L21-type Structure Research Articles

Wasp waisted-like hysteresis loops observed in the γ-Fe2MnGa compound

Bulk and local magnetic properties of the polycrystalline Fe2MnGa compound, submitted to cold-work and annealing, have been studied using DC magnetization measurements and 57Fe Mössbauer spectroscopy in a broad temperature range (15–400 K). Structural characterization has shown stabilization of an ordered L12-type structure for annealed materials. Mechanical cold-work induces grain refinement and internal stress enhancement in the L12-type structure, favoring antisite disorder. The chemical disorder favors competitions between antiferro and ferromagnetic interactions and, consequently, leading to a frustrated magnetic state when Fe and Mn sublattices ordered magnetically. Due to the chemical disorder intrinsically found in as-prepared sample or enhanced in annealed cold-work materials, we have shown (i) metamagnetic transition from antiparallel Fe and Mn coupling to a non-collinear magnetic state when applied field strength is increased and (ii) a wasp-waisted magnetic loop character (measured in annealed materials) as consequence of frustration in magnetic interactions between misplaced Fe and Mn neighbors. We also demonstrated that even under similar experimental annealing conditions, the chemical disorders in as-prepared, ribbon and powder samples of the same batch are different, producing distinct magnetic properties of the final product.

Electronic Properties of Cu‐Pd Ordered Alloys

SummaryThe Cu‐Pd alloy, for Pd concentration greater than 50 at% has a distorted FCC structure, while for Pd less than 18 at%, has a L12 type structure. The electronic structure of Cu3Pd and CuPd3, which are the ordered alloys of the Cu‐Pd system are investigated by the FP‐LAPW method based on the density functional theory. The temperature dependence of the electrical resistivity and Seebeck coefficients has been interpreted in terms of band structures and density of states.

In [formula omitted]-Fe2MnGa Heusler alloy do Fe and Mn sublattices magnetically couple parallel or antiparallel at low temperatures?

Bulk and local magnetic properties of the polycrystalline γ-Fe2MnGa Heusler alloy have been studied using X-ray diffraction, magnetization measurements and zero-field and in-field Mössbauer spectroscopy. X-ray diffraction data indicate stabilization of a L12-type structure and no structural phase transformation in the temperature range 30–300K. While Mn atomic magnetic moments order magnetically well above 300K according to magnetization data, 57Fe Mössbauer spectroscopy suggests that the Fe-sublattice ordering temperature is ca. 220K, when it couples antiparallel to the Mn-sublattice. Large vertical (magnetization-axis) and horizontal (field-axis) magnetization loop shifts are observed in field-cooling process (up to 70kOe); these effects may be associated with magnetic frustrated state that occurs by interaction of the two sublattices.

Structural and Magnetic Properties of Magnetic Shape Memory Alloy Ni<sub>46</sub>Mn<sub>41</sub>In<sub>13</sub> under Magnetic Fields

The structural and magnetic properties of Heusler alloy Ni46Mn41In13 were studied by magnetization and X-ray powder diffraction measurements in the 4.5­350K temperature range and in magnetic fields up to 5T. The alloy undergoes martensitic transformation from a L21-type cubic structure with ac = 0.600nm at 293K to a six-layered monoclinic (6M) structure in the transition temperature of 160­230K. The lattice parameters for the 6M structure were estimated to be am = 0.441nm, bm = 0.557nm, cm = 1.30nm, im = 90.0°, ¢m = 94.0°, and £m = 90.0° at 75K. In this transformation from the L21 to 6M structures, the cell volume contracts by 0.34% at 8K. In addition, we observed the magnetic field-induced reverse transformation of this alloy. [doi:10.2320/matertrans.M2013372]

Magnetocaloric effect and martensitic transition in Ni50Mn36−xCoxSn14

The structural, magnetic and martensitic transition properties are systematically investigated for Ni50Mn36−xCoxSn14 (x=0, 1, 2, and 3) alloys. X-ray diffraction reveals that all the compounds are in the cubic phase with the L21-type Heusler structure at room temperature. Temperature dependent magnetization indicates that the Curie temperatures of austenitic phases are almost unchanged with the substitution of Co for Mn. However, the martensitic transition temperatures firstly increase and then decrease with Co content increasing, which is different from the general dependency on the value of valence electrons per atom (e/a). The reason could be that the main driving force of martensitic transition for Ni50Mn36−xCoxSn14 is the hybridization between Ni and excess Mn rather than the value of e/a. Due to the strong coupling between magnetism and structure, Ni50Mn34Co2Sn14 exhibits a large magnetocaloric effect, thereby making it a good candidate for magnetic refrigeration materials.

