Modulated Magnetic Structure Research Articles

Magnetic and transport properties of Ho 7Rh 3

Magnetic and transport properties of Ho 7Rh 3 single crystals have been studied by measuring magnetization, magnetic susceptibility, electrical resistivity and by performing neutron-diffraction experiments. Ho 7Rh 3 has two magnetic transitions from paramagnetic to antiferromagnetic at T N=34 K and from antiferromagnetic to ferrimagnetic at T C=22 K. In the ferrimagnetic phase, a small spontaneous magnetization in the c-plane is observed. It is found by neutron-diffraction measurements that Ho 7Rh 3 has a modulated incommensurate helical magnetic structure with the propagation vector Q=(0,0,0.38) at 1.5 K.

First-principles study of the magnetic structures of ordered and disordered Mn-Ir alloys

Electronic and magnetic structures of \ensuremath{\gamma}-phase disordered ${\mathrm{Mn}}_{100\ensuremath{-}x}{\mathrm{Ir}}_{x}$ alloys including ${L1}_{2}$-type ordered ${\mathrm{Mn}}_{3}\mathrm{Ir}$ alloy have been investigated by the first-principles approach using the tight-binding (TB) linear muffin tin orbital (LMTO) method. For the ${L1}_{2}$-type ordered ${\mathrm{Mn}}_{3}\mathrm{Ir}$ alloy, a triangular $(T1)$ magnetic structure is considered to be stable, reflecting in a dip structure around the Fermi level in the density of states. For the \ensuremath{\gamma}-phase disordered ${\mathrm{Mn}}_{3}\mathrm{Ir}$ alloy, on the other hand, the most stable structure is suggested to be the $3Q$ structure among the multiple-Q spin density wave (MSDW) structures. The N\'eel temperature is estimated to be about 735 K from the effective exchange constant ${J}_{0},$ in good agreement with the experimental value of about 730 K. With decreasing Ir concentration in the \ensuremath{\gamma}-phase disordered ${\mathrm{Mn}}_{100\ensuremath{-}x}{\mathrm{Ir}}_{x}$ alloys, the transition from the $3Q$ to the $2Q$ structure takes place in the vicinity of $x=13$ under the constant lattice parameters with the axial ratio of $c/a=1.$ This critical concentration x is close to the observed concentration at which the axial ratio changes from $c/a=1$ to $c/ag1.$ However, it should be stressed that these two critical concentrations do not necessarily coincide with each other, that is, the critical concentration of the magnetic structure transition is lower than that of the lattice distortion. This theoretical expectation has been verified by the experimental structural and magnetic data.

Transition of ferromagnetism to superparamagnetism in Fe/Tb multilayers

Fe/Tb (1.2 nm) multilayer films with various Fe layer thicknesses were prepared by vapor deposition. Magnetic and microstructure of the films were characterized by various experimental methods. The results show that the films evolve from ferromagnetic to superparamagnetic, and Fe in the multilayers transforms from bcc crystalline to amorphous state. The transition of magnetic property and structure is found to be related to the rough interface associated with the amorphous phase formation in Fe/Tb interface. In the [Tb(1.2 nm)/Fe(0.5 nm)]30 multilayer, Fe layers are separated into discontinuous superparamagnetic clusters by three-dimensional Tb crystal aggregates, which are responsible for paramagnetic-like spins at the interface that strengthen the spin-dependent scattering.

Dimensional effects and intercluster interactions in nanosystems

Magnetic, structural-dynamic, catalytic properties of nanocluster systems including both isolated and mutually interacting clusters of oxides of iron and other metals were studied. The influence of dimensional effects and intercluster interactions was discovered for nanosystems including metal oxides both with no carrier and in polymeric and carbon matrices. These effects were displayed in decrease in the critical temperature of magnetic phase and structural transitions, appearance of a critical cluster size for first-order magnetic phase transitions, change of the character of magnetic phase transitions, and change of atomic and cluster dynamics, as well as of adsorption and catalytic properties.

