Browsing by Author "Mewes, T."
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Item Angular Dependence and Phase Diagrams of Exchange-Coupled Epitaxial Ni81Fe19/Fe50Mn50(001) Bilayers(2002) Mewes, T.; Nembach, H.; Rickart, M.; Demokritov, S. O.; Fassbender, J.; Hillebrands, B.; University of Alabama TuscaloosaThe exchange bias effect, especially the angular dependence of the exchange bias field and the coercivity of the epitaxial Ni81Fe19 /Fe50Mn50 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 StonerWohlfarth theory, discrepancies arise. The angular dependence of the possible orientations of the magnetization are summarized in a phase diagram.Item Correlation Between Topography and Magnetic Surface Anisotropy in Epitaxial Fe Films on Vicinal-to-(001) Au Surfaces with Different Step Orientation(2004) Rickart, M.; Mewes, T.; Demokritov, S. O.; Hillebrands, B.; Scheib, M.; University of Alabama TuscaloosaFor an epitaxial, vicinal surface of a magnetic film, the twofold step-induced symmetry about the film normal in the atomic coordination implies a corresponding twofold magnetic surface anisotropy. We show, that for epitaxial Fe films grown on Au surfaces vicinal to the (001) plane, tilted about the in-plane (100)Au or the (110)Au axis, a strong correlation exists between the in-plane structural anisotropy in the height-height correlation function and the magnetic surface anisotropy. This correlation is clearly evident although both anisotropies are complicated functions of the vicinal angle, the tilt axis and the film thickness due to appearing reconstructions.Item Detection of Higher Order Modulation Harmonics in Magnetic Resonance Force Microscopy(2007) Mewes, T.; Mewes, C. K. A.; Nazaretski, E.; Kim, J.; Fong, K. C.; Obukhov, Y.; Pelekhov, D. V.; Wigen, P. E.; Hammel, P. C.; University of Alabama TuscaloosaMagnetic resonance force microscopy measurements of the electron spin resonance of a thin film of 2,2-diphenyl-1-picrylhydrazyl were performed using a low doped silicon cantilever with a high coercivity SmCo particle glued to its end. The low doping level enables amplitude modulation of the microwave field with only small spurious driving of the cantilever. Besides amplitude modulation we use frequency modulation of the microwave field at integer fractions of the cantilever resonance frequency leading to derivative signals up to the fourth derivative of the amplitude modulation response signal. The influence of the modulation depth on the line shape of the first derivative response is also presented.Item Enhanced Coercivity of Exchange-bias Fe/MnPd Bilayers(1999) Tang, Y. J.; Roos, B.; Mewes, T.; Demokritov, S. O.; Hillebrands, B.; Wang, Y. J.; University of Alabama TuscaloosaWe present detailed studies of the enhanced coercivity of exchange-bias bilayer Fe/MnPd, both experimentally and theoretically. We have demonstrated that the existence of large higher-order anisotropies due to exchange coupling between different Fe and MnPd layers can account for the large increase of coercivity in the Fe/MnPd system. The linear dependence of coercivity on inverse Fe thickness is well explained by a phenomenological model by introducing higher-order anisotropy terms into the total free energy of the system.Item Ferromagnetic Resonance Force Microscopy on a Thin Permalloy Film(2007) Nazaretski, E.; Martin, I.; Movshovich, R.; Pelekhov, D. V.; Hammel, P. C.; Zalalutdinov, M.; Baldwin, J. W.; Houston, B.; Mewes, T.; University of Alabama TuscaloosaFerromagnetic resonance force microscopy (FMRFM) offers a means of performing local ferromagnetic resonance. The authors have studied the evolution of the FMRFM force spectra in a continuous 50 nm thick permalloy film as a function of probe-film distance and performed numerical simulations of the intensity of the FMRFM probe-film interaction force, accounting for the presence of the localized strongly nonuniform magnetic field of the FMRFM probe magnet. Excellent agreement between