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Magnetic birefringence under mechanical stress is useful for investigating antiferromagnetically ordered spins. Although the stress is expected to lead to a single magnetic domain, intrinsic distri...

Phase transitions in SrTiO3 between quantum paraelectric, coherent paraelectric, and electric-field-induced ferroelectric states are governed by tetragonal domains with quantum fluctuations. However, their characteristics are still unclear. To observe the electric-field-induced ferroelectric state using birefringence imaging techniques, we developed a suitable sample holder to apply high electric fields of up to \(E \simeq 5\) kV/cm and temperatures down to T = 20 K. From birefringence imaging measurements of the ferroelectric LiNbO3 with varying electric field, distributions of the electric field in the sample stage were found to be negligible. In SrTiO3, a huge-retardance area corresponding to the ferroelectric domains appears at E > 2 kV/cm and T ≤ 60 K even though the paraelectric domains partially remain. Furthermore, the fast-axis direction rotates by 90° at the ferroelectric phase transition because of an electrostrictive effect in ferroelectrics. The phase diagram of the critical electric field an...

In the layered compound (C3H7NH3)2CuCl4, both electrical and magnetic effects are impacted by rotations of CuCl6 octahedra and alkylammonium chains (C3H7NH3) at successive structural phase transiti...

The dependence of optical linear birefringence Δ n on temperature T was measured on a single crystal of (CH 3 ) 2 CHNH 3 CuCl 3 , which consists of ferromagnetic–antiferromagnetic (F–AF) alternating Heisenberg chains with S =1/2. The curve of dΔ n ( T )/d T showed a large peak near 25 K and a small shoulder near 45 K. The presence of these two anomalies is qualitatively similar to the theoretical curve of the magnetic specific heat in ferromagnetic-dominant F–AF chains.

Electron paramagnetic resonance measurements were performed at 24 GHz on the magnetic compound (CH 3 ) 2 CHNH 3 CuBr 3 , which consists of antiferromagnetic-antiferromagnetic alternating Heisenberg...

At ambient pressure, both the intralayer and the interlayer exchange interactions in the layered compound (CH 3 NH 3 ) 2 CuCl 4 are ferromagnetic. Magnetic susceptibility and neutron scattering experiments performed under pressures up to about 1 GPa revealed that the interlayer exchange interaction turns into an antiferromagnetic one at a pressure between 0.41 GPa and 0.61 GPa, while the intralayer exchange interaction remains ferromagnetic. The spin structure in the antiferromagnetic phase is still collinear at least up to 1.1 GPa.

In the two-dimensional Heisenberg antiferromagnet (C 2 H 5 NH 3 ) 2 CuCl 4 , the intralayer exchange interaction is ferromagnetic, whereas the interlayer exchange interaction, which is less than 10 -4 of the intralayer one, is antiferromagnetic, and the spins collinearly lie in each layer. To investigate the effect of pressure on its spin structure, magnetic susceptibility and neutron scattering measurements under pressures up to 2.5 GPa were performed. The magnetic susceptibility measurements revealed the appearance of a pressure-induced weak ferromagnetic moment along the c -axis around 0.6 GPa. The neutron scattering measurements clarified the noncollinear spin structure which causes the weak ferromagnetic moment mentioned above. That is, the ferromagnetically coupled spins in each layer cant from the layer, which brings about the weak ferromagnetic moment along the c -axis. The ferromagnetic coupling in each layer and the antiferromagnetic coupling among the nearest-neighboring layers are maintained a...

Electron spin resonance experiments were performed on α'-NaV 2 O 5 , a possible spin-Peierls compound, at 9.3 and 23.5 GHz over 1.3–700 K. Analyzing all factors of the absorption line above T sp =34 K, such as the value and the angular dependence of the linewidth at high temperatures, the temperature dependence of both the linewidth and the resonance field, as well as the high temperature lineshape, we find that the exchange-narrowing theory closely explains these factors when a linear Heisenberg chain with S =1/2 along the b -axis is assumed, and the antisymmetric exchange interaction, ∑ i d i i +1 ·( S i × S i +1 ) with d i +1 // c -axis, between nearest-neighbor spins on the b -axis is adopted as the origin of line broadening. As long as the results of this study are concerned, the model of V 4+ magnetic chains separated by nonmagnetic V 5+ chains in the a b -plane above 34 K is acceptable. However, the results do not necessarily deny a V-O-V molecular spin model that has been proposed to account for a...

