Harsh Deep Chopra

Ballistic magnetoresistance in nickel single-atom conductors without magnetostriction

Large ballistic magnetoresistance (BMR) has been measured in Ni single-atom conductors electrodeposited between microfabricated thin films. These measurements eliminate magnetostriction related artifacts. By making measurements on single atom conductors, the benchmark for the incontrovertible evidence against magnetostriction is set at the unyielding condition of the known quantum mechanical principles, namely,

Non-Joulian magnetostriction

1{G}_{o}=2{e}^{2}}{h=1}∕12\phantom{\rule{0.2em}{0ex}}900{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}

Role of magnetostatic interactions in micromagnetic structure of multiferroics

(for ferromagnetic contacts the unit of conductance being

Atomic engineering of spin valves using Ag as a surfactant

\frac{1}{2}{G}_{o}=1∕25\phantom{\rule{0.2em}{0ex}}800{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}

Channel saturation and conductance quantization in single-atom gold constrictions

) is the universal threshold ballistic conductance of an unbroken single atom contact below which even an angstrom separation of the contact due to magnetostriction is immediately signaled by an abrupt and large increase in tunneling resistance of several hundred thousand ohms across the gap. The present approach to electrodeposited point contacts between microfabricated thin films also provides an independent confirmation of Garcias original BMR experiments on atomic point contacts that were made by a mechanical method [N. Garcia, M. Mu\~noz, and Y.-W. Zhao, Phys. Rev. Lett. 82, 2923 (1999)]. There are many intricacies and subtleties to be resolved and understood in the highly interesting BMR phenomenon. Conclusive elimination of magnetostriction related artifacts, which is most easily invoked as a primary alternative explanation to the electronic origin of BMR, is one step towards a better understanding of these atomic scale entities. In addition, several explanations of null effects in some of the reported literature are given.

In situ study of temperature dependent magnetothermoelastic correlated behavior in ferromagnetic shape memory alloys

All magnets elongate and contract anisotropically when placed in a magnetic field, an effect referred to as Joule magnetostriction. The hallmark of Joulian magnetostriction is volume conservation, which is a broader definition applicable to self-accommodation of ferromagnetic, ferroelectric or ferroelastic domains in all functional materials. Here we report the discovery of ‘giant’ non-volume-conserving or non-Joulian magnetostriction (NJM). Whereas Joulian strain is caused by magnetization rotation, NJM is caused by facile (low-field) reorientation of magnetoelastically and magnetostatically autarkic (self-sufficient) rigid micro-‘cells’, which define the adaptive structure, the origin of which is proposed to be elastic gradients ultimately caused by charge/spin density waves. The equilibrium adaptive cellular structure is responsible for long-sought non-dissipative (hysteresis-free), linearly reversible and isotropic magnetization curves along all directions within a single crystal. Recently discovered Fe-based high magnetostriction alloys with special thermal history are identified as the first members of this newly discovered magnetic class. The NJM paradigm provides consistent interpretations of seemingly confounding properties of Fe-based alloys, offers recipes to develop new highly magnetostrictive materials, and permits simultaneously large actuation in longitudinal and transverse directions without the need for stacked composites.

Magnetoelastic dependence of switching field in TbFe–FeCo giant magnetostrictive spring-magnet multilayers

While it is well known that magnetoelastic coupling governs the magnitude of field-induced strain in magnetic shape memory alloys, the present study shows that the zero-field micromagnetic structure and the pathway leading to the field-induced strain is governed by magnetostatic coupling across martensite twins. The micromagnetic investigations reveal a new energy barrier to the motion of domain walls arising from magnetostatic coupling between walls across the twin planes.

Anisotropic Curie temperature materials

In this study, dc magnetron sputtered NiO (50 nm)/Co (2.5 nm)/Cu(1.5 nm)/Co (3.0 nm) bottom spin valves were studied with and without Ag as a surfactant. At Cu spacer thickness of 1.5 nm, a strong positive coupling >13.92 kA/m (>175 Oe) between NiO-pinned and “free” Co layers leads to a negligible giant magnetoresistance (GMR) effect (<0.7%) in Ag-free samples. In contrast, spin valves deposited in the presence of ≈1 monolayer of surfactant Ag have sufficiently reduced coupling, 5.65 kA/m (71 Oe), which results in an order of magnitude increase in GMR (8.5%). Using transmission electron microscopy (TEM), the large positive coupling in Ag-free samples could directly be attributed to the presence of numerous pinholes. In situ x-ray photoelectron spectroscopy shows that, in Ag-containing samples, the large mobile Ag atoms float out to the surface during successive growth of Co and Cu layers. Detailed TEM studies show that surfactant Ag leaves behind smoother interfaces less prone to pinholes. The use of surf...

Evaluation of mechanical properties of magnetic materials using a non-destructive method

It is shown that the finite elasticity of atomic-sized gold constrictions allows for a continuous and reversible change in conductance. This is achieved by superposition of atomic-scale or subatomic-scale mechanical oscillations on a steadily retracting/approaching gold tip against a gold substrate. Through these perturbation studies, we report the direct observation of channel saturation and conductance quantization in stable, single-atom gold constrictions. In addition, the origin of peaks in conductance histograms is explained, and the peaks alone are shown to be insufficient in evaluating the stability of atomic configurations.

Magnetization reversal in polycrystalline NiO–Co exchange anisotropy coupled bilayers

This study reports the first in situ observation of temperature-dependent micromagnetic and twin structure in oriented single crystals of Ni–Mn–Ga Heusler alloys. Micromagnetic measurements were made over a temperature interval of 50 to −35 °C covering both forward and reverse martensitic transformation. Magnetic domains in the martensite phase were found to be uniformly spaced (25–30 μm); the direction of the domain walls conforms to the changing direction of the magnetic easy axis as they traverse from one twin to another. The martensite twins could be reoriented in applied fields as low as 1300 Oe.