Ge2 Compounds Research Articles

Structural and magnetic properties of the RAl0.2Ge2 compounds (R = Tb, Dy, Ho, Er, Tm)

The structural and magnetic properties of the RAl0.2Ge2 (R = Tb-Tm) germanides were studied by X-ray and neutron diffraction, scanning electron microscopy and magnetometric techniques. The compounds crystallize in the CeNiSi2-type crystal structure (space group Cmcm). Magnetic measurements reveal an antiferromagnetic ordering below TN equal to 38 K (R = Tb), 23 K (Dy), 9 K (Ho) and 6.5 K (Er), and no magnetic ordering in the Tm-compound down to 1.9 K. Based on the neutron diffraction data the magnetic structures can be described by the propagation vector k = (0.4714(2), 0, 0.3128(2)) for TbAl0.2Ge2 and k = (0.4858(5), 0, 0.3214(6)) for DyAl0.2Ge2 while in HoAl0.2Ge2 and ErAl0.2Ge2 a simple collinear antiferromagnetic ordering of the G-type was observed. The electronic structures of the title compounds were calculated using the tight-binding linear muffin-tin orbital (TB-LMTO) method.

Electronic structure of the TbMn0.33Ge2 compound: Band calculation and optical experiment

The results of the investigation of the electronic structure and optical properties of the TbMn0.33Ge2 compound have been presented. The spin-polarized calculations of the band spectrum have been performed within the framework of the local spin density approximation (LSDA) with a correction for strong correlations in the 4f shell of the rare-earth ion (the LSDA + U method). The optical constants have been measured using the ellipsometric method and a number of spectral and electronic characteristics of the compound under investigation have been determined over a wide range of wavelengths. The interband part of the experimental dependence of the optical conductivity has been interpreted using the results of the calculation of the electron density of states.

Magnetic properties and low-temperature large magnetocaloric effect in the antiferromagnetic HoCu0.33Ge2 and ErCu0.25Ge2 compounds

Magnetic properties and magnetocaloric effect (MCE) of HoCu0.33Ge2 and ErCu0.25Ge2 compounds have been investigated. The compounds were determined to be antiferromagnetic with the Néel temperatures TN=9K and 3.9K, respectively. The critical transition magnetic fields for the metamagnetic transition from antiferromagnetic to ferromagnetic state below TN were determined to be 10kOe for HoCu0.33Ge2 at 5K and 6kOe for ErCu0.25Ge2 at 2K. Large MCE with the maximal values of magnetic entropy changes (ΔSM) as −10.2J/kgK at 10.5K were found in HoCu0.33Ge2 for field changes of 0–70kOe and −10.5J/kgK at 5.5K in ErCu0.25Ge2 for field changes of 0–50kOe, respectively. The large ΔSM around TN as well as no hysteresis loss made RCuxGe2 competitive candidates as low temperature magnetic refrigerant.

Magnetic, thermal and magnetocaloric properties of non-stoichiometric TbMn0.33Ge2 compound

Magnetic, heat capacity and magnetocaloric properties of non-stoichiometric TbMn0.33Ge2 compound have been studied. Different measurements show that it undergoes three magnetic transitions at TN, T1 and T2. The magnetocaloric effect has been estimated from magnetization data using Maxwell׳s relation. The existence of collinear and sine wave modulated components makes the magnetic structure complex at low temperatures. It is also found that this compound shows normal and inverse magnetocaloric effects in different temperature regimes.

Magnetic properties of RE2MnGe6 (RE = La, Ce) and YMn0.3Ge2 germanides

The magnetic data of RE2MnGe6 (RE = La, Ce) and YMn0.3Ge2 are reported. La2MnGe6 and Ce2MnGe6 crystallize in the orthorhombic Ce2CuGe6 structure type, space group Amm2 (No. 38). The non-stoichiometric YMn0.3Ge2 compound crystallizes with the orthorhombic CeNiSi2-type structure (space group Cmcm (No. 36)). The studied RE2MnGe6 (RE = La, Ce) intermetallics are characterized by ferromagnetic properties with Curie temperatures 177 (La) and 150 K (Ce), respectively. For YMn0.3Ge2 the low-field magnetic measurements indicate the antiferromagnetic property below 395 K with the small ferromagnetic component. The values of the magnetic moments in the ordered state indicate the ferromagnetic ordering in La2MnGe6 and complex magnetic order with the ferromagnetic component in YMn0.3Ge2 and Ce2MnGe6. The hysteresis loop and values of the coercivity field indicate that these compounds are soft magnetic materials.

