Search

Recent results showed that the ferrimagnetic compensation point and other characteristic features of Tb/Co ferrimagnetic multilayers can be tailored by He+ ion bombardment. With appropriate choices of the He+ ion dose, we prepared two types of lattices composed of squares with either Tb or Co domination. The magnetization reversal of the first lattice is similar to that seen in ferromagnetic heterostructures consisting of areas with different switching fields. However, in the second lattice, the creation of domains without accompanying domain walls is possible. These domain patterns are particularly stable because they simultaneously lower the demagnetizing energy and the energy associated with the presence of domain walls (exchange and anisotropy). For both lattices, studies of magnetization reversal show that this process takes place by the propagation of the domain walls. If they are not present at the onset, the reversal starts from the nucleation of reversed domains and it is followed by domain wall propagation. The magnetization reversal process does not depend significantly on the relative sign of the effective magnetization in areas separated by domain walls.

We show that it is possible to engineer magnetic multidomain configurations without domain walls in a prototypical rare-earth-transition-metal ferrimagnet using keV He^{+} ion bombardment. We additionally show that these patterns display a particularly stable magnetic configuration due to a deep minimum in the energy of the system caused by flux closure and a corresponding reduction of the magnetostatic energy without an increase in energy by exchange and anisotropy terms across the walls. This occurs because light-ion bombardment affects an element's relative contribution to the properties of the ferrimagnet differently. Therefore, it is possible to control the relative contribution from each magnetic subsystem. The selection of material and the use of light-ion bombardment allow us to engineer domain patterns in continuous magnetic films, which open a way to fabricate them in a much smaller scale than currently possible. Our Letter emphasizes that the right criterion to determine the presence or absence of a domain wall is whether there is a rotation of the spin for each sublattice and that changes of the direction of effective magnetization alone do not constitute an appropriate criterion.

The ability to perform wide-range tuning of the magnetic field required to switch the magnetization of ferromagnetic layers with perpendicular magnetic anisotropy is of great importance for many applications. We show that, for (Au/Co)2(3) multilayers, this field can be changed from minus several kOe to plus several kOe because of changes to the coupling with a ferrimagnetic multilayer [either (Tb/Fe)6 or (Tb/Co)6] across a Au spacer (either homogeneous 1 nm thick or wedge-shaped). The adjustable parameters are the ratio of sublayer thicknesses of the ferrimagnet and the sequence of layers around the Au spacer. The change of the sequence from Co/Au/Co to Tb/Au/Co is accompanied by both the reduction of the interaction energy and the change of the magnetic field sign necessary to switch the magnetization of ferromagnetic multilayers. For a 1 nm thick Au spacer this fields change from positive (negative) to negative (positive) if the ferrimagnet is dominated by the transition metal (rare earth) as a result of its composition. The characteristic oscillatory behavior of RKKY-like coupling is demonstrated using a system with a wedge-shaped Au spacer.

Results of domain structures observation of individual Ni80Fe20 and Co sublayers in sputter deposited (NiFe/Au/Co/Au)10 multilayers, using an element-sensitive method: the photoemission electron microscopy combined with soft X-ray magnetic circular dichroism, are presented. Also, overall domain structures were studied with magnetic force microscopy. The studies allowed us to reveal submicron stripe domains in the investigated samples and the replication of the stripe domains from the Co layers with perpendicular anisotropy to the NiFe layers with easy-plane anisotropy.

Material selective sensitivity of a magneto-optical polar Kerr effect to magnetizations of films from different materials in a multilayer system is presented. The method is supported by rigorous modeling of magneto-optic response from the multilayer system and by experimental demonstration on the periodic cobalt-permalloy multilayers [Ni80Fe20(2 nm)/Au(2 nm)/Co(0.4, 0.8, and 1.2 nm)/Au(2 nm)]10 .

The magnetic properties of an ultrathin cobalt film were modified by a focused femtosecond pulsed laser beam. The Co wedge, with a thickness ranging from 0 to 2 nm, sandwiched by Au films was prepared using ultra-high vacuum magnetron sputtering on a mica substrate. The modifications of the laser induced magnetic anisotropy were investigated using magneto-optic Kerr microscopy and MFM/AFM techniques. The laser induces: (i) local reorientation of magnetization from an in-plane to a perpendicular state and (ii) an increase of the coercivity field. A corresponding increase of the perpendicular magnetic anisotropy is discussed considering an improvement of the Co/Au interfaces.

