Professor Stephen Blundell

Abstract:

We present an investigation of magnetic ordering in the two-dimensional S= 2 quantum magnet Cu (Pz)2 (Cl O4) 2 using specific heat and zero-field muon-spin relaxation (μ+ SR). The magnetic contribution to the specific heat is consistent with an exchange strength of 17.7 (3) K. We find unambiguous evidence for a transition to a state of three-dimensional long-range order below a critical temperature TN =4.21 (1) K using μ+ SR even though there is no feature in the specific heat at that temperature. The absence of a specific heat anomaly at TN is consistent with recent theoretical predictions. The ratio of TN J=0.24 corresponds to a ratio of intralayer to interlayer exchange constants of J′ J=6.8× 10-4, indicative of excellent two-dimensional isolation. The scaled magnetic specific heat of [Cu (Pz)2 (H F2)] B F4, a compound with an analogous structure, is very similar to that of Cu (Pz)2 (Cl O4) 2 although both differ slightly from the predicted value for an ideal 2D S= 2 Heisenberg antiferromagnet. © 2007 The American Physical Society.

Abstract:

We report muon spin relaxation (μSR) and magnetic susceptibility investigations of two Ti3+ (S=1/2) chain compounds, NaTi Si2 O6 and TiOCl, each of which exhibits a spin gap at low temperature. From these we conclude that the spin gap in NaTi Si2 O6, which arises from orbital ordering at TOO =210 K, is temperature independent below TOO, with a value of 2Δ=700 (100) K. In TiOCl, we find thermally activated spin fluctuations corresponding to a spin gap 2Δ=440 (60) K below Tc1 =67 K. We can describe both the μSR and susceptibility data in terms of a model based on the dimerization of the Ti3+ chains. We also compare the methods used to extract the spin gap and the concentration of free spins within the samples from μSR and magnetic susceptibility data. © 2007 The American Physical Society.

Abstract:

We present the results of muon-spin relaxation (μ+ SR) measurements on the kagome staircase compounds Ni3 V2 O8 and Co3 V2 O8. The magnetic behavior of these materials may be described in terms of two inequivalent magnetic ion sites, known as spine sites and cross-tie sites. Our μ+ SR results allow us to probe each of these sites individually, revealing the distribution of the local magnetic fields near these positions. We are able not only to confirm the magnetic structures of the various phases proposed on the basis of bulk measurements but also to give an insight into the temperature evolution of the local field distribution in each phase. © 2007 The American Physical Society.

Abstract:

Using X-band pulsed electron-spin resonance, we report the intrinsic spin-lattice (T1) and phase-coherence (T2) relaxation times in molecular nanomagnets for the first time. In Cr7M heterometallic wheels, with M=Ni and Mn, phase-coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples T2 reaches 3μs at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications. © 2007 The American Physical Society.