Identification of a spin-glass and a cluster-glass by Mössbauer measurements: Fe0.5Mn0.5TiO3 and Fe0.2Mg0.8TiO3

Summary M?ssbauer measurements have been made on two kinds of spin-glass (SG) systems: one is the magnetically non-diluted mixed compound Fe_0.5Mn_0.5TiO₃ with the SG-freezing temperatureT _SG=21.5K, and the other is the diluted one Fe_0.2Mg_0.8TiO₃ withT _SG=6K. We have shown that the temperature variation of the M?ssbauer spectrum of Fe_0.2Mg_0.8TiO₃ above and around itsT _SG is essentially different from that of Fe_0.5Mn_0.5TiO₃: the former is typical of a cluster-glass and the latter of an ordinary spin-glass. The present work has clearly demonstrated that the M?ssbauer spectroscopy is the most useful and unique technique to distinguish a cluster-glass from an ordinary spin-glass. Paper presented at the ICAME-95, Rimini, 10–16 September 1995     

Form-factor measurements on chromium with high-energy synchrotron radiation

Results of high-energy magnetic X-ray diffraction on pure antiferromagnetic chromium are presented. The temperature dependence of the propagation vector of the spin-density wave (SDW) and the strain-wave (SW) could be reproduced. The temperature dependence of the magnetic integrated intensity could be measured in the transversally as well as in the longitudinally polarised SDW phase. The magnetic form-factor has been determined in the transversally polarised SDW phase with five magnetic satellites. For the first time a spin-orbit separation has been performed by comparing X-ray to neutron data. The small orbital contribution to the magnetisation density turns out to be negligible, in agreement to our relativistic band-structure calculations. In addition, measurements of strain-wave reflections have been undertaken, and the results complement previous studies. Received 17 August 1998 and Received in final form 10 August 1999     

Quantum spin dynamics of the bilayer ferromagnet La Sr Mn O

We construct a theory of spin wave excitations in the bilayer manganite La_1.2Sr_1.8Mn₂O₇ based on the simplest possible double-exchange model, but including leading quantum corrections to the spin wave dispersion and damping. Comparison is made with recent inelastic neutron scattering experiments. We find that quantum effects account for some part of the measured damping of spin waves, but cannot by themselves explain the observed softening of spin waves at the zone boundary. Furthermore a doping dependence of the total spin wave dispersion and the optical spin wave gap is predicted. Received 15 January 2002 Published online 6 June 2002     

High magnetic field properties and ESR of the compounds

compounds have a layered structure made of alternating Ni-O and Li-O slabs. An amount z of extra divalent Ni ions is always present in the Li-O layers. We show, using high field magnetisation, static and dynamic susceptibility and high frequency ESR, that the magnetic properties are driven by the z parameter. The compounds can be described as ferromagnetic Ni-O layers, bridged by clusters possessing a net ferromagnetic moment. Received: 24 July 1998 / Revised and Accepted: 23 September 1998     

Spin-spin correlations in the insulating and metallic phases of the Mott system V 2 O 3

The magnetic response in V ₂ O ₃ has been investigated using polarised neutron scattering with polarisation analysis. Measurements were carried out at three temperatures corresponding to the antiferromagnetic insulating ground state, the metallic phase and the high temperature metallic phase. At the first order metal insulator transition there is a dramatic change in the magnetic response with the metallic and high temperature metallic phases being characterised by ferromagnetic spatial correlations of the paramagnetic response. The establishment of ferromagnetic correlations at the metal insulator transition accounts for the abrupt jump in the uniform susceptibility. It is proposed that the differentiation of the V-V distances across the edges of VO ₆ octahedra is of critical importance for the change in electronic conductivity but also for the establishment of the spatial correlations. The gradual high temperature evolution of the conductivity then occurs by the reduction in the vanadium d overlap brought about by thermal expansion. The first order reduction in atomic volume which occurs on the establishment of the metallic phase results from an instability of the vanadium local moment arising from the change in electronic structure. Received 7 April 1999     

