Magneto-optical Kerr Effect Magnetometry Research Articles

Domain wall propagation in continuous thin films initiated by precessional reversal

Using time resolved magneto-optic Kerr effect magnetometry, the precessional reversal properties of continuous soft magnetic layers such as Permalloy and Finemet have been studied in the presence of a weak bias field applied parallel to the initial magnetisation direction. This configuration induces for appropriate pulse width and pulse height, a reversed domain with two domain walls. It will be shown how these time resolved studies can be used to investigate the wall velocity and wall mobility of in-plane magnetised thin films.

Separation of the first- and second-order contributions in magneto-optic Kerr effect magnetometry of epitaxial FeMn/NiFe bilayers

The influence of second-order magneto-optic effects on Kerr effect magnetometry of epitaxial exchange coupled Fe50Mn50/Ni81Fe19-bilayers is investigated. A procedure for separation of the first-and second-order contributions is presented. The full angular dependence of both contributions during the magnetization reversal is extracted from the experimental data and presented using gray scaled magnetization reversal diagrams. The theoretical description of the investigated system is based on an extended Stoner–Wohlfarth model, which includes an induced unidirectional and fourfold anisotropy in the ferromagnet, caused by the coupling to the antiferromagnet. The agreement between the experimental data and the theoretical model for both the first- and second-order contributions are good, although a coherent reversal of the magnetization is assumed in the model.

Vortex-type domain structure in Co-rich amorphous wires

Investigation of the surface magnetic domain structure has been performed in Co-rich, nearly zero magnetostrictive, amorphous wires using magneto-optical Kerr effect magnetometry and microscopy. The formation and motion of a multidomain vortex-type structure with curved domain walls have been observed in amorphous wires. The possible origins of the existence of the vortex-type structures in these amorphous wires are discussed.

Controlled switching of ferromagnetic wire junctions by domain wall injection

The switching of submicrometer ferromagnetic wire junctions is investigated by controlled injection of domain walls (DWs). A three-terminal continuous Ni/sub 80/Fe/sub 20/ structure is described, consisting of two input wires and one output wire. Separate structures were fabricated by focused ion beam (FIB) milling, to inject either zero, one, or two DWs to the junction inputs. Introduction of DWs to the junction was performed using one or two DW injection pads with low switching fields. Hysteresis loops measured by magnetooptical Kerr effect (MOKE) magnetometry on the device output wires showed the coercivity of the output is strongly dependent on the number of DWs incident at the junction. By injecting either one or two DWs into the junction, it is possible to switch the output wire at two distinct field values, each markedly lower than the nucleation field of the junction. Results presented are relevant to the future development of spintronics DW logic systems, and for fundamental studies into DW resistance and interaction between DWs and wire morphology.

Interface coupling and magnetic properties of exchange-coupled Ni81Fe19/Ir22Mn78 bilayers

Hysteresis loop measurements using magnetooptic Kerr effect magnetometry in the low frequency region and magnetization dynamics measurements using high frequency permeametry are carried out to study the exchange anisotropy in Ni81Fe19/Ir22Mn78 bilayers. These two measurement techniques provide different exchange anisotropies for bilayers with thin Ir22Mn78 films. The observations can be understood by assuming that the AF grains break into domains due to the interface random field. By analysing the results from these two techniques, the interface coupling strength and the magnetic properties of the bilayers can be quantitatively determined.

Exchange anisotropy and spin-wave damping in CoFe/IrMn bilayers

The magnetic properties and the spin-wave damping in FM/AF bilayers of CoFe/IrMn have been investigated with magneto-optic Kerr effect magnetometry, ferromagnetic resonance (FMR), and Brillouin light scattering (BLS). As observed in other systems, the values of the interlayer exchange field obtained with different techniques are discrepant, but they all exhibit the expected t−1 dependence with the FM film thickness t. On the other hand the spin-wave relaxation rates measured by BLS and FMR are fit with a t−2 dependence plus a constant term. This is interpreted as the sum of two independent contributions, an intrinsic mechanism dominated by Gilbert damping and an extrinsic mechanism dominated by two-magnon scattering due to fluctuations of the interlayer exchange coupling caused by interface roughness.

