Using Phase Changes to Store Information in the Magnetic Permeability

Authors: Alan S. Edelstein^*, John Timmerwilke, Jonathan R. Petrie 

Affiliation: US Army Research Laboratory, Adelphi, Maryland, USA

*Email: aedelstein@cox.net

Finding better methods for storing information was a serious problem for the earliest computers; considerable effort is still being devoted to decreasing the cost and increasing the density and lifetime of stored information. Initial methods of storage included using acoustic delay lines and pixels on the display of cathode ray tubes [1]. Though the current methods for storing information, which include magnetic hard disks, magnetic tape, and various forms of random access memory (RAM) are far superior to these early methods, they still have significant limitations.

As an example, the information on hard disks or magnetic tape is written by a magnetic field and stored as regions, i.e. bits, having different directions for their remanent magnetization. Thus, this information can be erased by exposure to a magnetic field. Furthermore, to avoid thermal upsets of the spin direction of the bits there is a trade-off between how long the information can be stored without acquiring too many incorrect bits and the information density on hard disks. Maintaining a magnetic hard disk lifetime of about seven years has made it difficult to increase the information density in hard disk drives. Magnetic tape degrades in about 20 years.

To avoid these problems, we have been developing a new approach for storing information that we call Magnetic Phase Change Memory (MAG PCM). Instead of using the direction of magnetic remanance, information is stored in bits of soft ferromagnetic material having different values for their magnetic permeability. The initial magnetic permeability of a soft ferromagnetic material is reversible and dependent only on its atomic structure. Thus, it is independent of the magnetic field. The permeability of iron rich FeGa single crystal alloys [2]  is an interesting example of soft magnetic behavior.

The property we use is that high permeability bits attract magnetic fields and low permeability bits do not. To read the information stored in bits with high or low permeability we measure their effect on a probe field. We have primarily focused on writing and changing the permeability of bits of amorphous ferromagnet, 2826 MB Metglas, films by thermally heating with a laser. The nearest neighbor atoms in amorphous ferromagnets materials do not have long range order. Thus, they do not have much crystalline anisotropy or coercivity and are soft ferromagnets with large values for their permeability. When amorphous materials are heated above their glass temperature, they crystalize and have larger values for their coercivity and smaller values for their permeability. The glass temperature of 2826 MB Metglas is 410^oC. At temperatures as high as 200^oC, 2826 MB Metglas will retain its high permeability for hundreds of years.

Figure 1: a) Microscope view of three 50 micron wide crystallized lines written into amorphous Metglas. b) Voltage of the MTJ reader as it moved over the three crystalline lines written in the presence of a 32 Oe probe field.

We have read the effect on a probe magnetic field near each bit using both magnetic tunnel junction (MTJ) sensors [3,4], and spin transfer oscillators [5]. Figure 1a shows a microscope image of three crystalline lines in Metglas written by a focused 1.966 micron (Tm fiber) laser. The output voltage of an MTJ sensor is plotted in Figure 1b as it is swept over the crystallized Metglas lines shown in Figure 1a. One sees that the crystallized lines affect a 32 Oe probe field. The crystallized lines do not attract the probe field as much as the amorphous Metglas film does which causes the magnetic sensor to measure a larger field. Smaller nanometer-sized bits were created using e-beam lithography. Figure 2a is a microscope image of two square 0.9 mm arrays of 300 nm bits of amorphous Metglas. Figure 2b shows a scanning electron microscope (SEM) image of the 300 nm diameter bits. Laser heating was used to crystallize all of the bits in the left array. Figure 3 shows the voltage output of the MTJ sensor when it is moved over the two arrays before and after the laser heating. One sees that the bits in the left array no longer attract the magnetic field lines of the probe field.

Figure 2. (a) Microscope image of two square 0.9 mm arrays of 300 nm diameter bits of Metglas; (b) scanning electron microscope image of the 300 nm diameter bits of Metglas.

