First-order Antiferromagnetic Transition Research Articles

First-order antiferromagnetic transitions of SrMn2P2 and CaMn2P2 single crystals containing corrugated-honeycomb Mn sublattices

SrMn2P2 and CaMn2P2 are insulators that adopt the trigonal CaAl2Si2-type structure containing corrugated Mn honeycomb layers. Magnetic susceptibility χ and heat capacity versus temperature T data reveal a weak first-order antiferromagnetic (AFM) transition at the Néel temperature [Formula: see text] K for SrMn2P2 and a strong first-order AFM transition at [Formula: see text] K for CaMn2P2 Both compounds exhibit isotropic and nearly T-independent [Formula: see text], suggesting magnetic structures in which nearest-neighbor moments are aligned at [Formula: see text] to each other. The 31P NMR measurements confirm the strong first-order transition in CaMn2P2 but show critical slowing down above [Formula: see text] for SrMn2P2, thus also evidencing second-order character. The 31P NMR measurements indicate that the AFM structure of CaMn2P2 is commensurate with the lattice whereas that of SrMn2P2 is incommensurate. These first-order AFM transitions are unique among the class of (Ca, Sr, Ba)Mn2 (P, As, Sb, Bi)2 compounds that otherwise exhibit second-order AFM transitions. This result challenges our understanding of the circumstances under which first-order AFM transitions occur.

Effect of Ga and Cd substitutions on the first-order antiferromagnetic transition in NdCo2Zn20

The cubic neodymium-based compound ${\mathrm{NdCo}}_{2}{\mathrm{Zn}}_{20}$ exhibits a first-order antiferromagnetic (AFM) transition at ${T}_{\text{N}}=0.53$ K. Strong magnetic fluctuations at temperatures up to 5 K were suggested by the downward curvature of the electrical resistivity $\ensuremath{\rho}(T)$ and the reduced magnetic entropy ${S}_{\text{m}}$ of $0.5R\mathrm{ln}2$ at ${T}_{\text{N}}$. In this study, we measured $\ensuremath{\rho}(T)$, the isothermal magnetization $M(B)$, magnetic susceptibility $\ensuremath{\chi}(T)$, and specific heat $C(T)$ of ${\mathrm{NdCo}}_{2}{\mathrm{Zn}}_{20\ensuremath{-}x}{\mathrm{Cd}}_{x}$ for $x=1$ and ${\mathrm{NdCo}}_{2}{\mathrm{Zn}}_{20\ensuremath{-}y}{\mathrm{Ga}}_{y}$ for $y=1$ and 2. The sharp peak of the magnetic specific heat ${C}_{\text{m}}(T)$ at ${T}_{\text{N}}$ for $x$ = 0 is changed to a weak and broad maximum at 0.55 K for $x$ = 1. This drastic change in ${C}_{\text{m}}(T)$ suggests that the isovalent Cd substitution for Zn disorders the exchange interactions between the Nd moments to hinder the first-order AFM transition. On the other hand, ${C}_{\text{m}}(T)$ for $y=1$ and 2 exhibits a lambda-shaped anomaly, which is a characteristic of a second-order AFM transition, at elevated temperatures of 0.78 and 1.5 K, respectively. The stabilization of the AFM order by the Ga substitution indicates that $4p$ electron doping strengthens the AFM interaction. We therefore propose that the first-order transition in ${\mathrm{NdCo}}_{2}{\mathrm{Zn}}_{20}$ is maintained by competitive magnetic interactions inherent in the Nd diamond sublattice.

