Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃
We report experimental results of the static magnetization, ESR and NMR spectroscopic measurements of the Ni-hybrid compound NiCl₃C₆H₅CH₂CH₂NH₃. In this material NiCl₃ octahedra are structurally arranged in chains along the crystallographic a axis. According to the static susceptibility and ESR data...
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| Zitieren: | Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ / F. Lipps, A.H. Arkenbout, A. Polyakov, M. Günther, T. Salikhov, E. Vavilova, H.-H. Klauss, B. Büchner, T.M. Palstra, V. Kataev // Физика низких температур. — 2017. — Т. 43, № 11. — С. 1626-1633. — Бібліогр.: 18 назв. — англ. |
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| author | Lipps, F. Arkenbout, A.H. Polyakov, A. Günther, M. Salikhov, T. Vavilova, E. Klauss, H.-H. Büchner, B. Palstra, T.M. Kataev, V. |
| author_facet | Lipps, F. Arkenbout, A.H. Polyakov, A. Günther, M. Salikhov, T. Vavilova, E. Klauss, H.-H. Büchner, B. Palstra, T.M. Kataev, V. |
| citation_txt | Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ / F. Lipps, A.H. Arkenbout, A. Polyakov, M. Günther, T. Salikhov, E. Vavilova, H.-H. Klauss, B. Büchner, T.M. Palstra, V. Kataev // Физика низких температур. — 2017. — Т. 43, № 11. — С. 1626-1633. — Бібліогр.: 18 назв. — англ. |
| collection | DSpace DC |
| container_title | Физика низких температур |
| description | We report experimental results of the static magnetization, ESR and NMR spectroscopic measurements of the Ni-hybrid compound NiCl₃C₆H₅CH₂CH₂NH₃. In this material NiCl₃ octahedra are structurally arranged in chains along the crystallographic a axis. According to the static susceptibility and ESR data Ni²⁺ spins S = 1 are isotropic and are coupled antiferromagnetically (AFM) along the chain with the exchange constant J = 25.5 K. These are important prerequisites for the realization of the so-called Haldane spin-1 chain with the spin-singlet ground state and a quantum spin gap. However, experimental results evidence AFM order at TN ≈ 10 K presumably due to small interchain couplings. Interestingly, frequency-, magnetic field-, and temperature-dependent ESR measurements, as well as the NMR data, reveal signatures which could presumably indicate an inhomogeneous ground state of co-existent mesoscopically spatially separated AFM ordered and spin-singlet state regions similar to the situation observed before in some spin-diluted Haldane magnets.
|
| first_indexed | 2025-12-01T23:54:16Z |
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11, pp. 1626–1633
Magnetic properties of the spin-1 chain compound
NiCl3C6H5CH2CH2NH3
F. Lipps1, A.H. Arkenbout2, A. Polyakov2, M. Günther3, T. Salikhov4, E. Vavilova4,
H.-H. Klauss3, B. Büchner1,3, T.M. Palstra2, and V. Kataev1
1Leibniz Institute for Solid State and Materials Research IFW Dresden, D-01171 Dresden, Germany
2Zernike Institute for Advanced Materials, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
3Institut für Festkörperphysik, TU Dresden, D-01069 Dresden, Germany
4Kazan E.K. Zavoisky Physical Technical Institute of RAS, 420029 Kazan, Russia
E-mail: v.kataev@ifw-dresden.de
Received June 1, 2017, published online September 25, 2017
We report experimental results of the static magnetization, ESR and NMR spectroscopic measurements of the
Ni-hybrid compound NiCl3C6H5CH2CH2NH3. In this material NiCl3 octahedra are structurally arranged in chains
along the crystallographic a axis. According to the static susceptibility and ESR data Ni2+ spins S = 1 are isotropic
and are coupled antiferromagnetically (AFM) along the chain with the exchange constant J = 25.5 K. These are im-
portant prerequisites for the realization of the so-called Haldane spin-1 chain with the spin-singlet ground state and a
quantum spin gap. However, experimental results evidence AFM order at TN ≈ 10 K presumably due to small
interchain couplings. Interestingly, frequency-, magnetic field-, and temperature-dependent ESR measurements, as
well as the NMR data, reveal signatures which could presumably indicate an inhomogeneous ground state of co-
existent mesoscopically spatially separated AFM ordered and spin-singlet state regions similar to the situation ob-
served before in some spin-diluted Haldane magnets.
PACS: 76.30.–v Electron paramagnetic resonance and relaxation;
76.60.–k Nuclear magnetic resonance and relaxation;
75.10.Pq Spin chain models;
75.50.Ee Antiferromagnetics.
Keywords: ESR, NMR, spin chains.
1. Introduction
Investigations of quantum magnetic phenomena in spin
networks with reduced spatial dimensions of magnetic in-
teractions is a well established and exciting field of re-
search in condensed matter physics (for reviews see, e.g.,
Refs. 1–4). In systems with reduced dimensionality quan-
tum effects become more relevant and ground states can be
established not observed in three-dimensional systems.
