Non-collinear Spin Arrangement Research Articles

Non-collinear magnetism in distorted perovskite compounds

Using results of the band structure calculations in the local-spin-density approximation, we demonstrate how the crystal distortions affect the magnetic structure of orthorhombically distorted perovskites leading to a non-collinear spin arrangement. Our results suggest that the non-collinearity of the spin magnetic moments, being generally small in LaMO 3 series with M  CrFe, is large in SrRuO 3.

Weak antiferromagnetism in YFeO 3 and HoFeO 3

We have performed neutron diffraction measurements of the weak antiferromagnetic component of the noncollinear spin arrangement of Fe 3+ ions in YFeO 3 and HoFeO 3 by means of the SNIM-2 neutron spectrometer at the IBR-2 pulsed reactor of JINR, Dubna. Single crystals were used. The ratio of the week antiferromagnetic component A b to the basic antiferromagnetic component G a was found A b G a = 0.014 for YFeO 3 and A b G a = 0.011 for HoFeO 3 within error limits of about 10%.

Structural peculiarities in magnetic small particles

Nanostructured magnetic materials, consisting of nanometer-sized crystallites, are currently a developing subject. Evidence has been accumulating that they possess properties that can differ substantially from those of bulk materials. This paper illustrates how Mössbauer spectroscopy can yield useful information on the structural peculiarities associated with these small particles. As illustrations, metallic iron and iron-oxide systems are considered in detail. The subjects discussed include: (1) phase stabilities in small particles, (2) deformed or nonsymmetrical atomic arrangements in small particles, and (3) peculiar magnetic structures or non-collinear spin arrangements in small magnetic oxide particles that are correlated with lower specific magnetizations as compared to the bulk values.

Symmetric and antisymmetric anisotropic exchange energies as crucial factors for the magnetic structures in MnP, FeP, CrAs and MnAs

The isotropic and anisotropic exchange energies in phosphide and arsenide compounds with helical or ferromagnetic structures have been studied by means of an anisotropic two-band model. The anisotropic term is separated into a symmetric and an antisymmetric contribution. It appears that, in crystals with non-vanishing spin-orbit interaction, a circular isotropic helix minimizes the antisymmetric contribution (AEI) to the exchange energy. The helix is stable if the AEI compensates for the increase in the symmetric anisotropic energy that the deviation from a non-collinear spin arrangement causes. The band model predicts the correct ground state, with the exception of MnP, where the helical (ground state) and the ferromagnetic (high-temperature phase) configurations correspond to the same energy. The results confirm that the AEI and the shape of the Fermi surface are crucial for the stability of a helical magnetic structure.

Electronic and magnetic states of γ-Fe

The electronic and magnetic structure of fcc (γ-)Fe is determined using the local spin-density functional approximation and the ASW method to solve the band-structure problem self-consistently. The calculations reveal that the detailed nature of the ground state of γ-Fe depends sensitively on the volume but quite unambigously consists of a spiral magnetic structure that is incommensurate with the lattice, in agreement with recent neutron-diffraction experiments. We give, in some detail, the theory necessary to implement the calculations. The noncollinear spin arrangement is described using for the spiral vector q a wide range of values that continuously cover ferromagnetic and antiferromagnetic configurations. The stability of the structure having minimum total energy is related to band-structure features and we emphasize the occurence of multiple magnetic states, which we study using a “constrained-moments” method.

Band Calculations for Non-Collinear Spin Arrangementsin Gamma-Phase Manganese-Iron Alloys

We perfomed total-energy band calculations for Mn, FeMn, Fe 3 Mn and Fe in gamma phase. In the calculations, we dealt with not only a collinear spin arrangement but also non-collinear spin arrangements in the local density approximation. Apart from gamma-Fe and FeMn, we obtained reasonable results in the ground-state spin arrangement and magnetic moments.

Spin waves and Heisenberg exchange constants for α-iron

Using a density functional theory for non-collinear spin arrangements, we model magnetization changes with well-defined wave vectors and calculate self-consistently the corresponding total-energy changes. From this we obtain the anisotropic spin wave dispersion relation for long and medium wavelengths, finding the spin wave stiffness constant to be in good agreement with experimental values. We also derive Heisenberg-exchange constants that give a reasonable mean-field estimate of the Curie temperature.

