June W. Lau

National Institute of Standards and Technology, Maryland, United States

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Publications (20)46.06 Total impact

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    ABSTRACT: The A1- L10 phase transformation has been investigated in (001) FeCuPt thin films prepared by atomic-scale multilayer sputtering and rapid thermal annealing (RTA). Traditional x-ray diffraction is not always applicable in generating a true order parameter, due to non-ideal crystallinity of the A1 phase. Using the first-order reversal curve (FORC) method, the A1 and L10 phases are deconvoluted into two distinct features in the FORC distribution, whose relative intensities change with the RTA temperature. The L10 ordering takes place via a nucleation-and-growth mode. A magnetization-based phase fraction is extracted, providing a quantitative measure of the L10 phase homogeneity.
    APL Materials. 08/2014; 2(8).
  • L. Krayer, J. W. Lau, B. J. Kirby
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    ABSTRACT: A detailed understanding of the interfacial properties of thin films used in magnetic media is critical for the aggressive component scaling required for continued improvement in storage density. In particular, it is important to understand how common etching and milling processes affect the interfacial magnetism. We have used polarized neutron reflectometry and transmission electron microscopy to characterize the structural and magnetic properties of an ion beam etched interface of a CoFeB film. We found that the etching process results in a sharp magnetic interface buried under a nanometer scale layer of non-magnetic, compositionally distinct material.
    Journal of Applied Physics 04/2014; 115(17). · 2.21 Impact Factor
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    ABSTRACT: Magnetization reversal mechanisms and depth-dependent magnetic profile have been investigated in Co/Pd thin films magnetron-sputtered under continuously varying pressure with opposite deposition orders. For samples grown under increasing pressure, magnetization reversal is dominated by domain nucleation, propagation, and annihilation; an anisotropy gradient is effectively established, along with a pronounced depth-dependent magnetization profile. However, in films grown under decreasing pressure, disorders propagate vertically from the bottom high-pressure region into the top low-pressure region, impeding domain wall motion and forcing magnetization reversal via rotation; depth-dependent magnetization varies in an inverted order, but the spread is much suppressed.
    Applied Physics Letters 01/2014; 104(15):152401-152401-5. · 3.52 Impact Factor
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    ABSTRACT: Brillouin light scattering was applied to measure the evolution of spin waves in exchange-coupled [Co/Pd]-NiFe films with out-of-plane/in-plane magnetic anisotropy. Tunable tilting of the magnetization has important effects on the spin wave frequency gap: a substantial increase is observed with decreasing the soft NiFe thickness, while seemingly insignificant tilting angles cause a strong reduction with respect to a film with only the hard [Co/Pd] component. The spin wave frequency is reproduced also in samples, with increased thickness of the upper Pd layer, where a sudden modification in the tilting at the hard/soft interface is caused by the weakening of interlayer exchange.
    Physical Review B 04/2013; 87(14):144426. · 3.66 Impact Factor
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    ABSTRACT: We compute the switching field in a disk-shaped polycrystalline exchange-coupled bit (similar material to those found in bit-patterned media (BPM)) with micromagnetics, by varying physical parameters of a test grain. It was found that the size and the anisotropy of the test grain have substantial effects on the switching field, while its location has only minor influence. Scaling of the bit and the test grain dimensions result in similar switching properties. Switching field was reduced when the number of the low anisotropy test grains increased. Additionally, it was established that the intergranular exchange coupling needs to be at least 10% of the intragrain exchange for the bit to behave as one exchange-coupled entity. This investigation provides some insights for optimizing the material microstructure for the BPM application.
