- [Show abstract] [Hide abstract] ABSTRACT: Here, we report on the experimental discovery of biskyrmion magnetic nanodomains at RT and the observation of a biskyrmion-derived topological Hall effect (THE) in the centrosymmetric hexagonal MnNiGa magnet. Using a phase reconstruction technique based on a transport-of-intensity equation (TIE), we established the texture of the biskyrmion spin. Results from Lorentz transmission electron microscopy (TEM) and the topological Hall effect revealed that the biskyrmion phase is stable over a much wider temperature range (100 K to ~340K) and a larger magnetic field range in our material than in skyrmion-hosting bulk crystals reported previously[1-3,6]. The super-wide operating temperature and the broad range of material options indicate important progress toward the realization of skyrmion-based spintronic devices.
- [Show abstract] [Hide abstract] ABSTRACT: Magnetostructural coupling, which is the coincidence of crystallographic and magnetic transition, has obtained intense attention for its abundant magnetoresponse effects and promising technological applications, such as solid-state refrigeration, magnetic actuators and sensors. The hexagonal Ni2In-type compounds have attracted much attraction due to the strong magnetostructural coupling and the resulted giant negative thermal expansion and magnetocaloric effect. However, the as-prepared samples are quite brittle and naturally collapse into powders. Here, we report the effect of particle size on the magnetostructural coupling and magnetocaloric effect in the Ni2In-type Mn-Fe-Ni-Ge compound, which undergoes a large lattice change across the transformation from paramagnetic austenite to ferromagnetic martensite. The disappearance of martensitic transformation in a large amount of austenitic phase with reducing particle size, to our best knowledge, has not been reported up to now. The ratio can be as high as 40.6% when the MnNi0.8Fe0.2Ge bulk was broken into particles in the size range of 5~15 μm. Meanwhile, the remained magnetostructural transition gets wider and the magnetic hysteresis becomes smaller. As a result, the entropy change drops, but the effective cooling power RCeffe increases and attains to the maximum at particles in the range of 20~40 μm. These observations provide constructive information and highly benefit practical applications for this class of novel magnetoresponse materials.
- [Show abstract] [Hide abstract] ABSTRACT: We investigated the martensitic transition and the magnetic properties of Mn1-xCoGe melt-spun ribbons. The as-prepared Mn1-xCoGe ribbons crystallize in austenite hexagonal phase with a textured structure. The postannealing process promotes the formation of the martensitic phase and homogenization of the alloy, resulting in a first-order magnetostructural transition in the annealed ribbons, and thus a giant magnetocaloric effect. The magnetic entropy change around the transition reaches 19 J/kgK for a magnetic field change of 0-5 T. Furthermore, it is found that the hysteresis loss around the magnetostructural transition is negligible in present annealed ribbons, which would facilitate the application of Mn1-xCoGe alloys.
- [Show abstract] [Hide abstract] ABSTRACT: Here, we report strain-induced high coercivity in La0.7Sr0.3CoO3 (LSCO) films, which suffer in-plane tensile strains due to the positive lattice mismatch between the substrate and the LSCO bulk. The films on (011)-0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 exhibit large uniaxial anisotropy, large coercivity, and high saturation magnetization at low temperature in contrast to the well soft magnetic behaviors in LSCO bulk. It is found that the coercivity of the 40 nm (001)-LSCO/SrTiO3 film can be as high as 1.45 T at 10 K and the observed coercivity decreases rapidly as the thickness increases, though the Curie temperature is below room temperature. The large coercivity and anisotropy should be closely related to the strain-induced structural changes and the different orbital ordering of Co3+ and Co4+ ions. Meanwhile, the enhanced domain wall pinning by the tensile strain may also contribute to the observed high coercivity.
