Tetragonal Mn Research Articles

Constituent contribution to the magnetocrystalline anisotropy in Mn(Al1−xGax)

The phase stability and magnetocrystalline anisotropy (MCA) of tetragonal Mn(Al1−xGax) with the L10-type structure (P4/mmm) has been studied using first-principles density functional calculations. The calculated decomposition energy indicates that partial replacement of Al by Ga suppresses the formation of Mn5(Al,Ga)8 and enhances the thermal stability of the L10 phase while the total magnetic moment per formula unit (f.u.) remains almost unchanged. The site- and atomic-resolved MCA calculations show that the MCA energy (MAE) comes mainly from the Mn atoms, and the total MAE increases from 0.25 meV/f.u. (x = 0) to 0.34 meV/f.u (x = 1). Spin resolved MCA and band structure calculations indicate that the high MCA is mainly due to spin flipping behavior near Fermi level. The derived effective magnetic anisotropy field increases from 37 kOe (x = 0) to 46 kOe (x = 1), in agreement with experiments. Doping with Ga improves the thermal stability of the L10 structure and enhances the magnetic anisotropy field, which facilitates developing high coercivity Mn-Al magnets.

Antiferromagnet-induced perpendicular magnetic anisotropy in ferromagnetic Co/Fe films with strong in-plane magnetic anisotropy

Ferromagnetic (FM) films with higher magnetic moment density typically exhibit sizable in-plane magnetic anisotropy as a result of strong dipolar interactions, which hinder their application in state-of-the-art perpendicularly based magnetic devices. This study reports the effects of triggering the perpendicular magnetic anisotropy (PMA) of a Co/Fe film with strong in-plane magnetic anisotropy through nearby antiferromagnetic (AFM) fcc ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ and face-centered tetragonal (e-fct) Mn films. Our results demonstrate that fcc ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ films with a three-dimensional quadratic AFM spin structure can trigger robust PMA in a Co/Fe film through collinearlike AFM-FM exchange coupling at room temperature. Furthermore, by incorporating a monolayered ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ film into the Mn-Co interface or by reducing the temperature, PMA can also be triggered in a Co/Fe film by using e-fct Mn films with an in-plane-oriented AFM spin structure at room or low temperature through noncollinear exchange coupling across the AFM-FM interface. Our results clarify the exchange-coupling mechanisms, characteristic behaviors, and critical conditions for increasing control over antiferromagnet-induced PMA in FM films with high magnetic moment density but strong in-plane magnetic anisotropy, which is helpful for the development of next-generation perpendicularly based spintronic devices.

Spin transfer torque in Mn3Ga -based ferrimagnetic tunnel junctions from first principles

We report on first-principles calculations of spin-transfer torque (STT) in epitaxial magnetic tunnel junctions (MTJs) based on ferrimagnetic tetragonal Mn$_3$Ga electrodes, both as analyzer in an Fe/MgO stack, and also in an analogous stack with a second Mn$_3$Ga electrode (instead of Fe) as polarizer. Solving the ballistic transport problem (NEGF + DFT) for the nonequilibrium spin density in a scattering region extended to over 7.6 nm into the Mn$_3$Ga electrode, we find long-range spatial oscillations of the STT decaying on a length scale of a few tens of angstroms, both in the linear response regime and for finite bias. The oscillatory behavior of the STT in Mn$_3$Ga is robust against variations in the stack geometry and the applied bias voltage, which may affect the phase and the amplitude of the spatial oscillation, but the wave number is only responsive to variations in the longitudinal lattice constant of Mn$_3$Ga (for fixed in-plane geometry) without being commensurate with the lattice. Our interpretation of the long-range STT oscillations is based on the bulk electronic structure of Mn$_3$Ga, taking also into account the spin-filtering properties of the MgO barrier. Comparison to a fully Mn$_3$Ga-based stack shows similar STT oscillations, but a significant enhancement of both the TMR effect at the Fermi level and the STT at the interface, due to resonant tunneling for the mirror-symmetric junction with thinner barrier (three monoatomic layers). From the calculated energy dependence of the spin-polarized transmissions at 0 V, we anticipate asymmetric or symmetric TMR as a function of the applied bias voltage for the Fe-based and the all-Mn$_3$Ga stacks, respectively, which also both exhibit a sign change below 1 V. In the latter (symmetric) case we expect a TMR peak at zero, which is larger for the thinner barriers because of a spin-polarized resonant tunneling contribution.

