Specific heat and magnetic susceptibility of the spinelsGeNi2O4andGeCo2O

Abstract

Specific-heat and magnetic-susceptibility measurements are reported for the polycrystalline spinel compounds GeNi 2 O 4 and GeCo 2 O 4 in magnetic fields up to 14 T and 0.5 K Յ T Յ 400 K. Both compounds have first-order antiferromagnetic transitions. There are two sharp closely spaced magnetic-ordering anomalies for GeNi 2 O 4 at Néel temperatures T N1 ͑0͒ = 12.080 K and T N2 ͑0͒ = 11.433 K in zero magnetic field. There is also a broad anomaly in the specific heat centered at ϳ5 K, which is present for all fields. Spin waves with an average gap of 10.9 K are associated with this anomaly, which is confirmed by neutron-scattering measurements. An unusual feature of the antiferromagnetism for GeNi 2 O 4 is the simultaneous presence of both gapped and ungapped spin waves in the Néel state, inferred from the specific-heat data. GeCo 2 O 4 has a single anomaly at T N ͑0͒ = 20.617 K in zero magnetic field. Spin waves with an average gap of 38.7 K are derived from fitting the low-temperature specific heat and are also observed by neutron scattering. For both compounds ϳ50% of the derived magnetic entropy is below the ordering temperatures, and the total magnetic entropies are only ϳ60% of that predicted for the Ni 2+ and Co 2+ single-ion ground-state configurations. The missing entropy is not linked to magnetic disorder in the ground state or hidden ordering below 0.5 K. It is postulated that the missing entropy is accounted for by the presence of substantial magnetic correlations well above the Néel temperatures. Fitting the GeNi 2 O 4 susceptibilities to the Curie-Weiss law yields parameters that are consistent with those found for Ni 2+ ions in a crystal-electric-field environment including octahedral and trigonal components. The application of the Curie-Weiss law to the GeCo 2 O 4 susceptibilities is not valid because of low-lying crystal-electric-field states.

Molecular spin-orbit excitations in theJeff=12frustrated spinel GeCo2OTomiyasu1, Crawford2, Adroja3 et al. 2011Phys. Rev. BSelf Cite

“…2(a). Below T N spin-wave-like dispersion rises up from around Q = |q m | in addition to the previous discovered 4-meV mode, 20 as shown in Fig. 2(b).…”

“…1(a). 19,20 In fact, as shown in Fig. 2(e), the experimental spectrum around 29 meV is asymmetrically spread up to 40 meV, suggesting this splitting.…”

Geometrically Frustrated Magnetic Materials

“…For the Co compound it has been shown that low lying crystal field excited states cannot be ignored and thus, application of the Curie-Weiss law is not justified within the usual temperature range measured. [186] This is the same situation which is obtained with many lanthanide ions, such as Nd 3þ , which has been documented in an earlier section. In fact the original report for GeNi 2 O 4 shows susceptibility data taken up to 1500 K and a true Curie-Weiss regime does not appear to be obtained unless T > 900 K!…”

“…For example c for GeCo 2 O 4 , is widely reported to be fairly large and positive, values range from 57 to 81 K with a strong dependence on the temperature range chosen for the Curie-Weiss fit. [185,186] Yet, this material shows order AF below 21 K. The corresponding GeNi 2 O 4 has c ranging from -4 to -6 K, at least suggesting net AF interactions but still smaller than T N of $ 11K. In both cases the temperature range investigated extends to $ 400 K, the upper limit for most SQUID devices currently available.…”

“…52 Heat capacity data for GeNi 2 O 4 showing two sharp maxima at low temperatures and a large magnetic contribution extending to very high temperatures [186]. Reprinted with permission fromLashley et al, 2008 [186].…”