Mn Planes Research Articles

Direct visualization of the Jahn-Teller effect coupled to Na ordering in Na5/8MnO2.

The cooperative Jahn-Teller effect (CJTE) refers to the correlation of distortions arising from individual Jahn-Teller centres in complex compounds. The effect usually induces strong coupling between the static or dynamic charge, orbital and magnetic ordering, which has been related to many important phenomena such as colossal magnetoresistance and superconductivity. Here we report a Na5/8MnO2 superstructure with a pronounced static CJTE that is coupled to an unusual Na vacancy ordering. We visualize this coupled distortion and Na ordering down to the atomic scale. The Mn planes are periodically distorted by a charge modulation on the Mn stripes, which in turn drives an unusually large displacement of some Na ions through long-ranged Na-O-Mn(3+)-O-Na interactions into a highly distorted octahedral site. At lower temperatures, magnetic order appears, in which Mn atomic stripes with different magnetic couplings are interwoven with each other. Our work demonstrates the strong interaction between alkali ordering, displacement, and electronic and magnetic structure, and underlines the important role that structural details play in determining electronic behaviour.

Orbital structure and magnetic ordering in stoichiometric and doped crednerite CuMnO2

The exchange interactions and magnetic structure in layered system CuMnO${}_{2}$ (mineral crednerite) and in nonstoichiometric system Cu${}_{1.04}$Mn${}_{0.96}$O${}_{2}$, with triangular layers distorted due to orbital ordering of the Mn${}^{3+}$ ions, are studied by ab initio band-structure calculations, which were performed within the GGA + $U$ approximation. The exchange interaction parameters for the Heisenberg model within the Mn planes and between the Mn planes are estimated. We explain the observed in-plane magnetic structure by the dominant mechanism of the direct $d\text{\ensuremath{-}}d$ exchange between neighboring Mn ions. The superexchange via O ions, with 90${}^{\ensuremath{\circ}}$ Mn--O--Mn bonds, plays a less important role for the in-plane exchange. The interlayer coupling is largely dominated by one exchange path between the half-filled $3{z}^{2}\ensuremath{-}{r}^{2}$ orbitals of Mn${}^{3+}$. The change of interlayer coupling from antiferromagnetic in pure CuMnO${}_{2}$ to ferromagnetic in doped material is also explained by our calculations.

Magnetic order in YbMn2Si2 — Neutron scattering investigation

Several mechanisms have been proposed to account for the structural and magnetic behaviour of the rare earth intermetallic compound YbMn2Si2 below the transition that occurs around ∼30 K. We have carried out detailed neutron diffraction measurements over the temperature range ∼0.3–52 K and confirm both an antiferromagnetic structure with ferromagnetic Mn (001) planes coupled antiferromagnetically along the c-axis above ∼30 K and the absence of ordering of the Yb sublattice down to 0.3 K. The decrease in intensity of the (111) reflection around ∼30 K together with the appearance of satellite reflections of propagation vector \(k = 00\tfrac{1} {2}\) confirm that the magnetic unit cell doubles along the c axis below TN2. The resultant molecular field acting on half of the ions below ∼30 K removes the degeneracy of the Kramers doublets of the Yb3+ ions and accounts for the additional excitations observed below ∼30 K compared with the inelastic neutron scattering above ∼30 K.

Lattice and spin excitations in multiferroich-YbMnO3

Lattice and spin excitations have been studied by Raman scattering in hexagonal YbMnO3 single crystals. The temperature dependences of the phonon modes show that the E2 mode at 256 cm-1 related to the displacement of Mn and O ions in a-b plane is coupled to the spin order. The A1 phonon mode at 678 cm-1 presents a soft mode behavior at the Neel temperature. Connected to the motion of the apical oxygen ions along the c direction, this mode controls directly the Mn-Mn interactions between adjacent Mn planes and the superexchange path. Crystal field and magnon mode excitations have been identified. The temperature investigation of the spin excitations shows that the spin structure is strongly influence by the Yb-Mn interaction. Under a magnetic field along the c axis, we have investigated the magnetic reordering and its impact on the spin excitations.

