Manh-Huong Phan

Excellent magnetocaloric properties of La0.7Ca0.3−xSrxMnO3(0.05⩽x⩽0.25) single crystals

This letter reports on the superior magnetocaloric properties of La0.7Ca0.3−xSrxMnO3 (x=0.05, 0.10, 0.20, and 0.25) single crystals. Upon 50 kOe applied field, the magnetic entropy changes (ΔSM) reach values of ∼10.5, 7.45, 6.97, and 6.86J∕kgK for x=0.05, 0.10, 0.20, and 0.25 compositions, respectively. The large magnetic entropy changes have been found to occur around 300 K, thus allowing magnetic refrigeration at room temperature. Due to the absence of grains in the single crystals, the ΔSM distribution here is much more uniform than that of gadolinium and polycrystalline manganites, which is desirable for an Ericson-cycle magnetic refrigerator. The single crystals have the large magnetic entropy changes induced by low magnetic field change, which is beneficial for the household application of active magnetic refrigerant (AMR) materials. These results indicate that the present single crystals are excellent candidates as working materials for AMR.

Magnetocaloric effect and refrigerant capacity in charge-ordered manganites

The influence of first- and second-order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity (RC) of charge-ordered Pr0.5Sr0.5MnO3 has been investigated. The system undergoes a paramagnetic to ferromagnetic transition at TC∼255 K followed by a ferromagnetic charge-disordered to antiferromagnetic charge-ordered transition at TCO∼165 K. While the first-order magnetic transition (FOMT) at TCO induces a larger MCE (6.8 J/kg K) limited to a narrower temperature range resulting in a smaller RC (168 J/kg), the second-order magnetic transition at TC induces a smaller MCE (3.2 J/kg K) but spreads over a broader temperature range resulting in a larger RC (215 J/kg). In addition, large magnetic and thermal hysteretic losses associated with the FOMT below TCO are detrimental to an efficient magnetic RC, whereas these effects are negligible below TC because of the second-order nature of this transition. These results are of practical importance in assessing the usefulness of charge-o...

Large magnetocaloric effect in a La0.7Ca0.3MnO3 single crystal

We report the results of a thorough study of the magnetocaloric effect (MCE) in a La0.7Ca0.3MnO3 single crystal, which undergoes a first-order magnetic phase transition at ∼227 K. The magnetic entropy change ΔSM and the adiabatic temperature change ΔTad reach, respectively, ∼6.42 J/kg K and 4.76 K for ΔB=5 T and even if both are smaller than those exhibited by gadolinium (∼9.8 J/kg K and ∼11.7 K), the ΔSM distribution here is much more uniform than that of gadolinium and polycrystalline manganites. This is desirable for an Ericson-cycle magnetic refrigerator. The MCE is larger in the single-crystalline manganite compared to the polycrystalline one. The manganite single crystal has large ΔSM induced by low magnetic field change, which is beneficial for the household application of active magnetic refrigerant (AMR) materials. All these make the lanthanum manganite single crystal an attractive candidate as a working substance for AMR. The molecular field model provides a fairly good description of the magnet...

Table-like magnetocaloric effect and enhanced refrigerant capacity in Eu8Ga16Ge30-EuO composite materials

A large reversible magnetocaloric effect (MCE) and enhanced refrigerant capacity (RC) were observed in multiphase composite materials composed of type-I clathrate Eu8Ga16Ge30 and EuO. Eu8Ga16Ge30 undergoes two successive ferromagnetic transitions at 10 K and 35 K, and EuO exhibits a ferromagnetic transition at 75 K. A large RC of 794 J/kg for a field change of 5 T over a temperature interval of 70 K was achieved in the Eu8Ga16Ge30–EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K-100 K. Adjusting the Eu8Ga16Ge30 to EuO ratio is shown to produce composites with table-like MCE, desirable for ideal Ericsson-cycle magnetic refrigeration. The excellent magnetocaloric properties of these Eu8Ga16Ge30–EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.

Electron spin resonance and Raman studies of Mn-doped ZnO ceramics

In the present work, the influence of annealing on structure, electron spin resonance (ESR), and Raman scattering (RS) spectra of x at. % Mn-doped ZnO (x=4 and 8) ceramic compounds has been systematically investigated. The samples were annealed at temperatures (Tan) between 400 and 1000°C for 12h. The obtained results revealed the strong dependence of x-ray diffraction, ESR, and RS spectra on the annealing temperature Tan. Mn2+ ions did not substitute into Zn2+ sites in samples annealed in the range of 400–600°C but started to substitute into Zn2+ sites for annealing temperatures Tan>600°C. The results of this investigation provide further insights into the physical processes occurring in Mn-doped ZnO materials due to annealing.

