Magneto-optical Kerr Effect Magnetometry Research Articles

Bulk and near-surface magnetic properties of FeRh thin films

Epitaxial Fe48Rh52 thin films of nominal thickness 500Å were grown on MgO (001) substrates via molecular beam epitaxy and capped with 20Å of either Au or MgO. The presence of the anticipated magnetostructural phase transition from antiferromagnetism to ferromagnetism at ∼350K was confirmed by superconducting quantum interference device and magneto-optic Kerr effect magnetometry. X-ray magnetic circular dichroism (XMCD) experiments were carried out at the L2,3 edges; measurements were done in both total electron yield (TEY) mode and indirect transmission mode. At room temperature, the indirect transmission XMCD data show no detectable dichroism, consistent with bulk antiferromagnetic behavior. However, room-temperature TEY data originating from the films’ surface reveal an appreciable dichroic signal indicating the presence of ferromagnetism. This near-surface/interfacial magnetism may be modified by choice of capping layer; the MgO-capped sample exhibited a considerably smaller ferromagnetic XMCD signal. The results have potential impact on the application of FeRh thin films as temperature-variable pinning layers in exchanged-biased systems.

Voltage-induced modification in magnetic coercivity of patterned Co 50Fe 50 thin film on piezoelectric substrate

A controlled modulation of magnetic properties through the inverse piezoelectric (PE) effect was investigated by means of the magneto-optical Kerr effect magnetometry in a periodic array of ferromagnetic (FM) Co 50Fe 50 stripes patterned on a commercial PE substrate. The coercive field ( H c ) rise (up to 80% in virgin cycle and 25% in subsequent cycles) when a DC electric field (up to 75 kV/cm) is applied on the PE substrate manifests a magnetoelectric coupling between the PE substrate and adjacent FM layer. The electric field dependence of H c resembles the shear strain response to electric field of the PE constituent. The differences in the hysteresis loops shape when the magnetic field is oriented parallel and perpendicular to the stripes, reveal the interplay of the shape and stress anisotropy of the ferromagnet. Besides the inherent difficulties on constructing, an epitaxial hybrid system is a promising candidate of future random access memories designation.

Effect of substrate roughness on the magnetic properties of thin fcc Co films

We present a study of the influence of substrate roughness on the magnetic properties of thin fcc Co films (7 and $17\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick) grown on Cu(001)/Si(001). A significant decrease in cubic anisotropy with increasing film roughness was observed with Brillouin light scattering and magneto-optical Kerr effect magnetometry. In addition, the rougher samples showed a substantial broadening of the spin wave peaks. Both effects were found to be more pronounced for the thinner Co layers. Our observations are discussed in the framework of a theoretical model which takes into account the morphology of the Co films as measured by atomic force microscopy. While roughness effects are usually discussed in the context of N\'eel's ``orange-peel'' model, we propose a qualitatively different effect in this work whereby the magnetization follows coherently the substrate morphology (``undulating'' state) resulting in the absence of magnetic surface charges. This magnetic configuration gives rise to a reduction in the magnetic anisotropy of epitaxial thin films, which is in good qualitative agreement with the experimental observations.

Magnetic Properties of EuS-SrS Semiconductor Multilayer Structures

Magnetic and structural properties of EuS–SrS semiconductor multilayers were studied by SQUID and magneto-optical Kerr effect magnetometry techniques and by X-ray diffraction method. The multilayers composed of monocrystalline, lattice matched ferromagnetic EuS layers (thickness 35–50 A) and nonmagnetic SrS spacer layers (thickness 45–100 A) were grown epitaxially on KCl (001) substrates with PbS buffer layer. Ferromagnetic transition in EuS–SrS multilayers was found at the Curie temperature Tc = 17 K. The multilayers exhibit only weak in-plane magnetic anisotropy with [110] easy magnetization axis. Coercive field of EuS–SrS multilayers shows a linear increase with decreasing temperature. Magneto-optical mapping of magnetic hysteresis loops of the multilayers revealed good spatial homogeneity of their magnetic properties.

