Dipolar Spin Liquid Ending with Quantum Critical Point in a Gd-based Triangular Magnet

Abstract

By performing experiment and model studies on a triangular-lattice dipolar magnet KBaGd(BO3)2 (KBGB), we find the highly frustrated magnet with a planar anisotropy hosts a strongly fluctuating dipolar spin liquid (DSL), which originates from the intriguing interplay between dipolar and Heisenberg interactions. The DSL constitutes an extended regime in the field-temperature phase diagram, which gets lowered in temperature as field increases and eventually ends with an unconventional quantum critical point (QCP) at Bc 0.75 T. Based on the dipolar Heisenberg model analysis, the DSL is identified as a Berezinskii-Kosterlitz-Thouless (BKT) phase with emergent U(1) symmetry, and the end QCP belongs to the 3D XY universality class. Due to the tremendous entropy accumulation that can be related to the strong BKT and quantum fluctuations, unprecedented magnetic cooling effects are observed in the DSL and particularly near the QCP, making KBGB a superior dipolar coolant to commercial Gd-based refrigerants. We establish the phase diagram for triangular-lattice dipolar quantum magnets where emergent symmetry plays an essential role, and provide a basis and opens an avenue for their applications in sub-Kelvin refrigeration.

Giant Low-Field Cryogenic Magnetocaloric Effect in a Polycrystalline EuB4O7 Compound

“…The same situation is also performed in the experiment and model studies on a triangular-lattice dipole magnet KBaGd(BO 3 ) 2 , which lead to a dipolar energy of about 660 mK and has the fitted Curie−Weiss temperatures of about 300 mK. 31 The isothermal magnetization M(H) curves of EuB 4 O 7 at applied fields of up to 50 kOe are shown in Figure 2b. The magnetization increases rapidly with an increasing applied magnetic field up to 20 kOe and reaches a saturation of 6.13 μ B /f.u.…”

Exceptional Magnetocaloric Responses in a Gadolinium Silicate with Strongly Correlated Spin Disorder for Sub‐Kelvin Magnetic Cooling

“…[20a] The low-temperature portion of the 0 T magnetic heat capacity C mag was fitted to a power law C mag (T)≈T 𝛾 , gives a 𝛾 = 1.90( 16), which does not follow the ∼ T 3 dependence that is usually observed in conventional ungapped antiferromagnetic magnon excitations, nor those with an anisotropy energy gap, C mag (T)≈exp(-ΔE/T). [21] Instead, the parameterized behaviour suggests the existence of unusual low-energy excitations, similar to those with an apparent persistence of spin dynamics down to very low temperatures, such as the pyrochlore Gd 2 M 2 O 7 (M = Ti, Sn), [21,22] the dipolar spin liquid KBaGd(BO 3 ) 2 , [17,23] and the two-dimensional Dirac spin liquid NaYbO 2 . [24] Integrating the C mag /T up to 30 K gives a magnetic entropy S mag reaching 95% of NRln8, where N = 9.33 and R denotes the gas constant (Figure 3b).…”

“…[16] Furthermore, the unconventionally quantum critical point in the dipolar spin liquid KBaGd(BO 3 ) 2 permits an efficient magnetic cooling to well below the ordering temperature, which is identified as a Berezinskii-Kosterlitz-Thouless phase with emergent U(1) symmetry. [17] However, it remains unanswered whether this strongly correlated spin disorder can contribute to the design of ADR materials with a substantial cooling capacity per unit volume. It is within this context that we present the magnetocaloric parameters of the oxyapatite Gd 9.33 [SiO 4 ] 6 O 2 (abbreviated to GSO henceforth), which exhibits correlated spin disorder from ≈1.5 K to at least 50 mK.…”