Neutron diffraction study on PrMn 2− xFe xGe 2 and general magnetic phase diagram for RMn 2− xFe xGe 2 (R: La–Sm)

We have investigated the magnetic structures and phase transitions of PrMn 2− x Fe x Ge 2 (0 ≤ x ≤ 1) by the temperature dependence of weak-field magnetization and powder neutron diffraction. For the x = 0.6 sample, a typical SmMn 2Ge 2-like magnetic behavior is observed. The magnetic structures of the samples with x = 0.6, 0.8 and 1 have been determined between 1.5 and 305 K. Above T C inter ( x = 0.6) and T N inter ( x = 0.8 and 1) up to T N inter , these samples exhibit an intralayer antiferromagnetic structure in (0 0 1) Mn planes. For x = 0.6 sample, the magnetic structure is canted ferromagnetic along the c-axis between T N inter and T C inter and below T C Pr . For the samples with x = 0.8 and 1, the magnetic structure is canted antiferromagnetic along the c-axis below T N inter . Pr moments order ferromagnetically along the c-axis for the x = 0.6 sample below T C Pr . The results are summarized in the PrMn 2− x Fe x Ge 2 magnetic phase diagram. The general magnetic phase diagram for RMn 2− x Fe x Ge 2 (R: La, Ce, Pr, Nd and Sm) is also constructed from previous and present works. For these compounds, the magnetic phase transition from ferromagnetic to antiferromagnetic occurs at different intralayer Mn–Mn distances for each system. These critical distances are larger than the corresponding threshold value d Mn – Mn a = 2.87 Å for the ternary end-member RMn 2X 2 compounds.

Magnetic properties of Cu substituted NdMn2Si2 intermetallics

AbstractThe structure and magnetic properties of NdMn2–xCuxSi2 (0.2 ≤ x ≤ 1) were studied by X‐ray powder diffraction and magnetization measurements. In this study, we investigate the variations in the magnetic properties of NdMn2–xCuxSi2 as a function of Cu concentration by examing the evolution of the features in the temperature dependence of the magnetization. Earlier neutron diffraction experiments showed that the ferromagnetic Mn planes are ordered antiparallel along the c‐axis below 380 K and the Nd sublattice orders at 33 K in NdMn2Si2. The ordering of the Nd sublattice reconfigures the ordering in Mn sublattice and leads to ferromagnetic ordering. With increasing amount of Cu, the Curie temperature has a maximum value of 120 K at x = 0.7 and decreases for the samples with x ≥ 0.8. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic ordering of the Pr sublattice with the substitution of Cu for Mn inPrMn2Si2 intermetallics

The magnetic properties of PrMn2−xCuxSi2 () were studied by field-cooled and zero-field-cooled magnetization measurements in thetemperature range 5 K K in low external fields (5 mT) and by magnetic-field-dependent magnetization measurementsin fields up to 5.5 T. Substitution of Cu for Mn leads to a linear decrease in the lattice constantc and the unitcell volume V and a linear increase in the lattice constanta. Earlier neutron diffraction experiments showed that Pr does not order down to 1.6 K inPrMn2Si2 while the ferromagnetic Mn planes are ordered antiparallel along thec axis. With the increasing Cu content, the magnetization increases rapidly at lowtemperatures for the samples with . Cu substitution strongly changes the magnetic properties and leads to the magneticordering of the Pr sublattice. This is mainly deduced from the discussion of the valuesof the magnetic moments at low temperatures. Below the Curie temperaturesTC, the spins in the Mn sublattice are arranged parallel to the Pr sublattice. With increasing Cu,TC(x) has a maximumvalue of 155 K at x = 0.6 and decreases for samples with .