Systematic tuning of magnetization reversal in Permalloy nanowires using sloped ends

The magnetization reversal of Permalloy (Ni81Fe19) nanowires has been investigated by magneto-optic Kerr effect (MOKE) magnetometry, where one end of the wire exhibits a slope in the thickness. Straight nanowires with a thickness of 7.5nm, widths of 150nm, and length of 100μm were prepared by electron-beam lithography. The sloped ends were achieved by using a penumbra shadow mask during NiFe deposition. The topography of the wires has been studied by atomic force microscopy. One finds that the slope profile can be tuned by the position under the mask, mask-to-sample distance, and angle of deposition. Corresponding MOKE hysteresis loops show a systematic reduction of the coercive field with increasing length of the sloped part. For example, wires where the slope has a length of 45μm exhibit a coercive field of 11Oe, whereas nanowires without sloped ends show 107Oe.

Preparation, characterization and magneto-optical investigations of electrodeposited Co/Au films

Au/Co(4–8 ML)/Au single magnetic layers and Au(8 ML)/Co(4 ML)/Au(8 ML)/Co(8 ML)/Au bilayer were sequentially grown by electrodeposition on an Au(1 1 1) buffer layer electrodeposited on Si(1 1 1). The technique used in this work provides full control on the structure and the chemical composition of the different layers (no alloying) as well as on the chemistry at interfaces. scanning tunneling microscopy (STM) and atomic force microscopy (AFM) imaging and X-ray diffraction measurements show that atomically flat continuous Co(0 0 0 1) layers (4–8 ML) can be grown in epitaxy with the Au(1 1 1) substrate and that the 2 nm-thick spacer is also a continuous Au(1 1 1) layer. The Co ultrathin layers (4 and 8 ML) exhibit perpendicular magnetic anisotropy. The lateral magnetic homogeneity and magnetization reversal process have been investigated by scanning magneto-optical Kerr effect (MOKE) magnetometry and global Kerr microscopy. The correlation between magnetization switching behaviour in each layer of the Co-bilayer stack has been evidenced from in-depth sensitive MOKE measurements and microscopy. The strong coupling observed between the two Co layers is attributed to magnetostatic interaction at domain wall boundaries.

Cross-over from coherent rotation to inhomogeneous reversal mode in interacting ferromagnetic nanowires

We have investigated the magnetization reversal mechanism and the effects of magnetostatic interaction in arrays of lithographically defined Permalloy nanowires with a fixed width of 185 nm, spacing of 35 nm and film thicknesses from 10 to 120 nm. The magnetization reversal has been investigated with vectorial Magneto-Optical Kerr Effect magnetometry. The vectorial hysteresis loops (in-plane magnetization components parallel and perpendicular to the applied field) recorded with the external field applied perpendicular to the length of the wires (hard magnetization direction), show a transition from coherent rotation to inhomogeneous reversal mode for wires’ thickness above 80 nm. The effects of dipolar interactions are evidenced by the variation of the saturation field in the hard-axis hysteresis loops as a function of wires thickness. In detail, the analysis of the saturation field vs. wires thickness shows that the dipolar interactions start to play a role for wires thickness ⩾20 nm.

Microwave assisted switching in a Ni81Fe19 ellipsoid

The authors demonstrate the stimulation of the magnetization switching process of a Ni81Fe19 ellipsoid, which is dominated by domain nucleation and propagation, by applying a transverse microwave field. The study of the quasistatic switching behavior under the influence of a microwave field was performed using longitudinal magneto-optic Kerr effect magnetometry. A strong reduction of the coercive field for microwave frequencies between 500 and 900MHz has been observed, which can be attributed to two different mechanisms: microwave stimulated enhancement of domain nucleation and microwave stimulated growth of the reversed domain. The authors prove that heating is not the origin of the reduction of the coercive field.

