Photon Factory Activity Report 2019 #37 (2020)

BL-16A/2018P011

Magnetic reversal in Mn4-xNixN films measured via X-ray magnetic dichroism
1Taro KOMORI1, Kenta AMEMIYA2, and Takashi SUEMASU1,*
University of Tsukuba, 1-1-1 Tennohdai, Tsukuba, Ibaraki, 305-8573, Japan2
Photon Factory, Institute of Materials Structure Science,High Energy Research Organization,
1-1 Oho, Tsukuba, 305-0801, Japan

1 Introduction
Ferrimagnets have attracted increasing attention for
their applications to spintronics devices. There are someferrimagnets possessing a magnetic (angular momentum)
compensation point since their key feature is that themagnetic moments of their sublattices are aligned
antiparallel to each other. Spin transfer torque (STT) andspin orbit torque (SOT) are composed of a flux of angularmomentum carried by an incoming spin current andabsorbed by the magnetization. In ferrimagnets close to the
angular momentum compensation point, the switching iseased and the critical current densities decrease, and thus
this can be applied to current-induced domain wall motion(CIDWM) assisted by STTs and SOTs. Thus far, there havebeen many studies on ferrimagnets consisting of bothtransition-metal and rare-earth (RE) elements. For instance,
a domain wall velocity of 1.3km/s assisted by both STTsand SOTs has been achieved in Gd44Co56 at 260 K [1].
However, there have been no reports of ferrimagnets
without RE elements possessing a compensation
temperature close to room temperature (RT). Under suchcircumstances, we have focused on RE-free ferrimagnet
Mn4N films [2-4].
Figure 1 shows the crystal structure of Mn4N. The cornerand face-centered atomic sites are labelled as I and II,respectively. The II sites are further divided into IIA andIIB sites in the presence of magnetization (arrow in Fig. 1).
N atoms are at the body-centered sites. Neutron diffraction
measurement shows the magnetic moments of the Mn
atoms in bulk Mn4N to be 3.53 μB at I sites and 0.89 μB at
II sites with the easy magnetization axis along the [111]direction [5,6], where μB is the Bohr magneton. Mn4N
films have perpendicular magnetic anisotropy (PMA) withrelatively a large magnetic anisotropy constant ofapproximately 105 J/m3. We have recently achieved avelocity of approximately 900 m/s with a current densityof 1.2×1012 A/m2 at RT [7], which is a record velocity for
CIDWM only by STT. Thus, mixed crystal nitride films
based on Mn4N are expected to have both PMA with low
saturation magnetization (MS) and a compensation point,
leading to more efficient domain wall motion. Comparedwith Mn4N, Mn4-xNixN films have shown an anomalous
decrease in MS of 45% at x = 0.1 and 75% at x = 0.25, while
the PMA is preserved. It is noted that the tensile stress ispresent in the Mn4-xNixN films with PMA. We further
concluded that Mn4-xNixN films have a magnetic
compensation composition at RT because the signs of theanomalous Hall resistivity and MS temperaturedependences were different between x = 0.1 and 0.25 [8].
The latter two trends were also similar to those of Mn4xInxN and Mn4-xSnxN, in which Mekata demonstrated theexistence of a magnetic compensation point [9]. However,
the magnetic structures and preferential occupation sites ofNi atoms in Mn4-xNixN epitaxial films are not yet known.
X-ray absorption spectroscopy (XAS) and X-ray magnetic
circular dichroism (XMCD) are powerful techniques fordetermining element-specific electronic structures.
In this work, we performed XAS and XMCD
measurements on Mn4-xNixN (x = 0.1, 0.25) epitaxial films,
and proposed a model describing the magnetic moments ofNi and Mn atoms below and above the Ni magnetic
compensation composition in Mn4-xNixN films.
I
I
IIA

