Successive Magnetic Transitions Research Articles

Magnetic properties ofEuPtSi3single crystals

Single crystals of EuPtSi$_3$, which crystallize in the BaNiSn$_3$-type crystal structure, have been grown by high temperature solution growth method using molten Sn as the solvent. EuPtSi$_3$ which lacks the inversion symmetry and has only one Eu site in the unit cell is found to be an antiferromagnet with two successive magnetic transitions at $T_{\rm N1}$ = 17 K and $T_{\rm N2}$ = 16 K, as inferred from magnetic susceptibility, heat capacity and $^{151}$Eu M\"ossbauer measurements. The isothermal magnetization data for $H \parallel$ [001] reveal a metamagnetic transition at a critical field $H_{\rm c}$ = 1 T. The magnetization saturates to a moment value of 6.43 $\mu_{\rm B}$/Eu above 5.9 T (9.2 T) for $H \parallel $ [001] ([100]), indicating that these fields are spin-flip fields for the divalent Eu moments along the two axes. The origin of this anisotropic behaviour is discussed. A magnetic (H, T) phase diagram has been constructed from the temperature dependence of isothermal magnetization data. The reduced jump in the heat capacity at $T_{\rm N1}$ indicates a transition to an incommensurate, amplitude modulated antiferromagnetic structure. The shape of the hyperfine field split M\"ossbauer spectrum at $T_{\rm N1}$ provides additional support for the proposed nature of this magnetic transition.

Magnetic structures of the ternary silicide Nd6Ni1.67Si3

The ternary silicides RE6Ti1.67Si3 (RE = Ce, Pr, Nd, Gd, Tb and T = Co, Ni) crystallizing with the hexagonal Ce6Ni1.67Si3-type structure has been recently extensively studied. It has been shown that some of these compounds exhibit interesting magnetocaloric properties. In this work, the magnetic structures of the Nd6Ni1.67Si3 compound were determined by means of neutron powder diffraction. According to previous magnetization and specific heat measurements, this compound exhibits two successive magnetic transitions at 84K and 38K. The first transition corresponds to a ferromagnetic arrangement of the Nd-moments along the c-axis and the second one to a non-collinear ferromagnetic arrangement yielding a conical structure. The magnetic structures of Nd6Ni1.67Si3 are compared to those of the Nd6Co1.67Si3homologous ternary silicide.

Low-energy excitations probed by 51V NMR in the multiferroic Ni3V208

We present results of the single-crystal 51V NMR measurements in the multiferroic compound Ni3V208. 51V Knight shifts in a magnetic field parallel to the b axis follow the Curie-Weiss law with the Weiss temperature, −9.5 K, comparable to the onset temperature of long-range magnetic order, THI = 9.4 K, where the high-temperature incommensurate magnetic order takes place. Non-linearity in the Knight shift vs magnetic susceptibility plot suggests the significant Dzyaloshinsky-Moriya interaction above THI. Despite three successive magnetic transitions below THI, the 51V nuclear spin-lattice relaxation rate measured along the b axis shows a critical slowing down only at THI.

Long-time variation of magnetic structure in CeIr3Si2

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 magnetc susceptibility and 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. This is the first observation of long-time variation of magnetic structure in non-diluted uniform magnets.

Enhanced polarization and magnetoelectric response in Tb1−x Ho x MnO3

A series of manganites Tb1-xHoxMnO3 (0<=x<=0.6) with orthorhombic structure are synthesized and detailed investigations on their multiferroicity are performed. Successive magnetic transitions upon temperature variation are evidenced for all samples, and both the Mn3+ spiral spin ordering and rare-earth spin ordering are suppressed with increasing x. Significant enhancement of both the polarization and magnetoelectric response within 0.2<x<0.4 is observed, which may be ascribed to the competition possibly existing between spiral and E-type spin orders. Theoretical calculation is given based on two eg-orbital double-exchange model, and the result supports the scenario of the multiferroic phase separation.

Pressure-induced superconductivity in non-centrosymmetric compound CeIrGe3

We have carried out electrical resistivity experiments on the non-centrosymmetric compound CeIrGe3 under high pressures up to 24GPa. CeIrGe3 is an antiferromagnet with the Néel temperature TN1=8.5K and a successive magnetic transition at TN2=4.7K at ambient pressure. With increasing pressure, TN1 and TN2 merge around 3GPa, becomes unchanged up to 10GPa, and indicates a step-like decrease up to 20GPa. Further increasing pressure, we have found superconductivity above 20GPa with a transition temperature of 1.6K at 24GPa. The upper critical field for the magnetic field along the tetragonal [001] direction is expected to be much larger than 15T.

