Magneto-optical properties of metallic ferromagnetic materials

The authors have studied the magneto-optical Kerr rotation in more than 200 metallic systems comprising alloys as well as intermetallic compounds of 3d transition metals. For all these materials the crystal structure, the lattice constants, the room temperature magnetization and the room temperature Kerr rotation at two different wavelengths are specified. For several series of ternary compounds, comprising Heusler alloys Ni₂ In-type compounds and Cr₂₃C₆ type compounds, we determined the saturation moment at 4.2 K. For a number of representative alloys or compounds a study was also made of the wavelength dependence of the complex polar Kerr effect. The values of the Kerr rotation obtained at 633 nm were compared with the corresponding values of the measured magnetization. Systematic trends were observed and have been used to classify metallic systems into systems where the Kerr rotation will not reach values much in excess of 1° and systems where higher values are likely to be found.     

Defects-driven appearance of half-metallic ferrimagnetism in Co-Mn-based Heusler alloys

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co₂MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co_1−xMn_x]₂MnSi is of particular interest since Mn₃Si is known to crystallize in the Heusler L2₁ lattice structure of Co₂MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.     

Band structure calculations for Heusler phase Co2YBi and half-Heusler phase CoYBi (Y=Mn, Cr)

We have studied the electron structure and magnetic properties of Heusler phase Co₂YBi and half-Heusler phase CoYBi (Y=Mn, Cr) by using the full-potential linearized-augmented plane-wave (FLAPW) method. Co₂MnBi and Co₂CrBi are predicted to be half-metallic magnetism with a total magnetic moment of 6 and 5 μ_B, respectively, well consistent with the Slater-Pauling rule. We also predict CoMnBi to be half-metallic magnetism with a slight compression. The gap origin for Co₂MnBi and Co₂CrBi is due to the 3d electron splitting of Mn (Cr) and Co atoms, and the gap width depends on Co electron splitting. The atom coordination surroundings have a great influence on the electron structure, and consequently the Y site in the X₂YZ structure has a more remarkable electron splitting than the X site due to the more symmetric surroundings. The investigation regarding the lattice constant dependence of magnetic moment shows that the Co magnetic moment exhibits an opposite behavior with the change of the lattice constant for Heusler and half-Heusler alloys, consequently leading to the different variation trends for total magnetic moment. The variation of total and atom magnetic moment versus lattice constant can be explained by the extent of 3d electron splitting and localization of Mn (Cr) and Co atoms for both the series of alloys.     

A neutron diffraction and magnetization study of Heusler alloys containing Co and Zr,Hf, V or Nb

Neutron diffraction, X-ray diffraction and saturation magnetization measurements have been made on a series of ferromagnetic alloys at the composition Co₂YZ, where Y is a group IVA or VA element and Z is a group IIIB or IVB element. The alloys are mainly ordered in the Heusler L2₁ type chemical structure, but some show the presence of a small amount of additional phase, or some preferential disorder.The magnetic results form two distinct groups in which the moments are confined to the Co sites but their magnitude depends on the electron concentration determined by the Y and Z atoms. Although the magnetic moments and Curie temperatures of the alloys in the same group are similar, their lattice parameters differ according to whether Z is a group IIIB or IVB element. Such behaviour indicates that, as in other alloy series with the Heusler structure, a change in lattice parameter has little effect on the magnetic properties in comparison with the effect of a change in electron concentration.     

Magnetic and chemical order in Heusler alloys containing cobalt and titanium

Saturation magnetization, X-ray and neutron diffraction measurements have been made on alloys at the compositions Co₂TiAl, Co₂TiSi, Co₂TiGa, Co₂TiGe, Co₂TiSn, Co₂TiSb and CoTiSb. The alloys containing Ga and Sn have fully ordered Heusler, L2₁, chemical structures. Co₂TiAl is similarly ordered but with some partial Ti-Al disorder. The alloys Co₂TiSi and Co₂TiGe each contain a secondary phase in addition to the primary Heusler phase. Co₂TiSb contains the two phases Co₁₂₂TiSb and Co and CoTiSb is ordered in the Cl_b structure.The alloys containing the group IIIB or IVB elements Al, Si, Ga, Ge or Sn are ferromagnetically ordered, with the magnetic moments associated with the ordered Co sites. The two alloys containing the group VB element Sb have vacant chemically ordered ‘Co’ sites but are paramagnetic.     

