Magneto-optical Kerr Magnetometry Research Articles

Влияние радиационного воздействия на магнитные свойства пленок ферромагнетик/IrMn с обменным сдвигом

In this work, the effect of the action of gamma quanta and neutrons on the magnetic properties of Ta/feromagnet/IrMn/Ta bilayer films with an exchange bias of the hysteresis loop is studied. The samples were fabricated by magnetron sputtering, and their structure was studied by small-angle X-ray reflectometry and X-ray diffractometry. The measurement of the magnetic hysteresis loops of the irradiated and non-irradiated samples was carried out by the methods of magneto-optical Kerr magnetometry. As a result, the effect of broadening of the magnetic hysteresis loop after irradiation was found. The maximum broadening of the hysteresis loop was observed in the NiFe/IrMn film under neutron irradiation; in this case, the loop width increased by more than 2.5 times. No noticeable change in the field of the exchange bias of the hysteresis loops was observed in the entire range of radiation exposure.

Integration of epitaxial La2/3Sr1/3MnO3 thin films on silicon-on-sapphire substrate for MEMS applications

We report the integration of high-quality epitaxial La2/3Sr1/3MnO3 (LSMO) thin films onto SrTiO3 buffered Silicon-on-Sapphire (SOS) substrates by combining state-of-the-art thin film growth techniques such as molecular beam epitaxy and pulsed laser deposition. Detailed structural, magnetic and electrical characterizations of the LSMO/STO/SOS heterostructures show that the LSMO film properties are competitive with those directly grown on oxide substrates. X-ray magnetic circular dichroism measurements on Mn L2,3 edges show strong dichroic signal at room temperature, and angular-dependent in-plane magnetic properties by magneto-optical Kerr magnetometry reveal isotropic magnetic anisotropy. Suspended micro-bridges were thus finally fabricated by silicon micromachining, thus demonstrating the potential use of integrating LSMO magnetic layer on industrially compatible SOS substrates for the development of applicative MEMS devices.

Viscous magnetization decrease in first-order reversal curves induced by rotatable magnetic anisotropy in polycrystalline exchange-biased bilayers

A prototypical polycrystalline in-plane exchange bias system exhibited a viscous decrease of the ferromagnetic magnetization upon increasing external magnetic field when measuring first-order reversal curves. The phenomenon is investigated by means of angular-resolved vectorial magneto-optical Kerr magnetometry complemented by fits of model calculations and by Kerr microscopy. It is found that the viscous magnetization decrease is mediated by a rotatable magnetic anisotropy arising from thermally unstable antiferromagnetic grains coupled to the probed ferromagnet. This additionally manifests itself by a creeping domain wall motion in the ferromagnet due to thermally activated processes in the antiferromagnet. The investigations are in agreement with a generalized description of polycrystalline exchange bias systems and emphasize the relevance of understanding minor loop behavior addressing nonsaturated magnetic states for systems susceptible to dynamic changes on the hysteresis loop timescale.

Interrelation between polycrystalline structure and time-dependent magnetic anisotropies in exchange-biased bilayers

For polycrystalline exchange-biased thin films the macroscopic magnetic properties evolve from a complex interplay of different individual magnetic anisotropies which are directly connected to the grain volume distribution and to the crystal and interface structure of the layer system. A quantitative comparison of models describing a macroscopically observed exchange bias by the contributing anisotropies has in most cases been hampered by neglecting their time dependence with respect to data acquisition and storage times. Using a recently developed model we are able to connect time-dependent parts of the prevailing anisotropies with parameters describing the polycrystalline structure of the layer system. The model will be compared to experiments on a prototypical IrMn/CoFe bilayer, where structural and magnetic parameters have been systematically altered by varying the deposition rate of the antiferromagnetic layer and the field-cooling temperature. The combination of angular-resolved measurements obtained by vectorial magneto-optic Kerr magnetometry and a systematic analysis of the polycrystalline structure enables the disentanglement of the different anisotropy contributions to the macroscopic exchange bias and coercive fields and serves as a verification of the utilized model.

Room temperature biaxial magnetic anisotropy in La0.67Sr0.33MnO3 thin films on SrTiO3 buffered MgO (001) substrates for spintronic applications

Spintronics exploits the magnetoresistance effects to store or sense the magnetic information. Since the magnetoresistance strictly depends on the magnetic anisotropy of a system, it is fundamental to set a defined anisotropy to the system. Here, we investigate half-metallic La0.67Sr0.33MnO3 thin films by means of vectorial Magneto-Optical Kerr Magnetometry and found that they exhibit pure biaxial magnetic anisotropy at room temperature if grown onto a MgO (001) substrate with a thin SrTiO3 buffer. In this way, we can avoid unwanted uniaxial magnetic anisotropy contributions that may be detrimental for specific applications. The detailed study of the angular evolution of the magnetization reversal pathways and critical fields (coercivity and switching) discloses the origin of the magnetic anisotropy, which is magnetocrystalline in nature and shows fourfold symmetry at any temperature.

