Magnetic excitations in a nanocontact

Abstract

The domain wall in a ferromagnetic nanocontact adopts a specific configuration-Néel-like, vortex, or Bloch-like-depending on the dipole-dipole interactions governed by the size and shape of the contact and its atomic structure. Spontaneous thermal fluctuations between these modes arise in a soft ferromagnet at room temperature when the dimensions of the contact are less than about 10 nm. The giant magnetoresistance of a nanocontact may be reduced, but not eliminated by the mode fluctuations. DOI: 10.1103/PhysRevB.64.020407 PACS number͑s͒: 75.60.Ch, 75.40.Mg, 75.20.Ϫg, 73.63.Rt Contacts between ferromagnetic electrodes which have their magnetization directed parallel or antiparallel to each other are the basis of the emerging science of spin electronics. The electrodes may be separated by a thin metallic layer ͑spin valve͒ or a thin insulating layer ͑tunnel junction͒, or else they may be in direct contact with each other ͑nanocon-tact͒. Much effort is being directed to perfecting spin valves and tunnel junctions as sensors for magnetic recording and as storage elements for magnetic memory. Some nanocontacts show impressive magnetoresistance effects at room temperature, 1 especially in half-metallic systems, 2 but little is known of their magnetic structure. It was recently predicted that very narrow domain walls with dimensions comparable to the length of the nanocontact itself should exist, even in soft magnetic materials. 3 There have been reports of domain walls in ferromagnetic thin films patterned with micron-size constrictions, 4,5 but the studies of domain walls in nanometer-scale constrictions have been restricted to micromagnetic calculations 3 and simulations, based on the continuum approximation, 6,7 or on lattice sums. 8 Here we point out that these nanowalls are subject to magnetic fluctuations, which may influence the spin polarization of electrons as they traverse the contact.To illustrate the idea, consider the simple ''isthmus'' nanocontact illustrated in Fig. 1. A thread of ferromagnetic material of length l and radius r 0 connects two semi-infinite slabs of the same material. We assume a common anisotropy axis Oz throughout, with anisotropy constant K. The atoms are arranged on a square lattice with lattice parameter a. Each atom has a classical spin ͉S͉ϭ1 and moment 1 B . There is nearest-neighbor exchange coupling of strength J. ͑1͒where Gϭ 0 B 2 /4 and the sums are over all atomic sites in the nanocontact. The three terms represent the exchange, anisotropy, and dipole-dipole interactions, respectively.Basically, three types of nanowalls can appear in the isthmus when the two ferromagnetic slabs are oppositely magnetized. These are ͑i͒ Bloch-type walls where the magnetization rotates in the yz plane, with ϭϮ /2, ͑ii͒ Néel-type walls where the magnetization rotates in the zx plane, with ϭ0 or , and ͑iii͒ walls with more complicated vortex structures, where is variable within a plane. The magnetization direction in the slabs adjacent to the isthmus will also be perturbed. The lowest-energy...

Spin-polarised currents and magnetic domain walls

“…Monte Carlo simulations of the magnetisation in nanocontacts confirm Bruno's idea of a constrained wall and showed the possibility of thermal fluctuations in the form of walls in a nanocontact between various different Bloch, Néel, and vortex states [331]. They estimate the height of the energy barrier separating these states as equivalent to about 80 mK per atom, meaning that structures smaller than about 10 nm in size will be unstable at room temperature [332]. This opens a new inelastic scattering mechanism in the contact, which can break phase, flip spins and otherwise reduce the magnetoresistance of a contact.…”

Magnetoresistance of nickel nanocontacts fabricated by different methods

“…Another issue is that thermally activated fluctuations between different magnetic modes are to be expected in constrictions smaller than about 10 nm at room temperature. 27 In conclusion, the sharp low-field feature in the resistance of nickel nanocontacts has been attributed to ballistic magnetoresistance. The smallness of the effect ͑Ϸ1%͒ in Ohmic or in tunnel-barrier contacts reflects the fact that dipolar interactions tend to impose an antiparallel spin alignment across the contact, which is little influenced by the magnetization direction of the electrodes.…”

“…24 While this model is certainly oversimplified, it can explain why no huge ballistic magnetoresistance is observed in this type of contact. Atomic Monte Carlo 25 and micromagnetic 26,27 simulations of different nanocontacts and nanoconstrictions indicate that there are vortex modes in small constrictions. Our own OOMMF simulations ͑Fig.…”

Giant magnetoresistance in quantum magnetic contacts

“…Coey et al concluded [29,30] that giant MR of a nanocontact may be reduced somewhat by these fluctuations, but not eliminated. In recent calculations Zhuravlev et al [31] also predicted giant values and fluctuations of MR in segmented nanowires, when conductance of the wire is quantizes.…”