Magnetic properties and magneto-optical spectroscopy of Heusler alloys based on transition metals and Sn

The formation and magnetic properties of Heusler compounds of the general formula X₂YSn were studied, where X represents a 3d transition metal or Cu and where Y represents a second 3d transition metal or a metal of group IV A, VA and VI A of the periodic table. The lattice constant was determined for all Heusler compounds studied. The Co moment in the Co₂YSn compounds was not found to scale in a sample manner to the electronegativity difference E φ^* between Co and the Y components as was previously observed in the series Co₂YAl and Co₂YGa. All compounds of the type Ni₂YSn were found to be Pauli paramagnetic when Y is non-magnetic metal. Compounds having a Curie temperature above room temperature were investigated by means of magneto-optical polar Kerr-effect spectroscopy. Experimental indications were obtained for charge transfer transitions in the series Co₂TiSn, Co₂HfSn.     

Properties of Heusler-type materials Fe2TSi and FeCo2Si

A number of 3d, 4d and 5d elements are substituted into Fe₃Si in an attempt to form Heusler-like alloys. It is found that with the heat treatment followed, only the 3d elements V, Mn and Co form Heusler alloys. Substitution of the other elements produces polyphase materials. For such elements the limits of solubility in Fe₃Si are determined. For the Heusler alloys the data on degrees of disorder, hyperfine fields and magnetization are presented. These studies, in addition, test the already known site preferences of the transition metal impurities in Fe₃Si at high impurity concentrations.     

Defect‐induced ferrimagnetism in the half‐metallic Co2CrAl and Co2CrSi compounds

Co₂CrAl and Co₂CrSi are amongst the most studied Heusler alloys due to their half‐metallic character. These compounds are also well‐known to present ferromagnetism with high Curie temperatures. We show using first‐principles calculations that the creation of Cr antisites (Cr atoms at the Co sites) induces ferrimagnetism in these compounds without destroying the half‐metallic character of these alloys. The reduction of the total spin moment causes lower external fields and thus smaller energy losses in realistic magnetoelectronic, also known as spintronic, devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical study of the electronic and magnetic properties of Co2Cr1−xVxAl

We have investigated the electronic and magnetic properties of the doped Heusler alloys Co₂Cr_1−xV_xAl(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 Co₂Cr_1−xV_xAl 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 Co₂Cr_1−xV_xAl alloys is more favorable than that of Co-Y-type disorder. Furthermore, we found that Co₂Cr_1−xV_xAl of the L2₁-type structure have a half-metallic character. And the stability of L2₁ 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 Co₂Cr_1−xV_xAl.     

Investigation of half-metallic and magnetic phase transition in Co2TiZ (Z = Al,Ga, In) Heusler alloys

ABSTRACT

The half-metallic and ferromagnetic properties of Co₂TiZ (Z?=?Al, Ga, In) Heusler alloys are investigated. The structural stability is analyzed among the two possible structures, namely, L2₁ (Cu₂MnAl phase) and XA (Hg₂CuTi phase) structures. It is found that these alloys are stable in L2₁ (Cu₂MnAl phase) structure. The electronic structure of Co₂TiZ reveals that these alloys exhibit half-metallic ferromagnetism with small spin-flip gap at the minority spin state. As the pressure is increased, ferromagnetic to non-magnetic phase transition is observed at the pressures of 380.9, 363.0 and 317.8?GPa for Co₂TiAl, Co₂TiGa and Co₂TiIn, respectively. Half-metallic to metallic phase transition occurs in Co₂TiAl, Co₂TiGa and Co₂TiIn at the pressures of 76.5?GPa, 73.1?GPa and 63.9?GPa respectively.     