Robust Antiskyrmion Phase in Bulk Tetragonal Mn–Pt(Pd)–Sn Heusler System Probed by Magnetic Entropy Change and AC‐Susceptibility Measurements

AbstractMagnetic skyrmions, topologically protected chiral spin textures having potential applications in data storage, are stabilized in certain magnetic materials with broken inversion symmetry. The existence of magnetic antiskyrmions has been recently demonstrated in thin plates of a tetragonal Heusler material with D2d crystal symmetry. Here, the robust nature of the antiskyrmion phase in bulk tetragonal Mn–Pt(Pd)–Sn compounds by utilizing magnetic entropy change and AC‐susceptibility measurements is shown. It is found that the formation of the antiskyrmion phase is accompanied by a positive magnetic entropy change, which is supported by the concomitant observation of an anomaly in AC‐susceptibility measurements. Supporting these findings, no anomalies are found in AC‐susceptibility and magnetic entropy change measurements for a Mn–Pt(Pd)–Sn compound that is stabilized in the cubic phase by slight changes in chemical composition, thereby showing the robustness of the antiskyrmion phase to the D2d crystal structure.

The frequency behavior of hysteresis loops in Mn:Fe:KTN ferroelectric single crystal

In this paper, the frequency (f) behavior of hysteresis loops in a tetragonal Mn:Fe:KTN crystal is investigated. We have found that the shape of P–E hysteresis loops experiences systemic f-dependent changes from an asymmetric double hysteresis loop to a normal single loop, as the frequency changes from high to low (150–0.01 Hz). The shape of P–E loop is strongly f-dependent only at low frequency (f < 1 Hz) and f-independent at the high one (f ≥ 10 Hz), and the maximum polarization stays unchanged with f varying at the same temperature and amplitude of electric field. The frequency-dependent properties of hysteresis loop stem from the forced vibration of two types of electric dipoles induced by the off-center displacement and the local composition gradient distribution, with different response time to applied electric field. The results are of great value for enhancing device performance and fabricating required devices with specific performance characteristics.

An optical method for characterizing domain wall motions and ferroelectric hysteresis in tetragonal Mn:Fe:KTN co-doped crystals

An optical method is proposed to extract the displacement vector of domain wall (DW) sidewise motion from ferroelectric domain configuration pattern on a tetragonal Mn:Fe:KTN crystal. The ferroelectric domain configuration evolution under an external electric field is observed in situ using optical microscope imaging system, and the relative displacement of DW sideways motion between two adjacent electric fields is extracted. By the combination of the new domain length vector extracted, the ferroelectric domain displacement vector as a function of field strength can be obtained; what is more, the hysteresis loop can be achieved during field cycling. The method provides an access to directly manipulating DW motion and exploring the properties of ferroelectric materials from ferroelectric domain configuration pattern.

Structural changes induced spin-reorientation of ultrathin Mn films grown on Ag(001)

The strained body centered tetragonal (bct) Mn ultrathin film from lattice parameter a=2.89Å to lattice value of 2.73Å induces anti-ferromagnetic behavior between Mn layers. The magnetic easy axis of Mn film was demonstrated theoretically to switch from the in-plane to out-of-plane by magneto-optical Kerr effect investigation. By including spin–orbit coupling in full potential linearized augmented plane waves and linearized muffin-tin orbitals methods, manganese ultrathin film displays different magnetic behaviors and the spin-reorientation transition is shown to be correlated to these structural changes. The calculated magnetic moment of manganese planes are enhanced and reach a value of ~4.02μB. The polar magneto-optical Kerr effect is calculated for a photon energy range extended to 15eV. It shows a pronounced peak in visible light.