Optical and magnetic properties of Al/NiFe and Al/Ge/NiFe nanosized films

Nanosized films with ferromagnetic layers are widely used in nanoelectronics, sensor systems and telecommunication. The physical and magnetic properties of nanolayers may significantly differ from those known for bulk materials due to fine crystalline structure, influence of interfaces, roughness, and diffusion. In this work, we are employing a spectral ellipsometry method, magneto-optical Kerr magnetometry and VSM to investigate the impact of layer thickness on the optical constants and magnetization processes for two and three layer films of the type Al/NiFe/sitall, Al/Ge/NiFe/sitall on sitall substrate for different thickness of the upper Al layers. The refractive indexes of two layer films are well resolved by spectral ellipsometry demonstrating their good quality. Modelling data for three-layer films show considerable discrepancy with the experiment, which can be related to a stronger influence of interfaces. The magnetization processes of two-layer films weakly depend on the type and thickness of the upper non-ferromagnetic layers. However, the coercivity of three layer films may significantly change with the thickness of the upper layer: more than twice when the thickness of Al layer increases from 4 to 20 nm.

Surface magnetic structures induced by mechanical stresses in Co-rich microwires

We report the results of both magneto-optical Kerr effect and magnetometry studies of torsion stress induced transformation of the surface domain structure and magnetization reversal process in amorphous microwires. The coexistence of different types of domain structures depends on amplitude and sign of the torsion stress. The obtained results were analyzed using model supposing the existence of surface domains of different types in inner core – outer shell of microwires. The experimentally determined volume contribution to magnetization reversal is comparable with the volume contribution obtained as a result of simulation.

Thickness and angular dependent magnetic anisotropy of La0.67Sr0.33MnO3 thin films by Vectorial Magneto Optical Kerr Magnetometry

We investigate the in-plane magnetic anisotropy in La0.67Sr0.33MnO3 thin films grown on SrTiO3 (001) substrate using angular dependent room temperature Vectorial Magneto-Optical Kerr Magnetometry. The experimental data reveals that the magnetic anisotropy symmetry landscape significantly changes depending upon the strain and thickness. At low film thickness (12 and 25 nm) the dominant uniaxial anisotropy is due to interface effects, step edges due to mis-cut angle of SrTiO3 substrate. At intermediate thickness, the magnetic anisotropy presents a competition between magnetocrystalline (biaxial) and substrate step induced (uniaxial) anisotropy. Depending upon their relative strengths, a profound biaxial or uniaxial or mixed anisotropy is favoured. Above the critical thickness, magnetocrystalline anisotropy dominates all other effects and shows a biaxial anisotropy.

Large area periodic ferromagnetic nanowires deposited onto a polymer substrate

There are various challenges associated with the fabrication of highly ordered magnetic nanostructures on flexible substrates due to the compatibility with lithography and deposition techniques. In this article, we present a nanofabrication technique to synthesize a large area (5 × 5 mm2) of ferromagnetic nanowires on top of a polymer substrate (Kapton®) using interference lithography and sputtering processes. We have systematically characterized their static and dynamic magnetic behaviors using magneto-optical Kerr magnetometry and broadband ferromagnetic resonance spectroscopy. To evaluate the quality of our approach, we also deposited an identical array of nanowires on Silicon substrates for comparison. The nanowires deposited on the two substrates display similar static and dynamic properties, including the identical magnetization reversal process, number of resonance modes, and comparable damping parameters. The results suggest the good quality of our nanowires and their suitability in future flexible spintronic devices.

Improvement of Magnetic Properties in Microfabricated FePt and Fe Nano-Composites Upon Post-Annealing

In order to evaluate the magnetic properties of cylindrical nanocomposite magnets, microfabrication, electron beam lithography, and Ar ion etching were used to prepare hard, ring-shaped, $L$ 10 ordered FePt structures embedded with body-centered cubic Fe cores. $L$ 10 FePt has high uniaxial magnetic anisotropy, moderate saturation magnetization, and excellent corrosion resistance. Magnetic properties were measured by magneto-optic Kerr effect, magnetometry, and magnetic force microscopy before and after annealing. Coercivity of the FePt/Fe structures with inner diameter 1.0 µm increased from 4.5 kOe to 8.4 kOe with post-annealing at 500 °C for 1 hour.

