arrow was Tilt in diagram, what i do for this problem?

Hi. I write :

Code:

---

\begin{tikzpicture}[auto,>=stealth']
\node (L) {$\frac{L}{Z(L)}\times \frac{L}{Z(L)}$};
\node[right=of L] (L1) {${L}^2$};
\node[below=of L] (M) {$\frac{M}{Z(M)}\times \frac{M}{Z(M)} $};
\node[below=of L1] (M1) {$M^2$};
\draw[->] (L) -- node[swap] {$\alpha \times \alpha$} (M);
\draw[->] (L1) -- node[] {$\beta$} (M1);
\draw[->] (L) -- node {} (L1);
\draw[->] (M) -- node {} (M1);
\path[use as bounding box] (-1.5,0) rectangle (0,0);
\end{tikzpicture}

---

but in pdf the arrows was tilted. What I do for this problem?

thanks

By default, when you use options like "below=of ...", the node distance is measured between the nodes' borders. Distance can be specified as in "below=1cm of ..." or using "node distance=..." option. You can see this if you give the option "draw" to all nodes: arrows start and end at the borders and the vertical arrows have the same length. To measure the distance between nodes' centers use the option "on grid". You can give it to the whole tikzpicture environment. In this case, you also need to increase the distance as described above.

This is described in section 16.5.3 of the TikZ manual version 2.10.