Sputtered Ni 80 Fe 20 /Au/Co/Au multilayers (MLs) exhibit perpendicular anisotropy of Co layers and easy-plane anisotropy of Permalloy layers. In these samples the changes of relative orientation of magnetization between Co and NiFe layers, with magnetic field, can be determined by magnetoresistance measurements. The magnetic structure is discussed for two types of Ni 80 Fe 20 /Au/Co/Au MLs characterized by a weak coupling between ferromagnetic layers: (i) MLs with rectangular hysteresis loops and small coercive field, (ii) MLs with dense stripe domain structure in Co layers.

The influence of Ar-ion irradiation on the microstructure, interlayer coupling and giant magnetoresistance (GMR) effect in the model Fe/Cr system was studied by the conversion electron Mossbauer spectrometry (CEMS), vibrating sample magnetometry (VSM), and magnetoresistivity, for the Fe-1.4 nm/Cr- t Cr multilayers with t Cr ranging from 0.73 to 1.85 nm. The loss of antiferromagnetic coupling and the simultaneous degradation of GMR, observed for increasing ion dose, was caused by the formation of pinholes due to the increase of interface roughness and at high ion doses, by the ion-beam mixing leading to the alloying of Fe and Cr at interfaces.

The influence of Ar-ion irradiation on the microstructure, interlayer coupling and magnetoresistance in Fe/Cr multilayers is studied. An increase of interface roughness of Fe/Cr multilayers caused by irradiation with 200 keV Ar ions and doses exceeding 5 × 1012 Ar/cm2 is clearly seen in conversion electron Mossbauer spectroscopy (CEMS) measurements, while the small angle X-ray diffraction (SAXRD) technique hardly detects such changes in microstructure even at the higher ion doses. This subtle modification of the microstructure induces distinct changes in magnetization reversal (increase of the remanence magnetization, decrease of the saturation fields) and strongly decreases GMR effect with increasing irradiation dose. The most prominent changes are observed for the samples with a small thickness of Cr layers. The increasing immunity of GMR effect to ion irradiation with the increasing thickness of Cr layers as well as correlation between changes in GMR and antiferromagnetically coupled sample fraction suggest that the main effect responsible for the decrease of GMR is caused by the pinholes creation. For doses exceeding 2 × 1013 Ar/cm2 the volume intermixing seems to be a dominating mechanism responsible for further degradation of GMR and antiferromagnetic interlayer exchange coupling.

The magnetic properties of the sputter-deposited Cu/NiFe/MnIr/CoFe/Al+Ox/NiFe/Ta system are investigated to create a magnetically optimized TMR system and to tailor the magnetic properties. The unidirectional exchange anisotropy was found to depend strongly on the MnIr layer thickness with no exchange bias for t(MnIr) less than or equal to 5 nm and decreasing bias for t(MnIr) greater than or equal to 10 rim. For t(MnIr) = 15 nm exchange coupling energy was found to be 0.17 mJ/m(2) and the blocking temperature reaches 500 K. The thickness of the CoFe layer was found to strongly influence the shift of the magnetization loop of the upper NiFe layer by magnetostatic coupling. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The influence of annealing on giant magnetoresistance (GMR) effect and magnetization reversal processes has been investigated in Py/Cu (Py = Ni 83 Fe 17 ) sputter-deposited multilayers (MLs) with a limited number of magnetic sublayers (N ≤ 6) and equal sublayer thicknesses both for Py and Cu (t Cu = 2 nm, t Py = 2 nm). Based on the magnetization and magnetoresistance measurements it has been shown that the magnetic behavior of such MLs is strongly influenced by annealing driven disruption of magnetic layers (lateral decoupling) and nonmagnetic layers (magnetic bridging between Py layers). A comparison of experimental hysteresis curves with model dependences indicates that annealing causes a change of the magnetization reversal from a local to an absolute energy minimum mode leading to quasi-two-state magnetoresistance characteristics.

Currently, much attention is being paid to patterned multilayer systems in which there exists a perpendicular magnetic anisotropy, because of their potential applications in spintronics devices and in a new generation of magnetic storage media. To further improve their performance, different patterning techniques can be used, which render them suitable also for other applications. Here we show that He(+) 10 keV and Ar(+) 100 keV ion bombardment of (Ni(80)Fe(20)-2 nm/Au-2 nm/Co-0.6 nm/Au-2 nm)(10) multilayers through colloidal mask enables magnetic patterning of regularly arranged cylindrical magnetic domains, with perpendicular anisotropy, embedded in a non-ferromagnetic matrix or in a ferromagnetic matrix with magnetization oriented along the normal. These domains form an almost perfect two-dimensional hexagonal lattice with a submicron period and a large correlation length in a continuous and flat multilayer system. The magnetic anisotropy of these artificial domains remains unaffected by the magnetic patterning process, however the magnetization configuration of such a system depends on the magnetic properties of the matrix. The micromagnetic simulations were used to explain some of the features of the investigated patterned structures.