Ground state properties and excitation spectra depend criti-
cally on the dimensionality of the interaction, the dimen-
sionality of the spin and the interplay between different
interactions. On the experimental side, the search for reali-
zations of the spin systems where magnetic exchange be-
tween the localized spins is restricted to one (1D) or two
(2D) spatial dimensions is important for the verification of
modern theories of quantum magnetism and for the explo-
ration of novel magnetic phenomena.
Indeed, in many naturally occurring or man made solids
the magnetic interactions are restricted to less than their
three dimensions. This is the case when the crystal struc-
ture assembles in such a way that the couplings between
spins along certain directions are much stronger than along
others. There are 2D systems in which interaction takes
place predominantly between magnetic ions arranged in a
plane. In other systems magnetic ions are arranged in 1D
structures, forming so-called spin chains.
One of the important classes of spin chains is the Hal-
dane chain. This is a one-dimensional Heisenberg chain
with integer spins and antiferromagnetic (AFM) nearest-
neighbor coupling. Haldane predicted that the ground state
© F. Lipps, A.H. Arkenbout, A. Polyakov, M. Günther, T. Salikhov, E. Vavilova, H.-H. Klauss, B. Büchner, T.M. Palstra, and V. Kataev, 2017
Magnetic properties of the spin-1 chain compound NiCl3C6H5CH2CH2NH3
of such a system would be a nonmagnetic singlet state
which would be separated in energy from the excited tri-
plet state by a gap ∆ [5]. This gap is not an anisotropy
gap, but is due to the quantum nature of the = 1S system.
Haldane considered the pure Heisenberg Hamiltonian for
an easy-axis-configuration [5]. In order to explore the lim-
its of the Haldane phase bi-quadratic exchange and single-
ion anisotropy, among other parameters, can be taken into
account. Already those simple extensions reveal rich phys-
ics involved in the quasi-one-dimensional antiferromagnet-
ic integer Heisenberg spin chains. A general Hamiltonian
of the Haldane system is given in [6]:
2
1 1= [ ( ) ]i i i i
i
J + ++ β +∑ S S S S
2
B[ ( ) ].z
i i
i
D S g S Hα α+ − µ∑ (1)
Here J is the energy coupling constant between neighbor-
ing spins S . β describes the bi-quadratic exchange. Uni-
axial single-ion anisotropy is considered: With z being the
chain direction either easy axis (spin along the chain,
< 0)D or easy plane (spin perpendicular to the chain,
> 0)D is favored. The interaction with a magnetic field
H is described by the typical Zeeman term where g is
the g-factor and Bµ is the Bohr magneton.
Besides the bi-quadratic exchange the single-ion anisot-
ropy plays a crucial role for the realization of a Haldane
system. The energy gap ∆ between the singlet ground
state | 0〉 and the excited triplet state |1〉 directly depends
on the value of D. The gap is largest for the absence of
single-ion anisotropy, but an energy difference exists with-
in a certain range of D. For >D J an anisotropy gap
opens.
The first material discovered to realize the Haldane sys-
tem was Ni(C2H8N2)2–NO2ClO4 (NENP) [7]. Similar to the
system studied in this present work NENP contains the tran-
sition element Ni realizing the chain structure in an organic
matrix. NENP exhibits a single-ion anisotropy [7], which
results in the splitting of the excited triplet state [8] and with
that an anisotropic Haldane gap. However the ground state is
still the singlet state. Since then a number of other com-
pounds featuring Ni-based chains have been discovered and
investigated (for details see, e.g., Refs. 6, 9).
In the present paper which summarizes some of the re-
sults of the PhD work in Ref. 10 we report the magnetic
properties of the inorganic-organic hybrid compound with
the chemical formula NiCl3C6H5CH2CH2NH3 as revealed
by static magnetic measurements, and ESR and NMR local
probe techniques. This compound contains structurally
well isolated Ni chains where Ni2+ spins = 1S are coupled
antiferromagnetically with the isotropic exchange coupling
constant = 25.5 K.J Despite showing typical signatures
of the 1D AFM behavior in the static susceptibility and
ESR at elevated temperatures, the Ni-hybrid compound
orders AFM at 10 K.NT ≈ The occurrence of the magneti-
cally ordered ground state and not of the expected Haldane
spin-singlet state might be presumably related to the pres-
ence of residual interchain magnetic couplings. Still, the
ESR and NMR data indicate a possible competition be-
tween these two different states which could be specula-
tively interpreted in terms of the spatially inhomogeneous
ground state with coexisting AFM order and the Haldane
state in the same sample. We speculate that such an
inhomogenous ground state, that was observed before in
some spin-diluted Haldane magnets, could be a conse-
quence of a small structural disorder that promotes coupled
AFM-ordered clusters around the defects in the spin chains
which are intertwined with the chain segments still exhibit-
ing a Haldane gap.
