Magneto-optical Kerr Effect Hysteresis Loops Research Articles

Exchange-coupled suppression of vortex formation in permalloy nanodot chain arrays

The effect of interdot exchange coupling on the magnetization reversal processes in nanodots has been investigated in a study on permalloy dot arrays with dot diameters of 300 nm and thicknesses of 40 nm. The dots, fabricated using e-beam lithography and ion beam deposition, are exchange coupled via 50 nm long permalloy bridges of widths ranging from 0 to 60 nm. Chains of five collinear coupled dots form the unit cell of the array structure. Magneto-optical Kerr effect hysteresis loops are reported with comparison to simulations. With field applied along the coupling direction, vortex nucleation occurs sequentially from the outer to the inner dots of the chain. Compared with uncoupled dots, nucleation is suppressed by the interdot exchange coupling resulting in highly correlated magnetization patterns. Buildup of exchange energy in the inner dots ultimately drives the bridge magnetization to switch, followed by abrupt, simultaneous vortex nucleation in these dots. Along the perpendicular-to-the-coupling direction, the exchange interaction has little effect and magnetic reversal is almost identical to arrays of isolated dots of the same geometry.

Magnetization Reversal of Exchange-biased Bilayers and Trilayers Probed using Front and Back LT-MOKE

Magneto-optical Kerr effect (MOKE) magnetometry was used to investigate magnetization reversal dynamics in 30-nm NiFe/15-nm FeMn, 15-nm FeMn/30-nm CoFe bilayers, and 30-nm NiFe/(2,10)-nm FeMn/30-nm CoFe trilayers. The in-plane magnetization components of each ferromagnetic layer, both parallel and perpendicular to the applied field, were separately determined by measuring the longitudinal and transverse MOKE hysteresis loops from both the front and back sides of the film for an oblique incident s-polarized beam. The magnetization of the FeMn/CoFe bilayer was reversed abruptly and symmetrically through nucleation and domain wall propagation, while that of the NiFe/FeMn bilayer was reversed asymmetrically with a dominant rotation. In the NiFe/FeMn/CoFe trilayers, the magnetic reversal of the two ferromagnetic layers proceeded via nucleation and domain wall propagation for 2-nm FeMn, but via asymmetric rotation for 10-nm FeMn. The exchange-biased ferromagnetic layers showed the magnetization reversal along the same path in the film plane for the decreasing and increasing field branches from transverse MOKE hysteresis loops, which can be qualitatively explained by the theoretical model of the exchange-biased ferromagnetic/antiferromagnetic systems.

Linear and nonlinear magneto‐optical response of ultrathin Co/Au/Mo and Co/Mo films grown on sapphire substrates

AbstractWe report studies of the linear and nonlinear magneto‐optical properties of epitaxial ultrathin Co films grown on sapphire substrates covered with Mo or Mo/Au buffers. Changes of the crystallographic symmetry and magnetization of buried interfaces were deduced from the magneto‐optical Kerr effect hysteresis loops and magnetization‐induced second‐harmonic generation (MSHG). Depending on the buffer, two‐fold (for Mo buffer) or six‐fold (for Mo/Au buffer) symmetries were observed in both inplane magnetic anisotropy and MSHG intensity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Magnetic response of Mn-doped amorphous porous Ge fabricated by ion-implantation

Ge substrates of (100) orientation were irradiated with 100keV Mn ions (2×1016at./cm2) at a temperature of 190°C. Pronounced swelling of the irradiated material up to 150nm, associated with the formation of a Mn-doped amorphous porous Ge layer, is investigated by scanning electron microscopy and X-ray photoelectron depth profiling. Magneto-optical Kerr effect hysteresis loops have been used to characterize the magnetic properties of the obtained sample. The details of the hysteresis loops reveal that the ferromagnetic property of the sample up to ∼20K and the subsequent strong non-linear paramagnetic response must be ascribed to Mn atoms dispersed in the amorphous porous implanted layer.

