Single Crystals Of Cr Research Articles

Optical properties and passive self Q-switching of floating zone grown Cr–Nd co-doped YVO4 crystal

Single crystals of Cr (0.7, 1.0, 1.4 and 2.0 at%) and Nd (0.7 at%) co-doped yttrium vanadate (YVO4) were grown by optical floating zone method. [100]-oriented elements were fabricated from the grown crystals for optical and lasing studies. The refractive index analysis revealed that the values of ordinary refractive indices (no) for 0.7, 1.0, 1.4 and 2.0 at% Cr doped samples were found to be almost constant except for a slight increase corresponding to 532 nm. But the extraordinary refractive indices (ne) was found to increase with Cr doping concentration and it is maximum at 1.4 at% for all the wavelengths. The thermal coefficient of ne and no increases slightly and becomes constant beynd 1.4 at% of Cr. The band gap of the crystal is polarization sensitive with the values in the range of 3.51–3.55 eV and 3.59–3.62 eV for electric field parallel and perpendicular to c axis, respectively. The band gap does not exhibit any significant change on Cr doping. The emission spectrum with excitation at 808 nm is characterized by three broad asymmetric bands centered at 896, 1064 and 1341 nm. The emission intensity for the peak around 1064 nm decreases with an increase in Cr due to a broad absorption around 1100 nm of Cr5+ ion and, therefore, this band is exploited for passive Q-switching in the crystal. Further, excitation at 400 nm of Cr band revealed that there is strong emission at 1064 nm, which may be explored for pumping the gain medium with solar radiation. Finally, a stable passive Q-switching at 1064 nm was demonstrated in [100] element fabricated from the gown crystal having 1.4 at% Cr. An output power of ∼108 mW having pulse-width ∼220 ns and repetition rate ∼239 kHz was achieved for a pumping of ∼14.6 W at 808 nm.

Strong uniaxial pressure dependencies evidencing spin-lattice coupling and spin fluctuations in Cr2Ge2Te6

Single crystals of Cr$_2$Ge$_2$Te$_6$ were studied by high-resolution capacitance dilatometry to obtain in-plane ($B\parallel ab$) and out-of-plane ($B\parallel c$) thermal expansion and magnetostriction at temperatures between 2 and 300 K and in magnetic fields up to 15 T. The anomalies in both response functions lead to the 'magnetoelastic' phase diagrams and separate the paramagnetic (PM), ferromagnetic low-temperature/low-field (LTF) and aligned ferromagnetic (FM) phases. The presence of two distinct thermal expansion anomalies at small fields $B\parallel ab$ of different magnetic field dependence clearly supports the scenario of an intermediate region separating PM and LTF phases and is indicative of a tricritical point. Simulations of the magnetostriction using the Stoner-Wohlfarth model for uniaxial anisotropy demonstrate that the observed quadratic-in-field behavior in the LTF phase is in line with a rotation of the spins from the preferred $c$ direction into the $ab$ plane. Both the LTF and the PM phase close to T$_{\rm C}$ exhibit very strong pressure dependencies of the magnetization, ${\partial}\ln{M_{\rm ab}}/{\partial}p_{\rm ab}$, of several hundred %/GPa and also the transition from the LTF to the FM phase strongly depends on $p_{\rm ab}$ ($\sim -$280%/GPa), indicating a strong decrease of the uniaxial anisotropy under applied in-plane pressure. Our data clearly demonstrate the relevance of critical fluctuations and magnetoelastic coupling in Cr$_2$Ge$_2$Te$_6$.

