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Register a new user3d-4f spin interaction induced giant magnetocaloric effect in zircon-type DyCrO4 and HoCrO4 compounds
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We have investigated the influence of 3d-4f spin interaction on magnetic and magnetocaloric properties of DyCrO$_4$ and HoCrO$_4$ compounds by magnetization and heat capacity measurements. Both the compounds exhibit complicated magnetic properties and huge magnetic entropy change around the ferromagnetic transition due to the strong competition between ferromagnetic and antiferromagnetic superexchange interactions. For a field change of 8 T, the maximum values of magnetic entropy change ($Delta S_{M}^{max}$), adiabatic temperature change ($Delta T_{ad}$), and refrigerant capacity (RC) reach 29 J kg$^{-1}$ K$^{-1}$, 8 K, and 583 J kg$^{-1}$, respectively for DyCrO$_4$ whereas the corresponding values for HoCrO$_4$ are 31 J kg$^{-1}$ K$^{-1}$, 12 K, and 622 J kg$^{-1}$. $Delta S_{M}^{max}$, $Delta T_{ad}$, and RC are also quite large for a moderate field change. The large values of magnetocaloric parameters suggest that the zircon-type DyCrO$_4$ and HoCrO$_4$ could be the potential magnetic refrigerant materials for liquefaction of hydrogen.
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We report the results of our investigation of the physical properties of mixed metal oxides RFe0.5Cr0.5O3 (R = Er and Yb). ErFe0.5Cr0.5O3 undergoes an antiferromagnetic ordering around 270 K followed by spin reorientation (SR) transitions around 150 and 8 K respectively. In contrast, in YbFe0.5Cr0.5O3 a single SR transition is noted at 36 K, below the AFM ordering temperature of 280 K. In ErFe0.5Cr0.5O3, a significant value of magnetic entropy change ({Delta}SM) ~ -12.4 J/kg-K is noted near the 2nd SR transition, however, this value is suppressed in YbFe0.5Cr0.5O3. Temperature dependent dielectric permittivity of ErFe0.5Cr0.5O3 and YbFe0.5Cr0.5O3 at different frequencies, reveal the presence of Debye-like relaxation behaviour in both compounds, which can be due to the effect of charge carrier hopping between localized states of Fe and Cr ions. Temperature dependent Raman scattering studies divulge that spin-phonon coupling plays a crucial role in defining the physical properties of these compounds.
A giant magnetocaloric effect across the ferromagnetic (FM) to paramagnetic (PM) phase transition was observed in chemically synthesized Co2FeAl Heusler alloy nanoparticles with a mean diameter of 16 nm. In our previous report, we have observed a significant enhancement in its saturation magnetization (Ms) and Curie temperature (Tc) as compared with the bulk counterpart. Motivated from those results, here, we aim to explore its magnetocaloric properties near the Tc. The magnetic entropy change shows a positive anomaly at 1252 K. Magnetic entropy change increases linearly with the magnetic field, and a large value of ~15 J/Kg-K is detected under a moderate field of 14 kOe. It leads to a net relative cooling power of 89 J/Kg for the magnetic field change of 14 kOe. To confirm the nature of magnetic phase transition, a detailed study of its magnetization is performed. The Arrott plot and nature of the universal curve conclude that FM to PM phase transition in the present system is of second-order.
Temperature dependence of magnetization curves of well homogenized samples of Ce(Co$_{1-x}$Cu$_{x}$)$_5$ ($0le x le 0.7$), a family of representative $4f$-$3d$ intermetallic magnets found in rare-earth permanent magnets, is measured. A remarkable enhancement of intrinsic coercivity is observed with $x=0.3$ and $x=0.4$, persisting to higher temperatures. This experimental observation is theoretically attributed to an effect of electronic correlation among $4f$-electrons. That is, an intrinsic pinning happens originating in an anomalously enhanced magnetic anisotropy energy contributed by an order of magnitude stronger charge-transfer process between $4f$-electrons and $3d$-electrons, than the conventional crystal field effects. It is demonstrated that the $4f$-$3d$ charge-transfer process depends on the direction of magnetization in the middle of a crossover of the valence state of Ce between CeCu$_5$ with robust Ce$^{3+}$ and CeCo$_5$ with the mixed valence state.
Single crystal of PrSi was grown by Czochralski method in a tetra-arc furnace. Powder x-ray diffraction of the as grown crystal revealed that PrSi crystallizes in FeB$-$type structure with space group $Pnma$ (no. 62). PrSi undergoes a ferromagnetic transition at 52 K with [010] direction as the easy axis of magnetization. Heat capacity data confirm the bulk nature of the transition at 52 K and exhibit a huge anomaly at the transition. A sharp rise in the low temperature heat capacity has been observed (below 5 K) which is attributed to the $^{141}$Pr nuclear Schottky heat capacity arising from the hyperfine field of the Pr moment. The estimated Pr magnetic moment 2.88 $mu_{rm B}$/Pr from the hyperfine splitting is in agreement with the saturation magnetization value obtained from the magnetization data measured at 2 K. From the crystal electric field (CEF) analysis of the magnetic susceptibility, magnetization and the heat capacity data it is found that the degenerate $J = 4$ Hunds rule derived state of Pr$^{3+}$-ion splits into nine singlets with an overall splitting of 284 K, the first excited singlet state separated by just 9 K from the ground state. The magnetic ordering in PrGe appears to be due to the exchange generated admixture of low lying crystal field levels. Magnetocaloric effect (MCE) has been investigated from magnetization data along all the three principal crystallographic directions. Large magnetic entropy change, $-Delta S_M = $22.2 J/kg K, and the relative cooling power, RCP = $460$ J/kg, characteristic of giant magneto caloric effect are achieved near the transition temperature ($T_{rm C}$ = 52 K) for $H =$~70 kOe along $[010]$. Furthermore, the PrSi single crystal exhibits a giant MCE anisotropy.
The criticality-enhanced magnetocaloric effect (MCE) near a field-induced quantum critical point (QCP) in the spin systems constitutes a very promising and highly tunable alternative to conventional adiabatic demagnetization refrigeration. Strong fluctuations in the low-$T$ quantum critical regime can give rise to a large thermal entropy change and thus significant cooling effect when approaching the QCP. In this work, through efficient and accurate many-body calculations, we show there exists a significant inverse MCE(iMCE) in the spin-1 quantum chain materials(CH$_3$)$_4$NNi(NO$_2$)$_3$ (TMNIN) and NiCl$_2$-4SC(NH$_2$)$_2$ (DTN), where DTN has substantial low-$T$ refrigeration capacity while requiring only moderate magnetic fields. The iMCE characteristics, including the adiabatic temperature change $Delta T_{rm ad}$, isothermal entropy change $Delta S$, differential Gruneisen parameter, and the entropy change rate, are obtained with quantum many-body calculations at finite temperature. The cooling performance, i.e., the efficiency factor and hold time, of the two compounds is also discussed. Based on the many-body calculations on realistic models for the spin-chain materials, we conclude that the compound DTN constitutes a very promising and highly efficient quantum magnetic coolant with pronounced iMCE properties. We advocate that such quantum magnets can be used in cryofree refrigeration for space applications and quantum computing environments.
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