Field-induced Magnetic Phase Transitions Research Articles

Magnetic quantum phase transitions and entropy in Van Vleck magnet

Field-induced magnetic quantum phase transitions in the Van Vleck paramagnet with easy-plane single-ion anisotropy and competing Ising exchange between ions with the spin S=1 have been studied theoretically. The description was made by minimizing the Lagrange function at zero temperature (Т=0) and the free energy at T≠0. Stable and unstable solutions of equations corresponding to the case ψ0=|0〉 asymptotically transform into those following from the Lagrange function at Т=0. First-order phase transitions from the Van Vleck paramagnet state into the ferromagnet one were found to take place at a sufficiently high single-ion anisotropy. The entropy of such a magnet was shown to grow with its magnetization, as it occurs for antiferromagnets. At the point of quantum phase transition, the entropy has a jump, which magnitude depends on the ratio between the Ising exchange and anisotropy constants, as well as on the temperature. The described magnetic phase transition was supposed to be accompanied by the magnetocaloric effect. In the case when the Ising exchange dominates over the single-ion anisotropy, the magnetization reversal of ferromagnetic state by an external field was shown to be a phase transition of the first kind, which does not belong to orientational ones and which should be regarded as a quantum order-order phase transition.

Chiral heterobimetallic chains from a dicyanideferrite building block including a π-conjugated TTF annulated ligand.

The π-conjugated tetrathiafulvalene (TTF) annulated ligand was introduced into a dicyanometallate for the first time, leading to the synthesis of the versatile redox-active dicyanideferrite building block [(n-Bu)4N][Fe(TTFbp)(CN)2] (H2TTFbp = N-(2-(4,5-bis(methylthio)-1,3-dithiol-2-ylidene)-5-(picolinamido) benzo[d][1,3]dithiol-6-yl) picol inamide). The incorporation of the new precursor with chiral MnIII Schiff-base complexes resulted in two enantiopure one-dimensional complexes, [Mn((R,R)-salphen)Fe(TTFbp)(CN)2]n (2-(RR)) and [Mn((S,S)-salphen)Fe(TTFbp)(CN)2]n (2-(SS)) (Salphen = N,N'-1,2-diphenylethylene-bis(salicylideneiminato) dianion), which were synthesized and structurally characterized. Circular dichroism (CD) and vibrational circular dichroism (VCD) spectra confirmed the enantiomeric nature of the optically active complexes, and structural analyses revealed the formation of neutral cyanide-bridged double chains in 2-(RR) and 2-(SS). Solution and solid state CV studies revealed the redox-active characteristics of the complexes. Antiferromagnetic couplings were detected between FeIII and MnIII centers within a chain, and a field-induced magnetic phase transition was observed (TN = 4.8 K). The introduction of electroactivity and chirality into cyanide-bridged complexes with interesting magnetic properties leads the way towards new multifunctional materials.

The magnetic properties of NdMnxCr2−xSi2C (0 < x < 2)

Carbon was successfully introduced into NdMnxCr2−xSi2 serial compounds as an interstitial atom. The compounds with x ⩽ 0.5 keep a CeMg2Si2-type structure while the structure changes slowly with increasing manganese content. Finally, a new superstructure was found and determined for x around 1.7. All the compounds for x = 1.1, 1.3, 1.5 and 1.7 have two magnetic phase transition temperatures around 43 and 314 K. What is more, a giant magnetocaloric effect has been observed around the lower phase transition temperature, TR ~ 51 K, in NdMn1.7Cr0.3Si2C. The maximum values of the magnetic entropy change −ΔSM for the field change of 2 T and 5 T are 14.9 J kg−1 K−1 and 26.0 J kg−1 K−1, which means the interstitial atom effect enhanced the MCE values for 85% and 63% compared to NdMn1.7Cr0.3Si2, respectively. NdMn1.7Cr0.3Si2C with first order magnetic transition but low hysteresis losses (thermal < ~0.58 K; magnetic field < ~0.12 T) can be a candidate for magnetic refrigerator applications in the temperature region near 50 K. Besides, two field-induced magnetic phase transitions were found in NdMn1.7Cr0.3Si2C among the temperature range from 53 to 60 K, and a linear relation between the phase transition field and temperature was found and analyzed.

