| 1. |
- Tseggai, M., et al.
(author)
-
Effects of magnesium substitution on the magnetic properties of Nd0.7Sr0.3MnO3
- 2005
-
In: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 178:4, s. 1203-1211
-
Journal article (peer-reviewed)abstract
- Effects of magnesium substitution on the magnetic properties of Nd0.7Sr0.3MnO3 have been investigated by neutron powder diffraction and magnetization measurements on polycrystalline samples of composition Nd0.7Sr0.3MnO3, Nd0.6Mg0.1Sr0.3MnO3, Nd0.6Mg0.1Sr0.3Mn0.9Mg0.1O3, and Nd0.6Mg0.1Sr0.3Mn0.8Mg0.2O3. The pristine compound Nd0.7Sr0.3MnO3 is ferromagnetic with a transition temperature occurring at about 210 K. Increasing the Mg-substitution causes weakened ferromagnetic interaction and a great reduction in the magnetic moment of Mn. The Rietveld analyses of the neutron powder diffraction (NPD) data at 1.5 K for the samples with Mg concentration, y=0.0 and 0.1, show ferromagnetic Mn moments of 3.44(4) and 3.14(4) μB, respectively, which order along the [001] direction. Below 20 K the Mn moments of these samples become canted giving an antiferromagnetic component along the [010] direction of about 0.4 μB at 1.5 K. The analyses also show ferromagnetic polarization along [001] of the Nd moments below 50 K, with a magnitude of almost 1 μB at 1.5 K for both samples. In the samples with Mg substitution of 0.2 and 0.3 only short range magnetic order occurs and the magnitude of the ferromagnetic Mn moments is about 1.6 μB at 1.5 K for both samples. Furthermore, the low-temperature NPD patterns show an additional very broad and diffuse feature resulting from short range antiferromagnetic ordering of the Nd moments.
|
|
| 2. |
- Brede, Jens, et al.
(author)
-
Adsorption and conformation of porphyrins on metallic surfaces
- 2009
-
In: JOURNAL OF VACUUM SCIENCE and TECHNOLOGY B. - : American Vacuum Society. - 1071-1023. ; 27:2, s. 799-804
-
Journal article (peer-reviewed)abstract
- Tetraphenyl porphyrins (TPP) belong to a highly interesting class of molecules with a large variety of electronic, magnetic, and structural properties. So far, local investigations by scanning probe techniques were primarily focused on larger agglomerates of TPP molecules. Here, experimental results of the observation and manipulation of isolated molecules adsorbed on cold metal substrates by means of low temperature scanning tunneling microscopy are presented. Depending on the surface geometry, i.e., Cu(111) VS, Cu(100) three distinct deformations of the molecular structure are identified reflecting the interaction of the phenyl periphery with the substrate. In a second step, controlled manipulation in terms of deformation of the porphyrin core, ligand dissociation, and lateral displacement of the phenyl periphery are demonstrated.
|
|
| 3. |
- Brede, Jens, et al.
(author)
-
Dynamics of molecular self-ordering in tetraphenyl porphyrin monolayers on metallic substrates
- 2009
-
In: NANOTECHNOLOGY. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 20:27, s. 275602-
-
Journal article (peer-reviewed)abstract
- A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
- Jönsson, Petra, et al.
(author)
-
Absence of conventional spin-glass transition in the Ising dipolar system LiHo(x)Y(1-x)F(4)
- 2007
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 98:25, s. 256403-
-
Journal article (peer-reviewed)abstract
- The magnetic properties of single crystals of LiHo(x)Y(1-x)F(4) with x=16.5% and x=4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While this system is considered as the archetypal quantum spin glass, the detailed analysis of our magnetization data indicates the absence of a phase transition, not only in a transverse applied magnetic field, but also without field. A zero-Kelvin phase transition is also unlikely, as the magnetization seems to follow a noncritical exponential dependence on the temperature. Our analysis thus unmasks the true, short-ranged nature of the magnetic properties of the LiHo(x)Y(1-x)F(4) system, validating recent theoretical investigations suggesting the lack of phase transition in this system.
|
|
| 9. |
|
|
| 10. |
|
|
| 11. |
- Mathieu, Roland, et al.
(author)
-
Bandwidth-disorder phase diagram of half-doped layered manganites
- 2006
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:2
-
Journal article (peer-reviewed)abstract
- Phase diagrams in the plane of rA (the average ionic radius, related to the one-electron bandwidth W) and σ2 (the ionic radius variance, measuring the quenched disorder), or bandwidth-disorder phase diagrams, have been established for perovskite manganites, with a three-dimensional (3D) Mn-O network. Here we establish the intrinsic bandwidth-disorder phase diagram of half-doped layered manganites with a two-dimensional (2D) Mn-O network, examining in detail the parent state of the colossal magnetoresistance phenomenon in crystals without ferromagnetic instability. The consequences of the reduced dimensionality, from 3D to 2D, for the order-disorder phenomena in the charge-orbital sectors are also highlighted.
|
|
| 12. |
|
|
| 13. |
- Mathieu, Roland, et al.
