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Journal of Applied Physics 01/2013; 113:17A904. · 2.17 Impact Factor
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Journal of Applied Physics 01/2013; 113:17E120. · 2.17 Impact Factor
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S. Vélez,
J. M. Hernandez,
A. García-Santiago,
J. Tejada, V. K. Pecharsky,
Gschneidner, Jr., K. A,
D. L. Schlagel,
T. A. Lograsso,
P. V. Santos
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ABSTRACT: Experimental evidence of the anisotropy of the magnetic deflagration associated with the low-temperature first order antiferromagnetic (AFM) --> ferromagnetic (FM) phase-transition in single crystals of Gd5Ge4 is reported. The deflagrations have been induced by controlled pulses of surface acoustic waves (SAW) allowing us to explore both the magnetic field and temperature dependencies on the characteristic times of the phenomenon. The study was done using samples with different geometries and configurations between the SAW pulses and the direction of the applied magnetic field with respect to the three main crystallographic directions of the samples. The effect of temperature is nearly negligible, whereas observed strong magnetic field dependence correlates with the magnetic anisotropy of the sample. Finally, the
role of the SAW pulses in both the ignition and formation of the deflagration front was also studied, and we show that the thermal diffusivity of Gd5Ge4 must be anisotropic, following \kappaa>\kappab>\kappac.
Physical Review B 02/2012; 85:054432. · 3.69 Impact Factor
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ABSTRACT: Transport properties of the intermetallic compound Dy5Si3Ge have been studied across its structural and magnetic transitions. The compound has a monoclinic crystal structure at room temperature, and orders magnetically at 74 K (TN) and 62 K (TC). Upon cooling it undergoes a structural transition to an orthorhombic structure near TC. Below ∼50 K, Dy5Si3Ge exists in a structurally phase separated state and is ferromagnetically ordered. The electrical resistivity and heat capacity of Dy5Si3Ge show interesting hysteretic behavior in the vicinity of the structural transition in zero field. First principles calculations suggest that Ge and Si atoms preferentially occupy intraslab andinterslab sites in Dy5Si3Ge, respectively, which leads to dominant ferromagnetic interactions within both the orthorhombic and monoclinic structures.
Journal of Applied Physics 03/2011; 109(7):07A923-07A923-3. · 2.17 Impact Factor
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ABSTRACT: We report the crystal structure, magnetic properties, and electronic structure of Gd2C. The compound crystallizes in the rhombohderal CdCl2–type structure and has a Curie temperature of 351 K, which decreases to ∼340 K after heat treatment at 1000 °C for 1 week. The magnetic ordering transition is of second order, and the saturation magnetic moment measured at 2 K in 70 kOe magnetic field is 7.26 µB/Gd which compares well with 7.34 µB/Gd calculated from first principles. The electronic structure calculations performed using the tight bonding linear muffin tin orbital method within the non local exchange correlation potentials show stronger exchange interactions compared to the local exchange correlation potentials leading to the high Curie temperature of Gd2C.
Journal of Applied Physics 03/2011; 109(7):07A924-07A924-3. · 2.17 Impact Factor
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ABSTRACT: From formation energy calculated as a function of shear displacement of neighboring slabs in Gd5Ge4, we confirm stability of Pu5Rh4-type and Tm5Si2Sb2-type structures in addition to the well known orthorhombic O(I) and O(II) structures. The former two structures are more stable when some of the Ge sites are substituted by Ga or Sb, respectively. The sizes as well as the valence electron concentrations of the substituting atoms play important role in the preferred replacements of the Ge atoms in the Pu5Rh4-type and Tm5Si2Sb2-type structures. The calculations of band structure and exchange interactions reveal that the O(I), Pu5Rh4-type, and Tm5Si2Sb2-type structures have ferromagnetic ground states, but the ground state of the O(II)-type structure is antiferromagnetic.
Phys. Rev. B. 10/2010; 82(14).
