Power spintronics–Stabilization of spinmotive force and invention of a magnetic power inverter

J. Ieda1,2), Y. Yamane1,3), and S. Maekawa1,2)
1) R. G. for Condensed Matter Theory, ASRC, JAEA 2) CREST, JST 3) IMR, Tohoku Univ.

Spin current, a flow of spin angular momentum, is themain player in spintronics, an emerging field of scienceand technology in condensed matter physics. To harness
the spin current, much experimental and theoretical effort
is devoted and many interesting phenomena have been
unveiled [1]. One of the key issues in this field is tounderstand the mutual interaction between the spincurrent and the magnetization of ferromagnets. This
interaction leads to the so-called “spinmotive force”(SMF), an electromotive force of the spin origin. The
SMF is responsible for the energy-transfer between thespin current and the magnetization, offering an importantbuilding block for the future spintronics devices.
Striking features of the SMF are listed as follows: (1)In contrast to the inductive voltage where the time
variation of a magnetic flux is required, solely a static
magnetic field can generate an electric voltage. (2) The
conversion rate is given by fundamental constants apartfrom the spin polarization of the sample, enabling highenergy-conversion efficiency. (3) Applications using thiseffect can operate as active devices with zero stand-by
power and such power-conversion ability between
magnetic and electric systems provides a uniquefunctionality in ferromagnetic nanostructures.
SMFs have been primarily demonstrated in permalloy(Ni81Fe19), a material that is known as a soft magnet.
However, the soft magnetic properties could occasionallybe a major disadvantage for a SMF demonstration; certain
magnetization profiles such as domain walls (DWs) thatare required for generating SMFs are easily disturbed byapplied magnetic fields, leading to an increased instability
in the resulting output voltage signals as shown in Fig.1(left). Therefore, the magnitude of the SMF induced by
DW motion in NiFe nanowires is limited to a few μV.
In Ref. [2], we propose a solution for the abovementioned difficulty encountered in the NiFe nanowiresby the materials design; by using high perpendicularmagnetic anisotropy (PMA) materials (hard magnets), we
have shown that rigid DW motion is achieved, and hence,
the associated SMF signals can be stabilized as shown inFig.1 (right). We have numerically confirmed that SMFs
in the order of tens of μV in a multilayered Co/Ninanowire and in the order of several hundreds of μV in an
L10-ordered FePt nanowire with the very large PMA canbe obtained. In addition, the narrow DW structure andlow mobility of a DW in PMA nanowires are suitable indownsizing of devices using SMFs.
Next, based on the above results, we have proposed abasic concept of a “magnetic power inverter,” a device
that converts DC magnetic fields to AC electric voltages,and investigated its operation characteristics using model
calculations [3]. This device consists of a magnetic
nanowire with the high PMA and the width modulation asshown in Fig.2. A DW introduced in the nanowire is
subjected to not only applied DC magnetic field HDC but
also an effective magnetic field arising from themodulation of the DW energy that is proportional to thewire width. This results in time variation of the DW
precession frequency and the associated voltage. We haveshown that by tuning the magnetic field and the wiregeometry the variable frequency ranging from MHz toGHz can be achieved. The proposed device operates as anactive element in future spin-based power electronics, orpower spintronics.

Fig.1 Numerical simulations of domain wall (DW) motion and theassociated output voltages in Ni81Fe19 (left) and Co/Ni (right).

Fig.2 Schematic illustration of a magnetic power inverter consistingof a periodically modulated magnetic nanowire with a DW.

References[1] S. Maekawa et al. Eds, Spin Current (Oxford U. Press 2012).
[2] Y. Yamane et al., Appl. Phys. Lett. 100, 162401 (2012).
[3] J. Ieda and S. Maekawa, Appl. Phys. Lett. 101, 252413(2012).