Crystal and magnetic structure of TbFe0.25Ge2 compound

The crystal and magnetic structure of polycrystalline TbFe0.25Ge2 sample have been determined. X-ray and neutron diffraction studies indicate that this compound has the orthorhombic CeNiSi2-type crystal structure (space group Cmcm). The magnetic ordering, based on the neutron diffraction data in low temperature, is described by two components: a collinear antiferromagnetic G-type and a cosine-wave modulated one. In the collinear G-type structure the Tb magnetic moment is equal to 3.81(5) µB and it is parallel to the c-axis. The modulated structure is described by the propagation vector k=(0.460(8), 0, 0.305(1)), the Tb magnetic moment equals 7.75(8) µB, lies in b–c and forms an angle 23(2)° with the c-axis. The collinear component decreases to zero at 22.6K while the modulated one at 190.8K.

The influence of substitution of Mn by Fe and Co on magnetocaloric effect and magnetoresistance properties of SmMn2Ge2

Magnetocaloric and magnetoresistance properties of SmMn2−xFexGe2 (x=0.05 and 0.10) and SmMn2−xCoxGe2 (x=0.05 and 0.15) compounds have been studied by magnetic and resistance measurements in the temperature range between 30 and 350K. All compounds exhibit metamagnetic transition from antiferromagnetism to ferromagnetism around the Sm moments ferromagnetic ordering temperature-TSm and Mn moments antiferromagnetic ordering temperature-TN1. The magnetic entropy changes of these compounds are estimated from the Maxwell equation, Maxwell Clausius Clapeyron equation, Landau theory and mean-field theory. The maximum magnetic entropy change values of SmMn1.90Fe0.10Ge2 and SmMn1.85Co0.15Ge2 compounds are −8.1Jkg−1K−1 and −5.1Jkg−1K−1 in a magnetic field change of 5T, respectively. These compounds show negative magnetoresistance around the magnetic phase transition temperatures. The magnetoresistance value of SmMn1.95Fe0.05Ge2 is −23% at TSm which is bigger than the magnetoresistance value of SmMn2Ge2 (−15%).

Electronic structure of RCo x Ge 2 (R=Ce, Pr, Gd) compounds

The electronic structure of the RCoxGe2 (R=Ce, Pr, Gd) compounds with the orthorhombic crystal structure of CeNiSi2-type was studied by X-ray photoemission spectroscopy. The valence bands of CeCo0.86Ge2 and PrCo0.85Ge2 compounds have the similar character. Near the Fermi level the broad peak corresponding to the Co 3d and R 4f and 5d6s states is observed. In the case of GdCo0.64Ge2, the strong intensity peak at 8.7 eV corresponding to Gd 4f states is observed. The analysis of the XPS spectra of Ce and Pr 3d5/2 and 3d3/2 in the Gunnarson-Schönhammer model give the information on the hybridization of the f orbital with the conduction band. The hybridization energy equal 93 eV for R=Ce and 143 eV for R=Pr, respectively indicate stability of the shell in these compounds. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic properties of PrMn2−xFexGe2—57Fe Mössbauer spectroscopy