    Journal of Applied Physics 01/2013; 114(12):123909-123909-5. · 2.21 Impact Factor
  • June W. Lau, Xiaoyong Liu
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    ABSTRACT: The characterization of switching field and its angular dependence has been tremendously useful in understanding the switching mechanisms in magnetic thin-films and patterned structures. However, the study of the distribution in switching fields (SFD) in arrays and its angular dependence is less common. In this work, we investigate this dependency in arrays of (Co/Pd)n multilayer nanostructures. Results from arrays with different element sizes and periodicities (pitches) are presented, and we found that, like the switching field, the SFD varies with applied field angle in a Stoner-Wohlfarth-like fashion. Furthermore, when the SFD is represented as a dependent variable of the switching field, we consistently found a linear relationship between the two, and that the slope depends on both array element size and pitch. In general, the SFD in arrays with the largest structures and the smallest pitch tends to have the strongest dependence on the switching field. For arrays with nanostructures of a fixed size, however, we found that SFD values are virtually identical, regardless of pitch, if the reversal field is applied near 45° with respect to the surface normal. That the minimum SFD depends only on the size of the elements and not the pitch has significant implications for the practical design of densely-packed magnetic nanostructure arrays.
    Journal of Applied Physics 11/2012; 112(9). · 2.21 Impact Factor
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    ABSTRACT: In (Co/Pd)n multilayer dots with submicrometer diameters, it is understood that reversal is initiated by the nucleation of a small reversed region, followed by uninhibited expansion of the nucleus. The fact that the reversed domain expands with relative ease has been attributed to the fact that the field required to free a domain wall is much smaller than the field required to create the nucleus. In this work, we measured the angular dependence of the coercivity in individual 500-nm and 1-μm (Co/Pd)n multilayer dots and show that the shape of this curve deviates from the Stoner-Wohlfarth-like form observed by others. We attribute this deviation to (1) the domain-wall depinning field exceeding the nucleation field when the reversal field has a large in-plane component and (2) exchange-spring-like behavior at the dots’ edges due to ion-mill damage.
    Physical review. B, Condensed matter 12/2011; 84(21). · 3.66 Impact Factor
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    ABSTRACT: Nanostructured Mn 67 Ga 33 films exhibiting high room temperature coercivity (H C  = 20.5 kOe) have been prepared by sputtering onto thermally oxidized Si substrates. Both the morphology and the coercivity of the films can be tuned by varying the growth parameters. The low deposition rate film, sputtered at a reduced power and working pressure, demonstrates a discontinuous island-like growth and the highest H C . The large H C is linked to the presence of the high anisotropy DO 22 Mn 3 Ga phase and the single domain character of the exchange isolated, dipolar interacting, single crystal islands.
    Journal of Applied Physics 12/2011; · 2.21 Impact Factor
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    ABSTRACT: The temperature-dependent coupling mechanisms in perpendicular pseudo-spin valves based on the following structure, [Ni/Co]5/Cu(tCu)/[Ni/Co]2, are investigated. Despite a thick (tCu≥3 nm) Cu spacer, room-temperature measurements reveal complete coupling of the [Ni/Co]5 and [Ni/Co]2 multilayers. This coupling can be attributed to strong long range magnetostatic stray fields that penetrate the spacer layer. This results in magnetic domain imprinting and vertically correlated domains throughout the reversal process. Surprisingly, when the temperature is reduced, a complete decoupling is observed. This somewhat counterintuitive result can be explained by a large difference in the [Ni/Co]5 and [Ni/Co]2 multilayer coercivities at reduced temperatures, which then impedes domain imprinting and promotes decoupling. Finally, the decoupling temperature is found to increase with spacer thickness.
    Physical review. B, Condensed matter 11/2011; 84(17). · 3.66 Impact Factor
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    June W Lau, Justin M Shaw
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    ABSTRACT: Magnetic nanostructures are an integral part to many state-of-the-art and emerging technologies. However, the complete path from parts (the nanostructures) to the manufacturing of the end products is not always obvious to students of magnetism. The paper follows this path of the magnetic nanostructure, and explains some of the steps along the way: What are the technologies that employ magnetic nanostructures? How are these nanostructures made? What is the physics behind the functional parts? How are the magnetic properties measured? Finally, we present, in our view, a list of challenges hindering progress in these technologies.
