Low temperature magnetic properties of NdCu2

μSR experiments on NdCu₂ give evidence for short range order below 25\ K above T_N=6.5\ K. No signal was detected between 16\ K and 1.2\ K where neutron scattering reveals an incommensurate spin structure. Below 1.2\ K the μSR signal is recovered and shows a rotation with 22 MHz. This is interpreted with a squaring up of Nd spins accompanied by a decrease of magnon excitations which is reflected in a decrease of damping of the muon signal. This revised version was published online in July 2006 with corrections to the Cover Date.     

The magnetic structures of NdCu2 determined by single crystal neutron diffraction

We investigated the magnetic structure of NdCu₂ by means of neutron diffraction as a function of temperature between 1.5 K and 8 K in zero external field. The diffraction data were obtained on two single crystals with different orientations using the triple-axis-spectrometer TAS6 at the DR-3 reactor at Risø. Two magnetic phases were observed between 1.5K andT _N =6.5K. From 1.5 K to 4.1 K the magnetic reflections can be described by the commensurate wave vector =(3/5 0 0) and its higher harmonics 3 and 5. Below 2.5K the structure is completely squared-up. For 4.1 KT6.5 K the magnetic structure is incommensurate with the chemical lattice and can be described by the wave vector=(3/5 0 0) and its higher harmonies 3 and 5M. Below 2.5 K the structure is completely squared-up. For 4.1 K T 6.5 K the magnetic structure is incommensurate with the chemical lattice and can be described by the wave vector ^*=(0.62 0.044 0). In both phases the Nd-moments are oriented along the easyb-direction.     

Magnetostructural transition in NdCu2

The magnetic (H , T)-phase diagram of the orthorhombic compound NdCu₂ was investigated for external magnetic fields up to 15 T parallel to the crystallographic c-direction. Magnetization and magnetostriction measurements reveal an anomalous change of the magnetic properties as well as giant magnetostriction (GMS) and large hysteretic effects. This behaviour is similar to that observed in some other RCu₂ compounds where it has been interpreted as a conversion of the magnetic Ising axis. In contrast to these other RCu₂ compounds, however, the easy axis of magnetization in NdCu₂ is the b-axis. The macroscopic measurements are compared with neutron diffraction experiments which reveal GMS along the b-axis and a new magnetic phase with propagation vector in the converted crystal. Received 27 March 2000 and Received in final form 11 September 2000     

Complex magnetic phases in LuFe2O4

DC magnetization and AC susceptibility measurements on LuFe₂O₄ single crystals reveal a ferrimagnetic transition at 240 K followed by additional magnetic transitions at 225 K and 170 K, separating cluster glass phases, and a kinetically arrested state below 55 K. The origin of giant magnetic coercivity is attributed to the collective freezing of ferrimagnetic clusters and enhanced domain wall pinning associated with a structural transition at 170 K. Magnetocaloric effect measurements provide additional vital information about the multiple magnetic transitions and the glassy states. Our results lead to the emergence of a complex magnetic phase diagram in LuFe₂O₄.     

Theory of magnetic field-induced charge-density-wave phases

We elaborate an analytical theory of a cascade of magnetic field-induced charge-density-wave (FICDW) phases. It is shown that the following features distinguish it from the well-known spin-density-wave cascade: (1) the FICDW phases exist at temperatures much lower than the characteristic CDW transition temperature at H=0; (2) the cascade of the FICDW phases dramatically changes at certain directions of a magnetic field due to an interplay of Zeeman spin-splitting and electron motion along open Fermi surfaces. Theoretical results are compared with the recent experimental attempts to reveal FICDW phases in the organic conductors α-(ET)₂MHg(SCN)₄(M=K, Tl, Rb, etc.).     

Separation of magnetic phases in alloys

In this paper we present a study of the separation of phases in multi-phase alloys. The proposed technique is based on the hyperbolic model of magnetization. By using this model it is possible to decompose the magnetic phases of alloys and determine their magnetic properties separately. Experimental verification was carried out on a transformer-like setup, constructed from layered samples representing the various magnetic phases. The samples were constructed from elements of strongly different magnetic properties. The results given by the model are in an excellent agreement with the experimental results, giving justification for the proposed method of decomposition. The proposed method is the first step towards the recognition and the separation of magnetic constituencies of different magnetic properties in an alloy by analytical means.     

Field-induced orbital and magnetic phases in Ca3Ru2O7

Magnetic-field- and temperature-dependent Raman scattering studies of Ca3Ru2O7 reveal dramatic field-dependent properties arising from transitions between various complex orbital and magnetic phases, including a field-induced orbital-ordered to orbital-disordered transition (H(o) // hard axis), and a reentrant orbital-ordered to orbital-disordered to orbital-ordered transition (H(o) // easy axis). We find that the dramatic magnetic-field properties are most prevalent in a "mixed"-magnetic and -orbital phase regime, providing evidence for a strong connection between orbital phase inhomogeneity and "colossal" field effects in the ruthenates.     

Electronic structure of spiral magnetic manganite phases

An approach has been developed using the tight-binding and double-exchange Hamiltonian methods for calculating the E(k) spectrum of e _g electrons in noncollinear (spiral) magnetic structures of R _{1 − x} A _x MnO₃ (R = La, Pr, Nd, Sm; A = Ca, Sr, Ba) manganites. A magnetic structure in the form of a flat cycloid observed in TbMnO₃ below 28 K has been considered for undoped manganites (x = 0).     

