Probing magnetostatic and magnetotransport properties of the antiferromagnetic iron oxide hematite
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Abstract
With spin dynamics in the THz regime, a lack of stray fields and high stability in the
presence of external magnetic fields, antiferromagnets have several benefits over ferromagnets for spintronics applications. The antiferromagnetic state can be used to store information and excitations of the magnetic order can be used for information transfer and processing in low power, high frequency devices. By utilising antiferromagnetic insulators with low magnetic damping as hosts for propagating spin currents, Joule heating in information transfer can be mitigated, overcoming one of the major limiting factors for device downscaling. The work presented in this thesis focusses on two aspects of antiferromagnetic spintronics for the development of real devices; utilising the spin Hall magnetoresistance to probe the magnetostatic properties of the antiferromagnetic state and demonstrating the long-distance transport of antiferromagnetic magnons for information transfer.
Making use of antiferromagnetic materials for magnetic bits relies on reliably encoding information into the antiferromagnetic state, parameterised by the Néel vector, which
then needs to be read out. The equilibrium position of the magnetic state is determined by the present antiferromagnetic anisotropies which need to be overcome in a controlled manner in order to alter the magnetic state. Approaches to change the magnetic state include spin-orbit torques, thermal processes, strain or magnetic fields. In this thesis the insulating antiferromagnetic iron oxide hematite, the main component of rust, is studied.
This readily available antiferromagnetic material is an attractive prospect for spintronics with a low magnetic damping and accessible spin flop field. By electrically detecting the equilibrium position of the Néel vector as a function of magnetic field and temperature in single crystals and thin films of this material, the key antiferromagnetic anisotropies are determined. Although the effective uniaxial anisotropy field in hematite thin films is found to be comparable to single crystals, the effective field generated by the antisymmetric exchange interaction is found to be four times larger due to distortions of the crystal lattice parallel to the c-axis. The growth of thin films of hematite on a substrate leads to an increase of the effective anisotropy field in the basal plane by two orders of magnitude. It is also found that the growth can also lead to a deviation of the antiferromagnetic anisotropy axis, introducing additional signals into the electrical measurements that need to be carefully considered when interpreting the equilibrium orientation of the Néel vector. In pursuing fully antiferromagnetic devices where information transport, processing and storage all take place concurrently, information needs to be preserved over sufficient distances. However, previous reports have demonstrated that magnons coherently travel through antiferromagnets across nanometre distances before all the information is lost, which is too short for logic operations to occur. Therefore in this work this key issueof spin-transport distance is investigated for the chosen hematite systems. The work reveals efficient long-distance spin transport in both the bulk and thin films of hematite, demonstrating that the low magnetic damping of this material is preserved even in thin films. The magnons are excited by either a polarised interfacial spin-bias or by local heating of the magnetic order and are found to be carried by different magnetic order parameters, either the Néel vector or a field induced magnetisation. This sharp distinction between the propagation mechanisms does not exist in ferromagnetic materials and presents an opportunity for the electrical excitation and detection of magnons without parasitic contributions from excess heat. In the presence of a multi-domain state, the magnon transport is observed to attenuate sharply due to the domain walls leading to scattering of incident magnons, where lower frequency magnons are observed to scatter more. Nevertheless, the intrinsic diffusion length scales of both thin film and bulk antiferromagnets are found to be orders of magnitude larger than previously reported. Overall these results demonstrate the feasibility and promise of antiferromagnetic hematite based spintronic devices