Magnetic frustration in the spinel compounds GeNi[sub 2]O[sub 4] and GeCo[sub 2]O[sub 4] (original) (raw)

Magnetic frustration in the spinel compounds GeCo2O4 and GeNi2O4

Physical Review B, 2006

a) J 2 , J 2 ' AFM FIG. 4: (Color online) Magnetic ions in the kagomé (K) and triangular planes (T). (a) Geometric frustration in the J2/J ′ 2 network when only the triangular planes are considered. The (non) satisfied (J2) J ′ 2 are depicted by (dis) continuous lines. (b) Competition between J1 (whatever its sign) and AFM J2, J ′ 2 and J3.

Study of competitive magnetic interactions in the spinel compounds GeNi2O4, GeCo2O4

Physica B: Condensed Matter, 2004

We will present here results on the magnetic properties of two compounds, GeCo 2 O 4 and GeNi 2 O 4 ; obtained by magnetic susceptibility and high magnetic field magnetization measurements. GeCo 2 O 4 shows an antiferromagnetic behaviour with an ordering temperature T N ¼ 23 K and a Curie-Weiss temperature y ¼ þ81 K: There is no apparent magnetic frustration. From the value of the Curie constant, we deduce that Co 2þ ions are in high spin S ¼ 3 2 state and that they are bounded in pairs. We will discuss the possible origin of these pairs. In GeNi 2 O 4 ; we observe a negative Curie-Weiss temperature ðy ¼ À15 KÞ: In this compound the ferromagnetic Ni-Ni direct interaction present in GeCo 2 O 4 cannot exist. Moreover, the nickel cations are not bounded in pairs as in the cobalt compound. We show that GeNi 2 O 4 is a good candidate for magnetic frustration.

Specific heat and magnetic susceptibility of the spinels GeNi2O4 and GeCo2O4

Physical Review B, 2008

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.

Magnetic properties of the spinel system MgxMn3-xO4 (0 ≤ x ≤ 2)

Boletín de la Sociedad Española de Cerámica y Vidrio, 2008

Temperature-dependent studies of the low-field magnetization of the polycrystalline spinel oxide Mg x Mn 3-x O 4 (0 ≤ x ≤ 2) are reported. With the use of Lotgering's model, which is equivalent to the Néel's two-sublattice model, a set of molecular field constants λ AB , λ BB and λ AA , has been obtained for 0 ≤ x ≤ 0.4 from the fit of χ between T C and room temperature. Moreover, this model fits consistently the low-temperature canted-spin angles ψ of the B-sublattice. The Curie temperatures T C , as well as the exchange parameters J AB , J BB and J AA , show that the BB interactions are much greater than AA and AB, which are of the same order, in contrast to the situation usually found in magnetic spinels. As the concentration of the non-magnetic ion Mg 2+ increases at the tetrahedral site (A), T C decreases from 42K (for x = 0) while the ferromagnetic behaviour diminishes at the expense of a magnetic frustration, thus highlighting the great influence of the non-magnetic ions located at the A and B-sites on the magnetic order.

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Physical Review B, 2006

Co 3 V 2 O 8 ͑CVO͒ has a different type of geometrically frustrated magnetic lattice, a kagomé staircase, where the full frustration of a conventional kagomé lattice is partially relieved. The crystal structure consists of two inequivalent ͑magnetic͒ Co sites, one-dimensional chains of Co͑2͒ spine sites, linked by Co͑1͒ cross-tie sites. Neutron powder diffraction has been used to solve the basic magnetic and crystal structures of this system, while polarized and unpolarized single crystal diffraction measurements have been used to reveal a rich variety of incommensurate phases, interspersed with lock-in transitions to commensurate phases. CVO initially orders magnetically at 11.3 K into an incommensurate, transversely polarized, spin density wave state, with wave vector k = ͑0,␦ ,0͒ with ␦ = 0.55 and the spin direction along the a axis. ␦ is found to decrease monotonically with decreasing temperature and then locks into a commensurate antiferromagnetic structure with ␦ = 1 2 for 6.9Ͻ T Ͻ 8.6 K. In this phase, there is a ferromagnetic layer where the spine site and cross-tie sites have ordered moments of 1.39 B and 1.17 B , respectively, and an antiferromagnetic layer where the spine-site has an ordered moment of 2.55 B , while the cross-tie sites are fully frustrated and have no observable ordered moment. Below 6.9 K, the magnetic structure becomes incommensurate again, and the presence of higherorder satellite peaks indicates that the magnetic structure deviates from a simple sinusoid. ␦ continues to decrease with decreasing temperature and locks in again at ␦ = 1 3 over a narrow temperature range ͑6.2Ͻ T Ͻ 6.5 K͒. The system then undergoes a strongly first-order transition to the ferromagnetic ground state ͑␦ =0͒ at T c = 6.2 K. The ferromagnetism partially relieves the cross-tie site frustration, with ordered moments on the spine-site and cross-tie sites of 2.73 B and 1.54 B , respectively. The spin direction for all spins is along the a axis ͑Ising-like behavior͒. A dielectric anomaly is observed around the ferromagnetic transition temperature of 6.2 K, demonstrating that there is significant spin-charge coupling present in CVO. A theory based on group theory analysis and a minimal Ising model with competing exchange interactions can explain the basic features of the magnetic ordering.