The chromium spin density wave: magnetic X-ray scattering studies with polarisation analysis

We report on X-ray magnetic diffraction studies of the spin density wave antiferromagnetism formed in the conduction electron band of chromium. Non-resonant X-ray magnetic scattering was used to directly determine that chromium has zero orbital magnetisation. Furthermore, the azimuthal dependence of this scattering provides unique evidence that chromium forms a linearly polarised wave. In the vicinity of the K absorption edge, resonant X-ray magnetic scattering was observed. A consistent model of the magnetic scattering has been derived from the resonant and non-resonant magnetic amplitudes. The enhancement of the magnetic intensity arises primarily from dipole transitions from the core 1s level to 4p states. Quadrupole transitions to the magnetic 3d states are essentially non-existent due to their sensitivity to (and the absence of) orbital moment. This effect is predicted from atomic considerations of the 3d⁵ ( = 0) transition metal ions. Received 22 September 2000     

On the evolution of the ground state in the system U x La 1 - x S: Polarized neutron diffraction and X-ray magnetic circular dichroism study

In the U_xLa_1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x _c ∼ 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x _c ) the shape of the induced form factor (f ( Q )) is comparable with that found for x = 1 and is well reproduced by either a U⁴⁺ or a U³⁺ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M₅ edge suggests an abrupt change in the distribution of the electrons (holes) in the 5 f density of states around x _c . Received 31 January 2001     

Double-Q magnetic structures and strong planar anisotropy in tetragonal ErRu2Ge2and ErRu2Si2

Single crystal magnetization measurements and powder neutron diffraction on tetragonal ErRu₂Ge₂ as well as anisotropy of the paramagnetic susceptibility and specific heat measurements on ErRu₂Si₂ are presented. Besides the huge crystal field contribution to the uniaxial anisotropy, which favors the basal plane, a strong in-plane anisotropy is evidenced. From these features and neutron diffraction experiments it is shown that magnetic structures of these materials are double-Q and accordingly non-colinear below their Néel temperature (5.2 and 6.0 K for Ge and Si based compounds, respectively). The magnetic structures induced during the metamagnetic processes are discussed. Received 24 December 1999     

Spin correlations in the two-dimensional spin-5/2 Heisenberg antiferromagnet

We report a neutron scattering study of the instantaneous spin correlations in the two-dimensional spin S =5/2 square-lattice Heisenberg antiferromagnet Rb₂MnF₄. The measured correlation lengths are quantitatively described, with no adjustable parameters, by high-temperature series expansion results and by a theory based on the quantum self-consistent harmonic approximation. Conversely, we find that the data, which cover the range from about 1 to 50 lattice constants, are outside of the regime corresponding to renormalized classical behavior of the quantum non-linear model. In addition, we observe a crossover from Heisenberg to Ising critical behavior near the Néel temperature; this crossover is well described by a mean-field model with no adjustable parameters. Received: 3 March 1998 / Received in final form: 4 May 1998 / Accepted: 19 May 1998     

Spontaneous surface magnetisation of single crystal MnF2 in the antiferromagnetic state

Spontaneous magnetisation of (100) and (010) surfaces of single crystal MnF₂ in the antiferromagnetic state has been discovered. The sign of the surface magnetisation is determined by the difference in dielectric constants of MnF₂ and ambient matter: magnetisation is directed to the substance with smaller . Received 28 August 1998 and Received in final form 15 December 1998     

Neutron scattering on the strongly correlated electron CeNi Cu system: from non-magnetic behaviour to long-range magnetic order

