# Why the summation is printed like this?

The values of the upper and lower limits of a summation are usually printed below and above sigma. But I find TeX something very unusual. How to correct this?

Here is the MWE:

\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
<\mu_{m}> = \frac{\sum_{-J}^{J}{-g\mu_{\beta}M_{J}B.\exp{\left(\frac{-g\mu_{\beta}M_{J}B}{kT}\right)}}}{\sum_{-J}^{J}{\exp{\left(\frac{-g\mu_{\beta}M_{J}B}{kT}\right)}}}
\end{align}
\end{document}

The output:

-
Don't use <>. Use \langle \rangle instead. – Sigur Jul 19 '14 at 16:49
It's because you have the sums in a fraction. Use \sum\limits_{-J}^{J} to change the limit position. – Paul Gessler Jul 19 '14 at 16:49
@Sigur \langle \rangle, makes it soooo thin – subham soni Jul 19 '14 at 16:54
OK, but < is a relation symbol and \langle is a delimiter. They are different in some sense and the space around them are different. – Sigur Jul 19 '14 at 16:56

$\langle \mu_{m} \rangle = \frac{{\displaystyle \sum_{-J}^{J}}{-g\mu_{\beta}M_{J}B.\exp{\left(\frac{-g\mu_{\beta}M_{J}B}{kT}\right)}}} {{\displaystyle \sum_{-J}^{J}}{\exp{\left(\frac{-g\mu_{\beta}M_{J}B}{kT}\right)}}}$