Physical Science

Research Frontiers 2017

X-ray magnetic circular dichroism spectroscopy
for voltage-controlled magnetic anisotropy
in FePt/MgO multilayer system
An electric field at a surface is known to exhibit
useful phenomena, such as the pinch-off phenomenon
in field-effect transistors and the induction of Motttransitions, superconducting phases, ferromagneticphases, and magnetic anisotropy. All of these
phenomena are attributable to one or both of two
factors: electron accumulation or electric dipoleinduction. In particular, electric-field control of magnetic
properties at room temperature has attracted muchattention because of its great potential for enabling theconstruction of ultralow-power-consumption electric
devices. Voltage-controlled magnetic anisotropy(VCMA) in Fe/MgO-based tunnel junctions [1] has
shown that the magnetization of nanomagnets canbe controlled in fast periods (down to 0.1 ns) byelectric fields, as indicated by bistable precessional
magnetization switching [2] and ferromagnetic
resonance excitation [3]. Because the microscopic
origin of VCMA should be purely electronic, the VCMA
effect can be an ultimate technology for the operationof spintronics devices, such as nonvolatile random
access memory, where high-speed operation with highwriting endurance is indispensable.
In this research [4], we showed a mechanism toenhance electronic VCMA proved by X-ray magnetic
circular dichroism (XMCD) spectroscopy at SPring-8
BL39XU and BL25SU beamlines. We preparedmonatomic layers of Pt at an Fe-MgO interface by
X-ray (Pt-L3, L2)
External voltage
1 mm

Cr 20 nm + Pd 10 nm
Fluorescence (Pt-Lα, Lβ )silicondriftdetector
fcc-MgO (0 0 1) 5 nm+ polymide (1500 nm)

L10-FePt (0 0 1)

bcc-Cr (0 0 1) 20 nm + fcc-Pd (0 0 1) 50 nm
fcc-MgO (0 0 1) substrate

Bottom electrode Pd2 nm

Fig. 1. Schematic of sample structureand experimental design.

422
ΔE ≅ λ' ( ΔmL,↓– ΔmL,↑ ) – 21 λ' Δ mT
4μB
2μ B Eex

L10-FePt(1)^^
Δm L,s (= mL,s – m L,s) and ΔmT (= mT – m T) express thechanges in the orbital magnetic moment and themagnetic dipole moment between the perpendicularly(^) and in-plane (//) magnetized electronic states,respectively. Here ΔmL,↓(↑) expresses the contribution
from the minority (majority) spin band. The measured^(//)^(//)+ ΔmL,↑.
orbital magnetic moment equals mL^(//) = ΔmL,↓
In this case, λ ʹ is the effective spin-orbit interaction
coefficient in the d-band.
//

Dielectric layer (MgO)

Pt: 1 atomic layer
Fe: 1 atomic layer
Pt: 1 atomic layer
Fe: 1 atomic layer

molecular-beam epitaxy in an ultrahigh vacuum
as shown in Fig. 1. Since the Pt atoms have aconsiderable amount of spin polarization becauseof their hybridization with Fe and large spin-orbit
interactions, the influence of the modified electron
orbitals of Pt on the level of magnetocrystalline
anisotropy energy (MAE) should be significant.
An L1 0 -FePt/MgO system was prepared toconduct XMCD spectroscopy as shown in Fig. 1.
X-ray absorption spectroscopy (XAS) and its XMCD
spectra were recorded in situ at the Pt L 3 and L 2
edges by detecting the X-ray fluorescence yields. In
this configuration, positive external voltages induce
electron accumulation at the Pt-MgO interface.
As shown in Fig. 2, a clear VCMA effect, that is, achange in the magnetization hysteresis curve, wasobserved. Then, conventional sum-rule analysis cancharacterize the magnetic moments from XAS/XMCD
spectra at the Pt absorption edge. The results confirm
clear external voltage induction of the effective spin
magnetic moment m S −7m T and an increase in thenumber of hole in Pt. While voltage application does
not contribute a significant difference to the orbital
magnetic moment m L, the effective spin magneticmoment increased by 13%, and the hole number of Ptin the 5d-orbital increased by 0.019 when the external
voltage was switched from positive to negative. Fromthe magnetization angle dependence, we conclude
that the voltage-induced changes in the effective spin
magnetic moment originate from voltage-inducedchanges in the magnetic dipole Tz term m T in the Pt
atom, which has been revealed by high-precision
XAS/XMCD measurements in SPring-8.
Equation 1 shows the perpendicular MAE (DE )when the spin-orbit interaction is treated in the second
order [5] to discuss the MAE.