The ground state of the strongly correlated electron CeNi_1-xCu_x compounds has been investigated by means of neutron scattering experiments. Thus, magnetic diffraction was performed for compounds showing long-range magnetic order (x > 0.2). An evolution from a collinear ferromagnetic structure for x =0.6 to a simple antiferromagnetic one for CeCu takes place through some more complex magnetic structures for intermediate compositions. The magnetic moments are continuously reduced when the Ni content increases reflecting the progressive enhancement of the Kondo screening. The large reduction found for x =0.6 compound is discussed and the existence of a spin glass like component of the magnetic moment cannot be discarded. From the quasielastic spectra, we have obtained the Kondo temperatures which are close to the magnetic ordering ones. The quasielastic line-width evolves from a linear temperature dependence to a T ^1/2 behaviour when approaching the non-magnetic limit. Then, this system provides an interesting example for the evolution of unstable 4 f shell relaxation regimes when modifying the hybridisation strength. Received 22 May 2000     

Chromium doped manganites: evidence for electronically phase-separated systems

X-ray absorption spectroscopy (XAS) and soft-X-ray magnetic circular dichroism (SXMCD) at the Mn L_2,3-, Cr L_2,3- and O-K edges of Sm_0.5Ca_0.5Mn_1-xCr_xO₃ () bulk polycrystalline samples have been performed at T=20 K below the ferromagnetic Curie temperature. We show the existence of a magnetic sublattice on each of the probed cations. Considering an electronically phase-separated system, results are compared with magnetization and resistivity measurements and a tentative correlation with magnetoresistance properties on such doped compounds is discussed. Received 7 January 2000     

Jahn-Teller distortion and magnetoresistance in electron doped Sr1-xCexMnO3 (x = 0.1, 0.2, 0.3 and 0.4)

Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized electron doped Sr_1-xCe _x MnO₃ (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d _x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size may be responsible for the absence of double exchange ferromagnetism and CMR effect. Received 10 September 1999     

Neutron scattering study of transverse magnetism in the metamagnet FeBr2

In order to clarify the nature of the additional phase transition at H ₁ ( T ) < H _c ( T ) of the layered antiferromagnetic (AF) insulator FeBr ₂ as found by Aruga Katori et al. (1996) we measured the intensity of different Bragg-peaks in different scattering geometries. Transverse AF ordering is observed in both AF phases, AF I and AF II. Its order parameter exhibits a peak at T ₁ = T ( H ₁ ) in temperature scans and does not vanish in zero field. Possible origins of the step-like increase of the transverse ferromagnetic ordering induced by a weak in-plane field component when entering AF I below T₁ are discussed. Received 27 September 1999 and Received in final form 6 December 1999     

Magnetic properties of ErFe6Fa6 studied by magnetization and neutron diffraction

Neutron-diffraction experiments reveal that ErFe₆Ga₆ forms in the tetragonal ThMn₁₂-type of structure (space group I₄/mmm). The Fe sublattice orders ferromagnetically below K. The Er moments order antiparallel to the Fe moments which, below about 250 K, leads to a decrease of the total magnetization. The easy magnetization direction of ErFe₆Ga₆ is perpendicular to the c-axis in the whole temperature range. Refinement at 2 K shows that ErFe₆Ga₆ orders ferrimagnetically with Er moments of 8.5 (2) and Fe moments at the 8(j) site of 1.9 (1) and at the 8(f) site of 1.7 (1) , respectively. At room temperature, ErFe₆Ga₆ exhibits the same type of magnetic order, however with substantially lower Er moments of 1.0 (4) and Fe moments at the 8(f) site of 1.2 (2) . The Fe moments at the 8(j) site amount to 1.9 (5) /Fe. Received 24 November 1999 and Received in final form 27 March 2000     

On the ground-state magnetic structure in Er2Ni2Pb

We have studied the unusual low-temperature magnetic phase of Er₂Ni₂Pb using powder neutron diffraction measurements in zero field down to 460 mK. Our previous neutron diffraction experiments down to 1.5 K showed that magnetic Bragg reflections seen in Er₂Ni₂Pb can be indexed by several propagation vectors that partially coexist. All the incommensurate propagation vectors seemed to disappear in the low temperature limit. The present study, however, shows that reflections belonging to the propagation vector q’ = (0.47 0 1/2) do not disappear but remain present down to 460 mK. This highly unexpected result suggests that the magnetic structure described by this propagation vector might not be a simple sine-wave modulation. One interesting possibility here is a spin-slip structure as the ground state.     

