Successive Magnetic Transitions Research Articles

Topological and magnetic phase transitions in Bi2Se3thin films with magnetic impurities

When topological insulators meet broken time-reversal symmetry, they bring forth many novel phenomena, such as topological magnetoelectric, half-quantum Hall, and quantum anomalous Hall effects. From the well-known quantum spin Hall state in Bi${}_{2}$Se${}_{3}$ thin films, we predict various topological and magnetic phases when the time-reversal symmetry is broken by magnetic ion doping. As the magnetic ion density increases, the system undergoes successive topological or magnetic phase transitions due to variation of the exchange field and the spin-orbit coupling. In order to identify the topological phases, we vary the spin-orbit coupling strength from zero to the original value of the system and count the number of band crossings between the conduction and valence bands, which directly indicates the change of the topological phase. This method provides a physically intuitive and abstract view to figure out the topological character of each phase and the phase transitions between them.

Large magnetoresistance and metamagnetic transition in PrGa

The magnetic phase transition and the magnetoresistance (MR) in bulk intermetallic PrGa compound are investigated experimentally. Two successive magnetic transitions, ferromagnetic (FM)-antiferromagnetic (AFM) transition and AFM-paramagnetic transition, are observed at T1 = 28 K and T2 = 36 K, respectively. It is found that the PrGa compound exhibits a field-induced metamagnetic transition from AFM to FM state and a considerable change in lattice constants in a temperature range of 28-36 K. Accompanied with the AFM-FM transition, a negative MR occurs, and the maximal MR values are ∼30% and ∼34% at 28 K under the fields of 1 T and 5 T, respectively.

Neutron Diffraction Study of Successive Magnetic Phase Transitions of Distorted-Triangular-Lattice Antiferromagnet RbFeBr3

The successive magnetic phase transitions of the distorted-triangular-lattice antiferromagnet RbFeBr 3 have been investigated by single-crystal neutron diffraction measurements. The intermediate magnetic phase ( T N2 = 2.0 K< T < T N1 = 5.6 K) appears as a partially disordered (PD) spin structure. An inclusive understanding of a distortion model of this crystal with the symmetry P 6 3 c m , which enables the spontaneous electric polarization and the PD spin structure with a weak ferromagnetic moment, was established.

Effects of Geometrical Spin Frustration on Triangular Spin Tubes Formed in CsCrF4 and α-KCrF4

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...

Successive Magnetic Phase Transitions of Component Orderings in DyB4

The successive magnetic phase transitions in DyB 4 have been studied in detail by microscopic measurements using resonant X-ray diffraction and neutron scattering. It is shown that the a b -plane component of the magnetic moment is short-range-ordered in the intermediate phase where the c -axis component is long-range-ordered. It is estimated that this short-range order is dynamically fluctuating with a time scale between ∼10 -8 to ∼10 -11 s. Crystal field excitation has also been investigated by inelastic neutron scattering. To qualitatively understand the phase-transition phenomenon, we have studied a simple two-sublattice model with an antiferromagnetic interaction by mean-field calculation. The calculation, though without a quadrupolar interaction, successfully explains the occurrence of double phase transition, magnetic specific heat and entropy, magnetic susceptibility, and the huge elastic softening in the intermediate phase. The general success of the mean-field calculation, except for the short-range order, suggests that there is little effect of geometrical frustration on the macroscopic properties at zero magnetic field.

