The energy requirements for this are pretty ridiculously high. The rotational energy has to go somewhere, and it will probably turn into heat and cook the Earth. If you can somehow store the energy in the Earth's core, you might be able to avoid catastrophe.
The earth has a rotational kinetic energy of 2×10^29 J. A tidally locked planet has negligible rotational energy. In order to stop the rotation in 100 years, you need to bleed that energy off at a rate of 2×10^27 J/year or 7×10^19 W. Lets take a look at how much power that is.
Currently, the entire human race produces 3×10^12 W of electricity, one ten millionth of the required energy. The Earth gets 1.74×10^17 W of energy from the sun, 1% of the energy required.
It should not come as a surprise that the energy requirements for changing the motion of a planet are astronomical. Planets are astronomical bodies. 100 years is less than an eye blink in astronomical time.
Hypothetically, you somehow find a way to turn the rotational energy of the earth into useful energy. I don't know how you did it. You probably used a machine powered by pure handwavium. Kinetic energy has a nasty tendency to turn into heat energy. This is one of the consequences of the second law of thermodynamics. If your hypothetical rotational energy harvester actually gets up and running, it somehow has to get 99.9% of that energy away from the Earth's surface. If 1% stays behind, it will produce heat equivalent to the heat coming from the sun, and the Earth will cook.
Instead of sending this energy to space, we could use a machine powered by pure handwavium to store the rotational energy in the Earth's core or mantle. The Earth has a mass of 6×10^24 kg, which means there are 33000 J of rotational energy per kilogram of mass. For simplicity, I will assume that the Earth's crust has negligible mass. I will also assume that the Earth's mantle and core have a specific heat of roughly 500 J/kg/C, roughly the same as iron at room temperature. That means that it takes 500J of heat to increase the temperature of 1kg by 1 degree C. This assumption is mostly just a guess. I can't find great numbers for specific heat of the Earth's mantle. If we store all of that energy in the Earth's core and mantle, its temperature will increase by 66C. This is not huge, because the mantle between 500-900C already. This might lead to increased earthquakes, but that is way way better than turning the surface into an oven.
Earth's magnetism comes from its core. Maybe the mechanism you use to slow the Earth has some connection to magnetism.