- [Show abstract] [Hide abstract] ABSTRACT: High performance (La0.35Ce0.65)xFe14B (x = 2.0, 2.4, 2.8, 3.2, 3.6, 4.0) ribbons were prepared by melt-spinning method, using industrial La-Ce mischmetal. Phase composition and room temperature permanent magnetic properties were investigated. The main phases of all samples crystallize in the tetragonal 2:14:1 type structure. A second-phase appears when x = 3.6 and 4.0, which is La-Ce alloy with a crystal structure of face-centered cubic but not CeFe2. Replacement of CeFe2 by La-Ce alloy enhances the temperature stability of the magnetic material at low temperature. The intrinsic coercivity increases with x monotonously, and the energy product reaches a maximal value of 8.29 MGOe at x = 2.4 when an optimal wheel velocity of 20 m/s was adopted, which is twice of that of Ce2Fe14B. The coercivity mechanism and intergrain exchange coupling were studied by using minor loops. The results indicated that strong pinning effect dominate the magnetization reversal process in all of the rare-earth-rich ribbons. The structure-property relationship was analyzed by using Henkel plots, and intergrian exchange coupling were found in all samples.
- [Show abstract] [Hide abstract] ABSTRACT: (La0.35Ce0.65)2Fe14-xSixB (x=0.0, 0.5, 1.0, 1.5, 2.0) ribbons were prepared by melt-spinning technique, and La-Ce mischmetal with impurities were used as the starting materials. The effect of Si substitution on phase formation and room temperature magnetic properties were investigated by X-ray diffraction and VSM. It was found that the optimal wheel velocity dropped dramatically when a small amount of Si was introduced. It dropped from 20 m/s to 15 m/s while Si concentration increases from x=0.0 to x=0.5. We also found that introducing more Si contents can damage the phase formation of 2:14:1 structure, which is different from the case in Nd2Fe14B and even in Ce2Fe14B. The substitution of Si for Fe does not enhance the coercivity of (La0.35Ce0.65)2Fe14B magnets effectively. The result indicates that the early findings about the element substitution and addition in Nd2Fe14B cannot be simply copied in researches of (La0.35Ce0.65)2Fe14B.
- [Show abstract] [Hide abstract] ABSTRACT: The (La1-yPry)1-xCaxMnO3 systems has been extensively studied in recent years as a typical electronic phase separation(PS) system. [1-4] As well known, the average radius of A sites in the ABO3 structure plays a key role in determining the band (W) and electronic characteristics of the perovskite manganites. Uehara et al has found that the chemical replacement of Pr for La (Pr is smaller than La) could reduce the W , as a result, the ferromagnetic metallic (FM) phase transforms into the charge/orbital ordering (COO) partially. Therefore, the coexistence and competition of the FM metallic phase with the COO insulating phase emerges upon Pr doping. On the other hand, the strain exerted by the lattice mismatch between the substrate and bulk also plays an important role in controlling the magnetic and transport properties of films, which can influence the balance of free energy between the coexistent COO and FM phases by adjusting the strength of the double exchange interaction and the Jahn-Teller (JT) electron-lattice coupling.  Here, we report the strain effects on the phase separation and the competition between COO and FM phases in La0.325Pr0.3Ca0.375MnO3 (LPCMO) thin films.
- [Show abstract] [Hide abstract] ABSTRACT: The mechanical properties and magnetocaloric effect (MCE) of bonded La(Fe, Si)13 hydrides have been studied in detail. The mechanical strength increases with increasing the grade of epoxy resin from E-20 to E-51. This occurs because more pores and boundaries are filled with high grade resin since high epoxide content increases the degree of crosslinking and reduces the viscosity and shrinkage of resin. The compressive strength reaches 162 MPa for the bonded LaFe11.7Si1.3C0.2H1.8 with 3 wt. % E-51, which is 35% higher than that of bulk LaFe11.7Si1.3C0.2 compound (120 MPa). The mass ΔSM values remain almost same in bonded hydrides and are in a good agreement with the theoretical value. The maximum volumetric ΔSM values are 61.8, 58.0, and 54.7 mJ/cm3 K for bonded hydrides with epoxy resins E-20, E-44, and E-51, respectively, much higher than those of some magnetocaloric materials in same temperature range. The improved mechanical properties and large MCE indicate that bonded LaFe11.7Si1.3C0.2H1.8 is a promising material for room temperature magnetic refrigeration.
