Relative Magnetization Research Articles

Stable and variable features of the magnetic structure of fcc Fe/Cu(001) films

After more than a decade of experimental and theoretical studies of the magnetic structure of the fcc Fe/Cu(001) films, most of the researchers agree that the atomic spins of two surface layers are ferromagnetically ordered whereas the magnetic structure of further layers is not ferromagnetic. The conclusions on the magnetic configuration of the nonferromagnetic part vary broadly from a simple collinear layer-by-layer antiferromagnet to complex incommensurate spin-density wave. We report detailed theoretical study of the fcc Fe/Cu(001) films with 6, 7, and 8 ML coverages. The study is based on the density-functional-theory calculations with the code allowing for the consideration of arbitrary noncollinear magnetic configurations. We suggest a view on the magnetism of the Fe/Cu(001) films that includes the grouping of the layers into blocks with robust collinear magnetic structure whereas the variation in the relative directions of the moments of different blocks is energetically relatively inexpensive. The robust magnetic structure of the three surface layers was found to be $\ensuremath{\Downarrow}\ensuremath{\Uparrow}\ensuremath{\Uparrow}$. Further layers form the pairs with robust antiferromagnetic structure. We demonstrate that the formation of the robust magnetic blocks corresponds to a certain hierarchy of the interlayer exchange interactions. We suggest that the individual defect pattern of each fabricated film influences before all the relative orientation of the block magnetizations keeping the intrablock structure intact. The calculated interatomic exchange parameters are used to study magnetic thermodynamics of the films within mean-field approximation. In particular we focus on possible physical reasons of the appearance of the steplike features in the temperature dependence of the magnetization detected experimentally. We emphasize the important consequences of the inequivalence of the Fe atoms belonging to different layers of the film. This inequivalence makes the properties of the films to be essentially different from the properties of the corresponding bulk system. Using a rigid-band model we investigate the dependence of the exchange parameters on the electron number. We critically discuss the possibility of the formation of the incommensurate SDW in the thin Fe/Cu(001) films. The brief formulation of the idea of robust blocking and its experimental confirmation was recently published as a letter [H. L. Meyerheim, J.-M. Tonnerre, L. M. Sandratskii et al., Phys. Rev. Lett. 103, 267202 (2009)].

Electrical resistivity and thermopower of Nd1−xTbxCo2 compounds

Electrical resistivity ρ and thermopower S of Nd1−xTbxCo2 Laves phase quasibinary alloys (0 ≤ x ≤ 1) are investigated at temperatures from 2 K to 300 K. The magnetic transition temperature TC, determined from resistivity magnetic anomaly, increases linearly with increasing x. The low temperature thermopower changes its sign from negative to positive at the critical composition where the relative orientation of total magnetization and cobalt 3d moment is changed. We propose that this change is related to the dependence of s-d scattering rate on relative polarization of conduction electrons and cobalt 3d band.

Nonlinear Giant Magnetoresistance in Dual Spin Valves

Giant magnetoresistance (GMR) arises from differential scattering of the majority and minority spin electrons by a ferromagnet (FM) so that the resistance of a heterostructure depends on the relative magnetic orientation of the FM layers within it separated by nonmagnetic spacers. Here, we show that highly nonequilibrium spin accumulation in metallic heterostructures results in a current-dependent nonlinear GMR which is not predicted within the present understanding of GMR. The behavior can be explained by allowing the scattering asymmetries in an ultrathin FM layer to be current dependent.

Control of the switching behavior of ferromagnetic nanowires using magnetostatic interactions

Magnetostatic interactions between two end-to-end Permalloy (Ni80Fe20) nanowires have been studied as a function of their separation, end shape, and width. The change in switching field increases as the wires become closer, with deviations from the switching field of an isolated wire of up to 40% observed. The sign of the change depends on the relative magnetization orientation of the two wires, with higher fields for parallel magnetization and lower fields for antiparallel magnetization. A wire end shape has a strong influence, with larger field variations being seen for flat-ended wires than wires with tapered ends. The micromagnetic modeling and experiments performed here were in good qualitative agreement. The experimental control of switching behavior of one nanowire with another was also demonstrated using magnetostatic interactions.

