Modulated Magnetic Structures Research Articles

Versatile strain-tuning of modulated long-period magnetic structures

We report a detailed small-angle neutron scattering (SANS) study of the skyrmion lattice phase of MnSi under compressive and tensile strain. In particular, we demonstrate that tensile strain applied to the skyrmion lattice plane, perpendicular to the magnetic field, acts to destabilize the skyrmion lattice phase. This experiment was enabled by our development of a versatile strain cell, unique in its ability to select the application of either tensile or compressive strain in-situ by using two independent helium-actuated copper pressure transducers, whose design has been optimized for magnetic SANS on modulated long-period magnetic structures and vortex lattices, and is compact enough to fit in common sample environments such as cryostats and superconducting magnets.

Magnetic Option in Jana2006: A Unified Approach by Shubnikov (Super)space Groups

The concept of Shubnikov (magnetic) symmetry becomes frequently used for description, solution and refinement of magnetic structures. Its growing importance is connected with the ease of application to various classes of magnetic structures having the translation periodicity identical, commensurate or incommensurate with the nuclear one. Recently generalized superspace approach [1] for incommensurately modulated magnetic structures allows for combination of nuclear and magnetic modulations. This unified description helps fully understand e.g. multiferroic phases. The program Jana2006 (http://jana.fzu.cz) combines the concept of Shubnikov (super)space groups with the representational analysis based on the decomposition of the magnetic configuration space into basis modes, which transform according to different physically irreducible representations (irreps) of the space group of the paramagnetic phase [2]. Moreover, Jana2006 can launch the recently developed program ISODISTORT [3] to obtain similar but more general analysis. The generalized symmetry concept facilitates data processing where symmetry related reflections for single crystal data can be merged and the list of generated reflections for powder data can be reduced to independent ones. Another benefit concerns calculation of magnetic structure factors, stability of refinement and logical way to describe twin domains. Unlike in the Fullproff program [6], Jana2006 can combine the nuclear and magnetic scattering internally without necessity to introduce two phases. It can also calculate magnetic structures with modulated parent phase where the modulation appears before the magnetic phase transition. The lecture shows manifold possibilities how to refine modulated magnetic structures from various experiments. Several recently solved magnetic structures will be presented.

Landau-Zener and Rabi oscillations in the spin-dependent conductance

We describe the spin-dependent quantum conductance in a wire where a magnetic field is spatially modulated. The change in direction and intensity of the magnetic field acts as a perturbation that mixes spin projections. This is exemplified by a ferromagnetic nanowire. There the local field varies smoothly its direction generating a domain wall (DW) as described by the well-known Cabrera-Falicov model. Here, we generalize this model to include also a strength modulation. We identify two striking diabatic regimes that appear when such magnetic inhogeneity occurs. 1) If the field strength at the DW is weak enough, the local Zeeman energies result in an avoided crossing. Thus, the spin-flip probability follows the Landau-Zener formula. 2) For strong fields, the spin-dependent conductance shows oscillations as a function of the DW width. We interpret them in terms of Rabi oscillations. Time and length scales obtained from this simplified view show an excellent agreement with the exact dynamical solution of the spin-dependent transport. These results remain valid for other situations involving modulated magnetic structures and thus they open new prospects for the use of quantum interferences in spin-based devices.This paper is dedicated to the memory of the lifelong collaborator Patricia Rebeca Levstein.

Positive magnetoresistance induced by fan-type phases in a spin-spiral magnet

We report on the positive magnetoresistance (MR) that accompanies the onset of fan-type magnetic structures in an epitaxially grown Dy/Y superlattice (SL). We find that MR ratios for current perpendicular to plane (CPP) and in-plane (CIP) geometries yield comparable values and a similar nonmonotonic dependence on the applied magnetic field $H$ and temperature, which result in a maximum $\text{MR}\ensuremath{\sim}0.45%$ at $T=110$ K. We demonstrate that the rise in resistance is due to the increase in the number of superzone band gaps that accompanies the magnetization process of the anisotropic spin-spiral magnet, which reflects on the increasing complexity of the field-induced modulated magnetic structures. Furthermore, we find that the suppression of the giant MR in Dy/Y SL is due mainly to the combined action of chemical modulation and epitaxial strain effects. Finally, MR presents a linear (negative slope) and unsaturated scaling with $H$ in the magnetized state, which is a clear fingerprint of magnon MR in highly anisotropic magnetic materials.

Magnetic structures of quaternary intermetallic borocarbidesRCo2B2C (R = Dy, Ho, Er)

The magnetic structures of the title compounds have beenstudied by neutron diffraction. In contrast to the isomorphousRNi2B2C compounds,wherein a variety of exotic incommensurate modulated structures has been observed, the magnetic structureof ErCo2B2C is found to be a collinear antiferromagnet with while those of HoCo2B2C and DyCo2B2C are observed to be simple ferromagnets. For all studied compounds, the momentsare found to be confined within the basal plane and their magnitudes arecomparable to the values obtained from the low-temperature isothermalmagnetization measurements. The absence of modulated magnetic structures in theRCo2B2C series(for ErCo2B2C, verified down to 50 mK) is attributed to the quenching of the Fermi surface nestingfeatures.