Surface, interface and bulk studies of NiFe nanometer films for magnetoresistive heads

Polycrystalline Ni81Fe19 films have been vapor-deposited in a forming field onto a 100-μm-thick borosilicate glass. The free surface and film/substrate interface magnetization has been probed using magneto-optical Kerr effect (MOKE) magnetometry, and bulk measurements have been taken using an alternating gradient force magnetometer. Saturation magnetostriction constants were derived from the strain dependence of the anisotropy field in the MOKE measurements, separating out for the first time free surface and interface effects. The free surface showed a clear uniaxial anisotropy with a low dispersion, while the interface showed a mixture of domain switching and moment rotation. We interpret the data in terms of a gradient in magnetic properties through the sample, and discuss consequences for magnetoresistive heads.

Switching field phase diagram of Co nanoring magnets

We have studied the magnetic switching behavior of arrays of Co ring elements as a function of film thickness (2⩽t⩽32 nm), ring width (0.15⩽w⩽0.7 μm), and external diameter (0.5⩽D⩽2.0 μm), using magneto-optical Kerr effect magnetometry. For thick rings, two stable magnetic states are observed, a high remanence state (called the “onion” state) and a low remanence state (called the vortex state). The switching field for the transition from the onion to the vortex state increases with increasing thickness t and external diameter D, and with decreasing width w. In particular, for thin rings, the switching occurs between the two oppositely magnetized onion states, i.e., no vortex states develop during the reversal process. The transition between these two regimes depends on the diameter and width of the rings, and phase diagrams for the dependence of the switching behavior on the geometric parameters are presented. The switching behavior is discussed in terms of the competition between the exchange and magnetostatic energy terms.

Exchange coupling in symmetric Co/Cr/Co trilayer: a Brillouin light scattering study

A detailed Brillouin light scattering (BLS) investigation of spin waves in Co(20 nm)/Cr(dCr)/Co(20 nm) symmetric trilayer with wedge-shaped Cr interlayer has been carried out. Results showed that the interlayer exchange coupling oscillates, with antiferromagnetic peaks near the Cr layer thickness of 0.7 and 2.5 nm. These oscillations can be satisfactorily reproduced by a calculation based on the RKKY theory. An independent confirmation of the results obtained by BLS has been derived by measuring the saturation field of the longitudinal hysteresis loop as a function of the Cr thickness by magneto-optic Kerr effect magnetometry. Using the magnetic parameter determined by the above analysis, we have calculated the Brillouin cross section for the different Cr spacer thickness obtaining a very good agreement with the measured spectra.

Correlation of tunnel magnetoresistance with magnetic switching in Co/AlOx/Ni81Fe19 cross-strip structures

We describe the fabrication and characterization of cross-strip Co/AlOx/Ni81Fe19 magnetic tunnel junction (MTJ) structures with area of 0.01 mm2. The MTJs were deposited by magnetron sputtering through a series of masks and the AlOx tunnel barrier was formed by plasma oxidation of a thin Al layer. Two sets of MTJs were fabricated in which the thicknesses of the magnetic electrodes were approximately 1000 and 100 Å. We have studied the magnetic switching of both the strip electrodes and the junction area by magneto-optical Kerr effect magnetometry, and have measured the anisotropic magnetoresistance characteristics of the electrodes, and the tunnel magnetoresistance (TMR) of the junction area. TMR values of up to 18% were observed at room temperature. The easy axes of the two electrodes are in general canted apart and the coupling is found to be significantly stronger in the junctions with thicker electrodes. A single domain model provides a good qualitative description of the switching and TMR for all field azimuths. However, the measured TMR is less sensitive to the orientation of the applied field than the model predicts. This is attributed to the presence of pinning that enhances the coercivity of the Co hard axis hysteresis loop.

Structure and magnetic properties of amorphous Co-Si alloy films

Co/sub x/Si/sub 1-x/ alloy films have been prepared by sputtering in the composition range 0.65 /spl les/ x /spl les/ 1 at room temperature, and their structural and magnetic properties have been characterized by X-ray diffraction, extended X-ray absorption fine structure spectroscopy, and by magnetooptical Kerr effect magnetometry. The samples are polycrystalline with a mixture of fcc and hcp phases down to x = 0.76. For lower Co content, there is a transition to an amorphous structure that can be described by a dense random packing of hard spheres model. For x = 0.65 a change is found in the atomic environment of the Co atoms in the amorphous structure, that becomes more similar to a disordered Co/sub 2/Si phase. These structural changes are reflected also on the magnetic properties of the films: essentially isotropic with high coercivity for x /spl ges/ 0.76, with well-defined uniaxial magnetic anisotropy in the 0.75 /spl ges/ x /spl ges/ 0.66 composition range, and non magnetic for x below 0.65.

