High-field Magnetization Studies Research Articles

High-field magnetization and electronic spin resonance study in the twisted honeycomb lattice α-Mn2V2O7

We report the single-crystal growth of Mn2V2O7 and the results of magnetic susceptibility, high-field magnetization up to 55 T and high-frequency electric spin resonance (ESR) measurements for its low-temperature α phase. Two antiferromagnetic (AFM) ordering at 17.5 K and 3 K and obvious magnetic anisotropy are observed in α-Mn2V2O7 upon cooling. In pulsed high magnetic fields, the compound reaches the saturation magnetic moment of ∼10.5 μ B for each molecular formula at around 45 T after two undergoing AFM phase transitions at H c1 ≈ 16 T, H c2 ≈ 34.5 T for H//[] and H sf1 = 2.5 T, H sf2 = 7 T for H//[]. In these two directions, two and seven resonance modes are detected by ESR spectroscopy, respectively. The ω 1 and ω 2 modes of H//[] can be well described by two-sublattice AFM resonance mode with two zero-field gaps at 94.51 GHz and 169.28 GHz, indicating a hard-axis feature. The seven modes for H//[] are partially separated by the critical fields of H sf1 and H sf2, displaying the two signs of spin-flop transition. The fittings of ω c1 and ω c2 modes yield zero-field gaps at 69.50 GHz and 84.73 GHz for H//[], confirming the axis-type anisotropy. The saturated moment and gyromagnetic ratio indicate the Mn2+ ion in α-Mn2V2O7 is in a high spin state with orbital moment completely quenched. A quasi-one-dimensional magnetism with a zig-zag-chain spin configuration is suggested in α-Mn2V2O7, due to the special neighbor interactions caused by a distorted network structure with honeycomb layer.

Dimensional reduction and incommensurate dynamic correlations in the S=frac{1}{2} triangular-lattice antiferromagnet Ca3ReO5Cl2

The observation of spinon excitations in the S=frac{1}{2} triangular antiferromagnet Ca3ReO5Cl2 reveals a quasi-one-dimensional (1D) nature of magnetic correlations, in spite of the nominally 2D magnetic structure. This phenomenon is known as frustration-induced dimensional reduction. Here, we present high-field electron spin resonance spectroscopy and magnetization studies of Ca3ReO5Cl2, allowing us not only to refine spin-Hamiltonian parameters, but also to investigate peculiarities of its low-energy spin dynamics. We argue that the presence of the uniform Dzyaloshinskii-Moriya interaction (DMI) shifts the spinon continuum in momentum space and, as a result, opens a zero-field gap at the Γ point. We observed this gap directly. The shift is found to be consistent with the structural modulation in the ordered state, suggesting this material as a perfect model triangular-lattice system, where a pure DMI-spiral ground state can be realized.

High-field magnetization studies and their analysis in RFe11Ti and RFe11TiH1 rare-earth intermetallics (an example: HoFe11TiHx, x = 0 and 1)

We present experimental high-field magnetization studies for the single-crystalline ferrimagnetic RFe11Ti compounds at example of holmium-based hydride in order to evaluate, compare and analyze the crystal-field and exchange parameters. We predict theoretically the magnetization behavior of HoFe11Ti up to 80 T magnetic field for the first time. The results are compared with data for the parent compound HoFe11TiH1 and those with erbium and thulium (x = 0 and 1).

High-Field Magnetization Study of Laves Phase (Gd,Y,Sm)Fe2-H

The crystal structure and magnetic properties of the multicomponent Laves phase compounds (Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i> Y <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_z</i> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{x}$</tex-math></inline-formula> = 0, 0.2, 0.4, 0.6, 0.8, 1; <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{z}$</tex-math></inline-formula> = 0 and 3.4) are investigated. These compounds crystallize into an MgCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> type of structure. Hydrogenation does not change the crystal structure type but boosts the unit cell volume of (Gd,Y,Sm)Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> by ∼25%. Both parent and the hydrogen-charged ferrimagnetic compounds demonstrate magnetic compensation (mutual cancellation of magnetic moments of the individual magnetic sublattices). The exact compositions with zero magnetization state (compensated compositions) predicted using a three-sublattice model are in good agreement with the experiment. In the hydrides, the magnetic compensation takes place at a lower Y content as compared to the parent alloys. Pulsed magnetic fields reveal a distinctly different behavior of magnetization in the hydrides. With the increase of magnetic field, magnetization increases much faster in the hydrides as compared to the parent compounds.