Large reversible magnetocaloric effect caused by two successive magnetic transitions in ErGa compound

Intermetallic compound ErGa exhibits two successive magnetic transitions: spin-reorientation transition at TSR=15 K and ferromagnetic-paramagnetic transition at TC=30 K. Both transitions contribute greatly to the magnetic entropy change (ΔSM), each yielding a significant peak on their ΔSM-T curve and thus a considerable value of refrigerant capacity (RC) without hysteresis loss. For a magnetic field change of 5 T, the maximal values of −ΔSM are 21.3 J/kg K at TC and 16.5 J/kg K at TSR, with an RC value of 494 J/kg. Large reversible magnetocaloric effect and RC indicate the potentiality of ErGa as a candidate magnetic refrigerant at low temperatures.

Studies of Multiferroic System of LiCu2O2: II. Magnetic Structures of Two Ordered Phases with Incommensurate Modulations

Neutron diffraction and 7Li-NMR have been applied to determine the multiferroic system LiCu2O2, which has four chains (ribbon chains) of edge-sharing CuO4 square planes in a unit cell. We have confirmed that there are successive magnetic transitions at TN1=24.5 K and TN2=22.8 K. In the T region between TN1 and TN2, the quasi one-dimensional spins (S=1/2) of Cu2+ ions within a chain have a collinear and sinusoidally modulated structure with Cu-moments parallel to the c-axis and with the modulation vector along the b-axis. At T < TN2, an ellipsoidal helical spin structure with the incommensurate modulation has been found. Here, we present detailed parameters, describing the modulation amplitudes, helical axis vectors as well as the relative phases of the modulations of four ribbon chains, which can well reproduce both the NMR and neutron results in the two magnetically ordered phases. This finding of the rather precise magnetic structures enables us to discuss the relationship between the magnetic structure and the multiferroic nature of the present system in zero magnetic field, as presented in our companion paper (paper I), and open a way how to understand the reported electric polarization under the finite magnetic field.

Studies of Multiferroic System LiCu2O2: I. Sample Characterization and Relationship between Magnetic Properties and Multiferroic Nature

Single-crystal samples of LiCu2O2 with spin 1/2 Cu2+ chains of edge-sharing CuO4 square planes (ribbon chains), have been characterized by X-ray diffraction, thermogravimetric analysis, and magnetic measurements. Neither the atomic deficiency nor the mixing of Cu and Li atoms has been found, indicating that lattice defects conceived as a possible origin of the reported multiferroic behavior can be excluded. Anomalies found in the data of specific heat and neutron magnetic Bragg reflections show clear evidence that the system exhibits successive magnetic transitions at TN1=24.5 K and TN2=22.8 K. Based on the magnetic structures in the intermediate (TN2<T<TN1) and low temperature (T<TN2) phases, determined by the combined studies of neutron scattering and 7Li-NMR measurements, we can consistently understand the fact that the multiferroic properties are observed only below TN2 by considering existing theories.

Magnetocaloric effect in Ho2In over a wide temperature range

The compound Ho2In exhibits two successive magnetic phase transitions: a spin-reorientation transition at TSR=32 K and a magnetic-ordering transition at TC=85 K. The maximum reversible −ΔSM values are 6.3 and 11.2 J/kg K at TSR and TC, respectively, for a field change of 5 T. These two −ΔSM peaks with the same sign are partly overlapping, which results in a wide temperature interval with appreciable magnetocaloric effect. The results on Ho2In indicate that materials with successive SR and magnetic-ordering transitions may constitute an important new class of magnetic refrigerants since they work in a wider temperature range than the conventional refrigerant materials.

Magnetic properties of the layered cobaltite

Abstract We present a DC magnetization, zero and longitudinal field μ SR investigation on the magnetic properties of NdBaCo 2 O 5.50 . It was found that NdBaCo 2 O 5.50 exhibits successive magnetic transitions from a high temperature paramagnetic phase to a ferromagnetic (FM) phase at T C = 290 ( 5 ) K , then to an antiferromagnetic (AF1) phase at T N 1 = 265 ( 5 ) K , and to another AF (AF2) phase at T N 2 = 160 ( 10 ) K . Even though the transition temperatures are similar to the ones of RBaCo 2 O 5.5 with R = Ho , Y, Dy, Tb, the microscopic magnetic nature of NdBaCo 2 O 5.50 is very different from the other compounds. That is, the FM phase is characterized by a temperature dependent mixture of dynamic and static phase volumes. The observed μ SR frequencies indicate that in the AFM phases different spin state ordered (SSO) magnetic structures are present in NdBaCo 2 O 5.50 than in the compounds with other rare earth ions.

