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ADVANCED STORAGE TECHNOLOGY CONSORTIUM RESEARCH PROPOSAL

TIME-RESOLVED, TEMPERATURE-DEPENDENT REMAGNETIZATION RATES IN

THIN FILM MEDIA SUBJECT TO ULTRAFAST DEMAGNETIZATION PULSES

[01] Front matter

a. Date: April 28, 2011

b. Abstract: We propose to experimentally measure the longitudinal relaxation rate for recording media as a function of sample temperature for different media. Landau-Lifshitz-Bloch (LLB) theory makes definite predictions for this situation. Understanding and measuring the longitudinal relaxation rate is essential for the prediction of how quickly the spin system can be affected by the heating of the lattice/electronic system. LLB theory predicts that the longitudinal relaxation times diverge near Tc. Such a divergence would adversely affect the effectiveness of heat-assisted magnetic recording (HAMR) when recording temperatures approach Tc. We have already measured relaxation rates in alloys on short timescales, with a time resolution that is sufficiently high (! 10 fs) to be able to probe how the response of individual elements differs due to the timescale of the exchange interaction. The proposed measurements will extend our current capabilities to higher magnetic fields, longer timescales, and different media.

c. Proponent(s) and affiliation(s):

Dr. Tom Silva Project Leader, Magnetodynamics National Institute of Standards and Technology Boulder, Colorado USA Phone: 303-497-7826; Email: silva@boulder.nist.gov Professor Henry Kapteyn, ProfessorMargaret Murnane JILA, University of Colorado at Boulder Boulder, CO 80309-0440 Phone: (303) 210-0396, E-mail: murnane@jila.colorado.edu

d. Designated contact person: Professor Margaret Murnane, JILA, University of

Colorado at Boulder, Boulder, CO 80309-0440, Phone: (303) 210-0396, E-mail: murnane@jila.colorado.edu

[02] Subject of research and relevance to issue(s) to be solved.

a. Complete description of the research matter and its connection with ASTC stated goals. We propose to develop a comprehensive understanding of the physics, issues, and limitations of the magnetic write process at or close to the Curie point for heat-assisted magnetic recording (HAMR). We will experimentally test predictions of Landau-Lifshitz-Bloch (LLB) for the remagnetization rate in thin film media subject to ultrafast demagnetization pulses from a femtosecond laser. The LLB equations predict a divergence of the longitudinal relaxation time near the Curie temperature [1]. (This divergence is related to the “critical slowing” process that is well understood in the context of phase transition theory.) The longitudinal relaxation time determines how quickly the spin bath comes into equilibrium with the electron and lattice systems.

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In ultrafast demagnetization experiments, a material is strongly demagnetized by exciting it with an ultrafast optical pulse [2 - 4]. The resultant demagnetization and recovery can then be probed with elemental-specificity using ultrafast x-ray pulses that span the absorption edges of the material. The demagnetization process itself requires only 100-200 fs in most transition metal alloys, as shown in Fig. 1. Once demagnetized, the spin system is severely out of equilibrium with the phonon system, and longitudinal relaxation is required as the spin and phonon/electron systems come back into equilibrium. We propose to characterize these longer timescale recovery dynamics using ultrafast x-rays. Initial measurements in our laboratory indicate that the longitudinal relaxation times (magnetization rebound times) can be much longer than 100 ps when the sample temperature is close to the Curie temperature. In these experimental data, the time required for the magnetization to restore to its thermal equilibrium value ("rebound time") for Permalloy is short, and estimated to be tens of picoseconds (see Fig. 1 (top)). In contrast, the rebound time is estimated to be greater than 100 ps for Permalloy-Cu alloy (see Fig. 1 (lower)). It is even possible that the relaxation times might exceed 1 ns, although this depends strongly on the exact nature of the damping in the material in question. We already have initial measurements of the dependence of the magnetization rebound timescale on the laser pump power. As shown in Fig. 2, the stronger the pump power, the greater the demagnetization (!m), and the slower the rebound process. This suggests that the closer the spin temperature approaches the Curie temperature, the slower the longitudinal relaxation rate, as expected from critical slowing near a 2nd order

Figure 1. Ultrafast, femtosecond timescale, laser demagnetization of Fe (red dots) and Ni (blue dots) in Permalloy (Tc = 850 K) (top) and Permalloy-Cu (Tc = 406 K) (lower). The element-specific timescales correspond to the short initial drop in magnetization. In our proposed work, the timescale of the remagnetization or “rebound” would be measured as a function of sample temperature, close to the Curie temperature.

