Devo Schlagel - Academia.edu (original) (raw)

Papers by Devo Schlagel

Physical Review B, 2019

The magnetocaloric effect of gadolinium has been measured directly in pulsed magnetic fields up t... more The magnetocaloric effect of gadolinium has been measured directly in pulsed magnetic fields up to 62 T. The maximum observed adiabatic temperature change is ∆T ad = 60.5 K, the initial temperature T0 being just above 300 K. The field dependence of ∆T ad is found to follow the usual H 2/3 law, with a small correction in H 4/3. However, as H is increased, a radical change is observed in the dependence of ∆T ad on T0, at H = const. The familiar caret-shaped peak situated at T0 = TC becomes distinctly asymmetric, its high-temperature slope becoming more gentle and evolving into a broad plateau. For yet higher magnetic fields, µ0H > ∼ 140 T, calculations predict a complete disappearance of the maximum near TC and an emergence of a new very broad maximum far above TC.

Thin Solid Films, 2017

The influence of thermal cycling on the microstructure, magnetic phase transition and magnetic en... more The influence of thermal cycling on the microstructure, magnetic phase transition and magnetic entropy change of a Gd 5 Si 1.3 Ge 2.7 thin film up to 1000 cycles is investigated. The authors found that after 1000 cycles a strong reduction of the crystallographic phase responsible for the magnetostructural transition (Orthorhombic II phase) occurs. This is attributed to chemical disorder, caused by the large number of expansion/compression cycles that the Orthorhombic II phase undergoes across the magnetostructural transition. The suppression of the magnetostructural transition corresponds to a drastic decrease of the thin film magnetic entropy change. These results reveal the importance of studying the thermal/magnetic cycles influence on magnetostructural transitions as they can damage a real-life device.

Physical Review B, 2004

Temperature and magnetic field dependencies of the magnetization of single crystal Gd 5 Ge 4 indi... more Temperature and magnetic field dependencies of the magnetization of single crystal Gd 5 Ge 4 indicate antiferromagnetic coupling along the c direction below 130 K. Both a reversible spin-flop transition when a magnetic field of ∼8.4kOe is applied along the cdirection, and irreversible metamagneticlike transitions when a 20 kOe or greater magnetic field is applied at 4.3 K along any of the three crystallographic axes are observed. Although Gd3+ ions have negligible single ion anisotropy, the metamagnetic transitions and magnetization of Gd 5 Ge 4 in the ferromagnetic state depend on the crystallographic direction reflecting the anisotropy of the exchange interactions due to the distinctly layered Sm 5 Ge 4-type crystal structure.

Solid State Communications, 2015

Momentum resolved inverse photoemission spectroscopy measurements show that the dispersion of the... more Momentum resolved inverse photoemission spectroscopy measurements show that the dispersion of the unoccupied bands of Ni 2 MnGa is significant in the austenite phase. In the martensite phase, it is markedly reduced, which is possibly related to the structural transition to an incommensurate modulated state in the martensite phase. Based on first principle calculations of the electronic structure of Ni-Mn-Ga, we show that the modification of the spectral shape with surface composition is related to change in the hybridization between the Mn 3d and Ni 3d-like states that dominate the unoccupied conduction band.

Physical Review B, 2001

A comprehensive characterization of the compound Fe 2 VAl was carried out. Samples grown by arc m... more A comprehensive characterization of the compound Fe 2 VAl was carried out. Samples grown by arc melting or the Bridgman method have Al and Fe deficiencies of up to 5 at. %. Czochralski-grown samples were Fe rich and Al deficient. X-ray diffraction implies appreciable antisite disorder in all of our samples. Fourier-transform infrared ͑FTIR͒ spectroscopy measurements showed that the carrier density and scattering time had little sample-to-sample variation or temperature dependence for near-stoichiometric samples. FTIR and dc resistivity suggest that the transport properties of Fe 2 VAl are influenced by both localized and delocalized carriers, with the former primarily responsible for the negative temperature coefficient of resistivity. Magnetization measurements reveal that near-stoichiometric samples have superparamagnetic clusters with at least two sizes of moments. We conclude that in Fe 2 VAl, antisite disorder causes significant modification to the semimetallic band structure proposed theoretically. With antisite disorder considered, we are now able to explain most of the physical properties of Fe 2 VAl. None of our data suggest heavy-fermion behavior in our samples.

MRS Proceedings, 2003

ABSTRACTSingle crystals of Co48Ni22Ga30 have been synthesized using a modified Bridgman method. T... more ABSTRACTSingle crystals of Co48Ni22Ga30 have been synthesized using a modified Bridgman method. The ability to solidify and retain single phase B2 austenite was found to depend not only on the starting composition and growth rate, but also the ability to maintain sufficiently high cooling rates to avoid the precipitation of a Co-rich FCC phase during post-solidification cooling. DSC measurements on the single crystal found the Ms, Mf, As, and Af to be 35.7°C, -1.8°C, 34.1°C and 72.2°C, respectively. On subsequent heating the B2 phase was found to partially decompose into the Co-rich phase at temperatures exceeding 380°C. Decomposition of the single phase B2 phase was tracked by microstructural observation, DSC, powder diffraction and low temperature heat capacity measurement. Restoration of the crystal to single phase B2 austenite required annealing of the crystal at temperatures above 1125°C followed by rapid cooling.