This new approach for storing information has several advantages. One can write bits with at least three different values [6]  for their permeability. The bits will not be corrupted by a magnetic field or thermal upsets and therefore should last decades. It should be possible to write nm sized bits economically by an adaption of heat assisted magnetic recording (HAMR) [7], a technology that is being developed by hard disk companies such as Seagate and Western Digital to cope with the problem mentioned above of maintaining stability against thermal upsets. In HAMR a laser and a near field transducer is used to heat nm sized regions to 710^oC to decrease the coercivity so that they can be written without using as large a magnetic field. What we need to do is simpler, in that we do not need a magnetic field and for archiving we do not need to rewrite.

Figure 3: Magnetic tunnel junctions scans before (red, o) and after (blue, x) the left array of 300nm Metglas bits in Fig. 2 were crystallized by heating with a laser.

MAG PCM has the potential for a combination properties not found in other storage technologies. It should have decades of longevity and the rapid access and high density of future hard disks. We have a clear path for developing MAG PCM into commercial products for long term storage applications such as archiving. Though it is unnecessary for archiving, we have found that we can rewrite our bit [8], i.e., return a crystallized bit to an amorphous state.

This work was done while we were at the US Army Research Laboratory.

References:
[1] D.R. Hartree, M.H.A. Newman, M.V. Wilkens, F.C. Williams, J.H. Wilkinson, A.D. Booth, ” A discussion on computing machines”, Proceedings of the Royal Society of London Series A- Mathematical and Physical Sciences, 195:1042, 265 (1948). Abstract.
[2] Harsh Deep Chopra, Manfred Wittig, “Non-Julian magnetostriction”, Nature, 521, 340-343 (2015). Abstract.
[3] J.R. Petrie, K.A. Wieland, R.A. Burke, G.A. Newburgh, J.E. Burnette, G.A. Fischer, A.S. Edelstein, “ A non-erasable magnetic memory based on the magnetic permeability”, Journal of Magnetism and Magnetic Materials, 361, 262 (2014). Abstract.
[4] John Timmerwilke, J.R. Petrie, K.A. Wieland, Raymond Mencia, Sy-Hwang Liou, C.D. Cress, G.A. Newburgh, A.S. Edelstein, “Using magnetic permeability bits to store information”, Journal of Physics D: Applied Physics, 48, 405002 (2015). Abstract.
[5] J.R. Petrie, S. Urazhdin, K.A. Wieland, A.S. Edelstein, “Using a spin torque nano-oscillator to read memory based on the magnetic permeability”, Journal of Physics D: Applied Physics, 47, 055002 (2014). Abstract.
[6] J.R. Petrie, K.A. Wieland, J.M. Timmerwilke, S.C. Barron, R.A. Burke, G.A. Newburgh, J.E. Burnette, G.A. Fischer, and A.S. Edelstein, “A multi-state magnetic memory dependent on the permeability of Metglas”, Applied Physics Letters, 106, 142403 (2015). Abstract.
[7] M.H. Kryder and Soo Kim Chang, “After hard drives—what comes next?” IEEE Transactions on Magnetics, 45, 3406 (2009). Abstract.
[8] Unpublished data.

A New Way To Weigh A Star

Nils Andersson (left) and Wynn Ho 

Authors: Nils Andersson, Wynn Ho

Affiliation: Mathematical Sciences and STAG Research Centre, University of Southampton, UK. 

You probably have a pretty good idea of your own weight. And if you need an exact answer it is relatively easy to find out. Get the bathroom scales out, step up and read off the result. But have you ever asked yourself what was actually involved in that measurement? Have you considered that what actually happened was that gravity’s pull was countered by the electromagnetic interaction of the atoms that make up the surface of the scale, and what you actually measured was how hard the atoms had to work to push back on your feet? Possibly not, and the chances are that you have never really considered how one would weigh a distant star, either.