The compound (CH 3 ) 2 CHNH 3 CuCl 3 has recently been found to be an ideal ferromagnetic-antiferromagnetic alternating Heisenberg chain with the energy gap Δ/ k =17∼18 K [H. Manaka et al. : J. Phys. Soc. Jpn. 66 (1997) 564]. When an external field H is applied to this compound, the energy gap should vanish at the critical field H c1 = Δ/ g µ B . Just above H c1 , the three-dimensional magnetic long-range order is expected, if the interchain coupling exists. Motivated by this expectation, we performed specific heat measurements over a temperature range from 0.5 to 5 K applying H up to 12 T. As a result, we found a λ-type anomaly of specific heat over 10∼12 T, which indicates a transition to the three-dimensional long range order. The temperature at which the λ-type anomaly appears increases with increasing H . The critical field obtained by extrapolating the curve of the λ peak position versus H is 9.0 T, which is close to that obtained from a magnetization measurement.

Since the ferromagnetic intralayer exchange interaction in K 2 CuF 4 is due to the antiferrodistortive (AFD) orbital ordering of the 3d holes of Cu 2+ ions, we expect that the ferromagnetism of this compound to disappear when the AFD order of the distortion of CuF 6 octahedra collapses. Recent magnetic susceptibility measurements performed at high pressures on this compound actually confirmed the disappearance of the ferromagnetism above approximately 9 GPa [M. Ishizuka et al. : J. Phys. Soc. Jpn. 65 (1996) 1927]. To obtain evidence of the pressure-induced structural change which corresponds to the magnetic change mentioned above, we measured the pressure dependence of the phonon-Raman spectra of this compound. As a result, we found that the scattering lines relevant to the vibrational modes of CuF 6 octahedra ordered in the AFD fashion disappeared above 9∼10 GPa. This result is consistent with the structure above 9 GPa revealed by recent X-ray diffraction experiments that we performed.

We report that antiferromagnetic and antiferromagnetic (AF-AF) alternating Heisenberg chains with S =1/2 are formed in (CH 3 ) 2 CHNH 3 CuBr 3 , which is in contrast to the almost isomorphous compound (CH 3 ) 2 CHNH 3 CuCl 3 , because the Cl compound forms ferromagnetic-dominant ferromagnetic and antiferromagnetic (F-AF) alternating Heisenberg chains [H. Manaka et al. : J. Phys. Soc. Jpn. 66 (1997) 564]. From the analysis of the dependence of magnetic susceptibility on temperature, χ( T ), the two different antiferromagnetic exchange interactions defined in the Hamiltonian H = -2 J 1 ∑ S 2 i · S 2 i +1 -2 J 2 ∑ S 2 i · S 2 i -1 are determined to be J 1 / k =-61 K and J 2 / k =-33 K. Applying the low temperature experimental data to the formula χ( T )∝ T -1/2 exp (-Δ/ k T ), we estimate the energy gap Δ to be 98 K. We discuss why AF-AF and F-AF alternating chains are formed in the Br and the Cl compounds, respectively, in spite of their almost isomorphous crystal structures.

On measuring the dependence of the magnetic susceptibility of (CH 3 ) 2 CHNH 3 CuCl 3 on the temperature, we find that this compound forms ferromagnetic and antiferromagnetic alternating linear chains with S =1/2. The magnetic susceptibilities observed in external fields applied along three different surfaces of an as-grown single-crystal sample show broad peaks at around 24 K and steeply decrease to zero with further decreases in temperature toward 0 K. Two types of superexchange interactions defined in the Hamiltonian H = -2 J 1 ∑ S 2 i · S 2 i +1 - 2 J 2 ∑ S 2 i · S 2 i -1 are determined to be J 1 / k =-23.5 K and J 2 / k =+54.1 K, which indicates that a pair of ferromagnetically coupled spins behaves as a spin of S =1 and thus this compound is regarded as antiferromagnetic linear chains with S =1. The present system can therefore be discussed in terms of the Haldane conjecture for integer spins. The energy gap is estimated to be 17.1∼18.1 K.

Birefringence imaging microscopy is well suited for comprehensively investigating various structural phase transitions but its weak point is that it imposes an inherent limit on the maximum retardance that can be detected. To detect retardance above this limit, we performed simultaneous birefringence measurements at multiple wavelengths on (CnH2n+1NH3)2MnCl4 with \(n = 1,2,3\). For n = 1, we successfully detected large jumps in retardance (over six times greater than the limit) in the first-order phase transitions at Tc1 \( \simeq \) 88 K and Tc2 \( \simeq \) 251 K. For n = 2, a gradual change in retardance appears in the second-order phase transition at Tc = 214 K. High-definition images of optical fast-axis azimuths provided information on the misorientation of the alkylammonium chains. For n = 3, a retardance kink appears at Tc4 \( \simeq \) 152–155 K, and a retardance jump appears in the commensurate-incommensurate phase transition at Tc5 \( \simeq \) 96–99 K.