//XMCD at Pt-L3 Edge (arb. unit)

Research Frontiers 201710−0.7 V/nm
+0.7 V/nm–1–20–10010

Perpendicular Magnetic Field (mT)

20Fig. 2. Voltage-controlled magnetic
anisotropy in FePt/MgO multilayer.

The first term is the MAE related to the orbital
magnetic moment. When the majority spin band is full,
^(//)
mL,↑
can be neglected. Then, the perpendicular MAE
is proportional to the measured changes in the orbital
magnetic moment; this relation is known as Bruno’smodel. This mechanism originates from the charge
doping inducing anisotropy in the orbital magnetic^(//)
cannotmoment as shown in Fig. 3(a). Note that mL,↑be neglected in the case of Pt because of the small
size of the exchange splitting.

The second term represents the perpendicular
MAE related to the changes in the magnetic dipole
Tz term m T. It accounts for the spin-flip excitations
between the exchange-split (Eex) majority and minority
spin bands. Because an electric field applied to ametal/dielectric is inhomogeneous because of thestrong electrostatic screening effect in metals, such
an electric field, including higher-order quadraticcomponents, can couple with the electric quadrupole
correlated to the magnetic dipole Tz term. The induced
energy splitting of each orbital changes the MAE
through spin-flip excitation, as shown by the second
term in Eq. 1 and in Fig. 3(b).
From our first-principles study, the VCMA effectin the FePt/MgO system mainly originates fromthe second term in Eq. 1 for the Pt atom, whichcorresponds to the experimentally observed changes
in the magnetic dipole Tz term m T. However, in ourFePt/MgO system, the VCMAs from the orbital
magnetic moment and the electric quadrupole partiallycancel one another. It should be noted that the VCMA
from the electric quadrupole mechanism is larger
than that of the orbital magnetic moment. However,
the total VCMA is only 9% of the VCMA from theelectric quadrupole mechanism. If a material with both
VCMAs having the same polarity is to be designed,
an electronic VCMA larger by more than a factor of 10
would be feasible [4,5].

(a) Orbital magnetic moment mechanism(Bruno model)

Shinji Miwa a,belectronferromagnet

aferromagnetCharge dopingb

Graduate School of Engineering Science, Osaka University
The Institute for Solid State Physics, The University of Tokyo

Email: miwa@issp.u-tokyo.ac.jp=Orbital magnetic moment(b)

ferromElectric quadrupole mechanism

agnet

ferromagne

tCharge redistribution(electric quadrupole)

References[1] T. Maruyama et al.: Nat. Nanotechnol. 4 (2009) 158.
[2] Y. Shiota et al.: Nat. Mater. 11 (2012) 39.
[3] T. Nozaki et al.: Nat. Phys. 8 (2012) 492.
[4] S. Miwa, M. Suzuki, M. Tsujikawa, K. Matsuda,T. Nozaki, K. Tanaka, T. Tsukahara, K. Nawaoka, M. Goto,Y. Kotani, T. Ohkubo, F. Bonell, E. Tamura, K. Hono,T. Nakamura, M. Shirai, S. Yuasa and Y. Suzuki: Nat.
Commun. 8 (2017) 15848.
[5] G. van der Laan: J. Phys. Condens. Matter 10 (1998)
3239.

=

Magnetic dipole Tz term

Fig. 3. Microscopic origin of VCMA
in ultrathin ferromagnetic metals.

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