free ionic value of g = 1.992.
6.31 f 0.15 -
f 0.05
Ho-GdIr2 GdJ.r2
6.96
+ 0.018 f 0.007 + 0.013 t 0.01
Psl~B
ESR parameters
Ho-GdRhz GdRh2
of the gadolinium
&
3
Compound
Summary
TABLE
19
8
18
Reference
P18
is very small. This is in accordance with the study of Re,&Zdo_ozPdess by Peter et al. [ 201, where R is a rare earth metal. They have found that the presence of a heavy rare earth impurity leads to a positive shift of the gadolinium g value from that for the binary PdGd alloy and that the presence of a light rare earth leads to a negative shift; the shift for 2% Ho is about 0.1. These shifts have been attributed to the strong response of the exchange-enhanced d band of the palladium. The shifts due to holmium in the compounds investigated here are much smaller than those in the palladium alloy. This is in keeping with the relatively weak enhancement of the 4d and 5d transition metals. Electron transfer from rare earths to the d bands of the transition metals in these compounds may also suppress the weak enhancement still further by filling the band. The saturation moment ps of the magnetic ion, in this case gadolinium, is related to the g shift through the relation ps =gp,Gs,)
= ELO + Agp,(Sz)
(71
where p. = gipk(Ss) is the bare moment of gadolinium and CS,> is the expectation value of its spin operator; gi is the ionic g value of gadolinium. The g shift therefore reflects the excess or deficiency in the saturation moment gs over the free ion value ,uo. Although it may be made somewhat less clear by the bottlenecking of the gadolinium ESR in these compounds, this correlation between g shift and saturation moment is sufficiency evident in Table 3. For gadolinium in GdRua a negative g shift and a value of 6.31,~~ is obtained for the saturation moment. In the remaining compounds the g shifts are positive and the saturation moment for gadolinium is approximatelY 7cLB. The g shift of gadolinium may be expressed as [ 211 &r = Ag,
+ Ag,‘d) = JRpR + J$d)~~(d)
(8)
where Aga is the g shift due to s and d electrons at the gadolinium site, AgTtd) is the shift due to d electrons at the T site, pR and prCd)are the densities of states and Ja and JTtd) are the exchange couplings of the gadolinium and conduction electron at the gadolinium site and the T site respectively. JR is positive since it is the intraionic exchange of gado~~ sites while Jr(“) is negative because of the non-o~ogon~~ of f and d electrons originating from different sites. The results in Table 3 are in satisfactory agreement with this statement. The g shift for gadolinium in LaRua is large, -0.17 [ 211, and much greater than our value for gadolinium in GdRus and Ho-GdRuz. Also the temperature gradient of the line has been found to be 24 + 5 G K-l for gadolinium in LaRua which is higher than the values we measured for our two ruthenium compounds. This implies that the gadolinium ESR in GdRua is bottlenecked as may be expected for an alloy so rich in gadolinium although the bottlenecking may not be as severe as in, say, GdAls. Indeed for gadolinium in YRua, where no bottlen~k~g is expected, Barberis et al. [21] have measured ag value of 1.96, i.e. a g shift of -0.032 f 0.02, a value not too different from ours.
P19
The ESR of gadolinium in GdMnz proved to be very complicated and the extraction of any meaningful values for the ESR parameters proved to be difficult, if not impossible. However, it confirms the transitions at 233 and 106 K found by magnetic measurements. In conclusion this investigation suggests that in the compounds under consideration the gadolinium ESR is bottlenecked. The saturation moment for gadolinium is approximately ‘7~~ and its g shift is positive except in GdRus. In this compound the g shift is negative and the saturation moment is 6.31~~. The ESR of gadolinium in GdMnz is difficult to analyse but it confirms the transitions revealed by magnetization and susceptibility measurements.
Acknowledgment We are grateful to Professor B. R. Coles for his critical reading of the manuscript.
References
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
W. E. Wallace, Rare-Earth Zntermetallics, Academic Press, New York, 1973, Chap. 12. J. Hrubeck and W. Steinen, J. Phys. (Paris), CoNoq. C5, 40 (1979) 198. A. Tari and C. Larica, J. Phys. (Paris), 41 (1980) 35. E. Dormann, M. Huck and K. H. J. Buschow, 2. Phys. B, 27 (1977) 141. J. Crangle and J. W. Ross, Proc. Znt. Conf. on Magnetism, Nottingham, 1964, Institute of Physics and The Physical Society, London, 1964, p. 240. R. M. Bozorth, B. T. Matthias, H. Suhl, E. Corenzwit and D. D. Davis, Phys. Rev., 115 (1959) 1595. R. H. Taylor, Adu. Phys., 24 (1975) 681. L. B. Chiu, P. R. Elliston, A. M. Stewart and K. N. R. Taylor, J. Phys. F, 9 (1979) 373. R. E. Gegenwarth, J. I. Budnick, S. Skaiski and J. H. Wemick, Phys. Rev. Lett., 18 (1967) 9. E. Dormann, M. Huck and K. H. J. Buschow, J. Magn. Magn. Mater., 4 (1977) 47. E. Dormann, L. Schaafhausen and K. H. J. Buschow, J. Magn. Magn. Mater., 2 (1976) 177. L. B. Chiu, P. R. Eiiiston, A. M. Stewart and K. N. R. Taylor, J. Phys. F, 9 (1979) 955. V. B. Compton and B. T. Matthias, Acta Crystallogr., 12 (1959) 651. B. T. Matthias, Z. Fisk and J. L. Smith, Phys. Lett. A, 72 (1979) 257. B. Bleaney, in R. J. Elliot (ed.), Magnetic Properties of Rare-Earth Metals, Plenum, New York, 1972, Chap. 8. I. A. Campbell, J. Phys. C, 2 (1969) 1338. J. R. Chamberlain, Physica B, 86 - 88 (1977) 138. R. H. Taylor and B. R. Coles, J. Phys. F, 5 (1975) 121. R. H. Taylor, I. R. Harris and W. E. Garden, J. Phys. F, 6 (1976) 1125. M. Peter, D. Shaitiel, J. H. Wernick, H. J. Williams, J. B. Mock and R. C. Sherwood, Phys. Rev. Lett., 9 (1962) 50. G. E. Barberis, J. P. Donoso, C. Rettori and D. Davidov, J. Less-Common Met., 70 (1980) P69.