Magnetic structure and anisotropy of YFe 6 Ga 6 and HoFe 6 Ga 6

The magnetic structure of RFe₆Ga₆ intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, ⁵⁷Fe M?ssbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn₁₂ structure (space group I4/mmm) with the magnetic structure of YFe₆Ga₆ consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from M?ssbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe₆Ga₆ is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures. Received 8 March 2001 and Received in final form 18 June 2001     

Collapse of the magnetic ordering and structural anomalies in the U La S system: neutron diffraction and specific heat measurements

We report specific heat and neutron diffraction measurements of seven samples in the solid solution system U_xLa_1-xS. All samples have the simple fcc NaCl crystal structure. Both specific heat and neutron diffraction confirm the suggestion from the earlier magnetic measurements that the ferromagnetism disappears abruptly at 0.57. Near there is a doubling of the electronic contribution to the specific heat, as compared to the value of 23 mJ mol^-1K^-2 in pure US. Around the widths of the nuclear Bragg peaks show a considerable broadening, as well as anomalies in the mean lattice parameter, as compared to those expected from Vegard's law. A preliminary analysis suggests this broadening may be due to a loss of long range lattice order near . However, these changes are independent of temperature, so that further experiments are necessary before they can be associated with the changes in magnetic behavior at . Received 18 September 1998     

Crystal structure and magnetic ordering of RNi Si compounds

The element distributions and the magnetic ordering behaviour of compounds RNi₁₀Si₂ (R = Tb, Dy, Ho, Er, Tm) have been studied by neutron powder diffraction down to temperatures of 1.6 K. The compounds crystallize in an ordered variant of the ThMn₁₂ structure type in the tetragonal space group P4/nmm. An ordered 1:1 distribution of Ni and Si on sites 4d and 4e, respectively, corresponds to a modulation vector [0, 0, 1] with respect to the space group I4/mmm of the ThMn₁₂ structure. TbNi₁₀Si₂ orders antiferromagnetically below T _N = 4.5 K with a magnetic propagation vector of [0, 0, 1/2]. The magnetic Tb moments, 8.97(2) /Tb atom at 1.6 K, are aligned along the c-axis. The Ni sites in TbNi₁₀Si₂ do not carry any ordered magnetic moments. The compounds with R = Dy, Ho, Er, and Tm are paramagnetic down to 1.6 K and 3.0 K, respectively. Received 10 July 2002 / Received in final form 12 September 2002 Published online 29 October 2002     

Canted antiferromagnetism and magnetoelastic coupling in metallic Ho0.1Ca0.9MnO3

Ho0.1Ca0.9MnO3 is a canted antiferromagnet with the magnetic space group Pn ' ma '. The magnetic structure is a superposition C x F y A z of the three types of order allowed in Pn ' ma '. In the Ca-rich corner of the system Ho _1-x Ca x MnO ₃ the title compound has a strong magnetoelastic distortion , the highest metallic conductivity and a ferromagnetic component F y close to the maximum in the series. Among the areas ab, bc, ca calculated from the lattice constants only ca shows a strong magnetoelastic effect below T N = 106 K. The x-, y-, z-spin components depend differently on the temperature. This gives rise to spin rotation which is particularly strong close to T N. MnO ₆ octahedra have short bond lengths with a temperature independent average . They are practically regular at room temperature and show a Jahn-Teller distortion of 3.5% in the magnetically ordered state. Above T N we find small polaron conductivity. The presence of the Jahn-Teller distortion due to the only small abundance (10%) of Mn ³⁺ in the t _2g ³ e g configuration is attributed to delocalised e g electrons. In the magnetically ordered state the averaged magnetic moment of Mn is reduced appreciably from the paramagnetic value due to spin disorder. Received 21 January 1999