Magnetic Characterization and Low-Temperature Heat Transport Properties of the Orthoferrites &lt;italic&gt;R&lt;/italic&gt;FeO&lt;sub&gt;3&lt;/sub&gt; (&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$R =$ &lt;/tex-math&gt;&lt;/inline-formula&gt; Rare Earth) Single Crystals

Orthoferrites RFeO 3 (except R=Y, La and Lu) possess two sets of magnetic sublattice, that is, Fe3+ sub-system and R3+ sub-system. Magnetic properties and interactions between the two sub-systems including Fe-Fe, R-Fe, and R-R interactions depend on external parameters, such as temperature, magnetic field, etc.[1-3] So real magnetic materials can exhibt rich low temperature characteristics. We have successfully grown the orthoferrites RFeO 3 (R = Dy, Ho and Y) and Ho 1−x Y x FeO 3 (x = 0.1 and 0.3) by the floating zone technique using a four-mirror image furnace. The halogen lamp power and crystal growth atmosphere were identified to be the most important parameters. High-quality single crystals have been demonstrated by the X-ray diffraction data and X-ray Laue photograph. The magnetic transitions with decreasing temperature and the field-induced magnetic phase transitions in RFeO 3 for the rare earth ions and Fe3+ can be characterized by low-temperature magnetization and specific heat measurements. In addition, we report the low-temperature heat transport measurements of these orthoferrites single crystals to probe the magnetic properties and find that they show strong magnetic field dependence. The characteristic fields have a good correspondence with some transformations of the magnetic structure. These special behaviors demonstrate the magnetic field-induced transitions of the Fe3+ and R3+ spin structure.

Magnetic order, phase transitions and electrical resistivity of Ho7Rh3 single crystals

Field-induced magnetic phase transitions have been studied on Ho7Rh3 single crystals with the hexagonal Th7Fe3 structure by measuring the magnetization and electrical resistivity as a function of temperature and applied magnetic field. The Ho7Rh3 compound having an antiferromagnetic (AF) order below TN = 32.0 K enters other magnetic states with decreasing temperature below Tt2 = 23.5 K and Tt1 = 9.0 K. Below Tt2, Ho7Rh3 possesses a small ferromagnetic component in the c-plane. Meanwhile, two successive metamagnetic transitions were observed in magnetic fields up to 100 kOe applied along the b-axis at T < Tt2. The metamagnetic phase transitions from AF to the forced ferromagnetic state in magnetic fields applied in the basal plane are observed to be accompanied by a large magnetoresistance (|Δρ/ρ| ∼ 42%). When the magnetic field is applied along the c-axis, the superzone gap effect on the electrical resistivity below TN does not disappear even under 100 kOe. In the paramagnetic state, the additional magnetic contribution to the electrical resistivity is found to persist apparently because of the conduction electron scattering by the short ranged antiferromagnetic clusters and spin fluctuations in the Rh 4d electron subsystem, which leads to the negative temperature coefficient of the electrical resistivity.

High-field magnetization study of a HoFe6Al6 single crystal

Magnetization measurements have been performed in static (up to 14 T) and pulsed (up to 60 T) magnetic fields on a HoFe6Al6 single crystal with a tetragonal crystal structure of ThMn12 type. HoFe6Al6 is a ferrimagnet with exact compensation of the Ho and Fe sublattices at low temperatures, the spontaneous magnetic moment is Ms = 0 at 4.2 K. At elevated temperatures, Ms passes through a wide maximum (4.2 μB at 200 K) and vanishes at the Curie temperature, TC = 315 K. The compound displays a high magnetic anisotropy of the easy-plane type, a noticeable anisotropy exists also within the easy plane with the [110] axis as the easy magnetization direction. HoFe6Al6 exhibits two field-induced magnetic phase transitions along the easy [110] axis and two more along the [100] axis, the hard direction in the basal plane. The critical fields of the lower- and higher-field transitions display qualitatively different temperature dependencies. A T–H magnetic phase diagram has been constructed for the [100] and [110] directions. The Ho–Fe inter-sublattice exchange constant has been determined to be nHoFe = 3.8 T/μB.

