Partial magnetic order in the itinerant-electron magnet MnSi

MnSi is an itinerant ferromagnet with a long-wavelength helical modulation of the spin structure. Macroscopic measurements suggest that the ordering temperatureT _c is reduced with increasing pressure fromT _c = 30 K atp = 0 to zero at the critical pressurep _c = 14.6 kbar. Resistivity measurements show that MnSi enters a non-Fermi liquid state atp _c, which remains to be understood. Neutron scattering techniques have been used to investigate the magnetic structure at and abovep _c, i.e. triple-axis spectrometry and small angle neutron scattering. Surprisingly, sizeable quasi-static moments were found to survive to pressures considerably abovep _c. They are, however, organized in a highly unusual way such that the magnetic Bragg reflections are sharp in the longitudinal direction but are very broad in the transverse direction, implying a partial magnetic order that was never seen before.     

Spin-wave contributions to nuclear magnetic relaxation in magnetic metals

The longitudinal and transverse nuclear magnetic relaxation rates 1/T ₁(T) and 1/T ₂(T) are calculated for three- and two-dimensional (3D and 2D) metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic moments in the spin-wave temperature region. The contribution of the one-magnon decay processes is strongly enhanced in comparison with the standard T-linear Korringa term, especially for the FM case. For the 3D AFM case this contribution diverges logarithmically, the divergence being cut at the magnon gap ω due to magnetic anisotropy, and for the 2D AFM case as ω^-1. The electron-magnon scattering processes yield T ²ln(T/ω) and T ²/ω^1/2-terms in 1/T ₁ for the 3D AFM and 2D FM cases, respectively. The two-magnon (“Raman”) contributions are investigated and demonstrated to be large in the 2D FM case. Peculiarities of the isotropic 2D limit (where the correlation length is very large) are analyzed. Received 29 November 1999 and Received in final form 6 June 2000     

Dimensionality crossover upon magnetic saturation in Fe, Ni and Co

The magnetic saturation process of iron, nickel and cobalt single-crystal spheres is studied using neutron scattering in a vertical magnetic field. It is observed that upon magnetic saturation, the scattering intensities decrease instead of increasing. This indicates a decreasing coherent scattering with field. The spin precession around the field axis therefore can be assumed to be incoherent along directions transverse to the field. Comparison of the temperature dependence of the spontaneous magnetization measured by zero field NMR on the one hand and by the macroscopic magnetization on the other hand shows that Fe, Ni and Co are three-dimensional (3D) in the zero field ground state but one dimensional (1D) in the magnetically saturated state. The observed decrease in neutron scattering intensity is consistent with this conclusion. The change in dimensionality is associated with a crossover. Our neutron scattering study shows that the crossover occurs at a field that is smaller than the demagnetization field. The dimensionality crossover, therefore, is driven not by the field but by the associated forced magnetostriction.     

Orbital correlations in doped manganites

We review our recent X-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO₃. For x=0.25, we find an orbital structure indistinguishable from the undoped structure and long-range orbital order at low temperatures. For dopings 0.3≤x≤0.5, we find scattering consistent with a charge and orbitally ordered CE-type structure. While in each case the charge order peaks are resolution limited, the orbital order exhibits only short-range correlations. We report the doping dependence of the correlation length and discuss the connection between the orbital correlations and the finite magnetic correlation length observed on the Mn³⁺ sublattice with neutron-scattering techniques. The physical origin of these domains, which appear to be isotropic, remains unclear. We find that weak orbital correlations persist well above the phase transition, with a correlation length of 1–2 lattice constants at high temperatures. Significantly, we observe similar correlations at high temperatures in La_0.7Ca_0.3MnO₃, which does not have an orbitally ordered ground state, and we conclude that such correlations are robust to variations in the relative strength of the electron–phonon coupling. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

