The photo above definitely looks like sun scorch. The leaves also look a little dry which would make them more susceptible to burning.
If you can, move the pot so that it gets shade for most of the day (the pot, that is) because being in full sun not only dries the potting mix out quite rapidly, it also causes it to get quite hot and that can in turn damage the roots.
Magnolias like a cool root run. Mulching the surface of the pot with sugar cane or other organic mulch would assist but ideally if the pot can be in the shade during the hottest hours of the day, the plant will do better.
Water regularly to ensure the roots are kept moist but not wet and feed with a fertiliser suitable for acid-loving plants (azaleas, camellias, rhododendrons, etc). - Noelle