Magnetic properties, structure and shape-memory transitions in Ni-Mn-Ga thin films grown by ion-beam sputtering

Ni/sub 1-2x/Mn/sub x/Ga/sub x/ (x/spl sim/0.2 and 0.25) alloys in thin film form were prepared by ion-beam sputtering from alloy targets. For Si[001] substrates the structure/texture of the films are a function of the growth temperature (T/sub g/). The tetragonal martensite structure with a [220] texture was obtained for an optimal T/sub g//spl sim/300-350/spl deg/C. At higher temperatures a cubic silicide (Mn/sub 12/Ni/sub 4/Si/sub 3/) was obtained. At room temperature, optimal films with the stoichiometric composition (x /spl sim/ 0.2) showed a ferromagnetic hysteresis behavior characterized by significant in-plane rotation. The coercivity of the films was 310 Oe and temperature-dependent measurements confirmed a Curie temperature, T/sub c//spl sim/340 K. At room temperature the films show a modulated 7M martensite structure which was observed in in situ heating experiments to transforms to the cubic austenite phase at 500/spl deg/C.

Study of the incommensurate–commensurate magnetic transition in HoMnO 3 perovskite

The low-temperature magnetic structures of the orthorhombic perovskite HoMnO 3 have been studied on a polycrystalline sample from neutron diffraction, specific heat and susceptibility data. With cooling, HoMnO 3 exhibits three singularities at T N=41 K, T≈26 K and T≈6.5 K, which suggests a rich magnetic phase diagram. At T N, the Mn 3+ magnetic moments become ordered in an incommensurate antiferromagnetic (AFM) arrangement, defined by a (C x ,0,0) mode, adopting a modulated sinusoidal magnetic structure which becomes commensurate with the unit cell below 29 K. Finally, below 22 K, an ordered magnetic moment appears on Ho 3+ cations in an antiferromagnetic arrangement defined by a (A x ,0,C z ) mode; this moment significantly increases below 6.5 K, reaching a value of 7.27(11) μ B at 1.8 K. The different magnetic structures are interpreted on the basis of competing superexchange interactions of opposite signs.

Magnetic structure and magnetic phase transitions inTbPtGe2

Magnetic and neutron diffraction measurements have been performed onTbPtGe2 at low temperatures. The compound crystallizes in theorthorhombic YIrGe2-type structure (space group Immm);crystal structure parameters have been refined on the basis of theneutron diffraction pattern collected at T = 30.7 K (paramagneticregion). TbPtGe2 is antiferromagnetic below TN = 24.2 K.Below this temperature only one of the two Tb sublattices is ordered;the Tb magnetic moments localized at the 4(i) sites order with thesimple propagation vector k = 0. Below Tt1 = 11.4 K themagnetic moments at the other Tb sites, 4(h), show an ordering withthe propagation vector k1 = [0.2677(8),0.1312(24),0.6989(27)]. At Tt2 = 7 K a further phase transition to a newmodulated phase described by the propagation vector k2 = [0.2584(5),0,0.5895(6)] is observed.

U(As1−xSex)solid solutions. I. Resonant x-ray and neutron scattering study of the magnetic phase diagram

We report on a study of the magnetic structure and phase transitions of the ${\mathrm{UAs}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}$ solid solutions employing x-ray and neutron scattering. We have focused this study on compositions $0.18<x<0.22,$ in which magnetic structures having double-$\mathbf{k}$ and triple-$\mathbf{k}$ configurations are found with increasing x. In general, our findings are in excellent agreement with those reported by Kuznietz et al. using solely neutron diffraction. However, our x-ray studies have added several important details about (1) the incommensurate transitions near ${T}_{N},$ (2) the nature of the $2\mathbf{k}\ensuremath{-}3\mathbf{k}$ phase boundary, and (3) the discovery of a new magnetic phase.

Complex Magnetism and Magnetic Structures of the Metastable HoMnO3 Perovskite

The magnetic properties and the low-temperature magnetic structures of the orthorhombic perovskite HoMnO3 (space group Pnma) have been studied on polycrystalline samples by magnetization, specific heat, and neutron diffraction measurements. By cooling, HoMnO3 exhibits three singularities at TN = 41 K, T ≈ 26 K, and T ≈ 6.5 K, suggesting a rich magnetic phase diagram. The neutron diffraction data show that below TN = 41 K, the Mn3+ magnetic moments become ordered in an antiferromagnetic arrangement, adopting a modulated sinusoidal magnetic structure characterized by the wave vector k = (kx,0,0) (kx = 0.40 at 41 K) and defined by the magnetic mode (Cx,0,0). When the temperature is decreased, the propagation vector varies and at T ≈ 29 K a transition to a commensurate magnetic structure defined by k = (1/2,0,0) takes place. Below T ≈ 22 K, a small ordered magnetic moment appears on the Ho3+ cations, strongly increasing below 9 K and reaching 7.3(1) μB at T = 1.8 K. The magnetic structure of the Ho3+ moments is defined by a (Ax,0,Cz) mode. The (H,T) phase diagram has been mapped out and the different magnetic structures interpreted on the basis of competing superexchange interactions.

