Melting of antiferromagnetic ordering in spinel oxide CoAl₂O₄

Abstract

The magnetic and thermal properties of the spinel oxides CoB2O4 have been studied for B = Rh, Co and Al. The compounds consist of magnetic Co2+ (S = 3/2) in the A site and non-magnetic B3+ in the B site, which form diamond and pyrochlore sublattices, respectively. We found that the ratio of the Curie–Weiss temperature to the magnetic ordering temperature, |ΘCW|/TN, depends strongly on B3+ cations. CoRh2O4 was a typical antiferromagnet with |ΘCW|/TN = 31 K/25 K = 1.2. The ratio was enhanced up to |ΘCW|/TN = 110 K/30 K = 3.7 for Co3O4 (B = Co) and |ΘCW|/T* = 89 K/9 K = 10 for CoAl2O4, indicating the evolution of magnetic frustration. The specific heat for CoAl2O4 exhibited a broad peak at T* = 9 K and T2.5 behaviour at low temperatures, suggesting the most frustrated magnet CoAl2O4 is in the critical vicinity of a quantum melting point of the antiferromagnetically ordered state.

Spin glass behavior in frustrated quantum spin system CuAl2O4with a possible orbital liquid state

“…Our estimate of the lattice parameter of 8.07683 (5) Å is close to those values reported in the previous works: 8.078 (1) ions, which occupy the otherwise forbidden octahedral B-sites (see Table 1). We note that a smaller (6 ~ 8 %) amount of site inversion was reported for other polycrystalline normal spinel oxides TAl2O4 (T = Co, Fe and Mn) [13,14]. It is to be noted too that a similar amount (8 %) of the site inversion was recently reported for CoAl2O4 single crystals [16].…”

“…For example, while CoAl2O4 and FeAl2O4 display spin glass-like ground states, FeAl2O4 shows a sign of an additional orbital freezing at lower temperatures. On the other hand, MnAl2O4 shows a long-ranged antiferromagnetic order below 40 K [13,14,15].…”

Magnetoelectric properties of BixCo2−xMnO4 (⩽x⩽0.3)

“…Therefore, the appearance of FM in Bi x Co 2−x MnO 4 ͑0 ഛ x ഛ 0.3͒ may be attributed to the canting of the antiferromagnetically ordered spins by the structural distortion 13,15 and a breakdown of the balance between the antiparallel magnetization at Co 2+ sublattices due to the substitution of Mn and Bi ions. 5,14 In the scenario of measurements above, the studies on magnetocapacitive effect for Bi x Co 2−x MnO 4 ͑x = 0.3͒ are shown in Fig. 3.…”

Low-temperature nuclear magnetic resonance investigation of systems frustrated by competing exchange interactions

“…The presence of short-range correlations below 100 K can be argued to be caused by frustration which suppresses the T N to 10 K. In addition shortrange correlations can arise due to disorder, associated with the site inversion. As pointed out previously, although a spin-liquid ground state has been proposed from magnetic susceptibility measurements for CoAl 2 O 4 with low values of x, [203,209] our NMR data conclusively proved the existence of a long-range AFM-ordered state below 10 K. Since site inversion plays a crucial role in determining the magnetic properties of this spinel compound, it will also be interesting to perform systematic NMR and neutron diffraction measurements in a highly disordered (high x) CoAl 2 O 4 to investigate how the system changes from the AFM state to a spin glass state. This chapter describes NMR work on BiMn 2 PO 6 published in [120].…”

“…On the other hand, Suzuki proposed a possible spin liquid ground state below 9 K [203] with f = 10 (where T M is taken to be T * , the temperature at which a broad peak of the heat capacity C is observed). It is also interesting to note that the value of frustration parameter f depends strongly on the site disorder x.…”