Room temperature magneto-structural transition in Al for Sn substituted Ni–Mn–Sn melt spun ribbons

Martensitic and magnetic transformations in Ni48Mn39.5Sn12.5−xAlx (x=0, 1, 2, 3) Heusler alloy ribbons were investigated. It is demonstrated that both magnetic and structural transformations occur in all of the studied samples. It is also shown that substitution of Sn with Al causes the martensitic transformation (MT) and the reverse martensitic transformation (RMT) temperatures to increase to room temperature (ΔTMT=49K; ΔTRMT=43K), whereas the Curie temperature of martensite TCM decreases (ΔT=36K) and the Curie temperature of austenite TCA remains practically insensitive to Al introduction. This then allows to tune TCA and the MT temperature leading to their coincidence at ambient temperature. The austenite phase with the L21 type structure has been identified to exist in all the samples regardless of composition. On the other hand the structure of martensite has been shown to be sensitive to composition. It has been determined as the 10M martensite with (32¯) stacking sequence in Al free samples and the 4O martensite with the stacking periodicity (31¯) in Al containing samples. In addition, the splitting of the field cooling (FC) and the field heating (FH) thermo-magnetic curves at low (50Oe) magnetic field and below the TCM has been attributed to intermartensitic transition. The application of large magnetic field (50kOe) has shown the existence of two distinct ferromagnetic states with a considerable hysteresis loop. The properties of these materials make them promising for magnetocaloric applications.

Structural, electronic and adsorption properties of V–Rh(111) subsurface alloy

V–Rh(111) bimetallic systems were prepared by deposition of 0.4 and 2.5ML of vanadium on Rh(111) surface at 823K and by annealing up to 1023K. Subsurface alloys of similar composition and electronic structure were formed. On the latter one, (2×2) surface reconstruction was observed. Electronic structure of the V–Rh(111) subsurface alloys was studied by photoelectron spectroscopy utilizing synchrotron radiation. Atomic structure was studied by means of LEED and X-ray photoelectron diffraction. Chemisorption properties were probed by CO and O2 adsorption. The topmost atomic layer with (2×2) reconstruction consisted exclusively of Rh atoms. The obtained results indicate formation of alloy with Rh3V composition and L12-type structure. Changes observed in the electronic structure were correlated with lower reactivity towards CO adsorption. The photoemission data indicate that the CO saturation coverage at 300K is reduced to less than half on the alloy surface as compared to clean Rh(111). Oxygen adsorbed on the alloy surface strongly interacted with the subsurface vanadium and induced formation of a surface vanadium oxide.

Electronic structure, magnetic and optical properties of [formula omitted] Heusler alloy

The article describes the electronic structure, magnetic and optical properties of bulk Fe2MnGa Heusler alloy. It is shown that unlike most stoichiometric full-Heusler alloys, the bulk Fe2MnGa crystallizes into the face-centered cubic (fcc) lattice of L12(Cu3Au)-type ordering, with a Curie temperature of 800K, with small admixture of B2-phase. The magnetic and optical properties of Fe2MnGa Heusler alloy are discussed in terms of energy band calculations for ordered fcc and L21-type structure. According to the results of our first-principles calculations, the full electron energy of ferromagnetic metallic fcc phase (μfcc=6.11μBf.u.-1) is lower by about 0.06meV (0.69K) than that of ferrimagnetic half-metallic phase with L21 structure (μL21=2.036μBf.u.-1).