Valence transition in topological Kondo insulator

We investigate the valence transition in three-dimensional topological Kondo insulator through slave-boson analysis of periodic Anderson model. By including the effect of intra-atomic Coulomb correlation between conduction and local electrons, we find a first-order valence transition from Kondo region to mixed valence state upon ascending of local f- level above a critical , and this valence transition usually occurs very close to or simultaneously with a topological transition. Near the parameter region of zero-temperature valence transition, rise of temperature can generate a thermal valence transition from mixed valence to Kondo region, accompanied by a first-order topological transition. Remarkably, above a critical which is considerably smaller than that generating paramagnetic valence transition, the original continuous antiferromagnetic transition is shifted to first order one, at which a discontinuous valence shift takes place. Upon increasing , the paramagnetic valence transition approaches then converges with the first-order antiferromagnetic transition, leaving a significant valence shift on the magnetic boundary. The continuous antiferromagnetic transition, first-order antiferromagnetic transition, paramagnetic valence transition and topological transitions are all summarized in a global phase diagram. Our proposed exotic transition processes can help to understand the thermal valence variation as well as the valence shift around the pressure-induced magnetic transition in topological Kondo insulator candidates and in other heavy-fermion systems.

Magnetic field-induced switching of magnetic ordering in SrFeO3−δ

The oxygen-deficient strontium iron oxide SrFeO3−δ (SFO) exhibits richness in the phase diagram over a broad range of temperatures and for other external parameters. Room-temperature X-ray diffraction and Raman spectrum reveals that the structure of synthesized SFO system consists of two mixed phases, i.e., major orthorhombic and minor tetragonal phases. The low-temperature Raman and vibrating sample magnetometer measurements indicated a structural transition below 253 K. The magnetic property of the synthesized SFO for various external magnetic field (up to 5 T) reveals possible variation in oxygen stoichiometry. Also, the application of external H increases Neel transition temperature (T N), suppresses the hysteresis width (W H), and thus weakens the first-order nature of the transition. Our analysis revealed the vanishing of hysteresis and the first-order antiferromagnetic transition becomes a crossover above a critical magnetic field H CR ≈ 5 T. Possible switching of magnetic ordering and oxidation state observed in same system enhances interest in related compounds which may be used in magnetic sensors and other magnetic switching devices.

Superconductivity in the vicinity of antiferromagnetic order in CrAs.

One of the common features of unconventional superconducting systems such as the heavy-fermion, high transition-temperature cuprate and iron-pnictide superconductors is that the superconductivity emerges in the vicinity of long-range antiferromagnetically ordered state. In addition to doping charge carriers, the application of external pressure is an effective and clean approach to induce unconventional superconductivity near a magnetic quantum critical point. Here we report on the discovery of superconductivity on the verge of antiferromagnetic order in CrAs via the application of external pressure. Bulk superconductivity with Tc≈2 K emerges at the critical pressure Pc≈8 kbar, where the first-order antiferromagnetic transition at T(N)≈265 K under ambient pressure is completely suppressed. The close proximity of superconductivity to an antiferromagnetic order suggests an unconventional pairing mechanism for CrAs. The present finding opens a new avenue for searching novel superconductors in the Cr and other transition metal-based systems.

Y and Gd Substitution Effects on Magnetic Properties of SmPd3S4and TbPd3S4

Measurements of magnetization were performed on polycrystalline samples of Sm 1- x T x Pd 3 S 4 and Tb 1- x T x Pd 3 S 4 (T = Y and Gd) to examine the origin of first-order-like antiferromagnetic (AFM) transition in SmPd 3 S 4 and TbPd 3 S 4 . With increasing Y (Gd) concentration, the AFM transition temperatures for both Sm 1- x T x Pd 3 S 4 and Tb 1- x T x Pd 3 S 4 decrease (increases) monotonically. First-order AFM transitions observed in SmPd 3 S 4 and TbPd 3 S 4 change into second-order ones by Gd substitution. Such a change in the order of transition implies that a contribution of magnetic interaction to the AFM transition overcomes that of multipolar interaction in Gd-rich compounds.

Low-temperature antiferromagnetic ordering in the heavy-fermion metal YbPd

We investigate the low-temperature properties of YbPd by specific heat and thermal expansion measurements down to 0.1 K. This material crystallizes in a cubic CsCl-structure and four phase transitions at 125, 105, 1.9 and 0.5 K have been reported previously. The two transitions at higher temperature were suspected to be of structural origin, whereas the two low-T transitions are magnetic, confirmed by susceptibility measurements and Mössbauer spectroscopy. Our low temperature specific heat and thermal expansion data prove antiferromagnetic ordering at TN=1.9 K and an additional first-order antiferromagnetic transition at 0.6 K. The entropy reaches approximately half of Rln2 at 5 K, confirming a doublet ground state. The enhanced value of the Sommerfeld coefficient at 0.1 K suggests a classification of YbPd as a magnetically ordered heavy-fermion metal.

