We have investigated the magnetic and transport behavior of MgO based magnetic tunnel junctions incorporating ${\text{Co}}_{40}{\text{Fe}}_{40}{\text{B}}_{20}$ free layers with thicknesses in the vicinity of $15\text{Å}$. The magnetic response of the free layer changes rapidly as its thickness decreases. Linear and hysteresis-free switching is obtained when the CoFeB free layer thickness is thinner than a critical thickness of $\sim 15\text{Å}$. The tunneling magnetoresistance and hysteresis properties of the free layer change abruptly around the critical thickness. We have analyzed the transfer curves of junctions below the critical thickness and show that they agree well with the Langevin equation describing superparamagnetism. At even lower thicknesses, the total magnetic moment of the magnetic clusters decreases rapidly, possibly due to the reduction in the magnetic ordering temperature to below room temperature.

• Received 26 November 2008

DOI:https://doi.org/10.1103/PhysRevB.79.014418