As it turns out, this question has a fairly straightforward answer, which also involves gravity, electromagnetism… and a bit of luck. If you want to figure out how heavy a particular star is, and you are lucky enough that this star has a close companion, then all you need to do is track the star’s motion. The orbital motion of a double-star system is dictated by gravity and you can figure out how much the stars weigh using the same arguments that we use to figure out the mass of the moon (going around the Earth) and the Earth (circling the Sun). If you want to be a bit more precise you should use Einstein’s curved spacetime theory for gravity rather than Newton’s inverse square-law, but this may be a luxury in this exercise.

Using this technique, astronomers have weighed many stars with great precision and they have also managed to work out the masses of a number of pulsars [1]. This is particularly exciting as these systems stretch our understanding of several aspects of fundamental physics.

A pulsar is a highly magnetised rotating neutron star formed when a massive star runs out of nuclear fuel. At this point it can no longer hold itself up against gravity and it starts falling in on itself. This leads to a spectacular explosion – a supernova – the remains of which may be either a neutron star or a black hole. Neutron stars are nature’s own counterparts to the Large Hadron Collider [2]. In essence, they provide a link between astronomy and laboratory work in both high-energy and low-temperature physics. By weighing these stars we gain insight into physics under extreme conditions [3].

Pulsars are named for their rotating beam of electromagnetic radiation, which is observed by telescopes as it sweeps past the Earth, just like the familiar beam of a lighthouse [4]. They are renowned for their incredibly stable rate of rotation [5], but young pulsars occasionally experience so-called glitches where they are found to speed up for a very brief period of time [6]. The prevailing idea is that these glitches provide evidence of exotic states of matter in the star’s interior [3]. The glitches arise when a rapidly spinning superfluid within the star transfers rotational energy to the star's crust, a solid outer layer like a bowl containing a mysterious soup; the component that is tracked by observations. Imagine the bowl spinning at one speed and the soup spinning faster. Friction between the inside of the bowl and its contents, the soup, can cause the bowl to speed up. Whenever this happens, the more soup there is, the faster the bowl will be made to rotate.

Interestingly, it seems that the superfluid soup provides us with a new way of weighing these stars. This new technique is very different from the usual approach as it is not based on gravity, but nuclear physics, and it can also be used for stars in isolation. The star does not have to have a companion.

In a recent paper in Science Advances [7], we have developed this exciting new idea, which relies on a detailed understanding of neutron star superfluidity and the dynamics of the quantum vortices - a kind of ultra-slim tornadoes - by means of which these systems mimic large-scale rotation. Our results are promising and have important implications for the generation of revolutionary radio telescopes, like the Square Kilometre Array (SKA [8]) and the Low Frequency Array (LOFAR [9]), that are being developed by large international collaborations. The discovery and monitoring of many more pulsars is one of the key scientific goals of these projects. We now have a set of scales that may allow us to figure out how much these stars weigh, as well.

References: 
[1] See, for example, the list maintained at stellarcollapse.org. 
[2] See Large Hadron Collider. 
[3] J.M. Lattimer, M. Prakash, "The Physics of Neutron Stars", Science, 304, 536-542 (2004). Abstract. 
[4] The discovery of pulsars was recognized with the Nobel prize in physics in 1974. 
[5] Gravity tests using the stability of the timing of pulsars was recognized with the Nobel prize in physics in 1993. 
[6] A catalog of glitches is maintained by Jodrell Bank Centre for Astrophysics. 
[7] W.C.G. Ho, C.M. Espinoza, D. Antonopoulou, N. Andersson, "Pinning down the superfluid and measuring masses using pulsar glitches," Science Advances, 1, e1500578 (2015). Full Article. 
[8] See Square Kilometre Array.  
[9] See LOFAR. 

A Continuously Pumped Reservoir of Ultracold Atoms

From Left to Right: (top row) Jan Mahnke, Ilka Kruse, Andreas Hüper, (bottom row) Wolfgang Ertmer, Jan Arlt, Carsten Klempt.