Patterned Magnetic Permalloy and Nickel Films: Fabrication by Electron Beam and X-Ray Lithographic Techniques

Electron beam (e-beam) and X-ray lithographic techniques have been used to fabricate permalloy (Ni80Fe20) and nickel rectangular and triangular dots and antidots on an area of (1×1) mm2. Dot dimensions and spacings range from 500 nm to 1 µm and from 250 nm to 50 nm, respectively. The changes of the magnetic properties induced by patterning have been studied by means of magneto-optic Kerr effect (MOKE) magnetometry and the Brillouin light scattering (BLS) technique.

Angular dependence and phase diagrams of exchange-coupled epitaxialNi81Fe19/Fe50Mn50(001)bilayers

The exchange bias effect, especially the angular dependence of the exchange bias field and the coercivity of the epitaxial ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}/{\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ bilayer system is investigated. In order to prepare a well defined layer sequence large emphasis is laid on the structural characterization of each layer. Low-energy electron diffraction and scanning tunneling microscopy are employed after each deposition step and the height-height-correlation function is analyzed to quantify the interface quality. Successively the angular dependence of the longitudinal and transverse magnetic component is investigated by magneto-optic Kerr effect magnetometry. In order to fully analyze the magnetization reversal behavior a special gray scale representation of the data is chosen. Theoretical results based on a modified Stoner-Wohlfarth model, which includes unidirectional and fourfold anisotropy contributions, are presented and compared to the experimental results. Although a homogeneous rotation of the magnetization is assumed the agreement between theory and experiment is surprisingly good. Only in a narrow angular range, where an ongoing rotation of the magnetization is predicted by the Stoner-Wohlfarth theory, discrepancies arise. The angular dependence of the possible orientations of the magnetization are summarized in a phase diagram.

Magnetic nanostructure fabrication by soft lithography and vortex-single domain transition in Co dots

Micron/nano scale Co dot and wire arrays have been fabricated by soft lithography patterning with a resolution of 100 nm. Magneto-optic Kerr effect magnetometry combined with micromagnetic calculation is used to study, in detail, the vortex-single domain transition in the Co dots. A metastable single-domain state, which is stabilized by a local minimum energy induced by vortex nucleation, is demonstrated.

Dependence of the coercive field on the Cu overlayer thickness in thin Co/Cu(001) and Ni/Cu(001) fcc epitaxial films

We have studied the effect of the Cu overlayer thickness tCu on the coercive field Hc of epitaxial ultrathin fcc Co/Cu(001) and Ni/Cu(001) films by magneto-optic Kerr effect magnetometry. The Cu/40 Å Ni/Cu(001) system shows perpendicular magnetic anisotropy (PMA) for the whole tCu range studied (20–180 Å) with Hc showing a large, linear increase with tCu (from ∼280 to ∼360 Oe). On the other hand, the Cu/36 Å Co/Cu(001) structure has dominant in-plane anisotropy and in contradistinction with the Ni case the coercive field decreases rapidly with tCu (from 140 Oe for tCu=20 Å to ∼90 Oe at tCu∼180 Å). The variation in coercivity is described in terms of the magnetoelastic energy contribution to the magnetic anisotropy induced by the Cu overlayers (via strain). In particular, the increase in the Hc of Ni is attributed to an increase in the PMA while the relation between the magnetic anisotropy and coercivity in the case of Co remains unclear.

Effect of the Cu capping thickness on the magnetic properties of thin Ni/Cu(0 0 1) films

We have studied the effect of the Cu capping thickness (in the range 20⩽ t Cu ⩽180 Å) on the magnetic and structural properties of ultrathin FCC 40 Å Ni/Cu(0 0 1) films by means of magneto-optic Kerr effect (MOKE) magnetometry, polarised neutron reflection (PNR) and grazing incidence X-ray surface diffraction (GID) measurements. MOKE measurements show that perpendicular magnetic anisotropy (PMA) is observed for the whole of the Cu thickness range studied, while an increase of the coercive field with t Cu is found. The variation of the in-plane strain in the Ni film is shown to increase monotonously between 20 and 90 Å Cu capping thickness from −1.60±0.03% to −1.86±0.07% as revealed by GID, while PNR measurements show a marked decrease in the Ni magnetic moment per atom with increasing Cu capping layer thickness, from 0.54±0.03 μ B for t Cu=50 Å to 0.45±0.03 μ B for t Cu=180 Å. These results clearly show that a strain-induced reduction in the Ni magnetic moment per atom occurs. The increase in the coercive field is also attributed to the increase in the strain of the Ni film.