Reversal magnetoresistance effect in ferromagnet/superconductor/ferromagnet/antiferromagnet heterostructure

The oxide heterostructure consisting of La0.7Sr0.3MnO3/YBa2Cu3O7-δ/La0.7Sr0.3MnO3/La0.33Ca0.67MnO3 is fabricated by using the technique of pulsed laser deposition. The magnetoresistance (MR) for the system is investigated in the region of mixed vortex state near the onset temperature of superconducting transition. The present experimental results indicate a reversal variation in MR that is observed with increasing temperature in the system, implying the competition between a positive MR of YBa2Cu3O7-δ and a negative MR of La0.7Sr0.3MnO3. Furthermore, our analysis further suggests that the large MR peaks are mainly caused by spin-dependent interface scattering, which depends on the relative orientation of magnetization in two ferromagnetic layers. And it is proved that the superconductivity is an essential factor for the large positive MR in the artificial heterostructure system.

Superconducting and magnetic properties of Ni/Pb multilayered nanowires

Nickel/lead multilayered nanowires were prepared using standard three-electrode electrodeposition into anodic aluminum oxide porous templates. The diameters of the nanowires ranged from 50 to 200 nm. By means of electric and magnetic measurements, we found that the superconducting transition temperature, Tc, of the Pb layer persisted down to the smallest diameter without any noticeable dimensional effect. Tc was affected by the relative orientation of magnetizations in neighboring Ni layers. When the superconducting layers were driven normal under a large magnetic field, unusual resistance increase was observed with decreasing temperature, which was consistent with the Kondo effect.

Josephson current in ballistic heterostructures with spin-active interfaces

We develop a general microscopic theory of dc Josephson effect in hybrid superconductor--normal-metal--superconductor structures with ballistic electrodes and spin-active normal-metal--superconductor (NS) interfaces. We establish a direct relation between the spectrum of Andreev levels and the Josephson current which contains complete information about nontrivial interplay between Andreev reflection and spin-dependent interface scattering. The system exhibits a rich structure of properties sensitive to spin-dependent barrier transmissions, spin-mixing angles, relative magnetization orientation of interfaces, and the kinematic phase of scattered electrons. We analyze the current-phase relations and identify the conditions for the presence of a $\ensuremath{\pi}$-junction state in the systems under consideration. We also analyze resonant enhancement of the supercurrent in gate-voltage-driven nanojunctions. As compared to the nonmagnetic case, this effect can be strongly modified by spin-dependent scattering at NS interfaces.

Bistable control of ferromagnetic resonance frequencies in ferromagnetic trilayered dots

The ferromagnetic resonance (FMR) properties of submicron-scale Ni81Fe19/Cu/Ni81Fe19 trilayered dots were investigated for application in programable microwave absorbers for broadband filter devices. The trilayered dot shows binary FMR frequencies, depending on the relative orientation of magnetization in the top and the bottom magnetic layers. A parallel configuration of magnetization, which exhibits a lower FMR frequency than that for an antiparallel configuration, can be metastable as the lateral aspect ratio of the dot is larger than the threshold value governed by the Cu layer thickness. The difference in the FMR frequencies between the parallel and antiparallel configurations is gradually decreased with elongation of the dot. This is associated with the suppression of the magnetostatic coupling energy that stabilizes the antiparallel configuration. It was also found that the magnetic configurations can be programed by the selective application of easy- and hard-axis magnetic fields used to saturate the magnetization. This is an appropriate manner to realize the antiparallel configuration of magnetization in the case of a dot with a plural stack of magnetic layers with the same coercive field.

Giant magnetocurrent in triple-barrier ferromagnetic resonant-tunneling diode with different magnetization configurations

The authors theoretically investigate a triple-barrier ferromagnetic resonant-tunneling diode composing nonmagnetic couple wells and three GaMnN magnetic barriers, in which two kinds of antiparallel configurations are formed by changing the relative orientation of magnetization in the barriers. Based on a two-band model, the achievement of large magnetocurrents in the resonant bias regime is proposed under an optimal magnetization configuration. The magnetocurrents in both antiparallel configurations of triple-barrier system are much higher than that in double-barrier structure.