the experimental data and the simulation results provides insight into the mechanism of FMR mode excitation in a FMRFM experiment.Item Ferromagnetic Resonance Force Microscopy Studies of Arrays of Micron Size Permalloy Dots(2007) Mewes, T.; Kim, J.; Pelekhov, D. V.; Kakazei, G. N.; Wigen, P. E.; Batra, S.; Hammel, P. C.; University of Alabama TuscaloosaFerromagnetic resonance force microscopy (FMRFM) offers a means of performing local ferromagnetic resonance. The authors have studied the evolution of the FMRFM force spectra in a continuous 50 nm thick permalloy film as a function of probe-film distance and performed numerical simulations of the intensity of the FMRFM probe-film interaction force, accounting for the presence of the localized strongly nonuniform magnetic field of the FMRFM probe magnet. Excellent agreement between the experimental data and the simulation results provides insight into the mechanism of FMR mode excitation in a FMRFM experiment.Item Frequency-selective Control of Ferromagnetic Resonance Linewidth in Magnetic Multiplayers(2012) Schafer, S.; Pachauri, N.; Mewes, C. K. A.; Mewes, T.; Kaiser, C.; Leng, Q.; Pakala, M.; University of Alabama TuscaloosaWe report on a frequency-specific linewidth broadening of the ferromagnetic resonance (FMR) mode of a NiFe free layer within a magnetic multilayer stack. The FMR studies reveal a significant broadening of the FMR linewidth of the free layer at frequencies where this resonance is degenerate with FMR modes stemming from other layers within the multilayer stack. By pinning part of the magnetic multilayer to an antiferromagnet, we tailor a ferromagnetic linewidth behavior that is anisotropic for a specific frequency.Item Influence of Capping Layers on CoFeB Anisotropy and Damping(2012) Natarajarathinam, A.; Tadisina, Z. R.; Mewes, T.; Watts, S.; Chen, E.; Gupta, S.; University of Alabama TuscaloosaMagnetic behavior of CoFeB at various thicknesses ranging from 2 nm to 8 nm capped with different materials, such as MgO, Ta, Ru, and V have been studied. The films were sputter-deposited and subsequently characterized by magnetometry and broadband ferromagnetic resonance (FMR). There are magnetically dead layers at the interface observed with Ru and Ta capping layers, while MgO and V have almost no effect on the magnetization of the CoFeB. As the ferromagnetic layer is made thinner, the effective magnetization decreases, indicating an interfacial perpendicular anisotropy. Particularly in the case of MgO, V/Ru, and V/Ta capping layers, interfacial perpendicular anisotropy is induced in CoFeB, and the Gilbert damping parameter is also reduced. The origin of this perpendicular magnetic anisotropy (PMA) is understood to be caused by the interface anisotropy between the free layer and the capping layer. The effect of post-deposition annealing and CoFeB thickness on the anisotropy and damping of V/Ta capped samples are reported. Doping CoFeB with vanadium (V) greatly reduced the 4pMs and 4pMeff values, resulting in an effective increase in the PMA.Item Inhomogeneous Perpendicular Magnetic Anisotropy as a Source of Higher-order Quasistatic and Dynamic Anisotropies(2018-01-26) Mohammadi, J. B.; Cole, K.; Mewes, T.; Mewes, C. K. A.; University of Alabama TuscaloosaWe investigate the influence of lateral variations of the second-order perpendicular anisotropy in thin films on the effective anisotropies required to represent this structure using a macrospin approximation. Second-order and fourth-order effective anisotropies are required for the macrospin approximation. In the case of quasistatic calculations, the fourth-order effective anisotropy is closely linked to deviations of the average magnetization angle from the field direction and lateral variations of the magnetization direction in the structure leading to dependence on the field strength and the lateral length scale of anisotropy variations of the effective anisotropies. We find that the field and lateral length-scale