60 research outputs found
Ab initio comparison of spin-transport properties in MgO-spaced ferrimagnetic tunnel junctions based on MnGa and MnAl
We report on first-principles spin-polarised quantum transport calculations
(from NEGF+DFT) in MgO-spaced magnetic tunnel junctions (MTJs) based on two
different Mn-based Heusler ferrimagnetic metals, namely MnAl and MnGa
in their tetragonal DO phase. The former is a fully compensated
half-metallic ferrimagnet, while the latter is a low-moment
high-spin-polarisation ferrimagnet, both with a small lattice mismatch from
MgO. In identical symmetric and asymmetric interface reconstructions across a
3-monolayer thick MgO barrier for both ferrimagets, the linear response
(low-voltage) spin-transfer torque (STT) and tunneling magneto-resistance (TMR)
effects are evaluated. A larger staggered in-plane STT is found in the MnGa
case, while the STT in MnAl vanishes quickly away from the interface
(similarly to STT in ferromagnetic MTJs). The roles are reversed for the TMR,
which is practically 100\% in the half-metallic MnAl-based MTJs (using the
conservative definition) as opposed to 60\% in the MnGa case. The weak
dependence on the exact interface reconstruction would suggest
MnGa-MnAl solid solutions as a possible route towards optimal trade-off
of STT and TMR in the low-bias, low-temperature transport regime.Comment: 6 pages, 4 figure
Quasi-static magnetization dynamics in a compensated ferrimagnetic half-metal -- MnRuGa
Exploring anisotropy and diverse magnetization dynamics in specimens with
vanishing magnetic moments presents a significant challenge using traditional
magnetometry, as the low resolution of existing techniques hinders the ability
to obtain accurate results. In this study, we delve deeper into the examination
of magnetic anisotropy and quasi-static magnetization dynamics in \mrg\,(MRG)
thin films, as an example of a compensated ferrimagnetic half-metal, by
employing anomalous Hall effect measurements within a tetragonal crystal
lattice system. Our research proposes an innovative approach to accurately
determine the complete set of anisotropy constants of these MRG thin films. To
achieve this, we perform anomalous Hall voltage curve fitting, using torque
models under the macrospin approximation, which allow us to obtain out-of-plane
anisotropy constants J m (\,T) and
J m (\,T), along with a weaker
in-plane anisotropy constant J m
(\,T). By additionally employing first-order reversal curves
(FORC) and classical Preisach hysteresis (hysterons) models, we are able to
validate the efficacy of the macrospin model in capturing the magnetic behavior
of MRG thin films. Furthermore, our investigation substantiates that the
complex quasi-static magnetization dynamics of MRG thin films can be
effectively modelled using a combination of hysteronic and torque models. This
approach facilitates the exploration of both linear and non-linear quasi-static
magnetization dynamics, in the presence of external magnetic field and/or
current-induced effective fields, generated by the spin-orbit torque and spin
transfer torque mechanisms.Comment: 14 pages, 10 figure
Magnetism of noncolinear amorphous DyCo3 and TbCo3 thin films
The magnetization of amorphous DyCo3 and TbCo3 is studied by magnetometry,
anomalous Hall effect and magneto-optic Kerr effect to understand the
temperature-dependent magnetic structure. A square magnetic hysteresis loop
with perpendicular magnetic anisotropy and coercivity that reaches 3.5 T in the
vicinity of the compensation temperature is seen in thin films. An anhysteretic
soft component, seen in the magnetization of some films but not in their Hall
or Kerr loops is an artefact due to sputter-deposition on the sides of the
substrate. The temperature-dependence of the net rare earth moment from 4-300K
is deduced, using the cobalt moment in amorphous YxCo1-x. The single-ion
anisotropy of the quadrupole moments of the 4f atoms in the randomly-oriented
local electrostatic field gradient overcomes their exchange coupling to the
cobalt subnetwork, resulting in a sperimagnetic ground state where spins of the
noncollinear rare-earth subnetwork are modelled by a distribution of rare earth
moments within a cone whose axis is antiparallel to the ferromagnetic axis z of
the cobalt subnetwork. The reduced magnetization (Jz)/J at T=0 is calculated
from an atomic Hamiltonian as a function of the ratio of anisotropy to exchange
energy per rare-earth atom for a range of angles between the local anisotropy
axis and -z and then averaged over all directions in a hemisphere. The
experimental and calculated values of (J-z)/J are close to 0.7 at low
temperature for both Dy and Tb. On increasing temperature, the magnitude of the
rare earth moment and the local random anisotropy that creates the cone are
reduced; the cone closes and the structure approaches collinear ferrimagnetism
well above ambient temperature. An asymmetric spin flop of the exchange-coupled
subnetworks appears in the vicinity of the magnetization compensation
temperatures of 175K for amorphous Dy0.25Co0.75 and 200 K for amorphous TbCo3.Comment: 23 pages, 12 figure
Au4Mn, a localized ferromagnet with strong spin-orbit coupling, long-range ferromagnetic exchange and high Curie temperature
Metallic Mn-based alloys with a nearest-neighbor Mn-Mn distance greater than
0.4 nm exhibit large, well-localized magnetic moments. Here we investigate the
magnetism of tetragonal Au4Mn with a Curie temperature of 385 K, where
manganese has a spin moment of 4.1 muB and its orbital moment is quenched.
Since 80% of the atoms are gold, the spin orbit interaction is strong and Au4Mn
exhibits uniaxial magnetocrystalline anisotropy with surface maze domains at
room temperature. The magnetic hardness parameter of 1.0 is sufficient to
maintain the magnetization along the c-axis for a sample of any shape. Au also
reduces the spin moment of Mn through 5d-3d orbital hybridization. An induced
moment of 0.05 muB was found on Au under a pulsed field of 40 T. Density
functional theory calculations indicate that the Mn-Mn exchange is mediated by
spin-polarized gold 5d and 6p electrons. The distance-dependence shows that it
is ferromagnetic or zero for the first ten shells of Mn neighbors out to 1.041
nm (64 atoms), and very weak and oscillatory thereafter
Fabrication and soft magnetic properties of rapidly quenched Co-Fe-B-Si-Nb ultra-thin amorphous ribbons
Ultra-thin soft magnetic amorphous ribbons of Co-Fe-B-Si-Nb alloy were synthesised by a single step rapid-quenching approach to acquire advantage of improved material performance and lower costs over commercial amorphous alloys. The amorphous ribbons of approximately 5.5 µm thicknesses were quenched by a single roller melt spinner in a single-step production process and characterised for their structural and magnetic properties. The disordered atomic structure of amorphous ribbons was confirmed by the X-ray diffraction. A surface morphology study revealed the continuity of ultra-thin ribbons without pores over a large scale. The amorphous alloy showed the ultra-soft magnetic properties in the as-quenched state. The observed thickness dependency of the magnetic properties was attributed to the increased surface roughness and possibly due to a lack of densely packed atomic structure resulting from the extremely high cooling rates experienced by ultra-thin ribbons. We propose that in-situ thinning process of amorphous ribbons significantly reduces the basic material cost and eliminates the need for post-processing steps; hence it provides the opportunity for mass production of high-performance soft magnetic amorphous ribbons at relatively lower costs
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