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The magnetization reversal process in the ferromagnetic layer of an exchange-biased Co[sub 90]Fe[sub 10](20 nm)/Ir[sub 23]Mn[sub 77](10 nm) film structure, deposited by dc-magnetron sputtering, is imaged by high-resolution Kerr microscopy. Additionally, high-resolution magnetization loops are measured by deriving the magnetization signal from the average image intensity. The magnetization reversal occurs first by magnetization rotation under the development of ripple-like structures. The modulated structures then partially switch, generating complicated multidomain configurations, which finally annihilate by large angle domain wall movement. The amount of magnetization rotation at different field directions is quantified by measuring the transversal magnetization components during reversal. A strong asymmetry, both in domain behavior and magnetization loop, between the forward and recoil branch of the magnetization reversal is found. The magnitude of asymmetry strongly depends on small angle misalignments between the direction of exchange-bias and the external magnetic field. The observed domain behavior is explained by anisotropy dispersion in the ferro- and antiferromagnetic layer. The observed differences for both branches of the hysteresis loop are described in terms of domain nucleation mechanisms due to changes in the antiferromagnetic layer leading to an effectively wider anisotropy distribution. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Different sputtering methods, viz. rf-diode, rf-magnetron, and dc-magnetron sputtering, have been used to increase the sensitivity of Co/Cu/Co/FeMn top spin valve systems by more than a factor of 2. This improvement is due to an enhanced magnetoresistive effect ΔR/R and a reduced anisotropy field H[sub k] for the best sample. In the present paper the transport and magnetic properties are discussed in the context of microstructure and interfacial quality of the multilayers evaluated by comprehensive transmission electron microscopy and x-ray reflectometry investigations. The sample with the highest sensitivity (dc-magnetron sputtered) shows less structural defects and locally smoother interfaces than the other samples. Additionally it is shown that the ferromagnetic interlayer exchange field is not affected by the applied sputtering methods and can be well understood in terms of orange peel coupling. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Time-resolved x-ray magnetic circular dichroism was used to investigate ferromagnetically coupled CoFe/Ru/NiFe bilayers. The magnetization dynamics was driven by a continuous wave excitation. The precessional motion of the individual layers was detected separately by tuning the x-ray photon energy to the L3 absorption edge of either Ni or Co. Using two different waveguide stack geometries in-phase and antiphase excitation could be selected showing its effect in the measured precessional signal of the individual layers. In exchange-coupled bilayer two precessional modes were observed for each layer. The relative phase angles of the magnetic response between the two layers were found to be 8° and 133°, for the two modes revealed their quasiacoustic and quasioptic character. [ABSTRACT FROM AUTHOR]

The magnetization dynamics of each layer of interlayer exchange coupled Ni81Fe19/Ru(t)/Co90Fe10 films was investigated by time resolved x-ray magnetic circular dichroism (TR-XMCD) after pulsed excitation. The coupling was changed from ferromagnetic to antiferromagnetic by variation of the Ru thickness t. The precessional motion of the individual layers was detected separately by measuring the XMCD signal at the L3 absorption edge of either nickel or cobalt. From the observation of two frequency components in the precession of both layers in samples with negligible interlayer exchange coupling, the presence of a different coupling mechanism was concluded. Using two different sample geometries, the influence of antiphase and in-phase excitation on the triggered dynamics was studied. [ABSTRACT FROM AUTHOR]

By direct x-ray photoemission electron microscopy imaging, we probe current-induced domain wall motion in 20 nm thick CoFeB wires. We observe transverse walls for all wire widths up to 1500 nm as a consequence of the small saturation magnetization of the material. High critical current densities above 1×1012 A/m2 for wall displacement due to the spin transfer torque effect are found. The critical current densities jc increase further with decreasing wire width indicating that jc is governed by extrinsic pinning due to edge defects. In addition to wall displacements, we observe wall transformations to energetically favorable wall types due to heating. Owing to the high Curie temperature though, the sample temperature stays below the Curie temperature even for the highest current densities where structural damage sets in. [ABSTRACT FROM AUTHOR]

The anisotropy constant K of antiferromagnetic (111) textured IrMn films was determined from the blocking temperature TB of the high field rotational losses of NiFe/IrMn(t) film systems of low thickness t≤2.5 nm. The losses are measured by torquemetry as a function of t and temperature T. TB has been calculated using a Stoner-Wohlfarth model adapted to the experiments. From TB=(0.04/kB)Kt[lowercase_phi_synonym]2 ([lowercase_phi_synonym]=grain size), we estimate a constant value of K=(3±1)×105 J/m3 for T=10–400 K. The method is applicable also to other measurements of TB(t) and can be modified for other film systems. [ABSTRACT FROM AUTHOR]

