We have studied the crystal structure, surface morphology, magnetic anisotropy, domain structure and tunnel magnetoresistance of spin-valves with a single and double MgO barrier layers. We have demonstrated the domain structure for soft and hard magnetic layers and observed significant changes after low temperature annealing. We have carried out magnetoresistance measurements using current-in-plane (CIP) four-probe technique and discovered a substantial difference in the values of TMR ratio for single and double MTJ spin-valves. It is shown that domain structure and magnetization reversal are the same for both systems, but otherwise the behavior of tunnel magnetoresistance is different, because of second MgO barrier effects on the system conductivity. The ability to manipulate the magnetization direction in MTJ systems using temperature annealing is demonstrated. It makes these structures possible for new applications in nanoelectronics as magnetic recording media and high sensitive sensors.