Jagat Lamsal

Structural evolution of ammonia borane for hydrogen storage

We have studied the crystal structure of fully deuterated BH3NH3 using powder neutron diffraction at different temperatures. It is evident that an order-disorder phase transition occurs around 225K. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature tetragonal structure with space group I4mm above 225K. At 16K, the BD3–ND3 unit stacks along the c axis with a tilt angle of about 16° between the N–B bond and the c axis. As the temperature is increased, the BD3–ND3 groups start to reorient along the c axis and the deuterium atoms become disordered, leading to the tetragonal phase transition.

Observation of spin glass state in weakly ferromagnetic Sr2FeCoO6 double perovskite

A spin glass state is observed in the double perovskite oxide Sr2FeCoO6 prepared through sol-gel technique. Initial structural studies using x rays reveal that the compound crystallizes in tetragonal I4/m structure with lattice parameters, a = 5.4609(2) A and c = 7.7113(7) A. The temperature dependent powder x ray diffraction data reveal no structural phase transition in the temperature range 10-300 K. However, the unit cell volume shows an anomaly coinciding with the magnetic transition temperature thereby suggesting a close connection between lattice and magnetism. Neutron diffraction studies and subsequent bond valence sums analysis show that in Sr2FeCoO6, the B site is randomly occupied by Fe and Co in the mixed valence states of Fe3 + /Fe4+ and Co3+/Co4+. The random occupancy and mixed valence sets the stage for inhomogeneous magnetic exchange interactions and in turn, for the spin glass state in this double perovskite, which is observed as an irreversibility in temperature dependent dc magnetization...

Neutron diffraction analysis of magnetic ordering in YSr2Cu2RuO8

The superconducting ruthenocuprates RESr2Cu2RuO8 (where RE is a rare earth or Y) have been interpreted as ferromagnetic superconductors, based on a strong response to applied field at the Ru Neel temperature (around 130K). However, neutron diffraction measurements on the Gd compound by Lynn et al. [Phys. Rev. B 61, 1214964 (2000)] (using a separated isotope) show evidence only of antiferromagnetic order, while very limited data by Takagiwa et al. [J. Phys. Soc. Jpn. 70, 333 (2001)] on the Y compound show one antiferromagnetic Bragg peak and possible coexisting ferromagnetism. We have studied a sample of YSr2Cu2.1Ru0.9O7.9 prepared by a high pressure-high temperature route. Temperature dependent neutron powder diffraction shows antiferromagnetic order, manifested by the observation of the 12, 12, 12 and 12, 12, 32 reflections, which appear at 135K [as suggested by the superconducting quantum interference device (SQUID) data]. Their intensities at the lowest temperature are consistent with a c-axis orientat...

Magnetic, transport, and magnetocaloric properties of double perovskite oxide LaCaMnCoO6

Magnetic, magnetoresistive, and magnetocaloric properties of a novel double perovskite oxide, namely, LaCaMnCoO6 have been studied. Polycrystalline sample of LaCaMnCoO6 has been synthesized by sol-gel technique. It has cubic crystal structure (space group Fm3¯m) at room temperature. The temperature variation in magnetization reveals a steep increase in magnetization around 168K (TC). The magnetization does not even saturate at 5K and a magnetic moment of 0.7μB∕f.u. is obtained at 5K in an applied field of 50kOe. The electrical resistivity measurement indicates that the material is semiconducting-like in the temperature range of ∼300–50K and below ∼50K the sample becomes insulating. A maximum magnetoresistance (MR) of about 8% is found at 200K in an applied field of 7T and MR has a negative sign. The magnetocaloric effect is calculated from the magnetization versus temperature data and a maximum magnetic entropy change of 3.1J∕kgK for a field change of 11kOe is obtained near TC. Thus a moderate magnetocalo...

β-Mn-Type Co8+xZn12–x as a Defect Cubic Laves Phase: Site Preferences, Magnetism, and Electronic Structure

The results of crystallographic analysis, magnetic characterization, and theoretical assessment of β-Mn-type Co-Zn intermetallics prepared using high-temperature methods are presented. These β-Mn Co-Zn phases crystallize in the space group P4(1)32 [Pearson symbol cP20; a = 6.3555(7)-6.3220(7)], and their stoichiometry may be expressed as Co(8+x)Zn(12-x) [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co(8+x)Zn(12-x) reveals that the β-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed β-Mn structure type, a result that offers greater insight into the β-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).

