Narrow Domain Walls Research Articles

Spin accumulation contribution to domain wall resistivity in SrRuO3

Abstract SrRuO3 is a 4d itinerant ferromagnet (T c ∼150 K ) characterized by stripe domain structure ( 200 nm wide) with no closure domains and very narrow (∼3 nm ) Bloch domain walls. The domain walls induce excess resistivity which we measure between 2 K and Tc. To distinguish the spin accumulation contribution to the domain wall resistivity (DWR), we measured the DWR for different angles between the current and the domain walls, and we identified a term proportional to the net spin-polarized current flowing perpendicular to the domain walls.

Electrons in ferromagnets with domain walls

Domain walls can significantly modify electronic properties of ferromagnetic metals. In this paper we consider theoretically the influence of domain walls on transport properties of ferromagnetic materials and the results are compared with recent experiments. In the case of diffusive transport through a thick domain wall, the semiclassical approximation is applied and a local spin transformation is performed, which replaces the system with a domain wall by the corresponding system without a domain wall but with an additional gauge field. Due to a redistribution of single-particle electron states at the wall, one obtains then either negative or positive contributions to resistivity. The situation is different for very narrow and/or constrained domain walls. In such a case, the semiclassical approximation is not valid. Instead of this the approach based on scattering matrix is applied. The domain wall then gives rise to a large positive contribution to electrical resistivity. The corresponding magnetoresistance can be therefore very large, which is in agreement with recent experiments. The limiting case of narrow domain walls in systems with a single conduction channel is analysed in detail, with the effects due to electron–electron interaction taken into account. In this particular case the magnetoresistance due to a domain wall can be extremely large.

Random telegraph noise in a nickel nanoconstriction

Nickel nanoconstrictions about 20 nm wide produced by electron beam lithography in a 60 nm nickel film exhibit resistivities in the kΩ range with a nonlinear and asymmetric I–V characteristic. Noise spectra of the contacts sometimes deviate from 1/f behavior due to random telegraph fluctuations at room temperature with a frequency in the 10 Hz range. The resistance fluctuations between the two states are about 0.1%. The time spent in the high resistance state increases as we increase the temperature, and the discrete fluctuations eventually disappear with an increase of the temperature of more than about 15 °C. An explanation is proposed in terms of electron-wind electromigration which interacts with the narrow domain wall formed at the nanocontact.

Angular dependence of domain wall resistivity inSrRuO3films

${\mathrm{SrRuO}}_{3}$ is a $4d$ itinerant ferromagnet ${(T}_{c}\ensuremath{\sim}150\mathrm{K})$ with stripe domain structure. Using high-quality thin films of ${\mathrm{SrRuO}}_{3}$ we study the resistivity induced by its very narrow (\ensuremath{\sim}3 nm) Bloch domain walls ${\ensuremath{\rho}}_{\mathrm{DW}}$ (DWR), at temperatures between 2 K and ${T}_{c}$ as a function of the angle $\ensuremath{\theta}$ between the electric current and the ferromagnetic domains walls. We find that ${\ensuremath{\rho}}_{\mathrm{DW}}(T,\ensuremath{\theta})={\mathrm{sin}}^{2}\ensuremath{\theta}{\ensuremath{\rho}}_{\mathrm{DW}}(T,90)+B(\ensuremath{\theta}){\ensuremath{\rho}}_{\mathrm{DW}}(T,0),$ which provides the first experimental indication that the angular dependence of spin accumulation contribution to DWR is ${\mathrm{sin}}^{2}\ensuremath{\theta}.$ We expect magnetic multilayers to exhibit a similar behavior.

Half-metallic oxide point contacts

A classification of half-metals is illustrated with examples of ferromagnetic and ferrimagnetic oxides. I : V characteristics and magnetoresistance of contacts between oxides with localized electrons (Fe3O4) and delocalized electrons ((La0.7Sr0.3)MnO3) are discussed. The magnetoresistance is large when the conductance of the contact is less than the quantum of conductance G0 = 2e2/h. Broad distributions of point contacts are investigated in pressed powder compacts, illustrated by powder magnetoresistance measurements of CrO2, (La0.7Sr0.3)MnO3 and Fe3O4. When exchange coupling exists across an atomic scale contact, a narrow domain wall of width comparable to the contact size may be pinned there. Some consequences are thermal magnetic mode fluctuations, nonlinear I : V characteristics due to electron pressure on the wall and current-induced switching in a bistable `peanut' device with two closely spaced nanoconstrictions.

