Quasicritical Behaviour of the NMR and µSR Relaxation in YMn₂D_x

Abstract

The magnetic and structural properties of YΜn 2 are strongly influenced by introduction of hydrogen (deuterium) in the structure, however, it only slightly modifies the noncollinear antiferromagnetic structure of the compound. The temperature dependences of the NMR relaxation times T1 and Τ2 of deuterium and the zero-field relaxation of muons in YΜn2 D x were measured at temperatures above the magnetic ordering temperature TN. The corresponding relaxation rates are described by the empirical expres- The C15 Laves phase compound YΜn2 and its hydrides (deuterides) have very intriguing properties above their magnetic ordering temperature TN. The NMR relaxation times Τι and Τ2 of deuterium and the muon depolarisation rate λ of these materials exhibit a critical divergence as TN is approached from above [1][2][3]. It is interesting that their temperature dependence above TN can be described by nearly the same function of temperature Τ where δ = (Τ -TN )/TN is the reduced temperature, the exponent n depends linearly on the deuterium concentration x and the constant prefactor of course depends on the quantity under consideration. The dependence of n on x is shown in Fig. 1. We call the behaviour described by Eq. (1) "quasicritical" , because the function F(δ) has a singularity when Τ approaches the critical temperature TN but on the other hand the true critical exponent should not depend on the chemical composition of the material but only on the model Hamiltonian [4]. Thus we have to look for another formula which would explain this behaviour. It is clear that spin fluctuations play an important role in the vicinity of the temperature where the (867)

Deuterium NMR inYMn2Dxcompounds

“…16 The model of relaxation caused by spin fluctuations, introduced by Moriya, 17 gives for antiferromagnets, at temperatures close above the ordering temperature, the following expression for the spin-lattice relaxation rate: For the YMn 2 D x deuterides ͑hydrides͒ the muon 11 and NMR relaxation rates show divergence at the ordering temperature, but instead of Eq. ͑3͒ with fixed exponential factor 1/2, the formula with the free exponential factor may be used:…”

“…However, this behavior cannot be understood in terms of existing theories which predict that such divergences of critical type at the ordering temperature should not depend on chemical composition, and here in particular on the hydrogen ͑deuterium͒ concentration in the compound. This leads to the idea of the model, 16 which improves the Moriya's approach based on spin fluctuations. 17 The relaxation rates are generally proportional to ͚"(k)/⌫ k … term.…”

“…Starting from this expresion, the proposed model takes into account different wave-vector dependencies ͑dispersion͒ of the staggered susceptibility (k) and the ⌫(k) parameter of the relaxing ͑correlation͒ function, which is a width of the spin-fluctuation excitation spectrum. In the result of calculations a formula for the relaxation rate was proposed: 16 1…”

Magnetic ordering of TbMn2D2- a nuclear magnetic resonance analysis

“…The magnetic ordering temperature of cubic Laves phase compounds RMn 2 H x increases by more than 30% with hydrogen charging for 1.5 < x < 3.5 and R = Y, Dy, Tb, Gd [1][2][3][4]. Recently, we showed that the accompanying lattice expansion-and especially the increased Mn-Mn separation-changes also the quasi-critical behaviour reflected by µSR and 1 H/ 2 D nuclear spin-relaxation on approach to the antiferromagnetic ordering temperature of YMn 2 (H/D) x [5,6]. In this respect the nuclei on the tetrahedral interstitial sites with two R and two Mn nearest neighbours are convenient probes for the spin fluctuations and antiferromagnetic correlations on the Mn and R sublattices.…”

“…In this respect the nuclei on the tetrahedral interstitial sites with two R and two Mn nearest neighbours are convenient probes for the spin fluctuations and antiferromagnetic correlations on the Mn and R sublattices. Increased hydrogen content and Mn-Mn spacing increases not only the ordering temperature T N , but modifies also the wave-vector dependence of exchange interaction or dynamic susceptibility [5,6]. Structural distortions and ordered H/D superstructures are additional problems of these hydrides, however, that have to be considered for a complete magnetostructural phase diagram.…”