Annealing temperature dependence of crystal structures and magnetic properties of Fe2CrAl and Fe2CrGa Heusler alloys

The crystal structures, spontaneous magnetic moment, Is, and the Curie temperature, TC, of Fe2CrAl and Fe2CrGa Heusler alloys were investigated. Single phases of A2, B2 and L21-type structures were obtainable in Fe2CrAl alloy by controlling the annealing temperature. The values of the Is and TC of the A2-type phase were 2.2μB/f.u. and 316K and those of the B2-type phase were 2.0μB/f.u. and 274K, larger than those of the L21-type phase of 1.6μB/f.u. and 210K, respectively. A single phase of the A2-type phase in Fe2CrGa alloy was obtained and the Is and TC were about 2.8μB/f.u. and 353K, respectively. The B2 or L21-type single phases of Fe2CrGa alloy could not be obtained because of the inevitable precipitants.

Stress-Temperature Phase Diagram of Ni<sub>2</sub>MnGa and Structural Relations between its Constituent Phases

The ferromagnetic shape memory alloy Ni2MnGa exhibits a successive martensitic transformation from the L21-type structure to the so-called intermediate phase and then to the martensite phase with an incommensurate structure during cooling under zero stress. In addition to these phases, a new phase, which we call the X-phase, appears when Ni2MnGa is cooled under compressive stress applied in the [001] direction. In this paper, we discuss the structural relations between the X-phase and the other phases on the basis of experimental results of compressive tests, transmission electron microscope observations and neutron diffraction patterns. It is likely that a multicritical point exists in the stress-temperature phase diagram of Ni2MnGa.

Exchange interaction in L1-ordered FePt and CoPt from first-principles

The exchange interactions have been studied from a first-principles density functional calculation in FePt and CoPt with the tetragonal L10-type structure. For FePt, the effective exchange parameters of Fe at the sites 1a and 1c, JFe1a and JFe1c, have the same negative value (−1.40 mRy) while that between Fe (1a) and Fe (1c), JFe1a-1c, have a large positive value (8.29 mRy) and JFe-Pt also has a positive value (1.20 mRy). Similarly, for CoPt, the exchange parameters are JCo1a = JCo1c = −0.86 mRy, JCo1a-1c = 8.47 mRy and JCo-Pt = 1.30 mRy. The estimated mean fields TC are about 802 K and 870 K for FePt and CoPt, respectively, in good agreement with the experiments. TC shows peak value at c/a = 1 in FePt and CoPt, respectively, resulting from the interplay between negative and positive exchange interactions.

Neutron diffraction studies on the Heusler alloy Ni50Mn37Sb13

The evolution of martensitic to austenitic transformation in Ni50Mn37Sb13 has been studied usingtemperature dependent neutron diffraction, thermal property, and magnetization studies. Differential scanning calorimetric studies reveal a martensitic transformation TM around 291 K. The magnetization data yield a ferromagnetic ordering temperature of 329 K in the austenitic phase and 230 K in the martensitic phase. The analysis of the powder neutron diffraction data in the temperature range of 325–12 K indicates a structural transition from a high temperature cubic L21 type structure to an orthorhombic structure. At 270 K, both cubic and orthorhombic phases coexist. Anisotropic unit cell changes are observed at the martensitic transformation: The unit cell expands by about 1.5% along the a axis, by about 2.5% along the c axis, and compresses by about 4.28% along the b axis. Both cubic and orthorhombic phases show commensurate collinear ferromagnetic ordering with a magnetic moment of ∼3.67 μB/Mn in Mn (2a and 2f) sites.

Spin polarization of Fe-rich ferromagnetic compounds in Ru2−xFexCrSi Heusler alloys

We report spin polarization P of Ru2−xFexCrSi Heusler alloys by the Andreev reflection technique. Ru2−xFexCrSi with L21-type structure and saturation magnetic moment of 2μB per formula unit is theoretically predicted to be half-metals in the wide range of the composition x. We had clarified that the experimental results of saturation magnetic moment in Fe-rich compounds had coincided with the theoretical prediction. Therefore, we have measured the differential conductance of Ru2−xFexCrSi/Pb planar-type junctions. The P value of Ru2−xFexCrSi was determined by fitting the differential conductance with the modified Blonder–Tinkham–Klapwijk theory. We have found that the behavior of P for Ru2−xFexCrSi was independent of the composition x in the Fe-rich region; P=0.53 for both of x=1.5 and 1.7. The spin polarization is the similar value to Co-based Heusler alloys.

