Site Split of Antiferromagnetic -Mn Revealed by55Mn Nuclear Magnetic Resonance

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Site Split of Antiferromagnetic α-Mn Revealed by 55Mn Nuclear Magnetic Resonance

Masahiro Manago,∗Gaku Motoyama, and Kenji Fujiwara

Department of Physics and Materials Science, Shimane University, Matsue 690-8504, Japan

Shijo Nishigori

ICSR, Shimane University, Matsue 690-8504, Japan

Katsuki Kinjo, Shunsaku Kitagawa, and Kenji Ishida

Department of Physics, Kyoto University, Kyoto 606-8502, Japan

Kazuto Akiba, Shingo Araki, and Tatsuo C. Kobayashi

Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan

Hisatomo Harima

Department of Physics, Kobe University, Kobe 657-8501, Japan

The magnetic structure of antiferromagnetic α-Mn has been unclariﬁed for almost 70 years since its mag-

netism was discovered. We measured the zero-ﬁeld nuclear magnetic resonance spectra of antiferromagnetic

α-Mn to obtain further insight into magnetism below TN=95 K. The site II spectra split into two sites with

ﬁve subpeaks owing to quadrupole interaction, and this shows that the ordered moments at site II are slightly

tilted from the [001] direction. The site III spectra revealed that this site splits into four sites below TN. These

ﬁndings clearly demonstrate that the antiferromagnetic α-Mn symmetry is lower than previously considered.

Contrary to most elemental metals, α-Mn, the stable form

of Mn metal at ambient temperature and pressure, has unique

crystallographic properties. It crystallizes into a complicated

cubic structure without an inversion center (I¯

43m, No. 217,

d) with four inequivalent Mn sites and 58 atoms in a unit

cell. α-Mn exhibits antiferromagnetic (AFM) ordering at

TN=95 K at ambient pressure. Neutron diﬀraction mea-

surements revealed the commensurate and non-collinear mag-

netic structure in the AFM state.[1, 2]. The AFM phase is

suppressed by the hydrostatic pressure at ∼1.5 GPa, and an-

other magnetic phase emerges[3–6]. This is a weak ferromag-

netic one with a small spontaneous magnetization and is sup-

pressed at 4.2 GPa. A remarkable feature is the anomalous

Hall eﬀect in the pressure-induced phase despite the absence

of large spontaneous magnetization. This eﬀect is caused by

the non-zero Berry curvature in the momentum space of non-

collinear antiferromagnets[7], which was observed in Mn3Sn

and Mn3Ge[8–10]. A key factor of these rich properties in

α-Mn is the competition of AFM interactions between neigh-

bor Mn sites arising from its unique crystal structure. Such

competing interactions are also observed in magnetically-

frustrated systems β-Mn [11], Y(Sc)Mn2[12], and Mn3P [13],

where application of hydrostatic pressure and atomic substitu-

tion can drastically alter the magnetic states. The magnetism

in α-Mn has been studied for several decades, however, it still

remains to be a fundamental subject.

The magnetic structure of α-Mn remains elusive even at

ambient pressure. Neutron diﬀraction studies revealed that

the magnetic moment is largest at site I, followed by sites

II, III, and IV[2, 14]. Sites III and IV split into two sites

in the AFM state with a tetragonal distortion of the crystal

structure[1, 2]. The site I magnetic moment points to [001],

and the site II moments slightly tilt from the [001] axis, show-

ing the non-collinear orientation. A previous nuclear mag-

netic resonance (NMR) study[15], conversely, revealed that

the magnetic structure is more complicated than expected: the

NMR spectrum arising from site II split into two sites in the

AFM state, and this suggested that site III split into four sites.

The recent NMR results [16, 17] agree with the previous re-

port. The site II split[15] suggests that the structure in the

AFM state is lower than tetragonal[18]. However, it has not

been taken into consideration in the analysis of the neutron

study[2], and thus, the true magnetic structure has not been

revealed. Because of the complex crystal structure, the mag-

netic structure of α-Mn remains unclear for almost 70 years

since the antiferromagnetism of this system was ﬁrst reported

by neutron diﬀraction[19].

In this paper, we report on the result of 55Mn zero-ﬁeld

NMR (ZF-NMR) measurements on a high-quality α-Mn sam-

ple at ambient pressure to obtain additional insight into the

AFM structure. It was identiﬁed that the two types of mo-

ments at split site II are parallel with a 6% diﬀerence in size,

and the moments direction is slightly (6°) tilted from the [001]

axis, in agreement with the previous neutron study. We clar-

iﬁed that site III split into four sites in the AFM state, with

the hyperﬁne ﬁelds varying in 2–3 T. In addition, site IV

also splits into more than two sites with complicated spec-

tra. These ﬁndings conﬁrm that the AFM α-Mn symmetry

is lower than that previously considered. This presents a key

indicator for AFM structure identiﬁcation.

The α-Mn sample was synthesized from the Pb-ﬂux method

in a horizontal conﬁguration with a temperature gradient[20]

using Mn (99.999%) and Pb (99.9999%), as in Ref. 6. A typ-

ical residual resistivity ratio between 2 and 300 K is ∼17 for

samples by this method[6], which is higher than the previous

values ∼2[3, 4]. The sample was moderately crushed into

arXiv:2210.02754v1 [cond-mat.str-el] 6 Oct 2022

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摘要：

SiteSplitofAntiferromagnetic-MnRevealedby55MnNuclearMagneticResonanceMasahiroManago,GakuMotoyama,andKenjiFujiwaraDepartmentofPhysicsandMaterialsScience,ShimaneUniversity,Matsue690-8504,JapanShijoNishigoriICSR,ShimaneUniversity,Matsue690-8504,JapanKatsukiKinjo,ShunsakuKitagawa,andKenjiIshidaDepartm...

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Site Split of Antiferromagnetic -Mn Revealed by55Mn Nuclear Magnetic Resonance

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