FeGa based tunneling magnetoresistance junctions and strain sensors
Tunnel magnetoresistance (TMR) and inverse magnetostriction based strain gauges have gained immense importance due to their high spatial resolution and extremely high gauge factors. A TMR junction comprises of two ferromagnetic electrodes separated by an insulating barrier layer. One of the ferromagnetic layer is soft magnetic which is free to rotate its magnetization under external magnetic field, the other ferromagnet is hard magnetic and is stable under the same external field. An intentional introduction of highly magnetostrictive thin film as a soft magnetic layer would result in stress induced rotation of the soft magnetic film. Applications based on this effect include pressure sensors and strain gauges. Galfenol (FeGa) is a highly magnetostrictive material and was chosen as the candidate for the magnetostrictive free layer. Investigation of FeGa based magnetostrictive TMR junction is therefore a very interesting topic and forms the basis of this work. For this purpose FeGa thin film was first deposited on MgO (100) oriented substrate by thin film sputter deposition technique. The structural and magnetic properties are investigated and optimized without embedding FeGa film into the tunnel junctions. It was shown that highly oriented FeGa thin film can be grown on MgO substrate. Furthermore the film showed strain sensitivity and magnetostriction as high as 100±10*10-6. Soft magnetic behaviour was observed with reducing sputter power suggesting that these films can be used as soft magnetic or sensing layer in the TMR junctions. Two basic types of TMR junctions based on FeGa thin film was proposed. First type of TMR junctions was based on the thick and thin film FeGa electrodes and MgO tunnel barrier sputter deposited on MgO substrate. The as prepared junctions showed TMR ratio as high as 9 %. Annealing of these junctions resulted in drastic reduction of the TMR ratio. It was found out by that the Ga diffusion into the MgO barrier layer during annealing as the reason for the reduction of the TMR ratio. The second concept was based on exchange biased TMR junctions on Si/SiO2 substrate. In these TMR junctions a 2-4 nm CoFeB interlayer was introduced between FeGa film and the barrier layer to avoid the diffusion of Ga into the MgO barrier layer due to annealing. As a result TMR ratio as high as 65% was achieved on annealed TMR junctions. A moderate strain sensitivity was measured making it possible for the exchange bias based TMR junctions to be utilized as strain sensors.
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