Telecommunications
Information and communication, IoT, AI hardware

Microwave spin devices utilizing heat-driven nano-magnet
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Division of Materials Physics, Graduate School of Engineering Science
Assistant Professor Minori Goto
Researchmap https://researchmap.jp/minori_goto?lang=en

Telecommunications

Abstract
We are studying the application of spins in nano-ferromagnetic materials to microwave devices by controlling them with heat.
Since ferromagnetic materials have a resonance frequency at themicrowave frequency used for communication (in this case, mainlyin the vicinity of the gigahertz band), they are expected to enablehighly sensitive detection and highly efficient output of electricalsignals at that frequency. The detection sensitivity and output
efficiency depend on the way in which the spin (direction of themagnetic pole) of the ferromagnetic material is made to resonate.
In this study, we focused on the heat of a nanoscale ferromagnetic
device called a magnetic tunnel junction. When an electric current
is applied to this device, the temperature of the ferromagnetic layer
rises due to Joule heat, and the spin direction changes. We haveshown that the spin direction moves significantly and rapidly byheat. Using this control method, we succeeded in achieving theworld's highest microwave detection sensitivity (diode sensitivity) inthe sub-gigahertz frequency band [1]. In addition, we have experimentally demonstrated for the first time in the world that a magnetic
material has a microwave amplification effect by controlling its spinwith heat [2]. By implementing this microwave amplification effectin two devices, it was found that the devices amplify each other's
microwaves and become microwave oscillators [3].

▍ Background & Results
In recent years, with the development of IoT and AI, as well as
the spread of COVID-19 infections, the importance of remotelyconnecting with various objects has increased. This has increased
the importance of information and communication elements, andthe need for higher performance of these elements. Microwaves,which are responsible for sending and receiving this information,
are detected using diode elements. In recent years, in the research
field of spintronics, which is electronics using magnets, our groupdiscovered a diode device (spin torque diode) using magnets in
2005, and in 2013, we reported a diode sensitivity that exceedsthat of semiconductor diodes.
Spin torque diodes have been expected to be applied to communication devices, IC tags, and other fields due to their highsensitivity, small size, fast tuning, low resistance, and frequency
selectivity. On the other hand, in the infrared frequency range,
bolometers, which have much higher diode sensitivity than semiconductor diode devices and spin torque diodes, have been used.
A bolometer is a device that converts incident electromagnetic
waves into heat and outputs the temperature change as a DC voltage, similar in operation to a diode device. Bolometers are used toobserve weak infrared in calorimeters, infrared imaging devices,
and astronomical observations. There are several types of bolometers, such as superconducting, graphene, and semiconductor
bolometers, and uncooled bolometers that can be used at room
temperature are known to be around 1 million V/W. However, bolometers are not used in the sub-gigahertz frequency range used

主要for
long-distance communications. Spin torque diodes operate in
the sub-gigahertz band, but no device has been reported to haveas high a diode sensitivity as the bolometer. We have succeededin achieving the world's highest microwave detection sensitivity (diode sensitivity) in the sub-gigahertz frequency band by controllingspins with high speed and high efficiency using Joule heating [1].

▍ Significance of the research and Future perspective
These results are still in the basic research stage, but we hope
to realize them as novel microwave devices. It has also been
磁石を使ったボロメータの原理を示す概略図。(a)マイクロ波を印加しない時と、(b)印
foundthat the thermal spin control efficiency can be increased by
加した時の素子の様子。赤と黒の矢印はそれぞれ金属磁石の自由層と固定層の磁極の向きを表し、自由層の磁極のみ自由に磁極が動きます。
improving
the thermal design [4], and 素子にマイクロ波が印加されると、that the spin direction canbeマイクロ波によって素子に交流電流が流れ、磁石の温度が変化して、磁極の向きが変わ
reversedby
using
fastJoule
heatpulses.
We expect that theseります。磁極の向きが変わると、素子の抵抗値が変わるため、直流電圧変化として検出
methods
can be used not only for microwave devices but also forされます。
主要spin-based
memory devices and AI hardware control methods.

Schematic
diagramshowingthe
theprinciple
principleofofa abolometerbolometerusing
usingaamagnet.
magnet. (a)(a) without
withoutmiSchematic
diagramshowing
microwaves and (b) with microwaves applied to the device. The red and black arrows
ダイオード感度と周波数に対する本研究の位置づけ。従来のボロメータとスピントルク
crowaves
with microwaves
appliedmicrowaves
indicateandthe (b)directionof the magnetic
polestoofthe
theelement.
free layerThe
andheatfixedfromlayertheof themetal
ダイオードの領域をそれぞれ青とオレンジの領域で示します。
changes
therespectively.
direction ofWhen
the magnetic
poles,and thevoltage change
is detected.
magnet,microwaves
are applied
to DC
the element,
the microwaves
cause
an alternating current to flow through the element, which changes the temperature of themagnet and changes the direction of the magnetic poles. When the direction of themagnetic poles changes, the resistance of the element changes, which is detected as achange in DC voltage.

positionof thisstudy
respect
to diode
sensitivity
andfrequency.
Theregions
TheThepositionof thisstudy
withwith
respect
to diode
sensitivity
andfrequency.
Theregions
ofofthethe conventional bolometer and spin torque diode are shown in blue and orange,
conventional
bolometer
andspintorquediodeareshown
inblueandorange,respectively.
respectively.

Schematic of the phenomenon of microwave amplification (left) and the magnetic
field dependence of the reflectivity spectra of microwave power (right). In this experiment, we succeeded in obtaining a microwave power reflectivity of more than
1.6 at an external magnetic field of 50 mT and a frequency of about 0.4 GHz.

P a t e n t[1] Goto, M; Yamada, Y; Shimura, A. et al. Uncooled sub-GHz spin bolometer driven by auto-oscillation. Nat Commun. 2021; 12(1): 536. doi: 10.1038/s41467-020-20631-0[2] Goto, M; Wakatake, Y; Oji, U, K. et al. Microwave amplification in a magnetic tunnel junction induced by heat-to-spin conversion at the nanoscale, Nat. Nanotechnol. 2018;
14: 40-43. doi: 10.1038/s41565-018-0306-9
Tr e a t i s e[3] Yamada, Y; Goto, M; Yamane, T. et al. Quasi-maser operation using magnetic tunnel junctions. Appl. Phys. Lett. 2021; 118: 192402. doi: 10.1063/5.0050151[4] Okuno, R; Yamada, Y; Goto, M. et al. Enhanced electric control of magnetic anisotropy via high thermal resistance capping layers in magnetic tunnel junctions. J PhysCondens Matter. 2020; 32(38): 384001. doi: 10.1088/1361-648X/ab94f3
https://resou.osaka-u.ac.jp/ja/research/2021/20210126_1
URL
https://resou.osaka-u.ac.jp/ja/research/2018/20181205_1
K e y w o r d heat-spin control, microwave, diode devices, magnetic tunnel junction