O. Tegus

Tunable thermal hysteresis in MnFe(P,Ge) compounds

Structural, magnetic, and magnetocaloric properties of the MnFe(P,Ge) compounds were systematically studied on both bulk alloys and melt-spun ribbons. The experimental results show that the critical behavior of the phase transition can be controlled by changing either the compositions or the annealing conditions. The thermal hysteresis is found to be tunable. It can reach very small values, while maintaining a large magnetocaloric effect in a large range of working temperatures and under field changes that may be produced by conventional permanent magnets. Consequently, an effective way in producing ideal magnetic refrigerants for room-temperature applications is suggested.

Structure, magnetism, and magnetocaloric properties of MnFeP1−xSix compounds

MnFeP1−xSix compounds with x=0.10,0.20,0.24,0.28,…,0.80,1 were prepared by high-energy ball milling and solid-state reaction. The structural, magnetic, and magnetocaloric properties are investigated as a function of temperature and magnetic field. X-ray diffraction studies show that the samples in the range from x=0.28 to 0.64 adopt the hexagonal Fe2P-type structure with a small amount of second phase which increases with increasing Si content. The samples with lower Si content show the orthorhombic Co2P-type structure. Magnetic measurements show that the paramagnetic-ferromagnetic transition temperatures range from 214to377K. Of much importance is the fact that these compounds do not contain any toxic components and exhibit excellent magnetocaloric properties.

Magnetic properties and magnetic-entropy change of MnFeP0.5As0.5-xSix (x=0-0.3) compounds.

The effects of Si substitution for As on the magnetic and magnetocaloric properties of MnFeP0.5As0.5−xSix compounds have been investigated. The Si-substituted compounds MnFeP0.5As0.5−xSix with x=0, 0.1, 0.2, and 0.3 have been prepared by high-energy ball milling and solid-state reaction. X-ray diffraction shows that all the MnFeP0.5As0.5−xSix compounds crystallize in the Fe2P-type structure. Magnetic measurements show that the compounds are ferromagnetic with a nonlinear dependence of TC on Si content. All compounds exhibit a field-induced first-order magnetic phase transition. The magnetic-entropy change is derived from the magnetization data by using the Maxwell relations. The maximum magnetic-entropy change observed in this system reaches about 32J∕kgK for a field change from 0to2T for x=0.2, at temperature around 294K.

Analysis of the first-order phase transition of (Mn,Fe)2(P,Si,Ge) using entropy change scaling

For materials with small size of the discontinuities of the free energy derivatives at transition, it is not so easy to determine the order of the transition using the criterion of the Arrott plot. In our previous paper, we found that the Mn2?xFexP0.6Si0.25Ge0.15 compounds with x?=?0.7 and 0.8 undergo first-order phase transitions but apparently shows some characteristics of a second-order phase transition. In this paper, we have employed the Arrott plot and entropy change scaling to analyse the nature of the transitions for those compounds. The results reveal that the use of entropy change scaling is more effective than the Arrott plot for determining the nature of the transition in such materials.

Spin-reorientation and anisotropy of the magnetization in single crystalline Ho2Co15Si2

Abstract We have studied the magnetic properties of a Ho 2 Co 15 Si 2 single crystal. The easy magnetization direction is parallel to the c -axis in an extended temperature region below the Curie temperature. A spin-reorientation transition takes place at 323 K, leading to an easy magnetization direction perpendicular to the c -axis below this temperature. We have compared the present results with those obtained previously on various R 2 Co 17 single crystals and found that Si substitution not only leads to a sign reversal in the Co sublattice anisotropy but also leads to a substantial anisotropy of the saturation magnetization. Sign and magnitude of the magnetization anisotropy are conserved during the spin-reorientation transition.

Optical properties of Yb-doped LaB6 from first-principles calculation

The optical properties of Yb-doped LaB6 have been investigated by first-principles calculations within the framework of density functional theory. The results show that the Yb 4f states at near Fermi surface affect their optical properties and the Yb-doping leads to a reduction of the plasmon energy of LaB6, i.e. a redshift of the position of transmission peak in the visible-near infrared region. This study offers a theoretical prediction for the design and application of Yb-doped LaB6 as an optoelectronic material.