Ferromagnetism and metallic state in digital (Ga,Mn)As heterostructures

We present an extensive density functional theory study of the electronic, magnetic and transport properties of GaAs and AlAs digital ferromagnetic heterostructures. These can be obtained by $\delta$-doping with Mn the GaAs layers of a GaAs/AlAs superlattice. Our analysis spans a range of Mn concentrations and considers the presence of compensating defects such as As antisites. In the defect-free case all the heterostructures studied present an half-metallic electronic structure. In contrast when As antisites are present the half-metallic state is destroyed and the heterostructures behave as dirty planar metals. In this case they show a large $p$-type metallic conductance in the Mn plane mainly due to majority spin electrons, and an $n$-type hopping-like conductance in the GaAs planes mainly due to minority spin electrons. This suggests that if the As antisites can be kept far from the Mn planes, spatial separation of the different spin currents can be achieved. Finally we show that in the case of AlAs/(Ga,Mn)As digital ferromagnetic heterostructures the AlAs/GaAs valence band offset produces an additional confining potential for the holes responsible for the ferromagnetism. Therefore the ferromagnetic coupling between the Mn ions becomes larger and more robust to the presence of As antisites.

A novel technique for measuring etch rate distribution of Si

Three-dimensional etch rate distribution of Si is necessary in analyzing the anisotropic etching of Si. As three-dimensional etch rate distribution can be composed by a series of two-dimensional distributions, a novel method is suggested in this paper for measuring the two-dimensional distributions with simple tools and high efficiency. For example, vertical sidewalls of a series of U-shaped trenches in (0 mn) silicon wafers are first created by deep reactive ion etching (DRIE) technique. By measuring the etch rates in normal directions of the vertical sidewalls, two-dimensional distributions of etch rate in (0 mn) planes can be found. As the height of vertical sidewalls formed by DRIE can be very large, the planes of sidewall can withstand a relatively long etch time before they are replaced by emerging slow etching planes. No special measuring facilities but a conventional microscope is needed for the measurement. Presented in this paper are etch rate distributions in (001) and (01̄1) crystal planes in 40% KOH and 25% TMAH.

Spin correlations in the bilayer manganite La1.2Sr1.8Mn2O7

We have investigated the spin correlations of the bilayer manganite La1.2Sr1.8Mn2O7, which shows colossal magnetoresistance (CMR) behaviour. Rod-like diffuse scattering parallel to c→∗ in the (hol) plane has been observed. We have determined the temperature variation of the in-plane correlation length from the (h,0,1.833) scan, which is perpendicular to the rod. The modulation of the diffuse-scattering intensity parallel to the rod in the scan (0.95,0,l) above Tc shows that the two ferromagnetic Mn planes in the bilayer are ferromagnetically correlated.

Electronic and magnetic structure of Mn-Ni alloys in two and three dimensions

The electronic and magnetic structure of face-centred tetragonal Mn-Ni compounds and of structurally related c(2 × 2) Mn-Ni alloy films on Ni(001) substrates has been investigated using ab initio local-spin-density calculations including generalized gradient corrections. For the intermetallic compound a layered antiferromagnetic high-spin ground state with Mn moments of ±3.2 µB (LSDA) and ±3.4 µB (GGCs) and non-magnetic Ni atoms is predicted, in good agreement with the estimates from magnetic neutron scattering. Calculations of the magnetic anisotropy energy show that the moments are aligned in the Mn planes, parallel to the edges of the unit cell. An alloy with unit cell dimensions of the Mn-Ni planes strained to match the lattice parameter of the Ni(001) substrate has a very similar magnetic structure, albeit with slightly reduced moments. A monolayer c(2 × 2) Mn-Ni alloy shows high-spin ferromagnetic order (µMn = 3.9 µB). Films with two and more monolayers show antiferromagnetic interlayer coupling, with a quite pronounced enhancement of the surface moments over those in the deeper layers. The predicted antiferromagnetic ordering of the films emphasizes the similarity of the atomic and magnetic structures of the two-dimensional films with that of the three-dimensional compounds and contradicts recent claims as to a ferromagnetic order of the films not only in the monolayer limit. Possible explanations of this discrepancy are discussed.

