Magnetic Nanostructures: A Playground for Fundamental Physics

Ran Cheng
Thursday, March 2, 2017 - 4:30pm to 5:30pm

Nature becomes amazingly different from what we perceive with our eyes when zoomed in to the nanometer scale, where atomic spins interact and form diverse magnetic configurations. Besides holding great technological promise, magnetic nanostructures have also enabled a vibrant playground for fundamental physics—a thriving field known as spintronics. In this talk, I will introduce selected recent progress in spintronics that has reshaped our understanding of transport phenomena occurring at the microscopic scale. Special attention will be paid to antiferromagnetic...

Nitin Samarth (Penn State): Topological Spintronics: From the Haldane Phase to Spin Devices

We provide a perspective on the recent emergence of “topological spintronics,” which relies on the existence of helical Dirac electrons in condensed matter. Spin- and angle-resolved photoemission spectroscopy shows how the spin texture of these electronic states can be engineered using quantum tunneling [1] or by breaking time-reversal symmetry [2]. Inappropriately designed systems, broken time-reversal symmetry transforms helical Dirac states into chiral edge states, a realization of Haldane’s Chern insulator phase of matter. This is characterized by a precisely quantized Hall conductance and dissipationless edge transport without a magnetic field. We show how these edge states can be quantitatively characterized by analyzing their giant anisotropic magnetoresistance [3]. At miilikelvin temperatures, the interplay between Chern states and disordered magnetism [4] results in surprising behavior, perhaps consistent with quantum tunneling out of a ‘false vacuum’ [5]. Finally, we show how these helical Dirac electrons provide a possible pathway toward a spin device technology that works at room temperature [6,7].

[1] M. Neupane, A. Richardella et al.,Nature Communications 5, 3841 (2014).
[2] S.-Y. Xu et al., Nature Physics 8, 616 (2012).
[3] A. Kandala,A. Richardella, et al.,Nature Communications 6, 7434 (2015).
[4] E. Lachman et al., Science Advances 1, e1500740(2015).
[5] Minhao Liu et al., Science Advances 2, e1600167(2016).

Topological Spintronics: From the Haldane Phase to Spin Devices

Nitin Samarth
Monday, November 28, 2016 - 4:30pm to 6:00pm

We provide a perspective on the recent emergence of “topological spintronics,” which relies on the existence of helical Dirac electrons in condensed matter. Spin- and angle-resolved photoemission spectroscopy shows how the spin texture of these electronic states can be engineered using quantum tunneling [1] or by breaking time-reversal symmetry [2]. Inappropriately designed systems, broken time-reversal symmetry transforms helical Dirac states into chiral edge states, a realization of Haldane’s Chern insulator phase of matter. This is characterized by a precisely...

Next-Generation Computing using Spin-Based Materials

Jean Anne C. Incorvia
Friday, October 28, 2016 - 12:00pm to 1:30pm

We are at a time where the electronics industry is feeling pressure from two sides on the small scale we are facing the fundamental physical limits of silicon, and on the large scare we are facing new, abundant-data and distributed-data applications, such as for the internet of things. The future of computing will require both more energy efficient electronics and more big-data driven, application-specific designs.

Magnetic devices are a promising candidate for future electronics, due to their low-voltage operation...

Energetic Molding of Chiral Magnetic Bubbles

  • By Aude Marjolin
  • 21 July 2016

When it comes to computers, people never look for “bigger and better,” but rather “smaller and faster.” How do we continue to keep up with that demand, making technology smaller, faster, and more energy-efficient? According to Vincent Sokalski, the answer may be in the fundamental origins of magnets—the spin of electrons.

Sokalski and his group studied the interaction of electron spins in magnetic materials poised for use in next-generation cellphones and computers and discovered how to better measure and predict the changing magnetic state of those materials. This new understanding, recently published in Physical Review B under the title "Energetic Molding of Chiral Magnetic Bubbles", is exciting for the future of computing technology because it will allow scientists to explore and develop materials that are more energy-efficient and faster than traditional semiconductor-based materials.