- [Show abstract] [Hide abstract] ABSTRACT: Magnetic properties and magnetocaloric effect (MCE) of intermetallic HoNiSi compound have been investigated systematically. It is found that HoNiSi exhibits antiferromagnetic (AFM) state below the Néel temperature TN of 3.8 K, which is quite close to the liquid helium temperature (4 K). A giant MCE without hysteresis loss is observed in HoNiSi, which is related to the field-induced first-order metamagnetic transition from AFM to ferromagnetic states. For a magnetic field change of 2 T, the maximum values of magnetic entropy change (−ΔSM ) and adiabatic temperature change (ΔTad ) are 17.5 J/kg K and 4.5 K, respectively. In addition, HoNiSi presents both large values of positive and negative ΔSM for the low field changes, i.e., the maximum −ΔSM values are 9.2 J/kg K around TN and −7.2 J/kg K below TN for the field changes of 1 and 0.5 T, respectively. A universal curve of ΔSM is successfully constructed by using phenomenological procedure, proving the applicability of universal ΔSM curve for AFM materials. The giant reversible MCE for relatively low magnetic field change makes HoNiSi attractive candidate for magnetic refrigerant materials around liquid helium temperature.
- [Show abstract] [Hide abstract] ABSTRACT: The microstructure, crystal structure, and magnetic properties of low-temperature phase (LTP) Mn-Bi nanosheets, prepared by surfactant assistant high-energy ball milling (SA-HEBM) with oleylamine and oleic acid as the surfactant, were examined with scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer, respectively. Effect of ball-milling time on the coercivity of LTP Mn-Bi nanosheets was systematically investigated. Results show that the high energy ball milling time from tens of minutes to several hours results in the coercivity increase of Mn-Bi powders and peak values of 14.3 kOe around 10 h. LTP Mn-Bi nanosheets are characterized by an average thickness of tens of nanometers, an average diameter of ̃1.5 μm, and possess a relatively large aspect ratio, an ultra-high room temperature coercivity of 22.3 kOe, a significant geometrical and magnetic anisotropy, and a strong (00l) crystal texture. Magnetization and demagnetization behaviors reveal that wall pinning is the dominant coercivity mechanism in these LTP Mn-Bi nanosheets. The ultrafine grain refinement introduced by the SA-HEBM process contribute to the ultra-high coercivity of LTP Mn-Bi nanosheets and a large number of defects put a powerful pinning effect on the magnetic domain movement, simultaneously. Further magnetic measurement at 437 K shows that a high coercivity of 17.8 kOe and a strong positive temperature coefficient of coercivity existed in the bonded permanent magnet made by LTP Mn-Bi nanosheets.
- [Show abstract] [Hide abstract] ABSTRACT: Bonded La(Fe, Si)13 magnetic refrigeration materials have been prepared, and the microstructure, mechanical properties, and magnetocaloric effect (MCE) of bonded LaFe11.7Si1.3C0.2Hx have been investigated systematically. Bonded materials show porous architecture, and the mechanical properties increase with the increase of epoxy resin content, which could fill more pores and boundaries and thus enhance the binding force between different particles. Bonded LaFe11.7Si1.3C0.2H1.8 with 3 wt. % epoxy resin exhibits a compressive strength of 162 MPa, 35% higher than that of bulk compound. The mass magnetic entropy change (ΔSM) remains nearly unchanged while the volumetric ΔSM reduces due to the decrease of density in bonded materials. For a low magnetic field change of 2 T, the maximum ΔSM value of bonded LaFe11.7Si1.3C0.2H1.8 is ∼10.2 J/kg K and ∼54.7 mJ/cm3 K, which is larger than those of some magnetocaloric materials in the same temperature range. Enhanced mechanical properties and great MCE suggest that bonded La(Fe, Si)13-based materials could be promising candidates of magnetocaloric materials for practical applications of magnetic refrigeration.
