Heat Assisted Magnetic Recording Media Based on Exchange Bias

A study of a new paradigm for a heat assisted magnetic recording (HAMR) media based on the use of exchange bias is presented. Exchange bias occurs when an antiferromagnetic (AF) layer such as IrMn is grown in contact with a ferromagnetic (F) layer and the system if field cooled resulting in a hysteresis loop shifted along the field axis. In this concept the temperature dependent anisotropy is provided by IrMn. Therefore the information is stored in the AF layer and the recording layer is actually part of the read/write structure when field cooled. The F layer when magnetised, serves to align the F layer in the direction required to store the information and then provides a read out signal indicating in which direction the AF layer is oriented. Hence in a complex way the “recording layer” is actually part of the read/write head. The key to achieve a structure of this kind is the control of the orientation of the Mn ions of the IrMn such that they are aligned perpendicular to the plane of the film. In this way a perpendicular exchange bias required for information storage in the AF layer has been achieved. Over 300 samples have been prepared and evaluated to determine the optimised structure. A segregated sample CoCrPt-SiO2 was sputtered using a pressed powder target in a HiTUS deposition system. Dual Ru seed layers of 8nm and 12nm were deposited using 3mTorr and 30mTorr process pressure, respectively. The median grain size of D_m = (6.2 ± 0.2)nm was achieved using these sputtering conditions. High resolution cross section TEM imaging has been used to show that the CoCrPt grains remains segregated by SiO2 after the deposition of IrMn. The key feature of this media is that the recorded information is impossible to be erased by a demagnetising field. In order to achieve this requirement the hysteresis loop has to be completely shifted to a negative field. The highest shift or exchange bias of H_ex = (240 ± 5)Oe was measured at room temperature. This was achieved by depositing an ultrathin (0.8nm) Co interlayer above a Pt seed layer

HAMR Media Based on Exchange Bias

In this work we describe an alternative strategy for the development of heat assisted magnetic recording (HAMR) media. In our approach the need for a storage material with a temperature dependent anisotropy and to provide a read out signal is separated so that each function can be optimised independently. This is achieved by the use of an exchange bias structure where a conventional CoCrPt-SiO2 recording layer is exchange biased to an underlayer of IrMn such that heating and cooling in the exchange field from the recording layer results in a shifted loop. This strategy requires the reorientation of the IrMn layer to allow coupling to the recording layer. This has been achieved by the use of an ultrathin (0.8nm) layer of Co deposited beneath the IrMn layer. In this system the information is in effect stored in the antiferromagnetic (AF) layer and hence there is no demagnetising field generated by the stored bits. A loop shift of 688 Oe has been achieved where both values of coercivity lie to one side of the origin and the information cannot be erased by a magnetic field

Fabrication of magnetic tunnel junctions with a metastable bcc Co3Mn disordered alloy as a bottom electrode

We fabricated MgO barrier magnetic tunnel junctions (MTJs) with a Co3Mn alloy bottom and FeCoB top electrodes. The (001)-oriented epitaxial films of the metastable bcc Co3Mn disordered alloys obtained showed saturation magnetization of approximately 1640 emu/cm3. The transmission electron microscopy showed that the MgO barrier was epitaxially grown on the Co3Mn electrode. Tunnel magnetoresistance of approximately 150% was observed at room temperature after the annealing of MTJs at 350◦C, indicating that bcc Co3Mn alloys have relatively high spin polarization

Current-induced crystallisation in Heusler alloy films for memory potentiation in neuromorphic computation

The current information technology has been developed based on von Neumann type computation. In order to sustain the rate of development, it is essential to investigate alternative technologies. In a next-generation computation, an important feature is memory potentiation, which has been overlooked to date. In this study, potentiation functionality is demonstrated in a giant magnetoresistive (GMR) junction consisting of a half-metallic Heusler alloy which can be a candidate of an artificial synapse while still achieving a low resistance-area product for low power consumption. Here the Heusler alloy films are grown on a (110) surface to promote layer-by-layer growth to reduce their crystallisation energy, which is comparable with Joule heating induced by a controlled current introduction. The current-induced crystallisation leads to the reduction in the corresponding resistivity, which acts as memory potentiation for an artificial GMR synaptic junction

Non-destructive imaging on synthesised nano-particles

Our recently developed non-destructive imaging technique was applied for the characterisation of nano-particles synthesised by X-ray radiolysis and sol-gel method. The interfacial conditions between the nano-particles and the substrates were observed by subtracting images taken by scanning electron microscopy at controlled electron acceleration voltages to allow backscattered electrons to be generated predominantly below and above the interfaces. The interfacial adhesion was found to be dependent on solution pH used for the particle synthesis or particle suspension preparation, proving the change in the particle formation/deposition processes with pH as anticipated and agreed with the prediction based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We found that our imaging technique was useful for the characterisation of interfaces hidden by nano-particles to reveal the formation/deposition mechanism, and can be extended to the other types of interfaces

Revealing the importance of interfaces for pure spin current transport

Spin transport phenomena underpin an extensive range of spintronic eects. In particular spin transport across interfaces occurs in most device concepts; but is so far poorly understood. As interface properties strongly impact spin transport, one needs to characterize and correlate them to the fabrication method. Here we investigate pure spin current transport across interfaces and connect this with imaging of the interfaces. We study the detection of pure spin currents via the inverse spin Hall eect in Pt and the related spin current absorption by Pt in Py-Cu-Pt lateral spin valves. Depending on the fabrication process to pattern the Cu bridge, we either determine a large (inverse) spin Hall eect signal and low spin absorption by Pt or vice versa. We explain these counter-intuitive results by the fabrication induced varying quality of the Cu/Pt interfaces, which is directly revealed via a special scanning electron microscopy technique for buried interface imaging

Heusler alloys for spintronic devices: review on recent development and future perspectives

Heusler alloys are theoretically predicted to become half-metals at room temperature (RT). The advantages of using these alloys are good lattice matching with major substrates, high Curie temperature above RT and intermetallic controllability for spin density of states at the Fermi energy level. The alloys are categorised into half- and full-Heusler alloys depending upon the crystalline structures, each being discussed both experimentally and theoretically in Section 2. Fundamental properties of ferromagnetic Heusler alloys are described in Section 3. Both structural and magnetic characterisations on an atomic scale are typically carried out in order to prove the half-metallicity at RT as described in Section 4. Atomic ordering in the Heusler-alloy films is directly observed by X-ray diffraction and is also indirectly probed via the temperature dependence of electrical resistivity. Element specific magnetic moments and spin polarisation of the Heusler alloy films are directly measured using X-ray magnetic circular dichroism and Andreev reflection, respectively. By employing these ferromagnetic alloy films in a spintronic device, efficient spin injection into a non-magnetic material and large magnetoresistance are discussed in Section 5. Fundamental properties of antiferromagnetic Heusler alloys are described in Section 6. Both structural and magnetic characterisations on an atomic scale are shown in Section 7. Atomic ordering in the Heusler-alloy films is indirectly measured by the temperature dependence of electrical resistivity. Antiferromagnetic configurations are directly imaged by X-ray magnetic linear dichroism and polarised neutron reflection. Section 8 explains applications of the antiferromagnetic Heusler-alloy films. The other non-magnetic Heusler alloys are listed in Section 9. A brief summary is provided at the end of this review