1:13 phase formation mechanism and first-order magnetic transition strengthening characteristics in (La,Ce)Fe13–x Si x alloys

The effects of the introduction of Ce to La1−xCexFe11.5Si1.5 alloys on 1:13 phase formation mechanism, the first-order magnetic phase transition strengthening characteristics, and magnetocaloric property were studied, respectively. The results show that the formation mechanisms of 1:13 and LaFeSi phases in La1−xCexFe11.5Si1.5 alloys are the same as those of Ce2Fe17 and CeFe2 phases in Ce–Fe binary system, respectively. The substitution of Ce in 1:13 phase which is limited can make the first-order magnetic phase transition characteristics strengthen, which can make thermal and magnetic hysteresis increase, the temperature interval of temperature-induced phase transition decrease, and the critical magnetic field of field-induced magnetic phase transition (HC) increase, respectively. Owing to the lattice shrink of 1:13 phase with the increase in Ce content, the Curie temperatures (TC) show a linear decrease. The maximum change in magnetic entropy gradually increases due to the decrease in temperature interval of temperature-induced phase transition, but the relative cooling capacities are all about 80 J·kg−1 at magnetic field of 2 T.

Magnetic, thermal and transport properties of Tb3Ru4Al12 with a distorted kagome lattice

The Tb3Ru4Al12 compound (hexagonal crystal structure) having a distorted kagome net of the Tb atoms is studied on a single crystal by measurements of magnetization, specific heat and electrical resistivity. The system is an antiferromagnet with the Néel temperature TN=22K. A substantial magnetic anisotropy is observed that persists at least up to 60T. Tb3Ru4Al12 displays field-induced magnetic phase transitions: one along [100] and [120] and two along the [001] axis. The transitions are accompanied by sharp peaks in magnetoresistance and subsequently lead to a large negative effect in it, −20% in the longitudinal and −40% in the transverse geometry. The peaks are explained by changes in conduction electron spectra at the transitions due to the interaction between the conduction electrons and localized magnetic moments. The large negative effect suggests field-induced magnetic phases with a reduced period as compared to zero field.

Giant magnetoresistance and field-induced magnetic phase transitions in Gd7Rh3 studied on single crystals

Magnetization measurements in steady and pulsed fields and magnetoresistance measurements in different field-current geometries on single crystalline samples have been employed to study the field-induced magnetic phase transitions in the antiferromagnetically (AF) ordered compound Gd7Rh3 with hexagonal crystal structure of the Th7Fe3 type. Below TN=141K, the magnetic structure transformations from AF to the forced ferromagnetic (FF) state, which are accompanied by a giant reduction of the electrical resistivity (up to 7 times at low temperatures), were observed under the magnetic fields along the c-axis and in the basal plane. This giant magnetoresistance is associated with magnetic superzone effects. The negative temperature coefficient of the electrical resistivity and a large magnetoresistance were also observed in the paramagnetic region in Gd7Rh3; which are suggested to result from the retention of short-range AF correlations in a wide temperature range above TN.

Magnetic anisotropy and magnetic phase transitions in RFe5Al7

RFe5Al7 (R – Gd, Tb, Dy, Ho, Er and Tm) single crystals have been studied by measurements of magnetization, sound propagation (in static and pulsed magnetic fields up to 60T) and specific heat. Fundamental magnetic properties have been determined and compared for all these materials. RFe5Al7 are highly anisotropic ferrimagnets. Spontaneous and field-induced magnetic phase transitions of anisotropic and exchange nature have been observed in RFe5Al7. Strong magnetoelastic interactions are manifested by pronounced acoustic anomalies at the phase transformations. The detected magnetization jumps provide important information on the R–Fe inter-sublattice exchange interactions.