Two-sublattice description of the dimer-trimer chain compound Li2Cu5Si4O14 : High-field magnetization and ESR studies

We carried out high magnetic field magnetization and electron spin resonance (ESR) measurements for the alternating dimer-trimer chain compound ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$. The magnetization curve measured at 2 K exhibits a spin-flop transition at 2.6 T followed by a nonlinear increase in magnetization at high field up to 55 T. The antiferromagnetic ordering at ${T}_{\mathrm{N}}=22\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is well characterized by the temperature-dependent ESR spectra, and the anisotropic $g$ factors at the paramagnetic phase are determined. Intriguingly, our frequency-dependent ESR spectra at 2 K reveal the development of two collinear antiferromagnetic sublattices with biaxial anisotropy, which is quite unusual because ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$ contains five sublattices. Thus, ${\mathrm{Li}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}$ is three-dimensionally ordered and its magnetism might be described by a pseudo-two-sublattice model composed of antiferromagnetically coupled dimer-trimer groups (\ensuremath{\uparrow}\ensuremath{\uparrow}, \ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\uparrow}) and (\ensuremath{\downarrow}\ensuremath{\downarrow}, \ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\downarrow}).

Magnetic phase diagram, magnetoelastic coupling, and Grüneisen scaling in CoTiO3

High-quality single crystals of ${\mathrm{CoTiO}}_{3}$ are grown and used to elucidate in detail structural and magnetostructural effects by means of high-resolution capacitance dilatometry studies in fields up to 15 T which are complemented by specific heat and magnetization measurements. In addition, we refine the single-crystal structure of the ilmenite ($R\overline{3}$) phase. At the antiferromagnetic ordering temperature ${T}_{\mathrm{N}}$ pronounced $\ensuremath{\lambda}$-shaped anomaly in the thermal expansion coefficients signals shrinking of both the $c$ and $b$ axes, indicating strong magnetoelastic coupling with uniaxial pressure along $c$ yielding six times larger effect on ${T}_{\mathrm{N}}$ than pressure applied in-plane. The hydrostatic pressure dependency derived by means of Gr\"uneisen analysis amounts to $\ensuremath{\partial}{T}_{\mathrm{N}}/\ensuremath{\partial}p\ensuremath{\approx}2.7(4)\phantom{\rule{4pt}{0ex}}\mathrm{K}/\mathrm{GPa}$. The high-field magnetization studies in static and pulsed magnetic fields up to 60 T along with high-field thermal expansion measurements facilitate in constructing the complete anisotropic magnetic phase diagram of ${\mathrm{CoTiO}}_{3}$. While the results confirm the presence of significant magnetodielectric coupling, our data show that magnetism drives the observed structural, dielectric, and magnetic changes both in the short-range ordered regime well above ${T}_{\mathrm{N}}$ as well as in the long-range magnetically ordered phase.

Magnetic and structural dimer networks in layered K2Ni ( MoO4)2

The magnetic and thermodynamic properties of layered single-crystal ${\mathrm{K}}_{2}\mathrm{Ni}{({\mathrm{MoO}}_{4})}_{2}$ having both structural and magnetic dimers have been investigated. The crystal structure of ${\mathrm{K}}_{2}\mathrm{Ni}{({\mathrm{MoO}}_{4})}_{2}$ is composed of edge-sharing ${\mathrm{NiO}}_{6}$-octahedral pairs bridged by the ${\mathrm{MoO}}_{4}^{2\ensuremath{-}}$ polyatomic ion groups in a plane, and the ${\mathrm{K}}^{+}$ ions sit in the van der Waals gap between the layers. The temperature dependence of magnetic susceptibility shows a spin-singlet ground state with an activation gap of $\mathrm{\ensuremath{\Delta}}/{k}_{B}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}38$ K. A high-field magnetization study at $T=1.5$ K exhibits a half-magnetization plateau at ${\ensuremath{\mu}}_{0}H\ensuremath{\sim}25$ T, corresponding to a level crossing of the singlet ground state with the lowest triplet state. Further, we have performed density functional theory calculations to determine magnetic exchange interactions. The nearest-neighbor coupling constant ${J}_{1}\ensuremath{\sim}10$ K between the Ni spins turns out to be an order of magnitude larger than all interdimer couplings. Our experimental and theoretical results suggest that ${\mathrm{K}}_{2}\mathrm{Ni}{({\mathrm{MoO}}_{4})}_{2}$ constitutes a nearly isolated two-dimensional $S=1$ dimer model.