IIA

I

IIB

N

I
IIA
IIA

I
I

I

IIB

IFig. 1: Crystal structure of Mn4N. Face-centered sites (II)can be further divided into IIA and IIB sites withmagnetization (arrow).
2 Experiment
We grew approximately 10-nm-thick Mn4−xNixN (x = 0.1and 0.25) epitaxial films on SrTiO3[STO](001) substrates
by molecular beam epitaxy, using high-temperature
Knudsen cells with solid Mn and Ni sources and a radiofrequency N plasma source [4]. After growth, the samples
were capped in situ with Ti to prevent oxidation. XAS andXMCD measurements were performed at the twin APPLEII undulator beam line BL-16A of KEK in Japan. Both amagnetic field of ±3 T and circularly polarized x rays withleft or right polarization were applied perpendicularly tothe plane. The x-ray polarization was switched at every
energy point with a frequency of 10 Hz using five kicker
magnets. Spectra were acquired at RT in total electron
yield mode and total fluorescence yield mode at the Mn
L2,3 and Ni L2,3 absorption edges, respectively, in order toprevent Ni L2,3 spectra from overlapping with backgroundsignals.

Photon Factory Activity Report 2019 #37 (2020)3 Results and Discussion
Figures 2(a)(e) and 2(b)(f) show the XAS and XMCD
spectra of Mn3.9Ni0.1N and Mn3.75Ni0.25N films at the Ni L2,3and Mn L2,3 absorption edges, respectively. Figures 2(c)and 2(d) are those for Ni3FeN and NiFe3N films at the Ni
L2,3 absorption edges, respectively. Concerning the XAS
spectra of Mn3.9Ni0.1N and Mn3.75Ni0.25N films in Figs. 2(a)and 2(b), only the sharp main peaks appear at the L3 edge(852853 eV) and the L2 edge (870 eV). In contrast, we see
shoulders in the XAS spectrum of Ni-rich Ni3FeN films in
Fig. 2(c) [10].
0.6Ni-L2,3 edges in
Mn3.9Ni0.1N

L30.2

L2A

A’

0.04

B

0.02
0.00
-0.02

830

840

850

860

870

880

890

900

A’
0.025
0.000-0.025

B
840

850

860

870

880

890

900

0.6(c)

Ni-L2,3 edges in
Ni3FeN

L30.4

XAS (a.u.)

A’A L20.2

Ni-L2,3 edges in
NiFe3NL3A

L20.2

A’

0.00
-0.04

B-0.08-0.12

850

860

870

890

900

C0.2L2C’β
α0.000-0.005-0.010630

640(f)

650660

Photon energy (eV)

670

840

850

860

870

880

890

900

Mn-L2,3 edges in
Mn3.75Ni0.25N

L30.4

C0.2
0.0

0.005

-0.2
-0.3

830
0.6

Mn-L2,3 edges in
Mn3.9Ni0.1N

L3

0.4

0.0880XAS (a.u.)

(e)

840

B

XMCD (a.u.)

830

0.1
0.0-0.1

L2C’

α0.005β0.000-0.005

630

XMCD (a.u.)

0.00.04

XMCD (a.u.)

0.0(d)
0.4

640

650

660

XMCD (a.u.)

XAS (a.u.)

L2A8300.6

XAS (a.u.)

0.2
0.0

0.06

0.6

Ni-L2,3 edges in
Mn3.75Ni0.25N

L3

XMCD (a.u.)

0.0(b)
0.4

XMCD (a.u.)

(a)
0.4

XAS (a.u.)

XAS (a.u.)