Journal of Applied Physics, 2015

Gd5(SixGe1−x)4 has mixed phases in the composition range 0.32 < x < 0.41, which have not be... more Gd5(SixGe1−x)4 has mixed phases in the composition range 0.32 < x < 0.41, which have not been widely studied. In this paper, we have synthesized and indexed single crystal samples of Gd5Si1.3Ge2.7 and Gd5Si1.4Ge2.6. We have investigated the first order and second order phase transition temperatures of these samples using magnetic moment vs. temperature and magnetic moment vs. magnetic field at different temperatures. We have used a modified Arrott plot technique that was developed and reported by us previously to determine the “hidden” second order phase transition temperature of the orthorhombic II phase.

Smart Structures and Materials 2003: Active Materials: Behavior and Mechanics, 2003

The growing interest in ferromagnetic shape-memory Ni-Mn-Ga for implementation in actuator applic... more The growing interest in ferromagnetic shape-memory Ni-Mn-Ga for implementation in actuator applications originates from the fact that this class of materials exhibits large strains when driven by a magnetic field. Large bidirectional strains up to a theoretical 6% are produced in these materials by twin boundary motion as martensite variants rotate to align respectively parallel or perpendicular to applied magnetic fields or stresses. These strains represent a significant improvement over piezoelectric and magnetostrictive materials. In this paper, we report on experimental measurements conducted on Ni-Mn-Ga cylindrical rods subjected to uniaxial stresses and uniaxial magnetic fields which were applied collinearly along the magnetic easy axis direction of the rods. To this end, a test apparatus was developed which consists of a water-cooled solenoid actuator and a loading fixture. Despite the lack of a readily recognizable mechanism for reversible deformations, bidirectional strains as large as 4300 ppm (0.43%) were observed, or three times the saturation magnetostriction of Terfenol-D. This paper presents room-temperature data including magnetization hysteresis, strain versus field and peak strain versus stress curves collected over a range of stresses between 0-65 MPa. From the latter set of curves, blocking force values are estimated as those for which the strain is 1% of the maximum (zero-load) strain. The results illustrate the sensitivity of material behavior with respect to composition at different driving conditions and offer insight on the choice of material compositions at which maximum actuation performance is achieved.

Physical Review B, 2007

The critical magnetic fields required to induce the magnetostructural transformation below ∼30 K ... more The critical magnetic fields required to induce the magnetostructural transformation below ∼30 K in Gd 5 Ge 4 are dependent on the size of the magnetic-field step employed during isothermal measurements of magnetization: the smaller the step, the lower the critical field. The influence of the magnetic-field step size on the character of the magnetostructural transition in Gd 5 Ge 4 diminishes as temperature increases, nearly disappearing above ∼30 K. Decreasing the size of the field step also leads to the formation of multiple steps in the magnetization. The steps are reproducible in the same sample at low temperatures (below ∼9 K) but they become stochastic and irreproducible at high temperatures (above ∼20 K). The varying dynamics of both the magnetization and demagnetization processes is associated with approaching true equilibrium states and, therefore, reduction of the size of the magnetic-field step at low temperatures plays a role similar to the dominant role of thermal fluctuations at high temperatures. Similar phenomena are expected to occur in other martensiticlike systems, e.g., the manganites.

$Research paper thumbnail of Neutron diffraction studies of the magnetoelastic compounds Tb_{5}Si_{x}Ge_{4−x} ( x=2.2 and 2.5)$

Physical Review B, 2005

We report the results of a neutron diffraction study, carried out on both single crystalline and ... more We report the results of a neutron diffraction study, carried out on both single crystalline and polycrystalline samples of Tb 5 Si 2.2 Ge 1.8 and polycrystalline Tb 5 Si 2.5 Ge 1.5. On cooling, at approximately 120 K, the Tb 5 Si 2.2 Ge 1.8 system undergoes a magnetoelastic transition from a high-temperature monoclinicparamagnetic to a low-temperature orthorhombic-ferromagnetic structure. Between 120 K and 75 K, the magnetic structure has a net ferromagnetic component along the a axis direction. The moments are slightly canted with respect to the a axis, while the components along the b and c axes are ordered antiferromagnetically. A second magnetic transition occurs at approximately 75 K. Below this temperature, the magnetic structure consists of ferromagnetically aligned μ x and μ z projections of the magnetic moments and an antiferromagnetic arrangement of theμ y moment components. Magnetic structures of Tb 5 Si 2.5 Ge 1.5 are nearly identical to those of Tb 5 Si 2.2 Ge 1.8 .

