Electrical currents flowing in ferromagnetic materials are spin-polarised as a result of the spin-dependent band structure. When the spatial direction of the polarisation changes, in a domain structure, the electrons must somehow accommodate the necessary change in direction of their spin angular momentum as they pass through the wall. Reflection, scattering, or a transfer of angular momentum onto the lattice are all possible outcomes, depending on the circumstances. This gives rise to a variety of different physical effects, most importantly a contribution to the electrical resistance caused by the wall, and a motion of the wall driven by the spin-polarised current. Historical and recent research on these topics is reviewed. Contents PAGE 1. Introduction5862. Spin-polarised current587 2.1. Tunnelling current spin polarisation589 2.2. Ballistic current spin polarisation592 2.3. Diffusive current spin polarisation5933. Magnetic Domain Walls598 3.1. Basics of domain walls598 3.1.1. Domain wall thickness and...

/pdf/spin-polarised-currents-and-magnetic-domain-walls-1cmsmewys0.pdf

Spin-polarised currents and magnetic domain walls

The mutual influence of superconductivity and magnetism in F/S systems, i.e. systems of alternating ferromagnetic (F) and superconducting (S) layers, is comprehensively reviewed. For systems with ferromagnetic metal (FM) layers, a theory of the proximity effect in the dirty limit is constructed based on the Usadel equations. For an FM/S bilayer and an FM/S superlattice, a boundary-value problem involving finite FM/S boundary transparency and the diffusion and wave modes of quasi-particle motion is formulated; and the critical temperature Tc is calculated as a function of FM- and S-layer thicknesses. A detailed analysis of a large amount of experimental data amply confirms the proposed theory. It is shown that the superconducting state of an FM/S system is a superposition of two pairing mechanisms, Bardin – Cooper – Schrieffer's in S layers and Larkin – Ovchinnikov – Fulde – Ferrell's in FM ones. The competition between ferromagnetic and antiferromagnetic spontaneous moment orientations in FM layers is explored for the 0- and π-phase superconductivity in FM/S systems. For FI/S structures, where FI is a ferromagnetic insulator, a model for exchange interactions is proposed, which, along with direct exchange inside FI layers, includes indirect Ruderman – Kittel – Kasuya – Yosida exchange between localized spins via S-layer conduction electrons. Within this framework, possible mutual accommodation scenarios for superconducting and magnetic order parameters are found, the corresponding phase diagrams are plotted, and experimental results are explained. The results of the theory of the Josephson effect for S/F/S junctions are presented and the application of the theory of spin-dependent transport to F/S/F junctions is discussed. Application aspects of the subject are examined.

https://iopscience.iop.org/article/10.1070/PU2002v045n02ABEH001025/pdf

Competition between superconductivity and magnetism in ferromagnet/superconductor heterostructures

We report the first magnetic study of pure and metastable hexagonal close-packed (hcp) Ni nanoparticles (sample 1). We also produced stable face-centered cubic (fcc) Ni nanoparticles, as mixtures with the hcp Ni nanoparticles (samples 2 and 3). We compared the magnetic properties of the hcp Ni nanoparticles with those of the fcc Ni nanoparticles by observing the evolution of magnetic properties from those of the hcp Ni nanoparticles to those of the fcc Ni nanoparticles as the number of fcc Ni nanoparticles increased from sample 1 to sample 3. The blocking temperature (TB) of the hcp Ni nanoparticles is ∼12 K for particle diameters ranging between 8.5 and 18 nm, whereas those of the fcc Ni nanoparticles are 250 and 270 K for average particle diameters of 18 and 26 nm, respectively. The hcp Ni nanoparticles seem to be antiferromagnetic for T TB. This is very different from the fcc Ni nanoparticles, which are ferromagnetic for T TB. This unusua...

Comparison of the magnetic properties of metastable hexagonal close-packed Ni nanoparticles with those of the stable face-centered cubic Ni nanoparticles.