Magnetic properties of terbium dihydrides

Magnetization measurements of terbium dihydride samples with different hydrogen concentration, TbH 1.92, TbH 1.99, TbH 2.03 and TbH 2.09, in the temperature range from 1.8 to 300 K and at an applied field of 1 kOe are reported. Two first-order antiferromagnetic transitions have been registered in the narrow temperature range between 15–18 K for all hydride compositions. Isothermal magnetization ( σ ( H ) T = const ) measurements at several temperatures in the vicinity of magnetic transitions have been carried out and used for the magnetic-entropy change, Δ S σ , calculations. The magnetic-entropy changes have negative values, and in an applied magnetic field of 5 T they reach 2.5 J/kg K, which is only about 26% of the Δ S σ ( H ) value expected from thermal measurements. A much larger magnetic field must be applied for terbium hydrides to be used as refrigerant materials for low-temperature needs.

Critical examination of heat capacity measurements made on a Quantum Design physical property measurement system

We examine the operation and performance of an automated heat-capacity measurement system manufactured by Quantum Design (QD). QD’s physical properties measurement system (PPMS) employs a thermal-relaxation calorimeter that operates in the temperature range of 1.8–395 K. The accuracy of the PPMS specific-heat data is determined here by comparing data measured on copper and synthetic sapphire samples with standard literature values. The system exhibits an overall accuracy of better than 1% for temperatures between 100 and 300 K, while the accuracy diminishes at lower temperatures. These data confirm that the system operates within the ±5% accuracy specified by QD. Measurements on gold samples with masses of 4.5 and 88 mg indicate that accuracy of ±3% or better can be achieved below 4 K by using samples with heat capacities that are half or greater than the calorimeter addenda heat capacity. The ability of a PPMS calorimeter to accurately measure sharp features in C p ( T) near phase transitions is determined by measuring the specific heat in the vicinity of the first-order antiferromagnetic transition in Sm 2IrIn 8 ( T 0=14 K) and the second-order hidden order (HO) transition in URu 2Si 2 ( T N =17 K). While the PPMS measures C p ( T) near the second-order transition accurately, it is unable to do so in the vicinity of the first-order transition. We show that the specific heat near a first-order transition can be determined from the PPMS-measured decay curves by using an alternate analytical approach. This correction is required because the latent heat liberated/absorbed at the transition results in temperature–decay curves that cannot be described by a single relaxation time constant. Lastly, we test the ability of the PPMS to measure the specific heat of Mg 11B 2, a superconductor of current interest to many research groups, that has an unusually strong field-dependent specific heat in the mixed state. At the critical temperature the discontinuity in the specific heat is nearly 15% lower than measurements made on the same sample using a semi-adiabatic calorimeter at Lawrence Berkeley National Laboratory.

Anisotropy in magnetic and transport properties of SmTSb 3 (T=Cr, V)

SmCrSb 3 has a highly anisotropic crystal structure (space group Pbcm) which results in a very complex temperature vs. magnetic field phase diagram. We report the first measurements of the anisotropy in magnetic and transport properties, as well as a newly discovered first-order antiferromagnetic (AFM) phase transition at T N=30 K. SmVSb 3 is presented due to lack of itinerant 3d-electrons. Magnetic susceptibility and magnetization measurements for fields up to 55 kG and in the temperature range of 2–350 K show that at T C=109 K, SmCrSb 3 undergoes an itinerant ferro-magnetic transition due to the 3d Cr ions. Electrical resistivity measurements in the range of 5–295 K show that within the b– c plane, SmCrSb 3 exhibits metallic behavior, while the a-axis has a change in the sign of the slope at the Curie temperature. The first-order AFM transition has also been investigated with the magnetization as a function of field up to 31 T, and by magnetoresistance up to a field of 6 T, showing that the magnetic anisotropy is never completely suppressed in this region.

