Magnetic-Field-Induced Transitions in Spinel GeCo₂O₄

Abstract

The spinel GeCo 2 O 4 has the pyrochlore lattice of Co 2þ ions, in which the kagomé and triangular planes stack alternately along the [111] direction. It shows antiferromagnetic ordering below T N $ 21 K with a characteristic wave vector of Q M ¼ ð1=2; 1=2; 1=2Þ. The spin arrangement is ferromagnetic in the kagomé and triangular planes and antiferromagnetic between the kagomé planes as well as between the triangular planes. GeCo 2 O 4 exhibits a magnetic phase transition at H $ 4 T. Powder neutron diffraction measurements were performed on GeCo 2 O 4 in ambient and magnetic field. Our results are consistent with the magnetic structure model in which the kagomé and triangular planes behave independently in magnetic field, that is, the spin arrangement between the triangular planes becomes ferromagnetic above $4 T while retaining the antiferromagnetic arrangement between the kagomé planes.

Field-driven magnetostructural transitions in GeCo2O4

“…The amplitude of this component decreases from 3.1 to 1.8 µ B between 5.5 and 9.4 T while the FM component increases. On the TRI site, the AFM component is found equal to zero above H c1 , as already proposed [10]. At µ 0 H=12 T, i.e.…”

“…Their microscopic origin was investigated by powder neutron diffraction under magnetic field up to 10 T [10]. A transition, observed at ≈ 4 T, was attributed to an antiferromagnetic-toferromagnetic spin rearrangement between the triangular planes while retaining the antiferromagnetic arrangement arXiv:1609.08805v2 [cond-mat.str-el] 25 Jan 2017 at µ0H=0 T for the k1 domain (one of the 3 equipopulated S-domains represented) (a), at µ0H=5.5 T for the k1 domain (b) and at µ0H=12 T for the k3 domain after the high-field spin-reorientation (c).…”

Magnetic field induced phase transition in spinel GeNi2O4

“…An appreciable feature can be clearly observed: the intensity of the (1/2 1/2 1/2) Bragg peak is significantly enhanced, while the intensity of the (3/2 1/2 1/2) Bragg peak is drastically suppressed. A similar feature was observed previously in polycrystalline GeCo2O4 sample, which was ascribed to a combination of magnetic domain reorientation and a change of magnetic structure with either spins in neighboring triangular planes or spins in both neighboring triangular and Kagome planes turned into ferromagnetic alignment[21]. Nevertheless, distinct from GeCo2O4 where the nuclear Bragg peak intensity is also enhanced due to the fieldinduced canted ferromagnetic moment, no increase in nuclear Bragg peak intensity is observed in GeNi2O4 at 6 T, suggesting negligible ferromagnetic spin component at high field in either Kagome or triangular planes.The temperature dependence of both (1/2 1/2 1/2) and (3/2 1/2 1/2) magnetic Bragg peak intensities measured at H = 7 T on CG4C is shown in Fig.5(b).…”

“…By simply comparing the ratio of TN/θW, both GeTM2O4 compounds should not be regarded as highly frustrated magnets, where the magnetic frustration arises due to the competition of magnetic interactions between neighboring spins and further neighbors [13]. Neutron diffraction studies have shown that the ground state spin structure of these two compounds are similar, with a single ordering k-vector of (1/2 1/2 1/2), both having spins within the Kagome (triangular) planes ferromagnetically coupled whereas spins between the adjacent Kagome (triangular) planes being antiferromagnetically coupled [8,13,20,21]. On the other hand, in contrast to GeCo2O4 in which spins in Kagome and triangular lattices order simultaneously at TN and have the same moment size, for GeNi2O4 spins in the Kagome planes partially order at TN2 < T < TN1 but spins in the triangular planes remain completely disordered, which is followed by the ordering of spins in the latter planes at T < TN2 [8,13,18,20].…”

“…This feature implies that the magnetic interaction between adjacent Kagome and triangular planes is more substantial in GeCo2O4 than that in GeNi2O4. However, recent magnetic susceptibility and neutron diffraction measurements on GeCo2O4 have found two magnetic phase transitions occurring upon applying a magnetic field of ~ 4.6 T and 9.8 T [10], which are associated with the polarized spin alignment of triangular planes and Kagome planes [21], respectively. Strikingly surprising is that no such field-induced anomaly was reported on polycrystalline GeNi2O4 samples up to 14 T [11] and that two weak kinks in the M (H) curves, which were ascribed to the spin reorientation, were seen at very high fields (H = 30 T and 37 T) [13].…”