Magnetic Moments Of Mn Atoms Research Articles

LOCAL ORDER OF LIQUID AND UNDERCOOLED TRANSITION METAL BASED SYSTEMS: AB INITIO MOLECULAR DYNAMICS STUDY

An overview of a recent series of ab initio molecular dynamics (MD) simulations for pure liquid transition metals as well as for transition metals (TM) based liquid alloys is presented. The aim is to investigate the local structure of these systems and their evolution upon undercooling, and our results are analyzed through a three-dimensional picture of the short-ranger order (SRO) by means of the common-neighbor analysis. Recent diffraction experiments indicate that the structure of both pure metals and alloys in undercooled states is dominated by an icosahedral SRO. We find that the five-fold symmetry is already present in the liquid state of all the studied systems. However our findings show that the five-fold symmetry in the liquid state as well as its evolution upon undercooling depends on the system under consideration. For Ni , Zr , and Ta , local configurations are more complex than that given by the simple icosahedron. For Al 80 Ni 20 and Al 80 Mn 20 alloys, local configurations are the result of a strong competition between chemical and topological effects; the key role played by the occurrence of localized magnetic moments of Mn atoms to interpret their short-range arrangements is emphasized, and the time evolution of the configurations is examined in terms of the mean square displacements.

First principles study of magnetism in Al–Pd–Mn approximant

Electron and magnetic properties of 1/1 approximant of quasicrystalline AlPdMn are studied by means of density functional theory and all-electron PAW potentials. Magnetic moments of Mn atoms are analyzed in dependence on Mn concentration and its chemical environment. We have found that large local magnetic moment of Mn atoms appears at 8 at% of Mn. Substitution of Al atoms in AlPdMn by boron atoms is resulted in the enhancement of local magnetic moment of Mn atoms. The effect can be explained in terms of sp–d hybridization of atomic orbitals.

First-principles study of thin magnetic transition-metal silicide films on Si(001)

In order to combine silicon technology with the functionality of magnetic systems, a number of ferromagnetic (FM) materials have been suggested for the fabrication of metal/semiconductor heterojunctions. In this work, we present a systematic study of several candidate materials in contact with the Si surface. We employ density-functional theory calculations to address the thermodynamic stability and magnetism of both pseudomorphic CsCl-like $M$Si ($M$=Mn, Fe, Co, Ni) thin films and Heusler alloy $M_2$MnSi ($M$=Fe, Co, Ni) films on Si(001). Our calculations show that Si-termination of the $M$Si films is energetically preferable during epitaxy since it minimizes the energetic cost of broken bonds at the surface. Moreover, we can explain the calculated trends in thermodynamic stability of the $M$Si thin films in terms of the $M$-Si bond-strength and the $M$ 3d orbital occupation. From our calculations, we predict that ultrathin MnSi films are FM with sizable spin magnetic moments at the Mn atoms, while FeSi and NiSi films are nonmagnetic. However, CoSi films display itinerant ferromagnetism. For the $M_2$MnSi films with Heusler-type structure, the MnSi termination is found to have the highest thermodynamic stability. In the FM ground state, the calculated strength of the effective coupling between the magnetic moments of Mn atoms within the same layer approximately scales with the measured Curie temperatures of the bulk $M_2$MnSi compounds. In particular, the Co$_2$MnSi/Si(001) thin film has a robust FM ground state as in the bulk, and is found to be stable against a phase separation into CoSi/Si(001) and MnSi/Si(001) films. Hence this material is of possible use in FM-Si heterojunctions and deserves further experimental investigations.

Magnetic properties of ferromagnetic shape memory alloys Ni _{2-x} Cu _{x} MnGa

Magnetic measurements on Ni2−xCuxMnGa with 0 x 0.8 are made in the temperature range between 4.2 K and 440 K. A phase diagram of Ni2−xCuxMnGa is determined from the results of the temperature dependence of the initial permeability. The Curie temperature TC increases with the Cu concentration x. The martensitic and reverse martensitic transformation temperatures decrease abruptly with x. It is deduced that the substitution for Ni by Cu in Ni2MnGa promotes to localize the magnetic moment of Mn atom.

