A HIGH-PRESSURE ULTRASONIC INVESTIGATION OF THE NEEL TRANSITION IN A CR-PERCENT IR ALLOY SINGLE-CRYSTAL(0.07 AT)

Measurements of the ultrasonic wave velocity, ultrasonic attenuation, thermal expansion and electrical resistivity of a Cr + 0.07 at.% Ir al loy single crystal are reported as functions of temperature through th e NeeL point. Ultrasonic wave velocities were also measured as a funct ion of temperature at different constant applied pressures and as a fu nction of pressure at different constant temperatures. The pressure de rivative of the Neel temperature corresponds closely to that observed for pure Cr and for several other dilute Cr alloys exhibiting a phase transition from an incommensurate spin-density wave to a paramagnetic phase at the Neel point. Hysteresis effects were observed in the tempe rature and pressure dependences of the ultrasonic wave velocity below the Neel temperature. These effects probably originate from a differen ce in the antiferromagnetic domain pattern existing in the alloy on co oling or heating through the Neel transition. No firm conclusions coul d be reached on whether the addition of 0.07 at.% Ir to Cr is enough t o drive the first-order Neel transition of Cr to a second-order one.