Collinear Magnetic Structure Research Articles

Structural and Magnetic Characterization of NdCaCrO4 Oxide

The structural and magnetic properties of NdCaCrO4 oxide have been studied mainly by means of neutron powder diffraction. This oxide presents orthorhombic symmetry, space group Bmab, with lattice parameters at room temperature a=5.3739(1) Å, b=5.54114(1) Å, and c=11.9536(3) Å. At around 973 K a reversible phase transition to a tetragonal (I4/mmm) structure has been determined from X-ray powder diffraction data. Below 180 K, NdCaCrO4 shows antiferromagnetic ordering in the Cr3+ sublattice that has been explained on the basis of a propagation vector k=0 and a collinear magnetic structure, with a symmetry mode Gx. At 30 K an antiferromagnetic ordered component along the z direction appears. This spin reorientation could be induced by the onset of local magnetic interactions between the neodymium and chromium sublattices.

A study of the structures of some (FeRu)B metallic glasses by x-ray and neutron scattering

The atomic scale structures of the ruthenium substituted metallic glasses Fe83-xRuxB17 and Fe80-xRuxB20 have been studied by x-ray and neutron scattering. The addition of up to 22% of ruthenium causes an 5% increase in the first neighbour distance, while the co-ordination number stays roughly constant around the value n112.5±0.5. The evolution in the shape of the first peak in the radial distribution function is consistent with the new atomic correlations introduced by the addition of ruthenium. The higher real space resolution in the neutron experiments allows the first neighbour transition metal-boron correlations to be identified. The substitution of the larger transition metal causes little change in the structural order in these glasses, which are confirmed to be suitable candidates for the study of collinear and non-collinear magnetic structures in ferromagnetic metallic glasses. The variation of the magnetization of these glasses with composition can be described in an empirical way or in the context of percolation theory. Neither model requires the presence of non-collinear moments. According to the latter description the magnetization M appears to have a power law dependence on composition M = (x - xc). The magnitude of = 0.39 is close to the expected value, although the critical concentration xc = 0.595 is much greater than the values predicted by percolation theory.

Surface magnetism of Sc-substituted Ba-M hexaferrites

A technique of simultaneous gamma-ray, x-ray, and electron Mossbauer spectroscopy is used to study the magnetic structure of the surface layer with direct comparison to the magnetic structure inside single crystal samples of hexagonal Ba-M ferrites, in which part of the iron ions have been replaced by diamagnetic Sc ions (chemical formula BaFe12−δScδO9). It is found that when the diamagnetic Sc ions are introduced into the crystal lattice of BaFe12−δScδO19 at concentrations (x=0.4 and 0.6) far below the level at which the collinear magnetic structure inside the sample is destroyed, a macroscopic layer of thickness ∼300 nm develops on the surface, in which the magnetic moments of the iron ions are oriented noncollinearly with respect to the moments inside the sample. The deviation 〈θ〉 of the magnetic moments in BaFe11.6Sc0.4O19 was 10° ± 62° for x=0.4, and when the Sc concentration was raised to 0.6, the angle 〈θ〉 increased to 17° ± 62°. The noncollinear magnetic structure in the surface layer in these crystals develops because of further reduction in the energy of the exchange interactions owing to the presence of a “defect,” such as the surface. For the first time, therefore, an anisotropic surface layer whose magnetic properties differ from those in the interior of a sample has been observed experimentally in ferromagnetic crystals, as predicted by Neel [L. Neel, Phys. Radium. 15, 225 (1954)].

Magnetic properties and exchange interactions in T 100− xEr x amorphous alloys (T=Fe, Co)

Amorphous T 100− x Er x (with T=Fe, Co and 6< x<30) thin films were prepared by RF sputtering and their magnetic properties have been studied. A mean field model has been used to explain a temperature dependence of the magnetization. This analysis yields the spin values of T and the effective exchange interaction between T–T and T–Er atoms. High-field magnetization studies performed at 4.2 K in magnetic fields up to 5.5 tesla on amorphous T 100− x Er x alloys have revealed for samples with stoichiometry close to that of compensated ferrimagnet, a magnetic behavior that is characteristic of a non collinear magnetic structure of Er and T sublattices.

Antiferromagnetic intermetallic Tb2AgIn3

Neutron diffraction and susceptibility measurements have been performed on intermetallic Tb2AgIn3 crystallizing in the CaIn2-type structure in space group P63/mmc. The unit cell constants are a=4.793(2) Å and c=7.322(3) Å. Tb2AgIn3 orders antiferromagnetically below TN=42(1) K. The collinear magnetic structure is described in an orthogonal unit cell with A=a, B=a√3, C=c. Spin orientations are along C. The magnitude of the Tb-moment is 5.1(2) μB at 4.2 K.

