Professor Stephen Blundell

Abstract:

TmAgGe is an antiferromagnet in which the spins are confined to distorted kagome-like planes at low temperatures. We report angle-dependent measurements of the magnetization M in fields of up to 55 T that show that there are two distinct and separate energy scales present in TmAgGe, each responsible for a set of step-like metamagnetic transitions: weak exchange interactions and strong crystalline-electric-field (CEF) interactions. Simulations of M using a three-dimensional, free-energy minimization technique allow us to specify the physical origin of the metamagnetic transitions in low, in-plane fields. We also show that the transitions observed with the field perpendicular to the kagome planes are associated with the CEF-split multiplet of Tm. © 2007 The American Physical Society.

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.