FDR banned the ownership of gold, except for jewelry. Whether that is abandonment of a gold standard idk, but you may be correct. I think gold certificates were no longer being printed after 1933. Silver certificates stopped being printed in 1964.It was FDR in March of 1933 who took the United States off the gold standard. Then on June 5, 1933, Congress enacted a joint resolution nullifying the right of creditors to demand payment in gold.
The 1946 the Bretton Woods System was enacted and there were fixed exchange rates where foreign governments could sell gold to the US.
In any event Nixon abandoned that in 71. I've always taken that as being the end of the gold standard and move to a complete fiat based USD.