Areal densities of perpendicular hard-disk drives over 500Gb/in2 have already been demonstrated, with 1Tb/in2 densities being forecasted in the near future. However, at these high areal densities the information bearing units on the magnetic storage medium are magnetically unstable at temperatures expected in hard-disk drives. To extend or bypass this limit, new developments in head and media technologies and understanding of their record, readout and noise performances are necessary. The aim of this project was to study the record, readout and noise properties of conventional and future perpendicular magnetic recording media, heads and their related technologies. The objectives were therefore to develop a flexible and robust experimental recording platform to test the performance of different heads and media, and develop the necessary readout theory to predict the replay performance. In line with the project objectives, a high-precision open contact recording tester was developed with 1nm resolution. The open nature of this system allows different heads and media combinations to be tested. A second, closed system was also developed based on a commercial perpendicular hard-drive, modified to serve as a spin-stand to provide signal and noise measurements in practical drive conditions. The readout process in perpendicular recording was modelled based on the reciprocity principle for a shielded TMR head to study the parameters that affect the readout signal performance, and for comparison with the experimental measurements. Measured signal roll-off curves showed a practical linear density of 450KFCI for the commercial perpendicular disk medium, and indicated that non-linear effects happen at linear densities approaching 550KFCI. These results were in agreement with the theoretical calculations of the replay process. Two-dimensional readout scans were found have similar or higher resolution than Magnetic Force Microscopy (MFM) images of the same recorded regions – indicating the versatility and precision of the developed contact tester. Inverse filtering employing the Wiener filter was used to extract the magnetic transition. The extracted transition profiles and transition extents from the replay signals had much higher resolution than MFM images measured for the same transition region, thus showing the applicability of the developed testers for in situ magnetic characterisation. The developed contact and non-contact testers allowed the investigation of a new proposed recording scheme, Shingled Magnetic Recording (SMR). Measured signal roll-off curves of shingled tracks indicated a rise in the signal amplitude at low densities. At higher linear densities the signal performance was the same as conventionally written tracks with guardbands. It was found that a 30% reduction in track width in SMR, increase the areal density by a factor of 1.58 above that in existing hard drives.

<div>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.</div><div>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. </div><div><br></div>

Advanced scanning magnetoresistive microscopy (SMRM) — a robust magnetic imaging and probing technique — is presented. It utilizes conventional recording heads of a hard disk drive as sensors. The spatial resolution of modern tunneling magnetoresistive sensors is nowadays comparable with more commonly used magnetic force microscopes. Important advantages of SMRM are the ability to detect pure magnetic signals directly proportional to the out-of-plane magnetic stray field, negligible sensor stray fields, and the ability to apply local bipolar magnetic field pulses up to 10 kOe with bandwidths from DC up to 1 GHz. The performance assessment of this method and corresponding best practices are discussed in the first section of this work. An application example of SMRM, the study on chemically ordered L10 FePt is presented in a second section. A constructed heater unit of SMRM opens the path to investigate temperature-dependent magnetic properties of the medium by recording and imaging at elevated temperatures. L10 FePt is one of the most promising materials to reach limits in storage density of future magnetic recording devices based on heat-assisted magnetic recording (HAMR). In order to be implemented in an actual recording scheme, the medium Curie temperature should be lowered. This will reduce the power requirements, and hence, wear and tear on a heat source — integrated plasmonic antenna. It is expected that the exchange coupling of FePt to thin Fe layers provides high saturation magnetization and elevated Curie temperature of the composite. The addition of Cu allows adjusting the magnetic properties such as perpendicular magnetic anisotropy, coercivity, saturation magnetization, and Curie temperature. This should lead to a lowering of the switching field of the hard magnetic FeCuPt layer and a reduction of thermally induced recording errors. In this regard, the influence of the Fe layer thickness on the switching behavior of the hard layer was investigated, revealing a strong reduction for Fe layer thicknesses larger than the exchange length of Fe. The recording performance of single-layer and bilayer structures was studied by SMRM roll-off curves and histogram methods at temperatures up to 180 °C In the last section of this work, SMRM advantages are demonstrated by various experiments on a two-dimensional magnetic vortex lattice. Magnetic vortex is a peculiar complex magnetization configuration which typically appears in a soft magnetic structured materials. It consists of two coupled sub-systems: the core, where magnetization vector points perpendicular to the structure plane, and the curling magnetization where magnetic flux is rotating in-plane. The unique properties of a magnetic vortex making it an object of a great research and technological interest for spintronic applications in sensorics or data storage. Manipulation of the vortex core as well as the rotation sense by applying a local field pulse is shown. A spatially resolved switching map reveals a significant "write window" where vortex cores can be addressed correctly. Moreover, the external in-plane magnet extension unit allow analyzing the magnetic vortex rotational sense which is extremely practical for magnetic coupling investigations of magnetic coupling phenomena.

Advanced scanning magnetoresistive microscopy (SMRM) — a robust magnetic imaging and probing technique — is presented. It utilizes conventional recording heads of a hard disk drive as sensors. The spatial resolution of modern tunneling magnetoresistive sensors is nowadays comparable with more commonly used magnetic force microscopes. Important advantages of SMRM are the ability to detect pure magnetic signals directly proportional to the out-of-plane magnetic stray field, negligible sensor stray fields, and the ability to apply local bipolar magnetic field pulses up to 10 kOe with bandwidths from DC up to 1 GHz. The performance assessment of this method and corresponding best practices are discussed in the first section of this work. An application example of SMRM, the study on chemically ordered L10 FePt is presented in a second section. A constructed heater unit of SMRM opens the path to investigate temperature-dependent magnetic properties of the medium by recording and imaging at elevated temperatures. L10 FePt is one of the most promising materials to reach limits in storage density of future magnetic recording devices based on heat-assisted magnetic recording (HAMR). In order to be implemented in an actual recording scheme, the medium Curie temperature should be lowered. This will reduce the power requirements, and hence, wear and tear on a heat source — integrated plasmonic antenna. It is expected that the exchange coupling of FePt to thin Fe layers provides high saturation magnetization and elevated Curie temperature of the composite. The addition of Cu allows adjusting the magnetic properties such as perpendicular magnetic anisotropy, coercivity, saturation magnetization, and Curie temperature. This should lead to a lowering of the switching field of the hard magnetic FeCuPt layer and a reduction of thermally induced recording errors. In this regard, the influence of the Fe layer thickness on the switching behavior of the hard layer was investigated, revealing a strong reduction for Fe layer thicknesses larger than the exchange length of Fe. The recording performance of single-layer and bilayer structures was studied by SMRM roll-off curves and histogram methods at temperatures up to 180 °C In the last section of this work, SMRM advantages are demonstrated by various experiments on a two-dimensional magnetic vortex lattice. Magnetic vortex is a peculiar complex magnetization configuration which typically appears in a soft magnetic structured materials. It consists of two coupled sub-systems: the core, where magnetization vector points perpendicular to the structure plane, and the curling magnetization where magnetic flux is rotating in-plane. The unique properties of a magnetic vortex making it an object of a great research and technological interest for spintronic applications in sensorics or data storage. Manipulation of the vortex core as well as the rotation sense by applying a local field pulse is shown. A spatially resolved switching map reveals a significant "write window" where vortex cores can be addressed correctly. Moreover, the external in-plane magnet extension unit allow analyzing the magnetic vortex rotational sense which is extremely practical for magnetic coupling investigations of magnetic coupling phenomena.