Wenyun Yang

Magnetic frustration and magnetocaloric effect in AlFe2-xMnxB2 (x = 0-0.5) ribbons

The crystal structure and magnetic properties of AlFe2−x Mn x B2 (x = 0–0.5) compounds were investigated. With increasing Mn content, the magnetic properties of these compounds evolve gradually and the lattice parameters change continuously: Curie temperature (T c) decreases from 312 K (x = 0) to 220 K (x = 0.5) and a new phase transition from the ferromagnetic state (FM) to spin-glass state (SPG) appears upon cooling. The lattice shrinks in the ab plane while it expands in the c direction. The formation of re-entrant SPG around 50 K was studied via temperature dependent ac and dc magnetization as well as the initial magnetization curves in 5 K. The reason for this glass state is the triangular configuration of magnetic atoms and the antiferromagnetic interactions introduced by Mn substitutions. The Curie transition leads to a conventional magnetocaloric effect (MCE). The Mn doping leads to a decrease in the MCE near room temperature, but there is a plane in entropy change (ΔS) for the x = 0.1 compound under low field which makes these compounds good candidates to produce composites used for magnetic refrigeration application in a wide temperature span.

Magnetic properties of AlFe2B2 and CeMn2Si2 synthesized by melt spinning of stoichiometric compositions

By suppressing the growth of impurities, a melt spinning method was successfully applied to the synthesis of typical layered CeMn2Si2 and AlFe2B2 compounds. X-ray diffraction analysis showed that CeMn2Si2 and AlFe2B2 had good purity and crystallized in ThCr2Si2- and AlFe2B2-type structures, respectively. The differences among three -type structures were also analyzed. The magnetic properties were investigated by magnetic measurements and electronic structure calculations. It was found that the Fe moment of AlFe2B2 reaches 1.32 μB at 5 K, which fits well with the calculated result of 1.44 μB at 0 K, and that the isothermal magnetic entropy change reaches 7.2 J kg−1 K−1 at 5 T, which shows great potential for room-temperature refrigeration applications.

Wide temperature span of entropy change in first-order metamagnetic MnCo1?xFexSi

The crystal structure and magnetic properties of MnCoxFe1?xSi (x?=?0?0.5) compounds were investigated. With increasing Fe content, the unit cell changes anisotropically and the magnetic property evolves gradually: Curie temperature decreases continuously, the first-order metamagnetic transition from a low-temperature helical antiferromagnetic (AFM) state to a high-temperature ferromagnetic state disappears gradually and then a spin-glass-like state and another AFM state emerge in the low-temperature region. The Curie transition leads to a moderate conventional entropy change. The metamagnetic transition not only yields a larger negative magnetocaloric effect at lower applied fields than in MnCoSi but also produces a very large temperature span (103?K for ??0H?=?5?T) of ?S(T), which results in a large refrigerant capacity. These phenomena were explained in terms of crystal structure change and magnetoelastic coupling mechanism. Because of the large isothermal entropy change, the wide working temperature span and the low cost, MnCo1?xFexSi compounds are promising candidates for near room-temperature magnetic refrigeration applications.

One step preparation of pure 𝝉-MnAl phase with high magnetization using strip casting method

Ferromagnetic phase of Mn-Al exhibits great potential in the rare-earth free permanent magnetic materials due to its high magnetocrystalline anisotropy, high magnetization, high Curie temperature and low cost. In this work, the strip casting technique was applied to prepare MnAl magnetic phase. X-ray diffraction and energy dispersive X-ray analyses indicate that the as-prepared Mn54Al46 strip sample consists of pure τ-MnAl magnetic phase. It is found that the composition of Mn54Al46 is suitable to prepare τ-MnAl phase during the strip casting process. The Mn54Al46 strip sample synthesized through the strip casting exhibits a fairly high magnetization of 114 emu/g under a field of 5 T, while the coercivity of iHc = 2.8 kOe, magnetization of M5T = 63.9 emu/g at room temperature can be obtained for Mn54Al46 powder sample. This preparation method can produce a large amount of τ-phase MnAl alloy and promote mass industrialized production.

The magnetic properties of NdMn x Cr2−x Si2C (0 < x < 2)

Carbon was successfully introduced into NdMn x Cr2−x Si2 serial compounds as an interstitial atom. The compounds with x ≤ 0.5 keep a CeMg2Si2-type structure while the structure changes slowly with increasing manganese content. Finally, a new superstructure was found and determined for x around 1.7. All the compounds for x = 1.1, 1.3, 1.5 and 1.7 have two magnetic phase transition temperatures around 43 and 314 K. What is more, a giant magnetocaloric effect has been observed around the lower phase transition temperature, T R ~ 51 K, in NdMn1.7Cr0.3Si2C. The maximum values of the magnetic entropy change −ΔS M for the field change of 2 T and 5 T are 14.9 J kg−1 K−1 and 26.0 J kg−1 K−1, which means the interstitial atom effect enhanced the MCE values for 85% and 63% compared to NdMn1.7Cr0.3Si2, respectively. NdMn1.7Cr0.3Si2C with first order magnetic transition but low hysteresis losses (thermal < ~0.58 K; magnetic field < ~0.12 T) can be a candidate for magnetic refrigerator applications in the temperature region near 50 K. Besides, two field-induced magnetic phase transitions were found in NdMn1.7Cr0.3Si2C among the temperature range from 53 to 60 K, and a linear relation between the phase transition field and temperature was found and analyzed.