Josephson current in diffusive multilayer superconductor/ferromagnet/superconductor junctions

We calculate the Josephson current in a diffusive superconductor/ferromagnet/superconductor junction, where the ferromagnetic region contains multiple layers (or domains). In particular, we study a configuration where there are two layers with an arbitrary relative in-plane magnetization orientation and also include nonideal interfaces and arbitrary spin-flip scattering. We study the $0\text{\ensuremath{-}}\ensuremath{\pi}$ oscillations of the critical current for varying junction width $d$ and find that the $\ensuremath{\pi}$ state vanishes entirely when the magnetic misorientation angle of the two layers exceeds a critical angle ${\ensuremath{\phi}}_{c}$. While ${\ensuremath{\phi}}_{c}\ensuremath{\rightarrow}\ensuremath{\pi}/2$ in the limit of high temperatures, we find that ${\ensuremath{\phi}}_{c}$ becomes smaller than $\ensuremath{\pi}/2$ at low temperatures compared to ${T}_{c}$. $0\text{\ensuremath{-}}\ensuremath{\pi}$ oscillations are also found when varying the temperature or the misorientation angle for fixed values of $d$, and we present phase diagrams that show qualitatively the conditions for the appearance of such oscillations. We also point out how one may obtain significant enhancement of the critical current in such a system by switching the magnetization for selected values of the junction width $d$, and comment on the necessary conditions for establishing a long-range triplet Josephson effect.

Influence of interlayer magnetostatic coupling on the ferromagnetic resonance properties of lithographically patterned ferromagnetic trilayers

The influence of magnetostatic coupling in a Ni81Fe19∕Cu∕Ni81Fe19 elliptical element on the ferromagnetic resonance (FMR) properties was experimentally and numerically investigated. Discontinuous variations in the FMR frequencies were observed when the relative orientation of magnetization was switched between parallel and antiparallel. Furthermore, the trilayer element exhibits field-insensitive FMR properties for magnetization of antiparallel orientation. This is attributable to the conflicted magnetic field dependence of the FMR frequency in the top and bottom Ni81Fe19 layers with antiparallel magnetization.

SPIN-DEPENDENT TRANSPORT THROUGH A FINITE INTERACTING QUANTUM DOT WITH INTRADOT SPIN-FLIP SCATTERING

Based on the non-equilibrium Green function method, we investigate the spin-dependent transport through a finite interacting quantum dot with intradot spin-flip scattering for non-collinear configuration. It is found that when the intradot spin-flip scattering is applied, the Kondo peaks in the local density of states and the conductance are split, and the splitting peaks are suppressed sharply. By changing the relative orientation of both magnetizations, amplitudes of the splitting peaks appear asymmetric, and reduce with the increasing relative angle θ of the two magnetic moments. It is uncovered that the presence of the spin-flip scattering tends to greatly suppress the spin-valve effect.

Temperature and angular dependences of dynamic spin-polarized resonant tunneling in CoFeB∕MgO∕NiFe junctions

The bias dependence of tunneling magnetoresistance oscillations due to dynamic resonant tunneling in CoFeB∕MgO∕NiFe magnetic tunnel junctions was studied as functions of temperature and the relative magnetization angle of the two magnetic layers. The effect of temperature is consistent with thermal smearing, while that of the relative magnetic orientation was typical of a spin valve. A model of tunneling between spin-split free electron bands using the exact solution of the Schrödinger equation for a trapezoidal tunnel barrier agrees with experiment, underscoring the simplicity of dynamic resonant tunneling.

Magnetic and tunneling magnetoresistive properties of an all-oxideFe3O4−Al2O3granular system

A systematic study on the structural, magnetic, and transport properties of an all-oxide granular system consisting of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ grains dispersed in an insulating ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ matrix is presented. X-ray diffraction and transmission electron microscopy analyses provide the structure of nanosized ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ grains embedded in an amorphous ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ matrix and confirm that the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ grain size decreases with increasing ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ volume fraction $x$. Magnetic measurements further show that the coercivity of the granular films decreases with increasing $x$ due to decreasing ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ grain size, with the magnetization curves exhibiting typical superparamagnetic characteristics for $x>0.19$. Temperature dependence of resistance and nonlinearity in current-voltage characteristics confirms that the spin dependent tunneling of conduction electrons across grains dominates the transport properties. This tunneling conductance depends on the relative orientation of magnetization between ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ grains, which results in an isotropic granular tunneling magnetoresistance (TMR) effect. We also observed that the granular TMR ratio has strong temperature dependence and decreases with increasing temperature for all the granular films.

