Linear and nonlinear elastic properties of antiferromagnetic ?-manganese-nickel alloys

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Abstract
The linear and nonlinear elastic properties of single crystals of f.c.c. γ-manganese-nickel alloys with compositions Mn73Ni27, Mn80.5Ni19.5 and Mn84Ni16 have been studied using ultrasonic pulse-echo techniques. The elastic stiffness tensor components of Mn73Ni27 have been determined as a function of temperature and pressure on both sides of the Nee1 temperature. The longitudinal moduli c 11, and c L(= (c 11 + c 22 + 2c 44)/2) and the bulk modulus B S O are larger in the antiferromagnetic than in the paramagnetic state; the reverse is true for the shear stiffness c 44 and c′(= (c 11 - c 12)/2). The hydrostatic pressure derivatives (∂c IJ/∂P) P= 0, of the elastic stiffness tensor components have been measured in both the antiferromagnetic and paramagnetic states of Mn73Ni27. In the antiferromagnetic state (∂c 11/∂P) P=0, and (∂c′/∂P) P=0, increase with temperature, while (∂c 44/∂P) P=0 decreases; in the paramagnetic state (∂c 11/∂P) P = 0 and in consequence (∂B S O/∂P) P=0 are much smaller. Large differences are found between the mode Grüneisen gammas of the longitudinal acoustic modes in the antiferromagnetic and paramagnetic states; the shear modes are less affected. From hydrostatic and uniaxial pressure effects on the velocities of ultrasonic modes, the six third-order elastic stiffness tensor components of Mn80.5Ni19.5 have been obtained at room temperature; c 111 is the largest, indicating the predominance of contributions from the nearest-neighbour repulsive forces to the higher-order elastic effects.

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