Synthesis, and magnetic and optical properties of nanocrystalline alkaline-earth hexaborides

Cubic-shaped ultrafine alkaline-earth hexaborides (MB6, M = Ca, Ba, Sr) have been synthesized via a solid-state reaction of MO with NaBH4 at 1150 °C. Phase composition, grain morphology, microstructure, magnetic and optical absorption properties were investigated using XRD, FESEM, HRTEM, a SQUID magnetometer and optical measurements. Results show that all of the synthesized hexaborides are composed of a CsCl-type single phase and the average grain sizes of nanocrystalline CaB6, SrB6 and BaB6 are 150 nm, 20 nm and 30 nm at a reaction temperature of 1150 °C. The magnetic measurement results show that all of the synthesized samples show a weak ferromagnetic behavior at room temperature. This is the first time that we have found ferromagnetic properties in nanocrystalline alkaline-earth hexaborides. Moreover, the HRTEM results strongly supported that the magnetic moment of the alkaline-earth hexaborides originated from the intrinsic defects of the nanocrystals. The optical absorption results show strong light absorption in the visible light region and transparency for near infrared rays for nanocrystalline alkaline-earth hexaborides. This interesting optical property should have important effects for extending optical applications, such as in near infrared filtering or detectors.

Magnetocaloric proporties of Mn5Sn3-xGax alloys

The structural and magnetocaloric properties of Mn/sub 5/Sn/sub 3-x/Ga/sub x/ alloys with x = 0.1, 0.3, 0.5, 1.0 and 1.5 are investigated. Ingots of the alloys were prepared by arc melting stoichiometric amounts of Mn (99.9%), Sn (99.9%) and Ga (99.99%) in an argon atmosphere. The ingots were annealed at 1073 K for one week in evacuated quartz ampoules, followed by quenching into water. The powder X-ray diffraction (XRD) data were collected at room temperature with Cu K/sub /spl alpha// radiation in the range of 2 /spl theta/ from 20/spl deg/ to 80/spl deg/. The results show that all alloys investigated, crystallize in the Ni/sub 2/In-type hexagonal structure with space group P63/mmc. The lattice parameters a and c decrease with increasing x. Magnetization measurements were performed in the temperature range from 5 to 400 K and in fields up to 5 T. The magnetization of the alloys initially increases with increasing temperature, and then becomes flat which is probably related to the magnetic anisotropy of the alloys. The Curie temperature of these alloys monotonously increases from 236 K for x = 0.1 to 281 K for x = 1.5. Ga substitution leads to an increase in the average moment of Mn atoms and therefore, an increase in the magnetocaloric effects, though the effects are relatively small due to the relatively small moment and the second order nature of the transition.

Optical Response of CeB6 Nanoparticles with Different Sizes and Shapes from Discrete-Dipole Approximation

The discrete dipole approximation is used to investigate the optical response of CeB6 nanoparticles with different sizes and different shapes. The extinction valley in the visible light range becomes narrower and the extinction peak at the near infrared region (NIR) is red-shifted with the increasing particle size. In addition, the extinction peak value of the spherical particle decreases more rapidly than that of cubic-shaped particle with an increase in the particle size, and the cubic-shaped particles exhibit better performance on blocking NIR radiation than spherical-shaped particles. The calculation results coincide well with the reported experimental results.

EFFECT OF ANNEALING TIME ON THE STRUCTURE AND ELECTROCHEMICAL PROPERTIES OF LA0.70MG0.30NI2.45CO0.75AL0.30 HYDROGEN STORA GE ALLOY

The as-cast alloy with the composition of La0.70Ni2.45Co0.75Al0.30was prepared by melting. La-Mg-Ni-based La0.70Mg0.3Ni2.45Co0.75Al0.30 hydrogen storage alloy has been synthesized by milling blending of the as-cast alloy andelemental Mg, followed by an annealing for x (x= 2, 4, 6 and 8 h) at 600°C. The effect of annealing time on the structure and electrochemical properties of La0.70Mg0.3Ni2.45Co0.75Al0.30 hydrogen storage alloy was investigated. The results showed that the La0.70Mg0.3Ni2.45Co0.75Al0.30 alloys contain the LaNi5 and (La,Mg)2Ni7. The maximum discharge capacity of the La0.70Mg0.3Ni2.45Co0.75Al0.30 alloy increases first and then decreases with increasing annealing time. The maximum discharge capacity of alloy reaches the optimumwhen x is 4 h. The cyclic stability of the La0.70Mg0.3Ni2.45Co0.75Al0.30 alloy for a longer annealing time is better than that of the alloy for a shorter annealing time.