R automatically use data table name in output

If I understand your question correct you want to use the name of an object as a part of a file name. You can use deparse(substitute(obj)):

    ftest <- function(df) {
paste0(deparse(substitute(df)), ".tif")
}

ftest(iris)

# Output:
# [1] "iris.tif"

See: How to convert variable (object) name into String

If you want to use a list of strings as file names:

ftest2 <- function(lst) {
  for (i in 1:length(lst)) {
    filename <- lst[[i]]
    filename <- paste0(filename, ".tif")
    print(filename)
  }
}
rasnames = list("Wheat","Sbarley","Potato","OSR","fMaize")
ftest2(rasnames)

# Output:
# [1] "Wheat.tif"
# [1] "Sbarley.tif"
# [1] "Potato.tif"
# [1] "OSR.tif"
# [1] "fMaize.tif"

Here is a alternative version without using deparse(substitute()). This code read files from a directory and saves them with the prefix "df_" in a directory called "mynewfiles".

# create some text files in your working directory using the the "iris" data
write.table(iris, file = "test1.txt", sep = "\t")
write.table(iris, file = "test2.txt", sep = "\t")

# get the file names
myfiles <-  dir(pattern = "txt")

# create a directory called "mynewfiles" in your working directory 
# if it doesn't exists

if (!file.exists("mynewfiles")) {
  dir.create("mynewfiles")
}

for (i in 1:length(myfiles)) { 
  dftmp <- read.table(myfiles[i], header = TRUE, sep = "\t")
  # insert code to do something with the data frame...
  filename <- paste0("df_", myfiles[i])
  print(filename)
  write.table(dftmp, file = file.path("mynewfiles", filename), sep = "\t")
}