Magnetic Properties of GeNi$_{2-x}$Mg$_{x}$O$_{4}$ Under High Pressure and Magnetic Dilution

TL;DR

This study investigates how magnetic dilution and high pressure influence the magnetic phases and structural properties of Ni$_{2-x}$Mg$_{x}$GeO$_{4}$, revealing complex magnetic transitions, high percolation thresholds, and a shift to two-dimensional behavior in the spin glass state.

Contribution

It provides new insights into the effects of magnetic dilution and pressure on frustrated spinel systems, highlighting the formation of a 'network of networks' and a dimensional crossover in magnetic behavior.

Findings

High percolation thresholds for magnetic order suggest strong coupling between kagomé and triangular spins.

Dilution induces a transition from antiferromagnetic to spin glass states.

Evidence of a shift from 3D to 2D magnetic behavior in the spin glass phase.

Abstract

The effects of magnetic dilution and applied pressure on frustrated spinel Ni$_{2-x}$Mg$_{x}$GeO$_{4}$ (0 $\leq x \leq 1.0$) are analyzed using specific heat, AC and DC magnetization and x-ray diffraction. The parent compound has two closely spaced antiferromagnetic transitions $T_{\text{N1}}=12.0$ K (kagom\'e planes) and $T_{\text{N2}}=11.4$ K (triangular planes). In the dilution range tested, the low temperature magnetic state takes three forms: antiferromagnetic ($0 \leq x \leq 0.05$), ill-defined ($x=0.10$ and 0.15), and spin glass ($0.30 \leq x \leq 1.0$). The AFM region shows an extreme vulnerability to dilution with a percolation threshold of $p_{\text{c1}}=0.74 \pm 0.04$ and $p_{\text{c2}}=0.65 \pm 0.05$ for the kagom\'e and triangular planes respectively, which are much larger than expected for 3D systems. We suggest that this behavior is possibly due to coupling between the kagom\'e and triangular spins forming a 'network of networks' (NON). Thermal expansion data on parent NGO indicates a field dependent lattice contraction during ordering events. Furthermore, there is a transition from contraction to expansion in an applied field of 6 T in the kagom\'e planes. For dilution levels $x \geq 0.30$, the system becomes a spin glass with canonical behavior. Specific heat results suggest that the triangular spins become disordered with increasing Mg$^{2+}$ substitution followed by the onset of glassiness in the kagom\'e planes. Furthermore, there appears to be a dilution driven shift from 3 dimensional to 2 dimensional behavior as the low temperature magnetic heat capacity scales as $T^{2}$ in the spin glass state.