Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J31: Focus Session: Magnetic Multilayers |
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Sponsoring Units: DMP GMAG Chair: Kathryn Krycka, National Institute of Standards and Technology Room: 335 |
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J31.00001: 400-Fold Reduction in Saturation Field by Stress Relief in Multilayers William Egelhoff, John Bonevich, Carlos Beauchamp, Gery Stafford, John Unguris, Philip Pong, Robert Mcmichael A common problem in soft magnetic thin films is increased saturation field due to stress buildup with increasing thickness. We have found a solution to the problem using multilayers of a magnetic thin film and a film that is either not lattice matched or has a different crystal structure. Reductions in the saturation field as large as 400 fold are found. The ultrasoft Ni$_{77}$Fe$_{14}$Cu$_{5}$Mo$_{4}$ alloy can have saturation fields as small as 0.005 mT (0.05 Oe) for 10 nm thick films. However, for films 400 nm thick (which are needed for some applications) the saturation field is typically 20 mT. Splitting this magnetic thin film up into segments 100 nm thick separated by a 5 nm Ag film reduces the saturation field to 0.05 mT. Alternatively, using a 2 nm CoFe film yields a saturation field of 0.1 mT. A tensile stresses of 7.35 x 10$^{9}$ dynes/cm$^{2}$ was measured in the 400 nm film and 3.7 x 10$^{7}$ dynes/cm$^{2 }$for the multilayer with Ag. The highly-stressed Ni$_{77}$Fe$_{14}$Cu$_{5}$Mo$_{4}$ develops a magnetostriction coefficient of $\sim $5 ppm, although in the unstressed state its magnetostriction coefficient is near zero. In conclusion, we have found a solution to the stress-induced large saturation fields in an otherwise soft magnetic film. The results should be important for ultra-low magnetic-field tunnel-junction sensors and magnetic flux concentrators. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J31.00002: Structure and magnetic properties of magnetron-sputtered FePt/Au superlattice films Yongsheng Yu, Haibo Li, XingZhong Li, Lanping Yue, Weili Li, Mei Liu, Yumei Zhang, Weidong Fei, David J. Sellmyer FePt/Au multilayer films were prepared with sputtering and the effects of Au thickness and annealing temperature on the structure and magnetic properties were investigated. Superlattice structure was induced by thicker Au layer. The interatomic spacing d(220) in the fcc FePt lattice increases with increasing Au thickness, indicating increasing strain energy in fcc FePt lattice. After annealing at 300$^{\circ}$C, FePt films with Au layer of 3.5nm became ordered and the multilayer structure were retained. The strain energy in fcc FePt lattice appears to be responsible for lowering the ordering temperature of the FePt phase. For films annealed at higher temperatures, thicker Au layer restrained the ordering of FePt phase, which led to a decrease of coercivities. -- This research is supported by DOE and NCMN. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J31.00003: Controlling Interlayer exchange coupling in ultra narrow Fe/Pt multilayered nanowire: an ab initio study Puspamitra Panigrahi, Ranjit Pati Interest in the study of magnetic/non-magnetic multilayered structures took a giant leap since Gr\"unberg and his group established that the value of interlayer exchange coupling (IEC) depends upon the non-magnetic spacer width. The recent increase in demand for device miniaturization compelled researchers to look for novel nanoscale multilayered structures. Towards this effort, we have studied IEC in one dimensional Fe/Pt multilayered nanowires using first principles density functional approach. Our result shows the exchange coupling energy (J) to switch sign as the width of the non-magnetic Pt spacer varies. The competition among short and long range direct exchange and the super exchange is recognized to play an important role for the non-monotonous sign in IEC depending upon the width of the Platinum spacer layer. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J31.00004: Design of Co/Pd multilayer system with antiferromagnetic-to-ferromagnetic phase transition Invited Speaker: Among the known magnetic material systems there are only very few examples of materials that undergo a temperature dependent antiferromagnetic-to-ferromagnetic phase transition, and of these only the chemically ordered alloy FeRh exhibits this transition near room temperature [1, 2]. Here we present a perpendicular anisotropy multilayer structure that mimics FeRh. The basic idea is to use two stacks of Co/Pd multilayers with large perpendicular magnetic anisotropy and high Curie temperature, T$_{C}$, separated by a layer providing antiferromagnetic coupling, and a CoNi/Pd multilayer with perpendicular anisotropy with a lower T$_{C}$, interlayer, in the range of the desired AF-FM transition temperature, T$_{AF-FM}$. At room temperature this system behaves as two antiferromagnetically coupled layers with a low perpendicular remanent magnetic moment. As the temperature is raised to approach T$_{C}$, $_{interlayer}$ the magnetization of the interlayer is gradually reduced to zero, and consequently its coupling strength is reduced. Eventually, the effective coupling between the two high-K$_{U}$, high-T$_{C}$ layers becomes dominated by their dipolar fields, resulting in a parallel alignment of their moments and a net remanent magnetic moment equal to the sum of the moments of the two high-T$_{C}$ layers [2]. \\[4pt] [1] J. S. Kouvel and C. C. Hartelius, J. Appl. Phys. \textbf{33} (1962) p1343 \\[0pt] [2] J.