Coexistence of conventional and inverse magnetocaloric effects and critical behaviors in Ni50Mn50−xSnx (x = 13 and 14) alloy ribbons

A systematic study of the conventional and inverse magnetocaloric effects and critical behaviors in Ni50Mn50−xSnx (x = 13 and 14) alloy ribbons has been performed. We show that although the magnetic entropy change around the second-order ferromagnetic-paramagnetic (FM-PM) transition (ΔSm ≈ −4 J/kg K) in the austenitic phase is about five times smaller than that around the first-order martensitic-austenitic (M-A) transformation (ΔSm ≈ 22 J/kg K), the refrigerant capacity (RC) – an important figure of merit – is about two times larger for the former case (RC ≈ 160 J/kg) than for the latter case (RC ≈ 75 J/kg). This finding points to an important fact that to assess the usefulness of a magnetocaloric material, one should not only consider ΔSm but also must evaluate both ΔSm and RC. Our critical analysis near the second-order FM-PM transition reveals that Sn addition tends to drive the system, in the austenitic FM phase, from the short-range (x = 13) to long-range (x = 14) FM order.

Long-range ferromagnetism and giant magnetocaloric effect in type VIII Eu8Ga16Ge30 clathrates

Long-range ferromagnetism and low-field giant magnetocaloric effect are observed in Eu8Ga16Ge30 with the type VIII clathrate crystal structure, a material that is better known for its thermoelectric properties. Magnetization and modified Arrott plots indicate that the system undergoes a second-order ferromagnetic-paramagnetic phase transition at ∼13K. The low-field giant magnetic entropy change (−ΔSM∼11.4J∕kgK for Δμ0H=3T) coupled with the absence of thermal hysteresis and field hysteresis makes the system very attractive for low temperature magnetic refrigeration. The giant magnetic entropy change originates from the large magnetization (7.97μB per Eu ion) and the sharp change with temperature at the paramagnetic-ferromagnetic transition.

Excellent magnetocaloric properties of melt-extracted Gd-based amorphous microwires

We report upon the excellent magnetocaloric properties of Gd53Al24Co20Zr3 amorphous microwires. In addition to obtaining the large magnetic entropy change (−ΔSM ∼ 10.3 J/kg K at TC ∼ 95 K), an extremely large value of refrigerant capacity (RC ∼ 733.4 J/kg) has been achieved for a field change of 5 T in an array of forty microwires arranged in parallel. This value of RC is about 79% and 103% larger than those of Gd (∼410 J/kg) and Gd5Si2Ge1.9Fe0.1 (∼360 J/kg) regardless of their magnetic ordering temperatures. The design and fabrication of a magnetic bed made of these parallel-arranged microwires would thus be a very promising approach for active magnetic refrigeration for nitrogen liquefaction. Since these microwires can easily be assembled as laminate structures, they have potential applications as a cooling device for micro electro mechanical systems and nano electro mechanical systems.

Magnetic anisotropy and field switching in cobalt ferrite thin films deposited by pulsed laser ablation

We report the observation of contrasting magnetic behavior in cobalt ferrite (CFO) thin films deposited on single crystalline magnesium oxide (MgO) and strontium titanate (STO). Epitaxial films on MgO (100) with a lattice mismatch of 0.35% showed out-of-plane anisotropy whereas the films on STO (100) with a lattice mismatch of 7.4% displayed in-plane anisotropy. Stress anisotropy calculated from angle-dependent x-ray diffraction analysis confirmed that the change in anisotropy originates from the lattice mismatch. An additional low-field switching characteristic is observed in the M-H loops of the CFO films, which became prominent with lowering temperature as also evidenced from the rf transverse susceptibility measurements. The obtained results revealed that the low field switching is associated with the film-substrate interface.

Novel nanostructure and magnetic properties of Co–Fe–Hf–O films

The influence of the partial pressure of oxygen (PO2) on the microstructural and magnetic properties of sputtered Co?Fe?Hf?O films has been studied. It is shown that the films prepared under PO2 = 6?11.5% with large saturation magnetization, 4?Ms~18?21?kG, large hard-axis anisotropy field, HkH~30?84?Oe, and high electrical resistivity, ?~1400?3600????cm, are excellent candidate materials for high-frequency applications of micromagnetic devices such as magnetic thin film inductors, transformers and thin film flux gate sensors. In particular, the good soft magnetic properties of Co19.35Fe53.28Hf7.92O19.35 nanocomposite films of 50?437?nm thickness, in addition to their high electrical resistivity, make them ideal for use in micromagnetic devices with an opening bandwidth of several gigahertz. This is attributed to the formation of a peculiar nanostructure in these samples. A strong magnetic phase separation appears to occur as the film thickness increases over 437?nm, which, in turn, modifies the high-frequency magnetic behaviour of the Co19.35Fe53.28Hf7.92O19.35 film.