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Magnetic storage means storage of data using magnetised medium and is widespreadover the world today, especially in hard disk drives. Using this kind of storagerequires knowledge about these materials. A way to study thin magnetic materials isto use MOKE(magneto optical Kerr effect). A Moke-system is a setup to measure thinmagnetic films by shooting a laser and analyze the reflected beam.The purpose of this report is to document and if possible improve a MOKE-system,named HOMER. This includes temperature regulation, filters, amplifiers, opticalchopper, Helmholtz coils and a laser. HOMER was documented and some changeswere made. The PID-parameters were set successfully. A low pass filter wasremoved, which decreased the noise. Using an optical chopper and lock in amplifierhowever did not decrease the noise. A labview program was written to demagnetizethe samples in a certain time which seemed to work properly. The hall probe in thesystem was successfully calibrated.

Time resolved scanning Kerr microscopy (TRSKM) has been used to study a number of different magnetic systems. Firstly, partially built hard disk writer structures, with a multilayered yoke formed from 4 repeats of a NiFe(~1 nm)/CoFe(50 nm) bilayer, and with three coil windings underneath, were studied by TRSKM with unipolar driving pulses. Dynamic images of the in-plane magnetization suggest an underlying closure domain equilibrium state. This state is found to be modified by application of a bias magnetic field and also during pulse cycling, leading to different magnetization rotation and relaxation behaviour within the tip region. Studies of a further three yokes with the same stack structure, but with only one coil winding at different positions beneath the yoke, yielded dynamic images of “flux beaming” in a channel parallel to the driving field. The magnetic contrast was strongest when the active coil was located near the centre of the yoke, while relaxation after removal of the excitation was most complete when the active coil was located near the confluence region. These results confirm the need for a multi-turn coil to ensure effective flux propagation along the entire length of the yoke. Furthermore, a structure with a NiFe/CoFe/Ru/NiFe/CoFe synthetic antiferromagnetic (SAF) yoke was studied as a bipolar current pulse with 1MHz repetition rate was delivered to the coil. The component of magnetization parallel to the symmetry axis of the yoke was compared at the pole and above a coil winding in the centre of the yoke. The two responses are in phase as the pulse rises, but the pole piece lags the yoke as the pulse falls. The Kerr signal is smaller within the yoke than within the confluence region during pulse cycling. This suggests funneling of flux into the confluence region. Dynamic images acquired at different time delays showed that the relaxation is faster in the centre of the yoke than in the confluence region, perhaps due to the different magnetic anisotropy in these regions. Although the SAF yoke is designed to support a single domain to aid flux conduction, no obvious flux beaming was observed, suggesting the presence of a more complicated domain structure. The SAF yoke writer hence provides relatively poor flux conduction but good control of rise time compared to single layer and multi-layered yokes studied previously. Secondly, vortex dynamics within arrays of square ferromagnetic nano-elements have been studied using TRSKM with coherent microwave excitation. It is shown that TRSKM can be used to detect vortex gyration in square nanomagnets with a lateral size (250nm) that is smaller than the diameter (300nm) of the focused laser beam. In an array with large element separation and negligible dipolar interaction, TRSKM images acquired at a fixed point in the microwave cycle reveal differences in the phase of the dynamic response of individual nanomagnets. While some variation in phase can be attributed to dispersion in the size and shape of elements, the circulation and polarization of the vortex are also shown to influence the phase. In an array with element separation smaller than the optical spot size, strong magneto optical response was observed within small clusters of elements. Micromagnetic simulations performed for 2 x 2 arrays of elements show that a certain combination of circulation and polarization values is required to generate the observed magneto-optical contrast. Thirdly, polar TRSKM has been used to directly observe magnetostatically coupled transverse domain walls (TDWs) in a pair of closely spaced, curved nanowires (NWs). Kerr images of the precessional response revealed a minimum in the Kerr signal due to the TDW in the region of closest NW separation. When the TDWs were ejected from the NW pair, the minimum in the Kerr signal was no longer observed. By imaging this transition, the static decoupling field was estimated to lie between 38 and 48 Oe, in good agreement with a simple micromagnetic model. This work provides a novel technique by which DC and microwave assisted decoupling fields of TDWs may be explored in NW pairs of different width, separation, and curvature. Fourth, time resolved magneto-optical Kerr effect and phase modulated X-ray ferromagnetic resonance measurements have been performed on a CoO/Py bilayer for different temperatures, RF frequency, and CoO thickness. Kerr hysteresis loops did not show any evidence of exchange bias for temperatures between 200K and 330K for any thickness of CoO, but the coercivity was found to increase with increasing CoO thickness and decreasing temperature. Magneto-optical FMR and XFMR data showed some asymmetry with respect to the sign of the bias field, but the amplitude of the signals decreased rapidly with decreasing temperature. The results are consistent with the appearance of frustrated antiferromagnetic order within the CoO during field cooling.