Possible mechanisms of formation of long-periodic magnetic structure in high-symmetry crystals

On the example of a crystal with point symmetry group and based on the method of integral rational basis of invariants, linear and nonlinear invariants of first spatial derivatives with respect to the order parameters of the exchange origin have been calculated. It is shown the competition of what kind of parameters can initiate modulated magnetic structures in crystals of this symmetry.

Rapid evaluation of oscillating lattice sums

AbstractWe have generalized a method for the rapid evaluation of oscillating lattice sums for layered geometries, based on the Ewald's summation technique. Different types of long range interactions as well as different lattice symmetries can be considered, in particular modulated magnetic structures, periodic domain patterns, or periodic lattice distortions. As examples the demagnetizing energies for different thin films systems and magnetic excitations in a single atomic layer are calculated.

Magnetic order in HoNiBC and ErNiBC

Neutron diffraction measurements have been performed to study the nature of magnetic ordering in the compounds HoNiBC and ErNiBC. HoNiBC exhibits antiferromagnetic ordering of the Ho moments with a simple commensurate magnetic structure below K. The magnetic unit cell, doubled along the c-direction with respect to the chemical unit cell, consists of ferromagnetic planes antiferromagnetically coupled along the c-axis. On the other hand, the Er moments in ErNiBC show ferromagnetic ordering with a Curie temperature of 4.5 K. In contrast to the case for the structurally related superconducting counterparts ( and ) no modulated magnetic structures were observed for any temperatures above 1.4 K.

Magnetic phase transitions in double hexagonal close packed neodymium metal-commensurate in two dimensions

In elemental neodymium, a series of phase transitions between multi-q modulated magnetic structures takes place below the magnetic ordering temperature TN. Based on extensive neutron diffraction studies of the temperature dependence of the length and orientation of the modulation vectors associated with these structures we suggest that there exists a simple phenomenological relationship between the symmetry of the double hexagonal crystal lattice and the symmetry of the corresponding magnetic lattice. The model has a resemblance to the situation found for monolayer films on solid surfaces (orientational epitaxy) when considering the spin system as the adsorbate and the atomic structure as the substrate. In one dimension, the modulated magnetic structures are sequences of a commensurate to incommensurate transition followed by incommensurate to incommensurate transitions followed by an incommensurate to commensurate transition. However, in two dimensions, all the modulated magnetic structures are equally well described as higher-order commensurate to commensurate transitions, where the magnetic unit cell is commensurate with the crystallographic unit cell, but rotated by some angle around the hexagonal axis with respect to the crystallographic unit cell. The rotation angle as well as the magnetic unit cell depend on temperature. The data suggest that the rotation angle is zero whenever there is a change from one type of multi-q structure to another, i.e., in this case, the magnetic and the crystallographic unit cells are commensurate in both one and two dimensions. Previous and recent results for the light rare earth metals neodymium and praseodymium and alloys thereof, which lend support to this interpretation, are reviewed.

Magnetic order in the ternary compound NdRu 2Ge 2

Heat capacity, electrical resistivity and neutron diffraction studies have been performed on the tetragonal ternary compound NdRu 2Ge 2. Between temperatures of 17 and 10 K, the compound exhibits two types of sine-wave modulated magnetic structures, having wave vectors k = (0.19, 0.05, 0.125) and k = (0.12, 0.12, 0.0) with the amplitude of the moment along the c axis. Below 10 K, a first-order transition occurs to a ferromagnetic state with a moment of 3.64(13)μ B aligned along the tetragonal axis.

Magnetic ordering in the β phase of solid oxygen

The influence of interplane magnetic interactions on the magnetic structure of the β phase of solid oxygen has been analysed. The stability condition of a hypothetical paramagnetic β-O 2 structure has shown that instabilities may lead to modulated magnetic structures and therefore disturb long-range, three-sublattice, antiferromagnetic order of the β-O 2 crystal.

Squaring-up of modulated magnetic structures in HoAg and TmAg

The temperature variation of the amplitude modulated magnetic structures of HoAg and TmAg has been investigated by neutron diffraction on single crystals. A beginning of squaring-up has been observed, but due to experimental conditions the expected antiphase regime could not be fully reached. No locking of the incommensurate propagation vector into commensurate values has been evidenced within the experimental accuracy and no lattice modulation through the magnetoelastic coupling has been detected.

Specific heat and modulated magnetic structures in the HoNi 0.5Cu 0.5 and ErNi 0.6Cu 0.4 compounds

Specific heat measurements performed on HoNi 0.5Cu 0.5 and ErNi 0.6Cu 0.4 have confirmed the role of the rare earth nature on the thermal stability of modulated magnetic structures. With non Kramers ions (Ho for instance) lying in a very low symmetry site a modulated structure can remain stable down to zero Kelvin; the magnetic moment being induced by the exchange field, no entropy is associated with the modulation. With Kramers ions (Er for instance), for which an intrinsic moment always exists, a modulated structure transforms at low temperature into an antiphase structure.