| 1. |
- Bonanni, Valentina, et al.
(författare)
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First order reversal curve analysis of graded anisotropy FePtCu films
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 97:20
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Tidskriftsartikel (refereegranskat)abstract
- The reversal mechanisms of graded anisotropy FePtCu films have been investigated by alternating gradient magnetometer (AGM) and magneto-optical Kerr effect (MOKE) measurements with first-order reversal curve (FORC) techniques. The AGM-FORC analysis, which clearly shows the presence of soft and hard components, is unable to resolve how these phases are distributed throughout the film thickness. MOKE-FORC measurements, which preferentially probe the surface of the film, reveal that the soft components are indeed located toward the top surface. Combining AGM-FORC with the inherent surface sensitivity of MOKE-FORC analysis allows for a comprehensive analysis of heterogeneous systems such as graded materials.
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| 2. |
- Dumas, Randy K., et al.
(författare)
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Graded Anisotropy FePtCu Films
- 2011
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Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 47:6, s. 1580-1586
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Tidskriftsartikel (refereegranskat)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 (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. 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 (L1(0)) 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 L1(0) ordering process is found to depend on the film thickness.
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| 3. |
- Dumas, Randy K., et al.
(författare)
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Probing vertically graded anisotropy in FePtCu films
- 2011
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Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 84, s. 054434-
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Tidskriftsartikel (refereegranskat)abstract
- Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (TA). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300°C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low TA. A fundamentally different mechanism is found after annealing at 400°C, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500°C, a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated TA.
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| 4. |
- Fang, Yeyu, 1984, et al.
(författare)
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A Nonvolatile Spintronic Memory Element with a Continuum of Resistance States
- 2013
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 23:15, s. 1919-1922
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Tidskriftsartikel (refereegranskat)abstract
- A continuum of stable remanent resistance states is reported in perpendicularly magnetized pseudo spin valves with a graded anisotropy free layer. The resistance states can be systematically set by an externally applied magnetic field. The gradual reversal of the free layer with applied field and the field-independent fixed layer leads to a range of stable and reproducible remanent resistance values, as determined by the giant magnetoresistance of the device. An analysis of first-order reversal curves combined with magnetic force microscopy shows that the origin of the effect is the field-dependent population of up and down domains in the free layer.
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| 5. |
- Fang, Yeyu, et al.
(författare)
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An In Situ anneal study of graded anisotropy FePtCu films
- 2011
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Ingår i: IEEE Magnetics Letters. - 1949-307X .- 1949-3088. ; 2
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Tidskriftsartikel (refereegranskat)abstract
- We provide a detailed study of how the anisotropy (Ku) gradient in a compositionally graded FePtCu film gradually develops as a function of the postannealing temperature (TA). By utilizing the in situ annealing and magnetic characterization capabilities of a physical property measurement system, the evolution of the induced Ku gradient is elucidated. For low TA, the sample primarily remains in the low-Ku A1 phase. At intermediate TA, the gradual development of an A1 to L10 anisotropy gradient occurs. As TA is further increased, a well-developed L10 gradient is realized. Finally, annealing temperatures greater than 475 ◦C reduce the gradient until the film is effectively uniform for TA ≥ 525 ◦C and higher, presumably due to interdiffusion of the Cu through the film thickness. The resulting coercivity shows a nonmonotonic dependence on TA with an initial steep increase as the L10 fraction of the sample increases, a local minimum at TA = 525 ◦C where the gradient vanishes, and a final increase as the uniform L10 film orders completely.
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| 6. |
- Fang, Yeyu, et al.
(författare)
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FORC studies of exchange biased NiFe in L1_0(111) FePt-based spin valve
- 2010
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Ingår i: Journal of Physics Conference Series. - BRISTOL : IOP Publishing. - 1742-6588 .- 1742-6596. ; 200:7, s. 072002-
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Tidskriftsartikel (refereegranskat)abstract
- We use First-Order Reversal Curves (FORC) to study the switching distribution and exchange bias in L10 (111) FePt-based spin valves with a layer structure of Ta (6)/Pt (3)/L10 (111) Fe53Pt47 (20)/Co50Fe50 (1.5)/Cu (4.5)/Co50Fe50 (2)/Ni80Fe20 (3) (in nm). We find that the switching of the free layer magnetization is strongly influence by the magnetic state of the FePt/CoFe fixed layer, as evidenced by tunable coercivity and exchange bias fields.