Antiferromagnetism of RPtX (R = Ho, Er; X = Si, Ge) compounds

X-ray and neutron diffraction as well as magnetometric measurements performed for HoPtX and ErPtX (X = Si, Ge) compounds indicate that they are crystalline and that they have the orthorhombic TiNiSi-type crystal structure. They are antiferromagnetic at low temperatures. A collinear magnetic structure with the propagation vector k = |1/2,0,1/2|, stable between 1.5 K and the Néel point at 3.1 K is observed in HoPtSi. HoPtGe is paramagnetic at 1.8 K. A collinear magnetic order described by the wavevector k1 = |0,1/2,0| has been found in ErPtSi and ErPtGe. As the temperature rises this structure transforms in both compounds into a sine modulated one with the wavevector k2 = |0,ky,0|. The Néel points of ErPtSi and ErPtGe are 4.0 and 4.1 K respectively.

Magnetic structure of the ferro–antiferromagnet interface. II. Compensated interface

A compensated ferro–antiferromagnet interface is studied in the framework of a discrete classical spin model. The phase diagram is presented for the possible collinear and canted magnetic structures of the interface. The change in magnetization upon a transition from a collinear structure to canted form is studied.

Electronic structure and related properties of metallic glasses: Linear muffin-tin orbital approach

We describe various aspects of electronic structure calculation for amorphous metals using the linear muffin-tin orbital (LMTO) scheme. We discuss calculations of the magnetic properties within the framework of multiple scattering formalism assuming collinear magnetic structure and using the linear muffin-tin orbital Green’s function method. Results for the effective exchange coupling parameters and local magnetic moments for amorphous Fe and Co are discussed with special emphasis on the dependence of these quantities on the local and global features of the structure. A method of calculating the electronic transport properties in transition metal glasses, based on the LMTO-recursion method and the Kubo-Greenwood formula, is presented. The practical limitations of the method and the ways to overcome them are briefly discussed.

Neutron diffraction studies of the magnetic structures of the HoRhGe and ErRhGe compounds

HoRhGe and ErRhGe compounds crystallize in the orthorhombic TiNiSi-type of structure with the space group Pnma. Neutron diffraction measurements at T = 1.6 K indicate collinear magnetic structures with the propagation vector k = (1/2,0,1/2) for HoRhGe and k = (0,1/2,0) for ErRhGe. After increasing the temperature, the change to the incommensurate sine modulated structure is observed for ErRhGe at . The temperature dependence of the magnetic peak intensities gives the Néel temperature 4.6 K for HoRhGe and 9.5 K for ErRhGe.

Magnetic structures of antiferromagnetic chain fluorides A 2MnF 5, xH 2O

Neutron powder diffraction has been used to determine the magnetic structures of antiferromagnetic chain pentafluoromanganates A 2MnF 5, xH 2O (A/ x = Li/0; Na/0 and Rb/0, 1). Collinear magnetic structures, with Mn 3+ magnetic moments parallel to the chain axis, have been observed in all compounds and can be characterized by different interchain magnetic interactions, depending on MnMn distances.

Crystal and magnetic structures of the colossal magnetoresistance manganates Sr2-xNd1+xMn2O7 (x=0.0, 0.1).

Neutron powder diffraction techniques have been used to refine the crystal and magnetic structures of the Ruddlesden Popper (RP) compounds ${\mathrm{Sr}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Nd}}_{1+\mathit{x}}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ (x=0.0, 0.1), which show colossal magnetoresistance below 100 K. Samples of both compositions have been shown to be biphasic, although in each case both phases are of the RP type and have very similar structural parameters. Only one phase in each sample shows long range magnetic order. For x=0.0, the Mn cations and the Nd cations in the rocksalt layers of the structure are antiferromagnetically ordered in a noncollinear magnetic structure below 28 K; the Nd cations within the perovskite blocks do not show long range magnetic order. In the temperature range 28\ensuremath{\leqslant}T/K\ensuremath{\leqslant}137, only the Mn cations are ordered. The decoupling of the Nd cations is accompanied by a rotation of the Mn moments from a direction close (\ensuremath{\varphi}=27\ifmmode^\circ\else\textdegree\fi{}) to the z axis into the xy plane. A similar temperature dependence is seen in the case x=0.1, although the ordered components of the cation moments are much reduced and the low temperature phase has a collinear magnetic structure with the moments aligned along z. The magnetotransport properties of these samples are discussed in the light of their crystal and magnetic structures. \textcopyright{} 1996 The American Physical Society.

