1/5 and 1/3 magnetization plateaux in the spin 1/2 chain system YbAlO3

TL;DR

This work reports the first observation of magnetization plateaux at 1/5 and 1/3 of the saturation value in the spin-1/2 chain YbAlO$_3$, a quasi-one-dimensional magnet with Heisenberg intrachain exchange and Ising-like interchain coupling. Using thermal transport and magnetostriction at sub-Kelvin temperatures, the authors detect and characterize these plateaux within a field-induced longitudinal spin-density wave (LSDW) state, and provide a phenomenological theory in which the plateaux arise from commensurate locking via umklapp processes stabilized by ferromagnetic interchain interactions ($J'<0$). The theory explains the relative stability and widths of the plateaux, the sequence of phases (LSDW, TAF, FP), and the absence of similar plateaux in antiferromagnetic interchain magnets like BaCo$_2$V$_2$O$_8$, attributing it to the sign of $J'$. They also quantify a small gap in the 1/3 plateau ($\Delta_{1/3}/k_B\approx0.19$ K) from thermal conductivity and identify a weak, high-field phase transition near $B^*$, enriching the magnetic phase diagram of YbAlO$_3$ and motivating further numerical studies of its microscopic spin Hamiltonian.

Abstract

Quasi-one-dimensional magnets can host an ordered longitudinal spin-density wave state (LSDW) in magnetic field at low temperature, when longitudinal correlations are strengthened by Ising anisotropies. In the S = 1/2 Heisenberg antiferromagnet YbAlO3 this happens via Ising-like interchain interactions. Here, we report the first experimental observation of magnetization plateaux at 1/5 and 1/3 of the saturation value via thermal transport and magnetostriction measurements in YbAlO3. We present a phenomenological theory of the plateau states that describes them as islands of commensurability within an otherwise incommensurate LSDW phase and explains their relative positions within the LSDW phase and their relative extent in a magnetic field. Notably, the plateaux are stabilised by ferromagnetic interchain interactions in YbAlO3 and consistently are absent in other quasi-1D magnets such as BaCo2V2O8 with antiferromagnetic interchain interactions. We also report a small, step-like increase of the magnetostriction coefficient, indicating a weak phase transition of unknown origin in the high-field phase just below the saturation.

Figures (8)

• Figure 1: (a) Sketch of the magnetic structure showing only Yb ions and two relevant exchange interactions, $J_c$ and $J_{\perp}$. (b) Crystal structure of YbAlO$_3$ viewed from [001] direction.
• Figure 2: Field dependence of different quantities in YbAlO$_3$ at low temperature for $H\parallel a$. Sharp anomalies occur at $B_\mathrm{c}$, $B^\ast$ and at the fields where the magnetization reaches 1/5 $M_\mathrm{s}$ as well as 1/3 $M_\mathrm{s}$ in all quantities: normalised magnetization $M/M_\mathrm{s}$ (a), and its derivative (b), magnetostriction coefficient $\lambda$ (c), and thermal conductivity $\kappa$, here shown as the conductivity change normalised by the zero-field value (d). The 1/3 plateau is also evidenced by a constant position $\mathbf{Q} = (0,0,Q)$ with $Q = \pi(1+\delta)$ and $\delta = 1/3$ of the magnetic Bragg peak in neutron scattering (e, right axis) associated with the LSDW. The Bragg peak intensity is finite and the LSDW state persists up to 0.85 T but becomes very small for fields above 0.75 T (e, left axis).
• Figure 3: $T$ evolution of anomalies in magnetostriction and thermal conductivity zoomed around the 1/5 (a,b) and 1/3 (c,d) plateaux. The plateau region was defined as the field region in which the magnetostriction lies below the high-temperature curve as indicated by the arrows and orange symbols for the curve at 70 mK. (e,f) show a zoom at the phase transition at $B^*=0.96$ T at lowest measured temperature indicated by an arrow and a green symbol in (e).
• Figure 4: The phase diagram of YbAlO$_3$. White points are from specific heat $C$ and magnetization $M$ in reference wu_tomonagaluttinger_2019. Orange and green points are from the magnetostriction $\lambda$ in this work showing the two plateau regions appearing within the LSDW state and the phase transition at $B^\ast = 0.96$ T. See text for details.
• Figure S1: a) Raw and corrected thermal conductivity data at $T_\mathrm{const} = 202$ mK. b) Variation of the average sample temperature during the field sweep (left axis) and the exponent $\alpha$ determined from temperature sweeps $\kappa (T) \propto T^\alpha$ around 202 mK (right axis). $T_\mathrm{const}$ used as a basis for calculation of the field sweep data is marked by a dotted line.