Exchange timescale

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!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"!Figure 2. Dependence of magnetization rebound timescale on laser pump power for Permalloy. The stronger the pump power, the greater the demagnetization (!m), and the slower the rebound process. This suggests that the closer the spin temperature approaches the Curie temperature, the slower the longitudinal relaxation rate, as expected from critical slowing near a 2nd order phase transformation.

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phase transformation. It is these long-timescale remagnetization that will be the focus of our proposed ASTC project. However, although the ultrafast demagnetization dynamics on sub-100 fs timescales are not yet relevant to recording media, it is worth noting that the probe techniques we have developed can measure the ultimate limits of switching speed in materials relevant to next-generation storage technologies - that may come online in 10 years. In very recent work shown in Fig. 3 (just submitted for publication), we significantly increased the time resolution in our experiments, to under 10 fs. This allowed us to see for the first time that the demagnetization of the different elements Ni and Fe in Permalloy and Cu-doped Permalloy are delayed with respect to each other by 10 fs and 66 fs respectively, despite the strong exchange coupling that aligns their magnetic moments in thermodynamic equilibrium. Thus, instead of the exponential fits shown in Fig. 1, that appear to point to different demagnetization timescales for Fe and Ni in Permalloy and Cu-doped Permalloy, instead we interpret this delay as due to a finite spin-flip scattering time given by the exchange energy in the material, which essentially measures the timescale of the exchange interaction for the first time. This effect is enhanced by lowering the exchange energy by doping the Permalloy with Cu, which increases the lag in demagnetization of Ni with respect to Fe from 10 fs in Permalloy to 66 ± 10 fs in Permalloy-Cu. Most importantly, we note that without the ability to measure the dynamics of Fe and Ni in parallel (i.e. at the same time as shown in Fig. 3), it would not have been possible to measure these ! 10 fs timescales to uncover the relevant physics needed to understand the limiting switching speeds in magnetism. This is because jitter between the laser demagnetizing pulse and the x-ray probe pulse, or slight changes in the delay line as the x-ray source is tuned from one element to another, can easily introduce timing errors that makes it impossible to probe these ultrafast < 10 fs timescales using other x-ray sources.

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Figure 2. Zoom-in on sub-picosecond times of ultrafast demagnetization of Fe (red dots) and Ni (blue dots) in Cu-doped Permalloy. The data is shown as a function of magnetic asymmetry on linear and log scales. A simple exponential decay fit yields the effective demagnetization constant, which results in different values for Fe and Ni (given in Fig. 1). However, the log scale plot reveals that there is actually a lag between when the demagnetization of Ni and Fe, of ! 66 fs. After this characteristic delay time corresponding to the exchange interaction energy, both elements demagnetize with the same decay constant.

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b. Proposed research approach(es) Experimental: Measurements will use 30 - 100 femtosecond laser pump pulses in the 800 nm wavelength range. The probe beam will utilize < 10 femtosecond EUV pulses in the 50-80 eV photon energy range produced by high harmonic generation (HHG), as shown in Fig. 4 (top) [5, 6]. Contrast is provided by the resonantly enhanced transverse magneto-optic Kerr effect (T-MOKE) at the M absorption edge (3s-3p) of the magnetic atoms (see Fig. 4 (lower)) [3, 4]. The T-MOKE effect manifests itself as a magnetization-dependent reflectivity for the magnetization component perpendicular to the plane of incidence. Such a strong contrast mechanism can independently measure the response of the different magnetic components in the recording media. It should be possible to measure the magnetization through any carbonaceous overcoats on the media, owing to the fact that the carbon K-edge is well above 100 eV (the C edge is around 284 eV).

