Question: Should the /W pin be tied to VDD if it is not used in SPI FRAM devices?
If it is not used, the /W pin should be tied to VDD. The /W pin write-protects the status register. If this pin is left floating, there is no way to guarantee the state of the voltage. The pin probably typically remains HIGH, in which case the part behaves normally. However, if the pin floated LOW, you would not be able to write to the status register. Because it is an input pin, the /W pin should not be left floating.