Muons and magnets

Abstract:

The magnetic properties of α-Cu(dca)2(pyz) were examined by magnetic susceptibility, magnetization, inelastic neutron scattering (INS), muon-spin relaxation (μSR) measurements and by first-principles density functional theoretical (DFT) calculations and quantum Monte Carlo (QMC) simulations. The χ versus T curve shows a broad maximum at 3.5 K, and the data between 2 and 300 K is well described by an S = 1/2 Heisenberg uniform chain model with g = 2.152(1) and J/kB = -5.4(1) K. μSR measurements, conducted down to 0.02 K and as a function of longitudinal magnetic field, show no oscillations in the muon asymmetry function A(t). This evidence, together with the lack of spin wave formation as gleaned from INS data, suggests that no long-range magnetic order takes place in α-Cu(dca)2(pyz) down to the lowest measured temperatures. Electronic structure calculations further show that the spin exchange is significant only along the Cu-pyz-Cu chains, such that α-Cu(dca)2(pyz) can be described by a Heisenberg antiferromagnetic chain model. Further support for this comes from the M versus B curve, which is strongly concave owing to the reduced spin dimensionality. α-Cu(dca)2(pyz) is a molecular analogue of KCuF3 owing to dx² - y² orbital ordering where nearest-neighbor magnetic orbital planes of the Cu²⁺ sites are orthogonal in the planes perpendicular to the Cu-pyz-Cu chains. © 2009 Elsevier Ltd.

Abstract:

We present the results of muon-spin relaxation (μ⁺ SR) measurements on the hexagonal manganite HoMnO3. Features in the temperature-dependent relaxation rate λ correlate with the magnetic transitions at 76, 38, and 34 K. The highest temperature transition, associated with the ordering of Mn3⁺ moments has the largest effect on λ. The application of a static electric field of E=5× 106 Vm^-1 below T=50K causes a small reduction in λ which is suggestive of coupling between ferroelectric and magnetic domain walls in the ordered state of the material. © 2010 The American Physical Society.