SanDisk has hired Morgan Stanley to help it consider a possible bid for rival memory card maker Lexar Media, which agreed last month to be bought by Micron Technology. Micron’s deal to buy Lexar currently values the company at $8.46 per share, or nearly $700 million. But Lexar’s Nasdaq stock closed above that price on Thursday, at $9.17, after rallying 4.32 percent as investors wagered that the company would receive a better offer.