Nuclear magnetic resonance study of La3X compounds and related phases2

Normal state nuclear magnetic resonance studies of the La₃In, La₃T1 compounds have been made in order to investigate the origin of the large temperature-dependent magnetic susceptibility. It is possible to analyse the different contributions to the susceptibility using Knight-shift and relaxation time T₁ measurements of In¹¹⁵ and T1²⁰⁵ nuclei. The exchange enhancement of the spin-susceptibility χ_pd is of the same order as that found in A-15 compounds and the strong temperature-dependence of χ(T) is attributed to the presence of a peak in the electronic density of states near the Fermi level. The variation of the Knight-shift in the ternary alloys La₃X_1?yX_y¹ is analogous to that observed in the corresponding La₃X phases, on the other hand the Knight-shift in the carbides La₃XC is temperature independent.     

Disordered magnetic phases and magnetic transitions in non anisotropic frustrated systems

Numerous studies have been devoted to disordered magnetic phases which show the existence of several types of disorder in non-anisotropic systems. Semi-disordered systems which retain at least partially long-range order (reentrant properties and randomly canted structures) and fully disordered systems (spin cluster and soft transition systems, true spin glass state) are shortly reviewed. Several characteristic experiments and results are presented and commented on, such as alternative, nonlinear and static susceptibilities, thermoremanence, ageing effects, neutron diffraction and Mössbauer spectroscopy. The possible types of disordered magnetic phases are discussed as a function of a sharp (or soft) transition and of a more or less fast dynamics. In true spin glasses, some problems are still open while in the other disordered phases the unresolved questions are numerous.     

Structural and magnetic properties of electrospun FeCoNi magnetic nanofibers with nanogranular phases

Structural and magnetic properties of silicon/aluminum-added and -free FeCoNi magnetic alloy nanofibers with nanogranular phases prepared by electrospinning and subsequent annealing of the PVP-blended ternary metal precursors in hydrogen atmosphere were investigated. The FeCoNi magnetic alloy nanofibers with evenly distributed nanocrystalline phases were formed, which are identified as γ-Fe_1−x Ni _x binary phase with face-centered cubic structure and α-CoFe phase with body-centered cubic structure. At elevated temperature, the α → γ structural martensitic transformation in the FeCoNi ternary alloys occurred due to the inhomogeneities in composition of the matrix during annealing of the alloy with metastable α-phase. In the Si/Al-added FeCoNi nanofibers, more than two phases with complicated-boundaries of the grains in and/or outside the nanofibers were formed as crystalline phases and amorphous phase. The amorphous phase consisted of Si and/or Al acted as an inhibitor diminishing α → γ transformation as well as an interparticle insulation layer. At low annealing temperature of 450 °C, the Si/Al-added nanofiber mainly consisted of metastable α-phase with a low-crystallinity surface structure and very small diameter of 13 nm was formed and showed an unexpectedly high coercivity, which attributed to the surface effects and/or high surface/volume ratio.     

Pinning mode resonances of new phases of 2D electron systems in high magnetic fields

A striking rf or microwave resonance is a generic feature of electron solid phases of two-dimensional electron systems. These resonances have served to identify and characterize such solids, in the insulator that terminates the series of fractional quantum Hall effects at high magnetic field, in the range of the integer quantum Hall effect, and in bubble phases in the first excited and higher Landau levels.     

Neutron diffraction determination of the magnetic phases of FeCl2·2H2O

The metamagnetic behaviour of FeCl₂·2H₂O has been determined. At T = 4.2 K the reflections of the antiferromagnetic zero-field-phase dissapear, when a magnetic field of H₁ = 39 kG is applied along the direction of sublattice magnetization. There is evidence for a ferrimagnetic phase with a unit cell in which the a axis is tripled. At H₂ = 46 kG this phase vanishes, and the magnetically saturated configuration is reached. As the temperature is increased, the three phases meet at a triple point at T_T = 11.7K and H_T = 42.0 kG.     

Neutron diffraction determination of the magnetic phases of CoCl2·2H2O

The metamagnetic behaviour of CoCl₂·2H₂O has been determined by means of neutron diffraction. At T = 4.2 K the reflections of the antiferromagnetic zero-field phase disappear, when a magnetic field of H_c1 = 31.8 kOe is applied along the b axis. The new ferrimagnetic phase vanishes at H_c2 = 46.1 kOe and the magnetically saturated configuration is reached. The three phases meet at a triple point at T_T = 8.9 K and H_T = 37.5 kOe. Two types of phase transition have been detected.     

Magnetic structures of field-induced magnetic phases of EuAs3

The magnetic field induced magnetic phases of EuAs₃ have been studied with neutron diffraction. Five different magnetic structures have been identified by their moment directions and propagation vectors, only one of them being commensurate with the crystallographic lattice. The change of the magnetic structures with increasing field gives some information on the magnetic exchange interactions.     

Strain-induced modification of magnetic structure and new magnetic phases in rare-earth epitaxial films

Rare earths exhibit complex magnetic phase diagrams resulting from the competition between various contributions to the magnetic energy: exchange, anisotropy and magnetostriction. The epitaxy of a rare-earth film on a substrate induces (i) a clamping to the substrate and (ii) pseudomorphic strains. Both these effects are shown to lead to modifications of the magnetic properties in (0 0 1)Dy, (0 0 1)Tb and (1 1 0)Eu films. In Dy and Tb films, spectacular variations of the Curie temperature have been evidenced. Additionally, Tb films exhibit a new large wavelength magnetic modulation. In Eu films, one of the helical magnetic domains disappears at low temperature whereas the propagation vectors of the other helices are tilted. The link between structural and magnetic properties is underlined via magnetoelastic models. Moreover, molecular beam epitaxy permits the growth of Sm in a metastable dhcp phase. The magnetic structure of dhcp Sm has been elucidated for the first time. In this review, neutron scattering is shown to be a powerful technique to reveal the magnetic structures of rare-earth films.