Academic literature on the topic 'AF magnetic order'

The problem of magnetic transitions between the low-temperature (macrospin ordered) phases in two-dimensional XY arrays is addressed. The system is modelled as a plane structure of identical single-domain particles arranged in a square lattice and coupled by the magnetic dipole–dipole interaction; all the particles possess a strong easy-plane magnetic anisotropy. The basic state of the system in the considered temperature range is an antiferromagnetic (AF) stripe structure, where the macrospins (particle magnetic moments) are still involved in thermofluctuational motion: the superparamagnetic blocking T b temperature is lower than that ( T af ) of the AF transition. The description is based on the stochastic equations governing the dynamics of individual magnetic moments, where the interparticle interaction is added in the mean-field approximation. With the technique of a generalized Ott–Antonsen theory, the dynamics equations for the order parameters (including the macroscopic magnetization and the AF order parameter) and the partition function of the system are rigorously obtained and analysed. We show that inside the temperature interval of existence of the AF phase, a static external field tilted to the plane of the array is able to induce first-order phase transitions from AF to ferromagnetic state; the phase diagrams displaying stable and metastable regions of the system are presented. This article is part of the theme issue ‘Patterns in soft and biological matters’.

Abstract Rhenium oxides belonging to the family AReO4 where A is a metal cation, exhibit interesting electronic and magnetic properties. In this study we have utilized the muon spin rotation/relaxation (µ +SR) technique to study the magnetic properties of the MgReO4 compound. To the best of our knowledge, this is the first investigation reported on this interesting material, that is stabilized in a wolframite crystal structure using a special high-pressure synthesis technique. Bulk magnetic studies show the onset of an antiferromagnetic (AF) long range order, or a possible singlet spin state at T C1 ≈ 90 K, with a subtle second high-temperature transition at T C2 ≈ 280 K. Both transitions are also confirmed by heat capacity (Cp ) measurements. From our µ +SR measurements, it is clear that the sample enters an AF order below T C1 = T N ≈ 85 K. We find no evidence of magnetic signal above T N, which indicates that T C2 is likely linked to a structural transition. Further, via sensitive zero field (ZF) µ +SR measurements we find evidence of a spin reorientation at T Cant ≈ 65 K. This points towards a transition from a collinear AF into a canted AF order at low temperature, which is proposed to be driven by competing magnetic interactions.

At ambient pressure, the Fe0.49(Rh1-xPdx)0.51 alloys with the Pd concentration within 0 < x < 0.12 exhibit an antiferromagnetic (AF) state below the critical temperature Tt, while above x = 0.12 the alloys have a ferromagnetic (F) order up to the Curie temperature TC 650 K. The temperature and field dependences of the magnetization in the alloys with x = 0.08; 0.13 were investigated under hydrostatic pressure up to 10 kbar. The application of pressure of about 8.5 kbar is observed to induce the F-AF phase transition in the ferromagnetically ordered compound with x = 0.13. The AF-F-AF phase evolution was revealed with increasing pressure in the AF-ordered alloy with x = 0.08.

Double perovskites (DPs) of the form A 2 BB ′ O 6 usually involve a transition metal ion, B, with a large magnetic moment, and a nonmagnetic ion B ′. While many DPs are ferromagnetic, studies on the underlying model reveal the possibility of antiferromagnetic (AF) phases as well driven by electron delocalization. In this paper we present a comprehensive study of the magnetic ground state and Tc scales of the minimal DP model in three dimensions using a combination of spin-fermion Monte Carlo and variational calculations. The effective magnetic lattice in three dimensions is face centered cubic (FCC) and so geometrically frustrated. This promotes noncollinear spiral states and "flux" like phases in addition to collinear AF order. We map out the possible magnetic phases for varying electron density, "level separation" ϵB - ϵB', and the crucial B ′ B ′ (next neighbour) hopping t′.

By simultaneous measurements of sample resistance and of X-ray resonant scattering, we tried to establish a direct correlation between magnetoresistance (MR) and antiferromagnetic (AF) order in a Co/Cu metallic multilayer. Field-dependent scattered intensity has been measured in a geometrical configuration corresponding to the Bragg peak coming from the AF coupling, at a photon energy close to the Co L 3 absorption edge (hν = 776.5 eV ). The comparison between measurements of the scattered intensity and of the resistance clearly shows a direct correlation between AF order and MR.

We demonstrate a probe of long-range antiferromagnetic (AF) order in FeRh thin films using non-resonant magnetic x-ray scattering. In particular, x-rays at energies below the Fe K-edge have been used for the observation of magnetic Bragg peaks. Due to the low efficiency of the magnetic scattering, a grazing incidence geometry was used to optimise the diffracted intensity from the thin film samples. Based on Scherrer analysis, we estimate a coherence length similar to previous reports from x-ray magnetic linear dichroism (XMLD) experiments, indicating that domain sizes are limited to 40 nm which is consistent with the grain size. The temperature dependent behaviour of the AF order shows an inverse correlation with the emergence of the ferromagnetic (FM) moment, as expected from the phase diagram.

While atomic disorder has provided a paradigm shift in crystalline materials because of unusual atomic arrangements and functional response, “partial” disorder is scarcely reported until now. We discovered partial cation disorder in Li2MnO3 with fewer stacking faults, which was synthesized under high pressure. Mn and Li atoms in a Mn2/3Li1/3O2 layer disorder while Li atoms in a Li layer order. Magnetization and specific heat measurements indicate a long-range antiferromagnetic (AF) order below 35 K. The irreversibility observed in the magnetization data and the hump observed for the specific heat data suggest the coexistence of an AF order and a partial magnetic disorder. Neutron diffraction measurements reveal that the coexisted state is formed instead of the Néel AF state that has previously been reported for conventional Li2MnO3. These results indicate that high pressure makes a breakthrough to introduce partial disorder within crystals and designs not only a unique magnetic structure but also other physical properties.