Physical Review B, 2006

The magnetization of single crystal Gd 5 Ge 4 , which in a zero magnetic field orders antiferroma... more The magnetization of single crystal Gd 5 Ge 4 , which in a zero magnetic field orders antiferromagnetically at 128 K, indicates a reversible spin-flop transition when the magnetic field is along the c axis and the absence of similar transformations when the magnetic field vector is perpendicular to the c axis. This anisotropic behavior is due to variation of magnetization energy between the c axis and the a or b axes of the orthorhombic crystal caused by a different alignment of the Gd moments with respect to the magnetic field vector. The anisotropy of the antiferromagnetic state diminishes with the increasing magnetic field and temperature. The critical magnetic field for the antiferromagnetic-ferromagnetic transition is the smallest and the ferromagnetic state is most stable when the magnetic field vector is parallel to the b axis, indicating an easy magnetization direction along this axis. The anisotropy of the magnetic field-induced transformation in Gd 5 Ge 4 is discussed in connection with the coupled magnetic and structural transitions. Anisotropic magnetic phase diagrams along the three major crystallographic axes are constructed.

$Research paper thumbnail of Anisotropic magnetic deflagration in single crystals of Gd_{5}Ge_{4}$

Physical Review B, 2012

We demonstrate the existence of a charge-density-wave (CDW) associated with an incommensurate per... more We demonstrate the existence of a charge-density-wave (CDW) associated with an incommensurate periodic lattice distortion on Ni2MnGa surface in the ferromagnetic state. Our temperature dependent photoemission spectra provide compelling evidence of a pseudogap at the Fermi level for TCDW ≤ 270 K that appears at the onset of the pre-martensite phase and persists in the martensite phase. While the width of the pseudogap is about 25 meV, a spectral weight transfer is observed over a much wider energy range that is associated with the CDW.

$Research paper thumbnail of Magnetic and thermal properties of Er_{75}Dy_{25} single crystals$

Physical Review B, 2011

We report on an experimental study of the thermal and magnetic properties of Er 75 Dy 25 single c... more We report on an experimental study of the thermal and magnetic properties of Er 75 Dy 25 single crystals with magnetic fields applied parallel to the [0001] and [101̅ 0] directions. The [0001] direction is the easy axis of magnetization, while the [101̅ 0] direction is the hard axis of magnetization. Three major transitions are observed in the heat capacity, magnetization, and alternating current magnetic susceptibility data. A firstorder transition is observed at ∼30 K, and two second-order transitions occur at ∼45 and ∼110 K. The H-T phase diagrams constructed from the experimental data reveal that several magnetic phases, including ferromagnetic cone, ferromagnetic fan, and c axis-modulated phases, exist in an Er 75 Dy 25 single crystal. Both similarities and dissimilarities are observed in the H-T phase diagrams of Er 75 Dy 25 when compared to the H-T phase diagrams of pure Er single crystals.

Journal of Applied Physics, 2004

We report results of thermal expansion (TE) and magnetostriction (MS) measurements on a single cr... more We report results of thermal expansion (TE) and magnetostriction (MS) measurements on a single crystal sample of Gd5(Si0.5Ge0.5)4 prepared by the Bridgman method. TE and MS were measured along the c axis by the strain gauge method and the temperature was controlled using a closed cycle helium refrigerator. From the TE measurements the magnetic structural phase transition temperature was found to be 259.5 K on cooling and 261.5 K on heating. The abrupt change in strain and the temperature hysteresis indicate that it is a first order transition. MS measurements were conducted at 15, 258, and 265 K. At 15 K, the magnetostriction amplitude was 3–4 ppm, whereas at 258 K it was 100 ppm. At 265 K, which is just above the Curie temperature, a giant magnetostriction of 2000 ppm was found. This unusual behavior is due to the fact that the external magnetic field can increase the transition temperature above 265 K, resulting in a first order magnetic/structural phase transition. The results re...

Journal of Applied Physics, 2006

The Tb5(SixGe4−x) alloy system is similar to the better known Gd5(SixGe4−x), except it has a more... more The Tb5(SixGe4−x) alloy system is similar to the better known Gd5(SixGe4−x), except it has a more complex magnetic and structural phase diagram. Gd5(SixGe1−x)4 has received much attention recently due to its giant magnetocaloric effect, colossal magnetostriction and giant magnetoresistance in the vicinity of a first order combined magnetic-structural phase transition. The magnetostriction changes that accompany the phase transitions of single crystal Tb5(Si2.2Ge1.8) have been investigated at temperatures between 20 and 150K by measurements of magnetostriction along the a axis. Over this temperature range the shape and slope of the magnetostriction curves change, indicative of changes in the magnetic state, crystal structure, and magnetic anisotropy. The results appear to indicate a phase transition that occurs near 106K (onset-completion range of 116–100K). The steepness of the strain transition, its unusual hysteresis, and its temperature dependence appear to indicate a first order...