Publications

The Comparison of Direct and Indirect Methods for Determining the Magnetocaloric Parameters in the Heusler Alloy Ni50Mn34.8In14.2B

Igor Dubenko et al. — Fri, 02 Nov 2012 12:05:46 PDT

The magnetocaloric properties of the Ni₅₀Mn_34.8In_14.2B Heusler alloy have been studied by direct measurements of the adiabatic temperature change (ΔT_AD(T,H)) and indirectly by magnetization (M(T,H)), differential scanning calorimetry, and specific heat (C(T,H)) measurements. The presence of a first-order ferromagnetic-paramagnetic transition has been detected for Ni₅₀Mn_34.8In_14.2B at 320 K. The magnetocaloric parameters, i.e., the magnetic entropy change (ΔS_M = (2.9-3.2) J/kgK) and the adiabatic temperature change (ΔT_AD = (1.3-1.52) K), have been evaluated for ΔH = 1.8 T from C_P(T,H) and M(T,H) data and from direct ΔT_AD(T,H) measurements. The extracted magnetocaloric parameters are comparable to those of Gd.

The Role of Ni-Mn Hybridization on the Martensitic Phase Transitions in Mn-rich Heusler Alloys

Mahmud Khan et al. — Fri, 02 Nov 2012 12:05:45 PDT

Room temperature x-ray diffraction, dc magnetization, and ac susceptibility measurements have been performed on a series of Mn rich Ni₅₀Mn_37-xCr_xSb₁₃ and Ni_50+xMn_37-xSb₁₃ Heusler alloys. Depending on the value of x, the room temperature crystal structures of the samples are either L2₁ cubic or orthorhombic. It is a commonly accepted idea that the martensitic transition temperatures in Ni-Mn-Z (Z = Ga, In, Sb, Sn) based Heusler alloys decrease (increase) with decreasing (increasing) valence electron concentration, e/a. However, the present work shows that regardless of the change in e/a, the martensitic transition temperature (T_M) decreases with increasing Cr or Ni concentration. These results support the model where, in the case of Mn rich Heusler alloys, it is the hybridization between the Ni atoms and the Mn atoms in the Z sites that plays the dominant role in driving the martensitic transformation.

Magnetostructural Phase Transitions and Magnetocaloric Effects in MnNiGe1-xAlx

Tampas Samanta et al. — Fri, 02 Nov 2012 12:05:44 PDT

The thermomagnetic and magnetocaloric properties of the MnNiGe_1−xAl_x system have been investigated by magnetization and differential scanning calorimetry (DSC) measurements. The presence of first-order magnetostructural transitions (MSTs) from hexagonal ferromagnetic to orthorhombic antiferromagnetic phases has been detected for x = 0.085 and 0.09 at 193 K and 186 K, respectively. The values of latent heat (L = 6.6 J/g) and corresponding total entropy changes (ΔS_T = 35 J/kg K) have been evaluated for the MST (x = 0.09) from DSC measurements. The magnetic entropy change for x = 0.09 (ΔS_M = 17.6 J/kg K for 5 T) was found to be comparable with well-known giant magnetocaloric materials, such as Gd₅Si₂Ge₂, MnFeP_0.45As_0.55, and Ni₅₀Mn₃₇Sn₁₃.

Direct Measurements of Field-Induced Adiabatic Temperature Changes Near Compound Phase Transitions in Ni–Mn–In Based Heusler Alloys

A. P. Kazakov et al. — Fri, 02 Nov 2012 12:05:43 PDT

The adiabatic temperature changes (ΔT_ad) in the vicinity of the Curie and martensitic transition temperatures of Ni₅₀Mn₃₅In₁₅ and Ni₅₀Mn₃₅In₁₄Z (Z = Al and Ge) Heusler alloys have been studied using an adiabatic magnetocalorimeter of 250–350 K temperature interval for applied magnetic field changes up to ΔH = 1.8 T. The largest measured changes were ΔT_ad = −2 and 2 K near the martensitic (first-order) and ferromagnetic (second-order) transitions for ΔH = 1.8 T, respectively. It was observed that |ΔT_ad| ≈ 1 K for relatively small field changes (ΔH = 1 T) for both types of transitions. The results indicate that these materials should be further explored as potential working materials in magnetic refrigeration applications.