- [Show abstract] [Hide abstract] ABSTRACT: La(Fe, Si)13-based compounds have been considered as promising candidates for magnetic refrigerants particularly near room temperature. Herein we review recent progress particularly in the study of the effects of interstitial H and/or C atoms on the magnetic and magnetocaloric properties of La(Fe, Si)13 compounds. By introducing H and/or C atoms, the Curie temperature T C increases notably with the increase of lattice expansion which makes the Fe 3d band narrow and reduces the overlap of the Fe 3d wave functions. The first-order itinerant-electron metamagnetic transition is conserved and the MCE still remains high after hydrogen absorption. In contrast, the characteristic of magnetic transition varies from first-order to second-order with the increase of C concentration, which leads to remarkable reduction of thermal and magnetic hysteresis. In addition, the introduction of interstitial C atoms promotes the formation of NaZn13-type (1:13) phase in La(Fe, Si)13 compounds, and thus reducing the annealing time significantly from 40 days for LaFe11.7Si1.3 to a week for LaFe11.7Si1.3C0.2. The pre-occupied interstitial C atoms may depress the rate of hydrogen absorption and release, which is favorable to the accurate control of hydrogen content. It is found that the reduction of particle size would greatly depress the hysteresis loss and improve the hydrogenation process. By the incorporation of both H and C atoms, large MCE without hysteresis loss can be obtained in La(Fe, Si)13 compounds around room temperature, for instance, La0.7Pr0.3Fe11.5Si1.5C0.2H1.2 exhibits a large |ΔS M| of 22.1 J/(kg·K) at T C = 321 K without hysteresis loss for a field change of 0–5 T.
- [Show abstract] [Hide abstract] ABSTRACT: Magnetocaloric effect (MCE) of RMn2 (R = Tb, Dy, Ho, Er) compounds are investigated. TbMn2 and DyMn2 crystallize in cubic Laves phase structure (C15 type), whereas HoMn2 and ErMn2 crystallize in hexagonal Laves phase structure (C14 type). For TbMn2 compound, the field-induced metamagnetic transition accompanying a spontaneous cell volume expansion is observed (inverse MCE), which leads to a large positive value (8.3 J kg−1 K−1) of magnetic entropy change around 36 K under the field change of 0–1 T, while the maximal values of magnetic entropy change (ΔSM) and the refrigerant capacity (RC) for other RMn2 (R = Dy, Ho, Er) compounds are −15.7, −18.4, −25.5 J kg−1 K−1 and 403.6, 404.3, 316.0 J kg−1 around their TC with negligible thermal and magnetic hysteresis loss for the field change of 0–5 T, respectively. The results suggest that RMn2 (R = Dy, Ho, Er) may be appropriate candidates for magnetic refrigerant working at low temperature region 10–80 K.
- [Show abstract] [Hide abstract] ABSTRACT: The magnetocaloric effect (MCE) of RNi2Si2 (R=Dy, Ho, Er) compounds with the ThCr2Si2-type body-centered tetragonal structure are investigated. RNi2Si2 compounds are antiferromagnetic (AFM) with Néel temperature TN=6.5 K, 4.9 K, and 3.5 K, respectively. A field-induced metamagnetic transition from AFM-to-ferromagnetic (FM) state is found below TN, which leads to a large MCE around the TN. The maximal values of magnetic entropy change (ΔSM) for RNi2Si2 (R=Dy, Ho, Er) reach −6.9, −10.9, and −15.1 Jkg−1 K−1 and −21.3, −21.7, and −21.3 Jkg−1 K−1 without thermal and magnetic hysteresis losses for the field changes of 0–2 T and 0–5 T, respectively. The large ΔSM is associated with the field-induced first-order AFM–FM metamagnetic transition and low critical field. The excellent MCE under low field change without hysteresis loss suggests that RNi2Si2 (R=Dy, Ho, Er) can be an appropriate candidate for magnetic refrigerant in liquid helium temperature ranges.
