Magnetic structure and phase diagram in a spin-chain system: Ca ₃ Co ₂ O ₆

Abstract

The low-temperature structure of the frustrated spin-chain compound Ca3Co2O6 is determined by the ground state of the 2D Ising model on the triangular lattice. At high-temperatures it transforms to the honeycomb magnetic structure. It is shown that the crossover between the two magnetic structures at 12 K arises from the entropy accumulated in the disordered chains. This interpretation is in an agreement with the experimental data. General rules for for the phase diagram of frustrated Ising chain compounds are formulated.PACS numbers: 75.25.+z, 75.30.Kz, 75.50.Ee A combination of a low dimensionality and frustration in magnetic compounds, which contain weakly bound Ising spin-chains packed into a two-dimensional (2D) frustrated lattice, gives rise to a complex magnetic behavior. Antiferromagnetic (AFM) Ising chains form the triangular lattice in well-known Ising-chain compounds CsCoCl 3 , CsCoBr 3 [1,2,3,4]. In these substances a partial AFM interchain order (honeycomb magnetic structure) appears just below the ordering temperature. While the temperature decreases, two magnetic phase transitions occur to low-temperature ferrimagnetic phases. A theoretical explanation of the magnetic structure of CsCoCl 3 , CsCoBr 3 was proposed by Mekata in the framework of a mean-field theory [1]. To describe the low-temperature phases an interaction between next nearest neighboring chains was included in the model. Recently, Ca 3 Co 2 O 6 has drawn considerable attention to a step-like magnetization curve [5,6,7,8,9]. The number of the steps in the curve depends strongly on a sweep rate of the external magnetic field and temperature [6,7,9]. Two steps become apparent in the temperature range from 12 K to 24 K [9]. The first step takes place at the zero magnetic field. Then the magnetic moment remains constant at about 1/3 of the full magnetization up to the magnetic field of 3.6 T where the second step occurs to the fully magnetized FM state. At least four equidistant steps are clearly visible below 12 K at a very low sweep rate. They are accompanied by a sizeable hysteresis. Similar phenomena were observed in other spin-chain compounds, e.g. Ca 3 CoRhO 6 [10,11].The structure of Ca 3 Co 2 O 6 consists of Co 2 O 6 chains running along the c axis. The Ca ions are situated between them. The chains are made up of alternating, facesharing CoO 6 trigonal prisms and CoO 6 octahedra. The crystalline electric field splits the energy level of Co 3+ions into the high-spin (S = 2) and low-spin (S = 0) states. The Co 3+ ions situated in the trigonal environment (CoI) are in the high-spin state and the octahedral Co sites (CoII) occurs in the low-spin state. In the last case the energy difference between the low-spin and * Electronic address: kudasov@ntc.vniief.ru high-spin states is very small and a tiny fraction of CoII sites is reported to be in the high-spin state. The crystalline electric field leads also to a very strong Ising-like anisotropy at the CoI sites. The chains form triangular lattice in the ab plane that is perpendicula...

Dynamics of magnetization in frustrated spin-chain systemCa3Co2O6

“…The present interpretation of the transition differs from that was assumed in Ref. 6. It arises due to a drastic increase in a domain boundary mobility at high temperatures that leads to establishing of the single-domain ferrimagnetic state during a short time.…”

“…However, it causes the frustration and a large variety of magnetic structures. [1][2][3][4][5][6] A steplike magnetization curve in Ca 3 Co 2 O 6 has drawn recently considerable attention. 3,5-13 The number of the steps depends on a sweep rate of the external magnetic field and temperature.…”

“…[1][2][3][4][5][6] A steplike magnetization curve in Ca 3 Co 2 O 6 has drawn recently considerable attention. 3,[5][6][7][8][9][10][11][12][13] The number of the steps depends on a sweep rate of the external magnetic field and temperature. 3,8,10 Two steps become apparent in the temperature range from 12 to 24 K. 10 The first step takes place at the zero magnetic field.…”

Exchange interactions in Ca3Co2O6probed locally by NMR

“…At lower temperatures, the M(H ) curves exhibit a strongly hysteretic multistep behavior, indicative of more complex magnetic configurations [6,[16][17][18][19]. Theoretical models predict a number of superstructures of the spin chains as a function of the applied field, qualitatively reproducing the magnetization steps [20][21][22].…”

Field-driven magnetisation steps in Ca 3 Co 2 O 6 : A single-crystal neutron-diffraction study

“…In zero field, Ca 3 Co 2 O 6 was recently shown to have a sinusoidally-modulated spin-density wave with a very long period propagating along the c axis [8,9]. A series of low temperature magnetisation steps [3,10,11], reminiscent of the quantum tunnelling observed in molecular magnets [12,13], has ensured continued experimental and theoretical interest in this system in recent years [14][15][16][17][18][19][20][21][22][23][24].…”