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The magnetic phase diagram of a dilute CrGa system
Solid State Communications, Printed in Great Britain.
Vol. 50, No. 12, pp. 1063-1064,
0038-1098/84 $3.00 + .OO Pergamon Press Ltd.
1984.
THE MAGNETIC PHASE DIAGRAM OF A DILUTE Cr-Ga
SYSTEM
H.L. Alberts and J.A.J. Lourens Department
of Physics, Rand Afrikaans University,
P.O. Box 524, Johannesburg
2000, South Africa
(Received 20 February 1984 by F.R.N. Nabarro) The boundaries of the magnetic phase diagram of a dilute Cr-Ga system have been determined from measurements of the temperature and pressure dependence of the electrical resistivity. The phase boundaries separating the observed magnetic states, namely the paramagnetic state and the incommensurate and the commensurate spin density wave states, meet at about 0.5 at.% Ga, the triple point for this system.
ACCORDING to measurements of the electrical resistivity (p) and thermal expansion of Cr-Ga alloys [l] the NCel temperature, TN, initially decreases with increasing Ga concentration and then increases abruptly, showing a minimum around 0.7 at.% Ga. These measurements did not reveal the existence of transitions from the incommensurate to the commensurate spin density wave states as was found in the recent neutron diffraction work of Booth et aE. [2] on Cr-Ga alloys which display three magnetic states, the paramagnetic (P), the incommensurate (I) and the commensurate (C) spin density wave (SDW) states. Similar magnetic states have also been found in dilute Cr-Si [3,4] and Cr-Ge [5]. In Cr-Ge both the thermal expansion measurements of Suzuki [5] and the electrical resistivity measurements of Arajs et al. [6] pointed to the existence of a magnetic triple point at ‘v 0.35 at.% Ge. Mtinch et al. [7], however, did not observe anomalies in the resistivity at the C-I transition of Cr-Ge alloys. Booth et al. [2] found no evidence of the existence of a triple point in either the Cr-Ga or the Cr-Ge magnetic phase diagram. Very recently [8] a resistivity anomaly was observed at the I-C SDW transition on the application of hydrostatic pressure in a Cr-Ga alloy containing 0.92 at.% Ga. It therefore became feasible to investigate the phase diagram of this system through measurements of the resistivity of a series of Cr-Ga alloys as a function of temperature and pressure. In this note we report such measurements which have enabled us to determine the boundaries of the phase diagram and the triple point for this system. Details of the preparation of the Cr-Ga alloys used in this investigation (Table 1) are given elsewhere [8]. Figure 1 shows the temperature dependence of the electrical resistivity at various applied pressures of the aBoy containing 0.19 at.% Ga. The temperature coefficient of the resistivity, (l/p) dp/dT, is negative near TN as for chromium, and the anomaly increases with
Table I. Transition temperatures of Cr-Ga alloys at.% Ga
TIC
0.19 0.29 0.52 0.73 0.92 1.18 1.50 2.56
1.69
TN
WI
_ _ 270 f 2 264 + 5 267 f 7
(K)
229 f 3 291+3 281 +3 350 + 3 354 + 4 368 5 8 384 f 3 418 + 4
-
F---J /...... ‘. .’
1.67
_.....”
l.65 t
1.53 1.51 260
.,....“.
270
.‘.‘(
280
. ... . ... ....
T(K)
:’
200
300
Fig. 1. The temperature dependence of the resistivity of the 0.19 at.% Ga alloy at different pressures in kbar. increasing pressure as was found to be the case for all the alloys. The alloy containing 0.29 at.% Ga behaved quite similarly. Alloys having c 2 0.52 at.% Ga all had positive resistivity coefficients near TN (or TIC). The anomalies corresponding to transitions at TIC and TN were observed in the 0.73,0.92 [8] and 1.18 at.% Ga alloys. The 0.92% alloy turned out to be the only sample where the TIC transition could not be 1063
310
1064
MAGNETIC PHASE DIAGRAM OF A DILUTE Cr-Ga
SYSTEM
Vol. 50, No. 12
I i Fig. 2. The temperature dependence of the resistivity of the 0.73 at.% Ga alloy at different pressures in kbar. The derivative of the curve at zero pressure shows two transitions at Tro = 270+2andTN=350f2K.
