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Journal Articles

Adiabatic demagnetization cooling well below the magnetic ordering temperature in the triangular antiferromagnet KBaGd(BO$$_3$$)$$_2$$

Jesche, A.*; Winterhalter-Stocker, N.*; Hirschberger, F.*; Bellon, A.*; Bachus, S.*; Tokiwa, Yoshifumi; Tsirlin, A. A.*; Gegenwart, P.*

Physical Review B, 107(10), p.104402_1 - 104402_8, 2023/03

Crystal structure, thermodynamic properties, and adiabatic demagnetization refrigeration (ADR) effect in the spin-1/2 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-$$frac{1}{2}$$ 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.

Journal Articles

Frustrated magnet for adiabatic demagnetization cooling to milli-Kelvin temperatures

Tokiwa, Yoshifumi; Bachus, S.*; Kavita, K.*; Jesche, A.*; Tsirlin, A. A.*; Gegenwart, P.*

Communications Materials (Internet), 2(1), p.42_1 - 42_6, 2021/04

Generation of very low temperatures has been crucially important for applications and fundamental research, as low-temperature quantum coherence enables operation of quantum computers and formation of exotic quantum states, such as superfluidity/superconductivity. One of the major techniques to reach milli-Kelvin temperatures is adiabatic demagnetization refrigeration. This method uses almost non-interacting magnetic moments of paramagnetic salts where large distances suppress interactions between the moments. The large spatial separations are facilitated by water molecules, with a drawback of reduced stability of the material. Here, we show that the water-free frustrated magnet KBaYb(BO$$_{3}$$)$$_{2}$$ can be ideal for refrigeration, achieving at least 22 mK. Compared to conventional refrigerants, KBaYb(BO$$_{3}$$)$$_{2}$$ does not degrade even under high temperatures and ultra-high vacuum. Further, its magnetic frustration and structural randomness enable cooling to temperatures several times lower than the energy scale of magnetic interactions, which is the main limiting factor for the base temperature of conventional refrigerants.

Oral presentation

Quantum bi-critical point in YbAgGe

Tokiwa, Yoshifumi; Garst, M.*; Gegenwart, P.*; Bud'ko, S. L.*; Canfield, P. C.*

no journal, , 

Bicritical points, at which two distinct symmetry-broken phases become simultaneously unstable, are typical for spin-flop metamagnetism. Interestingly, the heavy-fermion compound YbAgGe also possesses such a bicritical point (BCP) with a low temperature 0.3K at a magnetic field of 4.5T. In its vicinity, YbAgGe exhibits anomalous behavior that we attribute to the influence of a quantum bicritical point that is close in parameter space yet can be reached by tuning $$T_{BCP}$$ further to zero. Using high-resolution measurements of the magnetocaloric effect, we demonstrate that the magnetic Gruneisen parameter $$Gamma$$ indeed both changes sign and diverges as required for quantum criticality. Moreover, $$Gamma$$ displays a characteristic scaling behavior but only on the low-field side $$H_{BCP}$$, indicating a pronounced asymmetry with respect to the critical field. We speculate that the small value of $$T_{BCP}$$ is related to the geometric frustration of the Kondo lattice of YbAgGe.

Oral presentation

Frustrated magnet for ADR to milli-Kelvin temperatures

Tokiwa, Yoshifumi; Bachus, S.*; Kavita, K.*; Jesche, A.*; Tsirlin, A. A.*; Gegenwart, P.*

no journal, , 

In adiabatic demagnetization cooling, the entropy due to the fluctuation of the magnetic moment absorbs the ambient heat. However, at low temperatures, the entropy disappeared due to the ordering of the magnetic moment at the temperature corresponding to the energy scale (J) of the magnetic interaction, and the final temperature was limited. We have shown that a substance called KBaYb(BO$$_{3}$$)$$_{2}$$ in which Yb forms a triangular lattice can be cooled to a temperature below J, which is the original limit. In a cooling experiment in a nearly ideal adiabatic state, sweeping from a magnetic field of 5T to 0T at a temperature of 2 K reached 22 mK, well below the Wyeth temperature of -60 mK estimated from magnetic susceptibility. This is because the entropy maintains a large value even at low temperatures due to the suppression of magnetic order due to frustration. Normally, as the volume density of the magnetic moment decreases, the distance between the moments increases and J decreases. Therefore, the final temperature decreases as the volume density of the moment decreases. Due to the effect of frustration, this substance achieves a lower final temperature despite its higher magnetic moment density than the conventional paramagnetic salt, which is a cooling material. Such a high density moment results in a high amount of heat absorption per volume. In addition, paramagnetic salts contain many water molecules, which causes deterioration such as deliquescent and efflorescence. However, KBaYb(BO$$_{3}$$)$$_{2}$$ used in this study is stable in air because it does not contain water molecules. be. This substance is an ideal cooling material that has all of the final temperature, stability in air, and high magnetic moment density per volume.

Oral presentation

Frustrated magnet for adiabatic demagnetization cooling to milli-Kelvin temperatures

Tokiwa, Yoshifumi; Bachus, S.*; Kavita, K.*; Jesche, A.*; Tsirlin, A. A.*; Gegenwart, P.*

no journal, , 

Due to the supply insecurity of Helium-3, adiabatic demagnetization refrigeration (ADR) for cooling to milli-Kelvin temperatures has been attracting much attention. Conventional refrigerants for ADR, paramagnetic salts, contain water molecule, which causes lack of stability in the air. Therefore, special care has to be taken for the handling. Here, we show that an H$$_{2}$$O-free frustrated magnet KBaYb(BO$$_{3}$$)$$_{2}$$ can be ideal refrigerant for ADR, achieving at least 22mK. KBaYb(BO$$_{3}$$)$$_{2}$$ does not degrade under severe conditions like high temperatures and ultra-high vacuum. Further, its magnetic frustration enables refrigeration to temperatures several times lower than the energy scale of magnetic interactions, which is the limit for conventional refrigerants.

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