Authors: Jan Mahnke¹, Ilka Kruse¹, Andreas Hüper¹, Stefan Jöllenbeck¹, Wolfgang Ertmer¹, Jan Arlt², Carsten Klempt¹

Affiliation:
¹Institut für Quantenoptik, Gottfried Wilhelm Leibniz Universität Hannover, Germany
²Institut for Fysik og Astronomi, Aarhus Universitet, Aarhus C, Denmark

During the last decades, the quantum regime could be accessed through very different systems, including trapped ions, micromechanical oscillators, superconducting circuits and dilute ultracold gases. Mostly, these systems show the desired quantum-mechanical features at ultra-low temperatures only. The lowest temperatures [1] to date are reached in dilute atomic gases by a combination of laser cooling and evaporative cooling. This approach has two disadvantages: It relies on the internal structure of the atoms due to the laser cooling and it can only cool discrete samples instead of continuous beams due to the evaporative cooling.

However, many applications would greatly benefit from a continuous source of cold atoms, for example sympathetic cooling [2] of molecules. Here, the molecules are brought into contact with a bath of cold atoms to redistribute the thermal energy through collisions. Ideally, such a cold bath is realized in absence of disturbing laser light as the rich internal structure of the molecules results in a broad absorption spectrum and any photon can potentially harm the cooling process. Another application of a continuous beam of cold atoms is continuous matter interferometry. Atom interferometry is already in use for the precise measurement of many observables, including time [3], gravity [4] and rotation [5]. These measurements could greatly benefit from a continuous observation instead of the sequential interrogation of discrete samples. Even though continuous sources are highly desired, no continuous sources without the application of laser light have been demonstrated in the microkelvin regime yet.

One possible realization of an ultracold continuous sample was proposed theoretically [6], where a conservative and static trap is loaded by an atomic beam. In this scheme, pre-cooled atoms are guided towards the entrance barrier of an elongated trap with a finite trap depth (see figure 1). If the atoms pass the entrance barrier, they follow the elongated potential until they are reflected by the end of the trap. The strong confinement in the radial direction ensures that most atoms collide with another atom before they reach the entrance barrier again. These collisions allow for a redistribution of the kinetic energy. Consequently, some atoms acquire a kinetic energy larger than the trap depth and escape the trap. Other atoms lose energy and stay trapped. If the trap parameters are chosen well, an equilibrium condition with a surprisingly large phase-space density may be reached.

Figure 1: 3D plot of the static trapping potential in the x–z-plane through the point of the trap minimum.

In our recent publication [7], we demonstrate the first experimental implementation of such a continuous loading of a conservative trap. Our realization is based on a mesoscopic atom chip (see figure 2 and Ref. [8]), a planar structure of millimeter-sized wires. The mesoscopic chip generates the magnetic fields for a three-dimensional magneto-optical trap, a magnetic waveguide and the static trapping potential described above. The three-dimensional magneto-optical trap is periodically loaded with an ensemble of atoms. These ensembles are launched into the magnetic waveguide, where they overlap and produce a continuous atom beam with varying intensity. This beam traverses an aperture which optically isolates the loading region from the static trapping region. In this trapping region, the atom beam is directed onto the elongated magnetic trap, where the atoms accumulate.

Figure 2: Photograph of the mesoscopic atom chip with millimeter-scale wires. The magneto-optical trap is in the lower left area and the static trap is generated in the top right area. The bend wires create a guide connecting the two regions.

With this loading scheme, we create and maintain an atomic reservoir with a total number of 3.8 × 10⁷ trapped atoms at a temperature of 102 µK, corresponding to a peak phase-space density of 9 × 10^-8 h^-3. This is the first continuously loaded cloud in the microkelvin regime without the application of laser light. Such a continuously replenished ensemble of ultracold atoms presents a new tool for metrological tasks and for the sympathetic cooling of other atomic species, molecules or nanoscopic solid state systems. The scheme is also very versatile in creating cold samples of atoms and molecules directly, as it does not rely on any internal level structure.