High Temperature Ferromagnetism in GaAs-Based Heterostructures with MnδDoping

We show that suitably designed magnetic semiconductor heterostructures consisting of Mn delta (delta)-doped GaAs and p-type AlGaAs layers, in which the locally high concentration of magnetic moments of Mn atoms are controllably overlapped with the two-dimensional hole gas wave function, realized remarkably high ferromagnetic transition temperatures (T(C)). A significant reduction of compensative Mn interstitials by varying the growth sequence of the structures followed by low-temperature annealing led to high T(C) up to 250 K. The heterostructure with high T(C) exhibited peculiar anomalous Hall effect behavior, whose sign depends on temperature.

Dopant effects on martensitic transition and magnetic properties of Ni51.5Mn25Ga23.5 materials

Ni51.5Mn23M2Ga23.5 materials are to reveal the dopant effect on the martensitic transition and the magnetic propert ies. We found that the dopants Ti, Zr, Hf, V, Nb, and Ta lower the martensitic transition temperature of the sample, but W increases it. The martensitic trans ition temperature is very sensitive to the atomic size when the valence electron ic concentration keeps constant for the doping elements. The dopants Ti, Zr, Hf or V decrease the magnetic moment of Mn atom in the martensite at low temperatur e, while Nb, Ta or W enhance it. A corresponding variation of TC of t he materials is not observed, which indicates that the influence of the lattice change of martensitic transition on the magnetic properties should also be ta lken into account.

Magnetic properties of (Mn 1-x Ru x ) 3 Ga alloys

We found that the pseudo binary alloys Mn1-xRux 3Ga, with 0.33≤x≤0.67, have an ordered b.c.c. structure. The lattice constant a is almost constant with respect to x: a=6.000 A for x=0.33 and a=5.992 A for x=0.67. For the alloy with x=0.33, i.e. Mn2RuGa, the magnetization is almost saturated in a field of 20 kOe. The saturation magnetization at 4.2 K is 23 emu/g, and the Curie temperature, TC, is 460 K. The TC of (Mn1-xRux)3Ga decreases almost linearly with increasing x, and it vanishes around x=0.67 (MnRu2Ga). We also determined atomic and magnetic structures from neutron diffraction experiments. The alloy Mn2RuGa (x=0.33) has an ordered structure of CuHg2Ti type; the magnetic Mn atoms mainly occupy the 4a (0,0,0) and 4d \(\frac\Box3\Box\Box4\Box\),\(\frac\Box3\Box\Box4\Box\),\(\frac\Box3\Box\Box4\Box\) sites. We also observedthat the magnetic moments of Mn atoms on the 4a and 4d sites are antiparallel to each other; values of the magnetic moment are μa=4.6 and μd=3.3 μB per Mn atom.

Search for high magnetic moment recording head material: Manganese compounds

Electronic structure calculations of magnetic properties for Mn-based alloys are performed. The full-potential linear augmented-Slater-type-orbital implementation of the local-density approximation is employed. Theoretical predictions for magnetic moments of ${\mathrm{Mn}}_{x}{\mathrm{Fe}}_{4\ensuremath{-}x}Y$ $(Y=\mathrm{N}/\mathrm{C}/\mathrm{B}/\mathrm{B}\mathrm{e})$ are in good agreement with the reported experiments. Using the energy minimum criterion, the magnetic ordering of a particular phase has been determined. Simulations suggest that lattice expansions enhance magnetic moments of Mn atoms both in face-centered-cubic and body-centered-cubic crystal lattice structures with maximum magnetization ${M}_{s}$ found to be 25 and 28 KG corresponding to a 15% and 12% increase in the lattice constant, respectively. Simulations predict that an approximately 12% increase in the magnetization ${M}_{s}$ can be achieved with ${\mathrm{Mn}}_{3}\mathrm{FeN}$ compared to that with ${\mathrm{Fe}}_{4}\mathrm{N}.$

Electronic and magnetoresistive properties of MnAs(0001)/GaAs(111) heterostructures

Electronic band-structure calculations are carried out for the MnAs(0 0 0 1)/ GaAs(1 1 1) multilayers by using the linearized muffin-tin-orbital method within the atomic sphere approximation. The exchange splitting of Mn 3d states between spin-up and spin-down bands as well as the magnitude of local magnetic moment of Mn atoms are reduced remarkably for the Mn sites near the heterointerface compared with those in the bulk of MnAs. On the other hand, the spin-polarization at the Fermi level is enhanced for the MnAs monolayers near the heterointerface.

