December 11, 2015

Research Institute of Electrical Communication, Tohoku University
Center for Spintronics Integrated Systems, Tohoku University
Center for Innovative Integrated Electronic Systems, Tohoku University
WPI-Advanced Institute for Materials Research, Tohoku University

Tohoku University reveals the physics for the mechanism ofslow change in microscopic magnetic structures- Shedding light on a statistical physics of creep motion of elastic interfaces anddevelopment of high-performance magnetic memory devices -<Abstract>
The research group of Professor Hideo Ohno (Research Institute of Electrical Communication,
Center for Spintronics Integrated Systems (CSIS), Center for Innovative Integrated ElectronicSystems (CIES), WPI-Advanced Institute for Materials Research of Tohoku University) andAssociate Professor Shunsuke Fukami (CSIS, CIES) studied in detail a slow change of microscopic
magnetic structures in metallic wires induced by external driving forces, commonly called as“creep” motion, and clarified the physics regarding how the driving forces, magnetic field or electric
current, acts on the magnetic structure. Previous studies reported that the actions of magnetic fieldand current are fundamentally different for a semiconductor material, whereas they are the same fora metallic material. The present study revealed that in case when the sample satisfies a certain
condition, the current acts on the magnetic structure in a different manner from the magnetic field
case, irrespective of the intricacies of material. While a development of high-performance magnetic
memory device where the magnetic structure is manipulated by current has been intensively pursued
recently, the present findings are expected to facilitate the fundamental understanding to achieve thepractical application. This work will be published in online version of Nature Physics on December15 (JST).

<Background and challenges>
All around us, there are many phenomena in which an interface representing a boundary between
two configurations/domains gradually moves in one direction under the application of a driving
force. Such motion of the interface commonly called as “creep” motion can be observed in a varietyof physical systems ranging from fluid wetting/propagation, growth of bacterial colonies, plastic
deformation of metallic and polymer molecule materials, motion of earth’s plate, and so on. These
different physical systems can be described by a scaling law where the interface motion can be
expressed as a power of the driving force. The number of power is referred to as a scaling exponent
and this is determined from the competition of several microscopic interactions. Interestingly,
different diverse physical systems often show same value of the scaling exponent indicating theuniversality of the microscopic phenomena. In other words, systems showing the same value of thescaling exponent belong to the same universality class. Research on the universality class of the“creep” motion of the interface is thus an interesting problem in the statistical physics because themacroscopically observed results enable the understanding of the microscopic interactions.
Magnets are composed of many magnetic domains in which the direction of magnetization,
North Pole / South Pole, is aligned, and in between, a magnetic domain wall, in which themagnetization direction is gradually changed in nanometer scales, is formed. It has been known thatthe domain wall can be moved by magnetic field or electric current, and high-performance magnetic
memory devices utilizing the domain wall motion have been extensively developed. Also, recent
studies revealed that the domain wall show a “creep” motion described by the scaling law when aweak magnetic field or current is applied; however, there has no consensus on its universality class.
In particular, previous experimental results showed that the universality classes for the field- andcurrent-induced creeps are different for a magnetic semiconductor, whereas those are the same for amagnetic metal.

<Experimental method and results>
The group of Tohoku University studied the universality class of the domain wall creep driven bymagnetic field or electric current using a ferromagnetic metal CoFeB (Fig. 1). The stack structure isfrom the substrate side, Ta (0.5 nm)/ CoFeB(1.2 nm)/ MgO(1.5 nm)/ Ta(1 nm), where themagnetization is designed to point to the out of plane direction due to a specific electronic structure
formed at the CoFeB/MgO interface. They fabricated a micrometer-sized wire device throughlithography and observed domain wall motion induced by magnetic field or electric current using a
microscope with a magneto-optic effect*1.
They evaluated the domain wall velocity for various magnitudes of magnetic field or electriccurrent while keeping the device temperature constant, from which they derived the scalingexponent. The results indicated that the scaling exponent does not depend on factors such astemperature and wire width, for both magnetic field and current cases, confirming the universality ofthe observed feature (Fig. 3). Interestingly, unlike the previous study on metallic systems, they founddifferent universality classes between magnetic field and current driven domain wall creeps in thepresent metallic sample. This means that the actions of magnetic field and current on the domain
wall are fundamentally different from each other. The field-driven “creep” was found to belong to apreviously known universality class, whereas the current-driven “creep” was found to belong to adifferent universality class which cannot be explained by the present theories and the scaling
exponent was similar to the one observed in the magnetic semiconductor previously. From detailed
investigation on the behavior of the domain wall under the application of current, they found that thecurrent gives rise to an adiabatic spin-transfer torque*2 acting on the domain wall which has adifferent symmetry to the torque induced by magnetic field. In other words, it was clarified that, forsample in which stack structure is designed so that the adiabatic spin-transfer torque dominantly
affects the domain wall, universal creep characteristics appear irrespective of the nature of material,
such as metal or semiconductor, the details of microscopic structure.