Ab initio calculation of electronic structure and magnetic properties of R2Fe14BNx (R = Pr,Nd)

The site preference of N atom for R2Fe14BNx (R= Pr, Nd) and the interstitial nitrogen effect on the magnetic properties have been studied by the first-principles method. It was found that the nitrogen is more likely to occupy the 4e site for Pr2Fe14BNx compound, while 4f site for Nd2Fe14BNx. When N atoms entering some specific crystal sites (such as 2a and 4f), the total magnetic moments of these compounds are not reduced, but slightly increased. Although the doping of N may reduce the total magnetic moments of some R2Fe14B compounds in the cases of optimal occupancy, the volumetric effect caused by N doping can still change the electron density distributions of Fe near the Fermi level, improving the magnetic ordering temperature of such compounds.

Magnetic properties of Nd(Fe1-xCox)10.5M1.5 (M=Mo and V) and their nitrides

In this work, alloys of Nd(Fe1-xCox)10.5M1.5 (M=Mo and V) were prepared via arc melting and heat treatment. The nitrides of these alloys were synthesized using a gas-solid state reaction method. The influence of Co substitution for Fe in NdFe10.5Mo1.5 and NdFe10.5V1.5 alloys and their nitrides were investigated. It was found that the lattice parameters a, c, and unit cell volume V decrease with increasing Co content x for Nd(Fe1-xCox)10.5Mo1.5. As compared to their parent alloys, the lattice parameters and unit cells volume increase after nitrogenation, which gives rise to higher Curie temperature, magnetization and magnetocrystalline anisotropy field for nitrides. A small amount of Co substitution for Fe (x≤0.3) can enhance the magnetic properties including Curie temperature, saturation magnetization and magnetocrystalline anisotropy field of the alloys and their nitrides, while higher concentration of Co (x>0.3) will deteriorates these magnetic properties, especially for the nitrides, due to the modific...

Study of metamagnetism in Sm(Ni 0.5 Fe 0.4 Cu 0.1 ) 7

Metamagnetism, a sudden increase in the magnetization of a material with a small change of external magnetic field, are calling more attentions because of their rich magnetic phenomena and scientific significance . There are quite different physical causes for different types of metamagnets . In this work, metamagnetism has been found in arc-melting Sm(Ni0.5Fe0.4Cu0.1)7 . The physical property measurement system (PPMS) is used to measure the magnetic properties . The hysteresis loops at different temperatures (T>5K) are shown in Fig .1 . Both sides of the hysteresis loops exhibit obvious metamagnetic behaviors . The hysteresis loops show wasp-waisted character . At the beginning of the curves the magnetization increases rapidly and then gets saturation at about 1T . When field increases continuously to the critical magnetic fields (Hcm) metamagnetic behavior appears . The lower the temperature is, the higher the Hcm is . The magnetization appears to saturate when field increases to a higher value . The magnetization stays at high magnetization state until the field decreases to about 1T . The magnetizatic behaviors at another side of the hysteresis loops are the same . The critical magnetic fields (Hcm) corresponding to the metamagnetic points increases with the temperature decreasing by an exponential dependence. The spin reverse model with thermal activation (TA) is used to explain the relation of the critical field to temperature . The expression can be written as Hcm(T)=Hcm(0)exp(-kT/U), where Hcm(T) is the critical field at T, Hcm(0) is the calculated value at 0K, k is the Boltzmann constant, U is the energy needed to turnover one spin. The fitting results show U≈6 .6×10-15erg . At temperatures, below 5K, the smooth jumps turn into step-like jumps. The number of the steps and the values of critical fields vary with different samples . The inset in Fig .2 is the enlargement of one step with a field interval of 0.05T . When repeating the magnetization process, the hysteresis loops can coincide compactly, which is different from the usual Barkhausen jumps. The XRD results show that the main phase is hexagonal P6/mmm structure and the easy magnetization direction at room temperature is along c axis, which may show high anisotropy constants, where macroscopic quantum tunneling (MQT) may happen. As temperature gets extremely low, the thermal activation can be ignored and the quantum behavior becomes obvious . One possible explanation is as follows: the MQT happens first, which leads to a release of thermal energy and increase of sample temperature, followed by a huge magnetization reversal due to the external magnetic field . A step-like magnetic jump appears . The differences of step numbers and Hcm values between different polycrystalline samples are thought to be related to the relative orientation of the crystalline grain and the field direction . It seems that the MQT and TA models have solved the problem well . Another possible explanation is the narrow domain-wall pinning, but such a mechanism would have difficulty accounting for the presence of multistep jumps and the transition from one smooth jump to several sharp jumps just by changing few kelvins .

Influences of P doping on magnetic phase transition and structure in MnCoSi ribbon

The structure and magnetic properties of MnCoSi1− x Px (x = 0.05–0.50) are systematically investigated. With P content increasing, the lattice parameter a increases monotonically while both b and c decrease. At the same time, the temperature of metamagnetic transition from a low-temperature non-collinear ferromagnetic state to a high-temperature ferromagnetic state decreases and a new magnetic transition from a higher-magnetization ferromagnetic state to a lower-magnetization ferromagnetic state is observed in each of these compounds for the first time. This is explained by the changes of crystal structure and distance between Mn and Si atoms with the increase of temperature according to the high-temperature XRD result. The metamagnetic transition is found to be a second-order magnetic transition accompanied by a low inversed magnetocaloric effect (1.0 Jkg−1K−1 at 5 T) with a large temperature span (190 K at 5 T) compared with the scenario of MnCoSi. The changes in the order of metamagnetic transition and structure make P-doped MoCoSi compounds good candidates for the study of magnetoelastic coupling and the modulation of magnetic phase transition.