I. N. Goncharenko

Pressure-induced crystallization of a spin liquid

Liquids are expected to crystallize at low temperature. The only exception is helium, which can remain liquid at 0 K, owing to quantum fluctuations. Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie–Weiss temperature θCW (ref. 3). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry. This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below θCW (refs 5–7). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb2Ti2O7, under conditions of high pressure (∼9 GPa) and low temperature (∼1 K). This compound is a three-dimensional magnet with θCW = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb2Ti2O7 remains in a spin liquid state down to at least 70 mK (ref. 8). But we find that, under high pressure, the spins start to order or ‘crystallize’ below 2.1 K, with antiferromagnetic order coexisting with liquid-like fluctuations. These results indicate that a spin liquid/solid mixture can be induced by pressure in geometrically frustrated systems.

Neutron and X-ray diffraction study of the broken symmetry phase transition in solid deuterium

The solid hydrogen compounds D2, HD and H2 remain quantum molecular solids up to pressures in the 100 GPa range. A remarkable macroscopic consequence is the existence of a pressure-induced broken symmetry phase transition, in which the molecules go from a spherical rotational state to an anisotropic rotational state. Theoretical understanding of the broken symmetry phase structure remains controversial, despite numerous studies. Some open questions concern the existence of long- or short-range orientational order; whether a strong isotopic shift on the transition pressure should be assigned to the nuclear zero-point motion or to quantum localization; and whether the structures are cubic, hexagonal or orthorhombic. Here we present experimental data on the structure of the broken symmetry phase in solid D2, obtained by a combination of neutron and X-ray diffraction up to 60 GPa. Our data are incompatible with orthorhombic structures predicted by recent theoretical works. We find that the broken symmetry phase structure is incommensurate with local orientational order, being similar to that found in metastable cubic para-D2.

High pressure effects on the crystal and magnetic structure of La0.7Sr0.3MnO3

The crystal and magnetic structure of manganite La0.7Sr0.3MnO3 has been studied in the pressure range 0–7.5 GPa and the temperature range 4–300 K. The ferromagnetic state of La0.7Sr0.3MnO3 remains stable in the whole studied pressure range. The Curie temperature increases as d TC/d P = 4.3 K GPa−1. Unlike the manganites with orthorhombic crystal structure, the pressure-induced increase of TC in La0.7Sr0.3MnO3 having the rhombohedral crystal structure of symmetry may be explained by modification of structural parameters only. The difference between the properties of manganites with rhombohedral and orthorhombic structures under high pressure is discussed in terms of the symmetry of MnO6 octahedra.

Ferromagnetic Mixed-Valence and Kondo-Lattice State in TmTe at High Pressure

Neutron diffraction experiments on TmTe at pressures up to 7 GPa are reported. The semiconductor-to-metal transition occurring at 2 GPa in this compound is accompanied by the appearance of strong ferromagnetic exchange interactions producing a Curie temperature as high as 14 K. This behavior is characteristic for the incipient mixed-valence regime just above the transition pressure and traced back to the release of a small concentration of free charge carriers in the material. The steep decrease of both the Curie temperature and ordered magnetic moment occurring at higher pressures emphasizes the role of Kondo fluctuations, and raises the interesting possibility that a quantum critical transition to a non-magnetic ground state of mixed-valence Tm might take place around 6 GPa.

Pressure induced magnetic phase separation in La0.75Ca0.25MnO3 manganite.

The pressure dependence of the Curie temperature T(C)(P) in La(0.75)Ca(0.25)MnO(3) was determined by neutron diffraction up to 8 GPa, and compared with the metallization temperature T(IM)(P) (Postorino et al 2003 Phys. Rev. Lett. 91 175501). The behavior of the two temperatures appears similar over the whole pressure range, suggesting a key role of magnetic double-exchange also in the pressure regime where the superexchange interaction is dominant. The coexistence of antiferromagnetic and ferromagnetic peaks at high pressure and low temperature indicates a phase separated regime which is well reproduced with a dynamical mean-field calculation for a simplified model. A new P-T phase diagram has been proposed on the basis of the whole set of experimental data.

