Field induced magnetic density in the Pauli paramagnet YNi5

The induced magnetic density of the enhanced Pauli paramagnet YNi₅ has been measured by polarized neutron diffraction at 100 K and in an applied field of 48 kOe. The magnetic density is localized on Ni sites where a 3d form factor, similar to that of Ni metal, is observed. The induced Ni moment is different for the two crystallographic sites: (2.4 ± 0.6) × 10^-3 μ_B on Ni_I on the 2c site and (4.1 ± 0.8) × 10^-3 μ_B on Ni_II on the 3g site. This difference must be associated with the difference of the Y surroundings of each site. The results are discussed from the band structure of the alloy.     

Field induced magnetic structures in hexagonal HoAlGa

A neutron diffraction study on a single crystal of the HoAlGa compound under magnetic field applied along the c easy magnetization direction has been performed. The different field induced structures of the multistep metamagnetic process, as well as their thermal evolution, have been determined. Whereas the component of the propagation vector in the hexagonal plane is always (1/3, 1/3), along c it is 1/2 or 1/6 at low temperature and 1/2- τ with τ = 0.02, i.e. incommensurate, at high temperature. The stability of different structures of the H - T phase diagram can be discussed in terms of exchange interactions in a compound with a huge uniaxial anisotropy.     

Magnetism on the surface of the bulk paramagnetic intermetallic compound YCo2

Using full-potential electronic structure calculations, we predict that the (111) surface of the cubic Laves phase Pauli paramagnet YCo2 is ferromagnetic. The magnetism of the (111) surface is independent of the termination of the surface, does not extend beyond two Co layers, and is related to the field-induced metamagnetism of the bulk. YCo2 appears to be a prominent candidate to demonstrate the phenomenon of surface-induced itinerant magnetism localized in two dimensions.     

Field induced magnetic ordering transition in Kondo insulators

We study the 2D Kondo insulators in a uniform magnetic field using quantum Monte Carlo simulations of the particle-hole symmetric Kondo lattice model and a mean field analysis of the Periodic Anderson model. We find that the field induces a transition to an insulating, antiferromagnetically ordered phase with staggered moment in the plane perpendicular to the field. For fields in excess of the quasi-particle gap, corresponding to a metal in a simple band picture of the periodic Anderson model, we find that the metallic phase is unstable towards the spin density wave type ordering for any finite value of the interaction strength. This can be understood as a consequence of the perfect nesting of the particle and hole Fermi surfaces that emerge as the field closes the gap. We propose a phase diagram and investigate the quasi-particle and charge excitations in the magnetic field. We find good agreement between the mean-field and quantum Monte Carlo results.Received: 17 December 2003, Published online: 8 June 2004PACS: 71.27. + a Strongly correlated electron systems; heavy fermions - 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.30. + h Metal-insulator transitions and other electronic transitions - 75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena - 75.30.Fv Spin-density waves     

Decay of rabi oscillations induced by magnetic dipole interactions in dilute paramagnetic solids

Decay of Rabi oscillations of equivalent spins diluted in diamagnetic solid matrix and coupled by magnetic dipole interactions is theoretically studied. It is shown that these interactions result in random shifts of spin transient nutation frequencies and thus lead to the decay of the transient signal. Averaging over random spatial distribution of spins within the solid and over their spectral positions within magnetic resonance line, we obtain analytical expressions for the decay of Rabi oscillations. The rate of the decay in the case when the half-width of magnetic resonance line exceeds Rabi frequency is found to depend on the intensity of resonant microwave field and on the spin concentration. The results are compared with the literature data for E′₁ centers in glassy silica and [AlO₄]⁰ centers in quartz.     