Magnetic field-induced phase transitions in the antiferromagnet Nd0.6Dy0.4Fe3(BO3)4

Low-temperature studies of elastic and magnetic characteristics of the single crystal Nd0.6Dy0.4Fe3(BO3)4 have been performed. A transition to the antiferromagnetically ordered state of the magnetic subsystem has been manifested in the temperature behavior of the velocity and attenuation of acoustic modes and magnetization. Spin-reorientation phase transitions, which reveal themselves as anomalies in the behavior of elastic and magnetic characteristics of the crystal in the external magnetic field applied along the axis of the trigonal symmetry of the crystal have been discovered. The phase H–T diagram for H ‖ C3 has been constructed.

One pillared-layer α-Po framework with a rare tetracobalt-formate (4,4) sheet exhibiting a field-induced magnetic transition

A novel coordination polymer has been synthesized under solvethermal reaction, which represents a pillared-layer α-Po framework with a rare tetracobalt-formate (4,4) sheet. The magnetic properties have been well studied, and the results show an antiferromagnet and field-induced magnetic phase transition for the title complex.

Magnetic properties of the ternary aluminide TbFe2Al10

The magnetic properties of the ternary aluminide TbFe2Al10 have been studied with the help of magnetization measurements. From the temperature and field dependence of magnetization, a detailed magnetic phase diagram of TbFe2Al10 has been constructed. While the high- and low-temperature phases (in low fields) of TbFe2Al10 are paramagnetic and antiferromagnetic respectively, the signature of a field-induced ferromagnetic phase is obtained in the magnetization results in the intermediate temperature regime. While it was already known that TbFe2Al10 has a ferrimagnetic phase in between the low-field antiferromagnetic and the high-field ferromagnetic phases, the present results indicate the presence of a second intermediate-field-induced ferrimagnetic phase in the compound, in between the first ferrimagnetic and the high-field ferromagnetic phases. The possible magnetic structure for this second ferrimagnetic phase is proposed on the basis of existing neutron diffraction results. The successive field-induced or metamagnetic transitions in TbFe2Al10 are found to be induced by temperature as well, when the applied magnetic field is appropriate. The present magnetization results also indicate the presence of short-range magnetic correlations in TbFe2Al10 well inside the paramagnetic regime. Owing to the presence of successive temperature and field-induced magnetic phase transitions, TbFe2Al10 is found to exhibit a moderate magneto-caloric effect with a maximum of 7.86 J kg−1K−1 at 18.5 K. The magneto-caloric effect is found to persist well inside the paramagnetic regime because of the presence of short-range magnetic correlations at these temperatures. This leads to a substantial refrigerant capacity in the material, which could be useful information for future technology.

Giant magnetoresistance and field-induced phase transitions in Tb7Rh3 single crystal

High-field magnetization and magnetoresistance measurements have been performed to investigate the field-induced magnetic phase transitions in Tb7Rh3 single crystals. Metamagnetic transitions were observed along the a-, b- and c-axes of the hexagonal crystal lattice in the three magnetic states at temperatures below the Neel temperature TN = 91 K; magnetic field H — temperature T phase diagrams were constructed for b- and c-axes. At temperatures below TN, the magnetic structure transformations under application of a magnetic field along the b-axis are observed to be accompanied by the giant magnetoresistance effect (|Δρ/ρ| up to 65%). Magnetization curves measured at temperatures above TN show inflection points, which may be indicative of phase transitions within regions with an antiferromagnetic short-range order persisting in a paramagnetic matrix.