Modeling a Schottky-barrier carbon nanotube field-effect transistor with ferromagneticcontacts

In this study, a model of a Schottky-barrier carbon nanotube field-effect transistor(CNT-FET), with ferromagnetic contacts, has been developed. The emphasis is put onanalysis of current–voltage characteristics as well as shot (and thermal) noise. The methodis based on the tight-binding model and the non-equilibrium Green’s function technique.The calculations show that, at room temperature, the shot noise of the CNT-FET isPoissonian in the sub-threshold region, whereas in elevated gate and drain/source voltageregions the Fano factor gets strongly reduced. Moreover, transport properties stronglydepend on relative magnetization orientations in the source and drain contacts. Inparticular, one observes quite a large tunnel magnetoresistance, whose absolute value mayexceed 50%.

A detailed analysis of dipolar interactions in arrays of bi-stable magnetic nanowires

The investigation of the role of interactions in magnetic wire arrays is complex and ofteninvolves substantial simplifications. In this paper analytical expressions taking intoconsideration the geometry of the wires and dipolar interactions between them have beenobtained. An expansion of these terms, at first order, can be easily evaluated andshows a good agreement with the total expression for the energy. The extent ofthe interwire magnetostatic coupling has also been investigated, and it is shownthat the number of wires required to reach a size independent magnetic state inthe array strongly depends on the relative magnetic orientation of the wires.

Spin accumulation in lateral all-ferromagnetic spin valves

Spin accumulation in magnetic tunnel junctions and lateral all-ferromagnetic spin valves is theoretically investigated by using a spin-dependent diffusive transport model. When electric current tunnels through a barrier, spin accumulation occurs in the ferromagnetic layer and can be detected with nonlocal spin valve measurements. Depending on the relative orientation of magnetization, the nonlocal spin signal has three different values and can determine the bulk spin polarization of the ferromagnetic material.

Theory of digital magnetoresistance in ferromagnetic resonant-tunneling diodes

We propose a ferromagnetic spintronic system, which consists of two serial connected resonant tunneling diodes. One diode is nonmagnetic whereas the other comprises a ferromagnetic emitter and quantum well. Using a selfconsistent coherent transport model we show that the current-voltage characteristic of the ferromagnetic diode can be strongly modulated by changing the relative orientation of the magnetizations in the emitter and quantum well, respectively. By a continuous change of the relative magnetization angle the total resistance exhibits a discrete jump realizing digital magneto resistance. The interplay between the emitter's Fermi energy level and the relative magnetization orientations allows to tailor the current voltage characteristics of the ferromagnetic diode from ohmic to negative differential resistance regime at low voltages.

Pulsed inductive measurement of ultrafast magnetization dynamics in interlayer exchange coupled NiFe∕Ru∕NiFe films

Pulsed inductive microwave magnetometer (PIMM), conventional ferromagnetic resonance (FMR), and vector network analyzer FMR (VNA-FMR), as well as quasistatic methods such as a magneto-optic Kerr effect magnetometer and a vibrating sample magnetometer, have been used for complementary studies of interlayer exchange coupled Si∕Ta∕NiFe(30nm)∕Ru(t)∕NiFe(30nm)∕Ta films of variable Ru thickness t. The coupling constants were determined from the measured magnetization curves by fitting them to simulated ones. The dynamics measurements revealed two spin wave modes, which could be identified as optical and acoustical ones. The optical mode changes significantly with the coupling, whereas the slight change of the acoustical mode is due to different relative orientations of the layer magnetizations. Numerical simulations helped to identify optical and acoustical modes. The intensity of these two modes changes with the bias field and coupling. Using VNA-FMR the optical mode could be observed up to higher bias fields than with the PIMM at small excitation angles. While varying the pulse field up to 150Oe using the PIMM, the intensity ratio of optical and acoustical modes changed and additional modes occurred.

Spin-filtering effect of ferromagnetic semiconductor La 2NiMnO 6

Ferromagnetic semiconductor La 2NiMnO 6 was used as a barrier in a tunneling junction. The junction with a ferromagnetic electrode showed magnetoresistance (MR) of −0.12% at 150 K associated with the relative orientation of magnetization of the ferromagnetic layers. The result is interpreted as a spin-filtering effect, which is caused by the spin-dependent barrier height of the ferromagnetic tunneling barrier.