Fe and Cr co-intercalation in 2H-NbS2 single crystals for realization of perpendicular magnetic anisotropy and large anomalous Hall effect

Recently, many interesting magnetic and electrical properties have been demonstrated in transition-metal intercalated $2\text{H-Nb}{\mathrm{S}}_{2}/\mathrm{Ta}{\mathrm{S}}_{2}$ layered single crystals. In this work, single crystals of Cr and Fe co-intercalated $2\text{H-Nb}{\mathrm{S}}_{2}$, with the compositions of ${({\mathrm{Cr}}_{1--x}{\mathrm{Fe}}_{x})}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2} (x=0.0--1.0)$, are successfully grown and their magnetic and transport properties are systematically studied. With increasing Fe content, the magnetic transition temperature continually decreases from 117 to 37 K, and the magnetic coupling transforms from ferromagnetic to antiferromagnetic. Importantly, a rotation of the magnetic easy axis occurs in this process, with perpendicular magnetocrystalline anisotropy and considerable coercive field achieved in the $x=0.13--0.66$ compounds. Both $x=0.33$ and $x=0.50$ exhibit relatively low carrier concentration, stable anomalous Hall effect, and butterfly-shape magnetoresistance. Notably, large anomalous Hall conductivity is identified in $x=0.50$, whose origin is discussed from the calculated electronic structures. The realization of perpendicular anisotropy and large anomalous Hall conductivity in this two-dimensional ferromagnetic system can facilitate their future application in spintronic devices.

Cr-doped Ca3NbGa3Si2O14: A promising near-infrared tunable laser crystal

Broad tunable solid-state lasers in the near-infrared region are urgently demanded due to their critical applications in optical communications, eye-safe imaging, medicine and spectroscopy, and Cr4+-based lasers exhibit good application prospect in this area. Herein, we reported a new near-infrared tunable laser crystal of Cr-doped Ca3NbGa3Si2O14 (Cr:CNGS). By employing the Czochralski method, single crystal of Cr:CNGS with dimensions of Φ 20 × 30 mm3 was grown. Its crystal structure, spectral properties were investigated in detail., Cr:CNGS exhibits attractive properties, including broad emission band at 800–1400 nm, long fluorescence lifetime (18 μs at 298 K) and good adaptability to LD pumping source, all of these characteristics enable it to be a potential laser crystal for the near-infrared tunable lasers. Notably, we discovered that the multi-sites in CNGS crystal make Cr ions exist in different valence states simultaneously, including Cr3+, Cr4+ and Cr5+, which is also partly responsible for its broadband emission.

Debye temperature of single-crystal Cr with incommensurate and commensurate magnetic structure

Single crystals of Cr and Cr3%Mn doped with ∼0.2 at.% 119Sn were studied by means of 119Sn-site Mössbauer spectroscopy. Mössbauer spectra recorded in the temperature range of 78–300 K were analyzed using a hyperfine field distribution method. Additionally, the spectra recorded on the Cr3%Mn sample were analyzed in terms of three sextets. Derived therefrom values of a center shift, CS, were fitted to the Debye model yielded values of the Debye temperature, for Cr and for Cr3%Mn. The difference in the -value can be regarded as evidence that a spin-phonon coupling is different for the incommensurate and commensurate antiferromagnetic magnetic structure of chromium.

EPR Studies of the Order-Disorder Phase Transition in DMAAS Crystal

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr 3+ -doped dimethylammonium aluminium sulphate hexahydrate (DMAAS) are presented from 100 K to room temperature. The crystal undergoes a phase transition at 152 K from ferroelastic to ferroelectric phase. The spin Hamiltonian parameters have been determined for both phases. The spin Hamiltonian parameters in the ferroelectric phase are: g = 1.980 - 0.003, b 2 0 = (1140 - 15) 10 m 4 cm m 1, b 2 2 = (214 - 10) 10 m 4 cm m 1 . Remarkable EPR linewidth changes confirm the order-disorder character of the ferroelectric phase transition on a microscopic level and demonstrate that the dimethylammonium reorientation freezing-out is the prime reason for this transition.