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| 7. |
- Fang, Yeyu, 1984
(författare)
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Magnetic Thin Films with Graded and Tilted Anisotropy for Spintronic Devices
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis magnetic thin films with graded or tilted anisotropy are intensively studied for potential applications in spintronic devices. A continuum of stable remanent resistance states is realized in Co/Pd multilayers based on a perpendicularly magnetized pseudo spin valve (PSV). The Co/Pd multilayers have been deposited using magnetron sputtering. By varying the Co thickness in each repeating unit, a graded anisotropy through the multilayer is achieved. We then incorporate this graded Co/Pd multilayer into a PSV as the free layer. The remanent resistance states are systematically adjustable depending on the reversal field. The gradual reversal of the free layer with applied field and the field-independent fixed layer leads to a range of stable and reproducible remanent resistance values, as determined by the giant magnetoresistance of the device. An analysis of first-order reversal curves (FORCs) combined with magnetic force microscopy (MFM) shows that the origin of the effect is the field-dependent population of up and down domains in the free layer. Thus, these structures have great potential for applications such as field-tunable resistance trimming devices, memristive devices, or magnetic analog memories with a continuous number of states per memory cell, thereby allowing much higher information storage. We have also successfully realized FePtCu thin films with graded anisotropy. During deposition a compositional gradient is achieved by continuously varying the Cu content from the top to bottom. After annealing at a proper temperature, the top Cu-poor regions remain in the as-deposited soft A1 phase, while the bottom Cu-rich regions transform into hard L10 phase. Hence the gradient anisotropy is established through the film thickness. The critical role of the annealing temperatures (TA) on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both a reduced coercivity and moderate thermal stability. The reversal mechanism of graded anisotropy has been investigated by alternating gradient magnetometer (AGM) and magneto optical Kerr effect (MOKE) measurements in combination with the FORC technique. The AGM-FORC analysis clearly shows the soft and hard phases. MOKE-FORC measurement, which preferentially probes the surface of the film, reveals that the soft components are indeed located toward the top surface. We provide a detailed study of the how the anisotropy gradient in a compositional graded FePtCu film gradually develops as a function of the TA. By utilizing the in-situ annealing and magnetic characterization capability of a physical property measurement system (PPMS), the evolution of the induced anisotropy gradient is elucidated. These results are important and useful for the solving the magnetic ‘‘trilemma’’ of magnetic recording technology. The Co/Pd-NiFe exchange spring system is investigated. Due to the competition between the strong perpendicular anisotropy of the Co/Pd multilayer and the in-plane shape anisotropy of the NiFe, the magnetization in the NiFe tilts out of the film plane. Experimental data from conventional magnetometry, and MFM, along with one-dimensional simulations, show that the titling angle in the NiFe layer is highly tunable from 0 to 60° by simply changing the thickness of NiFe. We employed the Co/Pd-NiFe exchange spring system with appropriate NiFe thickness as the polarizer in nano-contact spin torque oscillators (STOs) which show vortex oscillations at low fields.
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| 8. |
- Fang, Yeyu, 1984-
(författare)
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Titled and graded anisotropy FePt and FePtCu thin films for the application of hard disk drive and spin torque oscillators
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The FePt and FePtCu thin films with graded anisotropy and titled anisotropy are utilized to solve both the magnetic recording ‘‘trilemma’’ of the hard disk drives (HDDs) and the large field operation problem of spin torque oscillators (STOs). We have successfully realized the FePtCu thin films with graded anisotropy. During deposition a compositional gradient is achieved by continuously varying the Cu content from the top to bottom. After annealing at proper temperatures, the top Cu-poor regions remain at soft A1 phase, while the bottom Cu-rich regions transform into hard L1 0phase. Hence the gradient anisotropy is established through the film thickness. The critical role of the annealing temperatures (TA) on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both the reduced coercivity and moderate thermal stability. In conjunction of the fabrication, the subsequent analysis of the graded material is not trivial. The reversal mechanism of graded anisotropy have been investigated by alternation gradient magnetometer (AGM) and magneto optical Kerr effect (MOKE) measurements with first order reversal curves (FORC) technique. The AGM-FORC analysis, which clearly shows the soft and hard phases, is not able to resolve how these phases are distributed through the film thickness. MOKE-FORC measurement which preferentially probes the surface of the film, reveal that the soft components are indeed located toward the top surface. The TA plays a critical role in the induced anisotropy gradient. We provide a detailed study of the how the anisotropy gradient in a compositional graded FePtCu film gradually develops as a function of the TA. By utilizing the in-situ annealing and magnetic characterization capability of a physical property measurement system, the evolution of the induced anisotropy gradient is elucidated. These results are important and useful for the application of HDDs. In order to achieve the zero-field operation for STOs, we have successfully fabricated pseudo spinvalves based on L1 0(111) textured FePt or FePtCu. We demonstrate magnetoresistance(MR) in excess of 4% in FePt/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10(111)-oriented FePt fixed layers with a 36 ° out-of-plane tilted magnetization. The high MR is achieved by increasing the spin polarization at the Cu interfaces, using thin CoFe, and optimizing the FePt growth and Cu interface quality using Ta and Ta/Pt underlayers.We observe well-separated switching of the FePt/CoFe fixed layer and the CoFe/NiFe free layer, suggesting that CoFe is rigidly exchange coupled to FePt and NiFe in the respective layers. Futuremore, through optimization of the Cu spacer thickness, we demonstrate MR up to 5% in FePtCu/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111) FePtCu fixed layers with a tilted magnetization. We find an optimum spacer thickness of about 2.4 nm which correlates with a clear onset of strong interlayer exchange coupling below 2.4 nm and spin-independent current shunting in the spacer above 2.4 nm. These results are an important milestone for future applications of tilted spin polarizers in STOs.
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| 9. |
- Fang, Yeyu, 1984, et al.
(författare)
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Utility of reactively sputtered CuNx films in spintronics devices
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:7, s. 073912-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied nitrified copper (CuNx) thin films grown by reactive sputtering in the context of spintronic devices. The Ar-to-N-2 flow ratio enables tunability of the electrical resistivity and surface roughness of the CuNx films, with the former increasing to nearly 20 times that of Cu, and the latter reduced to the atomic scale. Incorporating this into a Ta/CuNx/Ta seed stack for spin valves improves the current-in-plane (CIP) magnetoresistance; maximum magnetoresistance results with CuNx seed layer and Cu interlayer. Finally, finite element modeling results are presented that suggest the use of CuNx in nanocontact spin torque oscillators can enhance current densities by limiting the current spread through the device. This may positively impact threshold currents, power requirements, and device reliability.
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| 10. |
- Mohseni, S. M., et al.
(författare)
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Temperature-dependent interlayer coupling in Ni/Co perpendicular pseudo-spin-valve structures
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:17, s. 174432-
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Tidskriftsartikel (refereegranskat)abstract
- The temperature-dependent coupling mechanisms in perpendicular pseudo-spin valves based on the following structure, [Ni/Co](5)/Cu(t(Cu))/[Ni/Co](2), are investigated. Despite a thick (t(Cu) >= 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.
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