A large magnetoresistance (TMR) is expected when half metallic ferromagnets are used in magnetic tunneling junctions with polycrystalline AlOx barrier. We have recently observed a TMR of 101% (T = 4K) for a Co2Cr0.6Fe0.4Al/AlOx/CoFe tunneling junction (Jullière spin polarisation of the Heusler electrode 67%). However, the TMR was reduced at 300K to almost half of its low temperature value. Considering the high Curie temperature of 800K of the Heusler compound this strong temperature dependence of the TMR is surprising. Using polarised neutron reflectometry (PNR) experiments we propose to test the theory of a reduced interface magnetisation of the Heusler electrode at elevated temperatures. We will investigate the magnetisation profile of a technologically relevant tunneling junction stack with a highly spin polarized Heusler electrode.