Figure 3 Photograph of the experimental setup. The atom chip is visible outside of the vacuum chamber at the top. In the front is the glass cell and the optics for the two-dimensional magneto-optical trap, which is used to load the three dimensional magneto-optical trap on the chip.

References:
[1] A. E. Leanhardt, T. A. Pasquini, M. Saba, A. Schirotzek, Y. Shin, D. Kielpinski, D. E. Pritchard,  W. Ketterle. “Cooling Bose-Einstein condensates below 500 picokelvin”, Science, 301, 1513 (2003). Abstract.
[2] Wade G. Rellergert, Scott T. Sullivan, Svetlana Kotochigova, Alexander Petrov, Kuang Chen, Steven J. Schowalter, Eric R. Hudson, “Measurement of a large chemical reaction rate between ultracold closed-shell ⁴⁰Ca atoms and open-shell ¹⁷⁴Yb⁺ ions held in a hybrid atom-ion trap”, Physical Review Letters, 107, 243201 (2011). Abstract.
[3] R. Wynands and S. Weyers. “Atomic fountain clocks”, Metrologia, 42 (3), S64 (2005). Abstract.
[4] A. Louchet-Chauvet, S. Merlet, Q. Bodart, A. Landragin, F. Pereira Dos Santos, H. Baumann, G. D'Agostino, C. Origlia, “Comparison of 3 absolute gravimeters based on different methods for the e-MASS project”, Instrumentation and Measurement, IEEE Transactions on, 60(7), 2527-2532 (2011). Abstract.
[5] J. K. Stockton, K. Takase, and M. A. Kasevich. “Absolute geodetic rotation measurement using atom interferometry”, Physical Review Letters, 107, 133001 (2011). Abstract.
[6] C. F. Roos, P. Cren, D. Guéry-Odelin, and J. Dalibard. “Continuous loading of a non-dissipative atom trap”, Europhysics Letters, 61, 187 (2003). Abstract.
[7] J. Mahnke, I. Kruse, A. Hüper, S. Jöllenbeck, W. Ertmer, J. Arlt, C. Klempt. “A continuously pumped reservoir of ultracold atoms”, Journal of Physics B: Atomic Molecular and Optical Physics, 48, 165301 (2015). Abstract.
[8] S. Jöllenbeck, J. Mahnke, R. Randoll, W. Ertmer, J. Arlt, C. Klempt. “Hexapole-compensated magneto-optical trap on a mesoscopic atom chip”, Physical Review A, 83, 043406 (2011). Abstract.

A Magnetic Wormhole

(From left to right) Carles Navau, Alvaro Sanchez, Jordi Prat-Camps

Authors: Jordi Prat-Camps, Carles Navau, Alvaro Sanchez 

Affiliation: Departament de Física, Universitat Autònoma de Barcelona, Spain.

Link to Superconductivity Group UAB >>

Is it possible to build a wormhole in a lab? Taking into account that large amounts of gravitional energy would be required [1], this seems an impossible task. However, redefining a wormhole into a path between two points in space that is completely undetectable, Greenleaf and colleagues [2] suggested in 2007 a (theoretical) way of realizing an electromagnetic wormhole capable of guiding light through an invisible path. They demonstrated that this is topologically equivalent as if the light had been sent through another spatial dimension. However, such a wormhole required metamaterials with extreme properties, which prevented its construction.

In our work, we have constructed an actual 3D wormhole working for magnetostatic fields. It allows the passage of magnetic field between distant regions while the region of propagation remains magnetically invisible. Our wormhole takes advantage of the possibilities that magnetic metamaterials offer for shaping static magnetic fields [3]. These metamaterials can be constructed using existing magnetic materials that can provide extreme magnetic permeability values ranging from zero - superconductors - to effectively infinity - ferromagnets.