Magnetic anisotropy induced by patterning in (1 1 0) MBE-grown ternary Laves phase TbDyFe 2 films

Ternary rare-earth intermetallic Laves phase compounds Tb 0.1Dy 0.9Fe 2, grown by MBE, were characterized by a vibrating sample magnetometer (VSM) and magneto-optical Kerr effect magnetometry (MOKE). Striped arrays with 5 μm-width along an in-plane easy-axis [−1,1,0] direction were defined by a UV direct write system in 400 nm-thick (1 1 0) epitaxial Tb 0.1Dy 0.9Fe 2 films. An additional shape anisotropy of 2.7 × 10 5 erg/cm 3 in magnitude is induced. MOKE measurements reveal pronounced enhancement of the switching field along [−1,1,0] direction. However, the switching field along the hard-axis [0 0 1] direction is unchanged as the intrinsic anisotropy along the [0 0 1] direction is an order of magnitude larger than the induced one. The results demonstrate a possibility of engineering the magnetostriction-to-anisotropy ratio by patterning in epitaxial TbDyFe 2 magnetostrictive materials.

Switching behavior of individual pseudo-spin-valve ring structures

In this work, focused magneto-optic Kerr-effect magnetometry, magnetoresistance measurements, and three-dimensional micromagnetic simulations are combined to build a detailed picture of the magnetic switching behavior of micron-scale pseudo-spin-valve ring elements. We find that the reversal of the ring magnetization is dominated by the magnetostatic interaction of the higher magnetic-moment cobalt layer, which nucleates reverse domains in the NiFe free layer, causing it to switch in a manner that would not be possible in a single-layer ring. Measuring single pseudo-spin-valve rings has allowed the resolution of fine details in hysteresis loops, so that small differences between the magnetic states and switching of nominally identical elements can be observed. Furthermore, the high signal to noise ratio of the magnetoresistance measurements allows single-switching events to be observed in the structures. Using this technique we observe a range of phenomena involving both stable and metastable configurations of the ring, which would have been obscured in field-cycle-averaged data.

The effect of the in-plane demagnetizing field on films with weak perpendicular magnetic anisotropy

The effect of reduced lateral dimension on the magnetic behavior of long microstrips with weak perpendicular magnetic anisotropy has been studied. The hysteresis loops of the surface and the whole volume of microstrips and large films have been recorded by magneto-optic Kerr effect magnetometry and vibrating sample magnetometry. Also, the magnetic structure of the microstrips and its evolution under an applied field have been studied by magnetic force microscopy and the Bitter technique. It has been observed that the transversal dimension of the microstrips gives rise mainly to an in-plane demagnetizing field which affects their hysteresis loops and magnetic structure in a different manner. A great increase in the surface coercivity in the transversal direction of the microstrips is the main unexpected effect that has been observed. On the other hand, the magnetic structure in remanence and its evolution with increasing thickness have not revealed a significant dependence on the direction of the last saturating field. It has also been observed that the reduced size of the microstrips in their transversal direction does not prevent the occurrence of the property known as rotatable anisotropy, typical of films with stripe domain structure, although it makes this property very anisotropic. Finally, a two-dimensional qualitative model of closed flux stripe domain structure has been used for correlating the enlargement of the surface coercivity of the microstrips with the evolution of their magnetic structure.

In-plane anisotropy of coercive field in permalloy square ring arrays

Magnetic ring arrays are promising candidates for application in magnetic random access memory devices. The magnetic reversal processes and anisotropy of the coercivity in arrays of square-shaped nanorings with different spacings were investigated by vector magneto-optical Kerr effect magnetometry, magnetic force microscopy, and micromagnetic simulations. Two-step magnetization reversal demonstrates fourfold symmetry in the film plane resulting from the shape anisotropy in rings. Our numerical simulations show good agreement with the experiment.

