23 research outputs found
Temperature dependent dynamic and static magnetic response in magnetic tunnel junctions with Permalloy layers
Ferromagnetic resonance and static magnetic properties of CoFe/Al2O3/CoFe/Py
and CoFe/Al2O3/CoFeB/Py magnetic tunnel junctions and of 25nm thick
single-layer Permalloy(Py) films have been studied as a function of temperature
down to 2K. The temperature dependence of the ferromagnetic resonance excited
in the Py layers in magnetic tunnel junctions shows knee-like enhancement of
the resonance frequency accompanied by an anomaly in the magnetization near
60K. We attribute the anomalous static and dynamic magnetic response at low
temperatures to interface stress induced magnetic reorientation transition at
the Py interface which could be influenced by dipolar soft-hard layer coupling
through the Al2O3 barrier
Unexpected resonant response in [Fe(001)/Cr(001)]_(10) /MgO(001) multilayers in magnetic field
We observed unexpected resonant response in [Fe/Cr]_(10) multilayers
epitaxially grown on MgO(100) substrates which exists only when both ac current
and dc magnetic field are simultaneously applied. The magnitude of the
resonances is determined by the multilayer magnetization proving their
intrinsic character. The reduction of interface epitaxy leads to non-linear
dependence of the magnitude of resonances on the alternating current density.
We speculate that the existence of the interface transition zone could
facilitate the subatomic vibrations in thin metallic films and multilayers
grown on bulk insulating substrates.Comment: 6 pages, 5 figure
Enhancement of the Thermal Conductivity in gapped Quantum Spin Chains
We study mechanism of magnetic energy transport, motivated by recent
measurements of the thermal conductivity in low dimensional quantum magnets. We
point out a possible mechanism of enhancement of the thermal conductivity in
gapped magnetic system, where the magnetic energy transport plays a crucial
role. This mechanism gives an interpretation for the recent experiment of
CuGeO_3, where the thermal conductivity depends on the crystal direction.Comment: 4 pages, 2 figure
Anomalous thermal conductivity of NaV2O5 as compared to conventional spin-Peierls system CuGeO3
A huge increase of thermal conductivity k is observed at the phase transition
in stoichiometric NaV2O5. This anomaly decreases and gradually disappears with
deviation from stoichiometry in Na(1-x}V2O5 (x = 0.01, 0.02, 0.03, and 0.04).
This behavior is compared with that of pure and Zn-doped CuGeO3 where only
modest kinks in the k(T) curves are observed at the spin-Peierls transition.
The change of k at critical temperature Tc could be partially attributed to the
opening of an energy gap in the magnetic excitation spectrum excluding the
scattering of thermal phonons on spin fluctuations. However, the reason for
such a strong anomaly in the k(T) may lie not only in the different energy
scales of CuGeO3 and NaV2O5, but also in the different character of the phase
transition in NaV2O5 which can have largely a structural origin, e.g. connected
with the charge ordering.Comment: PostScript 4 pages, 4 PostScript pictures. Submitted to Physical
Review Letter
Thermodynamics of Spin S = 1/2 Antiferromagnetic Uniform and Alternating-Exchange Heisenberg Chains
The magnetic susceptibility chi and specific heat C versus temperature T of
the spin-1/2 antiferromagnetic alternating-exchange (J1 and J2) Heisenberg
chain are studied for the entire range 0 \leq alpha \leq 1 of the alternation
parameter alpha = J2/J1. For the uniform chain (alpha = 1), detailed
comparisons of the high-accuracy chi(T) and C(T) Bethe ansatz data of Kluemper
and Johnston are made with the asymptotically exact low-T field theory
predictions of Lukyanov. QMC simulations and TMRG calculations of chi(alpha,T)
are presented. From the low-T TMRG data, the spin gap Delta(alpha)/J1 is
extracted for 0.8 \leq alpha \leq 0.995. High accuracy fits to all of the above
numerical data are obtained. We examine in detail the theoretical predictions
of Bulaevskii for chi(alpha,T) and compare them with our results. Our
experimental chi(T) and C(T) data for NaV2O5 single crystals are modeled in
detail. The chi(T) data above the spin dimerization temperature Tc = 34 K are
not in agreement with the prediction for the uniform Heisenberg chain, but can
be explained if there is a moderate ferromagnetic interchain coupling and/or if
J changes with T. By fitting the chi(T) data, we obtain Delta(T = 0) = 103(2)
K, alternation parameter delta(0) = (1 - alpha)/(1 + alpha) = 0.034(6) and
average exchange constant J(0) = 640(80) K. The delta(T) and Delta(T) are
derived from the data. A spin pseudogap with a large magnitude \approx 0.4
Delta(0) is consistently found just above Tc, which decreases with increasing
T. Analysis of our C(T) data indicates that at Tc, at least 77% of the entropy
change due to the transition at Tc and associated order parameter fluctuations
arise from the lattice and/or charge degrees of freedom and less than 23% from
the spin degrees of freedom.Comment: 53 two-column REVTeX pages, 50 embedded figures, 7 tables. Revisions
required due to incorrect Eq. (39) in Ref. 51 which gives the low-T
approximation for the specific heat of a S = 1/2 1D system with a spin gap;
no conclusions were changed. Additional minor revisions made. Phys. Rev. B
(in press