Time-dependent rotatable magnetic anisotropy in polycrystalline exchange-bias systems: Dependence on grain-size distribution

Angular resolved measurements of the exchange bias field and the coercive field are a powerful tool to distinguish between different competing magnetic anisotropies in polycrystalline exchange bias layer systems. No simple analytical model is as yet available, which considers time dependent effects like enhanced coercivity arising from the grain size distribution of the antiferromagnet. In this work we expand an existing model class describing polycrystalline exchange bias systems by a rotatable magnetic anisotropy term to describe grain size correlated effects. Additionally, we performed angular resolved magnetization curve measurements using vectorial magnetooptic Kerr magnetometry. Comparison of the experimental data with the proposed model shows excellent agreement and reveals the ferromagnetic anisotropy and properties connected to the grain size distribution of the antiferromagnet. Therefore, a distinction between the different influences on coercivity and anisotropy becomes available.

Magnetic stripe domains of [Pt/Co/Cu]10 multilayer near spin reorientation transition

The dependence of magnetic anisotropy, magnetic domain patterns and magnetization reversal processes in [Pt/Co(tCo)/Cu]10 film stack epitaxied on Cu (111) substrate have been studied as a function of the Co layer thickness tCo, by magneto-optic polar Kerr magnetometry and microscopy. We find the film undergoes spin reorientation transition from out-of-plane to in-plane as tCo increases. The SRT thickness is verified by Rotating-field Magneto-Optic Kerr effect method. The film exhibits the stripe domain structures at remanence with the width decreasing while tCo approaches SRT. As demonstrated by the first order reversal curve measurement, the magnetization reversal process encompasses irreversible domain nucleation, domain annihilation at large field and reversible domain switching near remanence.

Growth, structural, and magnetic characterization of epitaxial Co2MnSi films deposited on MgO and Cr seed layers

We report detailed structural characterization and magneto-optical Kerr magnetometry measurements at room temperature in epitaxial Co2MnSi thin films grown on MgO(001) and Cr(001) buffered MgO single crystals prepared by sputtering. While Co2MnSi/Cr//MgO(001) films display the expected cubic anisotropy, the magnetization curves obtained for Co2MnSi//MgO(001) samples exhibit a superimposed in-plane uniaxial magnetic anisotropy. The evolution of magnetization with film thickness points to a relevant interfacial Co2MnSi-buffer layer (Cr or MgO) contribution which competes with magnetic properties of bulk Co2MnSi, resulting in a drastic change in the magnetism of the whole sample. The origin of this interfacial magnetic anisotropy is discussed and correlated with our structural studies.

High-Frequency Electric Current Influence on Magnetization Reversal and Domain Structure in Co-Rich Amorphous Microwires

We report investigations on influence of high-frequency electric current on surface magnetization reversal and surface domains nucleation in amorphous glass covered microwires. Magneto-optical Kerr magnetometry and microscopy have been used. A strong change of surface magnetization reversal and the transformation of the surface domain structure induced by electric current within a frequency range of 100 Hz-15 MHz have been found.

Spin reorientation transition and phase diagram in an He+ ion irradiated ultrathin Pt/Co(0.5 nm)/Pt film

A rich thermal magnetic phase diagram has been determined by magneto-optical Kerr magnetometry and microscopy in a He+ ion irradiated ultrathin Pt/Co(0.5 nm)/Pt film. The components of the net magnetization and the evolution and disappearance of the ribbonlike magnetic domain pattern have been studied in the perpendicular to in-plane spin reorientation transition temperature region. As observed in a dipolar frustrated ferromagnet, the ribbon pattern blurs progressively with increasing the temperature as due to efficient spatial fluctuations. We emphasize the limitation of present theories for interpreting such a type of dynamic transition.