Fig. 3. The incommensurate-commensurate SDW transition temperature as a function of pressure. unambiguously identified at zero applied pressure. The variation of the electrical resistivity of the 0.73% alloy with temperature at different pressures is shown in Fig. 2 from which Tic = 270+2KandTN=350+ 3 K. Fukamichi [l] also measured the resistivity of a 0.73% Ga alloy. His graph shows an inflection point near 270 K that was, in view of the current data, incorrectly identified as the NCel temperature of this alloy. Furthermore, in the neutron diffraction work of Booth et al. [2] TN of a 0.75% sample was found to be 335 K in reasonable agreement with our result. Figure 3 shows TIC as a function of pressure for these three alloys. dTro/dp z - 11 K kbar-’ while dTN/dp r _ 6.5 K kbar-’ for alloys containing up to 0.52 at.% Ga. Extensive searches for anomalies at Tic in the 1.5 and 2.56 at.% Ga alloys failed to detect their existence. In this regard it was noted that the width of transitions at TIC as determined from the dp/dT vs temperature
Fig. 4. The magnetic phase diagram for Cr-Ga
alloys.
curves tended to decrease with increasing concentration whereas the width at TN increased as may be expected from considerations of homogeneity. In addition the actual size of the IC-anomalies also decreased with increasing Ga content. The appearance of anomalies may also be affected by impurities as they may act to change the lattice parameters in much the same way as an applied pressure would. In fact we observed a large anomaly near TIC at atmospheric pressure in a 1 at.% Ga alloy which contained - 0.02 at.% W. No tungsten could, however, be detected by microprobe analysis in the alloys listed in Table 1. The magnetic phase diagram of the Cr-Ga system is depicted in Fig. 4. Our observations clearly illustrate the boundaries between the I, C and P states with a triple point occurring at - 0.5 at.% Ga and a temperature of 277 f 5 K. Acknowledgements - We thank the Council for Scientific and Industrial Research for financial support and Mr S.I. Wagener for technical assistance. REFERENCES 1. 2. :: 2: 7. 8.
K. Fukamichi, J. Phys. F: Metal Phys. 9, L85 (1979). J.G. Booth, M.M.R. Costa & K.R.A. Ziebeck, J. Magn. Magn. Mat. 31,285 (1983). T. Suzuki, J. Phys. Sot. Japan 43,869 (1977). H.U. Astrbm, G. Benediktsson & K.V. Rao, J. Phys. Colloq. 39, C-785 (1978). T. Suzuki,J. Phys. Sot. Japan 45,1852 (1978). S. Arajs, R. Aidun & C.A. Moyer, Phys. Rev. B22, 5366 (1980). R. Munch, H.D. Hochheimer, A. Werner, G. Mater&, A. Jayaraman & K.V. Rao, Phys. Rev. Left. 50, 1619 (1983). H.L. Alberts & S.I. Wagener, J. Phys. F: Met. Phys. 13, L253 (1983).
Vol. 50, No. 12, pp. 1063-1064,
0038-1098/84 $3.00 + .OO Pergamon Press Ltd.
1984.
THE MAGNETIC PHASE DIAGRAM OF A DILUTE Cr-Ga
SYSTEM
H.L. Alberts and J.A.J. Lourens Department
of Physics, Rand Afrikaans University,
P.O. Box 524, Johannesburg
2000, South Africa
(Received 20 February 1984 by F.R.N. Nabarro) The boundaries of the magnetic phase diagram of a dilute Cr-Ga system have been determined from measurements of the temperature and pressure dependence of the electrical resistivity. The phase boundaries separating the observed magnetic states, namely the paramagnetic state and the incommensurate and the commensurate spin density wave states, meet at about 0.5 at.% Ga, the triple point for this system.
ACCORDING to measurements of the electrical resistivity (p) and thermal expansion of Cr-Ga alloys [l] the NCel temperature, TN, initially decreases with increasing Ga concentration and then increases abruptly, showing a minimum around 0.7 at.% Ga. These measurements did not reveal the existence of transitions from the incommensurate to the commensurate spin density wave states as was found in the recent neutron diffraction work of Booth et aE. [2] on Cr-Ga alloys which display three magnetic states, the paramagnetic (P), the incommensurate (I) and the commensurate (C) spin density wave (SDW) states. Similar magnetic states have also been found in dilute Cr-Si [3,4] and Cr-Ge [5]. In Cr-Ge both the thermal expansion measurements of Suzuki [5] and the electrical resistivity measurements of Arajs et al. [6] pointed to the existence of a magnetic triple point at ‘v 0.35 at.% Ge. Mtinch et al. [7], however, did not observe anomalies in the resistivity at the C-I transition of Cr-Ge alloys. Booth et al. [2] found no evidence of the existence of a triple point in either the Cr-Ga or the Cr-Ge magnetic phase diagram. Very recently [8] a resistivity anomaly was observed at the I-C SDW transition on the application of hydrostatic pressure in a Cr-Ga alloy containing 0.92 at.% Ga. It therefore became feasible to investigate the phase diagram of this system through measurements of the resistivity of a series of Cr-Ga alloys as a function of temperature and pressure. In this note we report such measurements which have enabled us to determine the boundaries of the phase diagram and the triple point for this system. Details of the preparation of the Cr-Ga alloys used in this investigation (Table 1) are given elsewhere [8]. Figure 1 shows the temperature dependence of the electrical resistivity at various applied pressures of the aBoy containing 0.19 at.% Ga. The temperature coefficient of the resistivity, (l/p) dp/dT, is negative near TN as for chromium, and the anomaly increases with
Table I. Transition temperatures of Cr-Ga alloys at.% Ga
TIC
0.19 0.29 0.52 0.73 0.92 1.18 1.50 2.56
1.69
TN
WI
_ _ 270 f 2 264 + 5 267 f 7
(K)
229 f 3 291+3 281 +3 350 + 3 354 + 4 368 5 8 384 f 3 418 + 4
-
F---J /...... ‘. .’