Pressure and magnetic field effects on ferroelastic and antiferromagnetic orderings in honeycomb-lattice Mn2V2O7

The multiferroic nature of the honeycomb-lattice $\mathrm{M}{\mathrm{n}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ was investigated through detailed temperature, high-pressure, and magnetic-field-dependent measurements. A first-order martensiticlike structural phase transition with the thermal hysteresis associated with magnetic, heat-capacity, and dielectric anomalies was observed between ${T}_{Mh}$ (303 K) and ${T}_{Mc}$ (291 K). External pressure up to 15.41 kbar suppresses the thermal hysteresis in the magnetization data, indicating that the high-temperature \ensuremath{\beta}-phase persists down to the lower temperature under 15.41 kbar. Furthermore, isothermal capacitance-stress hysteresis loops along with crystallographic Aizu notation of $2/mF\overline{1}$ supports a martensitic phase transition driving ferroelastic ordering near or below room temperature. At low temperature, a long-range antiferromagnetic ordering was observed at ${T}_{N}\ensuremath{\sim}17\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. With increasing the external pressure up to 15.41 kbar, 100% enhancement of ${T}_{N}$ was observed and a metamagnetic transition at 5 K was enhanced near 3 T. High-field magnetization study up to 60 T induces multiple metamagnetic transitions below ${T}_{N}$. Below ${T}_{N}$, a magnetostriction induced magnetoelectric coupling was observed and further supported by the temperature-dependent x-ray studies. Taking these comprehensive research findings into account, we established that $\mathrm{M}{\mathrm{n}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ is a unique multifunctional material with the coexistence of ferroelastic and antiferromagnetic orderings and with weak magnetoelectric coupling.

High-field magnetization study of (Nd,Dy)2Fe14B: Intrinsic properties and promising compositions

The crystal-electric field and exchange parameters are determined for the (Nd0.5Dy0.5)2Fe14B compound by analyzing experimental magnetization curves obtained in high magnetic fields up to 58 T. The series (NdxDy1-x)2Fe14B compounds were analyzed theoretically to achieve a temperature stability while maintaining a large maximum energy product (BH)max. We demonstrate that computational simulations is an important tool in the materials design.

Crystal-Field and Exchange Parameters Obtained from the High-Field Magnetization of ErFe11Ti: Revisited

We perform a high-field magnetization study for the single-crystalline ferrimagnetic ErFe11Ti compound in order to redefine the crystal-field and exchange parameters. For the first time, we predict theoretically the magnetization behavior of ErFe11Ti up to 100 T magnetic field. The latter allows us to observe the full magnetization process (up to the forced-ferromagnetic state) in ErFe11Ti. The results are compared with data for the hydride ErFe11TiH1.

Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCo2F7

We report high-frequency/high-field electron spin resonance (ESR) and high-field magnetization studies on single crystals of the bond-disordered pyrochlore NaCaCo$_2$F$_7$. Frequency- and temperature-dependent ESR investigations above the freezing temperature $T_f \sim 2.4$ K reveal the coexistence of two distinct magnetic phases. A cooperative paramagnetic phase, evidenced by a gapless excitation mode, is found as well as a spin-glass phase developing below 20 K which is associated with a gapped low-energy excitation. Effective $g$-factors close to 2 are obtained for both modes in line with pulsed high-field magnetization measurements which show an unsaturated isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent magnetization in high magnetic fields, we propose an empirical model accounting for highly anisotropic ionic $g$-tensors expected for this material and taking into account the strongly competing interactions between the spins which lead to a frustrated ground state. As a detailed quantitative relation between effective $g$-factors as determined from ESR and the local $g$-tensors obtained by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still sought after, our work motivates further theoretical investigations of the low-energy excitations in bond-disordered pyrochlores.