We have investigated the magnetic behaviour ofPrMn2−xFexGe2 compoundswith x = 0.4, 0.6 and 0.8 over the temperature range 4.2–350 K using ac magnetic susceptibility, dc magnetizationand 57Fe Mössbauer effect spectroscopy, as well as neutron diffraction for thePrMn1.2Fe0.8Ge2 compound. Replacement of Mn with Fe leads to contraction of the unit cell and ashortening of the Mn–Mn spacing, resulting in modification of the magnetic structure.PrMn1.6Fe0.4Ge2 is an intralayer antiferromagnet at room temperature and ferromagnetic belowTCinter∼230 K with additional ferromagnetic ordering of the Pr sublattice detected belowTCPr∼30 K. Re-entrant ferromagnetism has been observed inPrMn1.4Fe0.6Ge2 with four magnetictransitions (TNintra∼333 K, TCinter∼168 K,TNinter∼152 K andTCPr∼40 K). Moreover, itwas found that TCinter and TCPr increase withapplied field while TNinter decreases. PrMn1.2Fe0.8Ge2 isantiferromagnetic with TNintra∼242 K and TNinter∼154 K. The magnetic transition temperatures for all compounds are also marked by changes in the57Fe magnetic hyperfine field and the electric quadrupole interaction parameters. The57Fe transferred hyperfinefield at 4.5 K in PrMn1.6Fe0.4Ge2 and PrMn1.4Fe0.6Ge2 is reduced (below the ordering temperature of the Pr sublattice) compared with that at 80 K (aboveTCPr), indicating that the transferred hyperfine field from Pr acts in theopposite direction to that from the Mn atoms. The neutron data forPrMn1.2Fe0.8Ge2 demonstrate that an anisotropic thermal expansion occurs within the interplanarantiferromagnetic range.

Neutron diffraction study of the magnetic structures ofPrMn2−xCoxGe2 (x = 0.4,0.5 and 0.8) with a new refinement procedure

The magnetic structures of PrMn2−xCoxGe2 (x = 0.4, 0.5 and0.8) with ThCr2Si2-type structure have been investigated by means of neutron diffraction measurementsbetween 2 and 312 K. We introduced a new refinement procedure to determine the magneticmoments of Pr and Mn sublattices below the rare-earth ordering temperatureTCPr because of the overlapping of the magnetic reflections of the Pr and Mnsublattices. Rietveld refinements demonstrated that above the Curie temperaturean intralayer antiferromagnetic ordering within (001) Mn layers is observed inPrMn1.6Co0.4Ge2 andPrMn1.5Co0.5Ge2, while theintralayer antiferromagnetic ordering within (001) Mn layers is found over the whole temperature range forPrMn1.2Co0.8Ge2. Below the Curietemperature the PrMn1.6Co0.4Ge2 and PrMn1.5Co0.5Ge2 compounds have a canted ferromagnetic structure with the canting angles62° and65° at2 K, respectively. Below 75 and 70 K, a ferromagnetic ordering of the Pr sublattice is observed alongthe c-axis for these compounds. Below 70 K, a ferromagnetic ordering of Prsublattices is found (not detected by magnetic measurements) along thec-axisin PrMn1.2Co0.8Ge2.

Magnetic properties of RCo2 Ge2 compounds (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y)

The magnetic properties of the RCo2 Ge2 compounds with R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y have been investigated. It was found that the compounds for which R=La, Ce, Sm, Er, Tm, Yb, and Y exhibit paramagnetic behavior down to 4.2 °K with ErCo2Ge2 and TmCo2Ge2 probably ordering at lower temperatures. The compounds for which R=Gd, Tb, Dy, and Ho order antiferromagnetically. PrCo2Ge2 and NdCo2Ge2 show an unusual magnetic behavior. PrCo2Ge2 exhibits a susceptibility which decreases from 4.2 °K, then rises to a peak at 27.5 °K. NdCo2Ge2 has two peaks in the susceptibility-vs-temperature curve at 13.5 and 27.5 °K. The Néel points for the heavy rare-earth compounds follow the de Gennes function, and the equation χM = χ0,M + CM/T−θ is used to interpret the susceptabilities in the paramagnetic region. χM is the molar susceptibility; χ0,M is a temperature independent contribution to the susceptibility; CM is the Curie constant; T is the temperature and θ is the Weiss constant.