Uncompensated Antiferromagnetic Research Articles

Imaging and quantifying perpendicular exchange biased systems by soft x-ray holography and spectroscopy

The magnetic properties of perpendicular exchange biased ferromagnetic (FM)/antiferromagnetic (AFM) [Pt/Co]n/IrMn systems were investigated by a new set-up combining element-selective soft x-ray holography and spectroscopy measurements. The holography experiments allow imaging the magnetization reversal of an exchange biased FM layer with an equivalent Co thickness below 5 nm in real space and in external magnetic fields, and provide direct evidence of its asymmetric nature. From the spectroscopy analysis, we have quantified the unpinned (pinned) uncompensated AFM moments, providing direct evidence of its parallel (antiparallel) alignment with respect to the FM moments.

Magnetic properties of the CuCoAlBO5 single crystal

High-quality CuCoAlBO5 single crystals have been grown, and their crystal structure, magnetic susceptibility, and magnetization have been studied. It has been established that the CuCoAlBO5 compound is an uncompensated antiferromagnet or ferrimagnet with a small magnetic moment and the magnetic ordering temperature T N = 28 K. A model has been proposed for the magnetic structure. A strong anisotropy of the magnetic properties has been revealed.

Mechanochemically assisted synthesis of yttrium–lanthanum orthoferrite: Structural and magnetic characterization

Polycrystalline Y 0.5La 0.5FeO 3 powder was synthesized by mechanochemical activation of Fe 3O 4–La 2O 3–Y 2O 3 reactive mixtures. X-ray diffraction was used to follow the evolution of the formation of this orthoferrite. Peaks corresponding to reactants were no longer observed after 3 h of activation, showing the completion of the reaction after this time. Magnetic hysteresis loops measured at room temperature revealed a gradual decrease of saturation magnetization, consistent with the consumption of ferrimagnetic Fe 3O 4 and the formation of antiferromagnetic Y 0.5La 0.5FeO 3. However, a small ferromagnetic contribution was observed even after 3 h of milling, which can be attributed to a canting in the alignment of the two sub-lattices in the orthoferrite structure. This uncompensated antiferromagnetism disappeared when the solid was heated, giving rise to a nearly antiferromagnetic structure.

Special features of structural organization and magnetic properties of quasi-one-dimensional organoiron nanostructures on a silica support

Special features of the synthesis (constructing) of highly organized quasi-one-dimensional organoiron nanostructures were considered. For the first time magnetic properties of ferromagnetic nanostructures with a specified topology based on quasi-one-dimensional organometallic structures (of “pile“ type) fixed on an inorganic matrix were studied. The presence of an uncompensated antiferromagnetism arisen on plotting organoiron groups on a silica surface was experimentally found, and the obtained data showed the study of quasi-one-dimensional organometallic nanostructures fixed on a support to be promising for the spin electronics engineering.

The study of spin ordering in FM/AFM bilayers with a mixed interface

Monte Carlo simulation is adopted to study the spin ordering in ferromagnet(FM)/antiferromagnet(AFM) bilayers with a mixed interface. We introduce the time-dependent autocorrelation function to describe the thermal stability of the spin ordering for each plane, especially the interface, in FM/AFM bilayers with uncompensated and compensated AFM surfaces. It is found that the thermal decay of spin ordering depends on the plane, the interface coupling and the interface roughness. For the uncompensated AFM surfaces, when the interface coupling is small, the thermal decay of spin ordering is faster at the interface than other planes, while the large interfae coupling makes the spin ordering at the interface become relatively stable. In the case of compensated AFM surfaces, the spin ordering at the interface is thermally disordered much before that at other planes, and the thermal decay becomes slow gradually as the interface coupling increases.

Half-metallicity at ferromagnetic∕antiferromagnetic interfaces in zincblende transition-metal chalcogenides: A full-potential linearized augmented plane-wave study within LDA+U

Electronic structures and half-metallicity at ferromagnetic∕antiferromagnetic (AFM) interfaces in zincblende transition-metal chalcogenides, CrSe∕MnSe and CrTe∕MnTe, are investigated by means of the first principles full-potential linearized augmented plane-wave method within the LDA+U, and the effect of correlation in the 3d states on the half-metallic interfaces is discussed. The uncompensated AFM interface with the antiparallel alignment of the Cr and Mn moments at the interfaces shows an excellent half-metallicity, where the correlation effect tends to manifest the half-metallic interfaces. This indicates that these interfaces offer a key ingredient as promising exchange bias candidates in having interfaces with 100% spin polarization at the Fermi level.

Crystallographic, Magnetic and Mössbauer Study of Phase Transition in LaVO3

Nature of phase transition in <TEX>$LaVO_3$</TEX> has been studied using X-ray diffraction, SQUID magnetometer, and <TEX>$M\"{o}ssbauer$</TEX> spectroscopy with 1% of <TEX>$^{57}Fe$</TEX> doped sample. The crystal structure was orthorhombic with space group Pnma. Antiferromagnetic phase transition temperature <TEX>$T_N$</TEX> was 140K, below which a weak ferromagnetic trace has been found. <TEX>$M\"{o}ssbauer$</TEX> spectra below <TEX>$T_N$</TEX> were single set of hyperfine sextet, which enabled us to discard the possibility of two inequivalent magnetic sites or uncompensated antiferromagnetism. Hyperfine magnetic field abruptly disappeared as low as about 90K, much below <TEX>$T_N$</TEX>.