(author)
-
Coexistence of long-ranged charge and orbital order and spin-glass state in single-layered manganites with weak quenched disorder
- 2007
-
In: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 80:3, s. 37001-
-
Journal article (peer-reviewed)abstract
- The relationship between orbital and spin degrees of freedom in the single crystals of the hole-doped Pr1-xCa1+xMnO4, 0.3 <= x <= 0.7 has been investigated by means of ac-magnetometry and charge transport. We show that in an intermediate underdoped region, with 0.35 <= x < 0.5, the "orbital-master spin-slave" relationship commonly observed in half-doped manganites does not take place. The long-ranged charge-orbital order is not accompanied by an antiferromagnetic transition at low temperatures, but by a frustrated short-ranged magnetic state bringing forth a spin-glass phase. We discuss in detail the nature and origin of this true spin-glass state, which, as in the half-doped manganites with large quenched disorder, is not related to the macroscopic phase separation observed in crystals with minor defects or impurities.
|
|
| 14. |
- Mathieu, Roland, et al.
(author)
-
Logarithmic growth law in the two-dimensional Ising spin glass state resulting from the electron doping in single-layered manganites
- 2007
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:1, s. 014436-
-
Journal article (peer-reviewed)abstract
- The ac susceptibility of the electron-doped single-layered manganite La1.1 Sr0.9 Mn O4 is analyzed in detail. A quasi-two-dimensional antiferromagnetic (AFM) order with Ising anisotropy is stabilized below TN ∼80 K. We show that below TN, a rare two-dimensional (2D) spin-glass (SG) correlation develops with the same Ising anisotropy as the AFM state. Using simple scaling arguments of the droplet model, we derive a scaling form for the ac susceptibility data of a 2D SG, which our experimental data follow fairly well. Due to simplifications in this 2D case, the proposed scaling form only contains two unknown variables, ψν and τ0. Hence, the logarithmic growth law of the SG correlation predicted by the droplet model is convincingly evidenced by the scaling of our experimental data. The origin and nature of this 2D SG state are also discussed.
|
|
| 15. |
|
|
| 16. |
|
|
| 17. |
- Mathieu, Roland, et al.
(author)
-
The nanoscale phase separation in hole-doped manganites
- 2007
-
In: Journal of the Physical Society of Japan. - 0031-9015 .- 1347-4073. ; 76:12, s. 124706-
-
Journal article (peer-reviewed)abstract
- A macroscopic phase separation, in which ferromagnetic clusters are observed in an insulating matrix, is sometimes observed, and believed to be essential to the colossal magnetoresistive (CMR) properties of manganese oxides. The application of a magnetic field may indeed trigger large magnetoresistance effects due to the percolation between clusters allowing the movement of the charge carriers. However, this macroscopic phase separation is mainly related to extrinsic defects or impurities, which hinder the long-ranged charge-orbital order of the system. We show in the present article that rather than the macroscopic phase separation, an homogeneous short-ranged charge-orbital order accompanied by a spin glass state occurs, as an intrinsic result of the uniformity of the random potential perturbation induced by the solid solution of the cations on the A-sites of the structure of these materials. Hence the phase separation does occur, but in a more subtle and interesting nanoscopic form, here referred as "homogeneous". Remarkably, this "nanoscale phase separation" alone is able to bring forth the colossal magnetoresistance in the perovskite manganites, and is potentially relevant to a wide variety of other magnetic and/or electrical properties of manganites, as well as many other transition metal oxides, in bulk or thin film form as we exemplify throughout the article.
|
|
| 18. |
|
|
| 19. |
- Ye, F, et al.
(author)
-
Electronic Self-Organization in the Single-Layer Manganite Pr1-xCa1+xMnO4
- 2009
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 103:16
-
Journal article (peer-reviewed)abstract
- We use neutron scattering to investigate the doping evolution of the magnetic correlations in the single-layer manganite Pr1-xCa1-xMnO4, away from the x = 0.5 composition where the CE-type commensurate antiferromagnetic (AF) structure is stable. We find that short-range incommensurate spin correlations develop as the system is electron doped (x < 0.5), which coexist with the CE-type AF order. This suggests that electron doping in this system induces an inhomogeneous electronic self-organization, where commensurate AF patches with x = 0.5 are separated by electron-rich domain walls with short-range magnetic correlations. This behavior is strikingly different than for the perovskite Pr1-xCaxMnO3, where the long-range CE-type commensurate AF structure is stable over a wide range of electron or hole doping around x = 0.5.
|
|
| 20. |
|
|
| 21. |
|
|