-U. Thiele, E. E. Fullerton, S. Maat, Appl. Phys. Lett. \textbf{82} (2003) p2859 \\[0pt] [3] J.-U. Thiele. T. Hauet. O. Hellwig, Appl. Phys. Lett. \textbf{92} (2008) 242502. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J31.00005: Graded Magnetic Anisotropy in Co/Pd Multilayers B. J. Kirby, J. E. Davies, S. M. Watson, R. D. Shull, J. A. Borchers, G. T. Zimanyi, Kai Liu As the magnetic recording industry looks beyond perpendicular recording, multilayered exchange coupled media have demonstrated potential for increased storage density. Recent work has shown further enhancements when the anisotropy is gradually increased from a soft top to a hard bottom region.[1] However, creating graded anisotropy structures is difficult, and convincingly demonstrating such a gradient is also challenging. Since the coercivity of Co decreases with increasing thickness, we attempted to create graded anisotropy structures by sputtering [Co/Pd] superlattices with progressively varying Co layer thicknesses. We probed the depth dependent anisotropy of the samples using polarized neutron reflectometry (PNR), a technique sensitive to the depth-dependent magnetic composition of thin films. The sample magnetization vector M was bent away from the out-of-plane easy axis direction by an applied magnetic field H, and the depth profile of the in-plane component of M(H) was measured. Our results clearly demonstrate that samples with graded Co thickness also exhibit graded anisotropy. Further, comparisons of samples with different levels of gradient discretization shed light on the nature of the interlayer exchange coupling in a graded anisotropy system. [1] D. Suess, Appl. Phys. Lett. 89, 189901 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J31.00006: Coercivity enhancement in (Co/CoO)$_{n}$ superlattices Srinivas Polisetty, Christian Binek The temperature dependence of the coercivity is studied in (Co/CoO)$_{n}$ periodic multilayer thin film superstructures below and above the exchange bias blocking temperature. The ferromagnetic Co thin films are grown with the help of MBE at a base pressure of 10E-10 m.bar whereas antiferromagnetic CoO thin films are grown from in-situ oxidized Co. The thicknesses of these films are monitored by reflection high energy electron diffraction (RHEED). A mean-field theory$^{1}$ is outlined which provides an analytic and intuitive expression for the enhancement of the coercivity of the ferromagnet which experiences the exchange coupling with a neighboring antiferromagnet. An experimental approach is developed allowing to determine the interface susceptibility of an individual antiferromagnetic pinning layer by systematic change in the thickness of the antiferromagnet thin films in various sets of superlattice samples measured at different temperatures, respectively. The experiment enables us to separate out the intrinsic coercivity from the contribution induced by exchange coupling at the interface. It is the goal of our study to evidence or disprove if it is simply this susceptibility of the reversible interface magnetization creating the spin drag effect and by that the coercivity enhancement. Financial support by NSF through CAREER DMR-0547887, NRI and Nebraska MRSEC. $^{1}$G. Scholten, K. D. Usadel, and U. Nowak, Phys. Rev B. \textbf{71}, 064413 (2005). [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J31.00007: Antiferromagnetic coupling in Fe/Si/Fe structures with Co ``dusting'' layers Rashid Gareev, Matthias Kiessling, Matthias Buchmeier, Georg Woltersdorf, Christian Back Artificial antiferromagnetic (AF) tunnelling Si-based structures are attracting a special interest due to extremely strong AF coupling, which exceeds 5mJ/m$^{2}$, low resistance-area product and resonant-type tunnelling magnetoresistance (TMR) [1]. A promising way to regulate the spin polarization in TMR structures is to insert Co ``dusting'' layers at interfaces [2]. We present AF coupling in Fe/Co/Si/Co/Fe epitaxial structures with sub-monolayer --thick Co ``dusting'' layers at interfaces and different thicknesses of the Si spacer. We determined the strength of AF coupling from spin-wave frequencies and angular dependence of the resonance field of the ferromagnetic resonance, as well as MOKE hysteresis. We found the AF coupling near 0.1mJ/m$^{2}$ which decays with the spacer thickness and detectable for 2nm-thick Si spacers. The presented results can open an avenue for magnetotransport studies in AF-coupled structures using interface engineering. [1]. R.R. Gareev \textit{et al}, JMMM \textbf{240}, 235 (2002), R.R. Gareev \textit{et al}, APL \textbf{88}, 172105 (2006). [2]. Y. Wang, X.F. Han, and X.