Theoretical study of the electronic and magnetic properties of Co 2Cr 1− xV xAl

We have investigated the electronic and magnetic properties of the doped Heusler alloys Co 2Cr 1− x V x Al( x=0, 0.25, 0.5, 0.75, 1) using first-principles density functional theory within the generalized gradient approximation (GGA) scheme. The calculated results reveal that with increasing V content the lattice parameter slightly increases; both cohesive energy and bulk modulus increase with increasing x. The magnetic moment of the Co(Cr) sites increases with V doping; the total spin moment of these compounds linearly decreases. We also have performed the electronic structure calculations for Co 2Cr 1− x V x Al with positional disorder of Co–Y(Cr,V)-type and Al–Y(Cr,V)-type. It is found that formation of Al–Y-type disorder in Co 2Cr 1− x V x Al alloys is more favorable than that of Co–Y-type disorder. Furthermore, we found that Co 2Cr 1− x V x Al of the L2 1-type structure have a half-metallic character. And the stability of L2 1 structure will enhance, however, the Curie temperature decreases as the V concentration increases. The disorder between Cr(V) and Al does not significantly reduce the spin polarization of the alloys Co 2Cr 1− x V x Al.

Measurements of spin polarization of [formula omitted] Heusler alloys by Andreev reflection

We report spin polarization P of Ru2-xFexCrSi Heusler alloys by Andreev reflection technique. Ru2-xFexCrSi with the L21-type structure is theoretically predicted to be half-metals in the wide range of the composition x. The experimental results of saturation moment in Fe-rich compounds measurements also support the theoretical prediction. We have measured the differential conductance of Ru2-xFexCrSi/Pb planar-type junctions. The P value of Ru2-xFexCrSi was determined by fitting the differential conductance with the modified Blonder–Tinkham–Klapwijk theory. The behavior of P was independent of the composition x in the Fe-rich region; P=0.53 for x=1.5 and P=0.52 for x=1.7, respectively. We have found that the spin polarization of Ru2-xFexCrSi in Fe-rich compounds was the similar values to Co-based Heusler alloys.

Observation of Reverse Transformation in Metamagnetic Shape Memory Alloy Ni<SUB>40</SUB>Co<SUB>10</SUB>Mn<SUB>34</SUB>Al<SUB>16</SUB> by High-Field X-Ray Diffraction Measurements

The structural properties of metamagnetic shape memory alloy Ni40Co10Mn34Al16 were investigated by X-ray powder diffraction measurements in the temperature range from 283 to 473 K and magnetic fields up to 5 T. With increasing temperature from 283 K, the alloy undergoes the reverse martensitic transformation from the tetragonal L10-type structure with the lattice parameter at ¼ 0:3853 nm and ct ¼ 0:3299 nm into the B2-type cubic structure with the lattice parameter ac ¼ 0:2924 nm at As ¼ 383 K. At 408 K (>As), we directly observed the field-induced reverse martensitic transformation. [doi:10.2320/matertrans.M2010150]

Site Selection and Pseudo-Clustering Behaviors of Alloying Elements in Aluminum-Lean γ-TiAl Intermetallics

Site selection and pseudo-clustering behaviors of the various M alloying elements in Al-lean Ti50Al50–XMX (X = 1, 2, 3, 4, and 5 at. pct) intermetallics have been investigated by means of the ordering energy-dependent and long-range-order forced fast Monte Carlo simulation method. The ordering energies have been calculated via pseudopotential approximation in the electronic theory of alloys up to the third coordination sphere (CS) taking the anisotropic nature of tetragonal L10-type structure of γ-TiAl into account. It was shown that the site occupation characteristics of the M alloying element atoms in γ-TiAl intermetallics are governed by the relative magnitude of partial ordering energies between Ti-M and Al-M atomic pairs. However, the sign of partial ordering energies of these atomic pairs at the first CS becomes important in determining the clustering behavior and controls the dissolution modes of alloying element atoms in the γ-TiAl matrix. The pseudo-clustering behavior of alloying elements reveals three dissolution modes, namely, random dissolution (mode I), planar clustering in two dimensions (mode II), and three-dimensional (3-D) clustering (mode III) of the M occupant atoms.