Origin of the structural and magnetic anomalies of the layered compound SrFeO2: a density functional investigation

The structural and magnetic anomaly of the layered compound SrFeO2 are examined by first-principles density functional calculations and Monte Carlo simulations. The down-spin Fe 3d electron occupies the d(z(2)) level rather than the degenerate (d(xz), d(yz)) levels, which explains the absence of a Jahn-Teller instability, the easy ab-plane magnetic anisotropy, and the observed three-dimensional (0.5, 0.5, 0.5) antiferromagnetic order. Monte Carlo simulations show that the strong interlayer spin exchange is essential for the high Néel temperature.     

Field induced order in magnetic systems: Marginal case

The bond operator representation and the one-loop renormalization group treatment are used to study the spin-1 Heisenberg antiferromagnetic with single-ion anisotropy and transversal magnetic fields in three-dimensional cubic lattices. We start from a disordered spin-liquid phase to an ordered phase, at a critical field H_c1 above which the system enters an XY-antiferromagnetic phase. This transition is interpreted as belonging to a universality class with a dynamical critical exponent z=1. In this marginal case logarithmic corrections are found to the physical quantities. These theoretical predictions are compared with the scaling of the magnetization as a function of field and temperature for the organic compound NiCl₂-4SC(NH₂)₂.     

Proton nuclear magnetic resonance in paramagnetic CoCl2.6 H2O

The present paper deals with a study of nuclear magnetic resonance (NMR) of protons of the crystal water of paramagnetic CoCl₂.6 H₂O. The measurements were carried out on powdered samples at room temperature, for values of the external magnetic field ranging from 0.3 to 1.0 T.The NMR signals of protons of the crystal water exhibit asymmetric shape which changes with the applied external magnetic field. We found that the second moment of the resonance line shows a linear dependence on the square of the induction of the externally applied magnetic field.Both the cause of the asymmetry of the NMR line of protons of the crystal water and the dependence of the second moment of the resonance line on the induction of external magnetic field are interpreted in the paper.     

Instability of the paramagnetic state towards incommensurate magnetic order in the 2-d Hubbard model

We determine the instability line separating the paramagnetic phase in the phase diagram of the 2-d Hubbard Model from a phase with incommensurate magnetic order. A mean-field approximation of the Kotliar-Ruckenstein slave boson representation is used to calculate the wave-vector dependent magnetic susceptibility. For largeU/t the instability occurs at a densityn0.37, and a wave-vector close toq=(0,). The dependence ofq onU andn is also given.     

Parallel magnetic field induced giant magnetoresistance in low density quasi-two-dimensional layers

We provide a possible theoretical explanation for the recently observed giant positive magnetoresistance in high mobility low density quasi-two-dimensional electron and hole systems. Our explanation is based on the strong coupling of the parallel field to the orbital motion arising from the finite layer thickness and the large Fermi wavelength of the quasi-two-dimensional system at low carrier densities.     

YCo2: intrinsic magnetic surface of a paramagnetic bulk material

Here we report on results of a spin-resolved photoelectron spectroscopic (SRPES) study of YCo2 thin films (150 A-thick) grown on a W(110) substrate. The films were prepared by co-deposition of stoichiometric amounts of Y and Co onto a clean W surface followed by thermal annealing leading to (2x2) overstructure with respect to W(110) in the low-energy electron diffraction pattern indicated formation of a structurally ordered YCo2(111) surface. While no clear spin asymmetry was observed for bulk-sensitive SRPES data taken at hnu=1253.6 eV, the more surface-sensitive SRPES data obtained at hnu=21.2 eV photon energy revealed a clear spin-asymmetry probing the validity of the recent theoretical prediction.     

Spin fluctuations and magnetic order in the heavy fermion compound CePt2Sn2

We present zero and longitudinal field μ SR measurements of single crystal and polycrystalline specimens of the heavy fermion compound CePt₂Sn₂. Above 1 K the behaviour of the two samples is indistinguishable; the muon 1/T_1 increases with decreasing temperature until 25 K when it plateaus. The 1/T_1 relaxation rate differs strongly for the two cases below \sim\,0.8\ K. At 0.1 K a rate of about 20 μ s^-1 is seen in the polycrystal while in the single crystal it is only about 5 μ s^-1. Even more revealing is the fact that longitudinal field decoupling spectra at very low temperatures demonstrate an essentially static spin system to be present in the polycrystalline material while the single crystal shows definite dynamic spin properties. We conclude that, in the presence of the distortion, long range magnetic order occurs below 0.9 K while in tetragonal symmetry long range order is suppressed (probably due to frustration) and spin fluctuations remain for T\rightarrow0. This revised version was published online in July 2006 with corrections to the Cover Date.     

