You actually do have 5 lines but they are on top of each other thats why you do not see them. So your Z matrix for k1(1) and k1(2) are the same. Also Z matrix for k1(3) to k1(5) are the same.
So here is you code for 4 points instead of 101 to visaulize easily. You can also click and delete the lines in your plot to see the underlying lines:
clear; clc; clf;
M = 700; J = 400; a1 = 1.85; a2 = 1.85; L = 3.7;
n = 4;
m = 50000;
k_a12 = linspace(-m, m, n);
k_a21 = linspace(-m, m, n);
[X, Y] = meshgrid(k_a12, k_a21);
v = [0 0];
clr = ['r','g','b','c','k'];
b = 100000;
a = 5;
k1 = linspace(-b, b, a);
k2 = 0;
LegendsStrings = cell(numel(k1),1);
Z = zeros(n, n);
for q = 1:a
for i = 1:n
for j = 1:n
B = k1(q)*(J + M*a1^2) + k2*(J + M*a2^2) + (X + Y)*(J - M*a1*a2);
D = k1(q)*k2 - X.*Y;
if B(i, j) >= 0 && D(i, j) >= 0
Z(i, j) = D(i, j);
else
Z(i, j) = -1;
end
end
end
fprintf('Z for k1 = %.2f is\n',k1(q))
disp(Z)
LegendsStrings{q} = ['k_1 = ', num2str(k1(q))];
contour(X, Y, Z, v, 'LineWidth', 1.2, 'LineColor', clr(q), 'DisplayName', LegendsStrings{q});
hold on
end
legend()
xlabel('k_a_1_2')
ylabel('k_a_2_1')
grid on