The perpendicular exchange bias in NiO(antiferromagnet)/Co(ferromagnet) polycrystalline layer films is studied. It is found that the NiO layer forces the Co layer magnetization to be oriented perpendicular to the film plane in a greater thickness range than is found in the Au/Co/Au system. Simultaneously, a large coercivity and a significant perpendicular exchange bias field were observed that are owing to the interlayer exchange bias coupling between NiO and Co, which supports the perpendicular magnetic anisotropy of the Co layer. These findings are confirmed by magnetometry and magnetoresistance measurements.

Regularly arranged magnetic out-of-plane patterns in continuous and flat films are promising for applications in data storage technology (bit patterned media) or transport of individual magnetic particles. Whereas topographic magnetic structures are fabricated by standard lithographical techniques, the fabrication of regularly arranged artificial domains in topographically flat films is difficult, since the free energy minimization determines the existence, shape, and regularity of domains. Here we show that keV He + ion bombardment of Au/Co/Au layer systems through a colloidal mask of hexagonally arranged spherical polystyrene beads enables magnetic patterning of regularly arranged cylindrical magnetic monodomains with out-of-plane magnetization embedded in a ferromagnetic matrix with easy-plane anisotropy. This colloidal domain lithography creates artificial domains via periodic lateral anisotropy variations induced by periodic defect density modulations. Magnetization reversal of the layer system observed by magnetic force microscopy shows individual disc switching indicating monodomain states. (Some figures in this article are in colour only in the electronic version)

The magnetisation reversal of polycrystalline permalloy/copper multilayers with antiferromagnetic couplingwas measured as a function of temperature. The magnetisation behavior is discussed in the relation to the exstence of ferromagnetic pinholes bridging neighbouring permalloy layers.

A concept of a magnetoresistive sensor for magnetic fields based on a Au/Co-wedge/Au/Co/Au multilayer is proposed. The wedged Co layer is characterized by a laterally changing coercivity resulting from a gradient of perpendicular magnetic anisotropy. Its magnetization reversal in a perpendicular magnetic field takes place by movement of a single domain wall in the direction parallel to the anisotropy gradient. The magnetization reversal of the multilayer has been investigated by magnetooptical and magnetoresistive measurements. The resistance of the proposed film system correlates well with the position of the domain wall and thus it can be used to sense magnetic fields.

We report on a specific magnetostatic coupling in sputter deposited (Ni80Fe20∕Au∕Co∕Au)10 multilayers of alternating in-plane and out-of-plane magnetic anisotropies. We demonstrate on the basis of complementary studies (magnetoresistance, conventional magnetometry, and element specific soft x-ray resonant magnetic scattering hysteresis measurements) that the magnetization reversal of the Ni–Fe layers is strongly influenced by a magnetostatic coupling originating from the out-of-plane stripe domain stray fields of the Co layers.

As a continuation of our investigation of FINEMET alloy thin films, Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , with Nb replaced by other high-melting metals W, Mo, Ta, V [1,5], in this work we report studies on the alloys in which these refractory elements were replaced by the following representatives of lanthanides: Yb, Ho, Tb, Gd, Sm, Nd, Pr and Y as an element without a magnetic moment [9]. Their structure was determined on the basis of electron diffraction studies and TEM observations. The structure of as-deposited films was found to be strongly dependent on the deposition conditions. The films deposited for less than 30 seconds were amorphous whereas those deposited for a longer time revealed microcrystallin structure (the temperature of the evaporation source was the same for all samples). The coercive field H c in the film plane and the Hall extraordinary coefficient R 1 together with the value of magnetic saturation induction E g for the films of both types were determined from the magnetooptical Faraday effect and the Hall effect, respectively. A strong correlation between the magnetic behaviour and structural changes of the films was found, i.e., a strong increase in H c and decrease in R 1 with increasing annealing temperature wereobserved.

Sputtered NiFe/Au/Co/Au layers with noncollinear magnetizations are of scientific interest for unusual magnetization behavior and their potential application in spintronics. In this paper, we propose to use a material selective sensitivity of magneto-optical Kerr effect for separation of signals from different materials in a bilayer system. The materials selective sensitivity is demonstrated on a sample with two mutually perpendicular Co and NiFe wedges.