-G. Zhang, APL \textbf{93}, 172501 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J31.00008: Probing Magnetic Configurations in Buried Cobalt/Copper Multilayered Nanowires Invited Speaker: Multilayered magnetic nanowires have been a model system for heterostructured junctions that exhibit a host of fascinating perpendicular spin transport phenomena, such as giant and tunneling magnetoresistance (MR), and spin-transfer torque effects. Due to the extremely small physical dimensions the magnetic components in these nanowires or junctions often exhibit complex magnetization reversal behaviors, which are difficult to probe by magnetic imaging since the entities are buried deep inside a matrix. Conventional hysteresis loop measurement alone cannot reliably distinguish the reversal mechanisms either. In this work we have captured magnetic and MR ``fingerprints'' of Co nanodiscs in Co/Cu multilayered nanowires as they undergo a single domain to vortex state transition, using a first-order reversal curve (FORC) method [1]. The nanowires have been electrochemically deposited into nanoporous polycarbonate membranes. In 50 nm diameter [Co(5nm)/Cu(8nm)]$_{400}$ nanowires, a 10{\%} MR effect is observed at 300 K. In 200 nm diameter nanowires, the magnetic configurations can be tuned by adjusting the Co nanodisc aspect ratio. Nanowires with thinnest Co exhibit single domain behavior. Those with thicker Co exhibit vortex states, where the irreversible nucleation and annihilation of the vortices are manifested as butterfly-like features in the FORC distributions, similar to those observed in arrays of Fe nanodots [2]. They also show a superposition of giant and anisotropic magnetoresistance, which corresponds to the specific magnetic configurations of the Co nanodiscs. \\[4pt] [1] J. E. Davies, \textit{et al}, Phys. Rev. B \textbf{70,} 224434 (2004); Appl. Phys. Lett. \textbf{86,} 262503 (2005); Phys. Rev. B \textbf{77}, 014421 (2008).\\[0pt] [2] R. K. Dumas, \textit{et al, }Phys. Rev. B \textbf{75}, 134405 (2007); Appl. Phys. Lett. \textbf{91}, 202501 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J31.00009: Controlled formation of double-vortex configurations in a shape-engineered F/N/F trilayer stack studied by quantitative off-axis electron holography Lei Huang, Marvin Schofield, Yimei Zhu Vortex domain state, widely existing in submicron size patterned magnetic structures, can be very useful in high density magnetic data storage devices. In this report, we designed a shape engineered ferromagnetic-nonmagnetic-ferromangetic (F/N/F) trilayer stack that would generate four different vortex-based remnant states by applying defined sequences of in-plane magnetic field. These four states are distinguished by different relative chirality orientations of two vortices stabilized in the ferromagnetic layers. Experimentally, we lithographically patterned 400nm sized prototype device, and studied in-situ the switching behavior by off-axis electron holography. Using the integrated approach including single element hysteresis loop, induction contour mapping and quantitative electron phase shift measurement, we revealed the underlying reversal mechanism as separate vortex formation and annihilation in two magnetic layers. We also confirmed the field-control feasibility of such structure by distinguishing unambiguously the presence of all four states after each field recipe was applied. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J31.00010: Magnetic properties of ion-etched magnetic nanodot arrays Ioan Tudosa, Keith Chan, Erik Shipton, Eric Fullerton One pathway for increasing the density in magnetic recording media is to have bits stored as single patterned magnetic islands. While promising, this method has been hindered by the failure to reduce the island to island variation of magnetic switching properties. We have prepared [Co/Pd], [Co/Pd]Co/Ni and CoO/[Co/Pd]/CoO multilayers to tune the anisotropy. The films were subsequently patterned into nanodot arrays by ion etching using self organized di-block copolymers as the etch mask. The resulting patterned islands have a 32-nm diameter and 65-nm pitch. We characterize the time and temperature dependence of the magnetic properties to extract the coercivity, switching field distribution and thermal stability parameters. The distribution of the switching fields, in the range of 10-12{\%} of the coercive field, was separated into intrinsic and dipolar contributions and find small dipolar contribution to the switching field distributions. The room temperature stability parameters are greater than 100 k$_{B}$T for all the samples. However, the magnetic switching volume extracted from the thermal stability is significantly less than the physical volume of the samples suggesting incoherent reversal. We will discuss the physical origin of the incoherent reversal. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J31.00011: Magnetic multilayers and Nanomagnetic Patterns P. Panyajirawut, M.S. Rzchowski We have grown Ni/Co magnetic multilayers by sputtering, finding that the multilayers have in-plane uniaxial magnetic anisotropy. This is induced during growth by the sputtering geometry, and by the interaction between layers. We pattern the multilayers into sub-micron dots and networks using the nanosphere lithography technique, forming well-ordered two dimension arrays of magnetic nanoparticles. We use an oxygen plasma etch to adjust the size of the polystyrene spheres after spin coating. Using self-assembled close-packed monolayer of polystyrene spheres as deposition mask, the magnetic material is deposited through the interstitial areas to form networks. We also form isolated nanoparticles using the polystyrene spheres as a etch mask. We discuss the magnetic behavior of patterned mulitlayers. [Preview Abstract] |
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