2. Experimental details
Samples of the Ni-hybrid compound
NiCl3C6H5CH2CH2NH3 were grown in ethanol solution and
consist of an anorganic backbone of Ni atoms in the octahe-
dral environment of six chlorine atoms. The general synthesis
procedure and primary characterizations are described in
[11]. The Ni–Ni distance can be subtly varied by choosing
different organic constituents [12]. The crystallographic
structure is shown in Fig. 1. An almost perfectly symmetric
octahedron is realized, with the angle between Cl–Ni–Cl
found to be 1 86β ≈ and 2 94 .β ≈ For Ni–Cl–Ni the angle
is about 75 .γ ≈ Along the c direction the individual Ni
chains are separated by a large organic complex consisting of
a benzene structure with an amino group connected to it by
two carbon atoms. In the b direction the NiCl-octahedra are
separated directly through hydrogen bonds between chloride
and the amino group.
Static magnetization was measured with a VSM-
SQUID magnetometer from Quantum Design Inc. which
allows measurements in the temperature range from 1.8 to
325 K, in magnetic fields up to 7 T. ESR measurements
with a microwave frequency of 9.6 GHz and fields up to
0.9 T were performed using a standard Bruker EMX X-band
Fig. 1. (Color online) Crystallographic structure of the Ni-hybrid
NiCl3C6H5CH2CH2NH3. Shown are the view on the ac plane (a),
bc plane (b) and a close-up (c) of two chains where the face-
sharing octahedra are highlighted.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11 1627
F. Lipps et al.
spectrometer. It is equipped with an ESR 900 He-flow-
cryostat from Oxford Instruments, which allows measure-
ments at variable temperatures between 3.6 and 300 K.
High-field/high-frequency ESR (HF-ESR) was measured
using a homemade spectrometer which is described in de-
tail elsewhere [13]. In the latter set-up a superconducting
magnet from Oxford Instruments can generate static mag-
netic fields up to 16 T while a variable temperature insert
enables measurements between 1.6 and 300 K. For the
generation and detection of microwaves with frequencies
up to 360 GHz a vector network analyzer from ABmm was
used. ESR measurements were performed with resonant
cavities at frequencies of 9.6, 50, 83 and 93 GHz on single
crystals of the Ni-hybrid and at frequencies up to 360 GHz
on a powder sample. For the measurements at 9.6 GHz,
several single crystals were aligned on a teflon bar to in-
crease the signal/noise ratio. 35Cl NMR experiments in a
temperature range 1.5 K < T < 150 K were performed with
conventional pulse NMR techniques using a Tecmag
LapNMR spectrometer and a 16 T field-sweep supercon-
ducting magnet from Oxford Instruments. The polycrystal-
line powder was placed in a glass tube inside a Cu coil
with a frequency of the resonant circuits of 41 MHz. The
spectra were collected by point-by-point sweeping of the
magnetic field and integration of the Hahn spin echo at
each field step. The nuclear spin-lattice relaxation rate was
measured with the saturation recovery method.
3. Experimental results and discussion
3.1. Susceptibility and magnetization
Static susceptibility χ of the powder Ni-hybrid sample
as a function of temperature at an external magnetic field
of 0.01 T is shown in Fig. 2. ( )Tχ increases with decreas-
ing temperature and shows a broad peak with a maximum
around 30 K. The susceptibility then decreases down to
about 10 K. This is the expected behavior for a one-
dimensional spin system. Indeed, as can be seen in Fig. 1,
Ni atoms enclosed in an octahedron of oxygen or chlorine
are usually in the Ni2+ 3d8 configuration with an effective
spin moment of = 1S [14]. The intra-chain coupling be-
tween Ni(II)-ions is mediated by the surrounding Cl atoms
of the face sharing octahedra. The angle of 75γ ≈ indi-
cates an overlap of the Cl orbitals, thus an AFM super-
exchange is expected for the Ni ions along the chain. Thus,
from the structural point of view alone, this system seems
to be a promising candidate for a Haldane system.
For a magnetically isotropic 1D system with AFM cou-
pling the Weng equation [15] for the susceptibility of iso-
tropic S = 1 ring systems can be used to fit the temperature
dependence of the static susceptibility [16]:
2 2 2
=1 2 3
2 0.019 0.777=
3 4.346 3.232 5.834
S
B
N g
k T
β + α + α
χ
+ α + α + α
(2)
with = /( ),BJ k Tα Bk is the Boltzmann constant, and N
is the number of spins. For the fit to the static susceptibility
data (Fig. 2), the g factor was kept fixed with = 2.25g
(see ESR results below) and no temperature independent
offset 0χ was assumed. The equation reproduces well the
static susceptibility. From the fit an exchange constant of
J = 25.5 K is extracted. For a Heisenberg spin chain with
integer spin moment = 1S a Haldane gap system is pre-
dicted. In the absence of single-ion anisotropy the Haldane
gap is maximal and can be calculated from the exchange
constant as = 0.411H J∆ [6]. With J = 25.5 K a Haldane
gap of = 10.5H∆ K is expected.
In a Haldane system the ground state is a nonmagnetic
singlet state. Therefore the susceptibility should go to zero
with decreasing temperature. The static susceptibility in-
deed decreases down to temperatures around 10 K, but
below that temperature a minimum is visible followed by
an increase in the static susceptibility with decreasing tem-
peratures (Fig. 2).
