You need to learn to debug your program. There are many things wrong with your code: First problem,
B = 2;
for i = 1:length(B)
length(B) is 1, the i loop is therefore only executed once.
2nd problem,
theta = 0
theta_initial = ones(nrows) * theta
theta is initialised as scalar, theta_initial as vector. Later,:
theta_initial goes from being a vector to being a scalar. I'm fairly certain that's not the intention.
3rd problem,
for i = ...
for j = ...
theta_new = theta + dtheta(j);
end
After the j loop, you have in effect
theta_new = theta + dtheta(100)
regardless of what happens in the loop. Hence the loop is not needed. This is followed by
magnetization_new(i) = magnetization * cosd(theta_new);
So, in effect
magnetization_new(i) = magnetization * cosd(theta + dtheta(100));
for all i.
Reread your code, and if it is not enough for you to spot the mistake step through it using the debugger.