Magnetic effects in Cr-Mn alloy single crystals containing 0.05 and 0.1 at. % Mn

Measurements of the thermal expansion, ultrasonic wave velocity, and ultrasonic attenuation are reported for very dilute Cr-Mn alloy single crystals. The threshold Mn concentration needed to drive the first-order antiferromagnetic transition in Cr to second order is found to be smaller than 0.05 at. % Mn. This result is discussed in terms of recent theories. The critical behavior of the sound velocity and ultrasonic attenuation of Cr+0.1 at. % Mn is well described by a power law near TN but not for the thermal expansion.

Specific heat and magnetic susceptibility of U1-xThxNiSn.

The specific-heat C and magnetic susceptibility \ensuremath{\chi} of ${\mathrm{U}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Th}}_{\mathit{x}}$NiSn were measured down to 1.5 K. Pure UNiSn (x=0) shows an antiferromagnetic (AF) transition accompanied by a semiconductor-metal transition. The magnetic entropy ${\mathit{S}}_{\mathit{m}}$ for UNiSn abruptly increases at ${\mathit{T}}_{\mathit{N}}$ suggesting a first-order phase transition. In addition to an enhancement of the density of states at the Fermi level associated with the metallic phase, the low-temperature specific heat suggests a magnon excitation in the AF state. In the paramagnetic state, the x-independent ${\mathit{S}}_{\mathit{m}}$(T) and ${\mathrm{\ensuremath{\chi}}}_{\mathit{m}}$(T) are explained by a model based on the localized 5f electrons in a crystalline electric field. Assuming the ${\mathrm{U}}^{4+}$ ion has a 5${\mathit{f}}^{2}$ configuration, we determined the energy scheme of ${\mathrm{U}}^{4+}$, where the ground state is a nonmagnetic ${\mathrm{\ensuremath{\Gamma}}}_{3}$ doublet. The first-order AF transition, reduction of the ordered moment, and enhancement of \ensuremath{\chi} in the AF state are discussed.

Neutron diffraction study of the Néel transition in Cr(0.5% V)

While pure chromium undergoes a first-order antiferromagnetic transition, thermal expansion measurements have indicated that the addition of small amounts of vanadium and molybdenum make the Néel transition continuous. A neutron diffraction study of a single crystal of Cr(0.5% V) showed no discontinuity in magnetic Bragg satellite intensity at the Néel temperature, TN=254 K, that would indicate a first-order transition. Complicated behavior of the satellite intensities near the transition temperature is attributed to inhomogeneous strain in the sample. The satellite intensity showed a linear temperature dependence over a range of at least 100 K below TN.

Magnetic properties of UT(Si, Ge) series

Numerous equiatomic uranium—transition metal ternary compounds of general formula UT(Si, Ge) have been identified in the orthorhombic CeCu 2-type structure. The magnetic characteristics of these compounds with T = Co, Ni, Ru, Rh, Pd, Ir and Pt have been studied in the range 4.2–300 K. Some systematic dependences in their magnetic behaviour have been revealed. Paramagnetism was observed for UCo(Si, Ge) down to liquid helium temperature. Typical spin-fluctuation susceptibility variation at low temperatures has been found for URu(Si, Ge) and UIrSi. Magnetic ordering was observed for all remaining compounds studied. UNi(Si, Ge) probably show first-order antiferromagnetic transitions, URh(Si, Ge) are itinerant-electron ferromagnets, and UPd(Si, Ge) and UPt(Si, Ge) exhibit some magnetic transitions at low temperatures and behave as local-moment compounds at higher temperature range. The observed magnetic characteristics of the series UT(Si, Ge) reflect the 5f origin of magnetic moment, however more or less influenced by the hybridization between the U-5f and ligand electrons.

Melting pressure and the phase diagram of magnetically ordered solid 3He.