Magnetic and Magnetocaloric Properties of the New Rare-Earth-Transition-Metal Intermetallic Compound Gd3Co29Ge4B10

P. Hill et al. — Fri, 02 Nov 2012 12:05:42 PDT

The compounds Gd_3-xY_xCo₂₉Ge₄B₁₀ (x = 0, 0.5, 1.0, 1.5, and 3.0), Gd₃Co₂₉Al₄B₁₀, and Gd₃Co₂₉Al₄B₁₀ were synthesized by arc melting, and their magnetic properties investigated as a function of temperature and applied magnetic field. X-ray measurements showed primarily single-phase samples with the tetragonal crystal structure P4/nmm. It was found that Gd₃Co₂₉Ge₄B₁₀ orders ferromagnetically at T_C = 212 K and shows a compensation point at 128 K, indicating a ferrimagnetic ordering of the Co and Gd moments. An entropy change of −ΔS = 0.5 J/kgK was observed in a 5-T field at T_C for this sample, while a change in sign for this quantity was observed both at the maximum value of magnetization (around 200 K) and then again at the compensation point. Substitution of Y for Gd in Gd₃Co₂₉Ge₄B₁₀ does not affect the Curie temperature, but shifts the compensation point to lower temperatures. This indicates that a decrease in Gd concentration does not affect the d-d exchange interaction, but has a pronounced effect on the f-d exchange interaction.

Induced Magnetic Anisotropy and Spin Polarization in Pulsed Laser-Deposited Co2MnSb Thin Films

Moti R. Paudel et al. — Fri, 02 Nov 2012 12:05:41 PDT

Co₂MnSb thin films were grown on glass and GaAs (001) substrates using pulsed laser deposition. The films were grown in magnetic fields (H_G = 500 Oe and 0 Oe) that were applied in the plane of the substrate during the deposition process. Angle-dependent magneto-optic Kerr effect measurements for films grown on glass revealed a uniaxial magnetic anisotropy in the direction of the applied growth field. Films grown on GaAs (001) exhibited more complicated magnetic anisotropy behavior, due to additional contributions from the substrate. Point contact Andreév reflection spectroscopy measurements indicated that the spin polarizations of the films were about P~ 50%, with negligible difference between films grown in zero and non-zero applied fields.

Ferromagnetic ZnO Nanocrystals and Al-induced Defects

Chinmay Phadnis et al. — Fri, 02 Nov 2012 12:05:40 PDT

ZnO nanocrystals (NCs) capped with polyvinyl pyrrolidone reveal room temperature ferromagnetism. Incorporation of Al³⁺-ions induce defects in ZnO NCs leading to quenching of excitonic luminescence of ZnO at the cost of an increase in the intensity of oxygen vacancy related emission. Photoluminescence excitation spectra exhibit an additional hump like feature attributed to Al-doping. Saturation magnetization of Al³⁺-doped ZnO NCs is the same as that of the undoped ZnO NCs. However, a remarkable decrease in the coercivity associated with change in the nature of M (T) curve and electron paramagnetic resonance signal with g = 1.96 is observed consequent to Al doping. The results provide direct evidence of the defects within the core of NCs that are responsible for the ferromagnetic ordering in the Al³⁺-doped ZnO. The M(T) curve unravels a typical exchange mechanism.

Electronic Transport Imaging in a Multiwire SnO2 Chemical Field-Effect Transistor Device

Sergei V. Kalinin et al. — Fri, 02 Nov 2012 12:05:39 PDT

The electronic transport and the sensing performance of an individual SnO₂ crossed-nanowires device in a three-terminal field-effect transistor configuration were investigated using a combination of macroscopic transport measurements and scanning surface-potential microscopy (SSPM). The structure of the device was determined using both scanning electron- and atomic force microscopy data. The SSPM images of two crossed one-dimensional nanostructures, simulating a prototypical nanowire network sensors, exhibit large dc potential drops at the crossed-wire junction and at the contacts, identifying them as the primary electroactive elements in the circuit. The gas sensitivity of this device was comparable to those of sensors formed by individual homogeneous nanostructures of similar dimensions. Under ambient conditions, the dc transport measurements were found to be strongly affected by field-induced surface charges on the nanostructure and the gate oxide. These charges result in a memory effect in transport measurements and charge dynamics which are visualized by SSPM. Finally, scanning probe microscopy is used to measure the current-voltage characteristics of individual active circuit elements, paving the way to a detailed understanding of chemical functionality at the level of an individual electroactive element in an individual nanowire.

Grain Boundary Sliding Mechanisms in ZrN-Ag, ZrN-Au, and ZrN-Pd Nanocomposite Films

S. M. Aouadi et al. — Fri, 02 Nov 2012 12:05:38 PDT

Nanocomposite films of ZrN-Me (Me = Ag, Au, or Pd) were produced by reactive unbalanced magnetron sputtering and were found to form a dense and homogeneous microstructure whereby nanocrystals of Me are distributed evenly throughout the ZrN matrix. Interestingly, the Young’s modulus was found to decrease much more dramatically with the increase in metal content for the ZrN-Ag system. A systematic ab initio study was undertaken to understand the mechanism of grain boundary sliding in these nanostructures. The maximum energy variation during the sliding was found to be the largest and the smallest for ZrN-Pd and ZrN-Ag, respectively.