Magnetic properties and magnetocaloric effect of GdCr2Si2 compound under hydrostatic pressure

The magnetic and magnetocaloric properties of GdCr2Si2 have been studied under hydrostatic pressures up to 1.58 GPa. The temperature and field dependence of magnetization and magnetic entropy change (−ΔSM) show a weak pressure dependence behaviour. The observed large reversible magnetocaloric effect in GdCr2Si2 is related to a field-induced first order magnetic phase transition. For a field change of 5 T, the maximum −ΔSM are 14.1, 15.0, and 14.9 J/kg K under the pressures of 0, 0.69, and 1.58 GPa, respectively, the corresponding relative cooling power are evaluated to be 212, 223, and 226 J/kg.

Field-induced magnetic phase transition in Pr3+ doped Sm0.5Sr0.5MnO3 manganites

Magnetic properties of Pr3+ doped Sm0.5Sr0.5MnO3 manganite with the variation of particle size down to 30 nm have been investigated in detail. We have critically examined the magnetic field (H) dependence of the order of the ferromagnetic (FM) to paramagnetic (PM) phase transition in this Sm0.35Pr0.15Sr0.5MnO3 manganite. It has been shown that all the particle sizes (bulk to nano) exhibit first order FM → PM phase transition under low magnetic field accompanied by magnetization with thermal hysteresis in the field cooled cooling and warming cycle. However, the samples exhibit a second order magnetic phase transition above a critical field HCR. Again with decreasing the particle size, the ferromagnetic transition temperature, the thermal hysteresis width in the magnetizations, and the critical field HCR significantly decrease, which indicate that the ferromagnetism is weakened and the first-order magnetic phase transition is softened. We have also used the Banerjee criteria to distinguish the first-order magnetic phase transition from the second-order one. A detailed analysis of the magnetization measurements of this manganite reveal the disorder-induced softening of the first order phase transition in this phase separated manganite.

NMR study on field-induced charge anomaly in Cu3Mo2O9

A quasi-one-dimensional quantum spin antiferromagnet Cu3Mo2O9, which possesses the two spin degrees of freedom, the linear chain site and dimer-like site weakly interacting with one another, has recently been reported by permittivity and ESR to show a charge instability in its magnetically ordered state. In this article, we report the study on NMR and magnetization to show the existence of the field-induced magnetic phase transition accompanied by the charge anomaly at 8 T.

NMR study on field-induced charge anomaly in Cu3Mo2O9

A quasi-one-dimensional quantum spin antiferromagnet Cu3Mo2O9, which possesses the two spin degrees of freedom, the linear chain site and dimer-like site weakly interacting with one another, has recently been reported by permittivity and ESR to show a charge instability in its magnetically ordered state. In this article, we report the study on NMR and magnetization to show the existence of the field-induced magnetic phase transition accompanied by the charge anomaly at 8 T.

11B-NMR study on Shastry-Sutherland system TbB4

11B-NMR experiments were performed in high magnetic fields applied along a-axis up to 17.5T to investigate the field-induced magnetic phase transitions and the magnetic structure in high magnetic fields microscopically for the single crystalline TbB4, Shastry-Sutherland-type frustrated antiferromagnet. It was found that the field-swept 11B-NMR spectra observed at low magnetic field changes drastically at HC = 15.9 T, where the magnetization jump occurs. Based on a simple model of four-spin cluster and the classical dipole-dipole interaction, we have calculated NMR spectra, which qualitatively reproduced the observation below HC but the one above HC showed discrepancy.

XμCT analysis of magnetic field-induced phase transitions in magnetorheological elastomers

The phase behavior of polymer solutions and composites is a complex issue and is of both technological and fundamental interest. For a better understanding of the microstructure formation in magnetorheological (MR) elastomers, x-ray micro-computed tomography (XμCT) investigations were carried out. Magnetorheological elastomers with 5% mass content of iron powder were prepared under different magnetic field strengths between 1 and 220 kA m−1. Through quantitative analysis, valuable information was obtained regarding the number, size and frequency distribution of column structures in MR elastomers, as well as the magnetic field required to force structure formation.