Optical spectra and magnetic field effects in Cr(III)( l-histidine) 2d(NO 3)

We report measurements of the optical absorption and emission spectra in single crystals of Cr(III)( l-histidine) 2d(NO 3), with applied magnetic fields up to 5 T, at 1.8 K. The absorption spectrum displays two strong, wide bands due to the electronic transitions 4A 2 → 4T 2 and 4A 2 → 4T 1 of the Cr 3+ ion in a nearly octahedral crystal field. Besides these bands, we observed weak and narrow absorption peaks due to the spin-forbidden transitions 4A 2 → 2T 1 and 4A 2 → 2E in at least two different crystalline sites, and also the corresponding vibronic lines. The 2 E → 4 A 2 transition and its vibronic lines were also observed in the emission spectrum. From these data, we evaluated the Racah and crystal-field parameters of Cr 3+ and the g factors of 4A 2 and 2E .

Refinement of the Superstructure of C-Type Chromium(III) Tris(metaphosphate), Cr(PO3)3

Single crystals of Cr(PO3)3 suitable for X-ray investigations were obtained under various experimental conditions that favour volatilization of the phosphate and its recrystallization from the gas phase. The compound belongs to the series of phosphates M(PO3)3 (M3+ = Al, In, Sc, Ti, V, Fe, Mo, Rh, Ru) which adopt the C-type structure, characterized by one-dimensional infinite metaphosphate chains and isolated MO6 octahedra. Associated with the superstructure generally found for this structure type, in which the monoclinic b axis is tripled, three crystallographically different CrO6 octahedra of almost ideal geometry [dave(Cr—O) = 1.96 (2) Å] were observed. Within the two independent metaphosphate chains distinctly different bond distances from P to bridging and terminal O atoms were found. [dave(P—OXX tha ing of , ­ phospho­rus oxygen , respectively -Ob) = 1.58 (1) , dave(P—Ot) = 1.49 (1) Å].

Crystal growth and optical characterization of Cr,Ca: Y3Al5O12

Single crystals of Cr,Ca: Y3Al5O12(YAG) with different Cr and Ca concentrations were grown by the Czochralski (CZ) and floating zone (FZ) method. The analysis of absorption and emission spectra of both as-grown and oxygen-annealed crystals in relation to concentration ratio of Ca and Cr (Ca/Cr) showed that Cr4+ concentration ratio to total Cr concentration increased up to about Ca/Cr = 6, and Cr6+ can also exist. Luminescence lifetime at 1400 nm was measured as a function of Cr4+ concentration.

Critical magnetic neutron scattering above T N in Cr+0.18 at. % Re (abstract)

Magnetic critical neutron scattering from a single crystal of Cr+0.18 at. % Re was observed above its Néel transition (323 K) to an incommensurate spin density wave state. The data were analyzed in terms of the critical susceptibility model that successfully fit analogous data from single crystals of pure chromium and Cr+0.2 at. % V. An additional complication in this case is the fact that a small portion of the sample becomes commensurate antiferromagnetic at a temperature well above 323 K, which causes a commensurate magnetic Bragg peak that partially obscures the critical scattering near 323 K. The ‘‘Sato-Maki’’ form of incommensurate critical susceptibility fits the data reasonably well. The intensity of magnetic critical scattering in Cr+0.18 at. % Re is about a factor 2 stronger than in pure Cr, whereas in Cr+0.2 at. % V it is about the same factor weaker.