Pure magnetic patterning by means of ion-beam irradiation of magnetic thin films and multilayers often results from a postdeposition local modification of the interface structure with only minor effects on the film topography. In the study presented here a 60 keV fine-focused Co ion beam was used to change the coupling in a Ni81Fe19/Ru/Co90Fe10 structure from antiferromagnetic to ferromagnetic on a micron scale. Thereby an artificial structure with locally varying interlayer exchange coupling and therefore magnetization alignment is produced. High-resolution full-field x-ray microscopy is used to determine the magnetic domain configuration during the magnetization reversal process locally and layer resolved due to the element-specific contrast in circular x-ray dichroism. In the magnetically patterned structure there is, in addition to the locally varying interlayer exchange coupling across the Ru layer, also the direct exchange coupling within each ferromagnetic layer present. Therefore the magnetization reversal behavior of the irradiated stripes is largely influenced by the surrounding magnetic film. [ABSTRACT FROM AUTHOR]

By coupling an artificial antiferromagnet (AAF)(CoFe/Ru/CoFe) to a ferro-/antiferromagnet (F/AF) system (NiFe/IrMn) we found a strong increase of the exchange bias blocking temperature, e.g., at 1.5-nm IrMn from 110 to 360 K. By magneto-optical Kerr effect and torquemetry at 10-340 K we determined the exchange bias field He, the coupling energy JF/AF between ferromagnet and antiferromagnet, and the rotational loss for thickness of IrMn from (0.3 to 3.5) nm. By comparison of both systems we concluded that for small thickness of the AF its superparamagnetic state was overcome due to the increased total anisotropy energy in the system with AAF. In this system exchange bias can be realized at room temperature with vanishing crystalline anisotropy of the antiferromagnet and without rotational loss. [ABSTRACT FROM AUTHOR]

Thin permalloy films without and with an 8 nm IrMn exchange layer have been grown by sputter deposition. Permalloy thickness ranges from 15 nm to 50 nm. The microwave permeability of these layers has been investigated in the 50 MHz to 3 GHz range using a permeameter based on a microstrip cell. A satisfactory measurement precision is observed on permalloy films as thin as 15 nm. The gyromagnetic resonance frequency increases from 670 MHz for the unbiased sample, to 2.7 GHz for the thinner biased sample. This increase in frequency is slightly larger than expected from the exchange-bias field measured by Kerr effect using a simple model for the gyromagnetic behavior. The properties of a two-period superlattice are also reported. [ABSTRACT FROM AUTHOR]

A new method for the characterizing of ferromagnet/antiferromagnet (F/AF) systems is proposed. By measuring the longitudinal Kerr effect during rotation of a constant field (ROTMOKE technique) we derive unknown magnetic features of the F/AF system. The distribution of the strength and direction of exchange bias field can be determined. Asymmetries of the ROTMOKE curve give clues to the different orientations of the uniaxial and unidirectional anisotropy. The large influence of uniaxial anisotropy on the reversal of F/AF systems is shown. [ABSTRACT FROM AUTHOR]

Deals with a study which calculated the magnetization behavior in antiferromagnetically coupled multilayer systems by using an atomic layer model. Background to the study; Magnetization reversal and giant magnetoresistance calculations; Influence of structural defects.

Conversion electron Mössbauer spectroscopy is used to determine the magnetic ground state at zero magnetic field of four-monolayer thick amorphous iron layers as part of a CoFeB-Fe multilayer stack. By comparing the intensities of the magnetic hyperfine field, an easy in-plane axis of the amorphous embedded Fe layer is verified, which is collinear to the uniaxial anisotropy axis of the neighboring amorphous CoFeB. Despite the soft magnetic character of the Fe layers, external fields up to 4 T perpendicular to the film plane are insufficient to completely align the embedded Fe moments parallel to the magnetic field due to a local disorder of the magnetic moments of the Fe atoms. [ABSTRACT FROM AUTHOR]

Metalloid-free magnetic amorphous layers have been searched for their presumably high magnetic density. Multilayers of iron and amorphous boron-doped cobalt-iron have been studied with this in mind. Samples with various Fe thicknesses and a constant thickness of amorphous boron doped cobalt-iron were prepared. The aim was to reduce the metalloid content as much as possible but retaining the amorphous structure. Transmission electron microscopy, x-ray diffraction studies, and Mössbauer spectroscopy have been applied to elucidate physical properties. The iron-partial phonon density of states was measured showing differences between the amorphous and crystalline films. [ABSTRACT FROM AUTHOR]

