We investigate the Dzyaloshinskii-Moriya interactions (DMIs) in perpendicularly magnetized thin films of Pt/Co/Pt and Pt/Co/Ir/Pt. To study the effective DMI, arising at either side of the ...ferromagnet, we use a field-driven domain wall creep-based method. The use of only magnetic field removes the possibility of mixing with current-related effects such as spin Hall effect or Rashba field, as well as the complexity arising from lithographic patterning. Inserting an ultrathin layer of Ir at the top Co/Pt interface allows us to access the DMI contribution from the top Co/Pt interface. We show that the insertion of a thin Ir layer leads to reversal of the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously changes the domain wall structure from the right- to the left-handed Neel wall. The use of two DMI-active layers offers an efficient way of DMI tuning and enhancement in thin magnetic films. The comparison with an epitaxial Pt/Co/Pt multilayer sheds more light on the origin of DMI in polycrystalline Pt/Co/Pt films and demonstrates an exquisite sensitivity to the exact details of the atomic structure at the film interfaces.
The microscopic magnetization variation in magnetic domain walls in thin films is a crucial property when considering the torques driving their dynamic behaviour. For films possessing out-of-plane ...anisotropy normally the presence of Néel walls is not favoured due to magnetostatic considerations. However, they have the right structure to respond to the torques exerted by the spin Hall effect. Their existence is an indicator of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Here we present direct imaging of Néel domain walls with a fixed chirality in device-ready Pt/Co/AlOx films using Lorentz transmission electron and Kerr microscopies. It is shown that any independently nucleated pair of walls in our films form winding pairs when they meet that are difficult to annihilate with field, confirming that they all possess the same topological winding number. The latter is enforced by the DMI. The field required to annihilate these winding wall pairs is used to give a measure of the DMI strength. Such domain walls, which are robust against collisions with each other, are good candidates for dense data storage.
The perpendicular magnetic anisotropy K(eff), magnetization reversal, and field-driven domain wall velocity in the creep regime are modified in Pt/Co(0.85-1.0 nm)/Pt thin films by strain applied via ...piezoelectric transducers. K(eff), measured by the extraordinary Hall effect, is reduced by 10 kJ/m(3) by tensile strain out-of-plane ε(z) = 9 × 10(-4), independently of the film thickness, indicating a dominant volume contribution to the magnetostriction. The same strain reduces the coercive field by 2-4 Oe, and increases the domain wall velocity measured by wide-field Kerr microscopy by 30-100%, with larger changes observed for thicker Co layers. We consider how strain-induced changes in the perpendicular magnetic anisotropy can modify the coercive field and domain wall velocity.
We have imaged Néel skyrmion bubbles in perpendicularly magnetised polycrystalline multilayers patterned into 1 µm diameter dots, using scanning transmission x-ray microscopy. The skyrmion bubbles ...can be nucleated by the application of an external magnetic field and are stable at zero field with a diameter of 260 nm. Applying an out of plane field that opposes the magnetisation of the skyrmion bubble core moment applies pressure to the bubble and gradually compresses it to a diameter of approximately 100 nm. On removing the field the skyrmion bubble returns to its original diameter via a hysteretic pathway where most of the expansion occurs in a single abrupt step. This contradicts analytical models of homogeneous materials in which the skyrmion compression and expansion are reversible. Micromagnetic simulations incorporating disorder can explain this behaviour using an effective thickness modulation between 10 nm grains.
We study the energy and creep velocity of magnetic domain walls in perpendicularly magnetised Pt/Co/Ir thin films under strain. We find that the enhancement of domain wall creep velocity under strain ...from piezoelectric transducers is largest in films with the thinnest Co layers (0.56 nm), in which the strain causes the smallest relative change in perpendicular magnetic anisotropy and the largest relative change in domain wall creep velocity. We show how domain wall energy is predictive of the sensitivity of domain wall creep velocity to changes in strain, and thus provide a route to designing magnetic thin film systems for optimum strain control.