The crystal structure of Cr 1.73Mo 6S 8

Single crystals of Cr 1.73Mo 6S 8 with triclinic symmetry were grown from a melt of an appropriate mixture of Mo 2S 3, molybdenum and chromium. A single crystal structure determination was performed in space group P 1 (C i 1, No. 2; R(F 0) = 0.032 ) including 2870 reflections with No. 2; R(F 0) = 0.032) including 2870 reflections with F 0 2>2 δ( F 0 2). The lattice parameters are: a = 652.2(2) pm, b = 649.7(2) pm, c = 644.9(1) pm, a = 94.68(2)°, β = 90.70(2)°, γ = 97.91(2)δ (Guinier data). The structure is isopointal with Fe 2Mo 6S 8. The chromium atoms are located in parallel channels [010] Fe 2Mo 6S 8. The chromium atoms are located in parallel channels [010] which are surrounded by Mo 6S 8 clusters linked via six intercluster Mo-S contacts. The coordination about the chromium atoms and the Cr-S distances are in accordance with the valence state +2 for chromium. Despite the low rotational symmetry the refinements indicate some disorder of the chromium atoms which is not resolved by releasing the centre of symmetry. In addition, indications of a lowering of translational symmetry do not exist.

Anomalous ultrasonic attenuation by the spin-density wave in a dilute Cr(V) alloy

A single crystal of Cr+0.5 at. % V was field cooled through its Néel temperature TN≂ 260 K, with a field B=12 T, along a cube axis to produce a single-Q state. The attenuation α was measured for 10-MHz longitudinal ultrasound propagating parallel and perpendicular to Q, and also in the non-field-cooled poly-Q state. The resultant anisotropy in the change in α at the spin-flip temperature TSF≂40 K, shows the sample to be largely single-Q, but its nature suggests that the change of the magnetic structure at the low-temperature phase transition may differ from that in pure Cr.

Magneto-Optical Spectra in Single Crystals of Cr-Te Compounds

Optical and magneto-optical properties of single crystals of Cr 2 Te 3 and Cr 3 Te 4 , grown by the chemical vapor transport technique, were studied. The reflectivity spectra at normal incidence of both samples were measured at room temperature, and the magneto-optical spectra of the Cr 3 Te 4 were also measured. Spectra of the diagonal and off-diagonal elements of the conductivity tensor were obtained from the measured spectra. These were compared with estimations based on density of state curves calculated by Dijkstra, and showed reasonably good agreement. From these results it is concluded that transitions from Cr 3d to the Conduction Band for majority spins and Te 5p to Cr 3d for minority spins, are responsible for the observed optical absorption and magneto-optical effects.

Pressure Dependence of Spin Density Wave in Cr–0.5 at.%Ge

The magnetic structures of a single crystal of Cr–0.5 at.%Ge as a function of pressure have been studied by neutron diffraction. The characteristics are quite different from those of Cr–1.4 at.%Si. Instead the magnetic phase diagram as a function of pressure resembles that of CrMn.

The EPR spectrum of Os(CO) 5+ trapped in a single crystal of Cr(CO) 6∗

Crystals of Cr(CO) 6 containing dissolved Os(CO) 5 are paramagnetic at 100 K after γ irradiation. The substitutional paramagnetic impurity is Os(CO) 5 +, in which the unpaired electron occupies an osmium 5 d z 2 orbital. The z direction is parallel to one of the two types of Cr-C bonds in the crystal, leading to two distinct trapping sites (A and B) for the impurity.

The Temperature Dependence of the Velocity of the Longitudinal Acoustic Mode of Chromium in the Spin Density Wave State

It is well-known that the dynamical matrix is represented by the inverse dielectric matrix. We show that the temperature dependent part of the velocity of the longitudinal acoustic phonon can be given by only one element of the dielectric matrix e ( q , q ) in the ordered state of Cr if the random-phase approximation applicable and the Bloch wave functions can be expressed by the atomic orbital functions for the system. The same result hold in other metals if above conditions are satisfied. In the limit q →0 the element e ( q , q ) has the intimate relation with the density of states at the Fermi energy even in the ordered state of Cr which decreases with decreasing temperatures. Our theory gives one of reasons of the remarkable increase of the compressional constant C 11 of single crystals of Cr in the spin density wave state with decreasing temperatures.