Specific features of magnetic ordering in the SmFeGe2O7 compound

The results of the experimental investigation of the magnetic properties of the SmFeGe₂O₇ compound have been presented. It has been found that the temperature dependence of the susceptibility exhibits two features that coincide with the anomalies in the temperature dependence of the specific heat and indicate magnetic phase transitions in SmFeGe₂O₇. The external magnetic field induces a magnetic transition, the critical field of which depends on the temperature.     

Field dependence of spin-lattice relaxation in paramagnetic salts

The field dependence of the Raman SLR rate is determined for H₀ of the order of the local field. Both S-state and non S-state transition salts are investigated when (i)H_dip > H_exch and (ii) H_exch > H_dip. Measurements are obtained from the response of the longitudinal magnetization to an amplitude modulated microwave field. The observed dependences agree with Orbach-Huang calculations.     

Observation of paramagnetic relaxation in the NpCu4Al8 intermetallic compound

²³⁷Np Mössbauer spectroscopy between 77 and 4.2 K was carried out in connection with neutron diffraction studies between 300 and 2 K on the tetragonal intermetallic NpCu₄Al₈. The Mössbauer spectra give an isomer shift of + 14.3 mm/s vs. NpAl₂ indicating the non-Kramers Np³⁺(⁵I₄)-ion to be present. Below 45 K the onset of magnetic hyperfine splitting is observed. It develops into a fully resolved Zeeman pattern at 4.2 K with B_eff≈330 T. Neutron diffraction finds no evidence for magnetic order over the whole temperature range scanned. All these results can consistently be explained by paramagnetic relaxation phenomena involving a low lying Γ^t₅ doublet closely followed by a Γ^t₄ singlet as the tetragonal crystalline field states of the Np³⁺ ion.     

59Co nuclear magnetic resonance study in GdCo2Si2 compound

The spin echo NMR spectra of⁵⁹Co for GdCo₂Si₂ are reported. A weak negative hyperfine field on Co nuclei was found in magnetically ordered state of the compound. The splitting of resonance lines in external magnetic field indicates on antiferromagnetic structure of Gd moments lying in thea-a plane with energetic minima in [100] and [110] directions as in GdFe₂Si₂ and GdNi₂Si₂.     

Giant magnetic refrigerant capacity in Ho3Al2 compound

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Ho₃Al₂ compound are investigated by magnetization and heat capacity measurements. Two successive magnetic transitions, a spin-reorientation (SR) transition at T_SR=31 K followed by a ferromagnetic (FM) to paramagnetic (PM) transition at T_C=40 K, are observed. Both magnetic transitions contribute to the MCE and result in a large magnetic entropy change (ΔS_M) in a wide temperature range. The maximum values of ?ΔS_M and adiabatic temperature change (ΔT_ad) reach 18.7 J/kg K and 4.8 K for the field changes of 0–5 T, respectively. In particular, a giant value of refrigerant capacity (RC) is estimated to be 704 J/kg for a field change of 5 T, which is much higher than those of many potential refrigerant materials with similar transition temperatures.     

抗磁和顺磁性材料在磁场中弱磁力实验测量

对抗磁性材料和顺磁性材料在强磁场中的弱磁力进行测量.利用设计的实验装置,成功测得抗磁性石墨片和顺磁性材料铝片在永磁铁阵列表面受到的弱磁力.研究证明弱磁力的大小与磁场及磁场梯度乘积成正比.