Susceptibility measurements on single crystals reveal
that the ( )Tχ is isotropic down to about 10 K. Below that
temperature, the static susceptibility shows a minimum and
an anisotropic increase, different for the magnetic field
applied along the chain direction and perpendicular to it
(Fig. 2, inset). A Curie-like increase in the susceptibility is
often associated with paramagnetic impurities present in
the sample. Susceptibility from impurities could dominate
over the vanishing susceptibility of a Haldane system.
However, this cannot explain the anisotropy observed.
The magnetization as a function of applied magnetic
field was determined at temperatures of 1.8, 8, 11 and 15 K
up to 7 T (Fig. 3). While at a temperature of 15 K the mag-
netization increases linearly with the applied field, at 1.8 K
it shows a nonlinear behavior with an inflection point at
about 3.5 T. This is clearly visible in the derivative of the
magnetization in Fig. 3(b). For the intermediate tempera-
Fig. 2. Static susceptibility as a function of temperature. A broad
maximum around T ≈ 25 K is visible. Inset shows the low-
temperature susceptibility measured on single crystals with magne-
tic field parallel and perpendicular to the chain direction.
1628 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11
Magnetic properties of the spin-1 chain compound NiCl3C6H5CH2CH2NH3
tures deviations from the linear increase can already be
observed, but the effect is drastically reduced. Such an
inflection is usually associated with a spin-flop transition
of a magnetically ordered antiferromagnetic system, i.e., a
reorientation of spins in the increasing external field. This
points to an AFM ordering.
An AFM ordering is also consistent with the susceptibil-
ity data at low temperatures. For the easy axis of an antifer-
romagnet the susceptibility should go to zero, while for the
easy plane it should stay constant with decreasing tempera-
ture. The susceptibility measured on the single crystals can
be interpreted as a sum of that of an antiferromagnetic state
and that of paramagnetic impurities. The direction perpen-
dicular to the chain is the easy axis of the system. Note here
that the Haldane system Pb(Ni1–xMgx)2V2O8 orders
antiferromagnetically, upon substitutional doping of Ni with
Mg (S = 0 impurities). In contrast to the investigations in this
work, the Curie tail observed in the susceptibility of doped
PbNi2V2O8 is suppressed at the onset of AFM order [17].
This indicates that in the Ni-hybrid compound (not all) im-
purities are involved in the magnetic ordering.
The total magnetization is quite small with 0.1 /NiBM ≈ µ
at the maximum field of 7 T. For the Ni2+ ions contributing
to the magnetization a saturation field of sat = =M gS
2.25( /Ni)B= µ is expected. This is consistent with one-
dimensional Heisenberg AFM as well as Haldane systems, in
which the saturation magnetization can often not be reached
even in fields up to 40 T [17,18].
Altogether, the results of static susceptibility and mag-
netization on the Ni-hybrid samples indicate a one-
dimensional spin chain which exhibits an antiferromag-
netically ordered ground state that develops below
10NT ≈ K with the easy axis perpendicular to the chain
direction and a spin-flop transition at = 3.5cH T. A cer-
tain amount of impurities is present in the sample.
3.2. Electron spin resonance
To get a more detailed picture about the physics of the Ni-
hybrid spin chain compound ESR measurements were con-
ducted. ESR is a valuable tool to probe the local static and
dynamic magnetic properties which also can give infor-
mation about the different energy states in one-dimensional
Heisenberg antiferromagnets [8,19–23]. A single-crystalline
ESR spectrum at 9.6 GHz (X-band) at T = 25 K is shown in
Fig. 4 (inset) for external magnetic fields applied along the
chain direction and perpendicular to it. The resonance signal
exhibits a Lorentzian line around a resonance field of about
0.3 T corresponding to a g factor of g = 2.25. This g factor is
typical for a Ni2+ ion in an octahedral crystal field [14]. The
ESR signals are almost perfectly isotropic over the whole
temperature range down to about 8 K. This indicates the ab-
sence of single-ion anisotropy [D = 0in Eq. (1)] in the Ni-
hybrid compound meaning that an isotropic Heisenberg spin
chain is realized in the paramagnetic state which is very fa-
vorable for the realization of a Haldane system. The absence
of single-ion anisotropy is most likely related to the regular
octahedral ligand coordination of Ni2+. In such high sym-
metry of the ligand crystal field the = 1S state of Ni2+ re-
mains 3-fold degenerate in zero magnetic field implying the
isotropic character of the Ni spin [14]. The integrated intensi-
ty ESRI of an ESR signal is determined by the intrinsic sus-
ceptibility spinχ of the spins participating in the resonance
[14]. From Lorentzian fits of the ESR signals of the Ni-
hybrid sample spinESRI ∝ χ can be evaluated and the tem-
perature dependence of spinχ is plotted in Fig. 4. The spin
susceptibility shows a maximum around 20 K with a sharp
Fig. 3. (Color online) Magnetic field dependences of the magneti-
zation (a) and its derivative dM/dH (b) on powder sample. For T =
= 15 K a linear increase in the magnetization with magnetic field is
observed. At the lowest temperature a spin-reorientation is visible.