When subject to an ultrafast pump pulse, the magnetic material rapidly demagnetizes on ! 200 fs timescales. The detailed physics of ultrafast demagnetization in different materials is of ongoing scientific interest, but likely not of significant utility in the short term for the purposes of HAMR. (We note however that our approach for characterizing magnetic dynamics has the highest time resolution of any x-ray probe to date, in the femtosecond-to-attosecond range). However, once demagnetized, the magnetic material “rebounds” as the magnetic system sheds heat into the phonon system. We have observed a significant variation in the rate of this rebound process, depending on how close the sample temperature is to the Curie temperature. Based on the theory of LLB proposed by Chubykalo-Fesenko, et al. [1], such a variation in the rebound rate is expected due to the temperature dependence of the longitudinal spin relaxation rate in magnetic materials. Thus, we will provide direct experimental determination of the longitudinal relaxation rate for comparison with the LLB theory. Since the sample magnetization will be tilted away from perpendicular

Figure 4. (top) Experimental high harmonic emission (HHG) (black line) in the 40 – 70 eV photon energy region spanning the M absorption edges of several magnetic materials (green lines). (bottom) HHG spectrum reflected from the sample at an angle of 45 deg, for two different magnetization directions. The strong magnetic asymmetry signal for the Fe and Ni M-edges is also plotted. The asymmetry signal spans a few eV [3, 4].

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with an applied magnetic field, we expect that the magnetization will also undergo precessional oscillations after ultrafast demagnetization due to the time-varying internal fields. Thus, it will also be possible to compare longitudinal and transverse relaxation times to test LLB theory.

Samples will be heated in two different ways. Lower temperatures will be accessible using a simple heater stage. However, to reach higher temperatures approaching the Curie temperature, we will us a cw laser beam focused onto the measurement spot to heat the sample. We will calibrate the laser heating system by comparing the sample precession frequency using the heater stage and the laser heater. We expect that the temperature dependence of the anisotropy and magnetization will be detectable from the spin precession frequency. Finally, the elemental specificity of M-edge T-MOKE will allow us to examine the relaxation rate of different components in the media, should the media be formed of multilayer laminates, e.g. an exchange spring system. Our current experimental setup will require some modification to accomplish these measurements with perpendicular anisotropy materials. In particular, a large, in-plane magnetic field will be required to tip the magnetization out of the optical plane of incidence for the purposes of T-MOKE detection. Moreover, since the timescale for recovery is much larger than the timescale of the initial drop in magnetization, a new, larger, delay stage will be required. In addition, samples will require some preparation before we can measure them. (1) Samples will be overcoated with diffraction gratings by NIST Boulder labs, to permit spectrally resolved observation of the T-MOKE signal at JILA. This is required owing to the resonant nature of the T-MOKE signal: the signals are only present at the photon energies of the M-edge absorption as shown in Fig. 3. We have already tested that this approach works for arbitrary samples. (2) In the event that the EUV photons are not able to penetrate the media overcoat, the overcoat will be stripped using various etching methods available in the NIST Boulder laboratories.

Looking to the future, it will be possible to extend these measurements to the L absorption edges of magnetic media, due to recent advances in generating bright, coherent, x-rays from femtosecond lasers in the keV region of the spectrum [7]. In

Figure 5. A. (left) Predicted bright harmonic emission as a function of the driving laser wavelength. Solid circles represent current experimental results and open circles – theoretically expected phase matching limits. B. (right) Experimental full phase matched X-ray supercontinuum up to >1.6 keV or 7.8 Å (note linear X-ray intensity scale).[7]!

