Enhancing the performance of magnetic refrigerants through tuning their magnetism from antiferromagnetism to weak ferromagnetism

Accurate Prediction of the Magnetic Ordering Temperature of Ultralow-Temperature Magnetic Refrigerants

“…Since previous studies on the magnetic property of GdOHCO 3 have shown that it exhibits moderate antiferromagnetic interaction, the substitution of F – for OH – in GdOHCO 3 is expected to further weaken its antiferromagnetic interactions. , Indeed, the mole magnetic susceptibility ( χ M ) of polycrystalline 1 under 1000 Oe investigated in 2–300 K indicates that antiferromagnetic interaction in 1 is obviously weaker than that in GdOHCO 3 . As depicted in Figure a, the χ M T value at 300 K (7.88 cm 3 K mol –1 ) remains almost constant from 300 to 80 K. As the temperature further decreases, the χ M T value gradually declines to 7.68 cm 3 K mol –1 at 14 K, followed by a sharp decrease in the χ M T value from 14 to 2 K, reaching a minimum of 6.83 cm 3 K mol –1 at 2 K. The downward trend of χ M T – T plots indicates that there is an antiferromagnetic exchange interaction in 1 , consistent with the products of the Curie–Weiss law ( C = 7.86 cm 3 K mol –1 , θ cw = −0.31 K).…”

“…Compound 1 represents a newly synthesized entity, while the other five are already established and recognized compounds. , 1 was synthesized through a hydrothermal process of hydrated gadolinium carbonate and sodium tetrafluoroborate at 140 °C. Utilizing cerium fluoride carbonate (Bastnasite) as the initial structural model, Rietveld refinement was conducted on the powder X-ray diffraction (PXRD) pattern of 1 at ambient temperature (Figure a), yielding a good agreement between experimental and simulated data with R p = 1.64, R wp = 2.15, and χ 2 = 1.43.…”

“…Considering that the −Δ S m values of the magnetic refrigerants drop to zero below the magnetic ordering temperature ( T 0 ) and the temperatures in different stages need to partially overlap, excellent magnetic refrigerants suitable for 1–4 K should not only have a large −Δ S m value, but also its T 0 should be at least less than 1 K. In the effort to prepare the magnetic refrigerants with large −Δ S m , it was found that both high magnetic density and weak magnetic interaction are the prerequisites and guarantees for obtaining magnetic refrigerants with large −Δ S m . − Based on the understanding of the factors affecting −Δ S m , especially the discovery that the substitution of F – for OH – in the frameworks can effectively reduce the Gd···Gd magnetic interaction, one has been able to predict if a magnetic refrigerant has a large −Δ S m from the equation −Δ S m = nR ln­(2 S + 1)/ M w and has successfully prepared a series of magnetic refrigerants with large −Δ S m , such as Gd­(HCOO) 3 , Gd­(OH) 3 , GdPO 4 , Gd­(OH)­CO 3 , GdF 3 , and Gd­(OH)­F 2 . Although great progress has been made in the study of magnetic refrigerants with large −Δ S m , owing to the T 0 of a magnetic refrigerant being related to not only magnetic exchange but also single-ion anisotropy and magnetic dipole interaction, the study of the T 0 of magnetic refrigerants is still lagging behind, despite the fact that the T 0 of magnetic refrigerants is the key to determine whether a magnetic refrigerant can be practicable in the temperature range of 1–4 K or below 1 K. The main reasons are as follows: (1) Because the T 0 of magnetic refrigerants used in 1–4 K should be at least lower than 1 K, it is difficult for physical measurements to reach the temperature below 1 K without a dilution refrigerator, which limits the exploration of their T 0 experimentally.…”

Gd(OH)F2: A Promising Cryogenic Magnetic Refrigerant

“…Comparing the magnetic interactions in 1 (θ = −1.31 K), 2 (θ = −0.95 K), 3 (θ = −0.53 K), and Gd­(OH) 3 (θ = −1.69 K), it is clear that the T o value of magnetic materials in this system is closely related to their magnetic interactions, and a weak magnetic interaction is favorable for obtaining a low T o . However, previous work shows that weak magnetic interactions do not necessarily lead to a low T o as observed in Gd­(HCOO) 3 (θ = −0.3 K and T o = 0.8 K) and Gd­(HCOO)­F 2 (θ = +0.14 K and T o = 1.0 K) . Therefore, the topology structures of 1 – 3 are analyzed to reveal the nature of T o in 1 – 3 .…”

“…However, previous work shows that weak magnetic interactions do not necessarily lead to a low T o as observed in Gd(HCOO) 3 (θ = −0.3 K and T o = 0.8 K) 46 and Gd(HCOO)F 2 (θ = +0.14 K and T o = 1.0 K). 58 Therefore, the topology structures of 1−3 are analyzed to reveal the nature of T o in 1−3. Based on the DFT calculation result (Figure S10), along the a axis direction, both 1 and 3 have the same connection topology of alternating pure μ 3 -OH and pure μ 3 -F bridges, while 2 presents the topology of alternating pure μ 3 -F and mixed bridges (μ 3 -OH and μ 3 -F).…”