Large noncollinearity and spin reorientation in the novel Mn2RhSn Heusler magnet.

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal Mn(2)RhSn Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn(2)RhSn is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family.

Mn K-edge X-ray absorption studies of oxo- and hydroxo-manganese(IV) complexes: experimental and theoretical insights into pre-edge properties.

Mn K-edge X-ray absorption spectroscopy (XAS) was used to gain insights into the geometric and electronic structures of [MnII(Cl)2(Me2EBC)], [MnIV(OH)2(Me2EBC)]2+, and [MnIV(O)(OH)(Me2EBC)]+, which are all supported by the tetradentate, macrocyclic Me2EBC ligand (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Analysis of extended X-ray absorption fine structure (EXAFS) data for [MnIV(O)(OH)(Me2EBC)]+ revealed Mn–O scatterers at 1.71 and 1.84 Å and Mn–N scatterers at 2.11 Å, providing the first unambiguous support for the formulation of this species as an oxohydroxomanganese(IV) adduct. EXAFS-determined structural parameters for [MnII(Cl)2(Me2EBC)] and [MnIV(OH)2(Me2EBC)]2+ are consistent with previously reported crystal structures. The Mn pre-edge energies and intensities of these complexes were examined within the context of data for other oxo- and hydroxomanganese(IV) adducts, and time-dependent density functional theory (TD-DFT) computations were used to predict pre-edge properties for all compounds considered. This combined experimental and computational analysis revealed a correlation between the Mn–O(H) distances and pre-edge peak areas of MnIV=O and MnIV–OH complexes, but this trend was strongly modulated by the MnIV coordination geometry. Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn–O(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity. For tetragonal MnIV=O centers, more than 90% of the pre-edge intensity comes from excitations to the Mn=O σ* MO. Trigonal bipyramidal oxomanganese(IV) centers likewise feature excitations to the Mn=O σ* molecular orbital (MO) but also show intense transitions to 3dx2–y2 and 3dxy MOs because of enhanced 3d-4px,y mixing. This gives rise to a broader pre-edge feature for trigonal MnIV=O adducts. These results underscore the importance of reporting experimental pre-edge areas rather than peak heights. Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported S = 1 MnV=O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

Site-specific order and magnetism in tetragonal Mn3Ga thin films

Mn${}_{3}$Ga bulk material and thin films deposited on several different substrates have been investigated using x-ray and neutron diffraction, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and electronic structure calculations using density-functional theory with the aim of determining the atomic site occupancy, magnetic moments, and magnetic structure of this tetragonal D0${}_{22}$-structure compound. The Mn${}_{3}$Ga has close to ideal site occupancy, with Ga on $2a$ sites and Mn on $2b$ and $4d$ sites. The magnetic structure is basically ferrimagnetic, with the larger Mn moment of about 3 ${\ensuremath{\mu}}_{B}$ on the $2b$ site, which is coordinated by 8 Mn $4d$ and 4 Ga, and the smaller one on the $4d$ site, which is coordinated by 4 Mn $2b$, 4 Ga, and 4 Mn $4d$. The Mn $d$-band occupancy is close to 5 on both sites, and the orbital moments are small, $l$$0.2$ ${\ensuremath{\mu}}_{B}$. The material nevertheless exhibits substantial uniaxial anisotropy, ${K}_{u}=1.0\phantom{\rule{0.28em}{0ex}}{\mathrm{MJ}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}3}$, which originates from the $4d$ site. The $2b$ site has hard axis anisotropy, which together with an oscillatory exchange coupling from the first and second nearest neighbors, leads to a soft component of the magnetization in the $c$ plane, coexisting with $c$-axis hysteresis loops exhibiting coercivity of up to 1.2 T, and magnetization in the range 110--220 kA m${}^{\ensuremath{-}1}$ at room temperature, depending on preparation conditions. Tetragonal Mn${}_{2}$Ga films behave similarly. Manganese is lost from both sites, but the films have substantially larger magnetization (480 kA m${}^{\ensuremath{-}1}$) and anisotropy constant (2.35 MJ m${}^{\ensuremath{-}3}$) than Mn${}_{3}$Ga.