We have observed the spatial distribution of magnetic flux in Nb, Cu/Nb, and Cu/Nb/Co thin films using muon-spin rotation. In an isolated 50-nm-thick Nb film, we find a weak flux expulsion (Meissner ...effect) which becomes significantly enhanced when adding an adjacent 40 nm layer of Cu. The added Cu layer exhibits a Meissner effect (due to induced superconducting pairs) and is at least as effective as the Nb to expel flux. These results are confirmed by theoretical calculations using the quasiclassical Green's function formalism. An unexpected further significant enhancement of the flux expulsion is observed when adding a thin (2.4 nm) ferromagnetic Co layer to the bottom side of the Nb. This observed cooperation between superconductivity and ferromagnetism, by an unknown mechanism, forms a key ingredient for developing superconducting spintronics.
Abstract Ferromagnetic films with perpendicular magnetic anisotropy are of interest in spintronics and superconducting spintronics. Perpendicular magnetic anisotropy can be achieved in thin ...ferromagnetic multilayer structures, when the anisotropy is driven by carefully engineered interfaces. Devices with multiple interfaces are disadvantageous for our application in superconducting spintronics, where the current perpendicular to plane is affected by the interfaces. Robust intrinsic PMA can be achieved in certain Co $$_x$$ x Pt $$_{100-x}$$ 100 - x alloys and compounds at any thickness, without increasing the number of interfaces. Here, we grow equiatomic Co $$_{50}$$ 50 Pt $$_{50}$$ 50 and report a comprehensive study on the structural, magnetic, and spin-polarisation properties in the $$L1_1$$ L 1 1 and $$L1_0$$ L 1 0 ordered compounds. Primarily, interest in Co $$_{50}$$ 50 Pt $$_{50}$$ 50 has been in the $$L1_0$$ L 1 0 crystal structure, where layers of Pt and Co are stacked alternately in the 100 direction. There has been less work on $$L1_1$$ L 1 1 crystal structure, where the stacking is in the 111 direction. For the latter $$L1_1$$ L 1 1 crystal structure, we find magnetic anisotropy perpendicular to the film plane. For the former $$L1_0$$ L 1 0 crystal structure, the magnetic anisotropy is perpendicular to the 100 plane, which is neither in-plane or out-of-plane in our samples. We obtain a value for the ballistic spin polarisation of the $$L1_1$$ L 1 1 and $$L1_0$$ L 1 0 Co $$_{50}$$ 50 Pt $$_{50}$$ 50 to be $$47\pm 3\%$$ 47 ± 3 % .
Ferromagnetic films with perpendicular magnetic anisotropy are of interest in spintronics and superconducting spintronics. Perpendicular magnetic anisotropy can be achieved in thin ferromagnetic ...multilayer structures, when the anisotropy is driven by carefully engineered interfaces. Devices with multiple interfaces are disadvantageous for our application in superconducting spintronics, where the current perpendicular to plane is affected by the interfaces. Robust intrinsic PMA can be achieved in certain CoFormula: see textPtFormula: see text alloys and compounds at any thickness, without increasing the number of interfaces. Here, we grow equiatomic CoFormula: see textPtFormula: see text and report a comprehensive study on the structural, magnetic, and spin-polarisation properties in the Formula: see text and Formula: see text ordered compounds. Primarily, interest in CoFormula: see textPtFormula: see text has been in the Formula: see text crystal structure, where layers of Pt and Co are stacked alternately in the 100 direction. There has been less work on Formula: see text crystal structure, where the stacking is in the 111 direction. For the latter Formula: see text crystal structure, we find magnetic anisotropy perpendicular to the film plane. For the former Formula: see text crystal structure, the magnetic anisotropy is perpendicular to the 100 plane, which is neither in-plane or out-of-plane in our samples. We obtain a value for the ballistic spin polarisation of the Formula: see text and Formula: see text CoFormula: see textPtFormula: see text to be Formula: see text.
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