Sara Majetich May Be Building the Computers of the Future

  • By Aude Marjolin
  • 24 February 2016

“The computers of the future may be born in Sara Majetich’s labs” reads the header of a recent news article.

For the past three years, Majetich has been a principal investigator for the Center for Spintronic Materials, Interfaces, and Novel Architectures (C-SPIN), which coordinates the research of 32 professors from 18 universities towards overcoming the limits of traditional computer design with spintronic technology.

Department of Materials Science and Engineering, Carnegie Mellon University
Ph.D., Materials Science and Engineering, Carnegie Mellon University, 2011

Our research is focused on the exploration of novel magnetic & spintronic materials for memory, logic, and HDD storage applications. We concentrate on the development of thin films & nanoscale devices that will enable improved energy efficiency, non-volatility, and scalability to ever-decreasing dimensions. Current research efforts include crystallographic & microstructural characterization of perpendicular magnetic recording media, materials for spin hall effect devices, spin transfer torque magneto-resistive random access memory (STT-MRAM), and magnetic interactions in soft nanogranular composite thin films.

Most Cited Publications: 
  1. "Optimization of Ta thickness for perpendicular magnetic tunnel junction applications in the MgO-FeCoB-Ta system," Vincent Sokalski, Matthew T. Moneck, En Yang, and Jian-Gang Zhu, Appl. Phy. Lett. 101, 072411 (2012)
  2. "Experimental modeling of intergranular exchange coupling for perpendicular thin film media," Vincent Sokalski, David E. Laughlin, and Jian-Gang Zhu, Appl. Phys. Lett. 95, 102507 (2009)
  3. "Noise Mechanisms in Small Grain Size Perpendicular Thin Film Media," J. G. Zhu, V. Sokalski, Y. Wang and D. E. Laughlin, IEEE Transactions on Magnetics 47, 74 (2011)
  4. "Characterization of Oxide Materials for Exchange Decoupling in Perpendicular Thin Film Media," V. M. Sokalski, J. G. Zhu and D. E. Laughlin, IEEE Transactions on Magnetics 46, 2260 (2010)
  5. "Magnetic anisotropy and stacking faults in Co and Co84Pt16epitaxially grown thin films," Vincent Sokalski, David E. Laughlin, and Jian-Gang Zhu,  J. Appl. Phys. 110, 093919 (2011)
Recent Publications: 
  1. "Increased boron content for wider process tolerance in perpendicular MTJs," J. P. Pellegren, M. Furuta, V. Sundar, Y. Liu, J.-G. Zhu, and V. SokalskiAIP Advances 7, 055901 (2017)
  2. "Magnetization dynamics and damping behavior of Co/Ni multilayers with a graded Ta capping layer," M. Jaris, D. Lau, V. Sokalski, and H. Schmidt, Jour. of Appl. Phys. 121, 163903 (2017)
  3. "Nonlocal stiffness governing Dzyaloshinskii domain wall mobility," Price Pellegren, Derek Lau, Vincent SokalskiarXiv:1609.04386
  4. "Dispersive Stiffness of Dzyaloshinskii Domain Walls," Price Pellegren, Derek Lau, Vincent SokalskiarXiv:1609.04386
  5. "Enhancement of Thermal Conductance at Metal-Dielectric Interfaces Using Subnanometer Metal Adhesion Layer," Minyoung Jeong, Justin P Freedman, Hongliang Joe Liang, Cheng-Ming Chow, Vincent M Sokalski, James A Bain, Jonathan A Malen, Phys. Rev. Applied 5, 014009 (2016)
Department of Electrical and Computer Engineering, Carnegie Mellon University
Ph.D., Physics, University of California, San Diego, 1989

Dr. Zhu’s research has been in the field of magnetic data storage technologies. His research work on the microstructure of thin film recording media has been pivotal for hard disk drives to reach today’s storage capacity. He has pioneered the research on utilizing micromagnetic modeling for MRAM memory design and established some of the most fundamental design principles used today.

