Metal Alloy Magnetic Nanodots and Magnetic Tunnel Junctions

2018-10-15T19:00:33Z (GMT) by Ahmed Abdelgawad
The rapid technological advancement in spintronics and data storage necessitates the development of novel fabrication techniques to make close-packed magnetic dots or devices that have a long-range order. Additionally, characterization<br>techniques on the single dot level are also needed to characterize switching in a single dot. In this thesis, I present a novel technique to fabricate ultrahigh density closepack nanodots with a long range order (~1 cm2) and sizes in the range of 6-20 nm on substrates of different materials. The technique relies on utilizing a monolayer of<br>synthesized highly-monodisperse spherical Fe3O4 nanoparticles to create a nanohole substrate, overfilling the nanohole substrate with the magnetic materials and backetching until the nanodot morphology is confirmed via scanning electron microscopy (SEM). The fabrication of FePt nanodots on SiOx, SiNx, and TiNx substrates both at<br>room temperature and elevated temperature is presented; and the effect of the substrate surface energy on wetting of FePt dots is demonstrated. Moreover, the magnetic<br>properties of FePt dots on different substrates are discussed. Electric transport measurements on single 16 nm diameter hemispherical FePt nanoparticle are conducted at room temperature via a conductive atomic force<br>microscope (C-AFM) . The telegraphing in the measured current shows two states and the frequency of telegraphing depends on the applied bias. The telegraphing is possibly<br>due to electron tunneling due to trapping/detrapping of electrons in localized defects within the tunneling layer.<br>Electric transport measurements on single CoFeB nanodiscs with diameters of 20 nm, 40 nm, 60 nm, and 80 nm are conducted at room temperature via C-AFM. The CoFeB nanodiscs are free layers in a magnetic tunnel junction (MTJ) stack in the presence of a magnetic reference layer having a well-define in-plane magnetization<br>direction. All nanodiscs show telegraphing of the measured current as their magnetization switches from a parallel state to an antiparallel state relative to the magnetic reference layer. The frequency of telegraphing show size dependence. The 20 nm and 40 nm discs telegraph much slower than the 60 and 80 nm discs. Object oriented micromagnetic framework (OOMMF) [1] simulations reveal that the<br>magnetization of the 20 and 40 nm discs switch mostly by coherent rotation of spins while the magnetization of the 60 and 80 nm discs switch incoherently and initiated by<br>spin curling at the edges, which lowers the energy barrier that needs to be surmounted in order for the magnetic switch to occur. This can be useful in designing fast switching, energy efficient devices for stochastic computing.<br>Magnetization of single 100 nm diameter, 10 nm thick permalloy caps is studied at room temperature using magnetic force microscopy (MFM). A magnetic vortex<br>structure is stable at room temperature. Alternating gradient magnetometry (AGM) of a monolayer and sparse array of caps shows evidence of vortex nucleation in the<br>presence of an in-plane field. Out-of-plane AGM reveals interesting transiting from a single domain state at high field to an onion state at intermediate field to a vortex state<br>at remanence. OOMMF simulations corroborate the experimental results and predict the stability of a vortex state in caps of diameters down to 15 nm.<br>