The structure of the glass phase of Rb1−x(ND4)xD2PO4

X-ray scattering techniques have been used to study the diffuse scattering from a single crystal of Rb_1–x(ND₄)_xD₂PO₄ withx=0.65. This system has a structural glass phase at low temperatures resulting from the competing ferroelectric interactions of RbD₂PO₄ and antiferroelectric interactions of (ND₄)D₂PO₄. The diffuse scattering shows a broad peak with a maximum occurring at a wavevector of about 0.3a ^*, and the intensity of these peaks is surprisingly different for wavevectorsq on opposite sides of the Bragg reflections. A model of the D bonding is developed which suggests that the diffuse scattering arises from the interaction between ferroelectric displacements alongc, ferroelectric displacements alongb, and transverse acoustic modes polarized alongb andc. The model accounts for the incommensurate wavevector and, qualitatively, for the intensity of the diffuse scattering around different Bragg reflections. The temperature dependence of the scattering is also measured.     

Transverse spin correlations in a random anisotropy system: Amorphous ErCo2

We provide a coherent interpretation of early small angle scattering experiments performed by some of us on amorphous ErCo₂ [9]. At low temperature the zero field transverse spin-spin correlation function is found to fit a simple exponential for large length scales (l >l_c), supporting the lower critical dimensionality d_c=4. For shorter length scales (l<l_c) the correlation function is of the Ornstein-Zernike type. These results are physically understood in terms of the breaking of ferromagnetism into Imry and Ma domains.A further physical interpretation leads us to consider the localization of ferromagnetic spin waves within Imry and Ma domains in zero field, and their delocalization by application of an external field.     

Hall effect and negative magnetoresistance in thin crystals of NbSe<Subscript>3</Subscript>

We have measured the Hall effect and the transverse magnetoresistance in NbSe₃ single crystals. In the liquid helium temperature range we observed an absolute negative magnetoresistance (NMR) — the value of the resistance under magnetic field being much lower than that at zero field — in NbSe₃ single crystals with a thickness less than 5 μm with the magnetic field oriented in the (b, c) plane. We show that this NMR effect is observed in the magnetic field range in which the Hall constant changes its sign. The results are qualitatively explained by the change of the surface scattering contribution to the magnetoconductance in the magnetic field range near the Hall voltage zero crossing.     

Magnetic interactions and media noise in rigid disks

This paper discusses the relationship between media-noise and magnetic properties. Two topics in longitudinal barium ferrite (Ba-ferrite) particulate rigid disks are mainly investigated; the relationship between media-noise characteristics and interparticle magnetic interactions, and the particle-size effect in read/write properties. For the first topic, a series of longitudinal Ba-ferrite disks with a variation in volumetric particle-packing densities are investigated. The magnetomotive force dependences or reverse DC-erase noise are measured and compared with interaction field factor. For the second topic, the particle-size effect in read/write properties of Ba-ferrite particulate disks are described compared with the those of Co–γFe₂O₃ particulate disks. The magnetic interactions in Ba-ferrite disks consist of two mechanisms: the interparticle positive interactions in stacks of Ba-ferrite particles, and the negative interactions between the stacks in which the magnetic moments of the Ba-ferrite platelet particles are aligned in the stack direction. H_c and switching field distribution values depend on interparticle interactions and extra stack-like particle interactions. Media noise depends on particle size. These ideas are effective for thin film media without exchange of magnetic interactions.     

Band structure and Fermi surfaces of alternate structural phases of Co and Rh

In this work, first-principles DFT scalar-relativistic calculations using the GGA functionals were performed to study the equilibrium properties of alternate structural phases of Co and Rh. The results show that cobalt orders ferromagnetically in the bcc, fcc and hcp phases, where the Co atoms carry magnetic moments of 1.80 μ_B, 1.71 μ_B and 1.69 μ_B, respectively. Rhodium is ferromagnetic only in the bcc phase where the Rh atoms carry a moment of 0.56 μ_B. The results yield evidence for the influence of the crystal symmetry in establishing ferromagnetic order in transition metals.     