Anisotropic thermal expansion and cooperative Invar and anti-Invar effects in mn alloys.

We have investigated thermal expansion of a tetragonal Mn(88)Ni(12) alloy by x-ray diffraction, Mn and Ni K-edge extended x-ray-absorption fine-structure spectroscopy, and the computational simulations based on the path-integral effective-classical-potential theory. It is found from the x-ray diffraction that the tetragonal lattice constant c exhibits almost no thermal expansion like an Invar alloy, while the lattice constant a shows even larger thermal expansion than usually expected from anharmonicity, implying significant anisotropy in thermal expansion. The extended x-ray-absorption fine-structure reveals that the Mn local environment is actually tetragonally distorted, while the Ni one retains its inherent cubiclike symmetry. Combined with the computational simulations, it is concluded that large thermal expansion along the a axis originates from the anti-Invar effect, while negligibly small thermal expansion along the c axis originates from the cooperative Invar effect. Namely, the tetragonally distorted more stable antiferromagnetic Mn state gives a significantly smaller (slightly longer) atomic radius along the a (c) axis than the radius of the spherical paramagnetic state.

Manganese Nitride Complexes in Oxidation States III, IV, and V: Synthesis and Electronic Structure

The synthesis and characterization of a series of manganese nitrides in a tripodal chelating tris(carbene) ligand framework is described. Photolysis of [(TIMEN(xyl))Mn(N(3))](+) (where TIMEN(xyl) = tris[2-(3-xylylimidazol-2-ylidene)ethyl]amine) yields the isolable molecular Mn(IV) nitride, [(TIMEN(xyl))Mn(N)](+). Spectroscopic and DFT studies indicate that this Mn(IV) d(3) complex has a doublet electronic ground state. The metal-centered one-electron oxidation of this Mn(IV) species results in formation of the pentavalent Mn(V) nitride, [(TIMEN(xyl))Mn(N)](2+). Unlike previously reported, tetragonal Mn(V) nitrides with a d(2), nonmagnetic S = 0 ground state, this trigonal bipyramidal complex has a triplet ground state S = 1. One-electron reduction of [(TIMEN(xyl))Mn(N)](+) produces the neutral, nonmagnetic trivalent [(TIMEN(xyl))Mn(N)] species with a d(4) low-spin, S = 0, ground state.

New phases in the system LiMnVO 4–Mn 3(VO 4) 2

The synthesis of orthorhombic Mn 3(VO 4) 2 via a simple solid state route as well as the description of a new tetragonal high temperature phase are reported in this paper. Additionally, the system LiMnVO 4–Mn 3(VO 4) 2 (=Mn 1.5VO 4), described by the formula Li x Mn 1.5− x/2 VO 4, is investigated in detail, showing that substitution of one Mn 2+ in Mn 1.5VO 4 by two Li + ions favors the formation of the tetragonal high temperature phase for x≤0.22. This substitution is facilitated by the statistical half occupancy of the 4 b site by Mn 2+ in tetragonal Mn 1.5VO 4, whereas additional crystallographic sites would have to be occupied in the orthorhombic phase. The inverse substitution of Li by Mn in orthorhombic LiMnVO 4 is also shown to be possible in the range 0.72≤ x≤1. For 0.22≤ x≤0.72, there is a large miscibility gap with a two phase mixture of the tetragonal Mn 1.5VO 4 type and orthorhombic LiMnVO 4 type phases.