Ferromagnetic exchange-spring nanocomposites of A1 + L1/sub 0/ CoPt

Natural ferromagnetic exchange-spring nanocomposite thin films have been synthesized from sputter-deposited CoPt. Depending upon the details of the annealing treatment, the films consist of two phases in varying proportions: the chemically disordered A1 phase with low coercivity and the chemically ordered L1/sub 0/ phase with high coercivity. Transmission electron microscopy studies reveal fiber texture formation accompanying grain growth in the annealed films and further show that each grain is comprised, on average, of six chemically ordered domains. Magnetic studies show single-phase magnetic character, signaling robust interphase exchange coupling, and indicate that the coercivity development becomes more pinning controlled as the volume fraction of high-coercivity L1/sub 0/ phase increases. The increase of domain wall pinning with increased L1/sub 0/ content is attributed to the microstructure of the L1/sub 0/-ordered phase, which contains nanoscale defects such as antiphase, c-axis variant, and grain boundaries that are a source of effective pinning sites for the narrow (/spl sim/5 nm) domain walls found in L1/sub 0/ CoPt.

Structural chemistry, magnetism and electrical properties of binary Pr-silicides

Binary praseodymium silicides have been thoroughly investigated with respect to their crystal chemistry and physical properties. Formation and structure types of all known binary phases have been confirmed. Moreover, the crystal structure determination of a phase, previously reported to exist with the approximate composition Pr 3Si 4, was successfully performed from X-ray powder data on the basis of the isotypic orthorhombic Ho 3Si 4 type of structure. The magnetic properties of these Pr-silicides are characterized by the onset of ferromagnetic order below 110 K. Pr 5Si 3 ( T C=44 K) shows a canted spin structure. Pr 5Si 4 ( T C=40 K) and PrSi ( T C=51 K) reveal a weak aniosotropy. Whereas Pr 3Si 4 ( T C=105 K) exhibits a magnetic behavior as typically observed in narrow domain wall ferromagnets favoring easy axis anisotropy, the reduced saturation moment suggests a ferrimagnetic spin structure. PrSi 2− x ( x=0.12, GdSi 2 type) exhibits an antiferromagnetic transition at a Neél temperature T N=11 K followed by a spin reorientation towards a parallel spin alignment at 9 K. The electrical resistivities resemble a temperature dependence of metals, with characteristic changes of slope of the ρ( T) curves at the Curie temperatures. The ρ( T) plots follow ∼ T 2 relations as predicted for isotropic ferromagnets in the ordered state for the compound PrSi 1.33, only, whilst for the other compounds a ∼ T 2.8–3.7 dependency was observed.

Domain walls in ferromagnetic nanoconstriction

180° domain walls in ferromagnetic nanoconstrictions are investigated by classical atomic Monte Carlo simulations. Two types of constrictions are considered; one is a uniform circular cylinder (isthmus), the other is a double-truncated cone (hour glass). The wall width is determined by the effective length of the constriction, which may be as little as a nanometer. The wall can have a Néel-like configuration for constrictions much narrower than the normal wall width, but there is a crossover region with vortex-type walls before reaching a Bloch-type wall at larger diameters. In very narrow constrictions, effects of the atomic-scale structure become evident as the domain-wall structure depends on the number of atoms in the cross section. The simulations confirm the prospect of creating very narrow domain walls in a nanoconstricted soft magnetic material, and they indicate the possibility of spontaneous thermal fluctuations between different magnetic modes in walls smaller than about 10 nm.

Magnetization study of a UIrAl single crystal

The field and temperature dependence of the magnetization has been studied for the first time on a single crystal of the hexagonal intermetallic UIrAl. The compound is a ferromagnet below the Curie temperature TC=62 K with a ground-state spontaneous magnetic moment of Ms=0.96 μB. The huge uniaxial anisotropy (the anisotropy field Ba exceeds 200 T) leads to characteristic hysteresis properties which obey the model of high intrinsic coercivity of narrow domain walls

Giant magnetoresistive effect in L1 0 ordered FePt films

GMR behavior has been studied in L1 0 Fe x Pt 1− x (0.4⩽ x⩽0.6) alloy films, which appears as the excess resistivity of about 0.2% at around H c. This GMR is attributed to the spin-dependent electron scattering combined with the narrow domain wall of about 9 nm thick because of huge magnetocrystalline anisotropy.

Atomic-scale magnetic domain walls in quasi-one-dimensional Fe nanostripes.

Fe nanostripes on W(110) are investigated by Kerr magnetometry and spin-polarized scanning tunneling microscopy (SP-STM). An Arrhenius law is observed for the temperature dependent magnetic susceptibility indicating a one-dimensional magnetic behavior. The activation energy for creating antiparallel spin blocks indicates extremely narrow domain walls with a width on a length scale of the lattice constant. This is confirmed by imaging the domain wall by SP-STM. This information allows the quantification of the exchange stiffness and the anisotropy constant.

