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Register a new userExtraordinarily large intrinsic magnetodielectric coupling of Tb member within the Haldane spin-chain family, R2BaNiO5
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E. V. Sampathkumaran Professor
Publication date
2017
fields
Physics
and research's language is
English
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The Haldane spin-chain compound, Tb2BaNiO5, has been known to order antiferromagnetically below (T_N= ) 63 K. The present magnetic studies on the polycrystals bring out that there is another magnetic transition at a lower temperature (T_2= ) 25 K, with a pronounced magnetic-field induced metamagnetic and metaelectric behavior. Multiferroic features are found below T_2 only, and not at T_N. The most intriguing observation is that the observed change of dielectric constant is intrinsic and largest (e.g., about 18% at 15 K) within this Haldane spin-chain family, R2BaNiO5. Taking into account that this trend (the largest change for Tb case within this family) correlates with a similar trend in T_N (with the values of T_N being about 55, 58, 53 and 32 K for Gd, Dy, Ho and Er cases), we believe that an explanation usually offered for this T_N behavior in rare-earth systems is applicable for this behavior as well . That is, single-ion anisotropy following crystal-field splitting is responsible for this extraordinary magnetodielectric effect in this Tb case. To our knowledge, such an observation was not made in the past literature of multiferroics.
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We report the magnetic, heat-capacity, dielectric and magnetodielectric (MDE) behaviour of a Haldane spin-chain compound containing light rare-earth ion, Nd2BaNiO5, in detail, as a function of temperature (T) and magnetic field (H) down to 2 K. In addition to the well-known long range antiferromagnetic order setting in at (T_N=) 48 K as indicated in dc magnetization (M), we have observed another magnetic transition near 10 K; this transition appears to be of a glassy-type which vanishes with a marginal application of external magnetic field (even H= 100 Oe). There are corresponding anomalies in dielectric constant as well with variation of T. The isothermal M(H) curves at 2 and 5 K reveal the existence of a magnetic-field induced transition around 90 kOe; the isothermal H-dependent dielectric constant also tracks such a metamagnetic transition. These results illustrate the MDE coupling in this compound. Additionally, we observe a strong frequency dependence of a step in T-dependent dielectric constant with this feature appearing around 25-30 K for the lowest frequency of 1 kHz, far below T_N. This is attributed to interplay between crystal-field effect and exchange interaction between Nd and Ni, which establishes the sensitivity of dielectric measurements to detect such effects. Interestingly enough, the observed dispersions of the T-dependent dielectric constant curves is essentially H-independent in the entire T-range of measurement, despite the existence of MDE coupling, which is in sharp contrast with other heavy rare-earth members in this series.
We have investigated the magnetic, dielectric and magnetodielectric (MDE) behavior of a geometrically frustrated spin-chain system, Ca3Co1.4Rh0.6O6, in the single crystalline form for different orientations. The results bring out that the magnetic behavior of this compound is by itself interesting in the sense that this compound exhibits an anisotropic glassy-like magnetic behavior with a huge frequency dependence of ac susceptibility peak for an orientation along the spin-chain in the range 30-60 K; this behavior is robust to applications of large external magnetic fields (H) unlike in canonical spin-glasses. The temperature dependence of dielectric constant also shows strong frequency dependence with similar robustness to H. The isothermal H-dependent dielectric results at low temperatures establishes anisotropic MDE coupling. It is intriguing to note that there is a step roughly at one-third of saturation values as in the case of isothermal magnetization curves for same temperatures (for orientation along spin-chain), a correlation hitherto unrealized for geometrically frustrated systems.
Frustrated quantum magnets are expected to host many exotic quantum spin states like quantum spin liquid (QSL), and have attracted numerous interest in modern condensed matter physics. The discovery of the triangular lattice spin liquid candidate YbMgGaO$_4$ stimulated an increasing attention on the rare-earth-based frustrated magnets with strong spin-orbit coupling. Here we report the synthesis and characterization of a large family of rare-earth chalcogenides AReCh$_2$ (A = alkali or monovalent ions, Re = rare earth, Ch = O, S, Se). The family compounds share the same structure (R$bar{3}$m) as YbMgGaO$_4$, and antiferromagnetically coupled rare-earth ions form perfect triangular layers that are well separated along the $c$-axis. Specific heat and magnetic susceptibility measurements on NaYbO$_2$, NaYbS$_2$ and NaYbSe$_2$ single crystals and polycrystals, reveal no structural or magnetic transition down to 50mK. The family, having the simplest structure and chemical formula among the known QSL candidates, removes the issue on possible exchange disorders in YbMgGaO$_4$. More excitingly, the rich diversity of the family members allows tunable charge gaps, variable exchange coupling, and many other advantages. This makes the family an ideal platform for fundamental research of QSLs and its promising applications.
We consider the one-dimensional spin chain for arbitrary spin $s$ on a periodic chain with $N$ sites, the generalization of the chain that was studied by Blume and Capel cite{bc}: $$H=sum_{i=1}^N left(a (S^z_i)^2+ b S^z_iS^z_{i+1}right).$$ The Hamiltonian only involves the $z$ component of the spin thus it is essentially an Ising cite{Ising} model. The Hamiltonian also figures exactly as the anisotropic term in the famous model studied by Haldane cite{haldane} of the large spin Heisenberg spin chain cite{bethe}. Therefore we call the model the Blume-Capel-Haldane-Ising model. Although the Hamiltonian is trivially diagonal, it is actually not always obvious which eigenstate is the ground state. In this paper we establish which state is the ground state for all regions of the parameter space and thus determine the phase diagram of the model. We observe the existence of solitons-like excitations and we show that the size of the solitons depends only on the ratio $a/b$ and not on the number of sites $N$. Therefore the size of the soliton is an intrinsic property of the soliton not determined by boundary conditions.
Inelastic neutron scattering was used to probe edge states in a quantum spin liquid. The experiment was performed on finite length antiferromagnetic spin-1 chains in Y_2BaNi_{1-x}Mg_xO_5. At finite fields, there is a Zeeman resonance below the Haldane gap. The wave vector dependence of its intensity provides direct evidence for staggered magnetization at chain ends, which decays exponentially towards the bulk (xi = 8(1) at T=0.1K). Continuum contributions to the chain end spectrum indicate inter-chain-segment interactions. We also observe a finite size blue shift of the Haldane gap.
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