From high-resolution measurements of the melting pressure of solid $^{3}\mathrm{He}$ in applied fields, we determine the entropy and order of the magnetic phase transitions. We find the entropy discontinuity at the first-order antiferromagnetic transition to decrease as the field increases. No entropy discontinuity is seen at the paramagnetic to high-field transition, indicating that it is not first order.

Smeared antiferromagnetic phase transitions and spin-glass properties in EuxSr1-xSe.

We report specific-heat and susceptibility measurements on ${\mathrm{Eu}}_{\mathrm{x}}$${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Se samples in the concentration range 0.5\ensuremath{\ge}x\ensuremath{\le}1.0. For x\ensuremath{\ge}0.85, smeared first-order antiferromagnetic transitions are observed; for x\ensuremath{\le}0.7, the system shows typical spin-glass properties. In the intermediate concentration range the phenomena characteristic for antiferromagnetic and spin-glass ordering coexist.

Stress-induced changes in the paramagnetic susceptibility and antiferromagnetic phase transition of DySb

The effects of [100] uniaxial compressive stress on the magnetic properties of DySb have been investigated for stress levels up to 420 MPa and temperatures between 4 and 65 K. Magnetic susceptibility measurements in low fields using a SQUID magnetometer show a reduction in the discontinuity at the first-order antiferromagnetic transition when stress is applied. Compression also shifts the transition upward in temperature at an initial rate of (1.16±0.07)×10−2 K/MPa. At a critical stress of 280±30 MPa the transition becomes second order. In the paramagnetic phase an increase in susceptibility is observed that becomes less pronounced at high temperatures. This result is in good agreement with the predictions of a mean-field theory that includes Heisenberg dipolar exchange and quadrupolar pair interactions.

Theory of the magnetic ordering and lattice internal rearrangement transition in UO2

The first-order antiferromagnetic transition in fluorite structure UO/sub 2/ coincides with a lattice transition to a transverse internal rearrangement (TIR). In the TIR, the oxygen planes have a periodic internal shear relative to the undistorted fcc uranium sublattice. We have investigated the selection mechanism that causes the TIR to occur in preference to other distortions. The driving mechanism that causes some distortion to occur is the splitting of the GAMMA/sub 5/ cubic crystal-field ground state and consequent lowering of crystal-field energy. Our detailed calculations, involving consideration of a number of ''candidate'' distortional modes, verify that the TIR occurs because the lattice is relatively soft for such a distortion. This elastic energy advantage more than makes up for the advantage of a competing conventional (homogeneous) internal distortion in lowering the crystal-field energy more for a given size relative distortion. Our calculations incorporate experimental elastic effects by using parameters from a rigid-ion model fit to the phonon behavior. We find a low-temperature oxygen displacement of approximately 0.021 A in close agreement with the experimental 0.014 A.

First-Order Phase Transition and Critical Neutron Scattering in Chromium

The critical behavior of Cr near the first-order antiferromagnetic transition at 38.5\ifmmode^\circ\else\textdegree\fi{}C has been studied by neutron scattering. This study reveals the existence of a sharp central peak at the satellite position in a narrow temperature range just above ${T}_{\mathrm{N}}$. It exhibits a very singular temperature dependence such as the two-dimensional planar model predicts. Such unusual behavior supports the conjecture that the two-dimensional planar model is realized in the critical region of Cr.

First-order antiferromagnetic transition in CrN

The antiferromagnetic structure of CrN is known to belong to so-called IV-type cubic antiferromagnets. We discuss transformation properties of order parameter and have found that it belongs to a 12-dimensional irreducible representation of $\mathrm{Fm}3m$ group. We construct corresponding Landau-Ginzburg model and study phase transition by examining fixed points of renormalization-group transformations to second order in $\ensuremath{\epsilon}$ expansion. The renormalization-group equations have no stable fixed points and therefore transition should be of first order, which is in agreement with experiment. Other materials of same symmetry are reviewed and some new physical systems are added to list of the physical realizations of $n\ensuremath{\ge}4$ component vector models.