This thesis presents a study of magnetic anisotropies of transition metal based thin films investigated by magnetooptic Kerr effect (MOKE) and x-ray radiation techniques. Thin Co films of different thickness deposited on glass are investigated to study the uniaxial magnetic anisotropy (UMA) of these films. The direction of the UMA is determined from the azimuthal dependence of the magnetic remanence and differs with increasing thickness of the Co film investigated by x-ray reflectivity (XRR). Strain in the substrate and possible textures in the film structure can be excluded as the origin of the magnetic behavior as studied by grazing incidence wide angle x-ray scattering (GIWAXS). Furthermore, a new combination of different vectorial magnetometry techniques using MOKE is developed (vector MOKE). The processing of the experimental data contains the separation of linear and quadratic parts of the magnetization curves and determination of all three components of the magnetization vector in units of Kerr rotation. The complete reversal process can be reconstructed and interpreted by monodomain states and domain splitting. The measurement procedure and the processing of the data is demonstrated for ultra-thin epitaxial Fe films on MgO(001), which are structurally characerized by XRR and (grazing incidence) x-ray diffraction (GI(XRD)). Magnetic properties of thin Fe films (of different thickness) and A2-type ordered Co50Fe50 films (prepared with different annealing temperatures) on MgO(001) are studied by this new combined vector MOKE technique. Here, a two-domain switching process can be distinguished from a four-domain switching process generated by the incoherent rotation between magnetization saturation and magnetic remanence. Furthermore, UMA constants and domain wall pinning energies are determined using the Stoner-Wohlfarth model. For the Fe films the domain wall pinning energies obtained from these analyses decrease with increasing lateral crystallite sizes determined by GIXRD. For the Co50Fe50 films the domain wall pinning energies decrease with the annealing temperature due to fewer defects in the film. The magnetic structure of a thin B2 ordered Co50Fe50 film reveal two in-plane magnetic easy axes of different strengths which are not orthogonal. This atypical magnetic structure can be explained by a cubic magnetic anisotropy (CMA) induced by the crystalline film structure superimposed by an additional UMA which is not parallel to one of the magnetic easy axes of the CMA.

Magnetic properties are becoming increasingly important, especially with the advent of new technologies such as data storage, spintronics, and magnetic random access memory (MRAM). These technologies all benefit from the ability to characterize and optimize magnetic materials, which can be accomplished through the implementation of the magneto-optical Kerr effect (MOKE). This probing technique is described in detail along with the results of a self-built, longitudinal MOKE (L-MOKE) experiment that is used to determine the Kerr angle and ellipticity for the Heusler compound Fe2MnSn. The results from L-MOKE are combined with raw X-ray Diffraction (XRD) data in order to develop a complete picture of the material by connecting the Kerr response with the crystal structure. Trends in the crystal structure and magnetic properties of Fe2MnSn are developed and analyzed in order to isolate crystallographic directions that are primarily responsible for unique magnetic properties such as perpendicular magnetic anisotropy. Complementary techniques are also introduced to further the capabilities from a single geometry MOKE experiment to a compound experiment using all three geometries of MOKE with the ability to probe micro-structures, determine magneto-optical parameters, and create magnetic domain images.

