Ferromagnetic Planes Research Articles

Relation between Crystal and Magnetic Structures of Layered Manganite La2-2xSr1+2xMn2O7(0.30≤x≤0.50)

Comprehensive neutron-powder diffraction and Rietveld analyses were carried out to clarify the relation between the crystal and magnetic structures of La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50). The Jahn-Teller (JT) distortion of Mn-O6 octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the equatorial Mn-O bond length, is Delta_JT=1.042(5) at x=0.30, where the magnetic easy-axis at low temperature is parallel to the c axis. As the JT distortion becomes suppressed with increasing x, a planar ferromagnetic structure appears at x =< 0.32, which is followed by a canted antiferromagnetic (AFM) structure at x =< 0.39. The canting angle between neighboring planes continuously increases from 0 deg (planar ferromagnet: 0.32 =< x < 0.39) to 180 deg (A-type AFM: x=0.48 where Delta_JT=1.013(5)). Dominance of the A-type AF structure with decrease of JT distortion can be ascribed to the change in the eg orbital state from d3z^2-r^2 to dx^2-y^2.

Structural properties and magnetic structure of the heavy-fermioncompound UCu5Sn

The heavy-fermion compound UCu5Sn has been studied by meansof neutron powder diffraction. This compound exhibits the hexagonal CeNi5Sn-type structure with space group P63/mmc. We havedetermined the lattice parameters and atomic positions at room temperature,using a high-resolution diffractometer. The neutron diffraction diagramswere also measured at various temperatures between 1.4 and 61 K, using an800-cell position-sensitive diffractometer. The observed minimum in theunit-cell volume at temperature of 30 K, being rather far below themagnetic ordering one (TC = 53 K), may suggest a change in the strength ofthe 5f-conduction electron hybridization which probably takes place at lowtemperatures. It was found that the magnetic phase transition at TCcorresponds to the onset of a ferrimagnetic order. The magnetic structureis characterized by forming the ferromagnetic planes (001) with the(+ - + -) stacking sequence, where the U moments are oriented parallel tothe hexagonal c-axis. However, the size of the U moments arranged in thez = 0 plane (MU1), and formed by the sites 2(a), is substantiallylarger than these arranged in the adjacent z = ¼ plane (MU2),and formed by the sites 2(c). At 1.4 K, they are MU1 = 2.14(2) µB/atom U and MU2 = 0.18(4) µB/atom U. We discussseveral mechanisms leading to a reduction of the magnetic moment at the onesite compared to the other one.

Magnetic phase diagram of UNi 2Si 2 in high fields

The tetragonal UNi 2Si 2 exhibits three different AF phases in zero magnetic field below T N=125 K. In all of them the U moment is parallel to the c-axis, forming ferromagnetic basal planes. Between T N and 108 K the incommensurate AF1 phase with q = ( 0, 0, q z ) is established ( q z ∼0.74). The phase AF2, between 108 and 40 K, is of the AF-I type with q=(0, 0, 1) and the ground-state phase AF3 is the uncompensated squared-up AF structure with the (++−) stacking of basal planes along the c-axis ( q=(0, 0, 2 3 ) ). Both 108 and 40 K transitions are of the first order. The neutron scattering study of UNi 2Si 2 single crystal in magnetic fields up to 15 T shows a complex magnetic phase diagram. The 108 K phase boundary splits above 3 T and the AF2 phase disappears above 3.8 T. The T N remains unchanged up to 15 T and the AF1 phase is getting narrower in high magnetic field, vanishing above 14 T. Only the AF3 phase exists up to T N in higher fields. Strong field and temperature dependence of the q z were observed in the AF1 phase, it shows steep decrease with applied field at the AF1–AF3 boundary.

2D visual design of permanent MRI magnets by a hybrid BEM-IEM computation

A hybrid boundary element method (BEM)-integral equation method (IEM) computation is proposed for the two-dimensional (2D) design of permanent MRI magnets. Over the finite element method (FEM), the hybrid BEM-IEM computation has the advantages of much less discrete elements and thus less computer memory and CPU time. As the curved shape of the ferromagnetic pole planes of permanent MRI magnets plays an important role in improving the uniformity of the magnetic field, a computer program using the hybrid computation is developed to optimize the pole plane shape visually and automatically. With a 0.2T Nd-Fe-B magnet, the proposed design method is carried out and the results are given in this paper.