On the magnetic properties of Y(FexCo1−x)2 compounds

The composition dependence of the mean magnetic moment of cobalt atoms in Y(Fe_xCo_1?x)₂ compounds is analysed in the local environment model. Cobalt has a magnetic moment of 1.56 μβ if there are least two Fe atoms as nearest neighbours. The maximum in the composition dependence of the transition metal moments is due to the magnetic contributions of iron atoms only. The thermal variation of reciprocal susceptibility obeys a Curie-Weiss behaviour, in addition to the Pauli paramagnetic term. Finally, the influence of the variable magnetic interactions on the transition metal moments is discussed.     

Spin dynamics in oriented ferromagnetic nanowires Ge<Subscript>0.99</Subscript>Co<Subscript>0.01</Subscript>

The magnetic properties of one-dimensional oriented nanowires Ge_0.99Co_0.01 grown in pores of anodized aluminum oxide membranes are investigate using ferromagnetic resonance spectroscopy. The electron spin resonance signals of the magnetically ordered cobalt subsystem and the charge-carrier subsystem are identified. It is revealed that the anisotropy field at 4 K is equal to 400 Oe and aligned parallel to the nanowire axis. The transverse relaxation time of spin waves at 4 K is estimated to be ～10^?10 s. It is shown that the magnetic properties of nanowires are predominantly determined by the ferromagnetism of Co and GeCo alloy clusters.     

Field-induced confinement in (TMTSF)2ClO4 under accurately aligned magnetic fields

We present transport measurements along the least conducting c direction of the organic superconductor (TMTSF)₂ClO₄  performed under an accurately aligned magnetic field in the low temperature regime. The experimental results reveal a two-dimensional confinement of the carriers in the (a, b) planes which is governed by the magnetic field component along the b^′ direction. This 2-D confinement is accompanied by a metal-insulator transition for the c axis resistivity. These data are supported by a quantum mechanical calculation of the transverse transport taking into account in self consistent treatment the effect of the field on the interplane Green function and on the intraplane scattering time.     

On the magnetic properties of Y(FexCo1-x)2 compounds

The composition dependence of the mean magnetic moment of cobalt atoms in Y(Fe_xCo_1-x)₂ compounds is analysed in the local environment model. Cobalt has a magnetic moment of 1.56 μ_B if there are at least two Fe atoms as nearest neighbours. The maximum in the composition dependence of the transition metal moments is due to the magnetic contributions of iron atoms only. The thermal variation of reciprocal susceptibility obeys a Curie-Weiss behaviour, in addition to the Pauli paramagnetic term. Finally, the influence of the variable magnetic interactions on the transition metal moments is discussed.     

X-ray resonant magnetic scattering by Fe/Cr superlattices

We have studied the structural and magnetic properties of an antiferromagnetically (AF) coupled Fe/Cr superlattice by means of soft x-ray resonant magnetic scattering. Strong and purely magnetic Bragg peaks are observed at the half-order positions in reciprocal space parallel to the [001] growth direction and in between the structural Bragg reflections from the superlattice periodicity. The magnetic hysteresis loops measured at the first-order and at the half-order Bragg peaks clearly demonstrate the strong AF coupling of the Fe/Cr multilayer. Transverse scans and off-specular reflectivity measurements confirm an AF domain structure of the superlattice in remanence with large perpendicular correlation. In addition, the transverse scan of the half-order Bragg peak exhibits a Lorentzian line shape at zero field, which diminishes in higher fields, indicative of a remanent multidomain state approaching a single-domain state towards saturation.     