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The magnetization damping properties of square permalloy elements were characterized. These 20 nm-thick permalloy squares were deposited by the electron beam evaporation. Time resolved magneto-optic Kerr effect microscopy (TR-MOKE) was used to measure the magnetization evolution in the sample. By curve fitting in Matlab, I obtained the value of damping constant that is consistent with the reference paper. A Landau confi guration in the square permalloy sample leads to the different trends of the damping constant with external bias field. The damping constant in the bottom domain is found to decrease with increasing bias field while the damping constant in the top domain has been saturated into the minimum value. The decreasing tendency of magnetization precession frequencies is consistent with the Kittel equation modi fied with an anisotropic energy term. Additionally, FePt thin films and patterned CoFeB disks were investigated but neither yielded conclusive dynamic data. / Graduate / 0611 / 0607 / 0752 / leizhang.summer@gmail.com

In this project, we have explored the possibility of growing high quality Heusler alloy thin films from multiphase targets. Bulk targets were found to be partially formed, i.e. not of homogeneous L21 structure, through x-ray diffraction measurements. Pulsed laser deposition is a technique that can provide a congruent transfer of material from the target to the substrate, even in some cases where the target is not of a single crystalline phase. It was the objective of this work to determine whether L21 structured thin films of Co2MnAsxGe(1-x) could be grown from multiphase targets. Measurements have been carried out to study the magnetic and structural properties of the Heusler alloys Co2MnAsxGe(1-x). The optimization parameters that were investigated were substrate type, growth temperature, laser parameters, film thickness, and other common deposition parameters. Temperature-dependent magneto-optic Kerr effect (MOKE) techniques were used to study the qualitative magnetic properties. Alternating current (AC) susceptibility (using a MOKE technique) measurements were made as a function of temperature to view the second-order transition and obtain the Curie temperatures. Frequency dependent AC susceptibility was measured to determine the frequency dependence of the AC susceptibility for Co2MnAsxGe(1-x).

Title: Optical and magneto-optical spectroscopy of materials with antiferro- magnetic interaction Author: Bc. Katarína Križanová Department: Institute of Physics of Charles University Supervisor: RNDr. Jakub Zázvorka, Ph.D., Institute of Physics of Charles University Abstract: One of the goals of spintronic research is the efficient external con- trol of magnetic moment. Non-collinear antiferromagnets in the antiperovskite structure, such as Mn3NiN, show a piezomagnetic effect that can be used to utilize these materials in applications. In the strain free state, the material ex- hibit zero net magnetic moment. Using strain induced by a lattice constant mismatch between the thin layer and a substrate on which the thin film is applied on a non-zero net magnetic moment can be registered. Magneto-optical Kerr effect spectroscopy is used to investigate the non-collinear magnetic thin films. The effect of two substrate layers with resulting opposite sign of strain influencing the magnetic ordering of the antiperovskite material is studied with respect to sample temperature. Results show comparable spectral dependence with opposite sign of the Kerr effect caused by the opposite direc- tion of net magnetization moments. Ellipsometry measurements depending on sample orientation are performed to study the material...

In our earlier research, we found the MOKE signal incident from 45 degree would cause perpendicular signal to couple with the longitudinal one. To distinguish the signal from one to another, we arranged 45 degree and 0 degree optical setup for the measurement of longitudinal and perpendicular respectively. However, hysteresis loops are only observed in the longitudinal direction. To measure smaller signal in our experiment, we reduced the noise level by one order and thus improved the signal to noise ratio. It¡¦s ten times better then previous result. Under smaller signal or worse S/N ratio condition, we still measured the MOKE signal. In order to quantitatively analyze the current-induced field, we made a metal coil and try to measure the bias produced by the field. And we injected the reverse current-induced field, , try to counteract the bias of hysteresis loop which is induced by sample current. Besides, we also dosed oxygen on the sample and measured the MOKE signal. There was no change comparing with which is measured in gas-free condition. But we found the coercivity became larger after annealing to 300K, and the current-induced bias in oxygen become smaller.