Ferromagnetic resonance (FMR) linewidth (LW) is a tool for studying the high frequency properties of magnetic materials for their application in high-speed devices. Here, we investigate different mechanisms which determine FMR damping in bilayer ferromagnetic/antiferromagnetic (F/AF and AF/F) exchange bias systems. Variations of FMR LW with the thickness and deposition order of the F and AF layers were studied, as well as their correlation with the exchange bias field and roughness of the sample surface. We observed much larger LW in AF/F structures compared with F/AF samples. It was found that neither the exchange bias nor surface/interface roughness in the samples could explain the difference in LW for F/AF and AF/F samples. Instead, the different underlayer microstructure influenced the grainsize, leading to different angular dispersion of magnetization and different internal stray field in F-layers, promoting a different intensity of magnon scattering and FMR damping in F/AF and AF/F samples.

Ces travaux qui s’inscrivent dans le cadre d’une collaboration entre l’ICMCB et l’institut Néel portent sur les nickelates (RE1-xSrxNiO2 ou RE = La et Nd) sous forme de poudres. Dans le cadre de cette thèse je présente la synthèse et design chimique de ces nouveaux matériaux par voie chimique innovante, incluant la chimie douce (voie citrate – nitrate), la synthèse sous haute pression d’oxygène (PO2 &lt; 650 bars) (phases RENiO3) à haute température (T = 900°C) et enfin la réduction topochimique utilisant l’hydrure de calcium CaH2 à basse température (T&lt;300°C), dans le but de stabiliser le Ni(+I) dans un environnement plan carré (RENiO2). La caractérisation physico-chimique des composés synthétisés, via les rayons X et de la microscopie électronique couplée à une sonde EDX a mis en évidence une limitation intrinsèque de la solubilité du Sr. Ensuite, nous avons étudié les propriétés physiques intrinsèques des matériaux synthétisés, par différentes techniques de laboratoire, telles que des mesures de transport électrique, de chaleur spécifique et de susceptibilité magnétique, mais aussi des mesures complémentaires qui font appel aux grands instruments : DRX sous pression physique (ESRF), diffusion élastique et inélastique de neutrons (ILL, ISIS, étude des excitations et ordre magnétique). Nous avons établi le schéma de champ cristallin et la possible présence d’un ordre magnétique intrinsèque à NdNiO2 et Nd1-xSrxNiO2. Enfin de nouvelles phases de type RENiO2.5 ont été stabilisées. Une structure cristalline originale qui stabilise trois états de valence du nickel a été proposée pour le composé NdNiO2.5. Ce dernier présente un ordre ferrimagnétique autour de 270 K<br>This work, which is part of a collaboration between the ICMCB and the Néel Institute, focuses on nickelates (RE1-xSrxNiO2 or RE = La and Nd) in the form of powder (bulk samples). Within this thesis, I present the synthesis and chemical design of these new materials using an innovative chemical route, including soft chemistry (citrate – nitrate route), synthesis under high oxygen pressure (PO2 &lt; 650 bars) (RENiO3 phases) at high temperature (T = 900°C) and finally topochemical reduction using calcium hydride CaH2 at low temperature (T&lt;300°C), with the aim of stabilizing Ni(+I) in a square planar environment (RENiO2). The physicochemical characterization of the as-synthetized compounds via laboratory X-ray diffraction and electron microscopy coupled to an EDX probe evidenced an intrinsic solubility limit of the Sr atoms. We then studied the intrinsic physical properties of the synthesized materials, not only by different laboratory techniques, such as electrical transport measurements, specific heat and magnetic susceptibility, but also using complementary measurements in large scale facilities instruments: XRD under physical pressure at the ESRF, elastic and inelastic neutron diffraction at the ILL and at ISIS for the study of magnetic excitations and magnetic order. We have established the crystal field diagram and the possible presence of an intrinsic magnetic order in NdNiO2 and Nd1-xSrxNiO2. Finally, new phases of the RENiO2.5 type have been synthetized. An original crystal structure that stabilizes three valence states of nickel has been proposed for the compound NdNiO2.5. The latter presents a ferrimagnetic order around 270 K

Intervertebral disc degeneration is characterized by a progressive cascade of structural, biochemical and biomechanical changes affecting the annulus fibrosus (AF), nucleus pulposus (NP) and end plates (EP). These changes are considered to contribute to the onset of back pain. It has been shown that mechanical properties of the AF and NP change significantly with degeneration [1,2]. Therefore, mechanical properties have the potential to serve as a biomarker for diagnosis of disc degeneration. Currently, disc degeneration is diagnosed based on the detection of structural and compositional changes using MRI, X-ray, discography and other imaging techniques. These methods, however, do not measure directly the mechanical properties of the extracellular matrix of the disc. Magnetic Resonance Elastography (MRE) is a technique that has been used to measure in vivo mechanical properties of soft tissue by applying a mechanical vibration and measuring displacements with a motion-sensitized MRI pulse sequence [3]. The mechanical properties (e.g., the shear modulus) are calculated from the displacement field using an inverse method. Since the applied displacements are in the order of few microns, fibers may not be stretched enough to remove crimping. Therefore, it is unknown if the anisotropy of the AF due to the contribution of the fibers is detectable using MRE. The objective of this study is twofold: to measure shear properties of AF in different orientations to determine the degree of AF anisotropy observable by MRE, and to identify the contribution of different AF constituents to the measured shear modulus by applying different biochemical treatments.