[U-Boot] SD card building

I want to boot a fresh nitrogen6x board with uboot which I compiled recently.After a successful compilation I obtained u-boot.imx file which I wanted to bunn on my SD card for booting. I used "dd if=u-boot.imx of=/dev/sdb bs=1k seek=1" copy u-boot.imx file on the SD card which gave me the following output 327+0 records in 327+0 records out 334848 bytes (335 kB) copied, 0.00272321 s, 123 MB/s

...

sync

but actually nothing is getting Copied in the SD card. Also there are no hidden file. What can be the solution for this.

Thanks in Advance!