Durga Paudyal

Magnetic transition in NiPt alloy systems: experiment and theory

We report here the preparation and measurements on the susceptibility, sound velocity and internal friction for NiPt systems. We then compare these experimental results with the first principle theoretical predictions and show that there is reasonable agreement with experiment and theory.

Origins of ferromagnetism and antiferromagnetism in Gd5Ge4

The total and partial density of states (DOS) of two polymorphic modifications of Gd5Ge4 show varying hybridization between the 5d states of Gd and the 4p states of Ge resulting in differences in the magnetism of the compound that are consistent with changes in the chemical bonding. The integrated numbers of electrons in the minority and majority bands differ substantially from one allotrope to another, showing different magnetic moments. A substantial decrease of indirect exchange interactions between 4f spins of specific Gd atoms, which is reflected in the decrease of 5d DOS and the 5d band splitting at the Fermi level, supports the antiferromagnetic ground state of Gd5Ge4. As follows from calculations of 4f–5d exchange interaction energies, total energy analysis of different spin configurations, and band energy analysis, short range interactions are ferromagnetic but long range antiferromagnetic coupling between Gd atoms prevails in the ground state allotrope of Gd5Ge4. On the other hand, only ferromagnetic coupling is found in the high field Gd5Ge4 structure.

Magnetic properties of X-Pt (X = Fe, Co, Ni) alloy systems

We have studied the electronic and magnetic properties of Fe–Pt, Co–Pt and Ni–Pt alloy systems in ordered and disordered phases. The influence of various exchange–correlation functionals on the values of equilibrium lattice parameters and magnetic moments in ordered Fe–Pt, Co–Pt and Ni–Pt alloys have been studied using the linearized muffin-tin orbital method. The electronic structure calculations for the disordered alloys have been carried out using an augmented space recursion technique in the framework of the tight binding linearized muffin-tin orbital method. The effect of short-range order has also been studied in the disordered phase of these systems. The results show good agreement with available experimental values.

Kondo Lattice and Antiferromagnetic Behavior in Quaternary CeTAl4Si2 (T = Rh, Ir) Single Crystals

We have explored in detail the anisotropic magnetic properties of CeRhAl4Si2 and CeIrAl4Si2, which undergo two antiferromagnetic transitions, at TN1 = 12.6 and 15.5 K, followed by a second transition at TN2 = 9.4 and 13.8 K, respectively, with the [001]-axis as the relatively easy axis of magnetization. The electrical resistivity at ambient and applied pressure provides evidence of Kondo interaction in both compounds, further supported by a reduced value of the entropy associated with the magnetic ordering. The Sommerfeld coefficient γ is inferred to be 195.6 and 49.4 mJ/(mol K2) for CeRhAl4Si2 and CeIrAl4Si2, respectively, classifying these materials as moderate heavy-fermion compounds. The crystal electric field energy levels are derived from the peak seen in the Schottky heat capacity. Furthermore, we have also performed electronic structure calculations by using the local spin density approximation + U [LSDA+U] approach, which provide physical insights on the observed magnetic behavior of these two co...

Phase stability and magnetism in NiPt and NiPd alloys

We show that the differences in stability of 3d?5d NiPt and 3d?4d NiPd alloys arise mainly due to relativistic corrections. The magnetic properties of disordered NiPd and NiPt alloys also differ due to these corrections, which lead to increase in the separation between the s?d bands of 5d elements in these alloys. For the magnetic case we also analyse the results in terms of splitting of majority and minority spin d?band centres of the 3d elements. We further examine the effect of relativistic corrections to the pair energies and order?disorder transition temperatures in these alloys. The magnetic moments and Curie temperatures have also been studied along with the short range ordering/segregation effects in NiPt/NiPd alloys.

Balancing structural distortions via competing 4f and itinerant interactions: a case of polymorphism in magnetocaloric HoCo2

The nature of multiple magnetostructural transformations in HoCo2 has been studied by employing magnetic and specific heat measurements, temperature and magnetic field dependent X-ray powder diffraction, and first-principles calculations. Unexpected increase of magnetization observed below the spin-reorientation temperature (TSR) suggests that the low-temperature transition involves a reduction of Co moment. First principles calculations confirm that the paramagnetic cubic to ferrimagnetic tetragonal transformation at TC is assisted by itinerant electron metamagnetism, and that the reduction of Co moment in HoCo2 occurs in parallel with the ferrimagnetic tetragonal to the nearly ferromagnetic orthorhombic transformation at TSRvia the rearrangement of both 3d states of Co and 5d states of Ho. The ac magnetic susceptibility measurements show significant magnetic frustration below TC. In contrast to earlier reports neither ac nor dc magnetic susceptibilities show anomalies in the paramagnetic region obeying the Curie–Weiss law.

Synthesis, Crystal and Electronic Structure of the Quaternary Magnetic EuTAl4Si2 (T = Rh and Ir) Compounds

Single crystals of the quaternary europium compounds EuRhAl4Si2 and EuIrAl4Si2 were synthesized by using the Al-Si binary eutectic as a flux. The structure of the two quaternary compounds has been refined by single crystal X-ray diffraction. Both compounds are stoichiometric and adopt an ordered derivative of the ternary KCu4S3 structure type (tetragonal tP8, P4/mmm). The two compounds reported here represent the first example of a quaternary and truly stoichiometric 1:1:4:2 phase crystallizing with this structure type. In light of our present results, the structure of the BaMg4Si3 compound given in literature as representing a new prototype is actually isotypic with the KCu4S3 structure. Local spin density approximation including the Hubbard U parameter (LSDA + U) calculations show that Eu ions are in the divalent state, with a significant hybridization between the Eu 5d, Rh (Ir) 4d (5d), Si 3p and Al 3p states. Magnetic susceptibility measured along the [001] direction confirms the divalent nature of the Eu ions in EuRhAl4Si2 and EuIrAl4Si2, which order magnetically near ∼11 and ∼15 K, respectively.

Low temperature crystal structure and magnetic properties of RAl2

Low temperature crystal structure and magnetic properties of RAl2 (R = Pr and Nd) have been studied using temperature dependent powder x-ray diffraction, magnetization, and heat capacity measurements. Unlike PrAl2, NdAl2 retains cubic MgCu2-type structure from room temperature down to 5 K, which is also confirmed from first principles electronic structure calculations. The magnetization measurements show both PrAl2 and NdAl2 order ferromagnetically at TC = 32 K and 77 K, respectively. However, the magnetization measurements show the former is a hard ferromagnet compared to the latter which is a soft ferromagnetic material. The magnetic entropy change obtained from heat capacity measurements at ΔH = 30 kOe for PrAl2 and NdAl2 are 3.15 J mol−1 K−1 and 1.18 J mol−1 K−1, respectively.

Phase stability analysis in Fe-Pt and Co-Pt alloy systems: an augmented space study

We have studied the problem of phase stability in Fe-Pt and Co-Pt alloy systems. We have used the orbital peeling technique in the conjunction of augmented space recursion based on the tight binding linear orbital method as the method for the calculation of pair interaction energies. In particular, we have generalized our earlier technique to take account of magnetic effects for the cases where the magnetic transition is higher than the order-disorder chemical transition temperature as in the case of Co 3 Pt. Our theoretical results obtained within this framework successfully reproduce the experimentally observed trends.