On the induced cobalt moments in (GdxY1−x)Co3 compounds

We report the results of magnetic measurements on (Gd_xY_1?x)Co₃ compounds with x ? 0.2, in the temperature range (4.2–1300) K. The compounds are ferrimagnetically ordered, the cobalt magnetization being anti-parallel oriented to that of gadolinium. The mean cobalt moment is dependent on composition. In the paramagnetic range the reciprocal susceptibility obeys a non-linear variation. By using a two sublattices model, the mean values of the exchange interactions inside and between sublattices are determined. The analysis of the data shows that the changes of the cobalt moments are due to the variations of the exchange fields acting on these atoms. Finally, the magnetic behaviour of cobalt atoms in these compounds is discussed.     

Quasi two-dimensional magnetic order of Tb3+ spins in Pb2Sr2Tb1−x Ca x Cu3O8 (x = 0 and 0.5)

Neutron diffraction experiments were performed to study the magnetic ordering of the Tb⁺³ sublattice in the high-T _c superconductor Pb₂Sr₂Tb_1?x Ca _x Cu₃O₈ (x = 0.5) and the undoped parent compound (x = 0). For the parent compound, a quasi two dimensional (2D) phase with a finite antiferromagnetic correlation along the c-direction and a three dimensional phase with ferromagnetic correlation along the c-direction were found. The coexistence of the two phases is likely to be related to structural imperfections such as stacking faults, strains, oxygen disorder or cation vacancies. The superconductor with a superconducting transition temperature of T _c = 71K exhibits a quasi 2D magnetic ordering of the Tb sublattice with a finite ferromagnetic correlation along the c-direction. The correlation lengths along the c-direction for the quasi 2D phases are 32 and 26Å for x = 0 and 0.5, respectively. The magnetic saturation moments are with 9.1 and 8.8 μ _B in excellent agreement with our mean-field crystal-field calculations. 2D short range correlation could be observed up to about 8T _N and is described by a Lorentzian distribution of magnetic intensity perpendicular to the 2D rod in reciprocal space.     

On the induced cobalt moment in RCo4 B-type compounds

The results of magnetic measurements performed on (Gd_xY_1-x)Co₄B compounds in the temperature range 4.2 and 900 K are reported. The compounds with x ⩾ 0.2 are ferrimagnetically ordered. The mean cobalt moment is dependent on composition. Above the Curie temperatures, the reciprocal susceptibilities obey a non-linear dependence as function on temperature. By using a two sublattices model, the mean values of the molecular field coefficients, characterizing the magnetic interactions inside and between sublattices are determined. The mean cobalt moments are linearly dependent on the exchange fields acting on these atoms.     

Electronic, magnetic, and vibrational properties of the molecular magnet Mn4 monomer and dimer

A new type of the single-molecule magnet [Mn₄O₃Cl₄(O₂CEt)₃(py)₃] forms dimers. Recent magnetic hysteresis measurements on this single-molecular magnet revealed interesting phenomena: an absence of quantum tunneling at zero magnetic field and tunneling before magnetic field reversal. This is attributed to a significant antiferromagnetic exchange interaction between different monomers. To investigate this system, we calculate the electronic structure, magnetic properties, intramolecular and intermolecular exchange interactions using density-functional theory within the generalized-gradient approximation. Our calculations agree with experiment. We find that the calculated threefold symmetric structure is vibrationally stable. We also calculate vibrational infrared absorption and Raman scattering intensities for the monomer which can be tested experimentally.     

Crystalline and magnetic structures in REAlxGa2 − x systems (RE = Nd,Er; 0 ⪕ x ⪕ 2): Part II: Magnetic structures

Part II describes results on bulk magnetic properties, transition temperatures, magnetic structures and magnetic moments found for the NdAl_xGa_{2 ? x} and ErAl_xGa_{2 ? x} pseudoternary solid solution systems (0 ? x ? 2). Characterization techniques include electron microprobe analysis, induction magnetometer, X-ray and neutron diffraction. We found ferromagnetic as well as antiferromagnetic order with parallel and incommensurate spiral structures. These spiral structures are of cycloidal or triangular screw axis type.