X-ray magnetic circular dichroism in quasicrystals

The spectra of X-ray magnetic circular dichroism (XMCD) and extended X-ray absorption fine structure (EXAFS) were measured in the ferromagnetic quasicrystals (a stable decagonal phase of Al40Mn25Fe15Ge20, a unstrained icosahedral phase of Al50Pd15Mn20B15, a strained icosahedral phase of Al42.5Pd7.5Mn30B20) and the orthorhombic approximant crystalline phase of Al30Mn40B30. In Al40Mn25Fe15Ge20, the intensity of XMCD at Mn K-edge and Fe K-edge were −7×10−5 and −5×10−5, respectively, which are normalized by the edge jump of the normal absorption spectra. The values suggest that Mn and Fe in this alloy contribute to the magnetism to the same degree. From the result of EXAFS, the nearest neighbors of Mn atoms and Fe atoms were analyzed to be at the same distance. The environment around Mn and Fe were found not so similar except for the nearest neighbor distance. In Al50Pd15Mn20B15, Al42.5Pd7.5Mn30B20 and Al30Mn40B30 alloys, the intensity of XMCD around the Mn K-edge were about −3×10−4. The magnetic moments of Mn atoms in these alloys are roughly estimated to be 0.8μB. The difference of a magnetic structure was detected between the quasicrystals and the approximant. The result of EXAFS analysis indicates that the distances from Mn to its first nearest neighbor and its second nearest neighbor are almost equal. The existence of phason strain field have no appreciable influence on the moment of Mn.

The Effect of Deposition Rate on the Low Temperature Resistivity and the Neel Point of \alpha-Mn Thin Films

The effect of deposition rate on low temperature resistivity and the Neel point of thin films of evaporated \alpha-Mn has been studied. Resistivity measurements were carried out between 300 and 1.5 K using the van der Pauw four probe technique. A sharp transition, which defines the Neel point of antiferromagnetic Mn, has been observed at 90 \pm1 K by plotting the temperature derivative of the resistivity against temperature for a sample whose resistivity temperature behaviour is typical of bulk \alpha-Mn. The results reveal that, as the rate of deposition increases, the resistivity temperature pattern changes in a way so as to increase the line width of the transition. However, at a deposition rate of 30\AA s^{-1}, the transition is not observed. The excess low temperature resistivity increases as the rate of deposition increases. The origin of these excess resistivities can be interpreted as due to the effect of a marked sensitivity of the magnetic moment of Mn atoms to their local environment.

Low-frequency NMR line of 55Mn in the intermetallic compound Y 6Mn 23

The NMR spin-echo spectrum of 55Mn in Y 6Mn 23 at 4.2 K is reinvestigated. The well-known zero-field NMR line at about 44 MHz is identified as a 55Mn signal. A new assignment of resonance line frequencies to the structural positions and magnetic moments of Mn atoms is proposed and discussed.

Recent developments in magnetism and magnetic materials

In the last decade we have obtained a deeper understanding of basic magnetism in alloys and a variety of new magnetic materials have been developed. In this review article two kinds of recent topics in these fields will be discussed. The first one is the magnetic properties of rare earth-3d transition intermetallics. After describing the magnetic properties of RMn2 very briefly, we discuss the magnetic properties of YMn2 in detail, particularly its magnetovolume effect which is closely related with the magnetic moment of Mn atoms below and above the magnetic transition temperature. Then, as an example of recent developments in soft magnetic materials, Fe-Ni-Cr alloys with very high permeability and low coercive force will be discussed including impurity effects on their magnetic properties.