Field dependence of orbital magnetic moments in the Heusler compounds Co<Subscript>2</Subscript>FeAl and Co<Subscript>2</Subscript>Cr<Subscript>0.6</Subscript>Fe<Subscript>0.4</Subscript>Al

Orbital and spin magnetic moments of the Heusler compounds Co₂FeAl and Co₂Cr_0.6Fe_0.4Al were measured by magnetic circular dichroism in X-ray absorption (XMCD). The orbital magnetic moments per spin are quite large (0.1–0.2) compared to bulk values of Fe and Co metals, indicating a considerable spin–orbit coupling in these Heusler compounds. A strong localization of the 3d electron states might be responsible for this observation. The Co and Fe orbital to spin moment ratio shows a distinct decrease of r(Fe)=0.04±0.02 and r(Co)=0.06±0.02 with increasing external field for the ternary compound Co₂FeAl, while the ratio is within error limits independent of the field for Co₂Cr_0.6Fe_0.4Al. This is discussed in terms of a relation to magnetocrystalline anisotropies. PACS 75.50.Cc; 71.20.Lp; 78.40.Kc     

Spin-Dependent Thermoelectric Transport in Cobalt-Based Heusler Alloys

Sitting at the intersection of spintronics and thermoelectricity, research investigating the coupling of thermoelectric, magnetic, and electrical transport properties in materials has recently found that the ferromagnetic Heusler alloys are the ideal testbeds. These materials have attracted a lot of attention due to their useful magnetotransport properties and the possibility of tailoring these properties by modifying their composition or producing heterostructures. With the diverse range of interesting phenomena in the Heusler alloys, ferromagnetic Heusler alloys are also ideal candidates for engineering spin caloritronic devices that can take advantage of the interplay of the physics of heat flow, magnetism, and electric potential. The fundamental physical concepts important to spin-dependent thermoelectrics research are introduced and recent studies of several ferromagnetic Heusler compounds are reviewed, highlighting some exceptional latest experiments on half-metallic Co₂TiSn and the ferromagnetic Weyl semimetal Co₂MnGa. Furthermore, the potential to generate useful magnetothermoelectric voltages in electronic devices based on the anomalous Nernst effect is discussed.     

Electronic, magnetic and transport properties of Co2TiZ (Z=Si, Ge and Sn): A first-principle study

We have studied the electronic structure, magnetic and transport properties of some Co based full Heusler alloys, namely Co₂TiZ (Z=Si, Ge and Sn), in the frame work of first-principle calculations. The calculations show that Co₂TiZ (X=Si, Ge and Sn) are to be half-metallic compounds with a magnetic moment of 2 μ_B, well consistent with the Slater-Pauling rule. The electronic structure results reveal that Co₂TiZ has the high density of states at the Fermi energy in the majority-spin state and show 100% spin polarization. Our results also suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The origin of energy gap in the minority-spin states is discussed in terms of the electron splitting of Z (Z=Si, Ge and Sn) and 3d Co atoms and also the d-d hybridization between the Co and Ti atoms. The transport properties of these materials are discussed on the basis of Seebeck coefficients, electrical conductivity coefficients and thermal conductivity coefficients.     

First-principle study of full Heusler Co2Y Si using PBE0 hybrid functional

First-principle self-consistent full potential linear augmented plane wave calculations based on density functional theory using hybrid functional PBE0 are performed to study magnetic moments, density of states and half-metallicity of L2₁ type full Heusler alloys with formula Co₂Y Si. Y is Ti, V, Mn and Fe. We have compared these results with those of the PBE-GGA exchange correlation functional and the LDA + U method. The results for Co₂FeSi and Co₂MnSi are completely different; using the PBE0 hybrid functional for Co₂FeSi predicts experimental magnetic moment and also predicts this material to be half-metallic ferromagnet, while using PBE-GGA predicts it not to be half-metal. The results of PBE0 are more similar to the ones obtained by LDA + U method.     

Laser-plasma deposited nanosized layers of ferromagnetic semiconductors and silicon- and germanium-based Heusler alloys

The electric, magnetic resonance, and magneto-optical properties of thin laser-plasma deposited 50–100-nm layers of diluted magnetic semiconductors Ge:(Mn, Al)/GaAs, Ge:(Mn, Al)/Si, and Heusler alloys Co₂MnSi/Si, Co₂MnSi/GaAs, and Fe₂CrSi/GaAs with T _c > 293 K were studied. Anomalous ferromagnetic resonance in Ge:(Mn, Al) layers, ferromagnetism in CoSi/Si characterized by strong hysteresis in the magneto-optic Kerr effect, and the anomalous Hall effect at 293 K were observed.     