- [Show abstract] [Hide abstract] ABSTRACT: The title compounds are prepared by arc melting of stoichiometric amounts of the elements followed by annealing (1073 K, 7 d).
- [Show abstract] [Hide abstract] ABSTRACT: The compound Mn1.1Fe0.9P0.76Ge0.24 has been studied using neutron powder diffraction (NPD), differential scanning calorimeter (DSC), and magnetic measurements, in order to clarify the nature of the magnetic and structural transition and measure the associated entropy change (ΔS). The strongly first order transition occurs from a paramagnetic (PM) to a ferromagnetic (FM) phase and can be induced either by temperature or by an applied magnetic field. Our investigations indicate that the two processes exhibit identical evolutions regarding the crystal and magnetic structures, indicating they should have the same entropy change. We, therefore, conclude that the ΔSDSC obtained by the DSC method (where the transition is temperature induced) is valid also for the magnetically induced transition, thus avoiding uncertainties connected with the magnetic measurements. We have obtained the ΔSDSC = 33.8 J/kg · K for this sample upon cooling, which would increase to 42.7 J/kg · K for a impurity-free and completely homogeneous sample. For comparison, the magnetic entropy changes (ΔSM) induced by magnetic field and calculated using the Maxwell relation yields a ΔSM = 46.5J/kg · K, 38% higher than ΔSDSC. These entropy results are compared and discussed.
- [Show abstract] [Hide abstract] ABSTRACT: The microstructure and magnetic properties for LaFe10.85 Co0.65Si1.5C0.2, LaFe11.5Si1.5C0.2 and LaFe10.85Co0.65Si1.5 compounds were investigated. The ingots were prepared by 5 kilogram Vacuum Induction Melting Furnace. Then LaFe10.85Co0.65Si1.5C0.2 and LaFe11.5Si1.5C0.2 thin strips were prepared by the strip-casting process. The DSC measurement shows that the maximum latent heat of LaFe11.5Si1.5C0.2 thin strips has been obtained after annealing at 1353 K for 12 h, longer time annealing seems to be no good for latent heat. Substitute Co for Fe could cause inhomogeneous distribution of Curie temperature (T-C) in the bulk ingots, but the inhomogeneity can be improved by preparing the strip from the ingot. Under magnetic field changes of 0-1.5 T, the magnetic entropy change near the Curie temperature was about -11.1 J/kg. K and -4.0 J/kg. K for LaFe11.5Si1.5C0.2 and LaFe10.85Co0.65Si1.5C0.2, respectively.
- [Show abstract] [Hide abstract] ABSTRACT: The effect of hydrogenating process on the homogeneity of hydrogen absorption in the La0.8Ce0.2 (Fe1−xMnx)11.5Si1.5 compounds was investigated. It is attractive that the distribution of hydrogen atoms in the compounds becomes more uniform with increasing hydrogen pressure and annealing temperature. Homogeneous hydrides of La0.8Ce0.2(Fe0.985Mn0.015)11.5Si1.5 were obtained by annealing them at the temperature of 773 K and under the hydrogen atmosphere of 0.5 MPa. With changes of Mn content, the Curie temperatures (TC) of the hydrides of La0.8Ce0.2 (Fe1−xMnx)11.5Si1.5 can be adjusted in the room temperature range from 279 to 312 K. Large magnetic entropy changes due to the itinerant-electron metamagnetic transition are obtained in the room temperature range for the hydrides of La0.8Ce0.2(Fe1−xMnx)11.5Si1.5.
Government of the People's Republic of ChinaPeping, Beijing, China
Technical Institute of Physics and ChemistryPeping, Beijing, China
Chinese Academy of Sciences
Peping, Beijing, China
- State Key Laboratory of Magnetism