Crystal structure under pressure of geometrically frustrated pyrochlores

We have studied by x-ray synchrotron diffraction under high pressure five pyrochlore compounds: Tb2Ti2O7 (up to 42 GPa), Tb2Sn2O7 and Tb2Mo2O7 (up to 35 GPa), Gd2Mo2O7 and (Tb0.8La0.2)2Mo2O7 (up to 10 GPa). At ambient pressure all compounds crystallize in the cubic symmetry group. This structure is stable for all compounds in the investigated pressure range. All three compounds having Mo as transition metal are described by the same equation of state, with the same bulk modulus B0 = 149. The bulk modulus is smaller in the Mo pyrochlores than in the Ti and Sn ones, in contrast with a priori expectations.

Novel magnetic structures of the low-carrier system CeP under high pressure

Abstract The results of neutron scattering experiments on the low-carrier system CeP under high pressure up to 1.7 GPa are presented. The magnetic structures derived from the scattering patterns at various pressures at low temperatures are similar to those of CeP under magnetic field, forming the periodic stacking of the double Γ8 Ce (0 0 1) planes with sandwiching the Γ7 Ce (0 0 1) planes. The difference is a wide and systematic change of the period of the stacking of Γ8 Ce (0 0 1) planes with changing pressure. These results support strongly the magnetic polaron model for the unusual properties of CeP.

Magnetic diffraction studies under very high pressures

Abstract In 1992, a joint project for studying magnetic materials at very high pressures by neutron diffraction was established between the Laboratoire Leon Brillouin (LLB), Saclay, and the Kurchatov Institute, Moscow. Crystal structure studies under pressures in excess of 10 GPa had already been made in Moscow since the early 198Os, and the Russian team had developed considerable experience in using sapphire or diamond anvils for neutron measurements with sample volumes of less than one cubic millimeter. It thus looked promising to combine this expertise with the powerful neutron facilities available at the LLB.

Three-dimensional magnetic ordering in the Rb2CuCl4 layer perovskite—structural correlations

This work investigates the magnetic structure of Rb2CuCl4 as a function of pressure and temperature using neutron diffraction. As in most A2CuCl4 layered perovskites, there is a 2D ferromagnetic order within the layers. This behaviour is due to the Jahn–Teller (JT) antiferrodistortive structure of the CuCl6 units. Rb2CuCl4 undergoes a 3D magnetic transition at TN = 16 K, which mainly depends on the weak antiferromagnetic interlayer interaction. The pressure slightly increases TN ,a s∂TN/∂P = 0.13 K kbar −1 .T his behaviour is interpreted in terms of pressure-induced tilts and reduction of interlayer distance, both effects increasing the antiferromagnetic exchange coupling between layers. The results are compared with previous magnetic studies under chemical and hydrostatic pressure along layered perovskites series of [CnH2n+1NH3]2CuCl4 (n = 1–3) and BMnF4 (B = Li, Na, K, Rb, Tl, Cs and NH4 )i nvol ving JT ions of Cu 2+ and Mn 3+ ,r esp ectively. We show that the ratio of the interlayer to intralayer coupling, and thus the nature of the magnetic order, can be tuned by chemical or hydrostatic pressure along the A2CuCl4 series. The present findings stress the relevance of octahedral tilts on the magnetic behaviour of layered perovskites.

Single-crystal neutron diffraction under high pressures: valence instabilities in Tm monochalcogenides

New high-pressure devices based on the use of sapphire anvils now allow single-crystal neutron diffraction experiments to be performed up to P=8–10 GPa. After giving a brief overview of the technique, we present its application to the study of pressure-induced valence instabilities in Tm monochalcogenides (TmX, X: S, Se, Te). A variety of new magnetic phases have been characterized, yielding a consistent picture of the evolution of magnetism through the series. The results indicate a striking interplay between magnetic order taking place at low temperature and different types of electronic ground states (classical semiconductor, narrow-gap Kondo insulator, metallic Kondo lattice, etc.) inferred from the transport properties.