dependence of the effective anisotropies extracted from simulations of the magnetization dynamics are profoundly different from those of the quasistatic simulations. This is caused by resonance localization that depends on the orientation of the external magnetic field.Item Local Manipulation and Reversal of the Exchange Bias Field by Ion Irradiation in FeNi/FeMn Double layers(2001) Mougin, A.; Mewes, T.; Jung, M.; Engel, D.; Ehresmann, A.; Schmoranzer, H.; Fassbender, J.; Hillebrands, B.; University of Alabama TuscaloosaBoth the direction and the strength of the exchange bias field Heb of an FeNi/FeMn double layer are manipulated locally by He ion irradiation in an applied magnetic field. The magnitude of Heb is enhanced over its initial value in the low ion dose regime. Above a threshold irradiation dose a reduction and eventually a suppression of Heb is found. The direction of Heb is initialized by the field direction during irradiation. These observations are discussed considering the structural modifications caused by the ion bombardment. The data are well described by a model, which is based on the competition between the enhancement of the exchange bias field strength due to defect creation in the antiferromagnetic layer and the decrease induced by intermixing at the ferromagnet/antiferromagnet interface.Item Magnetization Relaxation and Structure of CoFeGe Alloys(2009) Lee, H.; Wang, Y.-H. A.; Mewes, C. K. A.; Butler, W. H.; Mewes, T.; Maat, S.; York, B.; Carey, M. J.; Childress, J. R.; University of Alabama TuscaloosaItem Measurements of the exchange stiffness of YIG films using broadband ferromagnetic resonance techniques(2015) Klinger, S.; Chumak, A. V.; Mewes, T.; Khodadadi, B.; Mewes, C.; Dubs, C.; Surzhenko, O.; Hillebrands, B.; Conca, A.; University of Alabama TuscaloosaItem Microstructural and Ferromagnetic Resonance Properties of Epitaxial Nickel Ferrite Films Grown by Chemical Vapor Deposition(2012) Li, N.; Schafer, S.; Datta, R.; Mewes, T.; Klein, M.; Gupta, A.; University of Alabama TuscaloosaMicrostructural and ferromagnetic resonance properties of epitaxial nickel ferrite (NiFe2O4) films grown by direct liquid injection chemical vapor deposition are reported. While high-quality epitaxial growth of NiFe2O4 films on (100)-oriented MgAl2O4 substrate is confirmed by high resolution transmission electron microscopy, bright field (diffraction contrast) TEM studies reveal the presence of dislocations and also dark diffused contrast areas, which originate from antiphase domains. Angle and frequency-dependent ferromagnetic resonance (FMR) experiments are conducted to determine the magnetic anisotropy and the magnetic relaxation. A low out-of-plane FMR linewidth of 160 Oe has been observed at a frequency of 10 GHz.Item Modification of the exchange bias effect by He ion irradiation(2000) Mougin, A.; Mewes, T.; Lopusnik, R.; Jung, M.; Engel, D.; Ehresmann, A.; Schmoranzer, H.; Fassbender, J.; Hillebrands, B.; University of Alabama TuscaloosaItem A Novel Technique to Detect Effects of Electromagnetic Interference by Electrostatic Discharge Simulator to Test Parameters of Tunneling Magnetoresistive Read Heads(2015) Khunkitti, P.; Kaewrawang, A.; Siritaratiwat, A.; Mewes, T.; Mewes, C. K. A.; Kruesubthaworn, A.; University of Alabama TuscaloosaElectrostatic discharge (ESD) has been a significant problem in the manufacturing processes of the magnetic recording head technologies for many years. Besides direct discharge damage, ESD can also generate electromagnetic interference (EMI) which could possibly cause failure in magnetic read heads. The aims of this work are to measure the EMI from ESD simulator based on the standard IEC 61000-4-2 and to investigate the effects of EMI on tunneling magnetoresistive (TMR) read heads. The discharge current and the EMI generated by ESD simulator are measured in the experiment. Also, the EMI is applied to the TMR read heads at various amplitudes and distances in order to evaluate