Successive component-separated magnetic transition in TbCoGa5

We grew single crystals of TbCoGa5 and measured the magnetic susceptibility, magnetization curves, specific heat and ultrasonic properties. The results of the magnetic susceptibility and specific heat measurements suggest that TbCoGa5 shows successive component-separated magnetic transition and the degeneracy of quadrupolar degrees of freedom is not fully lifted in intermediate phase. The softening of the elastic constant C44 indicates that the degeneracy of electric-quadrupolar (orbital) degrees of freedom exists in spite of the formation of magnetic order. We have considered that the degeneracy of quadrupolar degrees of freedom is important for the occurrence of successive component-separated magnetic transition.

Magnetic and reversible magnetocaloric properties of (Gd1−xDyx)4Co3 ferrimagnets

Magnetic properties and magnetocaloric effect of the ferrimagnetic compounds Gd4Co3 and (Gd0.9Dy0.1)4Co3 have been studied. In Gd4Co3, two successive magnetic-entropy changes occur due to a spin-reorientation transition at TSR=163 K and a ferrimagnetic-magnetic ordering transition at TC=220 K. For a field change from 0 to 5 T, −ΔSMmax at TC is 5.74 J/kg K, the full width at half maximum (δTFWHM) of the −ΔSM versus T curve has a high value of 131 K, and the refrigerant capacity is as high as 575 J/kg. TC can easily be tuned by Dy substitution for Gd and the magnetic-entropy change is increased simultaneously. −ΔSMmax of (Gd0.9Dy0.1)4Co3 at TC=145 K for the same field change has increased to 8.47 J/kg K. Our studies suggest that the successive magnetic transitions could enlarge the temperature interval, which makes this type of material meaningful for magnetic refrigeration in wider temperature regions than conventional materials.

Theory of Successive Magnetic Transitions with Tilting of Moments–Application to NpFeGa5–

Mechanism of successive magnetic transitions in NpFeGa 5 is studied theoretically with use of an extended CEF model assuming Fe moments as the source of anisotropy. All possible dipolar and quadrupolar interactions are taken into account, which are allowed by the doublet–singlet subspace. Phase diagrams are obtained which give either two or three successive transitions from the paramagnetic phase. In the latter case, Np moments first order along [110], then tilt around the c axis, and finally tilt out of the a b -plane. The ensuing splitting pattern of 69,71 Ga nuclear magnetic resonance (NMR) is analyzed theoretically, and compared with experimental results.

Spin-driven ferroelectricity in the multiferroic compounds of [formula omitted

Neutron diffraction studies under magnetic field as well as under hydrostatic pressure for microscopic magnetism on multiferroic RMn 2 O 5 ( R = rare-earth , Bi, and Y) are reviewed to discuss about the relevance between the magnetic property and dielectric property in detail. Without any external fields, a series of RMn 2 O 5 shows successive magnetic phase transitions of incommensurate–commensurate–incommensurate as temperature decreases, in which the dielectric phase transitions concomitantly occur. At the lowest temperature phase of HoMn 2 O 5 , the magnetic transition from the incommensurate phase to the commensurate one is induced by applying both magnetic field and hydrostatic pressure. At this field-induced magnetic transition, a spontaneous electric polarization simultaneously induced, indicating that the ferroelectricity in this material is magnetically controlled. Competition of multiple magnetic ground states due to intensional magnetic frustration in this system can be easily tuned by applying external fields, which give rise to the rich variety of spin-driven dielectric transitions.

Ternary Erbium Chromium Sulfides: Structural Relationships and Magnetic Properties