Magneto-optical characterization of Mn xGe 1−x alloys obtained by ion implantation

Magneto-optical Kerr effect hysteresis loops at various wavelengths in the visible/near-infrared range have been used to characterize the magnetic properties of alloys obtained by implanting Mn ions at fixed energy in a Ge matrix. The details of the hysteresis loops reveal the presence of multiple magnetic contributions. They may be attributed to the inhomogeneous distribution of the magnetic atoms and, in particular, to the known coexistence of diluted Mn in the Ge matrix and metallic Mn-rich nanoparticles embedded in it [ Phys. Rev. B 73 (2006) 195207].

Strong uniaxial anisotropies of iron grown on UHV cleaved InAs(110) investigated by correlated scanning tunneling microscopy and ferromagnetic resonance

The magnetic properties of iron films grown under UHV condition are governed by the deposition condition and the substrate preparation. One way of producing almost perfect surface conditions with large terrace width and exact orientation is cleaving. With this method no additional cleaning process of the surface is needed. Here we present our investigations of iron films (thickness: 0.1–30nm) grown on UHV cleaved InAs substrates using in situ scanning tunnel microscopy (STM) investigations combined with in situ magneto-optical Kerr effect (MOKE) and ex situ angle and frequency dependent conventional ferromagnetic resonance (FMR). The (110) InAs surface allows the investigation of three magnetic axes: in-plane easy (001), hard (111), and intermediate (110) axes of the iron layer. The (110) symmetry of the magnetic cubic crystalline anisotropy should lead to angles of 110° and 70° between the hard axes, respectively. STM images of the clean InAs(110) show 500nm wide terraces with mono atomic steps predominately oriented parallel to the (001) direction. MOKE hysteresis loops were measured during the Fe deposition to prove the ferromagnetic state. The angle dependent in-plane FMR measurement revealed unexpected 90° symmetry. Deducing the anisotropy constants from the FMR resonance position, one finds a slightly increased bulk crystalline anisotropy and a uniaxial anisotropy of the same order of magnitude. This strong uniaxial anisotropy distorts the 110° symmetry to 90° symmetry. The correlation of frequency and angle dependent FMR and STM measurements evidences that the uniaxial anisotropy is due to the dislocations caused by the orientation of the terraces.

MOKE hysteresis loop method of determining the anisotropy constants of ferromagnetic thin films: fe on GaAs(1 0 0) with overlayers of Au and Cr

This paper presents a quantitative method used to determine the magnetocrystalline anisotropy constants of thin magnetic films from normalized magnetization data measured on a magneto-optic Kerr effect (MOKE) magnetometer. The method is based on a total magnetic energy density model, and incorporates higher order effects in the detected signal. By way of illustration, the method is used to determine the magnetocrystalline anisotropy constants of epitaxial thin Fe films on GaAs substrates, which have different overlayers. It is shown that a Cr overlayer on a 30 ML thick Fe film reduces the uniaxial contribution to the magnetic anisotropy compared with an Au overlayer.

In situ magneto-optical Kerr effect study of uncovered Fe films on ZnSe(0 0 1)

The magnetic properties of uncovered Fe/ZnSe/GaAs(1 0 0) ultrathin films have been determined in situ by magneto-optical Kerr effect (MOKE). Fe films up to 10 monolayers (ML) thick were deposited on c(2×2) Zn-rich ZnSe/GaAs(0 0 1) surfaces at 180 °C. We have studied the thickness dependence of the in-plane lattice parameter of the Fe films and of the MOKE hysteresis loops in the longitudinal geometry, at 150 K, under magnetic fields up to 0.1 T applied along the [1 1 0] and [1-1 0] directions of the ZnSe(0 0 1). Reflection high energy electron diffraction show that in the low thickness regime the Fe films present an in-plane structural anisotropy characterized by an expansion along the [1 1 0] direction. Hysteretic loops were obtained only starting from ∼5 ML Fe. We found the onset of an uniaxial magnetic anisotropy with [1 1 0] magnetic easy axis at 7 ML Fe.