0.6670

Photon energy (eV)Fig. 2: XAS (blue) and XMCD (black) spectra in (a)(e)Mn3.9Ni0.1N films and (b)(f) Mn3.75Ni0.25N films at Ni-L2,3and Mn-L2,3 absorption edges, respectively. (c) and (d) arethose for Ni3FeN and NiFe3N films at Ni-L2,3 absorption
edges.
Such shoulders are observed approximately 2 eV above theNi L2,3 main peaks, and are reported for otherantiperovskite ferromagnetic nitrides such as Fe4N andCo4N films above the Fe (Co) L2,3 main peaks [11,12].
These are interpreted to originate from the electric dipole
transition from the metal 2p core level to the hybrid state
between the orbitals of the N 2p and metal 3d at II sites.
These results indicate that the objective element is at IIsites [11]. However, we don’t see these shoulders in theXAS spectrum of NiFe3N films in Fig. 2(d). It is reported
that in NixFe4-xN, the preferential sites of Ni atoms
determined by XAS measurements agree well with thoseobtained via Mössbauer measurements and first-principles
calculations based on the configuration-dependent totalenergy [10]. Thus, we have a reasonable confidence in theresults obtained for Mn4-xNixN as well. From these results,
it can be safely stated that Ni atoms preferentially occupy
the I sites in Mn4N when x ≤ 0.25. In addition to theshoulders mentioned above, Figs. 4(a)4(d) show otherdistinct satellites marked as A and A’. Satellites A and A’
are observed at approximately 7 eV above the Ni L3 and L2edges. We initially considered their origin to be the hybrid
state between the Ni 3d and O 2p orbitals. However, these
are rather close to the satellite positions reported for the Ni
L2,3 edges of NixFe4-xN.
In contrast to XAS spectra, the XMCD spectra for Ni
are simple. The peak signs are opposite at the L2,3 edges
between Mn3.9Ni0.1N and Mn3.75Ni0.25N in Figs. 2(a) and2(b). Thus, the directions of magnetic moments of Ni(I) in
Mn3.9Ni0.1N films are different with that in Mn3.75Ni0.25N
films with respect to magnetizations. Regarding satellites
B in Figs. 4(a)-4(d), it has been found only experimentally.
This can be understood by considering that satellite B
results from the interaction between the final-state
multiples described with 2p53d9 and 2p53d10. Although no
distinct satellite is observed in the XMCD spectra above
the Ni L2 edges, it has been experimentally confirmedelsewhere [13]. We conclude that the satellite intensity is
weak owing to the small atomic ratio of Ni, and that thesatellites are buried in background signals.
We next move on to the XAS and XMCD spectra ofMn3.9Ni0.1N (Fig. 4(e)) and Mn3.75Ni0.25N films (Fig. 4(f))
at the Mn L2,3 absorption edges. Shoulders marked byarrows and satellites (C,C’) are observed at approximately
2 eV and 3.5 eV, respectively, above the Mn L2,3 main
peaks of the Mn3.9Ni0.1N and Mn3.75Ni0.25N films. Note thatsimilar features were observed in the XAS spectra of Mnrelated oxides such as MnFe2O4 [14], and are characterized
by the final state multiples of the 2p53d6 configurationfrom a highly localized Mn2+ ground state.
The XMCD spectra of these films also show somecomplexity. The signs of the XMCD signals around the Mn
L3 edge are positive, negative, and positive from low tohigh photon energy in Mn3.9Ni0.1N. The superposition ofspectra with different signs indicates that the magnetic
moments of Mn(I) and Mn(II) are aligned antiparallel toeach other. In our previous studies on Fe4N films, thetransition metals at the I(II) site exhibit localized (itinerant)
states supported by the first-principles calculation using
the all-electron full-potential linearized augmented-planewave (FLAPW) method and Fermi’s golden rule with E1
transitions [15]. Similar calculation results are also
supported for Mn4N by the FLAPW calculation [16].
Therefore, the XMCD spectra of Mn3.9Ni0.1N andMn3.75Ni0.25N films can also be viewed as an overlap ofsuch localized and itinerant components characterized byfeatures α and β, respectively, with opposite signs. InPhoton Factory Activity Report 2019 #37 (2020)

