Photon Factory Activity Report 2022 #40 (2023)

BL-8B/2019G533, 2021G534

Magnetic ground state dependent magnetostriction effects on chiral magnet CrNb3S6
1

Masaki MITO1,* and Takayuki TAJIRI2
Kyushu Institute of Technology, Kitakyushu 804-8550, Japan2
Fukuoka University, Fukuoka 814-0180, JapanThe mono-axial chiral magnet CrNb3S6 has multiple magnetic structures, such as helimagnetic, first chiral soliton lattice (CSL-1),
second CSL (CSL-2), CSL-2 with irreversibility, and forced ferromagnetic phases below the magnetic ordering temperature (Tc). We
performed powder x-ray diffraction analyses to investigate the effects of magnetic field and temperature on the magnetostriction. The
temperature dependence of the lattice constants reveals that below Tc, the magnetostriction depends on the magnetic structure.

1 Introduction
Crystallographic chirality can be converted into a spin
system by the Dzyaloshinskii-Moriya (DM) interaction
deviated from the spin-orbit coupling (SOC) [1,2]. The
DM interaction is allowed in a chiral space group without
any rotoinversion symmetry. Further competition
between the DM and exchange interactions results in thehelimagnetic (HM) structure as the magnetic ground state
at zero dc magnetic field (H). Magnetostriction (MS),which often appears in ferromagnets [3–5], also occurs in
the magnetic materials with the DM interaction [6–10]
because of a strong magnetostructural correlation.
In a prototype of the chiral magnet CrNb3S6 with amonoaxial DM vector, two types of MS have alreadybeen observed: (1) spontaneous MS due to the exchangeinteraction and SOC at zero H [6] and (2) paramagnetic(PM) MS due to the SOC and Zeeman energy at room
temperature [7]. In CrNb3S6, the SOC originates in thehybridization between Cr and Nb [11]. These MS
phenomena accompany the change in the interatomic
distance between Cr and Nb. Thus, these MS effects inCrNb3S6 [6,7] are examples of SOC-induced MS [8–10].
However, MS in CrNb3S6 has not been in detailinvestigated over wide H and temperature T ranges.
2 Experiment
We performed powder XRD analyses at various
temperatures using a synchrotron radiation XRD systemwith a cylindrical imaging plate at the Photon Factory atthe Institute of Materials Structure Science, High Energy
Accelerator Research Organization [12]. The energy ofthe incident x-rays was 16 keV. To produce the maximum
H value of 2.2 kOe, two facing NdFeB magnets withremanence value of 13.8 and 14.5 kG were placed in thediffractometer [6]. Because the remanence of the NdFeB
magnets depends on temperature, the temperature of themagnets was measured using a K-type chromel-alumel
thermocouple. All XRD measurements were conductedunder increasing temperature in the range 92.8–294.7 K,and the temperature of the magnets was maintained
within 289.2–298.2 K.
3 Results and Discussion
Figures 1(a), 1(b), and 1(c) show the lattice constantsa, c, and the unit cell volume V for CrNb3S6, respectively,at H = 0, 0.71, 1.23, and 2.16 kOe [13]. In the PM region(130 K < T < 170 K), all of the lattice parameters changevery little, suggesting a type of Invar effect due tocompetition between thermal expansion and magnetic

shrinkage. In the HM region (T < 130K), shrinkage along
the a-axis and elongation along the c-axis occursimultaneously, so that V is almost constant. Thus, thedecrease in volume on the easy plane is canceled by theincrease in volume along the hard axis. In the presentstudy, we consider two phenomena: (1) the PM Invar
effect for T > Tc and (2) MS for T < Tc depending on themagnetic ground states.
First, we investigated the PM Invar effect by observing
the effect of H on V [Fig. 1(c)]. The Invar effect at theunit-cell level at T > Tc was also observed at H = 0.71
kOe, where the T range is almost the same as that at H =0 Oe. The constant V characteristic of the Invar effect is
not observed at H = 1.23 and 2.16 kOe. At H = 0.71
kOe, the surface is FFM and the interior is mostly HM.
The HM spin alignment is assumed to stabilize the Invar
effect due to competition between thermal expansion andmagnetic shrinkage. Further application of H reduces thestability of HM structure, such that, for T > Tc, thermalexpansion should exceed magnetic shrinkage at H = 1.23
and 2.16 kOe.
Next, the MS below Tc, depending on various
magnetic ground states, was investigated by observing theT dependence of the unit cell parameters below Tc atvarious H values. According to the trend of the lattice
constant on the ferromagnetic plane parallel to themagnetic anisotropy vector [a(T) in Fig. 1(a)], the MS at
T < 103 K can be divided into two categories; the firstoccurs at 0 Oe (HM) and 0.71 kOe (CSL-1), and thesecond occurs at 1.23 kOe (CSL-2) and 2.16 kOe [CSL-2with irreversibility, CSL-2(NL)]. The value of a at T <103K is almost the same at H = 0 and 0.71 kOe. At T =103 K, a discretely changes at H = 0.71 kOe, and for 107
K < T < Tc, the T dependence of a at H = 0.71 kOe issimilar to that at H = 2.16 kOe. At T < 110K and H =2.16 kOe, the T dependence of a is opposite to that at H =0.71 kOe. An increase in a due to MS at T < Tc wasobserved at 1.23 kOe; it stabilizes the CSL-2 state at T <120K and FFM state in narrow T region of 120 K < T <Tc. The large increase in a during warming occurs in theCSL-2 phase. Figure 1(b) shows the T dependence of thelattice constant along the hard axis, c(T); the increase at T
< Tc and H = 0 Oe changes to a discrete increase at 103
K and H = 0.71 kOe. Considering the behavior at 93 K,
the HM and CSL phases should be placed in the same
category, as suggested in the analysis of a, whereas CSL2 and CSL-2(NL) should be categorized separately. Otherfeatures have already been observed in the results for a.
The unit-cell volume V is almost constant at T < Tc andPhoton Factory Activity Report 2022 #40 (2023)

H = 0 Oe and just below 103 K at H = 0.71 and 2.16
kOe. However, at H = 1.23 kOe, the volume exhibits
thermal expansion. Because the change in c is muchlarger than that in a, the T dependence of V below Tc isqualitatively the same as that of c. In addition to a, c, andV, the ratio of c and a, c/a, provides information on theuniaxial distortion of the hexagonal unit cell in the twoMS phenomena mentioned above [13].

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* mitoh@mns.kyutech.ac.jp

Fig. 1: Lattice parameters a (a), c (b), and V (c) at H = 0,0.71, 1.23, and 2.16 kOe [13]. The position of Tc isrepresented with a broken line. The qualitative behavior
above 200 K, characterized with a straight line, in all of a,c, and V is independent of H. The T region categorized asa group is presented with red rectangle or circle.
Acknowledgement
We thank many collaborators, Dr. Y. Kousaka, Prof. J.
Akimitsu, Prof. J. Kishine, and Prof. K. Inoue, in thepresent study. This work was also supported by the Centre
for Chiral Science at Hiroshima University (the MEXT
program for promoting the enhancement of research
universities, Japan).
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