Measurement of MOKE Hysteresis Loop Using Non-uniform Sampling

There are two parts in MOKE hysteresis loops, typical for materials with perpendicular magnetic anisotropy. One is due to a steeply falling edge or rising edge accompanied by the sign changes in magneto-optical Kerr effect (MOKE) signal. The other corresponds to a slow change of non-transition segment. A satisfying tradeoff between response to rapid change and noise rejection in non-transition segment of the loop is hardly obtained. The problem may be solved using a method based on non-uniform sampling. Its goals are to reduce distortion of the transitional waveform and noise, to obtain high resolution of transitional waveform and to decrease the number of sampling points. This paper describes the principle of non-uniform sampling and its implementation, and we designed a comparative experiment using a constant sampling frequency and multi-sampling frequency. The result shows that the measurement system with non-uniform sampling can reduce distortion of the MOKE loops, improve the accuracy of the coercivity, and decrease the number of sampling points.

Dynamic and static measurements on epitaxial Fe/Si/Fe

Strong antiferromagnetic interlayer exchange coupling across an insulating spacer is in increasing demand for high-density magnetic recording. We report here on the interlayer exchange coupling of epitaxial Fe(8 nm)/Si(t)/Fe(10 nm) trilayers as a function of Si thickness studied by ferromagnetic resonance (FMR), Brillouin light scattering, and magneto optic Kerr effect (MOKE) measurement techniques. A very strong antiferromagnetic (AFM) interlayer exchange coupling (>6 erg/cm2) was observed at a spacer Si thickness of 0.7 nm. The bilinear J1 and biquadratic J2 coupling constants were determined from (i) the fitting of the angular variation of the resonance field (Hres) in FMR experiments, (ii) the field variation of the frequencies of the Damon–Eshbach surface modes (both optic and acoustic) in BLS measurements, and (iii) the fitting of longitudinal MOKE hysteresis loops. We obtain a higher Hres along the easy axis than along the hard axis and the magnetizations of the two Fe films are canted. The eightfold-like symmetry of Hres as a function of the angle observed at room temperature is due to the competition between the Fe fourfold anisotropy and AFM interfacial coupling energy. This behavior vanishes at low temperatures due to a strong increase of AFM coupling (especially J2) in comparison to fourfold in-plane anisotropy. From the fitting of the temperature dependent FMR data, we obtain the temperature variation of the bilinear and biquadratic exchange coupling constants. We distinguish the existence of canted magnetization states at resonance by fitting the experimental Hres versus θH data to the model calculation.

Tunneling in epitaxial Fe/Si/Fe structures with strong antiferromagnetic interlayer coupling

Fe(5 nm)/Si(0.8–2 nm)/Fe(5 nm) structures are grown by molecular-beam epitaxy on Ag(001) buffered GaAs substrates. Ferromagnetic tunneling junctions with crossed electrodes and junction areas ranging from 22 to 225 μm2 are patterned using photolithography. Antiparallel alignment of the magnetizations due to antiferromagnetic interlayer coupling, which is confirmed by longitudinal magneto-optical Kerr effect hysteresis loops, exists for the whole range of spacer thicknesses. Transport properties in current perpendicular to the sample plane geometry are examined by the four-point method in the temperature range from 4 K to room temperature. As a function of spacer thickness, the junctions show a strong increase of the resistance times area product from ≈1 Ω μm2 to more than 10 kΩ μm2. The dI/dV−V curves are parabolic and asymmetric and thus characteristic for trapezoidal tunneling barriers. The mean barrier heights derived from Brinkman fits range from 0.3 to 0.8 eV. The zero-bias resistance of the tunneling junctions moderately decreases with temperature by less than 10% over the whole measured temperature range. All these transport properties fulfill the necessary and sufficient criteria for elastic tunneling.