Ferromagnetic–antiferromagnetic (F–AF) alternating Heisenberg chains with S 1⁄4 1=2 form the Haldane state because a pair of ferromagnetically coupled S 1⁄4 1=2 spins (S1 and S2) behaves as a Ŝ 1⁄4 1 spin which is defined as Ŝ S1 þ S2. In a high enough temperature T region, however, the spins behave as individual S 1⁄4 1=2 spins because the spin correlation is absent or very weak. Therefore, a crossover of the spin state from S 1⁄4 1=2 to Ŝ 1⁄4 1 is expected when T decreases and the spin correlation develops. We already found such crossover of the spin state in (CH3)2CHNH3CuCl3 (abbreviated as IPACuCl3), which consists of a ferromagnetic-dominant F–AF chains and no magnetic long-range order appears at a finite T . From the magnetic susceptibility ðTÞ measurements, two kinds of exchange interactions were estimated to be JF=k 1⁄4 þ54:1K and JAF=k 1⁄4 23:5K when the Hamiltonian is expressed as

We conducted X-ray diffraction, magnetic susceptibility, heat capacity, high-field magnetization, and electron spin resonance experiments on high-quality polycrystalline S =3/2 compounds consisting of equilateral and non-equilateral triangular spin tubes in CsCrF 4 and α-KCrF 4 , respectively. In both compounds, geometrical spin frustration in the triangular planes coexists with one-dimensionality along the tubes. In CsCrF 4 , no magnetic long-range order appeared above 1.5 K, which agrees with a prior report [H. Manaka et al. : J. Phys. Soc. Jpn. 78 (2009) 093701]. In a low temperature ( T ) regime, T -linear component of heat capacity appeared and magnetization curves showed finite initial slopes. As a result, CsCrF 4 consists of a gapless spin-liquid state encompassing resonating spin-singlet pairs not only in each equilateral triangle but also along the tubes. In contrast, for α-KCrF 4 , successive magnetic phase transitions occurred at T N1 = 2.5(1) K and at T N2 = 4.0(1) K because superexchange inte...

Topological effects on spin frustration in equilateral triangle S =3/2 spin tubes formed in CsCrF 4 were studied by magnetic susceptibility, heat capacity [ C ( T )], and electron spin resonance experiments. The experimental data show that there is no magnetic long-range order down to T =1.3 K, and that no spin gap due to a chiral spin structure appears. Furthermore, since the C ( T ) curve has a T -linear component, a gapless spin-liquid ground state in one-dimensional antiferromagnets, the so-called Tomonaga–Luttinger liquid, is realized. This is characteristic of equilateral triangle spin tubes with S = half-integer.

The spin–lattice relaxation rate T 1 -1 of 1 H-NMR has been measured in (CH 3 ) 2 CHNH 3 Cu(Cl x Br 1- x ) 3 with x = 0 and 0.35, in order to investigate their microscopic magnetism. Previous macroscopic magnetization and specific heat measurements suggested that these two exist in a singlet-dimer phase. The temperature dependence of T 1 -1 in an x = 0 system decreased exponentially toward zero, providing microscopic evidence of a gapped singlet ground state, which is consistent with results of macroscopic experiments. At the same time, in the x = 0.35 system, T 1 -1 showed a sharp peak structure at approximately 7.5 K but no splitting of 1 H-NMR spectra, indicative of magnetic ordering. We discuss the observed sharp peak structure in the x = 0.35 system with the soft mode toward the exotic magnetic ground state suggested by recent µSR experiments.

We synthesized a single-phase polycrystalline sample of a double-layer perovskite fluoride K 3 Cu 2 F 7 . This compound is the new member of the potassium copper fluoride family, which has an antiferrodistortive orbital order (AFDOO). Magnetic susceptibility measurements revealed that K 3 Cu 2 F 7 is a rare experimental realization of antiferromagnetically isolated spin dimers on a simple double-square lattice with no frustration, and that AFDOO causes ferromagnetic interactions in respective layers. The coexistence of spin gap and AFDOO is an exotic phenomenon both in quantum spin systems and strongly correlated electron systems.

Muon spin relaxation (µ + SR) measurements were performed on a S = 1 of composite Haldane spin chains in IPA-CuCl 3 , which consists of a spin-ladder network along the a -axis. At a zero external magnetic field, the µ + spin relaxation became slightly faster with the development of a magnetic short-range order, but, as expected, oscillating spectra, which indicate a magnetic long-range order, did not appear. From the µ + SR time spectra under transverse and longitudinal magnetic fields, fluctuation of the local field whose frequency was estimated to be 99 ±12 MHz was observed at 2.0 K, but it disappeared at 12 and 50 K. The appearance of the local field at a temperature between 12 and 2.0 K may suggest experimental evidence of the crossover from S = 1/2 spin state to S = 1 of composite Haldane spin state with decreasing temperature.