Mn3.9Ni0.1N, the broad positive peak (β) near 642 eV likely
originates from Mn(II) atoms because of the hybridization
between Mn(II) 3d and N 2p orbitals. In contrast, the sharp
negative peak (α) near 640 eV likely comes from Mn(I),which is remote from N, and therefore entails lesshybridization. Note that the signs of the XMCD signals
near the Mn L3 absorption edges are negative, positive, andnegative from low to high photon energy in Mn3.75Ni0.25N,
which is the opposite of the results obtained for theMn3.9Ni0.1N films. We ascribe this to magnetization
reversal at both I and II sites. It is also notable that theXMCD signal at the Ni L3 absorption edge has the opposite
sign of the sharp peak for the Mn L3 edge at 640 eV
regardless of x, and has the same sign as that of the broad
spectrum at 642 eV. The former mainly comes from Mn(I)atoms and the latter from Mn(II) atoms. Thus, the magnetic
moments of Ni(I) always align in parallel with those ofMn(II) or in antiparallel with those of Mn(I) in the studied
range of x.
On the basis of these discussions, we conclude that thepreferential occupation site of Ni is the I site in Mn4-xNixN
thin films, and that the magnetic moments of Ni atoms areparallel to those of Mn(II). As mentioned in Fig. 1, Mn4N
is a ferrimagnetic material in which the magnetic momentsof the Mn(I) atoms are aligned parallel to the magnetic
field H and antiparallel to the moments of the Mn(II) atoms.
The magnetic moments of the Mn(I) sites thus determine
the direction of MS. For x = 0.1, some Mn(I) atoms arereplaced with Ni atoms, and the magnetic moments of theNi(I) atoms are parallel to those of Mn(II). This results ina net decrease in MS [8], and the Mn(I) atoms still
determine the direction of MS. When x is increased furtherto 0.25, more Mn(I) atoms are replaced with Ni atoms tothe extent where the sum of the magnetic moments aligned
antiparallel to magnetic field becomes greater than that ofthose aligned parallel to it. In this state, the net
magnetization and magnetic field are antiparallel, causing
an unstable Zeeman energy state. Thus, all magnetic
moments need to flip to achieve the stable state. As a result,the magnetic moments of the Mn(II) sites determine thedirection of MS at x = 0.25. Therefore, we can safely statethat the magnetic compensation occurs at a value of x
between 0.1 and 0.25.
Acknowledgement
This work was supported by Grants-in-Aid for Scientific
Research from the Japan Society for the Promotion ofScience (Nos. 19K04499, 19K21954, and 19KK0104).
The XMCD experiment was performed at beam line BL16A of KEK-PF with the approval of the Photon Factory
Program Advisory Committee (Proposal No. 2018P011).

References[1] L. Caretta et al., Nat. Nanotechnol. 13, 1154 (2018).
[2] Y. Yasutomi et al., J. Appl. Phys. 115, 17A935 (2014).
[3] K. Ito et al., AIP Adv. 6, 056201 (2016).
[4] T. Komori et al., J. Cryst. Growth 507, 163 (2019).
[5] W. J. Takei et al., Phy. Rev. B 125, 1893 (1962).
[6] C. Li et al., J. Alloys. Compd. 457, 57 (2008).
[7] T. Gushi et al., Nano Lett. 19, 8716 (2019).
[8] T. Komori et al., J. Appl. Phys. 125, 213902 (2019).
[9] M. Mekata, J. Phys. Soc. Jpn. 17, 5 (1962).
[10] F. Takata et al., Phys. Rev. Mater. 2, 024407 (2018).
[11] K. Ito et al., J. Appl. Phys. 117, 193906 (2015).
[12] K. Ito et al., J. Appl. Phys. 115, 11C712 (2014).
[13] T. Jo and G. A. Sawatzky, Phys. Rev. B 43, 8771
(1991).
[14] J.-S. Kang et al., Phys. Rev. B 77, 035121 (2008).
[15] K. Ito et al., J. Appl. Phys. 117, 193906 (2015).
[16] K. Ito et al., Phys. Rev. B 101, 104401 (2020).

* suemasu@bk.tsukuba.ac.jp