Interlayer exchange coupling of epitaxial Fe/Al/Fe trilayer films: Dynamic and static measurements

This report deals with the interlayer exchange coupling of epitaxial Fe(3 nm)/Al(tAl=0.4–0.9 nm)/Fe(2 nm) trilayers studied by using ferromagnetic resonance (FMR), Brillouin light scattering (BLS) and magneto-optic Kerr effect (MOKE) measurement techniques. A very strong antiferromagnetic (AFM) interlayer exchange coupling (>3 erg/cm2) was observed at a spacer Al thickness of 0.7 nm. The bilinear (J1) and the biquadratic (J2) coupling constants were determined from (i) the fitting of angular variation (θH) of resonance field (Hres) from FMR, (ii) field variation of acoustic and optic surface mode frequencies from BLS and (iii) by fitting of longitudinal MOKE hysteresis loops. The samples with strong antiferromagnetic (AFM) coupling have a higher Hres along the easy axis than that along hard axis and their magnetizations are canted. The eightfold-like symmetry, of the acoustic Hres as a function of in-plane field angle at room temperature, is due to the strong competition between the fourfold anisotropy energy (HK) and the AFM interfacial coupling energy. This behavior vanished at low temperatures due to a strong increase of AFM coupling in comparison to HK. We distinguish the existence of canted magnetization states at resonance by fitting the experimental Hres(θH) data to the model calculation.

Element-specific magnetic properties of the epitaxial NiFe/Cr/NiFe trilayers

Ni and Fe L3,2-edge magnetic circular dichroism (MCD) and Cr K-edge x-ray absorption near-edge structure measurements have been performed for the Py/Cr(tCr)/Py (Py=Ni80Fe20) trilayers with tCr=5, 10, 15, 20, and 25 Å. The MCD asymmetry ratios in the Ni and Fe L3-edges spectra are found strongly affected by the thickness of the Cr layer. The moments of the Fe and Ni 3d states are found, respectively, to decrease and increase suddenly when tCr increases beyond 20 Å. The measured magneto-optical Kerr effect hysteresis loops show a ferromagnetic-to-antiferromagnetic phase transition at about the same Cr-layer thickness. The transition of the magnetic property is found to correlate with the structural transition of the Cr spacer layer from a pseudomorphic fcc-like structure to a bcc structure.

Metastable states during magnetization reversal in square permalloy rings

The magnetic reversal process in a two-dimensional array of permalloy square rings is presented. Rings of thickness of 25 nm, of lateral size of 2.1 \ensuremath{\mu}m, and with ring width of 240 nm were microfabricated using electron-beam lithography and lift-off techniques. Analysis of the diffracted magneto-optical Kerr effect hysteresis loops, magnetic force microscopy images, and micromagnetic simulations show that the magnetization reversal path depends on the direction of the in-plane applied magnetic field. On reducing the field from saturation, for fields along an edge or a diagonal of the square, the ``onion'' state is the stable state at remanence. In a narrow field range around reversal we find that the reversal occurs via a metastable intermediate state. For fields along the diagonal this intermediate state is a magnetic vortex. When the field is applied along an edge direction the intermediate state is a ``horseshoe'' state.

Magnetization reversal processes in amorphous and polycrystalline Co-Si patterned nanowires

Arrays of amorphous and polycrystalline Co/sub x/Si/sub 1-x/ nanowires have been prepared by electron beam lithography and a lift-off technique. The angular dependence of the magnetooptical transverse Kerr-effect hysteresis loops for both kind of samples has been compared in order to analyze the interplay between material microstructure and element shape at the submicrometric scale. The overall magnetic properties of the Co-Si nanowires are found to be dominated by patterning induced shape anisotropy, with the presence of coherent rotations in the magnetization reversal process in a wide angular range around the hard axis direction. It is found that the more homogenous microstructure of the amorphous samples results in a softer magnetic behavior and on a much better definition of the wires uniaxial anisotropy.