U2Ni2Sn and the origin of magnetic anisotropy in uranium compounds

${\mathrm{U}}_{2}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sn}$ is a member of a large family of intermetallic compounds with the tetragonal $\mathrm{M}{\mathrm{o}}_{2}\mathrm{Fe}{\mathrm{B}}_{2}$ crystal structure. It orders antiferromagnetically at 25 K with propagation vector $q=(0,0,\frac{1}{2})$. Magnetization, magnetoacoustic, and neutron-diffraction experiments on a single crystal provide evidence that the uranium moments align parallel to the $c$ axis with the anisotropy energy of \ensuremath{\approx}170 K, indicating that ${\mathrm{U}}_{2}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sn}$ can be classified as an Ising system. The results are at variance with previous studies on polycrystals, which indicated different magnetic structure, and which were incompatible with the $5f\text{\ensuremath{-}}5f$ two-ion anisotropy model dominant in most U band systems. High-field magnetization studies exhibit a weak linear response for fields along the basal plane up to the highest field applied (60 T), while the $c$-axis magnetization curve exhibits three metamagnetic transitions at approximately 30, 39, and 50 T. The U magnetic moments of $0.87\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, the low magnetic entropy, and the enhanced Sommerfeld coefficient $\ensuremath{\gamma}=187\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{f}.\mathrm{u}.\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{2}$ suggest that ${\mathrm{U}}_{2}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sn}$ can be classified as an itinerant antiferromagnet with strong electron-electron correlations.

High-field phase diagram and phase transitions in hexagonal manganite ErMnO3

We report high-field magnetization study on the complex spin system $h\text{\ensuremath{-}}\mathrm{ErMn}{\mathrm{O}}_{3}$ and propose an explanation for the $c$-axis phase transitions observed at 0.8, 12, and 28 T based on an analysis of the magnetization jumps, the complete $H\text{\ensuremath{-}}T$ phase diagram and the irreducible representations of the $\mathrm{E}{\mathrm{r}}^{3+}$ ($2a$ and $4b$) spin structure. Our measurements reveal that magnetic reorientation of the representation ${\mathrm{\ensuremath{\Gamma}}}_{2}$ induces the magnetic transition at 12 T, i.e., from a one-third plateau phase to a fully polarized phase. The magnetic transition at 0.8 T and the anomaly at 28 T can be understood by magnetic reorientations of ${\mathrm{\ensuremath{\Gamma}}}_{1}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Gamma}}}_{2}$ and a reflection of the $\mathrm{M}{\mathrm{n}}^{3+}$ moments triggered by the $\mathrm{E}{\mathrm{r}}^{3+}$ spins in strong magnetic fields. These scenarios are in good agreement with the magnetization process perpendicular to the $c$ axis and the magnetic susceptibility results. These findings are helpful for a complete understanding of the magnetic phase transitions and the underlying physics in this family compounds.

High-Field Magnetization Study of $$R_{2}\hbox {Fe}_{17}\hbox {N}_{2}$$ R 2 Fe 17 N 2 ( $$R = \hbox {Ho}$$ R = Ho and Er) Nitrides

High-Field Magnetization Study of $$R_{2}\hbox {Fe}_{17}\hbox {N}_{2}$$ R 2 Fe 17 N 2 ( $$R = \hbox {Ho}$$ R = Ho and Er) Nitrides

High Field Magnetization Investigation and Mean Field Theory in Amorphous Co75Er17B8 Ribbons

Amorphous Co75Er17B8 ribbons were prepared by the melt spinning technique, and their magnetic properties were studied. Mean field theory was used to describe the temperature dependence of magnetization. High-field magnetization studies performed in magnetic fields up to 15 T have revealed a magnetic behavior typical of a non-collinear magnetic structure of Er and Co sublattices. The simulated magnetization curves show the existence of two critical fields at H cri1 = 9.5 T and H cri2 = 94.2 T, corresponding to collinear ferrimagnet, and collinear field-forced ferromagnetic behaviors. The high value of H cri2 highlights the strong antiferromagnetic interaction between Er and Co sublattices. From the non-collinear regime, the inter-subnetwork molecular field coefficients of the ferrimagnetic alloy were accurately evaluated. In addition, it is shown that the region of canted moments can be satisfactorily described by a phase diagram in the H-T plane.

High-field magnetization study of a HoFe6Al6 single crystal

Magnetization measurements have been performed in static (up to 14 T) and pulsed (up to 60 T) magnetic fields on a HoFe6Al6 single crystal with a tetragonal crystal structure of ThMn12 type. HoFe6Al6 is a ferrimagnet with exact compensation of the Ho and Fe sublattices at low temperatures, the spontaneous magnetic moment is Ms = 0 at 4.2 K. At elevated temperatures, Ms passes through a wide maximum (4.2 μB at 200 K) and vanishes at the Curie temperature, TC = 315 K. The compound displays a high magnetic anisotropy of the easy-plane type, a noticeable anisotropy exists also within the easy plane with the [110] axis as the easy magnetization direction. HoFe6Al6 exhibits two field-induced magnetic phase transitions along the easy [110] axis and two more along the [100] axis, the hard direction in the basal plane. The critical fields of the lower- and higher-field transitions display qualitatively different temperature dependencies. A T–H magnetic phase diagram has been constructed for the [100] and [110] directions. The Ho–Fe inter-sublattice exchange constant has been determined to be nHoFe = 3.8 T/μB.