Charge reallocation in the tetragonal to monoclinic transition in Pr 1/2Sr 1/2MnO 3

The structural changes associated with the ferromagnetic (FM) metallic-to-antiferromagnetic (AFM) insulating transition of Pr 1/2Sr 1/2MnO 3 (I4/mcm→P2 1/n) have been analysed by means of high-resolution neutron diffraction data. Mn positions in the low-temperature phase have the same charge and identically deformed octahedra. The transition is the result of a stretching-like distortion of the octahedra that (i) contracts the Mn–O bond perpendicular to the FM planes by deviating β from 90°C and (ii) injects electrons into in-plane Mn–O bonds

Neutron diffraction and 57Fe Mössbauer study of the HfFe 6Ge 6-type RFe 6Ge 6 compounds (R=Sc, Ti, Zr, Hf, Nb)

Both neutron diffraction experiments and 57Fe Mössbauer measurements have been performed on powder samples of the HfFe 6Ge 6-type RFe 6Ge 6 compounds (R=Sc, Ti, Zr, Hf, Nb). These compounds are characterized by an antiferromagnetic stacking along the c-axis of ferromagnetic (001) Fe planes, the easy-direction of magnetization being along [001]. However, for R=Ti, Zr and Nb, we observe that, at 4.2 K, ≈30% of iron atoms have moments which deviate from the c-axis. Both the ordered magnetic moment and the hyperfine field on iron atoms are found to increase with the R valency. This observation is tentatively attributed to an increase of the hybridization of the d valence states of the R metal with the 3d iron states.

Evidence of spin reorientation in YbFe6Ge6 from neutron diffraction and 57Fe Mössbauer experiments

High-resolution neutron diffraction and 57Fe Mössbauer experiments have been performed on powder samples of the ternary intermetallic YbFe6Ge6. This compound crystallizes with a hexagonal structure (P6/mmm) which can be described as an ordered state intermediate between the HfFe6Ge6- and YCo6Ge6-type structures, with cell parameters suggesting that the Yb ion is in (or close to) a trivalent state. At room temperature, the Fe-sublattice magnetic arrangement consists of an antiferromagnetic stacking along the c-axis of the ferromagnetic (001) Fe planes with the easy direction of magnetization along [001]. Below about 85 K a spin-reorientation process occurs; a fraction (T) of the iron moments rotate abruptly from the c-axis to a given direction in or close to the basal plane. This phenomenon allows the observation of anisotropic contributions to the total hyperfine field at the Fe site. At 4.2 K, approximately 18% of the iron moments remain along the c-axis. No long-range magnetic order of the Yb sublattice is observed, at least above 1.5 K. We also report 57Fe Mössbauer investigations of the HfFe6Ge6-type LuFe6Ge6 compound.

Tetragonal to monoclinic transition in the metallic antiferromagnetPr0.5Sr0.5MnO3

The structural changes associated with the ferromagnetic (FM)/metallic to antiferromagnetic (AFM)/insulating transition of ${\mathrm{Pr}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3} (I4/mc\stackrel{\ensuremath{\rightarrow}}{m}{P2}_{1}/n)$ are described in detail. Based on high-resolution neutron-diffraction data, in the ${P2}_{1}/n$ phase Mn1 and Mn2 octahedra have the same charge and are identically deformed. The electronically induced transition to the antiferromagnetic A-type structure at ${T}_{N}$ is the result of a stretchinglike distortion of the octahedra that (i) contracts the Mn-O2 bond perpendicular to the FM planes by deviating $\ensuremath{\beta}$ from $90\ifmmode^\circ\else\textdegree\fi{}$ and (ii) injects electrons into Mn-O1 and Mn-O3 bonds. The observed microscopic changes are completely different to the structural transition in the A-type ${\mathrm{LaMnO}}_{3}$ and are discussed in the light of the orbital polarization previously proposed.

Helimagnetism and field-induced phases in random Gd64Sc36 single crystals

We report magnetization, neutron diffraction and elastic constant measurements of the single-crystal rare earth alloy Gd64Sc36 as a function of magnetic field. For an a-axis field the material exhibits transitions from helix to fan to basal plane ferromagnet. At low temperatures the looked-in magnetic structure is very sensitive to the application of small fields (~5 mT).

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.