Magneto-optical study of the magnetization reversal process of Fe nanowires

We discuss results of magneto-optical Kerr effect (MOKE) measurementsperformed on a thin Fe film of 13 nm thickness, which has been patterned into aperiodic arrangement of nanowires by means of optical interference lithography.The resulting array of nanowires consist of stripes having a width of 150 nm anda periodicity of 300 nm. MOKE hysteresis loops are measured withinmagnetic fields which are aligned in different directions, both parallel andperpendicular with respect to the direction of the nanowires as well as forvarious angles in between. A particular arrangement of the longitudinalKerr effect measurement allows us to identify both the longitudinal andthe transverse component of the magnetization of Fe nanowires. Fromthis both the angle and the magnitude of the magnetization vector M⃗ arederived. For a non-parallel alignment of the nanowires with respect to the direction of theexternal magnetic field, the hysteresis loops consist of a plateau region with two coercivefields Hc1and Hc2,which is discussed as resulting from an anisotropic pinning behaviour ofmagnetic domains in directions along and perpendicular to the nanowires.

Spin Transport in a Ferromagnet/Semiconductor/Ferromagnet Structure: a Spin Transistor

The magnetoresistance (MR) and the magnetization reversal of a lateral spin-injection device based on a spin-polarized field effect transistor (spin FET) have been investigated. The device consists of a two-dimensional electron gas (2DEG) system in an InAs single quantum well (SQW) and two ferromagnetic (Ni_(80)Fe_(20)) contacts: an injector (source) and a detector (drain). Spin-polarized electrons are injected from the first contact and, after propagating through the InAs SQW are collected by the second contact. By engineering the shape of the permalloy contacts, we were able to observe distinct switching fields (Hc) from the injector and the collector by using scanning Kerr microscopy and MR measurements. Magneto-optic Kerr effect (MOKE) hysteresis loops demonstrate that there is a range of magnetic field (20~60 Oe), at room temperature, over which the magnetization in one contact is aligned antiparallel to that in the other. The MOKE results are consistent with the variation of the magnetoresistance in the spin-injection device.

Modeling of intermediate behavior in Co/Pt vertical magnetization multilayers

Magneto-optical Kerr effect (MOKE) hysteresis loops were acquired at various radial positions on a 25 mm diameter disk of a (0.3 nm Co/1.2 nm Pt) 15 multilayer. Two distinct types of behavior: unimodal, with two stable states, and bimodal, with four stable states, were observed. At the center of the disk, the loop exhibited bimodal features. At the outer edge, the reversal behavior was significantly altered, and the loop was unimodal. The radial variations in the magnetization are attributed to diffusion and interface effects arising from nonuniform heating in the film deposition process. We model these media to understand the physics of the magnetization processes. The bimodal loops are characterized by columns of CoPt 3 and of Pt threading the multilayers, strongly coupling them, and fully polarizing the Pt layers. Then, this thread is lost due to the different growth conditions at the edge of the disk, altering the polarization of the Pt layers. The overall behavior is modeled by assuming that in the unimodal region, there are a certain number of smooth layers starting at the substrate, and the remaining layers are rougher. In the intermediate region, vertical threads start forming at random points whose length increases as the medium becomes more bimodal. When the medium is fully bimodal, it consists solely of CoPt 3 and Pt threads that pierce the entire medium.

Domain nucleation processes in mesoscopic Ni80Fe20 wire junctions

The magnetization reversal process in permalloy (Ni80Fe20) wire junction structures has been investigated using magnetoresistance (MR) measurements and scanning Kerr microscopy. A combination of electron beam lithography and a lift-off process has been utilized to fabricate wires consisting of two 200 μm length regions with distinct widths w1 and w2 in the range 1–5 μm. Longitudinal MR measurements and magneto-optic Kerr effect hysteresis loops demonstrate that the magnetization reversal of the complete structure is predominantly determined by the wider region for fields applied parallel to the wire axis. Magnetic force microscopy and micromagnetic calculations show that several domain walls nucleate in the wider part and are trapped in the junction area. This implies that domain nucleation at the junction of the wire initiates magnetization reversal in the narrow half. As a consequence, the switching fields are found to be identical in both halves in this case. These results suggest the possibility of designing structures which can be used to “launch” reverse domains in narrow wires within a controlled field range.