Magnetic Properties of Np2Ir2In Investigated by 237Np Mössbauer Spectroscopy

The present study of Np2Ir2In completes the 237Np Mossbauer spectroscopy investigations of Np2T2X (T = Co, Rh, Ir, Ni, Pd, Pt; X = In, Sn) compounds. Np2Ir2In is found to order at Tord≈30 K and the complex Mossbauer spectra suggest the occurrence of a noncollinear modulated magnetic structure with an average magnetic moment on the neptunium μNp≈0.76 μB. The magnetic properties of Np2Ir2In are consistent with the general trends observed in the An2T2X (An =U, Np) isostructural family.

Yb-Based Kondo Lattices: Insight from 170 Yb Mössbauer, Positive Muon and γ–γ Perturbed Angular Correlations Spectroscopies

The usefulness of local hyperfine techniques in the investigation of the physical properties of Kondo lattices is illustrated through three examples. First, we show how Mossbauer spectroscopy on 170Yb down to very low temperature (0.025 K) provides evidence for the existence of an incommensurate modulated magnetic structure close to T=0 in YbPtAl; a modulated structure at T=0 is in principle forbidden for Kramers ions, but it is made possible in YbPtAl due to the presence of the Kondo coupling. Second, we present a μSR study of the Kondo insulator YbB12, and we evidence the persistence of fluctuations of very small correlated Yb moments close to T=0. Third, we report on a study by Perturbed Angular Correlation spectroscopy on the isotope 172Yb of the Kondo lattice Yb2Co3Ga9, which is characterised by a high Kondo temperature (T0≃260 K). Our aim was to compare, on a large temperature range, the thermal variations of the 4f quadrupole moment and of the magnetic susceptibility, to check whether the scaling property predicted by theory could be observed.

Vortex structures in low-dimensional superconductors: equilibrium magnetic characteristics and structural transitions

The effect is reported of long-range Pearl–Abrikosov vortices interaction on the magnetic characteristics of the mixed state in thin-film superconductors. On the basis of one-dimensional model both the thermodynamical field and equilibrium magnetization curves are described analytically. Numerical simulation of two-dimensional fluxoid distribution in the magnetic field reveals a sequence of structural transitions between the vortex ensembles of different symmetry.

Incommensurate modulated magnetic structure in the heavy electron compound YbPtAl

170Yb Mössbauer spectroscopy measurements down to 0.025 K and neutron diffraction measurements down to 1.7 K have been performed in the heavy electron compound YbPtAl ( T N≃ 5.8 K). Below T N the neutron data show that the magnetic structure is incommensurate, with a wave-vector k=(0.30, 0, 0) . At 0.025 K, the Mössbauer data reveal that the magnetic structure is modulated and that it is not an antiphase structure. YbPtAl is a clear example of a Kramers local moment system where the Kondo singlet ground state allows a modulated magnetic structure to persist down to 0 K.

Magnetization in Mn-richγ−Cu100−xMnx (36<~x<~83)alloys in fields up to 75 kOe

The field dependence of dc magnetization $[M(H)]$ has been measured in$\phantom{\rule{0ex}{0ex}}\ensuremath{\gamma}\ensuremath{-}{\mathrm{Cu}}_{100\ensuremath{-}x}{\mathrm{Mn}}_{x}$ ($x$=36, 60, 73, 76, and 83) alloys in magnetic fields up to 75 kOe at different temperatures between 4.2 and 21 K. The $M(H)$ data are found to be almost linear with field beyond 10 kOe. This is expected since the present alloys are antiferromagnetically ordered. But, at lower fields $(H<10 \mathrm{kOe}),$ the variation is slower than linear. The change of slope in $M(H)$ beyond the critical field of 10 kOe is attributed to the spin-flop transition which marks the collapse of the helical spin structure and the onset of the linear spin-density-wave modulation. Another interesting observation is that the composition dependences of the magnetization and the electrical resistivity exhibit a minimum and a maximum, respectively, around $x=73$. This behavior is described in terms of the transition of magnetic structure from AF3 to AF1.