Fig. 4. (Color online) Temperature dependence of the spin suscep-
tibility as determined from ESR measurements around 9.56 GHz.
Inset shows the ESR spectra (absorption derivative) at T = 25 K
for magnetic fields applied parallel and perpendicular to the chain
direction.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11 1629
F. Lipps et al.
decrease at the low-temperature side. This decrease is associ-
ated with the broadening and the strong decrease in the am-
plitude of the resonance. It is clearly visible that, in compari-
son to the bulk static susceptibility (Fig. 2), the spin
susceptibility exhibits an enhanced maximum and a steeper
decrease when going to lower temperatures. From Fig. 4 it is
apparent that towards zero temperature spinχ would ap-
proach the zero value. Below 8 K the signal cannot be ob-
served at this frequency. This clear tendency towards zero
spin susceptibility indicates that the Ni spin system (giving
rise to this spectrum) seems to behave as a Haldane system
down to 8 K. This is in contrast to the observed magnetic
order at 10NT ≈ K in the static susceptibility and magnetiza-
tion measurements.
However, there can be other spin subsystems present
which cannot be observed by ESR at frequencies around
10 GHz. Especially ESR resonances associated with the
AFM ordering which was detected by susceptibility and
magnetization measurements may occur outside of the
field and the frequency ranges of the X-band spectrometer.
Thus, HF-ESR experiments at higher frequencies and
higher magnetic fields were performed as well.
Similar to the X-band results, at all higher probing
frequencies up to 360 GHz a single isotropic line is ob-
served for temperatures above 10 K. Figure 5 shows the
ESR spectra at a frequency of about 93 GHz for selected
temperatures between 20 and 2.5 K measured on one sin-
gle crystal. ESR spectra with the magnetic field perpen-
dicular to the chain direction are shown in Fig. 5(a). At
20 K only a single Lorentzian line is visible, in agreement
with the experiments at 10 GHz. With decreasing temper-
ature this line decreases in intensity and shifts to lower
fields. Interestingly, two additional lines appear at fields
of about 1 and 5 T and become stronger in intensity with
decreasing temperatures. This is a new feature not ob-
served at lower frequencies.
For the magnetic field parallel to the chain [Fig. 5(b)],
the central line also decreases, but shifts to higher fields. At
almost zero magnetic field another signal appears. However
it is not clear if the minimum of the spectrum is fully visible.
That is why the absolute value of the resonance field ex-
tracted cannot be very accurate. At around 5 T another fea-
ture is observed which appears phase-shifted with respect to
the central line. This probably does not originate from the
main crystal and could be a spurious effect due to some
fragment at another position in the resonator.
The above discussed ESR signals from single crystals of
the Ni-hybrid compound measured with the resonator-based
setups at 10–93 GHz as well as data for a powder sample
measured without resonators at higher frequencies up to
360 GHz are summarized in a frequency ν vs magnetic
field H chart in Fig. 6. The paramagnetic signals at tem-
peratures above 10 K follow a linear dependence
= ( / )Bg h Hν µ (dashed line) with the slope given by the g
factor g = 2.25. Here h is the Plank constant. The theoreti-
cal equations [24,25] for resonances of a collinear two-
sublattice antiferromagnet are shown by solid curves (for
H || easy axis) and by a dash-dot line (for H ⊥ easy axis):
H || easy axis, < :cH H
1,2 = B
a
g
H
h
µ
ν ∆ ± ; (3)
H || easy axis, > :cH H
2
2
1 2= 0, = B
a
g H
h
µ ν ν − ∆
; (4)
H ⊥ easy axis:
2
2= ,B
a
g
H
h
µ ν + ∆
(5)
where = 2.25g and the so-called magnetic anisotropy gap
at zero field = ( / )a B cg h H∆ µ with 0 = 3.5cHµ T being
the spin-flop magnetic field value obtained from magneti-
zation measurements.
As known from the susceptibility measurements on the
single crystal the easy axis is perpendicular to the chain.
The side peaks at 93 GHz for this direction [Fig. 5(a)]
roughly agree with the theoretical AFM resonance modes
(Fig. 6). A similar assignment can be made for the ESR
signals for H || chain axis as the hard-direction AFM
modes. For a powder sample, modes for both directions are
present due to the powder averaging. Notably, the signals
at all measured frequencies that follow the paramagnetic
resonance branch lose their intensity below 20 K as if
they corresponded to some excited magnetic state that gets
thermally less populated with decreasing temperature. The-
se signals are still detectable below TN ≈ 10 K where their
position shifts as if the resonating spins sensed the internal
magnetic fields caused by the antiferromagnetic order in
their vicinity.
Fig. 5. (Color online) ESR spectra at 93 GHz with magnetic field
perpendicular to the chain (a) and along the chain (b) for tem-
peratures from 20 K down to 2.5 K. While the central line de-
creases additional lines develop with decreasing temperature.
