Current-controlled bi-stable domain configurations in Ni81Fe19 elements: An approach to magnetic memory devices

Abstract

The discovery of current-switchable bi-stable remanent domain configurations on small ferromagnetic islands is reported. Rectangular NiFe islands with a thickness of 50 to 100 nm and lateral dimensions on the order of several microns were imaged using magnetic force microscopy after application of 10 ns current pulses through the material. The closure configuration can be set into either the 4 or 7 domain configuration by applying positive or negative current polarity at density on the order 10 7 A/cm 2 . The chirality of the closure patterns is fixed, implying that only two rather than four states are stable in these patterns. The possibility of using these configurations as a means of storing a logic state for memory applications is discussed. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1495883͔The general method to assign a ''1'' or ''0'' in solid state magnetic memory devices is to orient the remanent magnetization of ferromagnetic elements in one of two opposite directions. Thus, the magnetic properties of small particles as well as new ways to switch the magnetization other than by applying an external magnetic field are topics of intense research activity. [1][2][3][4] In this letter, we offer an alternative approach to storing and reading binary information in magnetic systems. Specifically, we introduce the idea that a bi-stable remanent domain configuration of a ferromagnetic island, which is selectable by a current pulse through the material, can be a simple and viable option for magnetic random access memory applications.In a previous investigation, we established that in small rectangular NiFe thin film islands, the number of possible configurations at remanence (H applied ϭ0) is finite. 5 In the case where the length-to-width ratio ͑referred to as the aspect ratio͒ is between 1 and 4, and for thickness ϳ100 nm thin or less, the remanent states are predominantly 4-or 7-domain configurations. The four-domain ͑4D͒ structure is comprised of four 90°walls emanating from the corners and one 180°N éel wall at the center and parallel to the shape-induced easy axis. The seven-domain ͑7D͒ configuration, on the other hand, contains four 90°walls as in the previous case, but the 180°wall is replaced by a diamond-shaped domain at the center of the island, with an internal magnetization along the hard axis. These two states represent local minima of the energy and are determined by competition between the reduction of the magnetostatic energy and the cost of forming domain walls. From numerical calculations, the 7D configuration is about 25% lower in magnetostatic energy. We discovered that reversible 7D-4D state selection can be achieved by driving current pulses through the element in a process that involves domain wall displacement, 6 and submit that these two distinct domain configurations can be used to encode binary information.

Effects of spin current on ferromagnets (invited)

“…In this paper, we focus our discussions on two effects: domain wall dynamics and magnetization instability. Recently, both topics have received considerably interests in experiments [1,2,3,4,5,6,7,8,9,10,11] and in theories [12,13,14,15,16,17,18,19]. It is shown that the current is able to displace magnetic domain walls in a spin valve [3], in a constricted nanowire [4], in U-shaped (or L-shaped) nanowires [5,6], in a ring structure [7] and in zigzag wires [8].…”

Current-driven domain-wall dynamics in curved ferromagnetic nanowires

“…7,21,23,24,25 While it has been originally suggested that the discrepancy could be due to thermal activation 11,25,26,27,28 or surface roughness 25 , it has recently been found that the domain-wall velocity depends on the type of the domain wall 26,29 which can be changed by a spin-polarized current. 8,26,30,31 Recently it has been observed that the velocity of field-driven domain-wall motion 32 can be altered by ± 100 m/s by a pulsed spinpolarized current 33 and that the motion can even be halted completely. 34 It is now assumed that the adiabatic term is largely responsible for the acceleration of the domain wall while the non-adiabatic term will cause the wall to continually move.…”

Adiabatic domain wall motion and Landau-Lifshitz damping

“…1,2,3,4,5,6,7,8,9,10 Most theories ascribe this behavior to the interplay between spin-transfer (the quantum mechanical transfer of spin angular momentum between conduction electrons and the sample magnetization) and magnetization damping of the Gilbert type. 11 Contrary to the second point, we argue in this paper that LandauLifshitz damping 12 provides the most natural description of the dynamics.…”