    Journal of Physics D Applied Physics 07/2011; 44(30):303001. · 2.53 Impact Factor
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    ABSTRACT: Magnetization reversal and the effect of patterning have been investigated in full-film and dot arrays of Co/Pd multilayers, using the first-order reversal curve and scanning electron microscopy with polarization analysis techniques. The effect of patterning is most pronounced in low sputtering pressure films, where the size of contiguous domains is larger than the dot size. Upon patterning, each dot must have its own domain nucleation site and domain propagation is limited within the dot. In graded anisotropy samples, the magnetically soft layer facilitates the magnetization reversal, once the reverse domains have nucleated.
    Journal of Applied Physics 05/2011; · 2.21 Impact Factor
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    ABSTRACT: We report deposition-order-dependent, and depth-dependent, magnetization reversal in Co/Pd with graded anisotropy, which are technologically important as they address both writability and thermal stability challenges. Multilayers of [Co(0.4nm)/Pd(0.6nm)]60 have been deposited by sputtering, where the Ar pressure has been varied from 5 to 12 and 20 mtorr in type A samples and in the reverse order in type B samples. An extensive set of structural and magnetization reversal studies with depth-resolution has been performed using XRD, cross-sectional TEM, magnetometry, PNR and XMCD. In type A samples, due to the larger grain size, lower interfacial roughness and less disorder in the magnetically softer layer, magnetization reversal proceeds via domain nucleation, propagation, and annihilation. Type B samples show a more localized reversal. Layers grown at higher pressure contain more disorder and rougher interfaces, which is carried into the magnetically softer layers deposited on top, thus impeding domain movement.
    03/2011;
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    ABSTRACT: The fabrication and subsequent analysis of continuously graded anisotropy films are discussed. During deposition, a compositional gradient is first achieved by varying the Cu concentration from Cu-rich (Fe Pt ) Cu to Cu-free Fe Pt . The anisotropy gradient is then realized after thermal post-annealing, and by utilizing the strong composition dependence of the low-anisotropy (A1) to high-anisotropy ordering temperature. The magnetic properties are investigated by surface sensitive magneto-optical Kerr effect and alternating gradient magnetometer (AGM) measurements. AGM first-order reversal curve (FORC) measurements are employed in order to provide a detailed analysis of the reversal mechanisms, and therefore the induced anisotropy gradient. At low annealing temperatures, the FORC measurements clearly indicate the highly coupled reversal of soft and hard phases. However, significant interdiffusion results in virtually uniform films at elevated annealing temperatures. Additionally, the A1 to ordering process is found to depend on the film thickness. Index Terms—FePtCu thin films, first-order reversal curve (FORC) measurements, graded anisotropy films, magnetic reversal mechanism.
    IEEE Transactions on Magnetics 01/2011; 47(6):1580-1586. · 1.42 Impact Factor
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    ABSTRACT: We showed that a chain of nanomagnetic tunnel junctions (MTJs) devices can be electrically addressed individually, in situ, in a transmission electron microscope, such that transport properties can be in principle, quantitatively correlated with each device’s defects and microstructure. A unique energy barrier was obtained for each device measured. Additionally, in situ tunneling magnetoresistance (TMR) measurements were obtained for a subset of devices. We found that TMR values for our nano-MTJs were generally smaller than TMR in the unpatterned film.
    Applied Physics Letters 06/2010; 96(26):262508-262508-3. · 3.52 Impact Factor
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    ABSTRACT: The application of spin transfer torque switching for the realization of the logic input in a reconfigurable spin logic based on two magnetic tunnel junctions is discussed. Two electrodes with orthogonal fixed magnetization directions are used to replace the magnetic fields generated by a current though leads close to the magnetic tunnel junctions which have been used so far. Initial results of micromagnetic simulations suggest the feasibility of this approach.