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recent exciting work, we demonstrated that bright coherent high harmonic x-rays from femtosecond lasers can extend to photon energies >1.6 keV (wavelengths <7.8 Å), promising to realize a coherent ultrafast implementation of the Roentgen X-ray tube in a tabletop-scale apparatus. Other applications of high harmonic soft x-rays in materials and molecular science at surfaces have also been demonstrated, including coherent diffractive (lensless) imaging with 22nm spatial resolution (with the potential for sub-10nm spatial resolution), and in following energy flow in nanostructures [8 - 10].

i. Computational: None.

c. Likely outcome of research: Quantitative determination of the longitudinal

relaxations in actual and/or proposed recording media. Rigorous testing of theoretical predictions provided by LLB theory. Presumably, the testing of LLB theory will be used to improve the accuracy of micromagnetic models for the simulation of HAMR.

[03] Resources required to perform project We already have unique resources available for the project (see Fig. 6).

a. Personnel, students, etc.: We are requesting support for a half-time graduate

student, Chan La-O-Vorakiat. Chan has extensive experience with the proposed measurements, and has developed the setup and taken the data shown in this proposal. No support is requested for Drs. Silva, Murnane or Kapteyn.

b. Equipment, lab, etc.: Significant laser, x-ray and vacuum equipment is already available for the proposed work through the NSF Center for EUV Science and Technology and through the NIST Boulder laboratory facilities (see Fig. 6).

c. Computational:

Computers for data acquisition and models are already available.

[04] Resources other than ASTC funding dedicated to perform project

a. Grants: Significant

facilities and setup support will draw on the infrastructure developed as part of the National Science Foundation Engineering Research Center in Extreme Ultraviolet Science and Technology. A contract from ASTC would allow ASTC to become a center industrial member, with many benefits (seat on industrial advisory board, paper preprints, graduating students etc.). Another grant funded by the DOE funds

Figure 6. Several unique setups are available for the proposed experiments: (upper left) high harmonic generation cell; (lower left) femtosecond driving laser with 3mJ, 2kHz, 25fs; (upper right) magnetics dynamics setup and x-ray CCD camera; (lower right) soft x-ray spectrometer.

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investigation of the short, femtosecond, demagnetization dynamics that is probing the timescale of the exchange interaction, and is very complementary.

b. Contracts: None

c. Other: This work will benefit greatly from collaborative support with the Magnetodynamics Project at NIST, Boulder, under the supervision of Dr. Thomas Silva. Our NIST partners have agreed to provide lithography support to prepare media samples for T-MOKE measurements.

[05] Resources requested from ASTC and how they will be utilized

a. Funding – see next page for budget details. i. Overhead – 52.5% or ! $21K per year

ii. Direct project cost ! $53.6K per year iii. Facility use fees ! $10K/year for center membership with benefits iv. Materials - $9.5K average per year v. Student stipends - $23K on average (1/2 time salary, benefits, tuition)

vi. Travel - $2K per year

b. Expected technical cooperation with sponsor(s): Initial materials can be the Permalloy and Cu-doped Permalloy materials provided by Tom Silva’s group at NIST Boulder. However, we are very interested is testing materials and collaborating with the members of the ASTC consortia, as well as their university collaborators. We have already demonstrated our interest in such collaborations: although our group is relatively new to the areas of ultrafast magnetics and data storage, we have tested samples from Hitachi Global Systems, and will be happy to expand these interaction to other ASTC members and their colleagues.

c. Sponsors’ facility utilization: Not required.

d. Expected students’ internships: Three current students would be available for internships if there is interest from the ASTC members. Our students are very interested in such internships. We not that the quality of the graduate student body in physics in Boulder is very high - over 600 students apply each year for the ! 40 slots in our incoming graduate class.

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Time line:

[06] Home institutions & resources: The home institutions are the University of Colorado and NIST. The significant and unique resources and grants that make the proposed work possible are described in Sections [03] and [04].