Magnetocaloric effects in RNiIn (R = Gd-Er) intermetallic compounds

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic RNiIn (R = Gd-Er) compounds have been investigated in detail. GdNiIn and ErNiIn compounds exhibit a ferromagnetic (FM) to paramagnetic (PM) transition around the respective Curie temperatures. However, it is found that RNiIn with R = Tb, Dy, and Ho undergo two successive magnetic phase transitions with increasing temperature. In addition, a field-induced metamagnetic transition from antiferromagnetic (AFM) to FM states is observed in RNiIn with R = Tb and Dy below their respective AFM-FM transition temperatures (Tt). The maximal values of magnetic entropy change (ΔSM) of HoNiIn are –9.5 J/kg K at Tt = 7 K and –21.7 J/kg K at TC = 20 K for a magnetic field change of 5 T, respectively. These two successive ΔSM peaks overlap partly, giving rise to a high value of refrigerant capacity (RC = 341 J/kg at 5 T) over a wide temperature span. It is noted that the RC value of GdNiIn is as high as 326 J/kg due to the relatively broad distribution of ΔSM peak. Consequently, this RNiIn system shows large reversible ΔSM and considerable RC values in the temperature range of 10–100 K.

Magnetocaloric effect and magnetic phase transition in Ho3Co

Magnetocaloric effect (MCE) and magnetic phase transition of Ho3Co compound have been investigated. The two successive magnetic transitions: spin–reorientation transition at TSR = 8.5 K and antiferromagnetic (AFM)–paramagnetic transition at the Néel temperature TN = 21 K are observed. The Ho3Co compound undergoes a field-induced metamagnetic transition from AFM to ferromagnetic states below TN, which leads to a large MCE. The maximal value of magnetic entropy change (ΔSM) is found to be −14.5 J/kg K with a refrigerant capacity (RC) value of 440 J/kg around TN for a field change of 0–5 T. The large ΔSM as well as considerable RC in Ho3Co is very useful for applying it to the magnetic refrigeration in low-temperature ranges.

Successive Magnetic Transitions on Single Crystal Cu&lt;sub&gt;3&lt;/sub&gt;(OH)&lt;sub&gt;4&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;

Cu3(OH)4SO4, a parallel Cu (S = 1/2) triple chain system, is an interesting magnet because an idle-spin state is expected on the central chain. We have succeeded in growing high-quality single crystals of Cu3SO4(OH)4 by a hydrothermal method from copper sulfate and copper hydroxide. We measured field dependence of the magnetization on a single crystal under various directions of magnetic field up to 7 T. We found a clear magnetic anisotropy and confirmed that the existence of the easy axis lies in the ac plane. Under magnetic fields parallel to the c-axis and the a-axis at 2.5 K, we observed field-induced successive magnetic transitions. In the case of B//c, two anomalies of the magnetization are observed at 0.9 and 1.7 T, On the other hand, there are broad anomalies in the magnetization at 3.7 and 5.6 T in the case of B//a.

Magnetic order in Pu2M3Si5 (M = Co, Ni)

The physical properties including magnetic susceptibility, specificheat, and electrical resistivity of two new plutonium compoundsPu2M3Si5 (M = Co, Ni) are reported.Pu2Ni3Si5 crystallizes in theorthorhombic U2Co3Si5 structure type, which can be considered a variant of theBaAl4 tetragonal structure,while Pu2Co3Si5 adopts the closelyrelated monoclinic Lu2Co3Si5 type. Magnetic order is observed in both compounds, withPu2Ni3Si5 orderingferromagnetically at TC = 65 K then undergoing a transition into an antiferromagnetic state belowTN = 35 K. Two successive magnetic transitions are also observed atTmag1 = 38 K andTmag2 = 5 K inPu2Co3Si5. Specificheat measurements reveal that these two materials have a moderately enhanced Sommerfeld coefficientγ ∼ 100 mJ/mol Pu K2 in the magnetic state with comparable RKKY and Kondo energy scales.

