Magnetic and magnetotransport properties of single-crystalline R_2PdGe_6(R = Pr,Gd and Tb)

Single crystals of R_2PdGe_6(R = Pr, Gd and Tb) compounds were grown by the Bi-flux method. Pr_2PdGe_6 is an antiferromagnetic compound with Néel temperature T_N= 15 K, in which a field-induced magnetic transition(spin flip) occurs when a magnetic field is applied along either a or b axis; a small magnetization and hysteresis loop were observed when a field is applied along c axis. Gd_2PdGe_6 is a collinear antiferromagnetic compound with T_N= 37 K along b axis. Tb_2PdGe_6 is an antiferromagnetic compound with T_N= 48 K and its hard magnetization direction is along b axis. The temperature dependences of the resistance of the entire three compounds present inflection points at the respective T_N. A large resistance(as well as magnetoresistance) change can be found at the spin flip transition of Pr_2PdGe_6, but the change is not obvious at the spin flop transition of Gd_2PdGe_6.     

Magnetic Structure and Metamagnetic Transitions in the van der Waals Antiferromagnet CrPS4

In 2D magnets, interlayer exchange coupling is generally weak due to the van der Waals layered structure but it still plays a vital role in stabilizing the long-range magnetic ordering and determining the magnetic properties. Using complementary neutron diffraction, magnetic, and torque measurements, the complete magnetic phase diagram of CrPS₄ crystals is determined. CrPS₄ shows an antiferromagnetic ground state (A-type) formed by out-of-plane ferromagnetic monolayers with interlayer antiferromagnetic coupling along the c axis below T_N = 38 K. Due to small magnetic anisotropy energy and weak interlayer coupling, the low-field metamagnetic transitions in CrPS_4, that is, a spin-flop transition at ≈0.7 T and a spin-flip transition from antiferromagnetic to ferromagnetic under a relatively low field of 8 T, can be realized for H∥c. Intriguingly, with an inherent in-plane lattice anisotropy, spin-flop-induced moment realignment in CrPS₄ for H∥c is parallel to the quasi-1D chains of CrS₆ octahedra. The peculiar metamagnetic transitions and in-plane anisotropy make few-layer CrPS₄ flakes a fascinating platform for studying 2D magnetism and for exploring prototype device applications in spintronics and optoelectronics.     

Structural and magnetic properties of LaFe0.5Cr0.5O3 studied by neutron diffraction, electron diffraction and magnetometry

The structural and magnetic properties of the perovskite type compound LaFe_0.5Cr_0.5O₃ have been studied by temperature dependent neutron powder diffraction and magnetization measurements. Rietveld refinement of the neutron diffraction data shows that the compound crystallizes in an orthorhombic perovskite structure with a random positioning of the Fe and Cr cations at the B sublattice. The magnetic structure at 10 K is a collinear antiferromagnetic one with the magnetic moment per site being equal to 2.79(4) μ_B. Magnetisation measurements confirm the overall antiferromagnetic behaviour. Moreover, it indicates a weak uncompensated magnetic moment close to the transition temperature T_N ≈ 265 K. This moment can be described by a magnetic cluster state, which remains up to 550 K. Electron diffraction patterns along with high-resolution transmission electron microscopy images reveal that the crystallites are composed by domains of different orientation, which share the same cubic perovskite sub-cell reflections.     

Pressure- and Field-induced Non-Fermi-liquid Behaviors in a Heavy-Fermion Antiferromagnet Ce7Ni3

We review recent investigations of the heavy-fermion antiferromagnet Ce₇Ni₃ with a geometrically frustrated structure under hydrostatic pressures and magnetic fields. Below T _N1=1.9 K, an incommensurate spin-density-wave (SDW) develops, and another antiferromagnetic transition occurs at T _N2 = 0.7 K. At a rather low critical pressure (P _c ), P _c = 0.39 GPa, both T _N1 and T _N2 vanish, and the specific heat divided by temperature (C/T) exhibits –lnT dependence (i.e., non-Fermi liquid behavior). At P = 0.43 GPa > P _c , a T-independent behavior of C/T, i.e., Fermi-liquid behavior, recovers below 0.2 K. However, the magnetic susceptibility continues to increase down to 0.09 K at 0.43 GPa, which is not consistent with a conventional Fermi-liquid theory. On the other hand, upon applying magnetic fields B along the hexagonal c axis, T _N1 decreases and vanishes at 0.3 T. Magnetoresistance, specific-heat, and magnetization measurements reveal that a field-induced magnetic (FIM) phase appears in a B–T region for B _{∥ c} > 0.7 T and T < 0.5 K. Neutron diffraction experiments indicate that the magnetic unit cell in the c plane for the FIM phase is treble that of the chemical unit cell. Moreover, the intensity of the magnetic reflection remains even in the region between the FIM phase and SDW phase. This observation indicates the presence of large spin fluctuations in the c-plane associated with the magnetic frustration, which should be responsible for the non-Fermi-liquid behavior of Ce₇Ni₃.     

Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr

The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compounds has attracted considerable interest in these materials for both fundamental research and technological applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temperatures, and poor charge transport properties. Here the magnetic and electronic properties of CrSBr are reported, an air-stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Néel temperature, T_N = 132 ± 1 K, CrSBr adopts an A-type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is Δ_E = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large negative magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin-based electronics.     

Sb Incorporation in Wurtzite and Zinc Blende InAs1−xSbx Branches on InAs Template Nanowires

The physical properties of material largely depend on their crystal structure. Nanowire growth is an important method for attaining metastable crystal structures in III–V semiconductors, giving access to advantageous electronic and surface properties. Antimonides are an exception, as growing metastable wurtzite structure has proven to be challenging. As a result, the properties of these materials remain unknown. One promising means of accessing wurtzite antimonides is to use a wurtzite template to facilitate their growth. Here, a template technique using branched nanowire growth for realizing wurtzite antimonide material is demonstrated. On wurtzite InAs trunks, InAs_1?xSb_x branch nanowires at different Sb vapor phase compositions are grown. For comparison, branches on zinc blende nanowire trunks are also grown under identical conditions. Studying the crystal structure and the material composition of the grown branches at different x_v shows that the Sb incorporation is higher in zinc blende than in wurtzite. Branches grown on wurtzite trunks are usually correlated with stacking defects in the trunk, leading to the emergence of a zinc blende segment of higher Sb content growing parallel to the wurtzite structure within a branch. However, the average amount of Sb incorporated within the branch is determined by the vapor phase composition.     

Low-Temperature Properties of CeTAl3 (T=Au, Cu, Pt) and CeAuGa3

Measurements of the magnetization, specific heat, and electrical resistivity on CeTAl ₃ (T=Au, Cu, Pt) and CeAuGa ₃ polycrystals are reported. For all alloys, magnetic order is found. In CeTAl ₃ the antiferromagnetic ordering temperature T_N increases from 1.5 K (T=Au) to 2.9 K (T=Cu) while spin-glass order is found at 0.8 K for T=Pt. This tendency resembles the nonmonotonic dependence of the ordering temperature on the hybridization between Ce moments and ligand atoms in terms of the Doniach picture. CeAuGa ₃ (T_N=2.4 K) behaves similarly to CeAuAl ₃ , thus underscoring the dominant role of transition metal ligands in ternary Ce compounds.     

Magnetic Properties of the Frustrated fcc – Antiferromagnet HoB12 Above and Below T N

We have investigated the magnetic ordering of the frustrated fcc – antiferromagnet HoB₁₂. Below T_N= 7.4 K antiferromagnetic order and a complex phase diagram is observed. Above T_N neutron scattering experiments show strong diffuse scattering. The diffuse signal indicates strong correlations between rare earth moments along the [111] direction well above T_N. The behavior of this component resembles low dimensional magnets which are known to show long range order only at T = 0. Close to T_N correlations perpendicular to the [111] direction get relevant, they diverge toward T_N. Thus we observe a complex ordering process where the frustration is lifted in steps. The experimental data and their interpretation are presented, some of the possible microscopic origins are discussed.      

Localized magnetism in YMn2 below the néel temperature

We have measured magnetic susceptibility, thermal expansion, electrical resistivity, and magnetoresistance of the intermetallic compound YMn₂. YMn₂ orders antiferromagnetically belowT _N 110 K. There is a large thermal hysteresis such that the ordering temperature isT _N =86 K on cooling andT _N =116 K on warming. This is a first-order phase transition from the paramagnetic to antiferromagnetic phase. Our electrical resistivity and magnetoresistance measurements in the ordered state show a localized magnetic moment for Mn atoms in YMn₂ belowT _N . The Mn moments collapse atT _N which is reflected in the sharp decrease in the thermal expansivity atT _N . AboveT _N the Mn moment gradually recovers with increasing temperature which is reflected as an increase in with increasing temperature in the paramagnetic state. This behavior atT>T _N could be described by weak itinerant electron magnetism.     

Strong and Tunable Electrical Anisotropy in Type‐II Weyl Semimetal Candidate WP2 with Broken Inversion Symmetry

A transition metal diphosphide, WP₂, is a candidate for type‐II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP₂, spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP₂ electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle‐resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first‐principle studies of WP₂ are reported. The energies of the observed Raman‐active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first‐principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP₂. Moreover, the measured ratio (R_c /R_a ) between the crystalline c‐axis and a‐axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in‐plane electrical resistivity anisotropy among the reported type‐II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first‐principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the R_c /R_a of WP₂ at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so‐far‐strongest and magnetic‐field‐tunable electrical resistivity anisotropy found in WP₂ can serve as a degree of freedom for tuning the electrical properties of type‐II WSMs, which paves the way for the development of novel electronic applications based on type‐II WSMs.     