A study of the new ferromagnetic YbMn 6Sn 6 compound by magnetization and neutron diffraction measurements

Bulk magnetization and neutron diffraction experiments have been performed on the new ternary stannide YbMn 6Sn 6. This compound crystallizes with a hexagonal structure which can be described as an intermediate ordered state between the HfFe 6Ge 6- and the YCo 6Ge 6-type structures, with cell parameters revealing the abnormal behavior of the Yb ion (close to a divalent state) in this compound. The magnetometric measurements show that YbMn 6Sn 6 is ferromagnetic below T c=300(5) K with a maximum magnetization value of M max=13.56 μ B/f.u. at 4.2 K. The neutron diffraction study leads to a simple ferromagnetic arrangement of ferromagnetic easy-plane Mn(0 0 1) planes without ordering of the Yb sublattice down to 2 K. The Mn magnetic moment increases from μ Mn=0.90(16) μ B at 295 K up to μ Mn=2.35(6) μ B at 2 K. Moreover, we also publish the results of the neutron diffraction study of the already known ferromagnetic ( T c=290(5) K) HfFe 6Ge 6-type MgMn 6Sn 6 compound which adopts the same magnetic arrangement of the Mn sublattice than YbMn 6Sn 6 with an Mn moment value of μ Mn=2.32(4) μ B at 2 K.

Neutron diffraction study of the ZrMn6Ge6, LuMn6Ge6 and ScMn6Ge6 compounds

The new hexagonal HfFe6Ge6-type ZrMn6Ge6 compound orders antiferromagnetically below TN=640K and undergoes a second transition at Tt=50K. Neutron diffraction experiments were performed on the three ZrMn6Ge6, LuMn6Ge6 and ScMn6Ge6 compounds. At 300K, they exhibit collinear easy-axis antiferromagnetic arrangements and the magnetic structure consists of ferromagnetic (001) Mn planes antiferromagnetically coupled along the c-axis with a (+−−+) sequence (μMn≈2.06, 1.86 and 1.94μB for Zr-, Lu- and ScMn6Ge6, respectively). At low temperature, spin reorientation processes occur, yielding incommensurate antiferromagnetic arrangements. Below Tt, in both ZrMn6Ge6 and LuMn6Ge6 (Tt∽30K), the Mn moments form a double-cone structure (μMn≈2.1μB, semi-cone angle α≈6°) as already found in YMn6Ge6 (Tt∽80K) whereas only preliminary results are available for ScMn6Ge6 below Tt=255K. These results lead us to strongly revise previous conclusions concerning LuMn6Ge6 and ScMn6Ge6 with more reasonable Mn magnetic moment values (∽2μB against the ∽1μB value previously published).

A study of the new HfFe6Ge6-type ZrMn6Sn6 and HfMn6Sn6 compounds by magnetization and neutron diffraction measurements

Bulk magnetization and neutron diffraction measurements have been performed on the new HfFe6Ge6-type (P6/mmm) ZrMn6Sn6 and HfMn6Sn6 compounds. These compounds order antiferromagnetically below 580(5) and 575(5) K for R=Zr and Hf, respectively. At 300 K, both compounds exhibit a collinear easy-plane antiferromagnetic arrangement and the magnetic structure consists of ferromagnetic (001) Mn planes antiferromagnetically coupled along the c-axis with the (+−−+) sequence, i.e. a doubling of the c-axis of the chemical cell (μMn=1.98(7) μB and μMn=2.05(6) μB for ZrMn6Sn6 and HfMn6Sn6, respectively). At approximately 60 K, a commensurate–incommensurate transition occurs for both compounds, yielding an helical magnetic structure characterized by a thermal dependent k=〈00qz〉 wave vector. At 2 K, qz=0.491(1) and 0.481(1) r.l.u., while μMn=2.11(3) μB and 2.18(2) μB for ZrMn6Sn6 and HfMn6Sn6, respectively. A comparison with other RMn6Sn6 compounds involving a non-magnetic R element (R=Mg, Sc, Lu, Y) allows us to underline the influence of the R valency on the magnetic behavior of the Mn sublattice in this class of materials.

