Are pay raises mandatory?

Unfortunately, the answers are no and no--raises are not mandatory, and there is no law requiring them.

Most employment relationships are what is known as "at will".  This means that basically the employer can hire or fire someone for any reason or no reason whatsoever, as well has increase/decrease salary/hours, promote/demote, and generally impose requirements as they see fit.  In turn, you can work for an employer, or not, your choice. 

The exceptions to the above would be if there is a stated company policy covering this, or there is a union/employment agreement that governs, or this situation has arisen due to some type of discrimination.

Therefore, in general pay raises are not mandatory.  However, if you are making minimum wage and there is an increase in that wage pursuant to federal law, then such a "pay raise" would be mandatory.