Photon Factory Activity Report 2016 #34 (2017)

BL-16A/2016G066, 2016S2-005

Perpendicular magnetic anisotropy of the ferromagnetic semiconductor(Ba,K)(Zn,Mn)2As2 studied by angle-dependent x-ray magnetic circular dichroism
Shoya Sakamoto1,*, Guoqiang Zhao2, Goro Shibata1, Zhen Deng2, Kan Zhao2, Bijuan Chen2, Yosuke
Nonaka1, Keisuke Ikeda1, Zhendong Chi1, Yuxuan Wan1, Masahiro Suzuki1, Tsuneharu Koide3,Sadamichi Maekawa4, Yasutomo Uemura5, Changqing Jin2, and Atsushi Fujimori1
1
Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academyof Sciences, Beijing 100190, China
3Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research
Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
4Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai,
Ibaraki 319-1195, Japan
5Department of Physics, Columbia University, 538 W 129th St, New York, NY 10027, USA
1 Introduction
Ferromagnetic semiconductors (FMSs) have attractedmuch attention in the field of spintronics since thediscovery of ferromagnetism in Mn-doped III-V
semiconductors such as (Ga,Mn)As and (In,Mn)As [1]. Insuch systems, ferromagnetic interaction is mediated byholes and, hence, can be controlled externally by changing
the number of carriers through applying gate voltage orimpinging light.
Recently, a new FMS Ba1-xKx(Zn1-yMny)2As2 wassynthesized in bulk form [2], which crystalizes in thetetragonal ThCr2Si2 structure (I4/mmm) and isisostructural to 122-type Fe-based superconductors, asshown in Fig. 1. The host compound BaZn2As2 is asemiconductor with a narrow band gap of 0.2 eV. Whenholes and spins are introduced by K and Mn substitution
for Ba and Zn, respectively, a ferromagnetic ground stateis realized. With 30% of K and 15% of Mn substitution,
the Curie temperature (TC) reaches 230 K for
polycrystalline samples (60 K for single crystals) and thecarrier concentration 8 × 1020 cm-3.

Fig. 1 Crystal structure of (Ba,K)(Zn,Mn)2As2.

Because the crystal structure is inherently anisotropic,
where the Ba ions are located between the quasi-twodimensional MnAs layers, and the MnAs4 tetrahedrons areelongated to the c-axis by ~ 6% (see Fig. 1), sizable
magnetic anisotropy would be expected. In fact, large
perpendicular magnetic anisotropy was observed by
SQUID measurements, which is advantageous for futureapplications such as magnetic memories. In the presentstudy, we have investigated the magnetic anisotropy of thismaterial by means of angle-dependent x-ray magnetic
circular dichroism (AD-XMCD) measurements [3].
2 ExperimentBa0.904K0.096(Zn0.805Mn0.195)2As2 single crystals weregrown by the arc-melting solid-state reaction method. ADXMCD measurements were performed at BL-16A2 ofPhoton Factory, where we have developed a new apparatus
equipped with two pairs of superconducting magnets sothat magnetic fields up to 1 T can be applied to anydirection. Prior to the measurements, the samples werecleaved in situ to obtain clean surfaces. Absorption signals
were collected in the total-electron-yield mode andtemperature was set to 20 K. Measurement geometry isshown at the top right of Fig.2.

Fig. 2 Mn L2,3-edge XAS and XMCD spectra of
Ba0.904K0.096(Zn0.805Mn0.195)2As2. Measurement geometry isalso drawn at the top right.

Photon Factory Activity Report 2016 #34 (2017)

3 Results and Discussion
Figure 2 shows XAS and XMCD spectra of
Ba0.904K0.096(Zn0.805Mn0.195)2As2 recorded at the Mn L2,3absorption edges. The XAS and XMCD spectra look
similar to those of (Ga,Mn)As [4] and exhibit multipletfeatures. This suggests the localized nature of the Mn 3delectrons supporting the idea of carrier-induced
ferromagnetism, where itinerant holes mediate
ferromagnetic interaction between the localized spins.
However, the magnetic moments of Mn (mMn = mspin + morb)
deduced using XMCD sum rules are about 0.4 μB at H =0.7 T and T = 20 K, which is by far smaller than those of(Ga,Mn)As of 4.5 μB [4]. This implies the existence of
antiferromagnetically coupled Mn pairs or magneticallyinactive Mn atoms.
Figure 3 shows the deduced magnetic moments of Mn,
projected onto light direction, as a function of magnetic
field direction. If there is no magnetic anisotropy, wheremagnetic moments always point to the magnetic field
direction, the data are expected to follow a sine curve asshown by the blue curve in Fig. 3. However, the dataclearly deviate from the sine curve suggesting aconsiderable magnetic anisotropy in this system. Here, wehave reproduced the data using Stoner-Wohlfarth model.
In this model, the total energy of the system is expressedas [3]2𝐸 = 𝜇% 𝑀sat 𝐻 cos 𝜃. − 𝜃0 + 𝜇% 𝑀sat− 𝐾4 cos 2 𝜃. ,Projected Moment [µB/Mn]

where μ0 denotes the permeability of free space, Msat thesaturation magnetization, H the magnitude of the magnetic
field and KU the uniaxial magneto-crystalline anisotropy
energy. As drawn in Fig. 2, θM and θH represent the anglesof the magnetic moment and the magnetic field relative tothe sample normal (c-axis direction). The first term
represents the Zeeman energy, the second term the shapeanisotropy energy and the third term the uniaxial
anisotropy energy. From this equation, one can calculateθM (or the projected moment Msat cos(45°-θM)) for given θH,H, Msat and KU by minimizing the total energy E. In thisway, we have fitted the data treating K and Msat as freeparameters, and results are shown by the red curve in Fig.
3. The fit has yielded KU = 5 x 105 erg/cc and the saturation
magnetization per Mn atom mMn = 0.40 μB. The positive
value of KU means that easy axis is along the c-axis.
0.4 (Ba,K)(Zn,Mn)2As2
K 9.6%, Mn 19.5%

0.25KU =5 x 10 erg/ccmMn = 0.40 µΒ0H = 0.7 T
T = 20 K-0.2-90-60-300
θH [deg.]

HK = 2KU / MS
Hk = 0.84 T
Hk = 0 T

3060Fig. 3 Magnetic moments of Mn projected onto the lightincident direction as a function of magnetic field angle θH.

References[1] T. Dietl et al., Rev. Mod. Phys. 86, 187 (2014).
[2] K. Zhao et al., Nat. Commun. 4, 1442 (2013)[3] G. Shibata et al., arXiv:1706.05183 (2017)[4] K. W. Edmonds et al., Phys. Rev. B 71, 064418 (2005)* shoya@wyvern.phys.s.u-tokyo.ac.jp