Ordered magnetic frustration: XIV. The magnetic structure of the tetragonal bronze KMnFeF 6

The magnetic structure of the tetragonal bronze-like KMnFeF 6 is solved by means of neutron powder diffraction. The structure (space group Pb' a2') consists of a non-collinear arrangement of spins lying in the ( a, b) plane of the crystal cell. Spins belonging to triangular platelets adopt a star arrangement very close to the ideal 120° configuration, whereas the arrangement in square platelets is almost exactly antiparallel. The magnetic structure of KMnFeF 6 has been successfully simulated with the program MCMAG by assuming isotropic AF interactions between nearest magnetic neighbours (superexchange interactions).

Magnetic phase transitions and structural distortion in Nd 2CuO 4

Neutron and X-ray diffraction have been used to study the magnetic and structural properties of single crystal Nd 2CuO 4. Long range magnetic order of the Cu moments develops at T N=245 K, with a noncollinear antiferromagnetic arrangement of spins. Additional abrupt transitions are observed at 75 K and 30 K, in which a spin reorientation takes place. Bragg peaks associated with the crystal structure are found at the same positions as the magnetic Bragg peaks, and indicate that a distortion of the basic tetragonal structure has occurred above 300 K.

Effects that can stabilise multiple spin-density waves

Antiferromagnetism on the face-centred cubic lattice is frustrated. The different competing spin arrangements are degenerate in the classical limit of the Heisenberg model. The author considers quantum-mechanical spin fluctuations, using the Holstein-Primakoff transformation, as a perturbation to lift this degeneracy and find that the collinear spin arrangement is stabilised. When he considers the inclusion of paramagnetic impurities, however, he finds that non-collinear arrangements are locally preferred. He suggests this scenario as a possible explanation for the fact that manganese, when quenched in a face-centred cubic lattice, can be driven from a collinear to non-collinear spin arrangement by doping with either nickel, iron or iridium.

Magnetisation, curie temperature and YK angle studies of Cu substituted and non substituted NiZn mixed ferrites

The compositional variation of saturization magnetization and Curie temperature for Ni 1− y− x Zn x Cu y Fe 2O 4 where x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 for y = 0 and x = 0.0, 0.2, 0.4, 0.6, 0.8 for y = 0.2 have been reported. It is observed that Néel type of spins are favoured in NiFe 2O 4, Zn 0.2Ni 0.8Fe 2O 4, Ni 0.8Cu 0.2Fe 2O 4 and Zn 0.2Ni 0.6Cu 0.2Fe 2O 4. However, in the remaining samples non-collinear spin arrangement is favoured. This is concluded from the computation of YK angles for both the ferrite systems. The Curie temperatures have been evaluated by the technique described by Loria et al. The non-collinear arrangement is also evidenced by the compositional variation of Curie temperatures for both the systems. On addition of copper, the Curie temperatures and magnetization of the NiZn ferrite system are lowered which is attributed to lower magnetic moment for Cu +2.

The m�ssbauer spectrum of almandine

Mossbauer spectra of a synthetic, pure almandine (Alm100) and a natural almandine-rich garnet (Alm86) were taken at temperatures between 295 and 4.2 K. Different widths and depths of the lines observed in the paramagnetic state require the spectra to be fitted with at least two components of slightly different quadrupole splittings and isomer shifts. Alm100 shows a gradual onset of magnetic order between 10.4 and 9.0 K, with about 50 percent of the sample magnetically ordered at 9.6 K. At 4.2 K both samples have complex spectra that can be fitted by two eight-line magnetic hyperfine patterns of equal intensities. These result from the existence of two different polar angles between the z-axis of the electric field gradient and the direction of the magnetic hyperfine field, or from two distinctly different electric field gradients. The former interpretation, in which the two angles are attributed to a modulated, non-collinear spin arrangement, is considered more likely. Both magnetic components have practically identical quadrupole splittings, 1/2eQV zz (1+η2/3)1/2, that average −3.70 mm·s−1, and a small asymmetry parameter of η=0.07. Alm100 has a magnetic hyperfine field of 25.7 T at 4.2 K, whereas Alm86 has a hyperfine field of only 23.3 T; the other features of the spectra of both samples are similar.

Oscillatory temperature dependence of Kerr rotation in ion implanted garnets

Abstract Unusual oscillations of polar Kerr rotation in ion implanted garnets versus temperature, changing from liquid nitrogen to Neel, are observed in magnetic fields up to 14 kOe. It is suggested that a noncollinear arrangement of spins is induced by the implantation.