Magnetic polarons in layered antiferromagnetic systems

The energy and stability conditions are found for free ferromagnetic polarons (ferrons) in the antiferromagnetic semiconductors with a layered magnetic structure. Unlike a similar problem for the staggered antiferromagnets, solved long ago, this geometrically different problem was not considered yet. In particular, the difference between these two problems consists in the fact that the ferromagnetic region, which traps the charge carrier, is well defined for the staggered structure, and is not for the layered structure. The latter is due to ferromagnetic planes inherent in this structure. It is established that, nevertheless, the ferromagnetic region is almost spherical for large ferrons. The De Gennes single-spin magnetic polaron is unstable at realistic values of parameters. The energy and magnetic moment of the ferron bound to a donor or acceptor are calculated for the case of the double exchange.

Low density artificial microwave magnetic composites

The high frequency permeability of composites made of hollow conducting planes or tubes is derived. It is shown that such composites may exhibit permeability levels significantly different from unity, even for nonmagnetic metal volume fractions lower than 1%. This behavior is related to the skin effect, but it can occur for metal shell thicknesses much lower than the skin depth. The influence of the shape of the hollow inclusion on the permeability is reported. The maximum of the imaginary permeability can be tuned over a wide frequency range by adjusting the conductivity of the metal and the characteristic dimensions of the inclusions. The properties of composites made of hollow ferromagnetic planes are also investigated using experimental values obtained from materials made of ferromagnetic laminations.

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.

Semitopological solitons in planar ferromagnets

We establish the existence of a finite-energy solitary wave in a two-dimensional planar ferromagnet which moves rigidly at any constant velocity v that is smaller than the magnon velocity c. The shape of the calculated soliton depends crucially on the relative magnitude of v and c. For v<<c, the soliton describes a widely separated vortex-antivortex pair undergoing Kelvin motion at a relative distance . There exists a crossover velocity v0 at which the vortex-antivortex character is lost and the energy-momentum dispersion develops a cusp. For v0<v<c, the soliton becomes a lump with no apparent topological features and solves the modified KP equation in the limit . We also describe briefly a similar calculation of a vortex ring in a three-dimensional planar ferromagnet. These results together with the analytically known one-dimensional kink provide an interesting set of semitopological solitons whose physical significance is yet to be explored.

Ordered magnetic moments in (A,A′)Co2Ge2 systems

The relative orientation of ordered magnetic moments of f-atoms (A,A′) occupying randomly the same site, has been studied by powder neutron diffraction in the magnetically-ordered state of (A,A′)Co2Ge2 solid solutions. These materials crystallize in the ThCr2Si2-type structure and order antiferromagnetically in the AF-I structure, characterized by ferromagnetic (A,A′) basal planes stacked alternately (+−+−) along the tetragonal axis, with A and A′ moments aligned along this axis. The `spin-charge' concept, which accounts for antiparallel orientation of light and heavy lanthanide moments in (Nd1−xTbx)Co2Ge2, is found to apply also to uranium and terbium moments in (U1−xTbx)Co2Ge2. The parallel orientation of uranium and neodymium moments, observed in (U1−xNdx)Co2Ge2, also follows the `spin-charge' concept. Uranium and terbium moment compensation (in basal planes of zero total moment) is anticipated for a (U1−xTbx)Co2Ge2 solid solution with x≈0.15.

Solitary Waves of Planar Ferromagnets and the Breakdown of the Spin-Polarized Quantum Hall Effect

A branch of uniformly propagating solitary waves of planar ferromagnets is identified. The energy dispersion and structures of the solitary waves are determined for an isotropic ferromagnet as functions of a conserved momentum. With increasing momentum, the structure undergoes a transition from a form resembling a droplet of spin waves to a Skyrmion--anti-Skyrmion pair. An instability to the formation of these solitary waves is shown to provide a mechanism for the electric field-induced breakdown of the spin-polarized quantum Hall effect.

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.

Microscopic model for the magnetic subsystem inHoNi2B2C

We demonstrate that the system of localized magnetic moments in ${\mathrm{HoNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ can be described by the four-positional clock model. This model, at a proper choice of the coupling constants, yields several meta-magnetic phases in magnetic field at zero temperature in full agreement with the experimental phase diagram. The model incorporates couplings between non-nearest neighbors in the direction perpendicular to the ferromagnetic planes. The same model leads to a $c$-modulated magnetic phase near the Curie temperature. The theoretical value of the modulation wave vector agrees surprisingly well with that observed by the neutron-diffraction experiment without new adjustable parameters.

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.