[07] Not more than one-half page per contributor: contact information and

biographical sketch of researcher. MARGARET MARY MURNANE JILA, University of Colorado at Boulder Boulder, CO 80309-0440 http://jilawww.colorado.edu/kmgroup Email: murnane@jila.colorado.edu; Ph. (303) 210-0396 Education and Professional Experience Ph.D. in Physics, University of California at Berkeley (1989) Professor of Physics and ECE, University of Colorado, Boulder, CO (August 1999 - present) Associate Professor, EECS and Physics, University of Michigan, Ann Arbor, MI (1996 - 1999) Assistant and Associate Professor of Physics, Washington State University, Pullman, WA (1995) Selected Honors 2010 Appointed to the President’s Committee for the National Medal of Science 2010 R.W. Wood Prize of the Optical Society of America (shared with Henry Kapteyn) 2010 Arthur L Schawlow Prize in Laser Science of the American Physical Society (shared with Henry Kapteyn) 2009 Ahmed Zewail Award of the American Chemical Society (shared with Henry Kapteyn) 2004 Member, National Academy of Sciences (USA) 2003 Fellow of the American Association for the Advancement of Science 2000 John D. and Catherine T. MacArthur Fellow Publication Summary (Updated March 2011) >175 publications in peer reviewed journals > 8767 citations

Q1 Q2 Q3 Q4 YEAR 1 Implement long time

delay measurements (10ps – 1ns)

Measure rebound times for current samples; test penetration depth for HAMR media coatings

Implement higher magnetic fields for perpendicular anisotropy materials

Implement first measurement of rebound times for perpendicular anisotropy materials

YEARS 2 and 3

Optimize geometry and continue measurements for different HAMR media

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HENRY CORNELIUS KAPTEYN Department of Physics and JILA University of Colorado, Boulder, CO 80309-0440 E-mail: kapteyn@jila.colorado.edu; Phone: (303) 492-8198 Education and Professional Experience Ph.D. in Physics, University of California at Berkeley, 1989 M.A. in Physics, Princeton University, 1984 Professor of Physics, University of Colorado at Boulder, 8/1999-present Associate Professor of Electrical Engineering, University of Michigan, 1996-99 Assistant and Associate Professor of Physics, Washington State University, 1995 Honors R.W. Wood Prize, Optical Society of America, 2010 (shared with Margaret Murnane) Schawlow Prize in Laser Science, American Physical Society, 2010 (shared with Murnane) Ahmed Zewail Award of the American Chemical Society, 2009 (shared with Murnane) Fellow, AAAS, APS, OSA, Sloan Adolph Lomb Medal of the Optical Society of America, 1993 Sample Recent Publications (> 9263 Total citations) 1. K. Raines et al., "Three-dimensional structure determination from a single view," Nature 463, 214 (2010). 2. T. Popmintchev et al., “The Attosecond Nonlinear Optics of Bright Coherent X-Ray Generation”, Nature Photonics

4, 822 (2010). Featured on cover. 3. M. Siemens et al., “Measurement of quasi-ballistic heat transport across nanoscale interfaces using ultrafast coherent

soft x-rays”, Nature Materials 9, 26 (2010). 4. M. Chen et al., “Bright, Coherent, Ultrafast Soft X-Ray Harmonics Spanning the Water Window from a

Tabletop Source”, Phys. Rev. Lett. 105, 173901 (2010). Featured on cover. 5. C. La-O-Vorakiat, T. J. Silva et al., “Ultrafast Soft X-Ray Magneto-Optics at the M-edge Using a Tabletop High-

Harmonic Source”, Physical Review Letters 103, 257402 (2009). THOMAS JOSEPH SILVA National Institute of Standards and Technology (NIST) Div. 818.03, 325 Broadway, Boulder, CO 80305 Ph. (303) 497-7826 (work); E-mail: silva@boulder.nist.gov Education and Current Appointment: Ph.D., UCSD, Electrical and Computer Engineering (Applied Physics), 1994. Staff Scientist, NIST, Boulder, CO. Electromagnetic Technology Division, (1996 – present). Honors and Awards: 1. National Science Foundation Creativity in Engineering Fellowship Award, 1987. 2. Presidential Early Career Award in Science and Engineering (PECASE), 1996. 3. IEEE Distinguished Lectureship, 2000. 4. U.S. Dept. of Commerce Bronze Medal for Superior Federal Service, 2004. 5. U.S. Dept. of Commerce Silver Medal for Meritorious Federal Service, 2007. 6. Fellow, American Physical Society, 2010. Sample recent publications: 1) T. Gerrits, M. L. Schneider, and T. J. Silva, "Enhanced ferromagnetic damping in Permalloy/Cu bilayers," J.

