Weakly Coupled Moments in the Strong Antiferromagnet, MnSn2

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Abstract
The tetragonal compound MnSn2 orders antiferromagnetically (in state I) at TN ≈ 325°K and transforms abruptly to another antiferromagnetic state (II) at Tcrit ≈ 73°K. Its initial susceptibility (χ), after decreasing from a peak at TN, rises (following a Curie law) to a much higher peak at 86°K (Tpeak), then drops precipitously at Tcrit. Pulsed-field measurements at 4.2°K show a steep increase in magnetization (σ) marking a II→I transition at Hcrit ≈ 120 kOe, above which σ approaches saturation at ∼1 μB/Mn (compared to ∼2.3 μB/Mn for the average sublattice moment). With increasing temperature, Hcrit decreases and the approach to saturation in state I becomes more gradual. Above Tcrit (even above Tpeak), the saturation effect persists, but only after σ vs H undergoes an inflection point near 45 kOe. These unusual variations in χ vs T and σ vs H for state I are consistent with a simple model in which the moments of superparamagnetic clusters of ∼55 Mn atoms are weakly coupled to each ordered sublattice. This superparamagnetic moment decreases with increasing c in isomorphous Mn1−cFecSn2, vanishing at c ≈ 14.

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