High-field magnetization and ESR in the triangular-lattice antiferromagnets Ba3MnSb2O9 and Ba3TNb2O9 (T=Ni, Co)

We report high-field magnetization and electronic spin resonance (ESR) study on the triangular-lattice antiferromagnets Ba3MnSb2O9 and Ba3TNb2O9 (T=Ni, Co). The antiferromagnetic (AFM) ordering at TN is clearly characterized by the temperature dependent ESR spectra. In Ba3CoNb2O9, the complicated ESR peaks are observed above TN. No obvious frequency–field relationship could be deduced. In Ba3NiNb2O9, the AFM resonance with easy-plane anisotropy is observed, and accordingly, the magnetic exchange interactions are estimated. For Ba3MnSb2O9, the remarkable finding is that single resonance peak at paramagnetic state splits into double peaks below the ordering temperature. Possible interpretations for these unusual AFM resonance modes have been proposed.

High-field magnetization study of spin-chain compounds Ca3Co2−xMnxO6

High-field magnetization measurements of spin-chain compounds Ca3Co2−xMnxO6 reveal the presence of two metamagnetic transitions within the Mn composition range of x=1.0–0.8. These transitions are associated with spin-configuration changes from zero-field ↑↑↓↓ to ↑↑↓↑, and finally to ↑↑↑↑ along the Cotrig–(Co/Mn)oct–Cotrig–(Co/Mn)oct spin chain. For x=0.6–0.3, however, the magnetization curve presents a single metamagnetic transition, which can be understood as a ↑↓↑↑ to ↑↑↑↑ transition. The zero-field ↑↓↑↑ structure is further supported by the results of total energy calculation of x=0.5.

Susceptibility, high-field magnetization and ESR studies in a spin-5/2 triangular-lattice antiferromagnet Ba3MnSb2O9

The polycrystalline Ba3MnSb2O9 system as a spin-5/2 quasi-two dimensional triangular-lattice antiferromagnet (TLAF), is investigated by means of magnetic susceptibility, specific heat, high field magnetization and electron spin resonance (ESR) measurements. The ac susceptibility and specific heat studies reveal that it undergoes an AFM transition at TN=10.2K rather than the spin-glass one. High-field magnetization at 4.2K exhibits a weak 1/3 magnetization step near ~16.5T indicative of the field-induced quantum phase transition behavior. The analysis of temperature dependence of ESR spectra suggests that the short-range antiferromagnetic (AFM) correlation appear at temperatures well above TN. Further high-field ESR measurement up to 25T at 4.2K shows a broad resonance line, and the obtained g value is 2.094±0.012. Combined with the magnetic susceptibility and specific heat, the magnetic state below TN is explained by an AFM long rang order or a well developed AFM short range order with small anisotropy.

Field-induced magnetic ordering and single-ion anisotropy in the quasi-one-dimensional Haldane chain compound SrNi2V2O8: A single-crystal investigation

Field-induced magnetic ordering in the Haldane chain compound SrNi$_{2}$V$_{2}$O$_{8}$ and effect of anisotropy have been investigated using single crystals. Static susceptibility, inelastic neutron scattering, high-field magnetization, and low temperature heat-capacity studies confirm a non-magnetic spin-singlet ground state and a gap between the singlet ground state and triplet excited states. The intra-chain exchange interaction is estimated to be $J \sim 8.9{\pm}$0.1 meV. Splitting of the dispersions into two modes with minimum energies 1.57 and 2.58 meV confirms the existence of single-ion anisotropy $D(S^z){^2}$. The value of {\it D} is estimated to be $-0.51{\pm}0.01$ meV and the easy axis is found to be along the crystallographic {\it c}-axis. Field-induced magnetic ordering has been found with two critical fields [$\mu_0H_c^{\perp c} = 12.0{\pm}$0.2 T and $\mu_0H_c^{\parallel c} = 20.8{\pm}$0.5 T at 4.2 K]. Field-induced three-dimensional magnetic ordering above the critical fields is evident from the heat-capacity, susceptibility, and high-field magnetization study. The Phase diagram in the {\it H-T} plane has been obtained from the high-field magnetization. The observed results are discussed in the light of theoretical predictions as well as earlier experimental reports on Haldane chain compounds.