Development and Characterisation of a Zero-Moment Half-Metal
Citation:
Siewierska, Katarzyna Estera, Development and Characterisation of a Zero-Moment Half-Metal, Trinity College Dublin.School of Physics, 2021Download Item:
Abstract:
A dream material for spintronics would have low/zero net moment, no stray fields, high resonance frequency, low damping and be 100 % spin polarised. Such materials combine the best features of a ferromagnet and an antiferromagnet and are called zero moment half-metals (ZMHM). The first ZMHM is a Heusler alloy consisting of Manganese, Ruthenium and Gallium(MRG). Thedemonstrationopenedanewfieldofspintronicswith
no net moment, which has the potential to revolutionise magnetic data
storage and data transfer.
One of the main challenges of incorporating MRG into a spintronic device is ensuring stability of MRG during annealing at temperatures up to T = 350°C. A detailed study of the effect of annealing of blanket MRG films epitaxially grown on MgO substrates, capped with a 3 nm amorphous layer of AlOx , revealed little changes in material properties up to T = 350°C. Mn diffusion is the main concern for MRG-based device
structures and this was addressed by selecting several potential diffusion barrier materials. Then their performance was tested by investigating the effects of annealing were studied in MRG based spin-valve structures with varying spacers. The most promising spacer was found to be Mo with a thickness of 2 nm which did not affect the crystal structure of MRG or the interfaces between the layers and maintained perpendicular magnetic anisotropy of the MRG. Mo also has a large spin diffusion length and can potentially be used as a dusting layer in magnetic tunnel junction devices.
Varying Mn content in MRG was found to vary Mn on 4c and 4d sites, thus increasing Mn content raises Tcomp. Addition of Ru decreases both the Anomalous Hall angle and spin polarisation meaning the Fermi level is pushed upwards in the density of states. Comparison of AHE and SQUID loops shows there is a soft component in net moment, absent in AHE loops where Rxy is proportional to the Mn4c sublattice magnetisation. This means the net moment is canted at zero fields. The canting of the net moment in zero-field could arise from sublattice non-collinearity due to competing exchange on the Mn4c sublattice, as suggested by Heisenberg exchange energies found using mean field theory. This was investigated further by XMCD measurements of sublattice moments.
The imaging of domains directly by polar magneto-optic Kerr effect in a ZMHM was demonstrated for the first time. A detailed study demonstrates two thermally activated mechanisms of magnetisation reversal. In films with weak pinning, magnetisation reversal is dominated by domain wall motion. Magnetisation reversal by nucleation was found in films with strong pinning. In between the energy ranges, the reversal is not dominated by either process, but is rather a mixture of the two. Analysis of virgin domain patterns after thermal demagnetisation allow to visualise the prominent pinning sites. Their spacings are ≈ 240 nm, which provides an upper bound for the track-width of spin-torque domain-wall motion-based devices.
The sublattice specific moments were obtained as a function of applied field and temperature from x-ray magnetic circular magnetism measurements. Crossing of compensation and a spin-flop transition near compensationwereobserved. Usingasimpleuniaxialanisotropymodel, anisotropy constants of sublattices were found to be of the order 10^6 Jm^{−3}. The Mn4a and Mn4c sublattice showed easy-axis and easy-plane anisotropies, respectively. Non-collinearity of sublattices was observed, with the canting angle of sublattices (θ4a,4c −180°) < ±10°. The non-collinearity,
unusually large anisotropy constant values as well as the apparent easy-plane anisotropy found for Mn4c sublattice are most likely to originate from competing exchange on that sublattice. Data from neutron diffraction measurements of sublattice moments in three dimensions and a more complicated model will be a test of this hypothesis.
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Irish Research Council (IRC)
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https://tcdlocalportal.tcd.ie/pls/EnterApex/f?p=800:71:0::::P71_USERNAME:SIEWIERKDescription:
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Author: Siewierska, Katarzyna Estera
Advisor:
Rode, KarstenPublisher:
Trinity College Dublin. School of Physics. Discipline of PhysicsType of material:
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