Photon Factory Activity Report 2022 #40 (2023)

BL-8B/2021G038

Effect of Magnetic field on crystal structure in antiferromagnetic NiO nanoparticles
Takayuki TAJIRI1,* and Masaki MITO2
Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
2Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi 804-8550, Japan

3 Results and Discussion
The crystallographic structure of the NiO bulk crystal is
the so-called rock-salt structure that undergoes atransformation from the cubic symmetry to therhombohedral one below the Néel temperature TN = 523 K[4]. We observed the powder XRD patterns for the NiOnanoparticles with d = 3.2–19.5 nm in magnetic field up
0.3 T at 300 K. The XRD patterns of all NiO nanoparticles
exhibited some broad Bragg peaks, which could be

1.001
4.3 nm

11.7 nm

1.001

0.999

a/a0

a/a0

1.000

0.998

1.000
0.999

0.997
0.998

0.996

1.001

1.004

1.000
c/c0

1.006

1.002
1.000

0.999
0.998

1.010

1.002

1.008

1.001

1.006

(c/a)/(c/a)02 Experiment
The NiO nanoparticles with particle size d = 2.6–22 nm
were synthesized in the pores of mesoporous silica [1].
Powder XRD measurements were carried out using the aDebye-Scherrer camera at room temperature in external
magnetic field up to 0.3 T. The incident X-ray energy was
16 keV and its wavelength was calibrated using the XRD
pattern of the CeO2 powder. Two facing NdFeB magnets
were used to produce the external magnetic field. The
magnetic field were changed by controlling the distance
between the sample and NdFeB magnets [3]. The XRD
patterns were analyzed to estimate the lattice constants byRietveld analysis.

attributed to the rhombohedral distorted rock-salt structure
in magnetic field up to 0.3 T. Fig. 1 shows magnetic fielddependence of lattice constants of NiO nanoparticles withd = 4.3 and 11.7 nm. The relative changes in lattice
constants increased with decreasing particle size. These
results indicated that the magnetostriction effects occurredin NiO nanoparticles and were enhanced with decreasing
particle size. The NiO nanoparticles with d < 10 nmexhibited the pronounced change in lattice constants andmagnetic properties [1]. The rhombohedral distortion c/afor 4.3 nm increased with magnetic field and its relativechange increased with decreasing particle size. Thevariation in c/a led to the change in magnetic exchangeinteractions in NiO nanoparticles. The theoretical study of
NiO nanoparticles concluded that the spin configuration ofmultiple sublattice structures changes with decreasing
particle size, leading to an increase in the net magnetic
moment [2]. The present results suggest that thepronounced changes in lattice constants for d < 10 nm were
attributed to the increases in the multiple sublattice
structures and net magnetic moment with decreasing
particle size.

c/c0

1 Introduction
Surface states of nanoparticles are changed by thedevelopment of the lattice strain owing to the existence ofedges and defects at the surface of a particle as well as thecrystallographic structure different from that of the bulk
crystal. We investigated the crystal structure and magnetic
properties of nanoparticles of an antiferromagnetic
compound NiO with particle sizes ranging from 2.6 to 22
nm. The crystal structure and magnetic properties of NiO
nanoparticles are sensitive to the particle size and haveexhibited unique size dependences [1]. The magnetic
properties of the NiO nanoparticles were explained by thecore−shell model and exhibited large shell moment at
surface of a nanoparticle. Kodama et al. calculated theequilibrium spin configurations in NiO nanoparticles,
which resulted in 8-, 6-, or 4-sublattice spin ordering withdecreasing particle size [2]. The size dependence ofmagnetic properties and crystal structure on NiO
nanoparticles indicated an existence of strong correlation
between spin configuration and crystallographic structure.
It is expected that the change in spin configuration by applyof external magnetic field result in the change in thecrystallographic structure. In this study, we investigatedthe correlation between spin configuration and crystal
structure through X-ray diffraction measurements in themagnetic field.

(c/a)/(c/a)01

1.004
1.002

1.000
0.999
0.998

1.000
0.998

0.997
01000

2000
H [Oe]

3000

01000

2000

3000

H [Oe]

Fig. 1: Magnetic field dependence of lattice constants for
NiO nanoparticle with d = 4.3 and 11.7 nm.
References[1] T. Tajiri et al., J. Phys. Chem. C 119, 1194 (2015).
[2] R. H. Kodama et al., Phys. Rev. B 59, 6321 (1999).
[3] M. Mito et al., Phys. Rev. B 105, 104412 (2022).
[4] M. T. Hutchings et al., Phys. Rev. B 6, 3447 (1972).
* tajiri@fukuoka-u.ac.jp