Magneto-dipole coupling in arrays of micron-size rectangular magnetic elements

Static magnetic properties of arrays of micron-size rectangular magnetic permalloy elements are investigated by means of magneto-optic Kerr-effect magnetometry and magnetic force microscopy. The influence of the size and the spacing between the elements on the magnetization curves of the arrays is studied for different orientations of the applied magnetic field. A sizeable magnetic dipole coupling between the elements is found, affecting strongly the magnetic properties of the arrays.

Measurements of exchange anisotropy in NiFe/NiO films with different techniques

One of the puzzles of the recent investigations on the exchange anisotropy in ferromagnetic (FM)/antiferromagnetic (AF) bilayers is the fact that different techniques yield different values for the exchange field (HE) between the layers. We report an investigation on sputtered NiFe/NiO carried out with three different techniques, namely, magneto-optical Kerr effect magnetometry (MOKE), Brillouin light scattering (BLS), and ferromagnetic resonance (FMR). In an attempt to reconcile the measurements obtained with the various techniques, we interpret the data with a model that includes the formation of a planar domain wall in the AF layer, giving rise to a torque on the FM moment represented by an effective domain wall field (HW). We find out that while the same pair of values of HE and HW provide equally good fits to the reversible FMR and BLS measurements, different pairs are necessary to fit the irreversible magnetometry data.

Magnetization of thin Fe films on V(1 1 0) and Cr(1 1 0)

The magnetization of thin Fe(1 1 0) films was measured by magneto-optical Kerr effect (MOKE) magnetometry and polarized neutron reflectometry (PNR). In order to further understand the influence of strain and roughness on the Fe magnetization different surfaces were prepared by evaporating Cr and V on V(1 1 0) single crystals by molecular beam epitaxy. By varying the growth temperature both smooth and faceted Cr and V surfaces were created. For the facets we obtained an up and down staircase structure with the ridges along the [0 0 1] direction that consisted approximately of subsequent (1 0 0) and (0 1 0) surfaces. For the thin Fe films that were evaporated on the smooth V(1 1 0) surface a relatively high reduction of the magnetization was obtained, independently of the film thickness. In terms of thickness the observed reduction is equivalent to an offset of Δ t=−(4.0±1.0) Å. A V layer evaporated on this structure does not further reduce the magnetization of the Fe. For comparison Fe evaporated on a smooth Cr film was investigated, and the reduction amounted to Δ t=−(1.4±0.4) Å. For thin Fe films on faceted V and Cr substrates Δ t=−(3.3±0.4) and −(2.6±0.3) Å were measured, respectively. For samples with a 2 Å Cr buffer layer between the V(1 1 0) single crystal and the Fe film we obtained Δ t=−(4.08±0.10) Å, a value which is close to the result obtained for the smooth V(1 1 0) substrate.

Ferromagnetic hcp Fe in Fe/Re(001) multilayers

Until now, the hexagonal closed-packed (hcp) phase for bulk Fe, Fe multilayers, or Fe alloys has always been associated with a paramagnetic state. We show evidence that hcp Fe can support a ferromagnetic moment in Fe/Re multilayers. The structure and magnetic properties of Fe/Re multilayers are studied using x-ray diffraction, energy dispersive x-ray analysis, magneto-optical Kerr-effect magnetometry, and x-ray magnetic circular dichroism. The Fe layer thickness ${t}_{\mathrm{Fe}}$ is maintained constant at 8 \AA{}, while $0<~{t}_{\mathrm{Re}}<~35\mathrm{\AA{}}\mathrm{}.$ For ${t}_{\mathrm{Re}}<8\mathrm{\AA{}}\mathrm{}$ the multilayer has a body centered tetragonal (bct) structure with a perpendicular interplanar spacing that increases with increasing Re thickness. Beyond the ${t}_{\mathrm{Re}}\ensuremath{\approx}9\mathrm{\AA{}}\mathrm{}$ the structure transforms to hcp. Fe has a quite stable magnetic moment for all ${t}_{\mathrm{Re}},$ before and beyond the structural phase transition.