Adiabatic demagnetization cooling well below the magnetic ordering temperature in the triangular antiferromagnet KBaGd(BO3)2

Abstract

Crystal structure, thermodynamic properties, and adiabatic demagnetization refrigeration (ADR) effect in the spin-7 2 triangular antiferromagnet KBaGd(BO 3 ) 2 are reported. With the average nearest-neighbor exchange coupling of 44 mK, this compound shows magnetic order below T N = 263 mK in zero field. The ADR tests reach the temperature of T min = 122 mK, more than twice lower than T N , along with the entropy storage capacity of 192 mJ K −1 cm −3 and the hold time of more than 8 h in the PPMS setup, both significantly improved compared to the spin-1 2 Yb 3+ analog. We argue that KBaGd(BO 3 ) 2 shows a balanced interplay of exchange and dipolar couplings that together with structural randomness and geometrical frustration shift T min to well below the ordering temperature T N , therefore facilitating the cooling.

K2RENb5O15 (RE = Ce, Pr, Nd, Sm, Gd–Ho): A Family of Quasi-One-Dimensional Spin-Chain Compounds with Large Interchain Distance

“…Under ΔB = 7 T, the maximum MCE appears at temperatures below 2 K; this is distinct from the dense Gd 3+based compounds exhibiting the optimal MCE at temperatures well above 2 K. 58,59 Moreover, the maximum value of ΔS m = 10.8 J/K/(mol-Gd) = 0.62 R ln(2S + 1) under ΔB = 2 T occurs at ∼1 K, which can be attractive for realizing sub-Kelvin cooling due to the lower T N of K 2 GdNb 5 O 15 . In an adiabatic demagnetizing process, as guided by the arrows in Figure 10b, magnetic entropy is reduced by 13.3(1) J/K/(mol-Gd) when a field of 5 T is applied at T = 2 K, and the material is supposed to cool down to the minimum temperature T min ∼ 175 mK; this is comparable to T min ∼ 170 mK for Mn(NH 4 ) 2 (SO 4 ) 2 •6H 2 O (MAS) 60 and T min ∼ 125 mK in triangular-lattice frustrated KBaGd(BO 3 ) 2 polycrystals, 61 as well as T min ∼ 135 mK in NaYbGeO 4 polycrystals. 62 Also, a large volumetric entropy density (S GS /vol.)…”

“…Ordering temperature ( T 0 ) versus the distance of NN Gd ions ( d NN ) for some of Gd-based oxides, ,,,,,− the dashed line denotes the position of d NN ∼ 3.91 Å of K 2 GdNb 5 O 15 .…”

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

“…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 ]…”

Sr14.06Gd14.63(BO3)24: A Gadolinium-Rich Borate with Magnetic Refrigeration Performance

“…13−15 For instance, rare earth borates, such as GdBO 3 16 and Gd 17.33 (BO 3 ) 4 (B 2 O 5 ) 2 O 16 , 17 and alkali metal rare earth complex borates, such as Li 3 K 9 Gd 3 (BO 3 ) 7 and LiGd 6 O 5 (BO 3 ) 3 , 19 exhibit good MCE performances, among which −ΔS m values of GdBO 3 and Li 3 K 9 Gd 3 (BO 3 ) 7 are 57.8 J kg −1 K −1 at 2 K and 9 T and 39.3 J kg −1 K −1 at 2 K and 7 T, respectively. Alkali−alkaline earth and rare earth complex borates, such as KBaGd(BO 3 ) 2 , also have a larger −ΔS m of 28.9 J kg −1 K −1 at 2 K and 5 T. 20…”

“…In recent years, different kinds of gadolinium-based borates as potential magnetic refrigerants have been attracting extensive attention due to their rich structural diversity, large heat conductivity, high mechanical strength, and good physicochemical stability. − For instance, rare earth borates, such as GdBO 3 and Gd 17.33 (BO 3 ) 4 (B 2 O 5 ) 2 O 16 , and alkali metal rare earth complex borates, such as Li 3 K 9 Gd 3 (BO 3 ) 7 and LiGd 6 O 5 (BO 3 ) 3 , exhibit good MCE performances, among which −Δ S m values of GdBO 3 and Li 3 K 9 Gd 3 (BO 3 ) 7 are 57.8 J kg –1 K –1 at 2 K and 9 T and 39.3 J kg –1 K –1 at 2 K and 7 T, respectively. Alkali–alkaline earth and rare earth complex borates, such as KBaGd­(BO 3 ) 2 , also have a larger −Δ S m of 28.9 J kg –1 K –1 at 2 K and 5 T . So far, there are few reports about Gd-based alkaline earth borate with a large MCE.…”