Temperature Variation of Magnetization and Electrical Resistivity in MnCoxSb [0.1 ≤x≤ 0.25

Magnetization measurements have been done from 77 K to 600 K on MnCo x Sb [0.1 ≤ x ≤ 0.25]. For all the concentrations other than x =0.25, the temperature variation of magnetization was found to follow the Stoner model. Anomalous behaviour observed in the case of MnCo 0.25 Sb is attributed to the exchange interaction between Mn–Co dominating over that of the exchange interaction between Mn–Mn. Electrical resistivity studies carried out in the temperature range 85–300 K show metallic behaviour for all samples excepting in the case of MnCo 0.25 Sb where an anomalous behaviour has been observed. This has been attributed to the scattering of conduction electrons due to the fluctuation of magnetic moment of Co atoms dominating over that due to the regular site magnetic moment of Mn atom.

MAGNETISM OF Mn ATOMS IN RARE EARTH INTERMETALLIC COMPOUNDS

In this work the magnetic properties (the values of magnetic moments localized on the Mn atoms, the Néel or Curie temperature) of several intermetallic RMn2X2, RMnX2 and RMnX compounds (where R-rare earth atoms, X-Si or Ge) are analysed. The obtained data are correlated with the data for the RMn2 Laves phases. In R—Mn—X intermetallic compounds two critical values of Mn—Mn distances are observed. The first is connected with a transition from a nonmagnetic to a magnetic state, and the second one with the transition from an antiferro- to a ferromagnetic state. The values of the magnetic moment of Mn atoms are analysed. The change in these values in ferro- and antiferromagnetic state is observed. The presented results indicate an itinerant character of magnetic properties of Mn sublattices in these compounds.

Magnetic properties of Al-Ge-Mn and Al-Cu-Ge-Mn icosahedral alloys.

${\mathrm{Al}}_{52.5}$${\mathrm{Ge}}_{22.5}$${\mathrm{Mn}}_{25}$, ${\mathrm{Al}}_{52.5}$${\mathrm{Ge}}_{22.5}$${\mathrm{Mn}}_{24.85}$${\mathrm{Fe}}_{0.15}$, ${\mathrm{Al}}_{40}$${\mathrm{Cu}}_{10}$${\mathrm{Ge}}_{25}$${\mathrm{Mn}}_{25}$, and ${\mathrm{Al}}_{40}$${\mathrm{Cu}}_{9.94}$${\mathrm{Fe}}_{0.06}$${\mathrm{Ge}}_{25}$${\mathrm{Mn}}_{25}$ icosahedral alloys and AlGeMn and ${\mathrm{AlGeMn}}_{0.995}$${\mathrm{Fe}}_{0.005}$ crystalline alloys have been studied with x-ray diffraction, differential thermal analysis, magnetization, and $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy. It is found that AlGeMn, a ferromagnet, is the major second phase present in the samples of icosahedral Al-Ge-Mn and Al-Cu-Ge-Mn systems, and is thus partially responsible for the observed ferromagnetism in these alloys. This ferromagnet is also the main crystallization product of these systems. Room-temperature $^{57}\mathrm{Fe}$ M\"ossbauer spectra show that Fe atoms in these systems bear no magnetic moment and are distributed among a multiplicity of sites, which is interpreted as evidence of intrinsic disorder present in icosahedral alloys. Analysis of 4.2-K M\"ossbauer spectra shows that less than half of the Fe atoms possess a very small magnetic moment in both systems, which is the first direct evidence supporting the notion of two classes of transition-metal sites in Al-based icosahedral alloys. It is argued that the magnetic moment of Mn atoms in these systems as well as the Curie temperatures must be very small. It is noted that annealing of rapidly quenched Al-Ge-Mn and Al-Cu-Ge-Mn alloys leads to the occurrence of high values of coercive force and saturation magnetization.