Figure 1: (Left) 3D sketch of the magnetic wormhole, showing how the magnetic field lines (in red) of a small magnet at the right are transferred through it. (Right) From a magnetic point of view the wormhole is magnetically undetectable so that the field of the magnet seems to disappear at the right and reappear at the left in the form of a magnetic monopole. (Image credit: Jordi Prat-Camps and Universitat Autònoma de Barcelona).

The magnetic wormhole requires three properties: (i) to magnetically decouple a given volume from the surrounding 3D space, (ii) to have the whole object magnetically undetectable, and (iii) to have magnetic fields propagating through its interior. The first two properties are achieved by constructing a 3D magnetic cloak. Based on previous ideas [4] such a cloak could be made by surrounding a superconducting sphere with a specially created ferromagnetic (meta)surface, such that the magnetic signature of the superconductor was cancelled by the ferromagnet. For the third property, we use magnetic hoses, also made by magnetic metamaterials, as developed in [5].

The parts composing the magnetic wormhole are shown in fig. 2: a central magnetic hose to guide the magnetic field from one end of the hose to the opposite one, and a magnetic cloak composed of a superconducting-ferromagnetic bilayer to make the hose magnetically invisible.

Figure 2: (a) 3D image of the magnetic wormhole, formed by concentric shells: from outside inwards, an external metasurface made of ferromagnetic pieces (b), an internal superconducting shell made of coated conductor pieces (c), and a magnetic hose made of ferromagnetic foil (d). (e) Cross-section view of the wormhole, including the plastic formers (in green and red) used to hold the different parts. (Image credit: Jordi Prat-Camps and Universitat Autònoma de Barcelona).

Experimental results clearly demonstrate [6] the two desired properties for the wormhole: (i) magnetic field from a source at one end of the wormhole appear at the opposite end (actually as a kind of isolated magnetic monopole), (ii) the overall device is magnetically undetectable (it does not noticeably distort an applied magnetic field, even a non-uniform one).

Besides the scientific interest per se in the realization of an object with properties of a wormhole, our device may have applications in practical situations where magnetic fields have to be transferred without distorting a given field distribution, as in magnetic resonance imaging.

Acknowledgements: We thank Spanish project MAT2012-35370 and Catalan 2014-SGR-150 for financial support. A.S. acknowledges a grant from ICREA Academia, funded by the Generalitat de Catalunya. J. P.-C. acknowledges a FPU grant form Spanish Government (AP2010-2556).

References:
[1] Michael S. Morris, Kip S. Thorne, Ulvi Yurtsever, "Wormholes, Time Machines, and the Weak Energy Condition", Physical Review Letters, 61, 1446 (1998). Abstract.
[2] Allan Greenleaf, Yaroslav Kurylev, Matti Lassas, Gunther Uhlmann, "Electromagnetic Wormholes and Virtual Magnetic Monopoles from Metamaterials", Physical Review Letters, 99, 183901 (2007). Abstract.
[3] Steven M. Anlage, "Magnetic Hose Keeps Fields from Spreading", Physics, 7, 67 (2014). Full Article.
[4] Fedor Gömöry, Mykola Solovyov, Ján Šouc, Carles Navau, Jordi Prat-Camps, Alvaro Sanchez, "Experimental realization of a magnetic cloak", Science, 335, 1466 (2012). Abstract.
[5] C. Navau, J. Prat-Camps, O. Romero-Isart, J. I. Cirac, A. Sanchez. "Long-Distance Transfer and Routing of Static Magnetic Fields", Physical Review Letters, 112, 253901 (2014). Abstract.
[6] Jordi Prat-Camps, Carles Navau, Alvaro Sanchez. "A Magnetic Wormhole", Scientific Reports 5, 12488 (2015). Full Article.