Room temperature magnetism in Ni-doped CdSe nanoparticles

Room temperature ferro-magnetism in Ni-doped CdSe nanoparticles (NPs) synthesized by a wet chemical precipitation method is reported. Transmission electron microscopy shows that the average particle size of Ni-doped CdSe NPs is about 8 nm. X-ray diffraction shows the zinc blende (cubic) structure of Cd_1-x Ni _x Se NPs. Superconducting quantum interference device was utilized to study the magnetic behavior of NPs. Magnetic studies revealed that pure CdSe NPs exhibit diamagnetic behavior at 300 K, whereas 5% Ni doped CdSe NPs shows the mixture of paramagnetic and ferromagnetic behavior.     

EPR and optical absorption studies of Cr3+ ions in alkaline earth alumino borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cr³⁺ ions in Calcium alumino borate (CaAB) glasses have been studied. The EPR spectra exhibit weak resonance signal at g ≈ 4.50 and intense resonance signal at g ≈ 1.98. A sharp resonance signal at g ≈ 1.97 was also observed at lower concentrations of chromium. The concentration dependence of the linewidth of the resonance signal at g ≈ 1.98 suggests the formation of Cr³⁺ ion clusters by magnetic superexchange interactions. The temperature dependence of the peak to peak intensity and the linewidth of the resonance signal at g ≈ 1.98 suggests that the exchange interactions between Cr³⁺ ions in the present sample were antiferromagnetic in nature with Néel temperature, T _N = 233 K. From the number of spins participating in the resonance at g ≈ 1.98, the paramagnetic susceptibility (χ) was calculated at different temperatures (233–295 K). A plot of 1/χ and T was found to obey Curie-Weiss law with negative Curie temperature. By measuring the relative intensities of the resonance signal at g ≈ 1.98, at different temperatures, the value of antiferromagnetic coupling constant (J) has been estimated. The optical absorption spectrum of chromium doped CaAB glass exhibits four bands, characteristic of Cr³⁺ ions, in nearly octahedral symmetry. From the band positions, the crystal field splitting parameter, Dq and the Racah interelectronic repulsion parameters, B and C were evaluated. The optical band gap (E_opt) and the Urbach energy (ΔE) were calculated from the ultraviolet absorption edges.     

Record‐High Superconductivity in Niobium–Titanium Alloy

The extraordinary superconductivity has been observed in a pressurized commercial niobium–titanium alloy. Its zero‐resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record‐high pressure up to which a known superconducting state can continuously survive. Remarkably, at such an ultra‐high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (T_C) increases to ≈19.1 K from ≈9.6 K, and the critical magnetic field (H_C2) at 1.8 K has been enhanced to 19 T from 15.4 T. These results set new records for both the T_C and the H_C2 among all the known alloy superconductors composed of only transition metal elements. The remarkable high‐pressure superconducting properties observed in the niobium–titanium alloy not only expand the knowledge on this important commercial superconductor but also are helpful for a better understanding on the superconducting mechanism.     

Crystal chemistry and magnetic properties of CrIIBIIIF5 compounds

Three new compounds isostructural with Cr₂F₅ have been prepared: CrAlF₅, CrTiF₅, CrVF₅. Their magnetic properties have been investigated: CrAlF₅ is paramagnetic above 4.2 K with a Weiss constant θ = ?3K, CrTiF₅ and CrVF₅ are ferrimagnetic with T_C = 26 K and 40 K, Cr₂F₅ is antiferromagnetic with T_N = 40 K. From the structure and the rules of super-exchange, the magnetic order and spin orientations below the magnetic-ordering temperature are predicted to exhibit weak, antiferromagnetic spin canting in Cr₂F₅ and strong ferromagnetic canting in CrTiF₅ and CrVF₅ where interatomic-exchange interactions are competitive.     

EuFe2As2: Magnetic Structure and Local Charge Distribution Anisotropies as Seen by Resonant X-ray Scattering

Non-resonant and element specific magnetic X-ray scattering has been used to determine the orientation of Eu and Fe magnetic moments in EuFe₂As₂ iron pnictide. Experiments have been carried out on single crystal samples at the ESRF. Resonant measurements on magnetic reflections at the Eu L₃ absorption edge indicate that the orientation of the Eu moments in the antiferromagnetic phase (T<T _N=19 K) lie parallel to the crystallographic a-axis. In addition, non-resonant magnetic X-ray measurements indicate that the Fe moments are aligned along the same direction in the spin density-wave ordered phase (T<T _S=190 K). The temperature dependence of the integrated intensities suggests that the Fe magnetic sublattice is barely affected by the onset of Eu ordering at T _N. The observation of non-zero resonant intensity on nuclear-forbidden reflections with wavevector corresponding to the Fe magnetic propagation vector at both the Eu L₃ and As K absorption edges may be interpreted as the result of the polarization of the Eu 5d and As 4p electronic bands via hybridization with the Fe 3d states.     

