Research

Instituto de Física dos Materiais da Universidade do Porto

IFIMUP

Physical characterization of magnetic tunnel junctions

 

Pinholes in thin low resistance MgO-based magnetic tunnel junctions probed by temperature dependent transport measurements

 

Magnetic tunnel junctions (MTJs) are the most promising candidates for magnetic sensors for high density recording media. To be implemented in read heads, MTJs must have a low resistance-area product (RxA), to achieve a high readout speed, but still be sensitive enough to read the ever smaller magnetic bit. The recent successful fabrication of tunnel junctions with crystalline MgO(001) barriers displaying very large tunnel magnetoresistive ratios opened a new opportunity to develop read heads for ultra-high density hard drives. In fact, a tunnel magnetoresistance (TMR) of over 50% with RxA as low as 0.4 Ohm um2 was recently obtained in thin MgO MTJs. However, a TMR-decrease with decreasing MgO thickness is usually observed and, for the thinnest barriers, the presence of pinholes cannot be excluded.

Here we study the transport properties of MgO-based low resistance MTJs, deposited by physical vapor deposition (PVD) on a glass substrate. The structure of the prepared MTJs was (thickness in nm) MnPt 20/CoFe 2/Ru 0.9/CoFeB 3/MgO 0.6/CoFeB 3/Ru 5. The samples display RxA values as low as 8 Ohm um2 and TMR as high as 80% at room temperature. Temperature dependent (300-20 K) R-measurements allowed us to observe different behaviors for different samples and magnetic states. All samples showed positive dR/dT for the parallel state, indicating metallic-dominated behavior without obvious conductance quantization features, so that laterally sizeable pinholes are already present in the barrier. However, three different behaviors were observed for the antiparallel state: negative dR/dT, positive dR/dT or, interestingly, a crossover from negative to positive dR/dT at intermediate temperatures (~150 K) in some cases. Besides the interesting competition between tunnel and metallic transport in the studied samples, the observed dR/dT crossover indicates that metallic and tunnel resistances can reach similar values in some of the samples.