Exchange interaction studies using epitaxially stabilized expanded fct-Mn(0 0 1)

It is demonstrated experimentally that expanded face centred tetragonal (e-fct) Mn, which is characterized by an axial ratio c/a > 1, can be stabilized by epitaxial growth on ferromagnetic (FM) fct Co(0 0 1) films. At room temperature this new metastable Mn phase is antiferromagnetically (AFM) ordered for thicknesses above the 2.5 monolayer, as inferred from AFM/FM magnetic exchange interaction studies. This critical thickness does not depend on the magnetization direction of the FM layer. Even though for out-of-plane magnetized FM layers the coercivities are distinctly enhanced by the exchange interaction, no exchange bias can be detected. On the other hand, both coercivity enhancement and exchange biasing is observed for the in-plane magnetized FM/e-fct Mn bilayer. These results provide experimental evidence that the AFM spin structure of e-fct Mn at the AFM/FM interface is confined to the film plane.

Investigations of the EPR zero-field splitting and the defect structure for Mn 2+ and Fe 3+ ions in anatase crystals

The EPR zero-field splittings D of Mn 2+ and Fe 3+ ions in anatase crystals at room and low temperatures are calculated from the high-order perturbation formula of zero-field splitting D for 3d 5 ions in tetragonal symmetry based on the dominant spin-orbit coupling mechanism. The calculated results are consistent with the observed values. From the calculations, Mn 2+ and Fe 3+ ions are suggested to substitute for Ti 4+ ions in anatase (in the previous paper, Mn 2+ ion was suggested at an interstitial site rather than substitutional site) and the defect structures (characterized mainly by the local oxygen parameter u) for both tetragonal Mn 2+ and Fe 3+ impurity centers are estimated. The different zero-field splitting at room and low temperatures are due mainly to the change of local oxygen parameter u with the temperature. These results are discussed.

Epitaxial (Mn 0.7Cr 0.3) 2As and (Fe 0.7Mn 0.3) 2As thin films: Structural and magnetic properties

We have grown (Mn 0.7Cr 0.3) 2As and (Fe 0.7Mn 0.3) 2As thin films directly on Si (1 0 0) substrate by molecular beam epitaxy at the substrate temperature, T S=300 °C. We found that the crystal structure of (Mn 0.7Cr 0.3) 2As and (Fe 0.7Mn 0.3) 2 As thin films was the tetragonal Mn 2As type with the lattice constants of a=3.745 and 3.70Å, respectively. (Mn 0.7Cr 0.3) 2 As thin film shows spin-glass behaviour below 130 K, while (Fe 0.7Mn 0.3) 2As alloy has ferrimagnetic ordering at above room temperature.

Ab initioinvestigation of the magnetism of tetragonal Mn: Bulk, surface, ultrathin films, and multilayers

The magnetic properties of the tetragonally distorted $\ensuremath{\gamma}$ and $\ensuremath{\delta}$ phases of Mn stabilized by epitaxial growth on metallic surfaces are still subject of a lively debate, but so far no consistent and generally accepted picture has emerged. We have performed detailed and comprehensive investigations of the geometric and magnetic properties of tetragonal Mn in the bulk, at a (100) surface, in ultrathin $\mathrm{Mn}∕\mathrm{Fe}(100)$ films, and in $\mathrm{Mn}∕\mathrm{Fe}$ multilayers using ab initio spin-density functional techniques. The cubic structures of both $\ensuremath{\gamma}$ (face-centered cubic) and $\ensuremath{\delta}$ (body-centered cubic) Mn are unstable against tetragonal distortion. Whereas for $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Mn}$ a structure contracted along the $c$ axis and with $c(2\ifmmode\times\else\texttimes\fi{}2)$ in-plane (100) antiferromagnetism (AFM) is the unique ground state, for $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Mn}$ a contracted tetragonal $(c∕a=0.945)$ phase with a layered (100) AFM, and an expanded $(c∕a=1.048)$ phase with in-plane (100) AFM are energetically almost degenerate. In addition we find that the antiferromagnetic phases of both $\ensuremath{\delta}$ and $\ensuremath{\gamma}$ Mn are susceptible to long-period helical modulations. At the (100) surface, the Mn moments are strongly enhanced, and the strong antiferromagnetic coupling between the high surface moments favors in-plane AFM in the surface layer even on top of the tetragonally compressed near-fcc phase stabilizing layered antiferromagnetism in the bulk. A similar result is found for ultrathin $\mathrm{Mn}∕\mathrm{Fe}(100)$ films with up to six monolayers. A strong ferromagnetic $\mathrm{Mn}∕\mathrm{Fe}$ coupling at the interface favors layered antiferromagnetism in the deeper layers, but the in-plane antiferromagnetic structure in the top layer is stable in any case. For the thinnest Mn films we have also examined noncollinear magnetic structures and found evidence for a perpendicular coupling between the Mn surface layer and the deeper layers of film and substrate. The strong ferromagnetic $\mathrm{Mn}∕\mathrm{Fe}$ interface coupling also determines the properties of $\mathrm{Fe}∕\mathrm{Mn}$ multilayers. The ferromagnetic interface coupling is not perturbed by $\mathrm{Fe}∕\mathrm{Mn}$ intermixing and stabilizes a layered antiferromagnetism in the Mn spacer. We discuss our results in the light of the available experimental data and of previous theoretical calculations.