Magnetic Recording Technology for Hard Disk Drives and Digital Tape Recording: Magnetic recording technology has been advancing in dramatically rapid pace over the past decade during which we have made some important contributions. At present, our research includes:

  • Development of novel recording mechanisms that enables area storage density exceeding 1 Tbits/in^2 for hard disk drive applications;
  • Development of novel perpendicular thin film media microstructures that capable of high area density applications;
    The research is supported by DSSC and its industrial sponsors.

Innovative Designs of Magnetic Random Access Memory (MRAM): MRAM has the potential to replace SRAM, DRAM, FLASH, and even a small disk drive to be the universal memory for computer data storage, enabling an entire computer system to be made on a single chip. Our research focuses on novel MRAM designs that offer robust and repeatable magnetic switching characteristic, low operation power capability, and sufficient thermal-magnetic stability. Micromagnetic modeling on computers is utilized to aid the design process and the devices are fabricated using the state-of-the-art e-beam and optical lithographic fabrication technology. Our collaborators include the Naval Research Laboratory and Nonvolatile Electronics Corporation. This research is current funded by the Office of Naval Research, Pittsburgh Digital Green House, STMicroelectronics, and DSSC.

Understanding Noise in Nano-Magnetic Systems: Thermally excited magnetization precession and spin current induced chaotic spin waves are two important causes of magnetic noise in advanced nano-scale magnetic sensors. We perform both theoretical analysis and experimental measurements to obtain a good understanding of the noise and the corresponding underlying physics. This research is supported by Seagate Technology and DSSC.

Most Cited Publications: 
  1. "Ultrahigh density vertical magnetoresistive random access memory," Jian-Gang (Jimmy) Zhu, Youfeng Zheng, and Gary A. Prinz, J. Appl. Phys 87, 6668 (2000)
  2. "Microwave assisted magnetic recording," Jian-Gang (Jimmy) Zhu, Xiaochun Zhu, and Yuhui Tang, IEEE Transactions on Magnetics 44, 125 (2008)
  3. "Micromagnetic studies of thin metallic films, Jian-Gang (Jimmy) Zhu and H. Neal Bertram, J. Appl. Phys 63, 3248 (1988)
  4. "Magnetoresistive random access memory: the path to competitiveness and scalability," Jian-Gang (Jimmy) ZhuProceedings of the IEEE 96, 1786 (2008)
  5. “Magnetic tunnel junctions,” Jian-Gang (Jimmy) Zhu and Chando Park, Materials Today 9, 36 (2006)


Recent Publications: 

  1. "Fabrication of bit patterned media using templated two-phase growth," Vignesh Sundar, XiaoMin Yang, Yang Liu, Zhengkun Dai, Bing ZhouJingxi ZhuKim LeeThomas ChangDavid Laughlin, and Jian-Gang (Jimmy) ZhuAPL Materials 5, 026106 (2017)
  2. "MgO-C interlayer for grain size control in FePt-C media for heat assisted magnetic recording," B. S. D. Ch. S. Varaprasad, Bing Zhou, Tong Mo, David E. Laughlin, and Jian-Gang (Jimmy) ZhuAIP Advances 7, 056503 (2017)
  3. "Dynamic Feedback in Ferromagnet–Spin Hall Metal Heterostructures," Ran Cheng, Jian-Gang (Jimmy) Zhu, and Di Xiao, Phys. Rev. Lett. 117, 097202 (2016)
  4. "Energetic molding of chiral magnetic bubbles," Derek Lau, Vignesh Sundar," Jian-Gang (Jimmy) Zhu, and Vincent Sokalski, Phys. Rev. B 94, 060401 (2016)
  5. "Distinguishing Random and Spatially Deterministic Noise Components in Heat-Assisted Magnetic Recording," Michael Alex, Hai Li, Gerardo Bertero, and Jian-Gang (Jimmy) Zhu, IEEE Transactions on Magnetics 52, 1 (2016)