    Applied Physics Letters 04/2010; 96(14):142508-142508-3. · 3.52 Impact Factor
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    ABSTRACT: There is a continuing need for more sensitive magnetic sensors. We report here an approach that leads to MTJ structures at the wafer level that have TMR values in excess of 100% and saturation field (Bsat) values below 1 Oe. The ratio appears to be the largest ever reported. The approach we have used is to fabricate and anneal MTJs of a rather conventional type, SiO2/5Ta/30Ru/5Ta/2CoFe/15IrMn/2CoFe/0.9Ru/ 3CoFeB/2MgO/ 5CoFeB/5Ta/10Ru (thicknesses in nm), then etch down into the free layer of the MTJ, and deposit a thick (100 nm) and very soft magnetic film to lower Bsat. We have found that incorporating the soft film in the initial structure lowers the TMR significantly upon annealing. Maintaining a large TMR depends on depositing the soft film after annealing. A static field of 1.8 Oe is applied perpendicular to the sweep field (i.e. in the hard axis) to suppress the hysteresis of 1.3 Oe. This technique is described in Ref. 1.^ The sweep field is in the easy axis of the free layer and the side field is in the hard axis of the free layer. The low-field loop is linear, non-hysteretic, and extrapolates to saturation at 0.8 Oe. The measured TMR is 118%. 1) X. Liu, C. Ren, and G. Xiao 92, 4722 (2002).
    03/2010;
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    ABSTRACT: Magnetic tunneling junctions (MTJs), the key components for many spin-based technologies, are commonly found on the order of 100nm in lateral dimensions due to the continuing trend of device miniaturization. Pinpointing the variations in transport properties due to local structural defects in these nano-MTJs is extremely difficult to accomplish using traditional experimental techniques. Here, we explore directly, the local structural-transport correlations on a series of 100nm by 100nm cross-sectional MgO MTJs by performing simultaneous structural characterization, magnetic imaging and in-situ point contact tunneling experiments inside a transmission electron microscope. By changing the magnetic field at the specimen region, the two ferromagnetic electrodes in the MTJs can be controllably toggled between parallel and antiparallel (low and high resistance) states. I-V curves of the two resistance states are measured and quantitatively compared with the Simmons model. We found different barrier heights among nominally identical MTJs patterned from a single continuous film. The correlation of such barrier variations with the unique microstructure of individual devices will also be presented.
    03/2010;
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    ABSTRACT: There is a continuing need for greater sensitivity in magnetic tunnel junction (MTJ) sensors. We have found a new approach to achieving large tunneling magnetoresistance (TMR) with a very soft free layer. The high TMR is achieved by conventional means of annealing a bottom pinned MTJ that has Ta and Ru capping layers. The soft free layer is achieved by etching almost to the MgO tunnel barrier and depositing a thick soft magnetic film. The results are far superior to annealing the MTJ with the thick soft layer already deposited.
    Journal of Applied Physics 01/2010; 107. · 2.21 Impact Factor
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    ABSTRACT: Intrinsic magnetic defects in perpendicularly magnetized nanostructures reduce the predictability of device and developing recording technologies. In addition to a distribution of local anisotropy fields, we show that such defects also exhibit variations in local anisotropy axes. The magnetic defects are identified by the application of in-plane and out-of-plane magnetic fields and magnetic force microscopy imaging. Those defects that control magnetization reversal in arrays of patterned Co/Pd multilayers are highly dependent on applied field orientation. The symmetry of the defects with respect to the applied field direction indicates that the anisotropy consists of a canted axis, deviating from the surface normal. Micromagnetic simulations confirm that variations in anisotropy axis can cause a significant change in reversal field depending on the location and orientation of the defects, consistent with experimental results.
    Physical review. B, Condensed matter 01/2010; 82(14). · 3.66 Impact Factor
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    ABSTRACT: We demonstrate using both structural and magnetic analyses that an antiferromagnetic MnRu intermediate layer can simultaneously increase the anisotropy constant and reduce the intergranular exchange coupling of a CoPtCr-SiO2 recording layer. The anisotropy constant of CoPtCr-SiO2 is increased by an exchange coupling with the adjacent antiferromagnetic MnRu intermediate layer. Additionally, the MnRu layer leads to better SiO2 segregation within the recording layer which then weakens intergrain exchange coupling. While the enhanced grain isolation leads to a reduction in the activation volume, a potential loss in thermal stability is avoided due to the enhanced anisotropy.
    Physical review. B, Condensed matter 01/2010; 82(1). · 3.66 Impact Factor