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MAGNETIC EFFECTS IN CHROMIUM-PLATINUM
ALLOYS
H. Alberts, J. Lourens
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988
MAGNETIC EFFECTS IN CHROMIUM-PLATINUM ALLOYS H. L. Alberts and J. A. J. Lourens
Department of Physics, Rand Afrikaans University, P.O. Box 524, Johannesburg 2000, South Africa
Abstract.
-
Velocity of sound and thermal expansion measurements on Cr-Pt alloys in the concentration range up to 1.5 at.% Pt are reported. The existence of a triple point and a mixed magnetic state on the magnetic phase diagram is suggested.The magnetic phase diagram [I] of dilute Cr-Pt al- loys contains a paramagnetic (P), an incommensurate (I) and a commensurate (C) spin density wave (SDW) phase. Rather few studies have been reported on the effects of magnetic ordering on the physical properties of these alloys, particularly effects of the I-C phase transition which have only been reported in neutron diffraction studies [I]. We studied the effects of mag- netic ordering on the velocity of sound and thermal expansion of alloys containing up to 1.5 at.% Pt.
Alloys with 0.10, 0.19, 0.31, 0.54, 0.99 and 1.43 at.% P t were prepared by arc melting from 99.99 %
-
pure starting material. The samples were annealed at 1000 OC for three days and then quenched in wa- ter. Atomic emission spectroscopy and electron mi- croprobe methods were used t o ensure that the sam- ples were of good homogeneity. No phases other than solid solutions of Cr-Pt were observed in the micro- probe analyses. The longitudinal velocity of sound (v) and coefficient of thermal expansion (a) were measured from 77 K to 500 K as described elsewhere [2]. The upper temperature limit was set by the properties of the bonding material used to
fix
quarts transducers (N 500 K) and strain gauges (N 400 K) to the sam- ples.Figure 1 shows the temperature dependence of v and a for c = 0.10, 0.19, 0.31 and 0.54 at.% Pt. The veloc- ity for c = 0.99 and 1.43 at.% P t is shown in figure 2. a of these two samples was not measured.
When com~ared t o v and a of Cr
+
5 at.% V that remains paramagnetic at all temperatures [3] (Fig. I), both v and a of Cr-Pt display relatively large anoma- lies of magnetic origin. Shear wave velocities decrease continuously with increasing temperature without any trace of an anomaly. This is in agreement with Steine- mann's [4] approach to the elasticity of itinerant mag- netic materials that if bulk effects are present, shear anomalies can be absent and vice versa. The I-C phase transition temperatures (TIC) and Nee1 temperatures (TN) as determined by Booth et al. [I] from neutron diffraction experiments are also shown in the figures.Trc and TN determined in the usual way [3] from the results of figure 1, compare reasonably well with the
Fig. 1. - Temperature dependence of the velocity of sound (v) and coefficient of thermal expansion (a) of Cr-Pt al-
loys containing 0.10, 0.19, 0.31 and 0.54 at.% Pt. The I-C transition temperatures, TIC, and N&l temperatures, TN, obtained from neutron diffraction measurements by Booth et al. [I] are also shown. The small anomalies marked by Tsf occur at the spin flip transition temperature. The bro-
ken lines are for Cr
+
5 at.% V that remains paramagnetic at all temperatures.neutron diffraction work. For Cr
+
0.54 at.% P t for whichTI^
was not previously determined, we obtain TIC = (240 f 20) K. The spin-flip transition tempera- ture (T.f), where the SDW changes from a transverse t o a longitudinal one, was observed on the v - T curves for c = 0.10 and 0.19 at.% P t but not for the other concentrations.The v
-
T curves show anomalies near TN but not near TIC. Deep minima occur in the curves below TN. A characteristic feature of the minima in v is that, ex- cept for c = 0.10 and 0.19 at.% P t , v remains fairlyJOURNAL DE PHYSIQUE
Fig. 2. - Velocity of sound for c = 0.99 and 1.43 at.% Pt. N6el temperatures shown are from reference [I].
constant in a temperature range of about 50 K near the minimum. No anomaly is, however, observed in ar at Tsf while only one well defined anomaly (a mini- mum) is observed for 0.10 at.% P t near TN. For 0.19, 0.31 and 0.54 at.% P t two well defined anomalies in a are observed, one characterized by a maximum point
near TIC and the other by a minimum point near TN.
This type of behaviour was also observed previously [5,
61 in other Cr alloys. Only one transition, of the type observed [5, 61 in other Cr alloys that do not show I-C transitions, is observed in cr of the 0.10 at.% P t sam- ple. T h 1 ~ suggests the absence of the C phase in this sample and the presence of a triple point, not previ- ously observed, where the P, I and C phases coexist on the magnetic phase diagram between 0.1 and 0.2 at.% Pt. As for the v
-
T curves, the a-
T curves tend to become fairly flat between TIC and TN for c = 0.31 and 0.54 at.% P t but not for c = 0.19 at.% Pt.The anomalously low dv
/
dT and d a / dT values betweenTI^
and TN for c 2 0.31 at.% P t are ascribed t o magnetic inhomogeneities as analyses of our sam- ples confirmed their good chemical homogeneity. Wesuggest that a mixed magnetic state, consisting of both the ISDW and CSDW states, is present in these alloys between TIC and TN. This suggestion seems plausible since the magnetic contribution to the bulk modulus and the magnetovolume is expected to be larger [7] for the CSDW than for the ISDW state resulting in a near constant a or v in a certain temperature range when the ratio of the CSDW t o the ISDW phase increases with increasing temperature between TIC and TN.
In conclusion, well defined magnetic anomalies ap- pear in the temperature dependence of the longitudinal ultrasonic wave velocity as well as in the coefficient of thermal expansion of Cr-Pt alloys near the magnetic transition temperatures. No anomalies occur in the shear velocities. A triple point on the magnetic phase diagram between 0.1 and 0.2 at.% P t is suggested as well as a mixed state consisting of ISDW and CSDW states between TIC and TN.
Acknowledgments
We thank Mrs. T. Germishuyse for technical assis- tance and the FRD for financial aid.
[I] Booth, J. G . , Ziebeck, K. R. A. and Chagnon, R., 3. Phys. F 8 (1978) 1303.
[2] Alberts, H. L. and Lourens, J. A. J., J. Phys. F 13 (1983) 873.
[3] Alberts, H. L. and Lourens, J. A. J., J. Phys. F
15 (1985) 2511.
[4] Steinemann, S. G., J. Magn. Magn. Muter. 7 (1978) 84.
[5] Alberts, H. L. and Lourens, J. A. J., J. Phys. f+
18 (1988) 123.
[6] Alberts, H. L. and Lourens, J. A. J., J. Phys. F 17 (1987) 727.