Ferrimagnetic properties of Co/(Gd–Co) multilayers

Co/(Gd–Co) multilayers have been prepared by rf-sputtering and investigated by means of Transverse Magnetooptic Kerr Effect (TMOKE), SQUID and VSM magnetometry. The composition of amorphous Gd 0.36 Co 0.64 layers was chosen so that their saturation magnetization was dominated by Gd moments in all the temperature range. Co and Gd–Co layers formed a macroscopic ferrimagnetically coupled system displaying a compensation temperature. Complete magnetic moment compensation was found at such point. An inversion of TMOKE hysteresis loops and a divergent behaviour of coercivity were also observed. By changing the layers thickness it has been possible to control the magnetic characteristics of the Co/(Gd–Co) structures, in particular the compensation takes place at different temperatures.

Fundamental magnetic states of disk and ring elements

We present here a review of our recent work on the equilibrium magnetic states in micrometer ring and disk elements (1.65μm in diameter) with varying thickness (5–38nm), ring width (110–730nm) and magnetic material (fcc Co, polycrystalline hcp Co and polycrystalline fcc Ni80Fe20). The magnetic states have been studied using a variety of techniques, including magneto-optic Kerr effect magnetometry and non-invasive imaging techniques, such as photoemission electron microscopy (PEEM). In particular, we consider the effect of the magnetocrystalline anisotropy on the equilibrium magnetic states and on the magnetic switching of ring and disk elements to show that significant changes occur in the case of disk elements and wide rings. A systematic study of the remanent magnetic states in disk elements shows that several magnetic configurations are stabilised at remanence, ranging from the vortex state to other well defined multidomain states, namely a state with two vortex cores (‘diamond’ state) and a high remanence ‘triangle’ state, which are mapped according to material and thickness range. In most instances, different magnetic states coexist in an array of disks subjected to the same field history. These observations are correlated with micromagnetic simulations to show that these magnetic metastable states are stabilised by defects or attained in the nucleation process following the removal of the applied magnetic field.

Stochastic switching in individual micrometre-sized Permalloy rings

The circulation of the low-field stable vortex states in a 5 μ m -diameter Permalloy ring, obtained by sweeping an applied field down from near-saturation, has been experimentally determined by focused magneto-optic Kerr effect magnetometry. The rings typically switch via the same circulation of the vortex state for both branches of the major hysteresis loop. Varying the applied field sinusoidally, the number of field cycles spent switching via one vortex state before changing to switch via the opposite vortex state is random. Micromagnetic simulations of the reversal in a 2 μ m -diameter Permalloy ring show that the history of the spin configuration plays a significant role in determining the vortex state circulation in these structures, and suggest that thermal fluctuations in a small region of edge spins are responsible for the stochastic switching observed over several applied field cycles.

Magnetostatic interaction in arrays of nanometric permalloy wires: A magneto-optic Kerr effect and a Brillouin light scattering study

Two arrays of permalloy parallel wires, $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick, having the same width of $175\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and different spacing of 35 and $175\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were prepared by means of deep ultraviolet lithography and lift-off process. The effect of magnetostatic interaction on both the static and dynamic magnetic properties of arrays of wires has been investigated by means of magneto-optic and Brillouin light scattering techniques, respectively. In particular, the magnetization switching of the samples, measured by vectorial magneto-optical Kerr effect magnetometry and microscopy shows the effects of dipolar interaction in the case of $35\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ spaced wires, while in the other sample the measurements show that the wires are substantially noninteracting. The Brillouin light scattering measurements showed that for the sample with interwire spacing of $35\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, dipolar coupling between magnetic wires leads to the formation of a collective mode which has a continuous spectrum and exists in a range of frequencies, while for the $175\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ spaced wires the spin modes are dispersionless. To quantify the investigated effects, a theory developed earlier for an isolated wire has been extended to the case of a one-dimensional array of ferromagnetic wires.