MgTGe (T≡Mn, Fe) compounds of the CeFeSi-type. Magnetic structure of MgMnGe from neutron diffraction study

The CeFeSi-type compounds MgMnGe and MgFeGe have been prepared by reaction of the elements at 1000 K. The magnetic properties of the MgMnGe compound have been investigated by neutron diffraction. MgMnGe orders antiferromagnetically above room temperature. Its magnetic structure is characterized by a staking of antiferromagnetic (0 0 1) Mn planes with the moments (≈3.3 μ B at 2 K) lying in this plane. The magnetic behavior of the Mn sublattice is compared to those observed in the isotypic RMnGe and in the closely related RMn 2Ge 2 and RMnSi 2 (R≡Ca, Sr, Ba, La and lanthanide) compounds. The magnetic arrangement observed in MgMnGe is connected with modifications in the chemical bond in this compound.

Investigations of the La 1− xCa xMn 2Ge 2 (0⩽ x⩽1) solid solution by magnetic measurements and neutron diffraction

The La 1− x Ca x Mn 2Ge 2 (0⩽ x⩽1) solid solution has been studied by means of X-ray powder diffraction, magnetic measurements and neutron diffraction study. For x⩽0.7, all samples exhibit an easy-axis ferromagnetic behaviour below T C (320 K> T C>80 K). The Curie temperature and the maximum magnetisation values (4.2 K, H appl.=17 kOe) decrease continuously with the La substitution rate from 320 K and 1.45 μ B per Mn atom for LaMn 2Ge 2, to 80 K and 0.4 μ B for La 0.3Ca 0.7Mn 2Ge 2. Futhermore, above T C, neutron diffraction study of the La 1− x Ca x Mn 2Ge 2 ( x=0.2, 0.5, 0.7 and 1.0) reveals the occurrence of an additional antiferromagnetic region (not detected by magnetometric measurements) characterised by antiferromagnetic (0 0 1) Mn planes. The Néel temperature increases from 420 K for LaMn 2Ge 2 up to 670 K for CaMn 2Ge 2. The magnetic structures of La 0.8Ca 0.2Mn 2Ge 2, La 0.5Ca 0.5Mn 2Ge 2 and La 0.3Ca 0.7Mn 2Ge 2 have been determined between 2 and 600 K. A new AF DC magnetic type ordering is found in La 0.5Ca 0.5Mn 2Ge 2 between T C and T t. The results lead to the construction of the x– T magnetic phase diagram and are discussed and compared with the results obtained for the La 1− x Y x Mn 2Ge 2 solid solution.

A study of the La 1− xY xMnSi (0≤ x≤0.8) solid solution by magnetization and neutron diffraction measurements

Magnetization measurements and neutron diffraction study have been performed on the CeFeSi-type La 1− x Y x MnSi solid solution (0≤ x≤0.8). The lanthanum-rich compounds ( x<0.2) are characterized by antiferromagnetic Mn (001) planes and the yttrium-rich compounds ( x≥0.8) by ferromagnetic Mn planes. The compounds of intermediate compositions (0.4≤ x≤0.6) order antiferromagnetically and display a second magnetic transition at lower temperature, yielding the formation of canted planes where both ferro- and antiferromagnetic components coexist. These magnetic behaviours are discussed and compared with those of isotypic ternary RMnSi and of related ThCr 2Si 2-type compounds.

