Magnetic Effects in Chromium-platinum Alloys

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 alloys 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 magnetic 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 water. Atomic emission spectroscopy and electron microprobe methods were used to ensure that the samples were of good homogeneity. No phases other than solid solutions of Cr-Pt were observed in the microprobe 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 samples. Figure 1 shows the temperature dependence of v and a for c = 0.10, 0.19, 0.31 and 0.54 at.% Pt. The velocity 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 to 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 anomalies of magnetic origin. Shear wave velocities decrease continuously with increasing temperature without any trace of an anomaly. This is in agreement with Steinemann's [4] approach to the elasticity of itinerant magnetic 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 alloys 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 broken lines are for Cr + 5 at.% V that remains paramagnetic at all temperatures. neutron diffraction work. For Cr + 0.54 at.% Pt for which TI^ was not previously determined, we obtain TIC = (240 f 20) K. The spin-flip transition temperature (T.f), where the SDW changes from a transverse to a longitudinal one, was observed on the v T curves for c = 0.10 and 0.19 at.% Pt 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, except for c = 0.10 and 0.19 at.% Pt, v remains fairly Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988894