B )2)+ 2 2 [ z1+ [1+2 L2L2 (1-2) 3)a>>L,a>>Z B=0,→>H+M=0H=-M=Mk3 3 0 0 2 2 0 0 2 2 2 2 0 0 2 2 2 2 2 2 0 0 2 )] 2 (1 2 2 [1 2 2 1 2 ) ] 2 1 1 ) ) ( 2 1 1 [( 2 1 2 ) 2 1 1 ) ) 1 ( 2 1 1 ( 2 1 1 (1 2 z M a L z L z L z M a z L z z L M a z z L a z z L M a B m m m m = × + - - = + - - = × ï þ ï ý ü ï î ï í ì + + - - = - ´ - 3) a >>L, a >>Z B=0, => H+M=0 H=-M =M0k