Magnetoresistance of Half-Metallic Oxide Nanocontacts

Magnetoresistive effects $(R(0)\ensuremath{-}R(H))/R(H)$ exceeding $500%$ are found at room temperature in a field of 7 mT in nanocontacts between ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ crystallites. The shape of the $I(V)$ curve depends on field and the magnitude of the magnetoresistance is correlated with the resistance, the largest effects occurring when $Rg100\mathrm{k}\ensuremath{\Omega}$. The explanation proposed involves hopping transport of spin-polarized electrons through a narrow domain wall pinned at the nanocontact; spin pressure on the domain wall pushes it out into the electrode, leading to the nonlinearity of the $I(V)$ characteristic. Application of current-induced wall motion in a simple fast-switching magnetic memory element is proposed.

Structural and magnetic properties of the rare-earth–borocarbides, R 5B 2C 6 (R=Y, Ce–Tm)

The compounds R 5B 2C 6, R=Y, Ce–Tm, were prepared by arc melting the elements, rare earth ingots, boron and carbon. From X-ray powder analyses these compounds were found to crystallize essentially with tetragonal symmetry, space group P4 (No. 75). The yttrium containing sample is a Pauli type paramagnet below room temperature down to 4.2 K. The magnetic properties of Ce 5B 2C 6 are characterized by an intermediate valence behavior. Gd 5B 2C 6 orders antiferromagnetically at a Neél temperature T N=26 K and reveals a metamagnetic transition above B=0.5 T. All the remaining samples investigated undergo ferromagnetic transitions in the temperature regime below T=40 K. These borocarbides exhibit a magnetic behavior typical for narrow domain wall ferromagnets. Both the Curie temperatures, T C, as well as the paramagnetic Curie temperatures, θ P, scale roughly with the de Gennes factor and hence the indirect exchange interaction via the conduction electrons (RKKY-interacting) is the dominating force of the R–R coupling mechanism.

Structural chemistry, magnetism and electrical properties of binary Nd silicides

Binary neodymium silicides were thoroughly investigated with respect to their crystal chemistry and physical properties. Formation and structure types of all known binary phases were confirmed. Moreover, the crystal structure determination of a phase previously reported to exist with approximate composition Nd 3Si 4 was successfully performed from X-ray powder data on the basis of the isotypic orthorhombic Ho 3Si 4-type structure. The magnetic properties of these Nd silicides are characterized by the onset of ferromagnetic order below 120 K. Nd 5Si 3 shows a canted spin structure, which might eventually also be the case for NdSi ( T C=45 K). Nd 5Si 4 ( T C=67 K) and NdSi 1.4 ( T C=75 K) exhibit magnetic behavior as typically observed in narrow domain wall ferromagnets favoring easy axis anisotropy. NdSi 2− x (I) ( T C=9 K) seems to adopt a more complex non-linear spin arrangement. NdSi 2− x (II) exhibits an antiferromagnetic transition at a Neél temperature of T N=10 K followed by a spin reorientation towards a parallel spin alignment at 9 K. The electrical resistivities resemble the temperature dependence of metals, with characteristic changes of slope of the ρ( T) curves at the Curie temperatures. The ρ( T) plots follow T 2 relations as predicted for isotropic ferromagnets, with the exception of NdSi 2− x (I) ( T 3.6, as usually observed for antiferromagnets).

Structure and magnetic properties of ErFe11-xGaxTi

ErFe11-xGaxTi compounds with ThMn12-type structure were synthesized for x = 0,1, 2, 3, 4, 5. The crystalline structure and Ga occupancy in the compoundswere analysed in terms of the Rietveld method. With increasing Ga content, thelattice constant and unit-cell volume increase monotonically. Ti atoms arefound to occupy 8i sites for all the compounds. Bond length and Ga occupancyfactor are given. The Curie temperature increases first, going through amaximum and then decreases with increasing x while saturation magnetizationdecreases monotonically. Spin re-orientation from the uniaxial to the easy-conetype is detected for all the compounds as the temperature decreases from 300to 5 K. A tentative spin phase diagram is constructed for the ErFe11-xGaxTi. Inthe ErFe11-xGaxTi compounds, a magneto-history effect is observed which can beattributed to irreversible movement of narrow domain walls that are easilypinned.

