Enhanced Magnetocaloric Effect from Cr Substitution in Ising Lanthanide Gallium Garnets Ln₃CrGa₄O₁₂ (Ln = Tb, Dy, Ho)

Abstract

A detailed study on the crystal structure and bulk magnetic properties of Cr substituted Ising type lanthanide gallium garnets Ln 3 CrGa 4 O 12 (Ln = Tb, Dy, Ho) is carried out using room temperature powder X-ray and neutron diffraction, magnetic susceptibility, isothermal magnetization, and heat capacity measurements. The magnetocaloric effect (MCE) where the geometry of the magnetic lattice prevents all the nearest-neighbor magnetic interactions from being satisfied simultaneously. [8] This suppresses or in some cases, completely inhibits magnetic longrange ordering. Geometrically frustrated magnets (GFMs) typically show ordering features at T 0 ∼ θ CW /10 where θ CW is the Curie-Weiss temperature, thereby suppressing the ordering temperature. [9] In complex lanthanide oxides, the highly localized 4f orbitals have weak magnetic interactions, i.e., θ CW is small and so when the magnetic lattice is frustrated, ordering is suppressed to even lower T. The theoretical magnetic entropy that can be extracted is much higher than in transition metal compounds. GFMs with Ln 3+ ions are therefore ideal candidates for sub 20 K magnetocaloric materials (MCMs). Another advantage is that the lanthanides are chemically very similar but their magnetic properties vary widely. This allows tuning of the properties for optimization of the MCE. [10][11][12] The lanthanide gallium garnets, Ln 3 Ga 5 O 12, are a family of materials, where the magnetic Ln 3+ spins form two interpenetrating networks of ten membered-rings of corner-sharing triangles leading to a high degree of geometrical frustration. [13] Of these, gadolinium gallium garnet (GGG), which shows no long-range ordering down to 25 mK, has been established as a MCM for magnetic refrigeration in the liquid helium temperature regime. The absence of long-range ordering, high density of magnetic ions, chemical stability, and lack of single ion anisotropy (L = 0 for Gd 3+ ) allowing for the full magnetic entropy) to be extracted in high magnetic fields makes it an ideal MCM for T < 20 K. [14][15][16] In recent years, a number of Gd containing MCMs with better performance at 2 K have been reported [17][18][19][20] but GGG continues to be used and serves as the benchmark for MCMs in this temperature regime. However, for all the Gd-based magnetocalorics, the change in magnetic entropy is maximized in fields of 5 T or higher. Such high magnetic fields can only be produced using a superconducting magnet which again requires cooling using cryogens. For more practical applications, we need to focus on developing materials with high MCE in fields ≤ 2 T, attainable by a permanent magnet. This has been discussed in a recent study on Tb(HCO 2 ) 3 where the MCE is significantly higher than Gd(HCO 2 ) 3 at higher temperatures and lower fields as the Tb 3+ have Ising-like spins contrasted with the Heisenberg nature of the Gd 3+ spins. [21] For Ln 3 Ga 5 O 12 , the MCE in fields ≤2 T is expected to be maximized for the Ln 3+ having Ising-like spins, Magnetic Materials

Ferromagnetic Ising chains in frustrated LnODCO3: the influence of magnetic structure in magnetocaloric frameworks

“…Solid state caloric refrigerants are one efficient alternative for such applications; 4 recent advances have been made in developing magnetocalorics with good performance at low temperatures under fields that can be generated using permanent magnets, required for energy efficient and practical magnetocaloric devices. [5][6][7][8][9] In recent years work on magnetocaloric materials has extended from purely ionic systems to those containing molecular building blocks and this appears a very promising route forward for developing improved materials. [10][11][12] The paramagnetic magnetocaloric effect (MCE) is an entropically driven cooling process that occurs when paramagnets are in a cycled magnetic field.…”

“…Recently, however, literature has shown that materials containing cations with strong Ising anisotropy improves the magnetocaloric effect in powders under the low applied magnetic fields that can be generated using a permanent magnet (o2 T). 5,7,17 This surprising result is an outcome of the greater ease of magnetisation of these materials under low applied fields, but the microscopic cause of this remains unknown. Uncovering how magnetic interactions in such compounds are best optimised to improve magnetocaloric performance requires an understanding of these materials at the microscopic level, which is most readily achieved using neutron scattering rather than indirect bulk property measurements.…”

Sensitivity of magnetic properties to chemical pressure in lanthanide garnetsLn3A2X3O12,Ln = Gd, Tb, Dy, Ho,A = Ga, Sc, In, Te,X = Ga, Al, Li

“…A and X are both restricted to non-magnetic cations as inclusion of magnetic cations on the A and X sites gives rise to additional magnetic phenomena [16][17][18][19][20][21]. The garnets crystallise in a cubic structure and contain three distinct cation sites based on the coordination with oxygen: dodecahedral occupied by Ln, octahedral occupied by A and tetrahedral occupied by X, Figure 1a.…”

Relieving the frustration through Mn3+ substitution in holmium gallium garnet

“…The Cr 3+ substituted lanthanide gallium garnets, Ln3CrGa4O12 (Ln = Tb, Dy, Ho), also show an increase in TN by a smaller factor than on Mn 3+ substitution. 40 Neutron diffraction is required to elucidate the magnetic structure in these cases but this hints at a universal mechanism for relieving the magnetic frustration in Ising lanthanide garnets which is tuneable through control of the extent and type of magnetic ion substitution. e) The Ho 3+ quasi-spin couples ferromagnetically with the Mn 3+ spins located above and below the triangle.…”