1630 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11
Magnetic properties of the spin-1 chain compound NiCl3C6H5CH2CH2NH3
Based on the ESR results, one could conjecture that in
the studied Ni-hybrid compound two spin subsystems
could be realized, the one which orders AFM at TN ≈ 10 K,
and the other one which shows signatures of thermally
activated paramagnetism. It is tempting to speculate that
the latter subsystem might develop the Haldane spin gap
and could be spatially separated but yet still coupled to the
AFM ordered subsystem.
Indeed, a coexistence between singlet quantum ground
state and classically ordered magnetic state was reported,
e.g., for the Haldane compound PbNi2V2O8 where the
spinless defects were introduced in the Ni-spin chain by Mg
doping [17,21]. The development of the AFM order was
attributed to the nucleation of the soliton-like AFM clusters
around the defect sites in the Haldane chain which couple
together due to residual interchain magnetic exchange. ESR
experiments have indicated that at small concentration of
defects Pb(Ni1–xMgx)2V2O8 ( 0.02)x ≤ develops a spatially
inhomogeneous state of co-existing large AFM ordered clus-
ters, small paramagnetic clusters, and spin-singlet Haldane
regions [21].
The Ni-hybrid compound studied in the present work
was not doped intentionally with nonmagnetic defects.
However, it is conceivable that there might be some
(small) structural disorder in the crystals, resulting in a
segmentation of the Ni-chains in fragments of different
length. At the chain ends uncompensated spins and/or
AFM correlated regions could develop and interact with
each other, and be responsible for the small Curie-like up-
turns of the static susceptibility at low temperatures and
AFM order at TN ≈ 10 K, as evidenced by the static mag-
netic and ESR measurements. On the other hand, one can-
not completely exclude the possibility that still a certain
amount of Ni-chains in the sample develop the Haldane
spin-singlet ground state which could explain the thermally
activated paramagnetic ESR signals which are detected on
the background of the AFM resonance modes.
Finally, it should be noted that ESR spectra were ob-
served in Haldane systems, which were attributed to sin-
glet-triplet transitions between the = 0S ground state and
the = 1S triplet state in NENP [20,26]. These transitions
are forbidden by the dipole selection rules. However, mix-
ing between pure = 0S and = 1S spin states is possible
through anisotropic exchange interactions or single-ion
anisotropy. Then the forbidden transitions can be observed
in an ESR experiment. For the Ni-hybrid compound ESR
data in the paramagnetic state evidence the absence of sin-
gle-ion anisotropy (D = 0). Therefore it is likely that the
mixing is too small to make an observation of the forbid-
den transitions possible.
3.3. NMR spectroscopy
Additional insights into the local magnetic properties
and the spin dynamics of the Ni-hybrid compound were
obtained by 35Cl NMR spectroscopy. The NMR spectrum
has a total width of more than 2 T and consists of two
structured peaks corresponding to two isotopes of 35Cl and
37Cl and a quadrupole background. The temperature evolu-
tion of the main line for 35Cl nuclei below a temperature of
50 K is shown in Fig. 7(a). The “two-horn” shape of the
spectrum is present approximately down to 30 K, trans-
forming with a further temperature decrease into the three
peaks structure where the additional central line shifts
gradually to higher fields towards the Larmor field. The
second isotope 37Cl line undergoes the same changes. Be-
low 10 K, the shape of the spectrum changes dramatically:
the signals identified above as the main lines for both Cl
isotopes disappear, and the total width of the spectrum
increases by approximately 1 T already at T = 8 K. Such a
sharp transformation of the NMR spectrum suggests the
occurrence of the antiferromagnetic transition around 9 K,
consistent with the Néel temperature TN ≈ 10 K determined
by the static magnetization measurements. The temperature
dependence of the nuclear relaxation rate 1
1T − [Fig. 7(c)]
measured at the midpoint of the spectrum exhibits a broad
maximum around 35 K similar to the behavior of the static
susceptibility. At TN ≈ 10 K the temperature dependence
has a weak maximum while with a further temperature
Fig. 6. (Color online) Summary of the ESR modes in the frequency
vs magnetic field plot. Open squares correspond to the isotropic
ESR signals observed in the paramagnetic regime at T > 10 K. Tri-
angles and diamonds denote the ESR modes detected at T < TN≈
≈ 10 K for the external field applied perpendicular and parallel to the
Ni-chain axis, respectively. Circles depict the signals of a powder
sample at T < TN ≈ 10 K. Dash line corresponds to the paramag-
netic branch = ( / ) ,Bg h Hν µ and the solid and dash-dot curves
represent the AFM branches for the easy- and the hard directions
of a collinear two-sublattice antiferromagnet according to
Eqs. (3), (4) and Eq. (5), respectively. Hc denotes the spin-flop
field determined by the magnetization measurements. Note that
the signals grouped around the paramagnetic branch strongly
decrease in intensity below 20 K, whereas the signals grouped
around the AFM branches appear first below TN ≈ 10 K and grow
in intensity at lower temperatures.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11 1631
F. Lipps et al.
decrease the relaxation rate drops sharply. A sharp change
in the temperature dependence at 10 K indicates a possible
phase transition at this temperature, again in agreement
with the static data, while the absence of a pronounced
maximum, which is typical for establishing the magnetic
order in 3D systems, suggests the magnetic quasi-one-
dimensionality of the Ni-hybrid compound.