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ABSTRACT: When the complexity of a metallic compound reaches a certain level, a specific location in the structure may be critically responsible for a given fundamental property of a material while other locations may not play as much of a role in determining such a property. The first-principles theory has pinpointed a critical location in the framework of a complex intermetallic compound--Gd(5)Ge(4)--that resulted in a controlled alteration of the magnetism of this compound using precise chemical tools.
Physical Review Letters 08/2010; 105(6):066401. · 7.37 Impact Factor
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ABSTRACT: The magnetic and magnetostructural properties of the polycrystalline Ce ( Fe <sub>0.975</sub> Ga <sub>0.025</sub>)<sub>2</sub> have been investigated as a function of temperature and magnetic field. In Ce ( Fe <sub>0.975</sub> Ga <sub>0.025</sub>)<sub>2</sub> the magnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM) state is accompanied by a structural transformation from rhombohedral to cubic structure. Phase coexistence is present during both the temperature and field driven transformations from AFM to FM phase.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: Using x-ray powder diffraction technique at various temperatures and applied magnetic fields, we have studied the magnetostructural properties of Ce(Fe0.95Si0.05)2. The x-ray diffraction data establish quantitative relationships between bulk magnetization and the evolution of structurally distinct phases with magnetic field and temperature, and confirm the distinct features of first order phase transition like supercooling and superheating, metastability, and phase co-existence of different structural polymorphs. We observe the lattice volume mismatch across the structural phase transition, which appears to be the cause for the step behavior of the magnetization isotherms at low temperatures. The present study shows that the lattice distortion has to be treated explicitly, like spin, along with the effects of lattice-spin coupling to account for the magnetization behavior of this system. This structure template can resolve the issue of kinetics in this material as observed in different time scale measurements and with different experimental protocols. Comment: 13pages,5 figures
05/2010;
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02/2010: pages 199-222;
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ABSTRACT: The crystallography, phase relationships, and magnetic properties of the Ce <sub>5</sub> Si <sub>4-x</sub> Ge <sub>x</sub> alloys with 0≤x≤4 have been investigated by using x-ray powder diffraction and isothermal magnetization measurements. There are three different crystal structures in the Ce <sub>5</sub> Si <sub>4-x</sub> Ge <sub>x</sub> system: the Zr <sub>5</sub> Si <sub>4</sub> -type tetragonal structure with space group P4<sub>1</sub>2<sub>1</sub>2 exists from 0≤x≪2.15 , the Gd <sub>5</sub> Si <sub>2</sub> Ge <sub>2</sub> -type monoclinic structure with space group P112<sub>1</sub>/a exists at x≈2.225 , and the Sm <sub>5</sub> Ge <sub>4</sub> -type orthorhombic structure with space group Pnma is found for 2.4≪x≤4 . The magnetic ordering temperature increases when the tetragonal phase changes to the monoclinic phase, and then it remains composition independent throughout the orthorhombic phase, which is the opposite trend compared to that observed in the heavy lanthanide 5:4 compounds when Ge content increases. Another distinct difference is that Ce <sub>5</sub> Si <sub>4</sub> exhibits an antiferromagnetic ground state while Ce <sub>5</sub> Ge <sub>4</sub> phase orders ferromagnetically, which is reverse compared to the R <sub>5</sub> Si <sub>4-x</sub> Ge <sub>x</sub> systems where R = Gd and Tb. The magnetocaloric effect-
has been calculated from the magnetization data. The Ce <sub>5</sub> Ge <sub>4</sub> has the maximum magnetic entropy change ΔS<sub>M</sub>(-11.6 J / kg K ) at the Curie temperature of 11.5 K for a field change of 5 T.
Journal of Applied Physics 02/2010; · 2.17 Impact Factor
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ABSTRACT: Magnetic, magnetocaloric and magnetoresistance properties of the intermetallic compound Nd7Pd3 have been investigated. It exhibits a first-order magnetic phase transition at its ferromagnetic ordering temperature (TC = 34 K). Just above the TC, the magnetization isotherms exhibit a metamagnetic transition from the paramagnetic to a ferromagnetic state. In the immediate vicinity of TC, a field change of only 10 kOe leads to the giant magnetocaloric effect of 13 J mol−1 K−1. For a field change of 50 kOe a large magnetoresistance of ~21% is observed near TC. First-principles electronic structure calculations indicate that the first-order phase transition in Nd7Pd3 may originate from the peculiar nature of the density of states near the Fermi level.