Appl. Phys. 99 (2006) 023901. 3) M. Schneider, J. Shaw, A. Kos, T. Gerrits, T. J. Silva, and R. D. McMichael, "Spin dynamics and damping in

nanomagnets measured directly by frequency-resolved magneto-optic Kerr effect," J. Appl. Phys., 102, 2007.

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4) T. Gerrits, P. Krivosik, M. L. Schneider, C. E. Patton, T. J. Silva, "Direct Detection of Nonlinear Ferromagnetic Resonance in Thin Films by the Magneto-Optical Kerr Effect," Phys. Rev. Lett., 98 (2007) 207602.

5) R. Heindl, S. E. Russek, T. J. Silva, W. H. Rippard, J. A. Katine, and M. J. Carey, "Size dependence of intrinsic spin transfer switching current density in elliptical spin valves," Appl.Phy. Lett., 92 (2008) 262504.

6) M. A. Hoefer, T. J. Silva, and M. D. Stiles, "Model for a collimated spin-wave beam generated by a single-layer spin torque nanocontact," Phys. Rev. B, vol. 77 (2008) 144401.

!"#"$"%&"' 1. Chubykalo-Fesenko, O., Nowak, U., Chantrell, R. W., & Garanin, D., "Dynamic approach

for micromagnetics close to the Curie temperature", Phys. Rev. B74, 094436 (2006). 2. Beaurepaire, E., Merle, J., Daunois, A. & Bigot, J. Ultrafast spin dynamics in ferromagnetic

nickel. Phys. Rev. Lett. 76, 4250 (1996). 3. C. La-O-Vorakiat, T. J. Silva et al., “Ultrafast Soft X-Ray Magneto-Optics at the M-edge

Using a Tabletop High-Harmonic Source”, Physical Review Letters 103, 257402 (2009). 4. Stefan Mathias, Chan La-O-Vorakiat, Patrik Grychtol, Justin M. Shaw, Roman Adam, Hans

T. Nembach, Mark E. Siemens, Steffen Eich, Claus M. Schneider, Thomas J. Silva, Martin Aeschlimann, Henry C. Kapteyn and Margaret M. Murnane, “Probing the timescale of the exchange interaction in a ferromagnetic alloy”, submitted (2011).

5. Henry C. Kapteyn, Margaret M. Murnane and Ivan P. Christov, “Coherent X-Rays from Lasers: Applied Attosecond Science”, invited article, Physics Today, page 39 (March 2005).

6. T. Popmintchev et al., “The Attosecond Nonlinear Optics of Bright Coherent X-Ray Generation”, Nature Photonics 4, 822 (2010).

7. T. Popmintchev et al., “Bright Coherent Attosecond-to-Zeptosecond Kiloelectronvolt X-ray Supercontinua”, Postdeadling paper, CLEO, Baltimore, MD (May 2011).

8. Margaret M. Murnane and John Miao, “Ultrafast X-Ray Photography”, Nature 460, 1088 (2009). 9. M. Seaberg, D. Adams, E. Townsend, D. Raymondson, W. F. Schlotter, Y. Liu, C. Menoni, H.

C. Kapteyn, and M. M. Murnane, "Ultrahigh 22 nm Resolution Coherent Diffractive Imaging using a Desktop 13 nm High Harmonic Source," submitted (2011); Invited talk, CLEO, Baltimore, MD (May 2011). .

10. M. Siemens, Q. Li, R. Yang, K. Nelson, E. Anderson, M. Murnane, H. Kapteyn, “Measurement of quasi-ballistic heat transport across nanoscale interfaces using ultrafast coherent soft x-ray beams”, Nature Materials 9, 26 (2010).