Spin‐polarized electron gas in Co2M Si/SrTiO3 (M = Ti,V, Cr,Mn, and Fe) heterostructures

Spin‐polarized density functional theory is used to study the TiO₂ terminated interfaces between the magnetic Heusler alloys Co₂Si (M = Ti, V, Cr, Mn, and Fe) and the non‐polar band insulator SrTiO₃. The structural relaxation at the interface turns out to depend systematically on the lattice mis‐ match. Charge transfer from the Heusler alloys (mainly the M 3d orbitals) to the Ti d_xy orbitals of the TiO₂ interface layer is found to gradually grow from M = Ti to Fe, resulting in an electron gas with increasing density of spin‐polarized charge carriers. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Composition dependence of magnetic anisotropy and quadratic magnetooptical effect in epitaxial films of the Heusler alloy Co2MnGe

Magnetic anisotropy and magnetooptic Kerr effect for epitaxial films of Co_xMn_yGe_1−x−y grown on Ge (1 1 1) substrates have been studied systematically in the compositional vicinity of the Heusler alloy Co₂MnGe. A large quadratic magnetooptic Kerr effect has been observed within a narrow region of composition centered around the Co to Mn atomic ratio of 2. The effect has been used to probe and quantify the magnetic anisotropy of the system, which is shown to have a strong sixfold in-plane component accompanied by a weak uniaxial component at room temperature. These properties are shown to depend sensitively on atomic ratio between Co and Mn, indicating the presence of an intrinsic composition-driven phenomenon.     

Electronic band structure matching for half- and full-Heusler alloys

We explore the lattice and the electronic band structures matching between the half-metallic Heusler alloys (half-Heusler NiMnSb and full-Heusler Co₂MnSi) and several hypothetical non-magnetic Heusler alloys by using first principle calculations. The lattice and band structure matching are almost perfectly satisfied between the two materials of similar crystal structures: (i) NiMnSb and XYSb and (ii) Co₂MnSi and X₂YSi, where X, Y=Ni or Cu. Owing to the high interface spin scattering asymmetry, these materials are promising to realize a high giant magnetoresistance at room temperature.     

Magnetic hyperfine fields in Heusler alloys Co YZ (Y=Ti,Zr; Z=Al,Ga,Sn)

Magnetic hyperfine fields (mhf) acting on Ta at the Ti and Zr sites have been measured in Heusler alloys Co₂TiAl(Ga,Sn) and Co₂ZrAl(Sn) by the TDPAC technique utilizing the 133–482 keV gamma cascade in¹⁸¹Hf. Curie temperatures of all the alloys have also been measured using a vibrating sample magnetometer. Present data together with the existing results on the Co₂HfAl(Ga,Sn) are discussed and compared with the mhf systematics in Heusler alloys.work partially supported by CNEN and CNPq     

Structural and magnetic properties of Co2CrAl Heusler alloys prepared by mechanical alloying

Mechanical alloying has been used to produce nanocrystalline samples of Co₂CrAl Heusler alloys. The samples were characterized by using different methods. The results indicate that, it is possible to produce L2₁-Co₂CrAl powders after 15 h of ball-milling. The grain size of 15 h ball milled L2₁-Co₂CrAl Heusler phase, calculated by analyzing the XRD peak broadening using Williamson and Hall approach was 14 nm. The estimated magnetic moment per formula unit is ∼2 μ_B. The obtained magnetic moment is significantly smaller than the theoretical value of 2.96 μ_B for L2₁ structure. It seems that an atomic disorder from the crystalline L2₁-type ordered state and two-phase separation depresses the ferromagnetic ordering in alloy. Also, the effect of annealing on the structural and magnetic properties of ball milled powders was investigated. Two structures were identified for annealed sample, namely L2₁ and B2. The obtained value for magnetic moment of annealed sample is smaller than the as-milled sample due to the presence of disordered B2 phase and improvement of phase separation.