1.67
_.....”
l.65 t
1.53 1.51 260
.,....“.
270
.‘.‘(
280
. ... . ... ....
T(K)
:’
200
300
Fig. 1. The temperature dependence of the resistivity of the 0.19 at.% Ga alloy at different pressures in kbar. increasing pressure as was found to be the case for all the alloys. The alloy containing 0.29 at.% Ga behaved quite similarly. Alloys having c 2 0.52 at.% Ga all had positive resistivity coefficients near TN (or TIC). The anomalies corresponding to transitions at TIC and TN were observed in the 0.73,0.92 [8] and 1.18 at.% Ga alloys. The 0.92% alloy turned out to be the only sample where the TIC transition could not be 1063
310
1064
MAGNETIC PHASE DIAGRAM OF A DILUTE Cr-Ga
SYSTEM
Vol. 50, No. 12
I i Fig. 2. The temperature dependence of the resistivity of the 0.73 at.% Ga alloy at different pressures in kbar. The derivative of the curve at zero pressure shows two transitions at Tro = 270+2andTN=350f2K.
Fig. 3. The incommensurate-commensurate SDW transition temperature as a function of pressure. unambiguously identified at zero applied pressure. The variation of the electrical resistivity of the 0.73% alloy with temperature at different pressures is shown in Fig. 2 from which Tic = 270+2KandTN=350+ 3 K. Fukamichi [l] also measured the resistivity of a 0.73% Ga alloy. His graph shows an inflection point near 270 K that was, in view of the current data, incorrectly identified as the NCel temperature of this alloy. Furthermore, in the neutron diffraction work of Booth et al. [2] TN of a 0.75% sample was found to be 335 K in reasonable agreement with our result. Figure 3 shows TIC as a function of pressure for these three alloys. dTro/dp z - 11 K kbar-’ while dTN/dp r _ 6.5 K kbar-’ for alloys containing up to 0.52 at.% Ga. Extensive searches for anomalies at Tic in the 1.5 and 2.56 at.% Ga alloys failed to detect their existence. In this regard it was noted that the width of transitions at TIC as determined from the dp/dT vs temperature
Fig. 4. The magnetic phase diagram for Cr-Ga
alloys.
curves tended to decrease with increasing concentration whereas the width at TN increased as may be expected from considerations of homogeneity. In addition the actual size of the IC-anomalies also decreased with increasing Ga content. The appearance of anomalies may also be affected by impurities as they may act to change the lattice parameters in much the same way as an applied pressure would. In fact we observed a large anomaly near TIC at atmospheric pressure in a 1 at.% Ga alloy which contained - 0.02 at.% W. No tungsten could, however, be detected by microprobe analysis in the alloys listed in Table 1. The magnetic phase diagram of the Cr-Ga system is depicted in Fig. 4. Our observations clearly illustrate the boundaries between the I, C and P states with a triple point occurring at - 0.5 at.% Ga and a temperature of 277 f 5 K. Acknowledgements - We thank the Council for Scientific and Industrial Research for financial support and Mr S.I. Wagener for technical assistance. REFERENCES 1. 2. :: 2: 7. 8.
K. Fukamichi, J. Phys. F: Metal Phys. 9, L85 (1979). J.G. Booth, M.M.R. Costa & K.R.A. Ziebeck, J. Magn. Magn. Mat. 31,285 (1983). T. Suzuki, J. Phys. Sot. Japan 43,869 (1977). H.U. Astrbm, G. Benediktsson & K.V. Rao, J. Phys. Colloq. 39, C-785 (1978). T. Suzuki,J. Phys. Sot. Japan 45,1852 (1978). S. Arajs, R. Aidun & C.A. Moyer, Phys. Rev. B22, 5366 (1980). R. Munch, H.D. Hochheimer, A. Werner, G. Mater&, A. Jayaraman & K.V. Rao, Phys. Rev. Left. 50, 1619 (1983). H.L. Alberts & S.I. Wagener, J. Phys. F: Met. Phys. 13, L253 (1983).