Correlation Between Interfacial Electronic Structure and Mechanical Properties of ZrN–Me (Me=Ag, Au, or Pd) Nanocomposite Films

S. M. Aouadi et al. — Fri, 02 Nov 2012 12:05:37 PDT

Nanocomposite films of ZrN–Me (Me=Ag, Au, or Pd) were prepared using reactive unbalanced magnetron sputtering. The hardness and elastic modulus were measured by nanoindention and were found to vary differently with composition for the three nanocomposite structures. Young’s modulus was found to decrease much more dramatically with the increase in Me content for the ZrN–Ag system. These findings were attributed to the weaker bonding mechanism at the interface between the ceramic and the metallic phases, which is more prone to grain-boundary sliding as shown using first-principles calculations of the electronic structure at the interface for the three systems.

Coupling Nanowire Chemiresistors with MEMS Microhotplate Gas Sensing Platforms

Douglas C. Meier et al. — Fri, 02 Nov 2012 12:05:36 PDT

Recent advances in nanotechnology have yielded materials and structures that offer great potential for improving the sensitivity, selectivity, stability, and speed of next-generation chemical gas sensors. To fabricate practical devices, the “bottom-up” approach of producing nanoscale sensing elements must be integrated with the “top-down” methodology currently dominating microtechnology. In this letter, the authors illustrate this approach by coupling a single-crystal SnO₂ nanowire sensing element with a microhotplate gas sensor platform. The sensing results obtained using this prototype sensor demonstrate encouraging performance aspects including reduced operating temperature, reduced power consumption, good stability, and enhanced sensitivity.

Direct Measurements of Field-Induced Adiabatic Temperature Changes Near Compound Phase Transitions in Ni–Mn–In Based Heusler Alloys

A. P. Kazakov et al. — Fri, 02 Nov 2012 11:43:52 PDT

Large Inverse Magnetic Entropy Changes and Magnetoresistance in the Vicinity of a Field-Induced Martensitic Transformation in Ni50−xCoxMn32−yFeyGa18

Arjun K. Pathak et al. — Tue, 31 Jan 2012 16:15:00 PST

Significantly large inverse magnetic entropy changes (ΔS_M) and magnetoresistance (MR) were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys: Ni_50−xCo_xMn_32−yFe_yGa₁₈. The crystal structures of alloys were tetragonal at 300 K and the phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion. The maximum peak values of the ΔS_M and MR at H = 5 T were determined as ≈ (+)31 J Kg⁻¹ K⁻¹ and ≈−21%, respectively, for x = 8 and y = 2. The relatively small hysteretic loss and large refrigeration capacity observed in this system make these compounds promising materials for applications.

Magnetoresistance and Magnetocaloric Effect at a Structural Phase Transition from a Paramagnetic Martensitic State to a Paramagnetic Austenitic State in Ni50Mn36.5In13.5 Heusler Alloys

Arjun K. Pathak et al. — Tue, 31 Jan 2012 16:14:58 PST

It is established, using magnetization measurements, that Ni₅₀Mn_36.5In_13.5 is in a paramagnetic state (PS) above and below the martensitic transition temperature (T_M). Magnetoresistance (MR) and magnetic entropy changes (ΔS_M) in the vicinity of T_M were studied. MR and ΔS_M at T_M were found to be ≈−8% and ≈ +24 J Kg⁻¹ K⁻¹, respectively, at ΔH = 5 T. Although MR and ΔS_M values were lower than compared to those found in other Heusler systems, the significantly smaller hysteresis observed in Ni₅₀Mn_36.5In_13.5 makes this compound, and other such compounds that undergo a martensitic transition in a PS, promising for the study and applications of magnetocaloric magnetic materials.

Exchange Bias Behavior in Ni–Mn–Sb Heusler Alloys

Mahmud Khan et al. — Tue, 31 Jan 2012 16:14:57 PST

The authors report the observation of exchange bias in bulk polycrystalline Ni₅₀Mn_25+xSb_25−x Heusler alloys. Shifts in hysteresis loops of up to 248 Oe were observed in the 5 T field cooled samples. The observed exchange bias behavior in Ni₅₀Mn_25+xSb_25−x is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions in the system. Such behavior is an addition to the multifunctional properties of the Ni₅₀Mn_25+xSb_25−x Heusler alloy system.

