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Incommensurate short-range order in the S=1 triangular antiferromagnet NiGa2S4
Authors:
C. Stock,
S. Jonas,
C. Broholm,
S. Nakatsuji,
Y. Nambu,
K. Onuma,
Y. Maeno,
J. -H. Chung
Abstract:
Neutron scattering is used to investigate spin correlations in ultra pure single crystals of the S=1 triangular lattice NiGa2S4. Despite a Curie-Weiss temperature of Theta_CW=-80(2) K, static (tau > 1 ns) short range (xi_ab=26(3) Å) incommensurate order prevails for T>1.5 K. The incommensurate modulation Q_0=(0.155(3),0.155(3),0), Theta_CW, and the spin wave velocity (c=4400 m/s) can be accounted…
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Neutron scattering is used to investigate spin correlations in ultra pure single crystals of the S=1 triangular lattice NiGa2S4. Despite a Curie-Weiss temperature of Theta_CW=-80(2) K, static (tau > 1 ns) short range (xi_ab=26(3) Å) incommensurate order prevails for T>1.5 K. The incommensurate modulation Q_0=(0.155(3),0.155(3),0), Theta_CW, and the spin wave velocity (c=4400 m/s) can be accounted for by antiferromagnetic third-nearest neighbor interactions J_3=2.8(6) meV and ferromagnetic nearest neighbor coupling J_1=-0.35(9) J_3. Inter-plane correlations are limited to nearest neighbors and weakened by an in-plane field. These observations show that the short range ordered glassy phase that has been observed in a number of highly degenerate systems, can persist near the clean limit.
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Submitted 16 June, 2010;
originally announced June 2010.
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Successive phase transitions and phase diagrams of the quasi-two-dimensional triangular antiferromagnet Rb4Mn(MoO4)3
Authors:
Rieko Ishii,
Shu Tanaka,
Keisuke Onuma,
Yusuke Nambu,
Masashi Tokunaga,
Toshiro Sakakibara,
Naoki Kawashima,
Yoshiteru Maeno,
Collin Broholm,
Dixie P. Gautreaux,
Julia Y. Chan,
Satoru Nakatsuji
Abstract:
Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified that Rb4Mn(MoO4)3 is a model system of a quasi-2D triangular Heisenberg antiferromagnet with an easy-axis anisotropy, exhibiting successive transitions across an intermediate collinear phase. As a rare case for geometrically frustrated magnetism, quantitative agreement between experiment and theory is…
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Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified that Rb4Mn(MoO4)3 is a model system of a quasi-2D triangular Heisenberg antiferromagnet with an easy-axis anisotropy, exhibiting successive transitions across an intermediate collinear phase. As a rare case for geometrically frustrated magnetism, quantitative agreement between experiment and theory is found for complete, anisotropic phase diagrams as well as magnetic properties.
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Submitted 24 December, 2009;
originally announced December 2009.
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Spin dynamics and spin freezing behavior in the two-dimensional antiferromagnet NiGa$_{2}$S$_{4}$ revealed by Ga-NMR, NQR and $μ$SR measurements
Authors:
H. Takeya,
K. Ishida,
K. Kitagawa,
Y. Ihara,
K. Onuma,
Y. Maeno,
Y. Nambu,
S. Nakatsuji,
D. E. MacLaughlin,
A. Koda,
R. Kadono
Abstract:
We have performed $^{69,71}$Ga nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) and muon spin rotation/resonance on the quasi two-dimensional antiferromagnet (AFM) NiGa$_2$S$_4$, in order to investigate its spin dynamics and magnetic state at low temperatures. Although there exists only one crystallographic site for Ga in NiGa$_2$S$_4$, we found two distinct Ga signals by…
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We have performed $^{69,71}$Ga nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) and muon spin rotation/resonance on the quasi two-dimensional antiferromagnet (AFM) NiGa$_2$S$_4$, in order to investigate its spin dynamics and magnetic state at low temperatures. Although there exists only one crystallographic site for Ga in NiGa$_2$S$_4$, we found two distinct Ga signals by NMR and NQR. The origin of the two Ga signals is not fully understood, but possibly due to stacking faults along the c axis which induce additional broad Ga NMR and NQR signals with different local symmetries. We found the novel spin freezing occurring at $T_{\rm f}$, at which the specific heat shows a maximum, from a clear divergent behavior of the nuclear spin-lattice relaxation rate $1/T_{1}$ and nuclear spin-spin relaxation rate $1/T_{2}$ measured by Ga-NQR as well as the muon spin relaxation rate $λ$. The main sharp NQR peaks exhibit a stronger tendency of divergence, compared with the weak broader spectral peaks, indicating that the spin freezing is intrinsic in NiGa$_2$S$_4$. The behavior of these relaxation rates strongly suggests that the Ni spin fluctuations slow down towards $T_{\rm f}$, and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multi-component $T_1$ were observed below 2 K, indicating that a static magnetic state with incommensurate magnetic correlations or inhomogeneously distributed moments is realized at low temperatures. However, the wide temperature region between 2 K and $T_{\rm f}$, where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below $T_{\rm f}$, but keep fluctuating down to 2 K with the MHz frequency range.
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Submitted 31 December, 2007;
originally announced January 2008.
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Unusual superexchange pathways in a Ni triangular lattice of NiGa$_2$S$_4$ with negative charge-transfer energy
Authors:
K. Takubo,
T. Mizokawa,
J. -Y. Son,
Y. Nambu,
K. Onuma,
H. Tonomura,
S. Nakatsuji,
Y. Maeno
Abstract:
We have studied the electronic structure of the Ni triangular lattice in NiGa$_2$S$_4$ using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2$p$ core-level spectrum shows that the S 3$p$ to Ni 3$d$ charge-transfer energy is $\sim$ -1 eV and the ground state is dominated by the $d^9L$ configuration ($L$ is a S 3$p$ hole). Cell perturbation analy…
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We have studied the electronic structure of the Ni triangular lattice in NiGa$_2$S$_4$ using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2$p$ core-level spectrum shows that the S 3$p$ to Ni 3$d$ charge-transfer energy is $\sim$ -1 eV and the ground state is dominated by the $d^9L$ configuration ($L$ is a S 3$p$ hole). Cell perturbation analysis for the NiS$_2$ triangular lattice indicates that the strong S 3$p$ hole character of the ground state provides the enhanced superexchange interaction between the third nearest neighbor sites.
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Submitted 16 June, 2007; v1 submitted 15 June, 2007;
originally announced June 2007.