Field-driven magnetostructural transitions in GeCo 2 O 4

We theoretically investigate spin-lattice coupling (SLC) effects on the in-field ordering properties of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. Here, we use the two possible simplified models describing the effect of local lattice distortions on the spin ordering via the SLC, the bond-phonon and site-phonon models. It is found by means of Monte Carlo simulations that in both models, the 1 2 plateau shows up in the magnetization curve being relatively robust against the breathing bond-alternation, although magnetic long-range orders (LRO's) are realized only in the site-phonon model. In the bond-phonon model, additional further neighbor interactions are necessary to induce a magnetic LRO. In the site-phonon model, it is also found that in addition to the low-field, middle-field 1 2 -plateau, and high-field phases appearing on both the uniform and breathing pyrochlore lattices, various types of unconventional phases which can be viewed as LRO's in units of tetrahedron are induced by the breathing bond-alternation just below the 1 2 plateau and the saturation field. The occurrence of these tetrahedron-based orders could be attributed to the nature characteristic of the breathing pyrochlore lattice, i.e., the existence of the nonequivalent small and large tetrahedra. Experimental implications of our result are also discussed.

Section: Summary and Discussioncontrasting

confidence: 69%

Hedgehog-lattice spin texture in classical Heisenberg antiferromagnets on the breathing pyrochlore lattice

Aoyama

Kawamura

2021

“…In experiments, although various classes of breathing pyrochlore antiferromagnets such as the chromium oxides Li(Ga, In)Cr 4 O 8 [53, 59-66, 69, 70] and sulfides Li(Ga, In)Cr 4 S 8 [67,68,71] and the quantum magnet Ba 3 Yb 2 Zn 5 O 11 [72][73][74][75][76] have been studied, the ( 1 2 , 1 2 , 1 2 ) spin correlation has not been reported so far. On the other hand, the uniform pyrochlore antiferromagnets GeB 2 O 4 (B=Ni, Co, Fe, Cu) [77][78][79][80][81][82][83][84][85][86][87] do exhibit ( 1 2 , 1 2 , 1 2 ) magnetic LRO, although the experimentally proposed spin structure as well as the estimated exchange interactions (e.g., ferromagnetic J 1 ) seems to be different from the ones in the present theoretical model. If one can introduce the breathing alternation in the GeB 2 O 4 family, modifying the exchange interactions, the ( 1 2 , 1 2 , 1 2 ) hedgehog lattice might be realized.…”

Section: Summary and Discussioncontrasting

confidence: 69%

Spin ordering induced by lattice distortions in classical Heisenberg antiferromagnets on the breathing pyrochlore lattice

Aoyama

Kawamura

2019

We theoretically investigate effects of lattice distortions on the spin ordering of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. In the model, local lattice distortions originating from the site vibration are taken into account to yield effective spin interactions via the spin-lattice coupling (SLC). The breathing lattice-alternation is characterized by the ratio of the nearest-neighbor interaction for large tetrahedra to that for small ones, J ′ /J. It is found by Monte Carlo simulations that the system exhibits a first-order transition into four different types of collinear magnetic ordered states. In the uniform case (J ′ /J = 1), the state realized at stronger SLC is cubic-symmetric characterized by the ( 1 2 , 1 2 , 1 2 ) magnetic Bragg peaks, while the one at weaker SLC is tetragonal-symmetric characterized by the (1, 1, 0) ones. With increasing the breathing distortion (decreasing J ′ /J), the ordering pattern of the ( 1 2 , 1 2 , 1 2 ) state becomes non-cubic with its magnetic Bragg reflections almost unchanged, while the (1, 1, 0) state is robust. The non-cubic state peculiar to the breathing pyrochlores is further categorized into two types. We demonstrate that these two non-cubic orderings result in the massive degeneracy of the ground state and that the associated residual entropy per spin takes characteristic values of ≃ ln(4) 16 kB and ≃ ln(6) 16 kB. Experimental implications of the results are discussed.

“…In GeCo 2 O 4 , the magnetic ordering occurs at T N = 21K and the neutron powder diffraction experiments exhibit the Bragg peak at π(1, 1, 1) due to magnetic reflection. [27][28][29] In addition to this Bragg peak, there exist additional peaks at π(−1, 1, 1), π(1,−1, 1) and π(1, 1,−1). [28] These wavevectors are nothing but Q 1 , Q 2 , Q 3 and Q 4 defined to describe the 'distorted cubic' phase.…”

Section: Magnetic Phase Diagrammentioning

confidence: 98%

Discovery of a new type of magnetic order on pyrochlore spinels

Sim

Lee

2018

Frustration in spin system can give rise to unique ordered states and as a consequence several physical phenomena are expected such as multiferroics, high temperature superconductors and anomalous hall effect. Here we report the "new magnetic orders" induced by anisotropic spin exchanges on pyrochlore spinels as the interplay of spin orbit coupling and geometrical frustration. Due to complicated superexchange paths of B-site spinels, we claim that anisotropic interaction between next-nearest neighbors play an important role. Based on the systematic studies of generic spin model, we argue that several classical spin states can be explored in spinel systems; local XY state, all-in all-out state, Palmer-Chalker state and coplanar spiral state. In addition, we reveal new types of magnetic phases with finite ordering wavevectors, labeled as 'octagonal (prism)' state and '(distorted) cubic' states. When the 'octagonal prism' state is stabilized, non-zero scalar spin chirality induces alternating net current in addition to finite orbital current and orbital magnetization even in Mott insulators. Finally, we also discuss the relevance of 'distorted cubic' state to the magnetic order of spinel compound GeCo2O4.Magnetic frustration originates from competing interactions between different spin exchanges. Despite the simple spin interactions such as Heisenberg or Ising type, lattice geometries can give rise to frustration, often termed as geometrical frustration. [1,2] Apart from geometrical frustration, anisotropic spin exchanges can also give rise to frustration due to competing interactions. [3][4][5] In general, such anisotropic spin exchanges are expected for spin-orbit coupled system described by the total angular momentum J. When anisotropic spin exchanges meet geometrical frustration, exotic magnetism emerges in Mott insulators.Magnetic properties of the B site spinel compounds (AB 2 X 4 ) own such interplay forming a corner shared tetrahedra, pyrochlore lattice structure.[6-8] Focusing on pyrochlore lattice structure, one can derive the most generic spin model on symmetry grounds and indeed such generic spin model for nearest-neighbor interactions has been studied. Especially for rare-earth pyrochlore magnets (A 2 B 2 O 7 ), A site rare-earth ions also form a pyrochlore lattice and their partially filled 4f electrons can give rise to non-negligible anisotropic spin exchanges between nearest-neighbors.[9-12] However, the situation in B-site spinels with 3d-5d magnetic ions is quite different from the case of rare-earth pyrochlores. In spinels, the superexchange paths induce anisotropic exchanges between next-nearest-neighbor to play an important role, thus generic spin model for B-site spinel compounds is significantly distinct from ones studied before and one could expect new types of magnetic orderings.In this paper, we study the frustrated spin model on a pyrochlore lattice and find new magnetic phases and their unique properties. Focusing on spin orbit coupled B-site spinels, we write down the generic spin exchan...