Neutron diffraction study of the La 1− xY xMn 2Si 2 solid solution (0≤ x≤1)

The La 1− x Y x Mn 2Si 2 compounds (0< x≤1) have been investigated by powder neutron diffraction. For x<0.5, the low temperature magnetic structures are characterized by mixed planes where both ferro- and antiferromagnetic components coexist while for x≥0.5, ferromagnetic Mn planes prevail. The in-plane ferromagnetic component couples ferromagnetically with those of adjacent planes for x<0.2 and antiferromagnetically for x>0.2. The La 0.8Y 0.2Mn 2Si 2 compound displays a F–AF transition as a function of the temperature. The in-plane antiferromagnetic Mn moment components are smaller than those measured in the corresponding germanides. These results are discussed and compared with previous results of the La 1− x Y x Mn 2Ge 2 compounds. The Mn magnetic behaviour is consistent with bibliographic data on NMR experiments and Mössbauer spectroscopy.

Neutron diffraction study of the TbFeSi2-type NdMnCu0.5Ge1.5−xSix (0≤x≤1.5) solid solution

Antiferromagnetic TbFeSi2-type NdMnCu0.5Ge1.5−xSix compounds have been investigated by bulk magnetometric measurements and neutron diffraction. The Néel temperatures decrease from 370 K (x = 0) to 200 K (x = 1.5). The (010) Mn planes are antiferromagnetic in the germanium rich compounds (x≤0.3), and become ferromagnetic in the silicon rich compounds (x > 1.2). In the intermediate range (0.4<x<1.2), the compounds display more complicated magnetic structures with “mixed” Mn planes and/or helimagnetic arrangements. Whatever the concentration x, the Nd sublattice orders around Tt K. The magnetic behaviour of the Mn sublattice is compared to that observed in the RMnCu0.5Ge1.5 and closely related RMnSi, RMn2Ge2, and RMnSi2 compounds. The various magnetic arrangements observed in NdMnCu0.5Ge1.5−xSix and RMnSi2 compounds are connected with modifications in the chemical bond in each series.

The x-T magnetic phase diagram of the LaMn 2 − xFe xGe 2 (0≤ x≤1) system by neutron diffraction study

The ThCr 2Si 2-type structure LaMn 2 − x Fe x Ge 2 ( x = 0.125, 0.25, 0.375, 1.0) compounds were studied by neutron diffraction. LaMn 1.875Fe 0.125Ge 2 is antiferromagnetic below T N = 435 K with antiferromagnetic (001) Mn planes. Below T C = 280 K, the moments are canted and hence display a ferromagnetic component on the (001) Mn planes. Simultaneously, the interplane coupling of the in-plane antiferromagnetic components becomes incommensurate ( T ic = 280 K). LaMn 1.75Fe 0.25Ge 2 displays identical magnetic phenomena ( T N = 420 K, T C = T ic = 225 K). At 2 K, the antiferromagnetic component ( μ MnAF = 2.65(9) μ B) lies in the (001) plane whilst the ferromagnetic component ( μ MnF = 1.34(39) μ B) is aligned along the c axis. The LaMn 1.625Fe 0.375Ge 2 compound is antiferromagnetic at room temperature and contains antiferromagnetic (001) Mn planes with the moments aligned along the c axis. Below T C = 200 K, the Mn moments become canted and, as before, display a ferromagnetic component in the (001) plane. Below T SF = 175 K, a rotation of the moment takes place. At 2 K, the antiferromagnetic component ( μ MnAF = 2.61(7) μ B) makes an angle of 55(11)° with the c axis while the ferromagnetic component ( μ MnF = 1.77(24) μ B) makes an angle of 35(11)° with the c axis. The antiferromagnetic component gives rise to a commensurate interplane arrangement down to 2 K. The LaMnFeGe 2 compound is antiferromagnetic below T N = 230 K and is also made up of antiferromagnetic (001) Mn planes. This magnetic behaviour remains unchanged down to 2 K. The moment ( μ Mn = 276 μ B) is aligned along the c axis. The x - T magnetic phase diagram is depicted and compared with those of the LaMn 2 − xCu x Ge 2 and NdMn 2 − x Fe x Ge 2 systems.