Synthesis of nanocrystalline Mn 3O 4 at 100 °C

A simple gel to crystal conversion route has been followed for the preparation of nanocrystalline tetragonal Mn 3O 4 powders at 80–100 °C under refluxing conditions. Freshly prepared manganese hydroxide gel is allowed to crystallize under refluxing and stirring conditions for 4–6 h. Formation of nano crystallites of Mn 3O 4 is confirmed by X-ray diffraction (XRD) study. Transmission electron microscope (TEM) investigations revealed that the average particle size is 50 nm for these powders.

Thermochemistry of iron manganese oxide spinels

Oxide melt solution calorimetry has been performed on iron manganese oxide spinels prepared at high temperature. The enthalpy of formation of (Mn x Fe 1− x ) 3O 4 at 298 K from the oxides, tetragonal Mn 3O 4 (hausmannite) and cubic Fe 3O 4 (magnetite), is negative from x = 0 to x = 0.67 and becomes slightly positive for 0.67< x<1.0. Relative to cubic Mn 3O 4 (stable at high temperature) and cubic Fe 3O 4 (magnetite), the enthalpy of formation is negative for all compositions. The enthalpy of formation is most negative near x = 0.2 . There is no significant difference in the trend of enthalpy of formation versus composition for cubic ( x < 0.6 ) and tetragonal ( x > 0.6 ) spinels of intermediate compositions. The enthalpies of formation are discussed in terms of three factors: oxidation–reduction relative to the end-members, cation distribution, and tetragonality. A combination of measured enthalpies and Gibbs free energies of formation in the literature provides entropies of mixing. Δ S mix , consistent with a cation distribution in which all trivalent manganese is octahedral and all other ions are randomly distributed for x > 0.5 , but the entropy of mixing appears to be smaller than these predicted values for x < 0.4 .

Magnetic excitations at the Brillouin zone boundary in Mn (17%Fe, 3%Cu) alloy

In tetragonal Mn (18%Cu) and Mn (13.7%Ni) alloys the low and medium energy excitations were observed previously near points at the magnetic Brillouin-zone (MBZ) boundary, geometrically equivalent to the MBZ centre. The properties of the inelastic neutron scattering (INS) on these excitations, e.g. the temperature dependence of its intensity, are different from those of spin waves observed near the MBZ centre. The nature of these excitations is not yet clearly understood. Here, the magnetic excitations at the MBZ boundary in the Mn (17%Fe, 3%Cu) alloy are investigated. In our sample, (68±8)% of the volume is occupied by domains with the c-axis oriented along one direction. The INS at the MBZ centre increases when the temperature is lowered. It is dominated by the spin waves but a small quasi-elastic scattering is also present. The INS intensity at the MBZ boundary shows a very weak temperature dependence.