the changes of read head parameters including the bit error rate, resistance, read back signal amplitude, and asymmetry parameter of the head. The results show that the discharge current waveform is consistent with the IEC standard current waveform. In addition, it is found that the EMI is insufficient to cause a permanent change of the read head parameters at distances above 2 cm indicating the minimal impact on the head performance. Further experiments are proposed to carry out more detailed studies of the EMI effects on head parameters in order to improve the methods to prevent the degradation of parameters which can be a latent failure in the magnetic read heads.Item Origin of Fourfold Anisotropy in Square Lattices of Circular Ferromagnetic DotsKakazei, G. N.; Pogorelov, Yu. G.; Costa, M. D.; Mewes, T.; Wigen, P. E.; Hammel, P. C.; Golub, V. O.; Okuno, T.; Novosad, V.; University of Alabama TuscaloosaItem Oscillatory Exchange Bias Effect in FeNi/Cu/FeMn and FeNi/Cr/FeMn Trilayer Systems(2000) Mewes, T.; Roos, B. F. P.; Demokritov, S. O.; Hillebrands, B.; University of Alabama TuscaloosaThe first experimental observation of a spacer-thickness dependent oscillatory exchange bias effect in ferromagnet~FM!/spacer/antiferromagnet trilayers is reported. The period of the oscillatory exchange bias field is found to be half of the period of the oscillatory interlayer coupling in the corresponding FM/spacer/FM systems with the same spacer, indicating that the observed effect is caused by an analogous coupling mechanism, being, however, sensitive to the absolute value of the coupling strength and not on its sign.Item Perpendicular Magnetic Tunnel Junctions Using Co-based Multilayers(2010) Tadisina, Z. R.; Natarajarathinam, A.; Clark, B. D.; Highsmith, A. L.; Mewes, T.; Gupta, S.; Chen, E.; Wang, S.; University of Alabama TuscaloosaCoFeB/MgO/CoFeB magnetic tunnel junctions MTJs with perpendicular magnetic anisotropy PMA free and reference layers composed of Co/M where M=Pd or Ni multilayers have been optimized for high PMA and high tunneling magnetoresistance TMR. The effects of Co thickness, Pd thickness, and the number of Co/Pd bilayers on the anisotropy and coercivity of the Co/Pd n multilayer films have been studied for both free and reference layers. The damping parameter of CoFeB capped multilayers was determined using broadband ferromagnetic resonance. The transport properties of the patterned MTJ stacks were measured from 10 to 400 K. A maximum TMR of 10% at 10 K 5%–10% at 300 K was obtained for these perpendicular MTJs, regardless of whether or not they were magnetically annealed for MgO–CoFeB crystallization. This indicates that the fcc-bcc-fcc transitions from the fcc multilayers to the bcc CoFeB/MgO/CoFeB do not promote the “giant MgO TMR effect” caused by symmetry filtering.Item Phase Diagrams and Energy Barriers of Exchange-biased Bilayers with Additional Anisotropies in the Ferromagnet(2003) Mewes, T.; Nembach, H.; Fassbender, J.; Hillebrands, B.; Kim, Joo-Von; Stamps, R. L.; University of Alabama TuscaloosaA general discussion of magnetization in exchange-biased bilayers with higher-order anisotropy contributions in an applied magnetic field is given. Allowed magnetic configurations for a ferromagnet in an applied magnetic field are shown in magnetic phase diagrams that illustrate competing effects of unidirectional, uniaxial, and fourfold anisotropies. The role and asymmetry of energy barriers during magnetization reversal are discussed in detail. A direct comparison with recent experimental results in epitaxial NiFe/FeMn bilayers is provided, and the influence of thermal activation on the magnetization reversal process is discussed.Item Separation of the First- and Second-order Contributions in Magnetooptic Kerr Effect Magnetometry of Epitaxial FeMn/NiFe Bilayers(2004) Mewes, T.; Nembach, H.; Rickart, M.; Hillebrands, B.; University of Alabama TuscaloosaThe 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.