Annealing Effect on Martensitic Transformation and Magneto-Structural Properties of Ni-Mn-In Melt Spun Ribbons

We report the effect of a short-time vacuum annealing (1073 K during 10 minutes) on structural phase transition temperatures and magneto-structural properties of as-quenched ribbons of the Heusler alloy Ni50.6Mn34.5In14.9. This alloy crystallizes in a single phase cubic B2-type austenite with a Curie point of TCA=284 K that with the lowering in temperature transforms into a martensite with TCM185 K. The direct and reverse martensitic phase transition temperatures were MS=257 K, Mf = 221 K, AS = 239 K, and Af = 266 K. After annealing austenite shows the highly ordered L21-type structure while the average chemical composition as well as the structural and magnetic transition temperatures were shifted to Ni50.2Mn34.3In15.5 and MS = 253 K, Mf = 238 K, AS = 257 K, Af = 265 K, ΔT = 13 K, TCA = 299 K and TCM207 K. In the annealed samples the magnetization changes associated to the magnetic and structural transitions are more abrupt and magnetization isotherms in both the austenitic and martensitic existence region show higher initial magnetic susceptibility and faster approach to saturation. Field-cooled hysteresis loops at 10 K were shifted along the negative H-axis for both samples, but a significant anomaly was evident on the left side of the hysteresis loop for as-quenched ribbons.

Martensite-austenite and Ferromagnetic-paramagnetic Transformations in Ni2-xAxMn1-yByGa1-zCz (A, B=Co; C=Al) Heusler Alloys

Ni2MnGa Heusler alloy is known as a potential smart material. At room temperature it has a L21 type structure, undergoing a martensitic transition (TM) at low temperatures. Some authors have classified Ni-Mn-Ga Heusler alloys on the values of martensitic transformation and Curie temperatures: first group formed by alloys with TM below room temperature and TC, second group with TM around room temperature and third group with TM]TC. The partial substitution of Ni with Mn leads to an increase of the transition temperature and a decrease of the Curie temperature. The Ni2-xMn1+xGa alloys have a complex tetragonal structure at room temperature. The substitution of Ga with Al can change the crystalline and magnetic structure of Heusler alloys: Ni2MnAl is antiferromagnetic for a B2 (CsCl type, with Mn and Al randomly distributed in the center of the cube) structure and ferromagnetic for a L21 (BiF3 type with Mn and Al ordered distributed in the cube center) structure. We intend to put in evidence a dependence between the applied magnetic field on a side, and the transition temperature, on other side, for the Ni2-xAxMn1-yByGa1-zCz (A, B=Co; C=Al) Heusler alloys.

Magnetocaloric Properties of Rapidly Solidified Ni51.1Mn31.2In17.7 Heusler Alloy Ribbons

AbstractMagnetic entropy change and refrigerant capacity have been determined for a field change of 20 kOe around the second-order magnetic transition of austenite in as-quenched Ni51.1Mn31.2In17.7 alloy ribbons produced by melt spinning technique. Samples crystallize in a single-phase austenite with the highly ordered L21-type crystal structure and a Curie temperature of 275 K. The material shows a maximum magnetic entropy change of ΔSMmax= - 1.7 Jkg-1K-1, an useful working temperature range of 78 K (δTFWHM) and a refrigerant capacity of RC=132 Jkg-1 (RC= │ΔSMmax│ x δTFWHM). The considerable RC value obtained together with the fabrication via a single-step process make austenitic Ni-Mn-In ribbons of potential interest as magnetic refrigerants for room temperature magnetic refrigeration.