It would last as long as the applied force continued, or until the accelerating object hit something.
So the initial velocity is 15 m/s, the final velocity is 0 since it's at a complete stop and time is 10 seconds. Therefore:

Therefore, the acceleration is -1.5 m/s^2. The reason it's negative is due to the fact that the vector is going against it's original movement since it's decelerating.
The magnetic particles are mixed with other substances to make it flexible.