Successive magnetic transitions and low temperature magnetocaloric effect in RE 2Ni 7 (RE=Dy, Ho)

Magnetic properties of rare-earth intermetallics RE 2Ni 7 (RE=Dy, Ho) are reported. Both the samples undergo two successive magnetic transitions at T h (paramagnetic to ferromagnetic) and T l (spin reorientation) below 100 K. The transitions are found to be second order in nature as evident from the Arrot plot analysis. Large reversible magnetocaloric effect (MCE) was observed at low temperature in the studied samples. The maximum value of the magnetic entropy change in Ho 2Ni 7 is found to be −12.5 J/kg K (for 0 to 50 kOe of field change) around 25 K with a high relative cooling power (RCP) of 534 J/kg. The Dy counterpart also shows moderately large values of MCE (−7.3 J/kg K) and RCP (475 J/kg) around the magnetic transition region for similar change in the magnetic field. RE 2Ni 7 compounds can be promising materials for magnetic refrigeration in the temperature range of helium and hydrogen liquefaction.

Powder Neutron Diffraction Study of TbCoGa5

We report powder neutron diffraction study on TbCoGa 5 in order to understand the successive components-separated magnetic transitions. TbCoGa 5 exhibits a collinear antiferromagnetic order with the propagation vector k 1 =(1/2, 0, 1/2) in the intermediate phase II. The magnetic moments are parallel to the c direction. In phase III, the a - or b -component of the magnetic moment is ordered, in addition to the c -component order in phase II, resulting in the formation of an antiferromagnetic order canted from the c -direction. The results of the present study strongly indicate that, in phase II, the c -component of the magnetic moment is ordered but the a - or b -component of the magnetic moment is non-ordered state.

Elastic Properties of TbCoGa5under Magnetic Field

The elastic properties of TbCoGa 5 single crystal were investigated by the measurement of the elastic constants under magnetic fields. The results of the measurement are consistent with successive component-separated magnetic orderings in TbCoGa 5 . The elastic properties in high magnetic fields indicate that the spin system is strongly affected by the quadrupole moment system in TbCoGa 5 . Such a mutual influence assumes an important role in the occurrence of the successive component-separated magnetic transitions.

Magnetic and Transport Properties ofROs2Al10(R=Pr, Nd, Sm, and Gd)

We report on the synthesis and measurements of the magnetic susceptibility, electrical resistivity, and specific heat of R Os 2 Al 10 ( R =Pr, Nd, Sm, and Gd). All compounds are isostructural with a Kondo semiconductor CeOs 2 Al 10 showing an unusual phase transition at T 0 =28.6 K. For R =Pr, no magnetic transitions were observed down to 0.4 K, whereas compounds with R =Nd, Sm, and Gd undergo antiferromagnetic transitions at T N = 2.2, 12.5, and 18 K, respectively. Moreover, successive magnetic transitions occur for R =Sm and Gd. The resistivity of the compounds with R =Sm and Gd increases on cooling below T N due to the superzone-gap formation. The fact that T N =18 K for R = Gd is lower than T 0 for R = Ce indicates that the phase transition in CeOs 2 Al 10 is not originated from the RKKY interaction alone but from the anisotropic hybridization of the 4 f state with conduction bands.

Successive magnetic transition in non-centrosymmetric CeCoGe3 probed by Co-NQR

Co-NQR measurements for non-centrosymmetric superconductor CeCoGe3 have been performed in ambient and under pressure up to 1.5 GPa to investigate the magnetic structures for the successive magnetic transition at TN1=21 K, TN2=12 K and TN3=8 K. In ambient pressure, the crystallographically equivalent one Co site becomes three sites at TN1 > T > TN2, and further becomes four sites at TN2 > T. Applying pressure at 4.2 K, the four Co sites becomes two sites above 0.3 Gpa. These Co sites acquire slightly different internal field parallel to the c-axis, consistent with the anisotropic magnetization with easy c-axis. The T-dependences of the internal fields and the spectral weights for respective sites are estimated. The magnetic structure in each phase is discussed referring to the recent results of neutron scattering.

