Physica B 329–333 (2003) 1215–1216

Weak ferrimagnetism, compensation point and magnetization
reversal in NiðHCOOÞ2  2H2 O
H. Kageyamaa, D.I. Khomskiib, R.Z. Levitinc, A.N. Vasilievc,*aMaterials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,Chiba 277-8581, Japanb
Solid State Physics Laboratory, Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
cPhysics Faculty, Moscow State University, Moscow 119899, Russia

AbstractNickel (II) format dihydrate NiðHCOOÞ2  2H2 O shows peculiar magnetic response at ToTN ¼ 15:5 K: The
magnitude of weak magnetic moment increases initially below TN ; equals zero at T n ¼ 8:5 K and increases again atlowering temperature. The sign of low ﬁeld magnetization at any given temperature is determined by the sample’s
magnetic prehistory and the signs are opposite to each other at ToT n and T n oToTN : This behavior suggests thatNiðHCOOÞ2  2H2 O is a weak ferrimagnet and T* is a compensation point.
r 2003 Elsevier Science B.V. All rights reserved.

The nickel (II) formate dihydrate NiðHCOOÞ2 
2H2 O crystallizes in monoclinic P21 =c spacegroup andinclude four formula units in the unit cell with a ¼( b ¼ 7:06 A;( c ¼ 9:21 A;( b ¼ 96:5 A( [1]. The8:60 A;
structure contains two nonequivalent subsystems ofNi2þ ions and the oxygen octahedrons coordinating theNi1 and Ni2 ions are tilted with respect to each otherwithin both subsystems.
The temperature dependencies of magnetization ofNiðHCOOÞ2  2H2 O powder sample taken in zero-ﬁeldcooling (ZFC) and ﬁeld-cooling (FC) regimes at H ¼
0:01 T are shown in Fig. 1. The inset to this ﬁgure shows
temperature dependence of inverse magnetic susceptibility in a wide temperature range. Evidently, the main
exchange interaction in NiðHCOOÞ2  2H2 O is antiferromagnetic. At heating in ZFC regime the sample showsﬁrstly large ‘‘paramagnetic’’ response at low temperatures, then the magnetic moment gradually decreases
with increasing temperature, changes to ‘‘diamagnetic’’at T n ¼ 8:5 K; and once again became paramagnetic
above TN ¼ 15:5 K: At subsequent cooling in FC regime
the magnetic behavior of the sample at low temperaturesappears to be mirror-like with respect to magnetization
sign as compared with ZFC measurements.
*Corresponding author.
E-mail address: vasil@lt.phys.msu.ru (A.N. Vasiliev).

The magnetization curves taken in ZFC regime in therange 2–15 K are ferromagnetic-like, i.e. they showspontaneous magnetic moment, are weakly non-linear at
low ﬁelds and tend to linear dependences at highmagnetic ﬁeld. The absolute values of magnetization
are by two orders of magnitude smaller than that
corresponding to parallel alignment of Ni2þ magneticmoments. Therefore, the experimental data presented
suggest that below TN a weakly ferrimagnetic state isrealized in NiðHCOOÞ2  2H2 O; and T n is a compensation temperature.
The analysis of the magnetic superexchange pathways
in nickel formate dihydrate shows that the nickel ions in
Ni1 -subsystem are connected to each other and to nickel
ions in Ni2 -subsystem. There are no superexchange
pathways within the Ni2 subsystem, meaning that in theabsence of Ni1 –Ni2 exchange interactions the Ni2
subsystem can be considered paramagnetic.
The model suggested for the appearance of weak
ferrimagnetism
andcompensation
pointinNiðHCOOÞ2  2H2 O is as follows. Weak ferrimagnetism
arises from the competition of weak ferromagnetic
moments of two nonequivalent antiferromagnetic Ni1and Ni2 subsystems. The compensation point is due tothe difference in temperature dependencies of weakferromagnetic moments of Ni1 and Ni2 subsystems. The
deviations from antiparallel alignment of magnetic

0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0921-4526(02)02140-3

H. Kageyama et al. / Physica B 329–333 (2003) 1215–1216

1216(mol/emu)

80
60
40

M/H (emu/mol)

-1

1.0

200.5

H=1 T
002040
T (K)

60

80

0.0ZFCFC-0.5
H=0.01 T

Ni(HCOO)22H2O

-1.0
0

1020

30

4050

the weak magnetization of Ni1 subsystem exceeds that ofNi2 subsystem. This occurs because M1 ðTÞ dependence
is much steeper than M2 ðTÞ dependence in vicinity ofTN : At further cooling the values of Ni1 and Ni2sublattice magnetizations approach each other. If
magnetic anisotropy in Ni2 subsystem is ‘‘larger’’ thanthat of Ni1 subsystem, at low temperatures the weakmagnetization of Ni2 subsystem will prevail.
In conclusion, the phenomena of weak ferrimagnetism
is found in nickel (II) formate dihydrate NiðHCOOÞ2 
2H2 O containing only one type of magnetic ions. Theseions constitute two canted antiferromagnetic subsystems
whose competition results in compensation point andmagnetization reversal.

T (K)Fig. 1. The temperature dependencies of magnetization ofNiðCOOHÞ2  2H2 O at H ¼ 0:01 T taken in zero-ﬁeld-coolingðJÞ and ﬁeld-cooling ðÞ regimes.

References

moments within these subsystems are caused by single
ion anisotropy. At cooling below the Neel temperature[1] von K. Krogmann, R. Mattes, Z. Kristallogr. 118 (1963)
291.