Magnetization reversal in individual micrometer-sized polycrystalline Permalloy rings

The magnetization reversal of individual 2 μm and 5 μm diameter polycrystalline Permalloy rings, with respective widths 0.75 μm and 1 μm, thickness 45 nm, has been investigated by focused magneto-optic Kerr effect (MOKE) magnetometry. Micromagnetic simulation of the reversal in the 2 μm diameter ring reveals that the onion-to-vortex state switching occurs by nucleation and subsequent annihilation of vortex walls that span the width of the ring, and that the vortex-to-reverse-onion state switching occurs by expansion of a reverse domain. The hysteresis loop shows good agreement with the experimental MOKE loop. Measurements of the switching through one-half of a 5 μm diameter ring enable the determination of the circulation of the vortex states accessed during one applied field cycle. The rings switch via one vortex state (either clockwise or anticlockwise) on both downward and upward applied field sweeps. The number of applied field cycles spent switching via one vortex state before changing to switch via the opposite vortex state is random, likely to be due to the history of the spin configuration and thermal fluctuations.

Mesofrequency switching dynamics in epitaxial CoFe and Fe thin films on GaAs(001)

Thin epitaxial Co27Fe73 and Fe films have been grown on GaAs(001), with thicknesses of 8.5 and 10nm, respectively. The mesofrequency switching dynamics have been investigated by ac magneto-optic Kerr effect magnetometry. Dynamic hysteresis loops have been measured along the four major in-plane crystallographic axes [100], [110], [010], and [11¯0] in each film in the sinusoidal applied field frequency range of 0.2–2kHz. The coercive field Hc for the CoFe film is 1.4–2.9 times larger than that for the Fe film, depending on the measurement axis, indicating a larger effective energy barrier to reversal. Examining the coercive field as a function of frequency Hc(f) it is found that the time scales for the domain nucleation and wall propagation are 3–10 times longer in the CoFe than in the Fe film. As previously shown in epitaxial Fe films, a local minimum in Hc(f) can occur at the crossover from the wall propagation-dominated to the nucleation-dominated dynamic regime, and here we find that it is more pronounced for the CoFe than for the Fe film, indicating a reduction in domain-wall pinning. We conclude that the magnitude of the effective energy barrier to reversal sets the time scale for the domain nucleation and wall propagation processes and determines the characteristics of Hc(f) for the two films.

Edge roughness and coercivity in magnetic nanostructures

Rectangular Permalloy nanostructures with a range of edge roughnesses were fabricated using electron beam lithography by controlling the exposure conditions of individual structures. The widths of the structures ranged from 200 nm to 611.5 nm. Each individual structure was characterized magnetically, using magneto-optical Kerr effect (MOKE) magnetometry, and structurally, using scanning electron microscopy (SEM). Increasing edge roughness enhanced the coercivity of structures with similar widths, except for the 200 nm wide structures, which showed no enhancement. The coercivity enhancement was independent of surface roughness. Large edge roughness more than doubled the coercivity of the widest structures. The experimentally determined structural edge profiles were used to model the effect of edge roughness on magnetization switching. Micromagnetic simulations showed that edge roughness distorted the spin-structure of rectangular nanomagnets prior to switching and that, compared to smooth-sided rectangles, edge roughness could lead to an increase in the switching field. However, the magnitude of the simulated increase was not as high as observed experimentally.

Analysis of the Ni[formula omitted]Fe[formula omitted]/Fe[formula omitted]Mn[formula omitted] exchange bias system with a varying Cu spacer thickness and position for partial decoupling

In order to study the role of the exchange interaction at and near the interface, Ni81Fe19/Fe50Mn50 bilayers have been studied, which have an intervening layer of varying thickness and position in the antiferromagnetic Fe50Mn50 layer. The role of the intervening layer is to generate partial exchange decoupling. As a result, samples were obtained, where in one in-plane direction the position of the intervening layer varies from the interface to the top surface of the Fe50Mn50 layer at a constant intervening layer thickness, and in the other in-plane direction the intervening layer thickness varies. Two-dimensional maps of the resulting exchange bias field and the coercive field were obtained from magneto-optic Kerr effect magnetometry measurements. The role of the position and strength of the partial decoupling within the antiferromagnetic layer on the exchange bias effect and the coercive field is discussed.