In this paper, we investigate mobile devices as interactive controllers to support the exploration of 3D data spaces in head-mounted Augmented Reality (AR). In future mobile contexts, applications such as immersive analysis or ubiquitous information retrieval will involve large 3D data sets, which must be visualized in limited physical space. This necessitates efficient interaction techniques for 3D panning and zooming. Smartphones as additional input devices are promising because they are familiar and widely available in mobile usage contexts. They also allow more casual and discreet interaction compared to free-hand gestures or voice input. We introduce smartphone-based pan & zoom techniques for 3D data spaces and present a user study comparing five techniques. Our results show that spatial device gestures can outperform both touch-based techniques and hand gestures in terms of task completion times and user preference. We discuss our findings in detail and suggest suitable techniques for specific AR navigation tasks.