Abstract
Randomness and frustration are believed to be two crucial criteria for the formation of spin glass state. However, the spin freezing occurs in some well-ordered crystals below the related temperature Tf due to the instability of each spin state, which induces the variation of either magnetic moment value or exchange energy. Here we explore the new mechanism of the in-site originated disorder in antiferromagnets Gd0.73La0.27B6 and GdB6, which is caused by the random mutual shifts of Gd3+ spins from the centrally symmetrical positions in the regular cubic lattice. The universal scaling of ESR linewidth temperature dependencies to the power law ΔH(T) ~ ((T − TD)/TD)α with α = − 1.1 ± 0.05 in the paramagnetic phase of both compounds demonstrates the identity of the origin of magnetic randomness. In Gd0.73La0.27B6 the resulting random spin configurations freeze at Tf ≈ 10.5 K where the maximum of magnetization is observed. Below Tf the splitting of ZFC and FC magnetization curves takes place as well as the magnetic state depends on the antecedent sample history. In the case of GdB6 the coherent displacement of Gd ions compete with these random shifts forming an antiferromagnetic (AFM) phase at TN = 15.5 K, which prevails over the spin freezing at Tf ≈ 13 K, expected from the ESR data. The observation of the hysteresis of the ESR spectrum in the AFM phase suggests that its properties may be determined by the competition of two types of AFM orders, which results in formation of stable magnetic domains with nonequivalent positions of AFM Gd pairs at T < 10 K.
Highlights
Randomness and frustration are believed to be two crucial criteria for the formation of spin glass state
In this work we explore the new mechanism of in-site originated disorder and the related spin glass (SG) behavior in the rich borides G dB6 and G d0.73La0.27B6 with high symmetry cubic lattice (Pm3m − Oh1)
The determination of μeff for the doped sample strongly depends on the exact actual value of the doping level x. In this case we assume that the effective magnetic moment of G d3+ ion in the doped crystal is the same as in G dB6 at high temperatures obtaining the value x = 0.27 which appear in reasonable correlation with the nominal doping
Summary
The ESR in this case can be considered as a sum of resonances from individual Gd3+ ions moving in random effective magnetic fields (static and microwave), which cause the line broadening and its shift Such disordered spin structures freeze in the case of G d0.73La0.27B6 when the temperature decreases below Tf. the transition to the AFM state with a coherent ion shift structure in GdB6 occurs at higher temperature TN = 15.5 K masking the expected SG temperature Tf = 13.2 K which can be estimated using the relation Tf/TD in G d0.73La0.27B6. In this respect G dB6 and G d0.73La0.27B6 are first systems where the origin of SG behavior is not caused by the inherent disorder in the magnetic Gd3+ ion system but is induced by ions shift with the formation of random spin configurations It leads to short range spin correlations in the PM phase and SG freezing in G d0.73La0.27B6 or SG effects in AFM2 phase of G dB6 with temperature lowering. The coherent displacement of Gd ions compete in G dB6 with random configurations leading first to first order phase transition and at T < TD to the onset of complicated low temperature phase with peculiar hysteretic behavior
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