The NMR measurements were performed in fields of the
order of 9 T. In such fields, the hypothetical Haldane gap,
which value at = 0H is estimated theoretically as 10.5 K,
is expected to be almost closed due to the lowering of the
energy of the = | 1zS − 〉 state of the excited S = 1 triplet.
We attempted to describe the transformation of the powder
spectra near the Néel temperature as a result of the change of
the symmetry of the local fields acting on the Cl nuclei due
to the development of the Ni spin correlations in the chain.
Though no perfect fit to the experimental spectra could be
achieved, the reasonable agreement between the model and
experiment requires an assumption of the appearance and
growth of an additional component with a different sym-
metry which is located between the components of the high-
temperature spectrum [Fig. 7(a)]. The appearance of this
signal can be speculatively explained assuming a phase sep-
aration in the chains into the regions where the growth of
AFM correlations leads to the ordering and the regions
where the Haldane nonmagnetic state could still develop
with decreasing temperature. The magnitude and symmetry
of the field at the Cl nuclei in the “Haldane-like” part of the
chains is determined by nearest magnetic regions, and the
intensity of this NMR signal increases with decreasing tem-
perature. Concomitantly, the intensity of the left and right
signals originating from the paramagnetic regions falls down
[Fig. 7(b)] despite increasing their width. Another possible
origin of this mid-signal in the NMR spectrum could be the
Cl nuclei near the chain defects, the response from which
becomes more pronounced when the Ni spins in the chains
begin to correlate. Also the behavior of the relaxation rate
1
1 ( )T T− suggests a situation more complex than a simple
AFM ordering model. In the AFM ordered state, 1
1 ( )T T− is
mainly driven by magnon scattering, leading to a power-law
temperature dependence [17,18]. In the limit where the tem-
perature is much higher than the anisotropy gap in the spin-
wave spectrum, 1
1 ( )T T− either follows a 3T dependence
due to a two-magnon Raman process or a 5T dependence
due to a three-magnon process. If the temperature is smaller
than the gap, the relaxation rate is proportional to
2 exp( / )a BT k T−∆ where a∆ is the spin-wave anisotropy
gap. In the Ni-hybrid compound the relaxation below 10 K
does not obey this combined power-exponential law. The
temperature dependence can be well described by the T3.89
power law down to 3 K suggesting that the relaxation is
mainly governed by the two-magnon process [Fig. 7(c)].
However, in order to fit also the low-temperature part of the
1
1 ( )T T− dependence, we need to add the second term, which
has an activation character with a gap of about 3.2 K
[Fig. 7(c)]. In this case the first term is proportional to T4.13
suggesting that the relaxation is likely governed mainly by
the three-magnon process. If this gap is a conjectured Hal-
dane gap, its value of 3.2 K would be presumably a bit too
large for such a high field of the measurement. Nevertheless,
within the speculative phase separation scenario one would
indeed expect that just below TN the nuclear spin relaxation
is determined by regions with a Néel order. As the tempera-
ture is lowered, this relaxation channel strongly slows down
and the contribution from the “Haldane-like” regions be-
comes noticeable.
4. Conclusions
In summary, we have studied magnetic properties of the
Ni-hybrid compound NiCl3C6H5CH2CH2NH3 by static
magnetic as well as ESR and NMR measurements. In this
material structurally well-defined Ni spin-1 antiferromag-
netic chains are realized. ESR data in the paramagnetic
state at elevated temperatures evidence an isotropic Hei-
senberg character of the Ni spins. The analysis of the tem-
perature dependence of the static susceptibility yields an
AFM intra-chain exchange interaction constant J = 25.5 K.
Though the above results suggest this Ni-hybrid compound
as a possible realization of the Haldane spin-1 AFM chain
that should develop a singlet ground state with the quan-
Fig. 7. (Color online) (a) Selected 35Cl NMR spectra (solid cir-
cles) in the temperature range 10 K < T < 50 K. Solid line repre-
sents a modelling of the three-component powder averaged spec-
tra, dash, dot and dash-dot lines show different spectral
contributions (see the text); (b) Temperature dependence of the
relative intensities of the central spectral contribution (solid cir-
cles) and of the left and right side spectral contributions [almost
coinciding up (blue) and down (green) triangles]; (c) Temperature
dependence of the 35Cl nuclear spin-lattice relaxation 1
1T − (solid
squares). Dash line represents a fit by the power law T3.89, solid
line shows a sum of the power law T4.13 and of the activation
law exp(–3.2/T) (see the text).
