Magnetoelasticity of a Cr+2.2 at.% Al-alloy single crystal

Abstract

The temperature dependences of the elastic constants and thermal expansion of a Cr-Al-alloy single crystal containing 2.2 at.% Al are reported. This is a critical concentration, being simultaneously close to the very steep incommensurate-commensurate spin-density wave transition line on the magnetic phase diagram as well as to the triple point, where the incommensurate and commensurate spin-density wave states coexist with the paramagnetic state. Very large magnetic contributions at 0 K of up to 28% for cL= 1/2 (c11+c12+2c44), 24% for c11 and 48% for the adiabatic bulk modulus, B, were observed, while the magnetic contributions to the shear elastic constants, c44 and 1/2 (c11-c12), amount to less than 1% at 0 K. The magnetovolume, the longitudinal elastic constants as well as B vary linearly with temperature over a large temperature range, This cannot be explained by existing theories. The absolute value of the zero-temperature magnetic Gruneisen parameter is exceptionally large, reflecting the much larger magnetic contribution to B than to the volume of the crystal at 0 K.

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“…As for most other Cr alloy systems thus far studied, the α-T curves of the Cr-Ga single crystals show very distinct minima at T N . In the case of Cr-Al alloys with x x L , α(T ) also shows [6] a minimum at T N that becomes smaller [6] for x close to x L , tends to disappear [2] at x L and changes to a steplike anomaly at T N for x > x L [6]. The Cr-Ga system differs from the Cr-Al system in this regard as it shows, for both of the present studies on a single crystal as well as previous [7] studies on polycrystalline alloys, a minimum at T N for x > x L instead of a steplike increase with increasing temperature.…”

Electrical transport and specific heat of a Cr+2.2at% Al single crystal

“…The Cr+ 2.2 at% Al single crystal of this study is of good quality and homogeneity and the same one previously used for magnetoelastic and neutron diffraction studies [3,4]. These studies gave T N =95 K (neutron diffraction) and T N E110 K (magnetovolume), and show that the crystal is in the CSDW phase down to 12 K, the lowest temperature of the neutron diffraction measurements.…”

“…(1) to the electrical resistivity data of Fig. 1, as well as with y D (EC)= 578 K at T =4 K calculated using previous [3] elastic constant measurements on the same Cr +2.2 at% Al crystal and standard methods of obtaining Debye temperatures from such measurements. Differences between y D (ER), y D (EC) and y D (SH) are attributed to approximations made in the theories used to calculate the former two y D -values.…”

“…Alloys with x near x t show anomalous magnetic behaviour [1,2,3,4], featuring amongst others exceptionally large magnetoelastic effects and a gigantic peak, centered around x=2.2 at% Al, in the x-dependence of the magnetoelastic Grüneisen parameters [1,2]. In contrast, signatures of SDW effects on electrical transport properties [1,5,6,7,8] are highly quenched for x Ex t .…”