Broadband ultrasonics applied to phase transition characterisation in chromium antiferromagnetic alloy

Abstract

LABOEND/PEMM/EE/COPPE, Federal University of Rio de Janeiro, P.O.Box 68505, CEP 21945-970 Rio de Janeiro-RJ, Brazil(Received October 4, 2000)(Accepted in revised form January 17, 2001)Keywords: Acoustic methods; Phase transformations; Nondestructive testing; Magnetic structureIntroductionUltrasonic pulse-echo techniques are being widely used in the investigation of magnetic phasetransitions in solid materials [1,2]. In general, changes in the ultrasonic pulse amplitude and velocity aremeasured, and then the ultrasonic attenuation and elastic constants are determined for fixed frequenciesgiven by the odd harmonics of the transducer coupled to the sample. It is difficult to reproduce the sameconditions of coupling between transducer and sample, therefore most analyses are limited to a fewfixed frequencies. An investigation of the ultrasonic pulse propagation close to a magnetic phasetransformation, for a continuous frequency range, is presented using wide band transducers and digitalspectral analysis. The material chosen is a single crystal of Cr-0.18at%Re, which has a magnetic phasetransformation from the paramagnetic (PM) to an antiferromagnetic (AFM) state at 318 K [3,4].Similarly to pure chromium, the ultrasonic attenuation and internal friction for this alloy have acontribution due to the coupling between the spin density wave in the AFM phase and the oscillatoryelastic strain introduced by the ultrasonic pulse. Since the sixties the ultrasonic broadband analysis isbeing widely used for many applications [5–8]. In this paper, we have shown that the ultrasonic spectralanalysis approach is highly advantageous in many aspects for the investigation of phase transitions.Experimental MethodThe Cr-0.18at%Re single crystal used was obtained from a larger ingot previously investigated byneutron scattering [9] and cut with faces parallel to the crystallographic directions [001], [110] and[110] with dimensions of 10 3 12 3 6 mm. The full half width at half maximum (FWHM) of the Braggpeak was found to be less than 0.5° after standard procedure of electropolishing with oxalic acid.The measurement was performed using the immersion in water technique with a broadbandKrautkra¨mer transducer model IAP 50.2.1, which has a good response in the frequency interval from5 MHz to 40 MHz. The generator/receptor excites the transducer with a wide band pulse, and thegenerated ultrasonic pulse propagates through the water/sample system. Part of the pulse reflects at