Component-separated magnetic transition in HoRh2Si2 single crystal

Magnetic susceptibility and specific heat measurements have been carried out on a HoRh2Si2 single crystal compound. Specific heat shows two λ-like anomalies at TN1=29.1 K and TN2=12.0 K, and an additional sharp anomaly indicating the first order transition at Tt=27.3 K. The temperature dependence of susceptibility along the [001] direction shows a clear cusp at TN1 but a tiny kink at TN2 and Tt. In contrast, those in the ab-plane show a clear cusp at TN2. HoRh2Si2 shows a successive component-separated magnetic transition coming from a new type frustration not geometrical one.

Long-time variation of magnetic structure in a geometrically frustrated magnet Ca3Co2O6

Magnetization and neutron scattering measurements have been made on a single crystal of geometrically frustrated compound Ca3Co2O6 which shows successive magnetic transitions at Tc1~25 K and Tc2~13 K and multi-step metamagnetic transitions in low magnetic field. In this compound without random magnetic interactions, a long-time variation of magnetic structure has been detected. The magnetic Bragg peaks corresponding to an incommensurate antiferromagnetic structure observed at (1 0 ± 6) reciprocal lattice positions after cooled below Tc2 move toward (1 0 0) with time. The characteristic time for these variations follows the Arrhenius law with an activation energy Ea/kB=57 K.

Real-time observation of magnetic structural change in the multistep metamagnet CeIr3Si2

A ternary intermetallic compound CeIr3Si2 shows successive magnetic transitions at TN1=4.1 K and TN2=3.3 K. At T < TN2 it shows three-step metamagnetic transitions below H=1.43 T. In this non-diluted compound a long-time variation of magnetic structure has been detected by means of time-resolved neutron scattering measurements. When a sample is rapidly cooled below TN2, the magnetic Bragg peaks corresponding to the intermediate temperature phase (TN2 < T < TN1) are observed. The amplitude of these Bragg peaks gradually decreases with time. On the other hand, another group of Bragg peaks corresponding to the low temperature phase (T < TN2) gradually grow with time. The characteristic time for these variations follows the Arrhenius law with an activation energy Ea/kB =4 K.

Self-doping effect and successive magnetic transitions in superconductingSr2VFeAsO3

We have studied a quinary Fe-based superconductor Sr$_2$VFeAsO$_3$ by the measurements of x-ray diffraction, x-ray absorption, M\"{o}ssbauer spectrum, resistivity, magnetization and specific heat. This apparently undoped oxyarsenide is shown to be self doped via electron transfer from the V$^{3+}$ ions. We observed successive magnetic transitions within the VO$_2$ layers: an antiferromagnetic transition at 150 K followed by a weak ferromagnetic transition at 55 K. The spin orderings within the VO$_2$ planes are discussed based on mixed valence of V$^{3+}$ and V$^{4+}$.

Infrared spectroscopic studies of [formula omitted

This paper reports infrared spectroscopic studies on GdBaCo 2O 5.5 layered perovskite which exhibits successive magnetic transitions from paramagnetic to ferromagnetic to antiferromagnetic states as well as high-temperature metal to insulator transition and a change in charge transport mechanism at low temperature. Infrared absorption spectra recorded at various temperatures in the range 80–350 K reveal changes in the positions of Co–O stretching and bending frequencies which provide an explanation to the magnetic and transport behaviour of this compound.

Magnetic properties and magnetocaloric effects in antiferromagnetic ErTiSi

Magnetic properties and magnetocaloric effects (MCEs) of the antiferromagnetic (AFM) ErTiSi compound with a Néel temperature TN=46 K are studied by magnetization measurements. Two successive magnetic transitions in the thermomagnetic M-T curves, an AFM-AFM transition followed by an AFM-paramagnetic transition with increasing temperature, are observed. ErTiSi undergoes a field-induced metamagnetic transition from AFM to FM state below TN. A sign change in MCE with increasing temperature or magnetic field in ErTiSi is observed near the critical field. The maximal value of magnetic entropy change ΔSm is −8.9 J/kg K around TN for a field change of 0–5 T. The modest ΔSm as well as no hysteresis loss around TN in ErTiSi may be useful for its application in magnetic refrigeration.