Personal | Department
Department of Physics, Carnegie Mellon University
Ph.D., University of Georgia

My research focuses on magnetic nanoparticles that have very uniform sizes, and we study their fundamental behavior, as well as possible applications in data storage media, permanent magnets, and biomedicine. One of the consequences of this monodispersity is that the particles can then self-assemble into arrays (shown below), just as atoms come together to form a crystal. We are investigating the collective behavior of the nanoparticle arrays that are analogous to those in crystals. Isolated iron atoms do not interact with each other and are paramagnetic, but in an iron crystal the interactions lead to ferromagnetism. Superparamagnetic-to-ferromagnetic and insulator-to-metal phase transitions are expected as the nanoparticles are brought closer together. We have also developed a method to replace the surfactant coating the particles with an inorganic matrix, and are exploring methods that exploit this approach to prepare functional nanocomposites.

Most Cited Publications: 
  1. "TCE Dechlorination Rates, Pathways, and Efficiency of Nanoscale Iron Particles with Different Properties," Yueqiang Liu, Sara A. Majetich, Robert D. Tilton, David S. Sholl, and Gregory V. Lowry, Environ. Sci. Technol. 39, 1338 (2005)
  2. "Superparamagnetism in carbon-coated Co particles produced by the Kratschmer carbon arc process," M. E. McHenry, S. A. Majetich, J. O. Artman, M. DeGraef, and S. W. Staley, Phys. Rev. B 49, 11358 (1994)
  3. "Synthesis and Utilization of Monodisperse Superparamagnetic Colloidal Particles for Magnetically Controllable Photonic Crystals," Xiangling Xu, Gary Friedman, Keith D. Humfeld, Sara A. Majetich, and Sanford A. Asher, Chem. Mater. 14, 1249 (2002)
  4. "Superparamagnetic photonic crystals," Xiangling Xu, Gary Friedman, Keith D Humfeld, Sara A Majetich, Sanford A Asher, Adv. Mater. 13, 1681 (2001)
  5. "Magnetization Directions of Individual Nanoparticles," S. A. Majetich, Y. Jin, Science 284, 470 (1999)
Recent Publications: 
  1. "Size and voltage dependence of effective anisotropy in sub-100-nm perpendicular magnetic tunnel junctions," Stephan K. Piotrowski, Mukund Bapna, Samuel D. Oberdick, Sara A. Majetich, Mingen Li, C. L. Chien, Rizvi Ahmed, and R. H. Victora, Phys. Rev. B 94, 014404 (2016)
  2. "Formation of FePt nanodots by wetting of nanohole substrates," Abdelgawad, Ahmed M.; Oberdick, Samuel D.; Majetich, Sara A. AIP Advances 6, 056114 (2016)
  3. "Tracking the Verwey Transition in Single Magnetite Nanocrystals by Variable-Temperature Scanning Tunneling Microscopy," Amir Hevroni, Mukund Bapna, Stephan Piotrowski, Sara A. Majetich, and Gil Markovich, J. Phys. Chem. Lett. 7, 1661 (2016)
  4. "Patterning of sub-50 nm perpendicular CoFeB/MgO-based magnetic tunnel junctions," Larysa Tryputen, Kun-Hua Tu, Stephan K Piotrowski, Mukund Bapna, Sara A Majetich, Congli Sun, Paul M Voyles, Hamid Almasi, Weigang Wang, Patricio Vargas, Nanotechnology 27, 185302 (2016)
  5. "Magnetostatic effects on switching in small magnetic tunnel junctions," Mukund Bapna, Stephan K. Piotrowski, Samuel D. Oberdick, Mingen Li, C.-L. Chien, and  Sara A. Majetich Appl. Phys. Lett. 108, 022406 (2016)