Magnetization and neutron diffraction studies on Sr2TiMnO6

Magnetic properties of the double perovskite oxide, Sr2TiMnO6, have been studied by means of bulk magnetization and powder neutron diffraction experiments. Low field magnetization data reveal transitions at ∼45 K (TC) and at ∼15 K (TN). A magnetic moment value of only ∼0.23 μB/F.U. is attained at 5 K in the 7 T field. Powder neutron diffraction studies suggest the possible antiferromagnetic order in this compound at 12 K with moments of ∼0.5 μB at the Mn site.

Competing magnetic interactions in the intermetallic compounds Pr5Ge3 and Nd5Ge3

Magnetic properties of polycrystalline Pr5Ge3 and Nd5Ge3 (hexagonal, Mn5Si3-type) compounds have been studied. Magnetization measurements in 0.5 T field indicate that the Pr5Ge3 orders antiferromagnetically at 18 K (TN). However, in an applied field of 10 mT, the zero-field-cooled and field-cooled magnetization bifurcates below ∼65 K. This and the positive paramagnetic Curie temperature, obtained from the Curie–Weiss fit to the paramagnetic susceptibility, suggest the presence of competing ferromagnetic and antiferromagnetic interactions. The magnetization versus field isotherm at 5 K shows an S-shaped curve and a weak tendency to saturation in fields of 9 T with negligible hysteresis. The magnetic moment attains a value of 1.6 μB/Pr3+ at 5 K in 9 T. The magnetic entropy change near the magnetic transition has been calculated by measuring magnetization versus field isotherms close to TN. The entropy change is found to be considerably large. Neutron diffraction study indicates that below ∼43 K the Nd5Ge3 h...

Structural and Magnetic Properties of

Magnetic and structural properties of La0.7Sr0.3Mn1-xCrxO3 (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50) have been studied using neutron and x-ray diffraction and magnetic measurements. All samples are found to be of single phase. At 12 K, we find only ferromagnetic ordering for x < 0.30 and coexisting ferromagnetic and antiferromagnetic domains for x ⩾ 0.30. Our magnetic data indicate temperature dependent ferromagnetic to paramagnetic, and likely antiferromagnetic to paramagnetic transitions. We find no evidence of spin glass or cluster glass behavior, only long-range magnetic order with a transition to antiferromagnetic behavior beginning with x = 0.30 and almost completely antiferromagnetic behavior at x = 0.50.

Size effect on the structural, magnetic, and magnetotransport properties of electron doped manganite La0.15Ca0.85MnO3

Nanocrystalline La0.15Ca0.85MnO3 samples of various grain sizes ranging from ∼17 to 42 nm have been prepared by sol-gel technique. Phase purity and composition were verified by room temperature x-ray diffraction and SEM-EDAX analysis. The bulk La0.15Ca0.85MnO3 is known to order antiferromagnetically around 170 K and to undergo a simultaneous crystal structural transition. DC magnetization measurements on 17 nm size La0.15Ca0.85MnO3 show a peak at ∼130 K (TN) in zero-field-cooled (ZFC) state. Field-cooled magnetization bifurcates from ZFC data around 200 K hinting a weak ferromagnetic component near room temperature due to surface moments of the nanoparticle sample. Low temperature powder neutron diffraction experiments reveal that the incomplete structural transition from room temperature orthorhombic to low temperature orthorhombic-monoclinic state also occurs in the nanoparticle sample as in the bulk. Magnetization in the ordered state decreases as particle size increases, thus indicating the reduction ...

Cluster formation in quantum critical systems

The presence of magnetic clusters has been verified in both antiferromagnetic and ferromagnetic quantum critical systems. We review some of the strongest evidence for strongly doped quantum critical systems (Ce(Ru0.24Fe0.76)2Ge2) and we discuss the implications for the response of the system when cluster formation is combined with finite size effects. In particular, we discuss the change of universality class that is observed close to the order-disorder transition. We detail the conditions under which clustering effects will play a significant role also in the response of stoichiometric systems and their experimental signature.