Structural and magnetic properties of frustrated GaxMn(3-x)O4(1.2 ≤ x ≤ 1.6) spinels

Journal of Alloys and Compounds, 2018

We report a systematic study of the structural and magnetic properties of frustrated compounds of Ga x Mn (3−x) O 4 (1.2 ≤ x ≤ 1.6) prepared by solid-state reaction. Using Rietveld refinement of X-ray diffraction patterns and O'Neill-Navrotsky model, we demonstrate that the system Ga x Mn (3−x) O 4 (1.2 ≤ x ≤ 1.6) is an inverse spinel with low inversion parameter, in which Ga 3+ replaces Mn 3+ cations located in B-sites. The inverse magnetic susceptibility, the shape of ZFC/FC magnetization curves at low temperatures, the existence of hysteresis in all compounds, the frustration parameter and the spontaneous magnetization analysis show that the compounds with x = 1.2-1.4 exhibit a non-collinear ferrimagnetic order and the compounds with x = 1.5-1.6 exhibit a frustrated non-collinear ferrimagnetic order. Spin wave stiffness parameters were determined for each composition using the fitting results of spontaneous magnetization curves. It is demonstrated that for the compounds x = 1.2-1.4 with a non-frustrated ferrimagnetic order, the change of spontaneous magnetization Ms(T) obeys to Bloch's law (T 3/2). For x = 1.5-1.6, the compounds exhibit a frustrated ferrimagnetic order, and the Ms(T) shows a deviation from Bloch's law.

Geometrical magnetic frustration in rare-earth chalcogenide spinels

Physical Review B, 2005

We have characterized the magnetic and structural properties of the CdLn 2 Se 4 (Ln = Dy, Ho), and CdLn 2 S 4 (Ln = Ho, Er, Tm, Yb) spinels. We observe all compounds to be normal spinels, possessing a geometrically frustrated sublattice of lanthanide atoms with no observable structural disorder. Fits to the high temperature magnetic susceptibilities indicate these materials to have effective antiferromagnetic interactions, with Curie-Weiss temperatures Θ W ~ -10 K, except CdYb 2 S 4 for which Θ W ~ -40 K. The absence of magnetic long range order or glassiness above T = 1.8 K strongly suggests that these materials are a new venue in which to study the effects of strong geometrical frustration, potentially as rich in new physical phenomena as that of the pyrochlore oxides.

Magnetic Frustration Driven by Itinerancy in Spinel CoV2O4

Scientific reports, 2017

Localized spins and itinerant electrons rarely coexist in geometrically-frustrated spinel lattices. They exhibit a complex interplay between localized spins and itinerant electrons. In this paper, we study the origin of the unusual spin structure of the spinel CoV2O4, which stands at the crossover from insulating to itinerant behavior using the first principle calculation and neutron diffraction measurement. In contrast to the expected paramagnetism, localized spins supported by enhanced exchange couplings are frustrated by the effects of delocalized electrons. This frustration produces a non-collinear spin state even without orbital orderings and may be responsible for macroscopic spin-glass behavior. Competing phases can be uncovered by external perturbations such as pressure or magnetic field, which enhances the frustration.

Geometric frustration in the cubic spinelsMAl2O4(M=Co, Fe, and Mn)

Physical Review B, 2005

X-ray diffraction, magnetic susceptibility, electron-spin resonance, and heat-capacity investigations were performed on MAl 2 O 4 compounds with M = Co, Fe, Mn, and Zn. All compounds crystallize in cubic spinel structure AB 2 O 4 with minor inversion between A and B sites. CoAl 2 O 4 and FeAl 2 O 4 reveal spin-glass-like ground states with freezing temperatures well below the Curie-Weiss temperatures indicating strong geometric frustration. Below the freezing temperatures, the heat capacities show a T 2 temperature dependence for T → 0 K as has recently been observed in some related geometrically frustrated magnets, in contrast to the linear dependence expected for common spin glasses. The heat capacity of FeAl 2 O 4 exhibits an additional orbital contribution. With the absence of a cooperative Jahn-Teller distortion, this points toward an orbital freezing. MnAl 2 O 4 orders antiferromagnetically below T N = 40 K with a reduced value of the ordered moment and a large paramagnetic component.

Magnetic susceptibility of the frustrated spinels ZnCr2O4, MgCr2O4and CdCr2O4

Journal of Physics: Conference Series, 2010

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