Paramagnetic impurities in hexacarbonylchromium crystals. 2. The EPR spectrum of tetracarbonylnitrosylmanganese(1-) (Mn(CO)4NO)-

An anisotropic EPR spectrum detected in ..gamma..-irradiated single crystals of Cr(CO)/sub 6/ doped with Mn(CO)/sub 4/NO is attributed to the radical anion of the latter. The unpaired electron in this molecule occupies an antibonding sigma* orbital composed primarily of Mn(3d/sub z/sub 2//) and N(2p/sub z/) atomic orbitals. There is some evidence suggesting that the Mn-N-O moiety in the free radical is bent.

The EPR spectrum of a single crystal of chromium hexacarbonyl doped with Fe(CO)5

Single crystals of Cr(CO)6 have been doped with 0.05% Fe(CO)5 and γ irradiated at 77 K. The irradiated crystals revealed EPR spectra at 95 K of a species identified as Fe(CO)5−, present in two modifications A and B. Using Fe(CO)5+ enriched in the isotope 13C, the hyperfine tensors of the five carbon atoms of A were obtained; the unique 13C had a∥ = 21 G and a⊥ = 18 G; the other four had a∥ = 6.6 G and a⊥ = 9.0 G. Measurement of the 57Fe hyperfine interaction indicated the presence of a single Fe nucleus with a∥ = l0 G and a⊥ = 6 G. The two modifiactions A and B correspond to Fe(CO)5+ molecules in which the unique Fe–C bond is parallel to Cr–C(1,2) or Cr–C(3,4,5,6) bonds, respectively, in the pure Cr(CO)6 crystal.

Magnetic Properties of Cr3Te4 in Ferromagnetic Region

Single crystals of Cr 3 Te 4 with the pseudo-NiAs type structure have been made by chemical transport method in order to investigate the anomaly below 80 K. The saturation moment at 4.2 K is equal to 1.79±0.03 µ B per Cr 1- x Te (x=0.25), and the Curie temperature is 316 K. The magnetization only in the direction to the a -axis shows the anomaly that the saturation field increases abruptly below 80 K, which corresponds to the Néel point proposed by Andresen. But on the contrary to his result, the easy axis of the moment is parallel to the c -axis in the whole ferromagnetic region and antiferromagnetic order appears only in the c -plane. By a spin echo method, two nuclear magnetic resonance absorption lines of Cr 53 -nuclei are observed at 45.3 and 57.5 MHz at 4.2 K. The resonance frequencies increase remarkably below 80 K. This fact may support subsidiarily the existence of the antiferromagnetic order below 80 K.

Antiferromagnetism in Chromium Alloys. II. Transport Properties

The electrical resistivity and thermoelectric power have been studied as a function of temperature in single crystals of Cr containing small concentrations of V, Mn, Mo, W, and Re. The onset of antiferromagnetism is marked by anomalies in the transport properties, and the N\'eel temperatures can be deduced from these. The variation of ${T}_{N}$ upon alloying is qualitatively in accord with the two-band model of Fedders and Martin, especially in its exponentially rapid dependence on electron concentration. In the incommensurable magnetic structures, the most important factors modifying the transport properties are the mutual annihilation of electron and hole regions of the Fermi surface and the consequent reduction in the probability of electron-phonon scattering. Field-cooling experiments in pure Cr indicate that the effect of magnetic superzones is much smaller than these contributions. The electron-phonon scattering depends sensitively on the relation between the energy gap at the Fermi surface and the maximum phonon energy, and consideration of the magnitude of the anomalies in the transport properties below ${T}_{N}$, in particular the variation with electron concentration, leads to an approximate value of $2.2{k}_{B}{T}_{N}$ for the mean energy gap, which is consistent with the theory. In the more concentrated commensurable magnetic structures, on the other hand, superzone effects appear to be dominant, since the change in resistance on ordering is proportional to the ordered moment. When different solutes are added to Cr, the low-temperature thermopower behaves in a very complicated way, but its large magnitude in the more concentrated Cr-Mn alloys can probably be ascribed to magnon drag.