Magnetic tunnel junctions (MTJ), consisting of highly ordered Co2MnSi epitaxial bottom electrode, Al-O tunnel barrier and Co75Fe25 top electrode show at low temperatures a tunneling magnetoresistance (TMR) ratio of 159 % and a value of 70 % at room temperature. MTJ structures consisting of Co2MnSi/AlO/Co2MnSi, having TMR ratio of 570 %, at 2 K, were also fabricated, but these also show a large decrease of TMR ratio at 300 K. The experimental and theoretical results confirmed the existence of so-called noquasiparticle states and their crucial contribution to finite-temperature spin polarization [1]. In order to improve the MTJ performances, at room temperatures, the effects of doping with rareearths was analysed [2]. According to electronic structure calculations, substituting Ho, on Co sites, introduces a finite density of states in the minority spin gap, while substitution on the Mn sites preserves the half-metallic character. Since the existence of non-quasiparticle state, just above the Fermi level, is related to electron-magnon interaction, the diminishing of the depolarization effect as effect of temperature was analysed. This high diminution of TMR ratio, at room temperature, can be avoided by quenching the magnon excitations as result of Ho doping, while keeping the minority spin channel gapped.