Properties of solids under high pressure—An electronic band structure approach

The predicting capability of various parameters related to solids by performing their high pressure electronic band structures is discussed in detail. The studies on ground state crystal structure, magnetic structure and magnetic phase transitions under pressures,s-d electron transitions, bulk modulus, Debye temperature, elastic constants, insulator to metal transitions and the phenomenon of pressure induced superconductivity will be discussed with examples. Especially the method of calculating the Debye temperature for a large number of ternary charlcopyrite systems using their recently evaluated band structure will be presented. The limitations of the band theory when used with the local density approximation with respect to the determination of band gaps and magnetic properties will also be discussed.

Magnetic Structure and Spin Reorientation Transition in ScMnO3

The magnetic structure of ScMnO3was investigated using polycrystalline samples. Bulk magnetic measurements were carried out revealing a Neel temperature of 130 K, followed by two maxima at 58 K and 40 K. The high-temperature data indicate strong antiferromagnetic coupling between Mn3+ions in the form of a large, negativeθ=−943(7) K. Powder neutron diffraction as a function of temperature disclosed magnetic long-range ordering below 130 K, where the chemical and magnetic unit cell have the same volume. A 120° magnetic structure was found consistent with the triangular Mn3+sublattice and frustrated antiferromagnetic nearest neighbor coupling. The orientation of the magnetic spins is temperature dependent below 70 K.

Oxides: neutron and synchrotron X-ray diffraction studies

We shall review some results from several areas of oxide science in which neutron scattering and X-ray synchrotron scattering exercise a complement- ary role to high-resolution transmission electron microscopy. The very high- resolution time-of-flight neutron diffraction technique and its role in studies of the magnetic structure of oxides will especially be reviewed. The selected topics of structural studies for the chosen oxides are: crystal and magnetic structure of the so-called cellular random systems, magnetic structure and phase transitions in ferrites and the behaviour of water in non-stoichiom etric protonic conductors and in the opal silica-water system.

Thickness-dependent magnetic domain structures of ultrathin Co/Au(111) films studied by means of magnetic force microscopy in ultrahigh vacuum

We report on magnetic domain structure transition in ultrathin Co films on Au(111) as a function of thickness as observed by magnetic force microscopy (MFM) in ultrahigh vacuum (UHV). The transition from out-of-plane to in-plane anisotropy has been observed at a Co film thickness between 4 monolayers (ML) and 5 ML. Additionally, we have resolved the different contrast within domain walls on the in-plane domain structure which we attribute to different polarization of the domain wall.

A modulated magnetic structure in CaMn 7O 12

The magnetic structure of CaMn 7O 12 was investigated by neutron powder diffraction measurements in temperatures from 1.7 K to RT. We observed several magnetic reflections which appear around 86 K. Their 2θ positions shift considerably and get broader between 30 and 50 K, where an anomaly in magnetic susceptibility exists. These phenomena can be due to a modulated magnetic arrangement of the magnetic moments of Mn ions with a strongly temperature-dependent modulation period. An estimate of the value of the magnetic moments is given.

A neutron diffraction study of the magnetic structure and phase transition in Ni3 11B7O13Cl boracite

The nuclear and magnetic structures and the magnetic phase transition of Ni3 11B7O13Cl [Ni-Cl] boracite have been investigated by means of high resolution and high flux neutron powder diffraction between 1.5 and 30K. The temperature dependence of the magnetic intensities shows an unsharp onset at T N = 9 K, indicating the existence of a second-order magnetic phase transition. Refinements of the magnetic structure give rise to the magnetic space group Pc'a21′ for the magnetic phase below T N , with the magnetic modes C x , F y and A z for the three Ni sublattices. The three Ni sites display two kinds of magnetic moment with different values and orientations: Ni(1) has a moment parallel to the main antiferromagnetic axis along [100]or, while Ni(2) and Ni(3) have their antiparallel moments along [010]or-directions. The total corresponds to a coplanar two-dimensionally canted antiferromagnetic arrangement, which is characteristic for the magnetic structure of Ni-Cl. The refined magnetic moment values at 1.5 K are: μ1x = 1.65(2) μ B for Ni(1), and μ2y = -μ3y = 0.79(4) μ B for Ni(2) and Ni(3), respectively, showing a weak magnetic superexchange interaction compared with the spin only moment 2μ B /Ni2+ observable by neutrons. The magnetic ordering process and the possible frustration effects due to the interwoven configurations between the three Ni sites and the formation of isolated Ni3CI groups, are discussed based on the proposed magnetic structure.