Orientation growth modulated magnetic-carbon microspheres toward broadband electromagnetic wave absorption

We present measurements of domain wall resistivity, pinned by nanoconstrictions in single layer ferromagnetic wires of Ni80Fe20 and Ni. Unpinning domain walls from the constriction by current-induced switching allows for an unambiguous measurement of their resistivity changes, namely, 1.7% in Ni80Fe20 and 1.82% in Ni and both positive, which supports the theory of spin-dependent impurity scattering. By deriving an empirical relation for the various constriction widths, the large percentage changes of resistivity in ballistic nanocontacts are reproduced, showing a correlation between domain wall magnetoresistance and ballistic magnetoresistance.

Direct observation of domain wall scattering in patterned Ni80Fe20 and Ni nanowires by current-voltage measurements.

We studied the structure and magnetic properties of molecular-beam epitaxy grown 300 A thick Fe/Pt multilayers with different bilayer thickness and annealing temperature. The Fe/Pt multilayers were deposited on 100 A thick Pt buffer layers at 100 °C on Al2O3 (0001) substrates. The structure of as-deposited Fe/Pt films was fcc(111). While the postannealing temperature ⩾400 °C, an additional FePt(100) orientation was observed. A large coercivity range, namely, 200–16 000 Oe can be tuned by varying the bilayer thickness and annealing temperature.

Annealing effect on the Fe/Pt multilayers grown on Al2O3 (0001) substrates

Thickness dependence on the domain wall resistivity of zig zag thin permalloy wires was studied from 10 to 300 K. The maximum domain wall resistivity was obtained in wire with 100-nm-film thickness. The multidomain state resistivity was 14.29 μΩ cm, while single-domain state resistivity was 14.36 μΩ cm at 10 K. The ratio of domain wall magnetoresistance was measured to be 0.034%, 0.112%, and 0.258%, and the magnetic field where the domain wall started to switch was measured as −70, −40, and +80 Oe for wires with thicknesses of 20, 40, and 100 nm, respectively, at 250 K. Domain wall resistivity was nearly independent of temperature for wire with 40-nm-film thickness but varied significantly with temperature for 100-nm-thick wire between 10 and 300 K.

/pdf/magnetoresistance-study-in-thin-zig-zag-nife-wires-48zaglqngg.pdf

Magnetoresistance study in thin zig zag NiFe wires

Face-centered-tetragonal L10 Fe∕Pt multilayers have been prepared by molecular-beam epitaxy on MgO (001) substrates at 500°C. An ultrathin MgO layer, 1, 3, and 5nm in thickness, respectively, was inserted into the middle of Fe∕Pt multilayers by using an e-beam evaporation without introducing any oxygen source. The insertion of MgO layer effectively reduced the average grain size and the size of magnetic domains. Out-of-plane coercivity of the FePt films decreased with increasing thickness of the MgO insert layer. It was found through the studies of angular dependent coercivity that upon MgO insertion the decrease in coercivity is originated from the weakening of domain-wall motion, while domain rotation is enhanced depicting a decoupling of intergrain interaction.

Effects of an ultrathin MgO inserted layer on the magnetic properties of FePt films

Magnetic interactions between permalloy elliptical rings in different arrangements of two-dimensional arrays were investigated. The edge-to-edge distances between units along the long axis were changed from 2μm down to 100nm. Formation of domain walls during magnetization reversals results in magnetostatic coupling between adjacent units. When two neighboring rows were shifted one-half pitch along the short axis to form a shift-type array, large stable field range for vortex states was obtained.

C. C. Yu

Papers

Annealing effect on the Fe/Pt multilayers grown on Al2O3 (0001) substrates

Magnetoresistance study in thin zig zag NiFe wires

Effects of an ultrathin MgO inserted layer on the magnetic properties of FePt films

Magnetic interaction in nanometer line width elliptical ring arrays

Microstructure and magnetoresistance of MgO thin film with CoFeB and CoFeC underlayers