Solitary excitations and domain-wall movement in the two-dimensional canted antiferromagnet
„C
2
N
2
H
10
…
1/2
FePO
4
„OH…
Natasha A. Chernova,* Yanning Song, Peter Y. Zavalij, and M. Stanley Whittingham
Institute for Materials Research, State University of New York at Binghamton, Binghamton, New York 13902-6000, USA
(Received 31 March 2004; revised manuscript received 16 July 2004; published 13 October 2004)
The magnetic properties of the layered compound C
2
N
2
H
10
1/2
FePO
4
OH have been studied using dc
magnetization and ac susceptibility measurements. The compound orders as a canted antiferromagnet at T
c
= 29.6 K. The crossover in critical exponent of magnetization from
1
=0.19 to
2
= 0.30 is observed and
attributed to the change of magnetic lattice dimensionality from 2 at low temperature to 3 just below T
c
. The
formation of net magnetic moment and magnetic domains below T
c
is evidenced by an irreversible behavior of
the dc magnetization, the presence of absorption component in the ac susceptibility, and complicated relaxation
phenomena observed down to 15 K. Applying Cole-Cole analysis to the frequency dependences of dispersion
' and absorption , two relaxation processes were distinguished in the paramagnetic and ordered phases; the
temperature dependences of their relaxation times were analyzed. The faster relaxation process is described by
a scaling law above T
c
, and by a steady decrease of the relaxation time in the ordered phase. This process is
related to the existence of solitons in the magnetic chains forming the two-dimensional layers. The slower
process follows the Vogel-Fulcher law in the paramagnetic phase and the Arrhenius law with the activation
energy E
a
83 K in the ordered phase. This process is attributed to the formation of magnetic domains and
domain-wall movement. The mechanism of these processes is proposed and related to the crystal structure of
the material.
DOI: 10.1103/PhysRevB.70.144405 PACS number(s): 75.50.Ee, 75.60.Ch, 75.40.Gb, 75.40.Cx
I. INTRODUCTION
Layered transition metal compounds are of great interest
for their fundamental physical properties arising from quasi-
two-dimensional (2D) character of the metal layers,
1–3
and
for their practical applications in battery electrodes,
4
chemi-
cal sensing, separations, and catalysis
5,6
that utilize their abil-
ity to intercalate various species into the interlayer space.
Classic examples of such compounds include layered copper
oxide materials with extremely high superconducting transi-
tion temperature;
7
CuGeO
3
,
8
SrCu
2
O
3
,
9,10
and intercalated
V
2
O
5
(Ref. 11) that are low-dimensional magnets demon-
strating quantum spin phenomena; TiS
2
, the prototype lay-
ered intercalation compound, which gave rise to the devel-
opment and commercialization of lithium batteries now
employing another layered compound, LiCoO
2
.
12,13
As more
and more new compounds are synthesized in search for bet-
ter electrochemical performance in these devices, their mag-
netic and electronic properties should not remain unattended.
In this paper, the magnetic properties of layered ethylene-
diammonium templated iron phosphate hydroxide
C
2
N
2
H
10
1/2
FePO
4
OH were studied. This compound was
first synthesized by Cavellec et al.;
14
it consists of inorganic
layers built of FeO
6
octahedra and PO
4
tetrahedra separated
by ethylenediammonium cations (Fig. 1). In the b-axis direc-
tion, the FeO
6
octahedra form edge-sharing chains that are
connected through the phosphate tetrahedra into 2D layers
[Fig. 1(a)]. The magnetic transition to a canted antiferromag-
netic state was found near 30 K.
15
The spin structure in the
ordered phase of C
2
N
2
H
10
1/2
FePO
4
OH was determined
by Yang et al. using neutron diffraction.
16
It has been found
that the spins lie in the b-c crystallographic plane and the
spin alignment is nearly antiferromagnetic with the c axis as
the easy axis [the spins are shown as arrows in Figs. 1(b) and
1(c)]. However, the spins are not exactly antiparallel, each
forming 170° angle with the next spin in the chain running
along the b axis [Fig. 1(b)]. As a result of the spin canting, a
small net magnetic moment is formed along the b axis. In the
a-axis direction the FeO
6
octahedra are connected in chains
through the PO
4
tetrahedra, and a ferromagnetic alignment of
spins is realized in these chains [Fig. 1(c)].
The cause of the spin canting is a competition of antifer-
romagnetic superexchange with ferromagnetic direct ex-
change between the Fe
3+
S =5/2 ions,
16
so-called frustration.
The frustration has two far-reaching consequences. First, the
spin configuration which holds the net magnetic moment
may not be stable with respect to formation of magnetic
domains.
17–22
Second, the frustration, even disorder-free,
may cause a spin-glass-like behavior, including reentrance
into a spin-glass-like state from the ordered phase.
23–27
Ei-
ther of the above mechanisms would result in a similar
“glassy” behavior, namely, magnetic irreversibility and com-
plicated spin relaxations near and/or below the transition
temperature. These phenomena are often observed in
quasi-2D layered compounds, as their magnetic configura-
tion is likely to be frustrated by competing interactions or
geometrically, leading to a noncollinear ground state. Be-
sides, 2D systems are very sensitive to random fields, and
even infinitesimal random contribution introduced by defects
may drastically change their behavior.
2
In this work, we
study the mechanisms of the spin relaxations in
C
2
N
2
H
10
1/2
FePO
4
OH using dc magnetization to study the
irreversible magnetic behavior caused by the relaxation pro-
cesses and ac susceptibility to characterize dynamical re-
sponse of the system as a function of ac field amplitude and
frequency, temperature, and applied dc field.
28,29
PHYSICAL REVIEW B 70, 144405 (2004)
1098-0121/2004/70(14)/144405(10)/$22.50 ©2004 The American Physical Society 70 144405-1