Yes. A spell is anything that you cast to put on the stack. Basically every card except lands have the potential to be spells by casting them. As a planeswalker isn't an artifact (one could be, but none currently are), it is a nonartifact spell.
111.1. A spell is a card on the stack. As the first step of being cast (see rule 601, “Casting Spells”), the card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually its owner’s hand. (See rule 405, “Stack.”) A spell remains on the stack as a spell until it resolves (see rule 608, “Resolving Spells and Abilities”), is countered (see rule 701.5), or otherwise leaves the stack.