![]() PMA in magnetic thin film and Multilayers
Magnetic Nanostructure by tunable Electrodeposition
Magnetic thin film and nanowires are promising candidates for future spintronics and data storaga applications. Fabrication of such materials with a larger area and low-cost can be acheived by tunable electrodepostion. Correlation of different growth mechanism with structural, magnetic and morphological proerties of the materials are important for potential applications. In recent years, ferromagnetic nanowires got considerable attention due to its large aspect ratio and uniaxial anisotropy. Magnetic nanowires have potential applications in ultra-high density data storag. Template assisted electrodeposition is widely used to fabricate such nanowires due to its raliability, reproducibility and low cost.
Multiferroic
and Magneto-electric Materials
Carbon Based Nanosystem:
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No single, current imaging technique can satisfy all of these demands. In order to get a more complete picture of the magnetic structure, therefore, multiple complementary imaging methods should be used. Coherent Scattering: We are interested in studying the evolution of magnetic domains and effect of pinning centers in thin film magnetic systems as a function of magnetic field, temperature, and doping to identify the role the disorder in formation and stability of the magnetic domainsusing resonant magnetic x-ray coherent scattering. This technique allows us to use element specificity as well as depth resolution to understand the magnetic domains and their dynamics. In particular, we are interested in Fe/Gd and FeGdTb multilayer exhibiting ordered stripes due to perpendicular magnetic anisotropy (PMA).This system was chosen in order to have maximum magnetic contrast, negligible charge scattering and minor multiple scattering. We propose to study the effects of field pinning of the local magnetic structure of these systems through their magnetization hysteresis loops and their temperature driven dynamics.Comparison of the magnetic speckles (in momentum space) provides information on correlation between the magnetic structures (in real space). This metrology approach has been recently applied to study Return and Complimentary Point Memory in other magnetic system. If the speckles are dynamic, which indicates that domains are fluctuating rather than frozen, we can use speckle intensity fluctuations in X-ray Photon Correlation Spectroscopy mode to investigate the effects of temperature and magnetic field on these domain wall fluctuation. We then plan to study the Barkhausen noise spectrum following the applied field, and to extend this study to Tb doping in these systems, which would induce strong PMA. Tuning the x-ray energy to the sharp magnetic resonances can provide information about the magnetic contributions to the domains. Fe/Gd multilayers and higher periodicity of Fe/Gd layer] were grown at room temperature by sputtering Fe and Gd onto a 100nm thick Si3N4 membrane. Standard magnetic hysteresis measurements [magneto-optic Kerr effect (MOKE) and vibrating sample magnetometer (VSM)] show perpendicular anisotropy. From these measurements the anti-ferromagnetic coupling between Fe and Gd is evident.
Resonant Magnetic Speckle: X-ray scattering is non-invasive as far as domain mapping is concerned. Element sensitivity and depth resolution are the key advantages of x-rays to study magnetic materials.To study the dynamics two most tuning parameter, temperature and magnetic field can easily be added to existing set-up with nominal modification. By contrast, MFM is an invasive technique in which one may influence the magnetic domains of the sample by the stray field of the tip or vice versa.Imaging in the presence of moderate magnetic field (above 200 Gauss) is difficult as it changes the magnetization of the tip. Moreover, one observes an after-effect in response to the tip magnetization or the external field.Using resonant magnetic x-ray scattering we can probe fluctuations in the domains and their dynamics. Spin-orbit splitting at the L and M absorption edges affords direct access to the domain magnetization vector. Different type of domains may result depending on the magnetic history of the sample. Application of magnetic field in the perpendicular direction results in randomly oriented "maze" domains, whereas in-plane field creates stripes preferentially oriented along the field direction. Increasing magnetic field results in change of the stripe structure to bubbles and then saturated at higher field value. At saturation, film transfer to single domain state. MFM shows only bright or dark contrast depending on whether one measure at the positive or negative saturation field following the hysteresis loop.Formation, shape and stability of perpendicularly magnetized domains can be explained by the competition of the short range exchange interaction and the long range magnetostatic interaction. At higher magnetic fields a lattice of bubble domains has lower energy than stripe domains. Thus, a critical external field exists, where stripe domains become instable and collapse into bubble domains. It will be interesting to see the evolution of the domain pattern as a function of temperature. As the saturation magnetization goes down one can predict that at higher temperature the magnetization goes in-plane which would result in weak magnetic contrast.