1632 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11
Magnetic properties of the spin-1 chain compound NiCl3C6H5CH2CH2NH3
tum spin gap = 0.411 = 10.5 K,H J∆ experimental data
show that this material orders AFM at TN ≈ 10 K and the
excitations below TN, as probed by NMR, are predomi-
nantly magnon-like. The AFM order and not the Haldane
spin-singlet ground state is possibly due to the non-
negligible interchain interactions. Interestingly, besides the
AFM resonance modes detected at T < TN, a thermally
activated paramagnetic ESR signal is observed in the spec-
tra measured at different frequencies. It could be compati-
ble with the thermally activated signal of a Haldane chain.
To explain this signal, as well as the unusual shape trans-
formation of the 35Cl NMR spectrum and the peculiar be-
havior of the nuclear spin relaxation rate, we speculate that
a spatially inhomogeneous ground state might be realized
in the Ni-hybrid compound. Assuming the occurrence of a
small amount of structural defects in the Ni-chain that cut
the spin chain in fragments of different length one could
conjecture the nucleation of the soliton-like AFM clusters
at the chain ends which couple together and order AFM.
We further speculate that besides the ordered phase there
might be mesoscopically spatially separated regions in the
sample where the Ni-chains still exhibit a Haldane spin
gap, similar to the situation which was reported for the
spin-diluted Haldane magnet Pb(Ni1–xMgx)2V2O8.
Acknowledgments
This work was supported in parts by the Deutsche
Forschungsgemeinschaft (DFG) through projects FOR912
and KA 1694/8-1, by the Dieptestrategie of the Zernike Insti-
tute for Advanced Materials, and by the Erasmus + ICM
Program of the European Union.
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11 1633
1. Introduction
2. Experimental details
3. Experimental results and discussion
3.1. Susceptibility and magnetization
3.2. Electron spin resonance
3.3. NMR spectroscopy
4. Conclusions
Acknowledgments
|
| id | nasplib_isofts_kiev_ua-123456789-175321 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0132-6414 |
| language | English |
| last_indexed | 2025-12-01T23:54:16Z |
| publishDate | 2017 |
| publisher | Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| record_format | dspace |
| spelling | Lipps, F. Arkenbout, A.H. Polyakov, A. Günther, M. Salikhov, T. Vavilova, E. Klauss, H.-H. Büchner, B. Palstra, T.M. Kataev, V. 2021-01-31T18:06:30Z 2021-01-31T18:06:30Z 2017 Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ / F. Lipps, A.H. Arkenbout, A. Polyakov, M. Günther, T. Salikhov, E. Vavilova, H.-H. Klauss, B. Büchner, T.M. Palstra, V. Kataev // Физика низких температур. — 2017. — Т. 43, № 11. — С. 1626-1633. — Бібліогр.: 18 назв. — англ. 0132-6414 PACS: 76.30.–v, 76.60.–k, 75.10.Pq, 75.50.Ee https://nasplib.isofts.kiev.ua/handle/123456789/175321 We report experimental results of the static magnetization, ESR and NMR spectroscopic measurements of the Ni-hybrid compound NiCl₃C₆H₅CH₂CH₂NH₃. In this material NiCl₃ octahedra are structurally arranged in chains along the crystallographic a axis. According to the static susceptibility and ESR data Ni²⁺ spins S = 1 are isotropic and are coupled antiferromagnetically (AFM) along the chain with the exchange constant J = 25.5 K. These are important prerequisites for the realization of the so-called Haldane spin-1 chain with the spin-singlet ground state and a quantum spin gap. However, experimental results evidence AFM order at TN ≈ 10 K presumably due to small interchain couplings. Interestingly, frequency-, magnetic field-, and temperature-dependent ESR measurements, as well as the NMR data, reveal signatures which could presumably indicate an inhomogeneous ground state of co-existent mesoscopically spatially separated AFM ordered and spin-singlet state regions similar to the situation observed before in some spin-diluted Haldane magnets. This work was supported in parts by the Deutsche Forschungsgemeinschaft (DFG) through Project Nos. FOR912 and KA 1694/8-1, by the Dieptestrategie of the Zernike Institute for Advanced Materials, and by the Erasmus + ICM Program of the European Union. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Специальный выпуск. К 80-летию со дня рождения А.И. Звягина Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ Article published earlier |
| spellingShingle | Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ Lipps, F. Arkenbout, A.H. Polyakov, A. Günther, M. Salikhov, T. Vavilova, E. Klauss, H.-H. Büchner, B. Palstra, T.M. Kataev, V. Специальный выпуск. К 80-летию со дня рождения А.И. Звягина |
| title | Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ |
| title_full | Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ |
| title_fullStr | Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ |
| title_full_unstemmed | Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ |
| title_short | Magnetic properties of the spin-1 chain compound NiCl₃C₆H₅CH₂CH₂NH₃ |
| title_sort | magnetic properties of the spin-1 chain compound nicl₃c₆h₅ch₂ch₂nh₃ |
| topic | Специальный выпуск. К 80-летию со дня рождения А.И. Звягина |
| topic_facet | Специальный выпуск. К 80-летию со дня рождения А.И. Звягина |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/175321 |
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