There is nothing magical about a magnetostrictive rod. A nickel or a nickel alloy rod, or even a soft-iron rod should work. There are also rare-earth alloys, Terfenol-D for example, that work better, but the Chinese have pretty much cornered the market on rare-earth elements. Prices are increasing.
The biggest problem is coupling the linear expansion and contraction of the rod to your water environment. A practical diameter rod might be only one inch diameter, and with a reasonable magnetizing current may only increase or decrease in length by a few thousandths of an inch. You will need to couple this to a horn that "magnifies" and impedance-matches the actuator rod to the water tank load. You need the services of an acoustical engineer (not me) to figure out how to do that.
I only suggested magnetostrictive transducers because you were going down the wrong path to generating infrasonic waves in water with piezoelectric transducers, which typically resonate at high kilohertz (ultrasonic) frequencies and have very limited ranges of motion. Others have posted reasonable suggestions for alternative means of producing infrasonic waves based on conventional loudspeaker technology, i.e., a voice coil moving in an axial magnetic field. I would try some of these suggestions first before engineering a new magnetostrictive transducer.
The 10 Hz to 30 Hz range is just below the low-frequency limits of human hearing, but this range of frequencies resonates well with the adult chest cavity and can easily be felt while other, higher, frequencies are simultaneously heard from large "woofer" loudspeakers. A lot of air is being moved by these speakers, but air is compressible whereas water basically is not. So you need to move an actual volume of dense water back and forth with your transducer, which is going to be a power-hungry task if you want a large variation is pressure.
There is a
commercial audio transducer development that uses
Terfenol-D mangnetostrictive rods to drive large surfaces at low frequencies. Unlike voice-coil based loudspeakers, their transducer moves a much larger area over a much smaller distance to make sound. It might be worth your while to contact them and explain what you want to do. They are marketing their "driver" as an attention-getter, turning (for example) display windows into loudspeakers to attract passersby. I see no reason why their devices couldn't be re-purposed to your needs. The fact that this new company chose to use magnetostrictive transducers probably is related to the mass of the things they are driving. They emphasize that the driver exciting the "sound producing" surface makes very small motions, but there is nothing mentioned about the physics involved. You should probably look into that.