Single crystals of four erbium-chromium sulfides have been grown by chemical vapor transport using iodine as the transporting agent. Single-crystal X-ray diffraction reveals that in Er(3)CrS(6) octahedral sites are occupied exclusively by Cr(3+) cations, leading to one-dimensional CrS(4)(5-) chains of edge-sharing octahedra, while in Er(2)CrS(4), Er(3+), and Cr(2+) cations occupy the available octahedral sites in an ordered manner. By contrast, in Er(6)Cr(2)S(11) and Er(4)CrS(7), Er(3+) and Cr(2+) ions are disordered over the octahedral sites. In Er(2)CrS(4), Er(6)Cr(2)S(11), and Er(4)CrS(7), the network of octahedra generates an anionic framework constructed from M(2)S(5) slabs of varying thickness, linked by one-dimensional octahedral chains. This suggests that these three phases belong to a series in which the anionic framework may be described by the general formula [M(2n+1)S(4n+3)](x-), with charge balancing provided by Er(3+) cations located in sites of high-coordination number within one-dimensional channels defined by the framework. Er(4)CrS(7), Er(6)Cr(2)S(11), and Er(2)CrS(4) may thus be considered as the n = 1, 2, and infinity members of this series. While Er(4)CrS(7) is paramagnetic, successive magnetic transitions associated with ordering of the chromium and erbium sub-lattices are observed on cooling Er(3)CrS(6) (T(C)(Cr) = 30 K; T(C)(Er) = 11 K) and Er(2)CrS(4) (T(N)(Cr) = 42 K, T(N)(Er) = 10 K) whereas Er(6)Cr(2)S(11) exhibits ordering of the chromium sub-lattice only (T(N) = 11.4 K).

Spin-lattice model of magnetoelectric transitions inRbCoBr3

Extensive Monte-Carlo simulations are performed to analyze a recent neutron-diffraction experiment on a distorted triangular lattice compound ${\text{RbCoBr}}_{3}$. We consider a spin-lattice model where both spin and lattice are Ising variables. This model explains well successive magnetic and dielectric transitions observed in the experiment. The exchange interaction parameters and the spin-lattice coupling are estimated. It is found that the spin-lattice coupling is important to explain the slow growth of a ferrimagnetic order. The present simulations were made possible by developing a modified Monte-Carlo algorithm, which accelerates slow Monte-Carlo dynamics of quasi-one-dimensional frustrated systems.

Anisotropic Applied Field Dependency of Two Successive Magnetic Transitions in LiCu2O2

The anisotropy of two successive transitions of the spin-ladder compound LiCu2O2 is studied by the specific heat (C) under magnetic fields with H// c and H// ab (written as Hc and Hab in the following) up to 14 T. The peak of specific heat at 24.5 K in zero field shifts to lower temperature when the field is increased and the magnitude of the peak is suppressed by the field. On the contrary, the peak of 22.5 K shifts to higher temperature, especially at 14 T. Its magnitude increases in the field of Hc, whereas it decreases in the field of Hab. We calculate the entropy change between 21 K and 63 K. The different influence of the spin ordering by fields of different direction is obtained. Our experimental results suggest a mixed state between the long range incommensurate helimagnetic ground state and the higher-T short-range dimer liquid state. The temperature range of mixed state is shrunk with the increasing field. Possible mechanism is discussed.

Competition of Magnetic and Quadrupolar Order Parameters in HoB4

The successive magnetic phase transitions of HoB 4 with T N1 =7.1 K and T N2 =5.7 K have been investigated in detail by neutron and X-ray diffraction. Since Ho ions in the c -plane form a Shastry–Sutherland lattice, geometrical frustration has been expected in this compound. Below T N1 , HoB 4 exhibits an incommensurate antiferromagnetic order with q =(δ, δ, δ') (δ=0.022, δ'=0.43), followed by a first-order transition at T N2 into a commensurate order with q =(0, 0, 0), where four magnetic moments are antiferromagnetically coupled in a unit cell. After determining the magnetic structures of the two phases by neutron powder diffraction, we pay particular attention to diffuse scatterings responsible for the critical behavior. Very broad diffuse scatterings, extending almost to the zone boundary, are observed already in the paramagnetic phase at around q =(0, 0, 0) and (0, 0, 1/2) with magnetic correlation lengths of ξ< 10 A. Below T N1 , a strong diffuse scattering develops in a more concentrated region at ...

Coexistence and Coupling between Dielectric Order and Magnetic Order in Multiferroic Material of RMn2O5 (R=rare-earth, Bi, Y)

Microscopic magnetism and dielectric properties in the multiferroic rare-earth manganites of RMn2O5 studied by neutron diffraction and dielectric measurements are reviewed.It has been revealed that RMn2O5 shows successive dielectric and magnetic phase transitions, which concomitantly take place and are induced by temperature, magnetic field, and hydrostatic pressure.This coexistence and mutual coupling between dielectric and magnetic order, as well as the induced 4f magnetic moment on rare-earth ions, are ingredients essential for realizing a rich variety of colossal magnetoelectric effect.