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Register a new userSpin-flop and antiferromagnetic phases of the ferromagnetic half-twist ladder compounds Ba3Cu3In4O12 and Ba3Cu3Sc4O12
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The title compounds have dominant ferromagnetic (FM) exchange interactions within one-dimensional (1D) half-twist ladders of s =1/2 Cu2^{+} ions and antiferromagnetic(AFM) interactions between ladders, leading to ordered 3D phases at temperatures below 20K. Here we show that a microscopic 1D model of the paramagnetic (PM) phase combined with a phenomenological model based on sublattice magnetization describes the observed temperature and field dependent magnetism. The model identifies AFM, spin-flop (SF) and PM phases whose boundaries have sharp features in the experimental magnetization M(T,H) and specific heat CP(T,H). Exact diagonalization (ED) of the 1D model, possible for 24 spins due to special structural features of half-twist ladders, yields the magnetization and spin susceptibility of the PM phase. AFM interactions between ladders are included at the mean-field level using the field, HAF, obtained from modeling the ordered phases. Isotropic exchange J1 = -135K and g-tensor g = 2.1 within ladders, plus exchange and anisotropy fields HAF and HA, describe the ordered phases, and are almost quantitative for the PM phase.
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The magnetic properties of polycrystalline samples of Ba3Cu3In4O12 (In-334) and Ba3Cu3Sc4O12 (Sc-334) are reported. Both 334 phases have a structure derived from perovskite, with CuO4 squares interconnected to form half-twist ladders along the c-axis. The Cu-O-Cu angles, ~ 90o, and the positive Weiss temperatures indicate the presence of significant ferromagnetic (FM) interactions along the Cu ladders. At low temperatures, T < 20 K, sharp transitions in the magnetic susceptibility and heat capacity measurements indicate three-dimensional (3D) antiferromagnetic (AFM) ordering at TN. TN is suppressed on application of a field and a complex magnetic phase diagram with three distinct magnetic regimes below the upper critical field can be inferred from our measurements. The magnetic interactions are discussed in relation to a modified spin-1/2 FM-AFM model and the 334 half-twist ladder is compared to other 2-rung ladder spin-1/2 systems.
Two-leg spin-1/2 ladder systems consisting of a ferromagnetic leg and an antiferromagnetic leg are considered where the spins on the legs interact through antiferromagnetic rung couplings $J_1$. These ladders can have two geometrical arrangements either zigzag or normal ladder and these systems are frustrated irrespective of their geometry. This frustration gives rise to incommensurate spin density wave, dimer and spin fluid phases in the ground state. The magnetization in the systems decreases linearly with $J^2_1$, and the systems show an incommensurate phase for $0.0<J_1<1.0$. The spin-spin correlation functions in the incommensurate phase follow power law decay which is very similar to Heisenberg antiferromagnetic chain in external magnetic field. In large $J_1$ limit, the normal ladder behaves like a collection of singlet dimers, whereas the zigzag ladder behaves as a one dimensional spin-1/2 antiferromagnetic chain.
The orthorhombic antiferromagnetic compound CuMnAs was recently predicted to be an antiferromagnetic Dirac semimetal if both the Ry gliding and S2z rotational symmetries are preserved in its magnetic ordered state. In our previous work on Cu0.95MnAs and Cu0.98Mn0.96As, we showed that in their low temperature commensurate antiferromagnetic state the b axis is the magnetic easy axis, which breaks the S2z symmetry. As a result, while the existence of Dirac fermions is no longer protected, the polarized surface state makes this material potentially interesting for antiferromagnetic spintronics. In this paper, we report a detailed study of the anisotropic magnetic properties and magnetoresistance of Cu0.95MnAs and Cu0.98Mn0.96As. Our study shows that in Cu0.95MnAs the b axis is the easy axis and the c axis is the hard axis. Furthermore, it reveals that Cu0.95MnAs features a spin-flop phase transition at high temperatures and low fields when the field is applied along the easy b axis, resulting in canted antiferromagnetism. However, no metamagnetic transition is observed in Cu0.98Mn0.96As, indicating that the magnetic interactions in this system are very sensitive to Cu vacancies and Cu/Mn site mixing.
We calculate the excitation spectrum and spectral weights of the alternating antiferromagnetic-ferromagnetic spin-half Heisenberg chain with exchange couplings $J$ and $-|lambda|J$ as a power series in $lambda$. For small $|lambda|$, the gapped one-particle spectrum has a maximum at $k=0$ and there is a rich structure of bound (and anti-bound) states below (and above) the 2-particle continuum. As $|lambda|$ is increased past unity the spectrum crosses over to the Haldane regime, where the peak shifts away from $k=0$, the one particle states merge with the bottom of the continuum near $k=0$, and the spectral weights associated with the one-particle states become very small. Extrapolation of the spectrum to large $|lambda|$ confirms that the ground state energy and excitation gap map onto those of the spin-one chain.
Low-temperature electronic states in SrRu_{1-x}Mn_xO_3 for x <= 0.6 have been investigated by means of specific-heat C_p measurements. We have found that a jump anomaly observed in C_p at the ferromagnetic (FM) transition temperature for SrRuO_3 changes into a broad peak by only 5% substitution of Mn for Ru. With further doping Mn, the low-temperature electronic specific-heat coefficient gamma is markedly reduced from the value at x=0 (33 mJ/K^2 mol), in connection with the suppression of the FM phase as well as the enhancement of the resistivity. For x >= 0.4, gamma approaches to ~ 5 mJ/K^2 mol or less, where the antiferromagnetic order with an insulating feature in resistivity is generated. We suggest from these results that both disorder and reconstruction of the electronic states induced by doping Mn are coupled with the magnetic ground states and transport properties.
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