STUDY OF MAGNETIC PROPERTIES (SPIN REORIENTATION PHASE TRANSITIONS) IN Sm2(TM)17 ALLOYS

The magnetic properties of pseudobinary Sm2(FeNiCoM)17 alloys in which M are nonmagnetic elements have been studied. The sample consists of disordered hexagonal 2:17 phase (TbCu7-structure) coexisting with FeNi. Anomalous magnetic properties along the c-axis (easy magnetization direction) have been observed. Below the room temperature, the magnetization and reverse magnetization curves exhibit obvious jumps. The magnetic fields corresponding to the jumps Hr increase with the temperature decreasing. The hysteresis loops show wasp-waisted character. The curve of magnetization vs. temperature has a maximum, the corresponding transition temperature Tt decreases with the magnetic field increasing. During cooling, the thermal magnetic hysteresis phenomena have been found at some values of magnetic field. On the basal plane (hard magnetization direction) and at higher Co concentrations (>18at%), the alloys show normal ferromagnetic behaviors. Most of the phenomena observed can be explained in terms of spin reorientation phase transition leading to non-collinear spin arrangements. The reversal of spin direction must cross over an evergy barrier, the height of which is U = 9.2×10-15 erg. The calculated values of magnetization are in good agreement with the observed ones.

Magnetic structures and magnetoelastic phenomena observed in Tb 3Ga xFe 5− xO 12 garnets

A non-collinear spin arrangement escorted with a crystallographic distortion (from cubic to rhombohedral symmetry) was observed in the terbium iron garnet: Tb 3Fe 5O 12 at low temperature. A strong magnetostrictive effect ( λ 111 = 2400 x10 −6) was detected. X-ray and neutron diffraction experiments were performed at various temperatures (1.5 K < T < 300 K) on the substituted garnets: Tb 3Ga x Fe 5− x O 12 to get information on the dependence of the spin configuration and the magnetostrictive effects with the main (Tb-Fe) exchange interaction.

Mössbauer Studies of Very Thin Oxide Layer on Fe Surface

Very thin oxide layers on Fe metal surfaces with the thickness of ca. 10 Å, are investigated by means of Mössbauer absorption spectroscopy using surface-selectively enriched samples. The oxide is similar to γ-Fe2O3 but has non-collinear spin arrangement at 4.2 K and is superparamagnetic at 300 K. The reduction of the recoilless fraction at 300 K is not drastic. Oxidation of Fe surfaces coated by Sb layers is also studied.

Mössbauer studies on Fe2MoO4

The Mossbauer spectrum of Fe2MoO4 at 4.2 K was fitted to two six line hyperfine patterns with field values of 218±5 k0e, due to Fe2+ on A-sites, and 414±5 k0e due to the presence of Fe2+, along with small amounts of Fe3+ on the B-sites. The spectrum with an applied transverse field of 50 k0e was fitted to three hyperfine patterns. The field values of these patterns suggest non-collinear spin arrangement in Fe2MoO4.

Surface Magnetic Properties of γ-Fe2O3by57Fe Mössbauer Emission Spectroscopy

Surface magnetic properties of γ-Fe 2 O 3 were studied with the Mossbauer emission spectra of the decayed 57 Fe. The results suggested two surface anomalies: a steep temperature dependence of the surface hyperfine field and a non-collinear spin arrangement in the surface region.

Anisotropic magnetoelastic properties of terbium compounds with garnet and zircon structure

Jahn-Teller transitions, noncollinear spin arrangements, magnetostriction and thermal anomalies result from the sensitivity of the structure of the low-lying energy levels of the Tb 3+ ions to various anisotropic influences. The similarity between the observed properties in garnets and in zircons is to be related to the similitude of the two structures.

X-ray powder diffraction at low temperature applied to the determination of magnetoelastic properties in terbium iron garnet

An x-ray powder diffraction apparatus has been built to perform experiments between 2.6 and 350 K. The experimental procedure and the data analysis allow the measurement of small lattice distortions and very small parameter variations with a good accuracy. Relative variations of the crystal parameters of terbium iron garnet, with an accuracy up to 10−5, have been observed at low temperature. A very small lattice distortion has been determined: TbIG becomes rhombohedral below 100 K; at 4.2 K the rhombohedral angle of the distorted cell is αR=89°52′30″, while the cell side is slightly contracted. A noncollinear spin arrangement, an anomalous thermal expansion (Δa=28×10−4 at 4.2 K), and a rhombohedral distortion (β=90°−αR=7′30″ at 4.2 K) appear simultaneously at about 45 K. These phenomena are to be connected with a very large magnetostriction (λIII=2400×10−6 at 4.2 K).