<Significance of the results>
The present findings possess twofold significance from the viewpoints of fundamental physics
and device application.
Significance in terms of fundamental physics includes the fact that fundamental difference in theaction of magnetic field and current on the magnetic structure becomes clear, and it is a universal
nature being independent of the material and microscopic structure. Also, the general understandingon the creep motion of elastic interfaces becomes deepened. This is expected to give important
insight upon the understanding of the motion of magnetic flux in type II superconductors and so on.
Significance in terms of device application includes the fact that important insights for thedevelopment of magnetic domain wall motion devices were obtained. Previous studies showed thatthe thermal stability*3 of domain wall is determined independently of the critical driving current incase that the domain wall is driven by adiabatic spin-transfer torque; this property is favorable forthe applications. From the present study, the way to design the material so that the adiabatic
spin-transfer torque effectively acts on the domain wall becomes clear, which is expected to further
facilitate the device development.

This work is conducted in a Project for Research and Development of Spintronics Material andDevice Science and Technology for a Disaster-resistant Safe and Secure Society (ResearchRepresentative: Hideo Ohno), supported by Research and Development Project for ICT Key
Technology to Realize Future Societies by MEXT, and the Distributed IT System Project (ProjectLeader: Hideo Ohno): A project adopted from “Achieving Ultimate Green IT Devices with Long
Usage Times without Charging,” (Program Manager: Masashi Sahashi), a part of the “Impulsing
Paradigm Change through Disruptive Technologies Program (ImPACT),” from JST initiated by theCouncil for Science, Technology and Innovation, Cabinet Office.

<Figures>
Pulse width

MagneticfieldDefectDomain wallElectriccurrent

Domain wall motion
Current

Fig. 1: Schematic of sample. Magnetic domain wall
formed at the boundary between two domains having
opposite magnetization direction moves very slowly
under the application of magnetic field or electric
current.

Fig. 2: Example of observation result of the change in magnetic
structure induced by current pulse. The region swept by domain
wall is visualized by a microscope with a magneto-optic effect.
As the pulse width increases, the displacement of domain wall
becomes long.

Scaling exponent 1.2
1.0

= 1Field
0.80.65 mCurrent
5 m
10 m

= 0.50.4
= 0.25

0.2
0.0
260

280

300

320

340

Absolute temperature (K)360

Fig. 3: Device temperature dependence of the scaling
exponent for the field and current-induced domain wall
creep. For the current-induced creep, samples with thewidth of 5 and 10 m were used. Scaling exponent
independent of temperature and wire width indicates theuniversality. Horizontal broken lines indicate thetheoretically known scaling exponent  (= 0.25, 0.5, 1).

<Notes>*1: Magnet-optic effectPhenomena in which the polarization of transmission or reflection light is changed due to the magnetic state of thematerial or applied magnetic field.
*2: Adiabatic spin-transfer torque
A torque acting on magnetization through the adiabatic transfer of the spin angular momentum of conduction electron to
the angular momentum of orbital electrons.
*3: Thermal stability
An index to represent how stably the system can retain the state under the thermal fluctuation.

<Paper information>
S. DuttaGupta, S. Fukami, C. Zhang, H. Sato, M. Yamanouchi, F. Matsukura, and H. Ohno
“Adiabatic spin- transfer-torque-induced domain wall creep in a magnetic metal,” Nature Physics,
DOI: 10.1038/NPHYS3593 (2015).

For inquiries:*About the research
Shunsuke Fukami, Associate Professor
Center for Spintronics Integrated Systems, Tohoku University
E-mail: s-fukami <at> csis.tohoku.ac.jp*About the press
General Affairs Section
Research Institute of Electrical Communication, Tohoku University
E-mail: somu <at> riec.tohoku.ac.jp