Noncollinear and collinear magnetic structures in exchange coupled Fe/Cr(001) superlattices.

The magnetic and structural properties of molecular beam epitaxy grown Fe/Cr(001) superlattices were studied as a function of the growth temperature ${\mathit{T}}_{\mathit{g}}$ using polarized neutron reflectometry (PNR) with polarization analysis, magneto-optic Kerr effect (MOKE), and x-ray-scattering techniques. From MOKE and PNR as a function of external field we find strong noncollinear coupling between the Fe layers and a so far unexpected coupling angle of 50\ifmmode^\circ\else\textdegree\fi{} near remanence for a sample grown at ${\mathit{T}}_{\mathit{g}}$=250 \ifmmode^\circ\else\textdegree\fi{}C. A detailed discussion of the domain structure of the sample near remanence confirms the modeling. On the other hand, an otherwise equivalent sample grown at room temperature exhibits completely ferromagnetic or uncoupled behavior. Using diffuse x-ray-scattering methods these distinct differences in the magnetic structure are found to be correlated with a growth temperature dependent length scale of constant Cr interlayer thickness ${\mathit{l}}_{\mathrm{Cr}}$. We find that ${\mathit{l}}_{\mathrm{Cr}}$ increases significantly with ${\mathit{T}}_{\mathit{g}}$. These results are discussed in the framework of current theories of noncollinear exchange. It is demonstrated that the bilinear-biquadratic formalism used so far is inconsistent with the data. The Cr specific proximity magnetism model is discussed which explains the occurrence of noncollinear coupling for systems with Cr interlayer thickness fluctuations on the length scale observed here for ${\mathit{T}}_{\mathit{g}}$=250 \ifmmode^\circ\else\textdegree\fi{}C. The model yields an exchange energy different from the bilinear-biquadratic formalism used so far, explaining the asymptotic approach to saturation observed by MOKE.

Magnetic properties of the noncollinear antiferromagnetKNiCl3

Neutron-scattering measurements are reported on the magnetic structure of the noncollinear quasi-one-dimensional antiferromagnet ${\mathrm{KNiCl}}_{3}$. At 4.5 K there is a commensurate triangular structure. From the magnetization measurements a magnetic phase diagram for a field applied within the basal plane was detrmined. At low temperatures there is a phase transistion at field (2.3\ifmmode\pm\else\textpm\fi{}0.1) T to a collinear magnetic structure. From measurements of the spin-wave dispersion along the chain direction the intrachain exchange coupling J=(0.310\ifmmode\pm\else\textpm\fi{}0.0006) THz and the single-ion anisotropyD=(0.13\ifmmode\pm\else\textpm\fi{}0.01) THz were deduced after taking into account a correction for quantum fluctuations.

Magnetism in amorphous Fe(CO)Y alloys

Abstract We present calculations of the structural, electronic and magnetic properties of amorphous FeY and CoY alloys. We show that over a wide range of compositions both a-FeY and a-CoY are ferrimagnetic, but with some significant differences: a-CoY retains a simple collinear magnetic structure, but the disorder-induced broadening of the d-band leads to enhanced magnetism. In Fe-rich a-FeY alloys competing exchange interactions lead to complex spin structures.

Magnetic properties of (Mn1−xFex)3+δGe alloys

Abstract We have made X-ray, magnetization and electric resistivity measurements for (Mn 1−x Fe x ) 3+δ Ge with the D0 19 -type structure, and determined the x -dependence of the Neel temperature, the Curie temperature and a transition temperature from the triangular to the collinear magnetic structure for the alloys with δ=0.1 and 0 ⩽ x ⩽0.34. An abrupt change at the transition shows a remarkable thermal hysterisis.

Magnetization measurement of the Co moment induced by the molecular field in Y1-tGdtCo3

The magnetization of polycrystalline Y1-tGdtCo3 with 0<or=t<or=1 has been measured up to 40 T, and the effect of the molecular field from the Gd sublattice on the itinerant metamagnetism of the Co sublattice is studied. The compounds with t<0.25 show a field-induced metamagnetic transition of the Co sublattice. A transition from the collinear magnetic structure to the non-collinear structure is observed in compounds with 0.50<or=t<or=0.55. The spontaneous Co moment, which is evaluated from the magnetization curve, exhibits a stepwise increase at t approximately=0.25 when the Gd concentration is increased. This means that the metamagnetic transition of the Co sublattice is induced by the molecular field from the Gd sublattice in Y1-tGdtCo3. The average intersublattice molecular-field coefficient lambda GdCo, between the Gd and Co sublattices is estimated to be lambda GdCo=40.9+or-0.4 T formula units/ mu B; the corresponding exchange coupling parameter JGdCo is JGdCo=-(1.64+or-0.03)*10-22 J.