Journal of Physics Condensed Matter 10/2009; 21(45):456004. · 2.55 Impact Factor
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ABSTRACT: LV4O8 (L = Yb, Y, Lu) compounds are reported to crystallize in a structure similar to that of the orthorhombic CaFe2O4 structure-type, and contain four inequivalent V sites arranged in zigzag chains. We confirm the structure and report the magnetic, thermal, and transport properties of polycrystalline YV4O8 and LuV4O8. A first-order like phase transition is observed at 50 K in both YV4O8 and LuV4O8. The symmetry remains the same with the lattice parameters changing discontinously. The structural transition in YV4O8 leads to partial dimerization of the V atoms resulting in a sudden sharp drop in the magnetic susceptibility. The V spins that do not form dimers order in a canted antiferromagnetic state. The magnetic susceptibility of LuV4O8 shows a sharp peak at ~ 50 K. The magnetic entropies calculated from heat capacity versus temperature measurements indicate bulk magnetic transitions below 90 K for both YV4O8 and LuV4O8. Comment: 14 pages, 13 figures, 5 tables; submitted to Phys. Rev. B
09/2009;
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W. Tian,
A. Kreyssig,
J. L. Zarestky,
L. Tan,
S Nandi,
A. I. Goldman,
T. A. Lograsso,
D. L. Schlagel,
K A Gschneidner, V K Pecharsky,
R. J. McQueeney
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ABSTRACT: We present a single crystal neutron diffraction study of the magnetic short-range correlations in Tb$_5$Ge$_4$ which orders antiferromagnetically below the Neel temperature $T_N$ $\approx$ 92 K. Strong diffuse scattering arising from magnetic short-range correlations was observed in wide temperature ranges both below and above $T_N$. The antiferromagnetic ordering in Tb$_5$Ge$_4$ can be described as strongly coupled ferromagnetic block layers in the $ac$-plane that stack along the b-axis with weak antiferromagnetic inter-layer coupling. Diffuse scattering was observed along both $a^*$ and $b^*$ directions indicating three-dimensional short-range correlations. Moreover, the $q$-dependence of the diffuse scattering is Squared-Lorentzian in form suggesting a strongly clustered magnetic state that may be related to the proposed Griffiths-like phase in Gd$_5$Ge$_4$. Comment: 6 pages, 5 figures
08/2009;
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ABSTRACT: First-principles calculations predict that, in the antiferromagnetic state, Ho5Ge4 should adopt a unique monoclinic structure with an unusual distortion in the ac plane, making it a unique member of a broadly researched R5T4 family of compounds that are best known for their giant magnetocaloric, magnetoresistive, and magnetostrictive effects. Experiments prove that, in Ho5Ge4, the magnetic transition from the paramagnetic to the antiferromagnetic state is indeed accompanied by a structural transformation from the Sm5Ge4-type orthorhombic to the predicted monoclinic structure. Surprisingly, a magnetic field can partially reconstruct the high-temperature paramagnetic Sm5Ge4-type structure of Ho5Ge4 when applied to the magnetically ordered compound.
Phys. Rev. B. 03/2009; 79(9).
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ABSTRACT: We investigate the emergence of ferromagnetism (FM) in low-Si-content Gd5(SixGe1−x)4 alloys (x=0.025,0.05,0.075) from within the antiferromagnetic (AFM) phase of the Gd5Ge4 parent compound. X-ray magnetic circular dichroism (XMCD) and bulk magnetization measurements show that all samples exhibit partial FM order at low temperature, but their saturation magnetization is reduced relative to higher-Si-content samples (x=0.125,0.5). This reduced magnetization is due to an incomplete AFM orthorhombic(II)→FM orthorhombic(I) magnetostructural phase transition upon cooling, as evidenced by x-ray diffraction. High-pressure XMCD measurements in a diamond-anvil cell show that applied pressures of 5.0, 3.0, and 2.0 GPa restore the full saturation magnetization in x=0.025, 0.05, and 0.075 samples, respectively, by stabilizing the FM-O(I) phase. The mixed-phase behavior is also evidenced in dTc/dP, which strongly varies with silicon concentration in these samples at low pressures but becomes independent of x at higher pressures where values typical of higher-x samples (0.125<x<0.5) are found.