Large Magnetic Entropy Change in Ni50Mn50−xInx Heusler Alloys

Arjun Kumar Pathak et al. — Tue, 31 Jan 2012 16:13:30 PST

The magnetocaloric properties of polycrystalline Ni₅₀Mn_50−xIn_x (15 ⩽ x ⩽ 16) associated with the second order magnetic transition at the Curie temperature and the first order martensitic transition were studied using magnetization measurements. The refrigeration capacity and magnetic entropy change were found to depend on the In concentration and reach a maximum value of refrigeration capacity of 280 J/kg with a magnetic entropy change of −6.8 J/kg K at 318 K for a magnetic field change of 5 T. These values of the magnetocaloric parameters are comparable to that of the largest values reported near the second order transition of metallic magnets near room temperature.

Magnetocaloric Properties of Ni2Mn1−xCuxGa

Shane Stadler et al. — Tue, 31 Jan 2012 15:56:16 PST

We present the magnetocaloric properties of the substituted Heusler alloy Ni₂Mn_1−xCu_xGa, which shows a maximum magnetic entropy change of ΔS_M ≈ −64 J/Kg K = −532 mJ/cm³ K at 308 K for a magnetic field change ΔH = 5 T. The dependence of ΔS_M on ΔH is approximately linear and does not reach saturation at 5 T. It is demonstrated that the temperature at which ΔS_M occurs can be tuned through subtle variations in composition.

Magnetic Phase Diagrams of Erbium

B. H. Frazer et al. — Wed, 11 Jan 2012 15:19:14 PST

The magnetic phase diagrams of erbium in the magnetic field–temperature plane have been constructed for applied magnetic fields along the a and b axes. For an a-axis applied field our H–T phase diagrams determined from magnetization and magnetoresistance data are in good agreement and consistent with that of Jehan et al. for temperatures below 50 K. A splitting of the basal plane Néel temperature (T_N⊥) above 3.75 T introduces two new magnetic phases. Also a transition from a fan to a canted fan phase as suggested by Jehan et al. is observed in an increasing field below T_C. Our phase diagram for a b-axis applied field constructed from magnetization data is very similar to the phase diagram of Watson and Ali using magnetoresistance measurements. However, the anomaly at 42 K reported by Watson and Ali is not observed in the present study. No splitting of the T_N⊥ transition is observed in either work for a field applied along the b axis.

Colossal Magnetoresistance in Ce Doped Manganese Oxides

J. R. Gebhardt et al. — Wed, 11 Jan 2012 15:11:13 PST

In this study we investigate the effects of Ce doping in R_1−xA_xMnO₃ (R = La, Ce, and A = Sr, Ce) on the magnetic and transport properties of this system. For La_1−xCe_xMnO₃ (LCMO), an increase in Ce concentration is accompanied by an increase in T_C from 225 to 236 K, as well as an increase in the electrical resistivity. An extremely high resistivity is observed in the new system Ce_1−xSr_xMnO₃ (CSMO) which becomes insulating below its Curie temperature of 43 K. A maximum magnetoresistance (MR) ratio of 40% for CSMO and 53% for LCMO is observed. A larger change in resistivity is seen to correspond to an increase in the Ce concentration, however this is offset by an overall resistivity increase which keeps the MR ratio low. The high resistivity may be due to unreacted oxides in the samples. If true, the amount of impurity appears to be proportional to the Ce doping. If this impurity level can be reduced, a significant colossal magnetoresistance effect could be exhibited by these systems.

Magnetic Properties of RTSb3

Michael Leonard et al. — Wed, 11 Jan 2012 12:54:41 PST

Magnetization, electrical resistivity, and thermal expansion measurements have been performed on polycrystalline RTSb₃ (R=La, Ce, Pr, Nd, Gd, Tb, and Dy; T=Cr, V) samples in order to study their magnetic properties. Depending on the rare-earth element, RTSb₃ has been found to have either a purely ferromagnetic (or antiferromagnetic) phase or combination of antiferromagnetic (lower-temperature) and ferromagnetic (higher-temperature) phases. The antiferromagnetic order evolves from the ferromagnetic order as a result of the competition between R³⁺ and Cr³⁺ ions. As R is changed from La to Dy, the antiferromagnetic ordering of the R³⁺ ions becomes more prominent and predominate over the ferromagnetic ordering of Cr³⁺ for R=Gd, Tb, and Dy. Thermal expansion measurements show that the antiferromagnetic transition accompanies a drop in the sample dimension. The order of the ferromagnetic transition is found to be of the second order.