We report on the coupling of spin vortices in magnetic multilayer elements. The magnetization distribution in thin film disks consisting of two ferromagnetic layers separated by a nonmagnetic spacer is imaged layer-resolved by using x-ray microscopy. We directly observe two fundamentally different vortex coupling states, namely antiferromagnetic and ferromagnetic orientation of the flux directions. It is found that these states are predetermined for systems that involve a sufficiently strong interlayer exchange coupling, whereas for the case of a purely dipolar interaction both states are transformable into each other. [ABSTRACT FROM AUTHOR]

We report a combined experimental and theoretical study on the influence of slanted nanostripe edges on the magnetic domain wall (DW) dynamics in permalloy nanostripes. The DWs show a significantly different behavior in the same sample depending on the magnetization configuration in the DW. We explain these characteristics via a difference of the stray field generated by the out-of-plane magnetization inside the DW in the nanostripe edge region during DW motion. [ABSTRACT FROM AUTHOR]

We report the real time study of field driven domain wall (DW) motion in giant magnetoresistance nanostrips under the influence of both longitudinal and transverse fields. We have found clear evidence that transverse fields influence the DW dynamics below and above the Walker field [N. L. Schryer and L. R. Walker, J. Appl. Phys. 45, 5406 (1974)] drastically. We show the suppression of the Walker breakdown process and a huge enhancement in DW velocity below the Walker field. The maximum velocity reported here is 4500 m/s. [ABSTRACT FROM AUTHOR]

We report the real-time study of a domain wall motion in giant magnetoresistance nanostrips. We have visualized the Walker breakdown process (WBP) [N. L. Schryer and L. R. Walker, J. Appl. Phys. 45, 5406 (1974)] in single shot experiments. The domain wall motion above the Walker breakdown is highly nonperiodic. Surprisingly, the time intervals of movement are equal or larger than those where the domain wall nearly stops. When an additional transversal magnetic field is applied, domain wall motion becomes more regular, enabling the study of the WBP in more detail. A sufficiently large transverse field can suppress the WBP completely. [ABSTRACT FROM AUTHOR]

We report an experimental study dealing with the influence of in-plane transverse fields on the domain wall (DW) pinning in ferromagnetic nanostripes. We analyzed the pinning probability and depinning fields for several fabrication induced pinning sites. For all measurements reported here, the depinning field decreases with increasing transverse field independently from the actual domain wall type and the shape of the pinning site. The pinning probability decreases with increasing transverse fields for weak pinning sites. Stronger pinning sites can be active for large field ranges and show a complex dependence of the pinning probability on the applied transverse field. The occurrence of different domain wall types as well as the influence of a transverse field on the domain wall dynamics can explain this behavior. [ABSTRACT FROM AUTHOR]

Two concepts for new types of a magnetic domain wall (DW) based multiturn counter with true power on functionality are presented. Both counters use several closed loops of magnetic nanowires, each with different numbers of cusps. The turns are counted by the motion of domain walls through the loops, whereas a DW will move through one cusp during 90° external field rotation. Two concepts are introduced: a system based on binary logic, which is easy to integrate in digital electronics, and a system based on coprime numbers, which is useful to count large numbers. We have performed micromagnetic simulations in order to optimize the geometry of the cusp with respect to the operation margin of the device. Experimental verification of the domain wall motion through a giant magnetoresistance stack cusp is given. Furthermore, the read-out scheme for both systems is shown. [ABSTRACT FROM AUTHOR]

The tensor of the paramagnetic susceptibility χ of NdGaO3 single crystals was determined with high accuracy in the temperature range of T = (5-300) K. A vibrating sample magnetometer was used to determine the three main χ tensor components in the diagonal form. Torquemetry was additionally employed as it especially allows for the characterization of small deviations from isotropy. In the (100) plane the anisotropy Δχ was found to be indeed small (about 0.1% at 300 K and 1% at 30 K). It changes nonmonotonically with T, and reverses its sign at 250 K. In the planes (010) and (001) Δχ is much stronger (about 5% at 300 K and 22% at 30 K). Around 100 K it also changes its sign. Our results allow determing the anisotropy Δχ for any desired crystal orientation and temperature (5 K < T < 300 K). Furthermore, the conditions for disappearing magnetic torque (Δχ = 0) are deduced. [ABSTRACT FROM AUTHOR]