51ÁÔÆæÈë¿Ú

Abstract:

We show evidence that a structural martensitic transition is related to significant changes in the electronic structure, as revealed in thermodynamic measurements made in high magnetic fields. The effect of the magnetic field is considered unusual as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons. We provide a theoretical framework, which can be used to describe the effect of the magnetic field on the lattice dynamics in which the field dependence originates from the dielectric constant.

Abstract:

Specific-heat and magnetic-susceptibility measurements are reported for the polycrystalline spinel compounds GeNi2 O4 and GeCo2 O4 in magnetic fields up to 14 T and 0.5 K≤T≤400 K. Both compounds have first-order antiferromagnetic transitions. There are two sharp closely spaced magnetic-ordering anomalies for GeNi2 O4 at Néel temperatures TN1 (0) =12.080 K and TN2 (0) =11.433 K in zero magnetic field. There is also a broad anomaly in the specific heat centered at ∼5 K, which is present for all fields. Spin waves with an average gap of 10.9 K are associated with this anomaly, which is confirmed by neutron-scattering measurements. An unusual feature of the antiferromagnetism for GeNi2 O4 is the simultaneous presence of both gapped and ungapped spin waves in the Néel state, inferred from the specific-heat data. GeCo2 O4 has a single anomaly at TN (0) =20.617 K in zero magnetic field. Spin waves with an average gap of 38.7 K are derived from fitting the low-temperature specific heat and are also observed by neutron scattering. For both compounds ∼50% of the derived magnetic entropy is below the ordering temperatures, and the total magnetic entropies are only ∼60% of that predicted for the Ni2+ and Co2+ single-ion ground-state configurations. The missing entropy is not linked to magnetic disorder in the ground state or hidden ordering below 0.5 K. It is postulated that the missing entropy is accounted for by the presence of substantial magnetic correlations well above the Néel temperatures. Fitting the GeNi2 O4 susceptibilities to the Curie-Weiss law yields parameters that are consistent with those found for Ni2+ ions in a crystal-electric-field environment including octahedral and trigonal components. The application of the Curie-Weiss law to the GeCo2 O4 susceptibilities is not valid because of low-lying crystal-electric-field states. © 2008 The American Physical Society.

Abstract:

Although the isotope effect in superconducting materials is well documented, changes in the magnetic properties of antiferromagnets due to isotopic substitution are seldom discussed and remain poorly understood. This is perhaps surprising given the possible link between the quasi-two-dimensional (Q2D) antiferromagnetic and superconducting phases of the layered cuprates. Here we report the experimental observation of shifts in the Néel temperature and critical magnetic fields (Δ TN / TN ≈4%; Δ Bc / Bc ≈4%) in a Q2D organic molecular antiferromagnet on substitution of hydrogen for deuterium. These compounds are characterized by strong hydrogen bonds through which the dominant superexchange is mediated. We evaluate how the in-plane and interplane exchange energies evolve as the atoms of hydrogen on different ligands are substituted, and suggest a possible mechanism for this effect in terms of the relative exchange efficiency of hydrogen and deuterium bonds. © 2008 The American Physical Society.

Abstract:

Though long-range magnetic order cannot occur at temperatures T > 0 in a perfect two-dimensional (2D) Heisenberg magnet, real quasi-2D materials will invariably possess nonzero inter-plane coupling J⊥ driving the system to order at elevated temperatures. This process can be studied using quantum Monte Carlo calculations. However, it is difficult to test the results of these calculations experimentally since for highly anisotropic materials in which the in-plane coupling is comparable with attainable magnetic fields J ⊥ is necessarily very small and inaccessible directly. In addition, because of the large anisotropy, the Néel temperatures are low and difficult to determine from thermodynamic measurements. Here, we present an elegant method of assessing the calculations via two independent experimental probes: pulsed-field magnetization in fields of up to 85 T, and muon-spin rotation. We successfully demonstrate the application of this method for nine metalorganic Cu-based quasi-2D magnets with pyrazine (pyz) bridges. Our results suggest the superexchange efficiency of the [Cu(HF2) (pyz) 2]X family of compounds (where X can be ClO4, BF 4, PF6, SbF6 and AsF6) might be controlled by the tilting of the pyz molecule with respect to the 2D planes. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.