Zoom In an Image in MatLab

%zoom in by block
close all;
clear all;
aa=imread('r.jpg');
a=rgb2gray(aa);

c=size(a)

m=1;
n=1;

for i = 1:1:c(1)
    for j = 1:1:c(2)
         b(m,n)=a(i,j);
         b(m,n+1)=a(i,j);
         b(m+1,n)=a(i,j);
         b(m+1,n+1)=a(i,j);
         n=n+2;        
    end
    n=1;
    m=m+2;
end

figure()
imshow(a)
figure()
imshow(b)
imwrite(b,'b.jpg');