Anomalous magnetoelastic waves in nickel wire

An anomalous magnetoelastic wave is observed in a 0.127-mm-diameter nickel wire using ordinary magnetostrictive transducers. Its group velocity is constant over a frequency range of 1 to 3.5 MHz and equal to the shear wave velocity of the material (3 km/s for nickel), suggesting it is of a shear wave nature. However, the phase velocity of this wave is extremely high (up to 416 km/s was observed) and in the opposite direction to the group velocity. This anomaly disappears when an axial DC current is applied in order to establish a magnetic bias; a Wiedemann-type signal with normal phase velocity appears at the same group delay. Such an anomalous mode is not expected in conventional elastic theory, nor has it been observed in previous work on magnetoelastic effects. In parallel with the well-known MSBVW, this anomalous wave could have interesting applications in signal processing due to its backward propagation, its low group velocity and damping properties, as well as the low cost of the material