Geometrically-frustrated magnetism on triangular and tetrahedral lattices

The combination of simple, nearest neighbor antiferromagnetic interactions and certain lattice symmetries leads to phenomena known broadly as frustration. In such systems, the formation of a collinear magnetic structure cannot occur at low temperature. The material may undergo a phase transition to an unusual magneticallyordered state, or it may not undergo a conventional transition at all-entering a glass-like state as the temperature is lowered. This paper will review recent work, mostly from neutron scattering studies, on three such magnetic materials. CsMnBr3 and CsCoBr3 are XY-like and Ising-like stacked-triangular lattice antiferromagnets, respectively. Both undergo magnetic phase transitions with interesting properties due to the presence of geometricalfrustration within the triangular layers. TD2Mo2O7 is a pyrochlore antiferromagnet, in which the magnetic moments reside on a network of corner-sharing tetrahedra. This material enters a magnetic glass-like state at low temperature.

Magnetic and elastic properties in the paramagnetic state of NdIn 3 antiferromagnet

The cubic AuCu 3 type system NdIn 3 exhibits complex magnetic structures at low temperature. Immediately below the Néel temperature, T N = 6 K, the spins assume a sine wave modulated arrangement; for 5.5 K > T > 4.7 K, the modulation tends towards a square wave. Finally, below 4.7 K, a simple commensurate structure is stabilized. We first confirm here the complex magnetic phase diagram by means of resistivity measurements. In order to quantitatively understand the low temperature properties, the crystalline electric field is determined by inelastic neutron spectroscopy. An extensive study of the magnetic, magnetoelastic and quadrupolar properties in the paramagnetic phase by magnetization, ultrasonic velocity and parastriction measurements is then presented. The values of one-ion magnetoelastic coefficients agree with previous determinations in isomorphous systems. Quadrupolar pair interactions are found to be positive for the tetragonal symmetry lowering mode, in agreement with the symmetry of collinear magnetic structures. However the only quadrupolar model fails to coherently describe the whole set of paramagnetic properties, in particular the third order magnetic susceptibility. The existence of two-ion magnetoelasticity is then discussed in relation to the ultrasonic velocity and parastriction results. The description of the magnetic phase diagrams will be reported elsewhere.

Crystal structure and magnetic properties of a new ferrimagnetic chain containing manganese(II) and a nitronyl-nitroxide radical. Magnetic ordering in Mn(hfac)2NITR compounds

A new nitronyl–nitroxide radical, 2-(p-dimethylaminophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hereafter denoted NIT(p-NMe2)Ph, has been synthesized. It crystallizes in the P21/c, space group, with a= 15.742(2)A, b= 10.875(4)A, c= 19.011(5)A and β= 113.26(2)°, the asymmetric unit containing two independent radical molecules. Its reaction with manganese hexafluoroacetylacetonate, Mn(hfac)2, in the presence of chloroform leads to the solvent-containing compound Mn(hfac)2[NIT(p-NMe2)Ph]·0.25HCCl3, which crystallizes in the C2/c space group, with a= 28.183(4)A, b= 13.899(2)A, c= 23.425(5)A, β= 125.28(2)°. Its structure presents chains of 21 symmetry running along the b, axis, where the manganeses are bridged by the nitronyl-nitroxide radicals. The magnetic properties in the 5–300 K range are typically those of a ferrimagnetic chain where S= 1/2 and S= 5/2 spins alternate regularly. Below 5 K, a three-dimensional (3D) ferrimagnetic ordering occurs. Rationalization of the ferrimagnetic character of Mn(hfac)2[NIT(p-Nme2)Ph]·0.25HCCl3 and of some other Mn(hfac)2NITR chain compounds on the basis of magnetic dipolar energy calculations has been attempted. Results show that, as far as only collinear magnetic structures and localized moments are considered, the dipolar interaction alone cannot explain the type of magnetic order adopted by these compounds. This questions the possible role of other physical parameters, such as single-ion anisotropy, in the 3D ordering.

One-magnon light scattering in exchange-noncollinear magnets

Abstract The one-magnon light scattering in different types of magnets whose magnetic structure is noncollinear in exchange approximation has been studied theoretically with the help of symmetry analysis. The role of the exchange mechanism of scattering by acoustic and exchange modes has been elucidated. It is shown that the intensity of Raman light scattering by exchange magnons exceed far the intensity related to acoustic ones in opposite to the situation found in collinear magnetic structures.