Optimization

Hi all, I need to minimize following function :

dat <- matrix(rnorm(20000), ncol=2)
      targetFn <- function(x) {
            dat <- as.matrix(dat)
            dat1 <- 1*dat[,1] - (x^2)*dat[,2]
            return(sd(dat1)) }

i.e. I want ro find for which "x" the value of "targetFn" will be minimum,
depending on current dataset "dat". Is there any optimization routine
available for this type of optimization?