Magnetic Properties of EuS/Co Multilayers on KCl and BaF₂ Substrates

EuS/Co multilayers are hybrid structures composed of a model ferromagnetic semiconductor EuS (Curie temperature Tc=16.5 K) and a ferromagnetic transition metal Co. Recent investigations of these multilayers revealed variety of important spintronic effects, such as an antiferromagnetic interlayer coupling in EuS/Co bilayers [1] and a very efficient spin filtering by EuS electron barrier in Al/EuS/Co tunneling structures [2]. These effects are expected to originate from the exchange interactions across EuSCo interface. In previous studies, polycrystalline or amorphous EuS/Co multilayers were grown on glass or on Si substrate with Pt/Co metallic buffer layer. In this work, we exploit the technological method developed for the epitaxial growth of EuS/PbS multilayers on KCl and BaF2 substrates, in order to study the magnetic properties of EuS/Co multilayers with monocrystalline EuS layer. EuS/Co/EuS trilayers and Co/EuS bilayers were grown on cleaved KCl (001) and BaF2 (111) substrates using high vacuum deposition technique employing electron guns for Co and EuS. Epitaxial 20-60 nm thick PbS buffer layer was deposited on the substrates from heated tungsten boat. The layer thickness covered the range 35-55 A for EuS and 40-250 A for Co. The structural properties of the layers were examined by xray diffraction method revealing monocrystalline PbS buffer and bottom EuS layer. Co and top EuS layers are expected to be polycrystalline. Magnetic properties of the layers were examined using magneto-optical Kerr (MOKE) and superconductor (SQUID) magnetometry techniques. In the case of MOKE experiments, the linearly polarized light from He-Ne laser was used in longitudinal MOKE geometry. Measurements of magnetic hysteresis loops were performed in temperature range T = 4-250 K and in external magnetic fields up to 2 kOe applied in the plane of the multilayer. All the investigated multilayers showed ferromagnetic properties at room temperature due to Co layer with the ferromagnetic transition in EuS layer clearly marked upon cooling below 16 K. In EuS/Co/EuS trilayers grown on KCl substrate the antiferromagnetic alignment of magnetization vectors of Co and EuS layers was experimentally observed as a characteristic low field plateau on magnetic hysteresis loops and a decrease of multilayer magnetization below 16 K. In contrast, no interlayer coupling was found in multilayers grown on BaF2 substrate. In Co/EuS bilayers, the characteristic temperature dependent shift of the magnetic hysteresis loops was found indicating the well known exchange bias effect. We assign this experimental finding to the exchange coupling between Co layer and an ultrathin (about 2 nm) top antiferromagnetic CoO layer known to spontaneously form on Co layers in the air.

Determination of the micromagnetic parameters in (Ga,Mn)As using domain theory

The magnetic domain structure and magnetic properties of a ferromagnetic (Ga,Mn)As epilayer with perpendicular magnetic easy axis are investigated. We show that despite strong hysteresis domain theory at thermodynamical equilibrium can be used to determine the micromagnetic parameters. Combining magneto-optical Kerr microscopy, magnetometry, and ferromagnetic resonance measurements, we obtain the characteristic parameter for magnetic domains ${\ensuremath{\lambda}}_{c}$, the domain wall width and specific energy, and the spin stiffness constant as a function of temperature. The nucleation barrier for magnetization reversal and the Walker breakdown velocity for field-driven domain wall propagation are also estimated.

Investigation of magnetization reversal in micron-sized stripes of epitaxial-grown (1 1 0) Laves phase DyFe 2 films

A large-scale striped array of width 3.4 μm has been fabricated in 400-nm-thick epitaxial (1 1 0) DyFe 2 films with Laves phase structure using a UV direct write system. The magnetic and magneto-optical properties of the sample were characterized by a vibrating sample magnetometer and magneto-optical Kerr magnetometry, respectively. The close easy-axis [−1 1 0] direction, confirmed by VSM, was chosen for patterning. The MO Kerr loops reveal that an additional anisotropy can be induced by the shape of the stripes, which can obviously enhance the coercivity of the close easy-axis [−1 1 0] loop. The magnitude of this additional anisotropy was determined to be 4.2 × 10 5 erg/cm 3, which is not sufficient to change the coercivity of the hard-axis loop due to a large intrinsic anisotropy along the hard-axis [0 0 1] direction.

Ion beam smoothing with low-energy argon ions and reduction of Néel “orange peel” coupling in magnetic tunnel junctions

The influence of low-energy argon bombardment with an ion energy of 30–70 eV and normal incidence on the morphology of a 15-nm-thick Ni 81Fe 19 film on SiO 2 has been studied at room temperature. Scanning tunneling microscope measurements show a reduction of the root-mean-square roughness of over 40%. For magnetic tunnel junctions prepared by involving ion-beam smoothing process, the Néel coupling is substantially reduced and measured by magneto-optic Kerr magnetometry.

Evidence for interdot coupling in an array of micrometric Fe dots

Abstract The magnetization reversal of both the continuous and patterned Fe films, grown in ultra high vacuum by atomically controlled evaporation through a mask, was studied using magneto-optic Kerr magnetometry. The hysteresis loops recorded from continuous Fe film as a function of the direction of the applied field, evidence a well-defined uniaxial anisotropy, induced during the film deposition. The patterned area shows a magnetic anisotropy completely different from that of the continuous film and related to the lattice symmetry. Scanning electron microscopy images of the patterned area allows us to exclude effects related to deviations from circularity of the shape of the single dots. The close relationship between the anisotropy directions and the pattern lattice axes indicates that the new anisotropy originates from the interdot dipolar coupling interaction.