Synthetic Ferrimagnets and Magneto-Plasmonic Structures for Ultrafast Magnetization Switching
thesisposted on 25.06.2020, 16:55 by Bradlee K Beauchamp
The response time of magnetization switching in current spintronic devices is limited to nanosecond timescales due to the precessional motion of the magnetization during reversal. To overcome this limit two routes of investigation leading to novel recording and logic devices are considered in this thesis: 1) Magnetic tunnel junction structures where the recording and reference layers are replaced by synthetic ferrimagnets and switching is induced by spin transfer torque and 2) Hybrid magneto-photonic devices where switching is induced by plasmon-enhanced all-optical switching. To circumvent limitations of the materials and magnetic properties of CoFeB, the most utilized alloy in spintronics, hcp-CoCrPt, a material that exhibits superior perpendicular anisotropy and thermal stability, is chosen as the ferromagnetic electrode in this work. Whereas actual devices based on the two schemes aforementioned are still in the process of being fabricated, through collaborative work with our international collaborators, this thesis describes fundamental magnetic and structural characterization needed for the realization of said ultrafast switching devices. The magnetic switching behavior of CoCrPt-Ru-CoCrPt synthetic ferrimagnets with perpendicular magnetic anisotropy have been studied in the temperature range from 2K to 300K. It was found that two sets of magnetic transitions occur in the CoCrPt-Ru-CoCrPt ferrimagnet systems studied. The first set exhibits three magnetization states in the 50K – 370K range, whereas the second involves only two states in the 2K and 50K range. The magnetic hysteresis curves of the synthetic ferrimagnet are assessed using an energy diagram technique which accurately describes the competition between interlayer exchange coupling energy, Zeeman energy, and anisotropy energy in the system. This energy diagram analysis is then used to predict the changes in the magnetic hysteresis curves of the synthetic ferrimagnet from 200K to 370K. This represents the potential operation temperature extrema that a synthetic ferrimagnet could be expected to operate at, were it to be utilized as a free layer in a memory or sensor spintronic device in the device configuration described in this dissertation.
Circularly polarized fs laser pulses generate large opto-magnetic fields in magnetic materials, through the inverse Faraday effect. These fields are attributed to be largely responsible for achieving ultrafast all-optical magnetization switching (AOS). All experimental demonstrations of AOS thus far have been realized on thin films over micron-sized irradiated regions. To achieve magnetization switching speeds in the ps and potentially fs time regimes, this work proposes the use of surface plasmon resonances at the interface of hybrid magneto-photonic heterostructures. In addition to the ability of plasmon resonances to confine light in the nm scale, the resonant excitation can largely enhance induced opto-magnetic fields in perpendicular magnetic anisotropy materials. This requires strong spin-photon coupling between the plasmonic and the magnetic materials, which thus requires the minimization of seed layers used for growth of the magnetic layer. This work reports on the development of ultrathin (1 nm thick) interlayers to control the growth orientation of hcp-Co alloys grown on the refractory plasmonic material, TiN, to align the magnetic axis out-of-plane. CoCrPtTa seed layers down to 1 nm were developed to seed the growth of CoCrPt, and the dependence of the quality of the CoCrPt is investigated as Ta composition is varied in the seed layer. Whereas bismuth iron garnet (BIG) meets the magneto-optical requirements for a hybrid magneto-photonic material, its magnetic and structural properties are highly sensitive to the Bi:Fe ratio and must be grown epitaxially on single crystalline substrates. Therefore, in this work we have investigated alternative materials that offer superior magnetic properties and are amenable to growth on inexpensive substrates. Opto-magnetic field enhancements up to 2.6x in Co-ferrite magneto-photonic heterostructures have been obtained via finite element analysis modelling. Alternative materials for plasmon-enhanced all-optical switching such as Co/Pd multilayers have also been investigated. Successful growth of Co/Pd multilayers on TiN using ultrathin Ti interlayers has been achieved.