Magnetic structures of RMnSi 2 compounds (R = La, Ce, Pr, Nd) from neutron study

Investigations by neutron diffraction measurements are reported on the ternary silicides RMnSi 2 (R = La to Nd) with the orthorhombic structure of TbFeSi 2 type (SG:Cmcm). This structure, closely related to the ThCr 2Si 2 type structure, can be described as isolated ThCr 2Si 2 blocks connected via α ThSi 2 slabs. Each of the R, Mn and Si atoms are arranged in alternate layers stacked (along the b-axis) with the sequence RSi(Mn 2)SiRSiSiRSi(Mn 2)SiR. The Mn-Mn intralayer distances (∼2.9Å) are considerably shorter than the Mn-Mn interlayers distances (∼9Å). At room temperature, all four compounds order ferromagnetically. The magnetic moments of Mn atoms (∼2μ B) are parallel to the b-axis. At lower temperatures PrMnSi 2 and NdMnSi 2 show an additional magnetic transition which corresponds to the ordering of the rare-earth sublattice. PrMnSi 2 orders antiferromagnetically below T N = 35 K ; its magnetic structure consists of ferromagnetic (010) layers of Pr and Mn with moments perpendicular to the layers. Each Mn layer is antiferromagnetically coupled to the two adjacent ferromagnetically coupled Pr layers (μ Mn = 2.35(10)μ B, μ Pr = 2.04(4)μ B). Below T C = 40 K , NdMnSi 2 is still ferroma gnetic. Nd and Mn magnetic moments lie in the (011) plane, along [001] and at 45° of this direction for Nd and Mn, respectively ( μ Mn = 2.25(10)μ B , μ Nd = 1.80(7)μ B ). At 2 K, a preliminary study shows that, in CeMnSi 2, the cerium sublattice would be ferromagnetic and antiferromagnetically coupled with the manganese sublattice according to the following scheme: -(+)--(+)-, reference to the stacking sequence previously defined ( μ Mn = 2.24(15)μ B , μ Ce = 0.23(13)μ B ). The results are discussed in terms of RKKY exchange interactions and compared with those obtained previously on the corresponding ThCr 2Si 2 type ternary silicides and germanides.

Average Magnetic Moment of Mn Atoms in an Fe90Mn5Cr5Ternary Alloy

Average magnetic moment of Mn in bcc Fe 90 Mn 5 Cr 5 alloy has been studied by means of polarized neutron diffuse scattering in order to investigate the anomalous magnetic behavior of Mu impurity in Fe. By assuming the average magnetic moment of Fe and Cr to have the same value as in an Fe 95 Cr 5 alloy, we have determined the average magnetic moment of Mn at temperatures of 10 K, 150 K, 295 K and 383 K. The value of Mn average magnetic moments obtained for this alloy is larger than that in an Fe 95 Mn 5 alloy at all temperatures. A simple nearest neighbor model has been discussed in which Mn atoms in Fe have either positive or negative moments according to the various environmental conditions. In Fe(Mu, Cr) alloys, the negative Mn moments will always flip to positive direction if it has at least one Cr atom in its nearest neighboring shell. This model has explained the experimental results at least qualitatively.

Magnetic moment of Mn atom in Fe-Ni alloys

The magnetic moment of Mn atoms embedded in Fe-Ni alloys and the magnetic disturbance in the host alloy were obtained at room temperature by means of diffuse scattering of polarized neutrons. These can be explained by introducing the flipping of the Mn moments with increasing Fe concentration.

Average Magnetic Moment of Mn Impurities in Ferromagnetic Ni–Co Alloys

The average magnetic moments of Mn atoms dissolved in fcc Ni–Co alloys have been determined as a function of concentration of host alloys. The measurements were made with alloys containing 2 atomic percent Mn at about 300 K and 100 K by means of diffuse scattering of polarized neutrons. The average magnetic moment of Mn, which is about 3 µ B in pure Ni, decreases with increasing Co concentration and changes its direction at about 50 atomic percent Co from parallel to antiparallel to the bulk magnetization of host alloys. This behavior agrees qualitatively with the theoretical results obtained under the assumption of the extended coherent potential approximation, but a quantitative discrepancy is found especially near Co concentration of about 20 to 40 atomic percent.