Chemical substitution of Cd ions by Hg in CdSe nanorods and nanodots: Spectroscopic and structural examination

The chemical substitution of cadmium by mercury in colloidal CdSe quantum dots (QDs) and nanorods has been examined by absorption, photoluminescence and Raman spectroscopy. The crystalline structure of original CdSe QDs used for Cd/Hg substitution (zinc blende versus wurtzite) shows a strong impact on the optical and structural properties of resultant Cd_xHg_1−xSe nanocrystals. Substitution of Cd by Hg in isostructural zinc blende CdSe QDs converts them to ternary Cd_xHg_1−xSe zinc blende nanocrystals with significant NIR emission. Whereas, the wurtzite CdSe QDs transformed first to ternary nanocrystals with almost no emission followed by slow structural reorganization to a NIR-emitting zinc blende Cd_xHg_1−xSe QDs. CdSe nanorods with intrinsic wurtzite structure show unexpectedly intense NIR emission even at early Cd/Hg substitution stage with PL active zinc blende Cd_xHg_1−xSe regions.     

Magnetic ordering and Kondo compensation in the ternary heavy-fermion compound CeCu5Au

The new ternary compound CeCu₅Au is derived from the heavy-fermion system CeCu₆ by replacing the Cu(2) site by Au. Measurements of the dc magnetization, the specific heat, the magnetocaloric effect, and the electrical resistivity on Czochralski-grown single crystals reveal a complex (B, T) phase diagram with probably two antiferromagnetic phases and one spin-flop phase. The Néel temperature was determined toT _n = 2.3 K. The magneto-resistance exhibits several features which can partly be explained by the different antiferromagnetic phases. In addition to the antiferromagnetic order the observation of a large specific heat at very low temperatures ( = C/T = 0.64 J/molK² at T = 0.1 K) indicates the coexistence of antiferro-magnetism and Kondo compensation. An analysis of the specific heat using the resonance-level model yields a Kondo temperature ofT _k = 1.8 K. The analysis for lower fields is complicated by the presence of magnetic order. In terms of a homogeneous model,T _k = 1.0 K forB = 0 is inferred. The increase ofT _k with field and also the concentration dependence ofT _K can be qualitatively explained.     

High Field and Low Temperature Magnetization of ErCu2Si2 Single Crystal

The magnetic measurements have been performed for high fields up to 18 T and low temperatures down to 0.5 K on the single crystal compound ErCu₂Si₂. In the temperature dependence of magnetic susceptibility, two anomalies are seen at T _N =1.51 K and T _t =0.97 K which is corresponding to an antiferromagnetic ordering temperature and may come from a change of magnetic structure. Low temperature magnetization shows a metamagnetic transition around 0.4 T along the easy magnetization direction of [001]. The magnetization is almost saturated above 3 T and reaches 8.8μ _B /f.u. at 18 T. A metamagnetic transition appears around 0.5 T in the basal plane magnetization processes as well. The anisotropy within the basal plane is fairly large for high fields. These behaviors have been explained from crystalline electric field effects and magnetic interaction in terms of molecular field.     

Characterization of monoclinic Fe2TiO5 by Mössbauer spectroscopy and magnetic susceptibility

Mössbauer spectra and EFG calculations have shown that iron spins in monoclinic Fe₂TiO₅ are directed along the b axis. Below Neel temperature (T_N = 342 K) an anti-ferromagnetic ordering with weak ferromagnetism has been predicted. The ordering temperature was represented with the anomaly in the magnetic susceptibility and with the changes in Mössbauer spectra.     

Magneto-Acoustic Properties of UCuGe Single Crystal

We report on results of sound velocity and sound-attenuation measurements performed on the antiferromagnetic (T _N =48 K) UCuGe. The measurements have been done on a UCuGe single crystal at different frequencies for longitudinal ultrasound waves propagating along the [001] direction in static (up to 18 T) and pulsed (up to 60 T) magnetic fields applied along the same direction. The temperature dependences of the sound velocity and attenuation display a pronounced anomaly at T _N , which is evidence for a strong magneto-elastic interaction. The pulse-field measurements at 4.2 K show a minimum in the sound velocity followed by a jump-like anomaly at 37 T, and another kink-like anomaly at 48–49 T. These anomalies are due to field-induced spin rearrangements as measured in magnetization studies. In the paramagnetic state (T>T _N ), both acoustic characteristics show large frequency-dependent changes revealing the presence of an unusual relaxation mechanism which might be due to vacancy dynamics.