Structural chemistry, magnetic properties and electrical resistivity of the ternary germanides R 2RuGe 2 (R=Y, La–Nd, Gd–Er)

We have investigated the magnetic properties (magnetization, DC and AC susceptibility) of a new series of ternary rare-earth ruthenium germanides R 2RuGe 2, which crystallize in the monoclinic structure type Sc 2CoSi 2 (space group C2/m, No. 12). The two compounds containing yttrium or lanthanum are Pauli-type paramagnets. All the remaining compounds undergo ferromagnetic transitions, except Gd 2RuGe 2 and Tb 2RuGe 2, which seem to possess an antiferromagnetic ground state. The former finally exhibits a metamagnetic transition above 2 T and the latter, however, proves to be a highly anisotropic narrow domain wall ferromagnet, the critical field being close to 3 T. In the paramagnetic region the effective moments deduced are comparable with the ideal tripositive R 3+ ion moments and the θ p values are generally positive. The temperature dependence of the electrical resistivity is metallic like and reveals pronounced changes of slope near the ordering temperatures. The maxima of the d ρ/d T versus T plots are in agreement with the magnetic data ( T ord). The resistivity of Ce 2RuGe 2 was found to be determined by an interplay of Kondo scattering and crystal-field effects. The temperature dependence of the spin disorder resistivity in the paramagnetic regime for the remaining samples could be attributed to the influence of the crystalline field split 4f levels. Not knowing the exact level schemes, we use a single spacing ( δ) crystal-field model for an order of magnitude estimate.

SmNi 4 B : A narrow domain-wall ferromagnet

We have investigated magnetic properties of SmNi4B. The material undergoes a ferromagnetic ordering at TC≈39 K, which, surprisingly, is higher than that of isostructural GdNi4B (≈36 K). The saturation magnetic moment is rather small (0.15 μB per Sm at 5 K), suggesting the presence of crystal-field effects. Below TC, the material exhibits a large thermomagnetic irreversibility and also a large magnetic hysteresis at 5 K with a coercive field ≈28 kOe, which is one of the largest observed in a Sm-based material. We suggest that the large coercive field is intrinsic, associated with narrow domain walls of thickness of only one atomic layer, which resembles a ferroelectric wall.

Magnetic viscosity in a UPtAl single crystal

UPtAl is a ferromagnet with the Curie temperature 43.5 K, the rectangular hysteresis loop and the coercive force H c=0.7 T at 1.7 K. Due to the strong magnetocrystalline anisotropy, the hysteresis properties are likely caused by the intrinsic coercitivity of narrow domain walls. The magnetic viscosity in UPtAl has been studied by magnetization measurements. In a field slightly lower than H c (by absolute value) the magnetization decreases with time, changes the sign, and tends to saturate in the negative direction. The magnetic relaxation curves measured at different temperatures and fields have been analyzed within the thermal activation mechanism of the movement of narrow domain walls. The thermally activated volume and the expected coercive force for UPtAl without taking into account the thermal fluctuation mechanism have been determined as functions of temperature. The linear size of thermally activated areas was determined to be 1.2 nm at 1.8 K, a value close to the estimated domain walls width.

Strain-induced magnetic properties ofPr0.67Sr0.33MnO3thin films

We report the temperature dependence of the magnetic anisotropy in both compressive and tensile strained films of ${\mathrm{Pr}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$ (PSMO). Compressive strain induced by growth on ${\mathrm{LaAlO}}_{3}$ (LAO) substrates results in a spontaneous out-of-plane magnetization, while tensile strain (grown on ${\mathrm{SrTiO}}_{3})$ results in in-plane magnetization. The coefficient of linear proportionality between the magnetic anisotropy energy and the tetragonal strain for both compressive and tensile strained PSMO films is larger than that found previously in strained ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ films. From the data, we estimate a 20 unit-cell magnetic domain wall width for PSMO/LAO. Scattering from such a narrow domain wall could produce a potentially significant contribution to the resistivity.

Strain-Induced Magnetic Properties of Pr0.67Sr0.33MnO3 Thin Films

AbstractWe report the temperature dependence of the magnetic anisotropy in both compressive and tensile strained films of Pr0.67Sr0.33MnO3 (PSMO). Compressive strain induced by growth on LaAlO3 (LAO) substrates results in a spontaneous out-of-plane magnetization, while tensile strain (grown on SrTiO3) results in in-plane magnetization. The coefficient of linear proportionality between the magnetic anisotropy energy and the tetragonal strain for both compressive and tensile strained PSMO films is larger than that found previously in strained La0.67Ca0.33MnO3 films. From the data, we estimate a 20 unit cell magnetic domain wall width for PSMO / LAO. Scattering from such a narrow domain wall could produce a potentially significant contribution to the resistivity.