Phys. Rev. B. 12/2008; 78(21).
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ABSTRACT: Measurements as a function of both magnetic field and temperature along with first principles spin polarized calculations explain the remarkable magnetoelastic properties exhibited by GdNi below its Curie temperature. The lattice constants a and b elongate continuously by 0.35% and 0.49%, respectively, while the c axis contracts by 0.78%, all without phase volume change. Calculations and experiment confirm a relatively shallow magnetization-dependent energy landscape modified by the increased spin splitting of the conduction band as the 4f moments order.
Phys. Rev. B. 11/2008; 78(18).
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ABSTRACT: An x-ray powder diffraction study of polycrystalline Dy was performed over the temperature interval from 5 to 300 K in applied magnetic fields up to 40 kOe. A complex first-order magnetostructural transformation, which occurs below ∼8 kOe and above ∼20 kOe, is decoupled into a first-order magnetic (helix-fan) transition and a second-order magnetostructural (hexagonal-orthorhombic) transition between ∼135 and ∼175 K in magnetic fields between ∼8 and ∼20 kOe. The transition to the ferromagnetic (FM) state is always accompanied by an orthorhombic distortion of the hexagonal lattice, including a transition from the paramagnetic (PM) to the FM state in magnetic fields exceeding 20 kOe.
Phys. Rev. B. 03/2008; 77(9).
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L. Tan,
A. Kreyssig,
S. Nandi,
S. Jia,
Y. B. Lee,
J. C. Lang,
Z. Islam,
T. A. Lograsso,
D. L. Schlagel, V. K. Pecharsky,
Jr. K. A. Gschneidner,
P. C. Canfield,
B. N. Harmon,
R. J. McQueeney,
A. I. Goldman
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ABSTRACT: X-ray resonant magnetic scattering was employed to study a fully reversible spin-flop transition in orthorhombic Gd5Ge4 and to elucidate details of the magnetic structure in the spin-flop phase. The orientation of the moments at the three Gd sites flop 90° from the c axis to the a axis when a magnetic field, Hsf=9 kOe, is applied along the c axis at T=9 K. The magnetic space group changes from Pnm′a to Pn′m′a′ for all three Gd sublattices. The magnetic anisotropy energy determined from experimental measurements is in good agreement with the calculations of the magnetic anisotropy based on the spin-orbit coupling of the conduction electrons and an estimation of the dipolar interactions anisotropy. No significant magnetostriction effects were observed at the spin-flop transition.
Phys. Rev. B. 02/2008; 77(6).
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V. K. Pecharsky
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ABSTRACT: Magnetic and magnetocaloric properties of polycrystalline (Er1−xGdx)5Si4 with x=0.1 and 0.25 have been studied. The temperature dependencies of the dc magnetization and ac susceptibility indicate two magnetic transitions: the high temperature transitions are antiferromagnetic in character, whereas the low temperature ones are of ferromagnetic nature. The isothermal magnetization of these compounds shows metamagneticlike transitions at 5 K and no saturation in fields up to 70 kOe. Time dependencies of the magnetization data below the Néel temperature exhibit unusually strong relaxation effects with logarithmic time dependence. The ac susceptibility data reveal that magnetic state of these compounds is distinctly different from spin glasses and the relaxation behavior seen in the magnetization data is a consequence of complex magnetic interactions. The existence of complex magnetic interactions leads to an easy formation of field induced short range ferromagnetic correlations in the paramagnetic state and greatly affects both the magnetic and magnetocaloric properties of these compounds.
Phys. Rev. B. 02/2008; 77(5).
