133 research outputs found
Structural investigation of GaInP nanowires using X-ray diffraction
AbstractIn this work the structure of ternary GaxIn1−xP nanowires is investigated with respect to the chemical composition and homogeneity. The nanowires were grown by metal–organic vapor-phase epitaxy. For the investigation of ensemble fluctuations on several lateral length scales, X-ray diffraction reciprocal space maps have been analyzed. The data reveal a complicated varying materials composition across the sample and in the nanowires on the order of 20%. The use of modern synchrotron sources, where beam-sizes in the order of several 10μm are available, enables us to investigate compositional gradients along the sample by recording diffraction patterns at different positions. In addition, compositional variations were found also within single nanowires in X-ray energy dispersive spectroscopy measurements
Multiple-stable anisotropic magnetoresistance memory in antiferromagnetic MnTe
Commercial magnetic memories rely on the bistability of ordered spins in ferromagnetic materials. Recently, experimental bistable memories have been realized using fully compensated antiferromagnetic metals. Here we demonstrate a multiple-stable memory device in epitaxial MnTe, an antiferromagnetic counterpart of common II–VI semiconductors. Favourable micromagnetic characteristics of MnTe allow us to demonstrate a smoothly varying zero-field antiferromagnetic anisotropic magnetoresistance (AMR) with a harmonic angular dependence on the writing magnetic field angle, analogous to ferromagnets. The continuously varying AMR provides means for the electrical read-out of multiple-stable antiferromagnetic memory states, which we set by heat-assisted magneto recording and by changing the writing field direction. The multiple stability in our memory is ascribed to different distributions of domains with the Neel vector aligned along one of the three magnetic easy axes. The robustness against strong magnetic field perturbations combined with the multiple stability of the magnetic memory states are unique properties of antiferromagnets
Spontaneous anomalous Hall effect arising from an unconventional compensated magnetic phase in a semiconductor
The anomalous Hall effect, commonly observed in metallic magnets, has been
established to originate from the time-reversal symmetry breaking by an
internal macroscopic magnetization in ferromagnets or by a non-collinear
magnetic order. Here we observe a spontaneous anomalous Hall signal in the
absence of an external magnetic field in an epitaxial film of MnTe, which is a
semiconductor with a collinear antiparallel magnetic ordering of Mn moments and
a vanishing net magnetization. The anomalous Hall effect arises from an
unconventional phase with strong time-reversal symmetry breaking and
alternating spin polarization in real-space crystal structure and
momentum-space electronic structure. The anisotropic crystal environment of
magnetic Mn atoms due to the non-magnetic Te atoms is essential for
establishing the unconventional phase and generating the anomalous Hall effect.Comment: 34 pages, 14 figure
Antiferromagnetic CuMnAs multi-level memory cell with microelectronic compatibility
Antiferromagnets offer a unique combination of properties including the radiation and magnetic field hardness, the absence of stray magnetic fields, and the spin-dynamics frequency scale in terahertz. Recent experiments have demonstrated that relativistic spin-orbit torques can provide the means for an efficient electric control of antiferromagnetic moments. Here we show that elementary-shape memory cells fabricated from a single-layer antiferromagnet CuMnAs deposited on a III–V or Si substrate have deterministic multi-level switching characteristics. They allow for counting and recording thousands of input pulses and responding to pulses of lengths downscaled to hundreds of picoseconds. To demonstrate the compatibility with common microelectronic circuitry, we implemented the antiferromagnetic bit cell in a standard printed circuit board managed and powered at ambient conditions by a computer via a USB interface. Our results open a path towards specialized embedded memory-logic applications and ultra-fast components based on antiferromagnets
Magnetic anisotropy in antiferromagnetic hexagonal MnTe
Antiferromagnetic hexagonal MnTe is a promising material for spintronic devices relying on the control of antiferromagnetic domain orientations. Here we report on neutron diffraction, magnetotransport, and magnetometry experiments on semiconducting epitaxial MnTe thin films together with density functional theory (DFT) calculations of the magnetic anisotropies. The easy axes of the magnetic moments within the hexagonal basal plane are determined to be along ⟨1¯100⟩ directions. The spin-flop transition and concomitant repopulation of domains in strong magnetic fields is observed. Using epitaxially induced strain the onset of the spin-flop transition changes from ∼2 to ∼0.5 T for films grown on InP and SrF2 substrates, respectively
Altermagnetic lifting of Kramers spin degeneracy
Lifted Kramers spin-degeneracy has been among the central topics of
condensed-matter physics since the dawn of the band theory of solids. It
underpins established practical applications as well as current frontier
research, ranging from magnetic-memory technology to topological quantum
matter. Traditionally, lifted Kramers spin-degeneracy has been considered to
originate from two possible internal symmetry-breaking mechanisms. The first
one refers to time-reversal symmetry breaking by magnetization of ferromagnets,
and tends to be strong due to the non-relativistic exchange-coupling origin.
The second mechanism applies to crystals with broken inversion symmetry, and
tends to be comparatively weaker as it originates from the relativistic
spin-orbit coupling. A recent theory work based on spin-symmetry classification
has identified an unconventional magnetic phase, dubbed altermagnetic, that
allows for lifting the Kramers spin degeneracy without net magnetization and
inversion-symmetry breaking. Here we provide the confirmation using
photoemission spectroscopy and ab initio calculations. We identify two distinct
unconventional mechanisms of lifted Kramers spin degeneracy generated by the
altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization.
Our observation of the altermagnetic lifting of the Kramers spin degeneracy can
have broad consequences in magnetism. It motivates exploration and exploitation
of the unconventional nature of this magnetic phase in an extended family of
materials, ranging from insulators and semiconductors to metals and
superconductors, that have been either identified recently or perceived for
many decades as conventional antiferromagnets
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