Use of resonant coherent x-rays will enable us to gain information about the local disorder on the nano-scale. By analyzing correlations between speckles as a function of q, we can understand spatial distribution of correlations responsible for memory effects as a function of magnetic field and temperature, and identify the role the disorder plays in formation and stability of magnetic domains. Measurement of the speckle as a function of time will enable investigation of the effects of temperature and magnetic-field on domain wall fluctuations. The timescale of the fluctuations generally increases with the wave vector q (directly related to spatial frequencies or length scales for which fluctuations are observed). Analysis involving higher-order intensity correlation functions can yield information on the intermittent dynamics of magnetic switching (as opposed to continuous uniform domain wall propagation), while two-time correlation functions can be used to investigate non-equilibrium (aging) behavior of the system induced by a rapid change in tuning parameter, either the magnetic field or temperature. In future, using x-ray lensless imagingcoupled with scanning `methods to study extended sampleswe can access not only the order-parameter (out-of-plane magnetization), but also the phase, which may provide crucial "hidden" information that is not accessible with STXM, PEEM, MFM or other techniques. Reference: Jyoti Mohanty, Ashish Tripathi, Erik Shipton, Keith Chan, SangSoo Kim, Ian McNulty, Eric Fullerton and Oleg Shpyrko, APS March Meeting, Pittsburg, March 16-20, 2009. View Abstract A. Tripathi, J. Mohanty, O. G. Shpyrko, E. Shipton, E. E. Fullerton, S. S. Kim and I. McNulty, Dichroic coherent diffractive imaging, Proc Natl Acad Sci USA, 108(33)}, 13393-13398 (2011). Fe/Gd Related: Stability
of
stripe magnetic domain in magnetic multilayers: Prediction and
observation [1] Talk and Thesis: Thomas
Eimüller thesis [8],
Jorge Miguel
thesis [9],
Stan Konings
thesis [10],
J. B.
Goedkoop Talk [11],
Jan
Lüning Talk [12], ![]() Starting from the discovery of the electric charge, it has been used as a source of information transfer in electronics devices. However, the spin degree of freedom of the electron can be used as a medium for information exchange and transfer. The concept of utilizing spin of the electron along with its charge nature for more functionalityare known as spin-based electronics or so called, spintronics. Certainly, one can ask the question what is the advantage of using spin of the electron in comparison to its charge. To answer this question we have to take into account that spin is non-volatile and again spin has large life time compare to electric charge, therefore we can expect o have devices with higher performances and on the other hand the benefit of the non-volatile data storage technology.The generic spintronics scheme on a prototypical device, the Datta-Das spin field-effect transistor (SFET; Datta and Das, 1990). The scheme shows the structure of the usual FET, with a drain, a source, a narrow channel, and a gate for controlling the current. The gate either allows the current to flow (ON) or does not (OFF). The spin transistor is similar in that the result is also a control of the charge current through the narrow channel. The difference, however, is in the physical realization of the current control. In the Datta-Das SFET the source and the drain are ferromagnets acting as the injector and detector of the electron spin. The drain injects electrons with spins parallel to the transport direction. The electrons are transported ballistically through the channel. When they arrive at the drain, their spin is detected. In a simplified picture, the electron can enter the drain (ON) if its spin points in the same direction as the spin of the drain. Otherwise it is scattered away (OFF). The role of the gate is to generate an effective magnetic field, arising from the spin-orbit coupling in the substrate material, from the confinement geometry of the transport channel, and the electrostatic potential of the gate. This effective magnetic field causes the electron spins to precess. By modifying the voltage, one can cause the precession to lead to either parallel or antiparallel (or anything between) electron spin at the drain, effectively controlling the current. MnAs on GaAs: In the frame work of ferromagnetic semiconductor and spintronics I did my Ph.D thesis work on the MBE grown III-V ferromagnetic semiconductor system i.e. MnAs on GaAs substrates. MnAs exhibits a first-order phase transition from the hexagonal, ferromagnetic alpha-phase to the orthorhombic, paramagnetic beta-phase at around 40 degree centigrade. In epitaxial MnAs films on GaAs, it is known that the ferromagnetic and the paramagnetic phases coexist over a wide temperature range of about 30 degree centrigade below the phase transition temperature. In this temperature range, MnAs films on GaAs (001) and (311)A substrates show a regular array of ferromagnetic stripes due to the involved strain. The width of the ferromagnetic stripes can be tuned by varying the temperature, whereas the periodicity of the stripe pattern is found to be a linear function of film thickness. The in part complex micromagnetic domain structure strongly depends on the width and the distance of the ferromagnetic stripes, as it directly affects the shape anisotropy and magnetic coupling, respectively. The films exhibit a strong uniaxial magnetic anisotropy, where the magnetic easy axis is the a-axis (along MnAs [11¯20]) and the magnetic hard axis is the c-axis (along MnAs [0001]) and both axes lie in the film plane. More details can be found in the thesis. Related Publications: Jyoti Mohanty et.al, Appl. Phys. Lett. 82, 2308-2310 (2003), Appl. Phys. A 77, 739-742 (2003), Appl. Phys. Lett. 83, 2829-2831 (2003), Appl. Phys. Lett. 83, 2850-2852 (2003). Appl. Phys. Lett. 84, 1132 (2004). Physica E 24, 115 (2004). J. Appl. Phys. 96, 5056 (2004). Appl. Phys. A 81, 1359 (2005). J. Appl. Phys. 98, 013907 (2005). J. Vac. Sci. Technol. B 23, 1759 (2005).J. Appl. Phys. 98, 063909 (2005). Phys. Rev. B. 73, 104441 (2006). J. Magn. Magn. Mater. 305, 457 (2006).Appl. Phys. A 84, 231 (2006).Further Reading: [1]
I. Zutic et al. Rev.
Mod. Phys., 76, 323, 2004 |