Effects of Cu Dilution in IrMn on the Exchange Bias ofCoFe/IrMnBilayers

The effect of nonmagnetic dilution in metallic antiferromagnets (AFMs) on the exchange bias (EB) has been investigated from a structural, magnetic, and Monte Carlo simulation point of view in bilayers of CoFe/(IrMn)1-xCux. Dilution by Cu atoms throughout the volume of the AFM IrMn gives rise to an enhanced EB field (HEB) for 5 K<or=T<or=350 K. The thermoremanent magnetization of the sole AFM layer is also enhanced with dilution, shows qualitatively the same temperature dependence as HEB, and is at the origin of the EB mechanism. Monte Carlo simulations based on a Heisenberg model and temperature dependent uncompensated AFM moments confirm well the experimental results.

X-ray magnetic circular dichroism studies of (001)-oriented NiFe∕Mn100−xPtx exchange bilayers

(001)-oriented Ni80Fe20∕Mn100−xPtx and Co90Fe10∕Mn100−xPtx epitaxial bilayers were prepared using the molecular beam epitaxy method. Their exchange anisotropies were measured using a torque magnetometer, while their interfacial Mn uncompensated moments were measured by means of x-ray magnetic circular dichroism. The bilayers exhibited both one- and fourfold components in their anisotropy torque curves, which are difficult to explain using a simple model assuming the coherent rotation of ferromagnetic (F) and antiferromagnetic (AF) spins. Uncompensated Mn moments were confirmed to exist in the Mn100−xPtx layer due to exchange coupling with the adjacent F layer, and the Mn moment of CoFe∕Mn100−xPtx was found to be larger than that of NiFe∕Mn100−xPtx. In order to understand the experimental results, we extended the Mauri et al. [J. Appl. Phys. 62, 3047 (1987)] domain wall model by assuming cubic anisotropy in the AF and four AF domains whose interfacial moments are oriented along the principal axis of (001)-oriented Mn–Pt. The model predicted the uncompensated AF moment resulting from the domain wall formed in the AF layer and well reproduced the coexistence of one- and fourfold anisotropies in the in-plane torque curves. The uncompensated moment and torque curve were found to be dependent on the ratio of F/AF interfacial exchange coupling and the AF domain wall energy. By changing the ratio, it was possible to reproduce exchange anisotropy for various systems, such as NiFe∕MnPt, NiFe∕MnIr, and CoFe∕MnPt.

Magnetic structures and half-metallicity at zincblende ferromagnetic/antiferromagnetic interfaces: Role of tetragonal distortions

In our search for half-metallic exchange bias interfaces, we determined magnetic structures at ferromagnetic (FM)/antiferromagnetic (AFM) interfaces in zincblende transition–metal chalcogenides, CrSe/MnSe and CrTe/MnTe, by means of the first principles full-potential linearized augmented plane-wave method, including the effect of a tetragonal distortion that may be induced in epitaxial growth. The tetragonal distortion with c / a < 1 in both MnSe and MnTe is found to prefer the uncompensated AFM interface while that with c / a > 1 , the compensated AFM interface. With the uncompensated AFM interface, an antiparallel alignment of the Cr and Mn moments induces an excellent half-metallicity at the interface even when the tetragonal distortion is included.

Half-Metallic Exchange Bias Ferromagnetic/Antiferromagnetic Interfaces in Transition-Metal Chalcogenides

To investigate half-metallic exchange bias interfaces, magnetic structures at ferromagnetic (FM)/antiferromagnetic (AFM) interfaces in the zinc blende transition-metal chalcogenides, and with compensated and uncompensated AFM interfaces, were determined by the full-potential linearized augmented plane-wave method. With the uncompensated AFM interface, an antiparallel alignment of the Cr and Mn moments induces an excellent half-metallicity. More striking still, in the compensated AFM interface the Cr moments in the FM layer lie perpendicular to the Mn moments in the AFM layer but the Mn moments strongly cant to induce a net moment so as to retain the half-metallicity. These findings may offer a key ingredient for exchange biased spintronic devices with 100% spin polarization, having a unidirectional anisotropy to control and manipulate spins at the nanoscale.

Uncompensated antiferromagnetism and phase boundary with variable curvature in the mixed magnetMn1−xNixCl2·4H2O

The magnetic properties of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{Cl}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}4{\mathrm{H}}_{2}\mathrm{O}$ are examined by dc magnetization and susceptibility measurements on a wide range of compositions. The pure components are isomorphous three-dimensional Heisenberg antiferromagnets, ordering at $1.62\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}({\mathrm{MnCl}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}4{\mathrm{H}}_{2}\mathrm{O})$ and $2.99\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}({\mathrm{NiCl}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}4{\mathrm{H}}_{2}\mathrm{O})$. So far as is known, exchange interactions in either material are antiferromagnetic only. The Curie and Weiss constants, in ${\ensuremath{\chi}}_{M}=C∕(T\ensuremath{-}\ensuremath{\theta})$ fits to high temperature data, vary with composition in somewhat unexpected ways. $C(x)$ is not as linear as often seen in mixed magnets; $\ensuremath{\theta}(x)$ exhibits a high degree of scatter. Remarkably, for compositions from $x=0.10$ through $x=0.95$, the susceptibility shows no standard antiferromagnetic maximum; rather, sharp upturns appear, at temperatures which vary systematically with composition. Only very near the composition extremes does the susceptibility exhibit somewhat oddly shaped maxima. Ordering temperatures are estimated, and a ${T}_{c}(x)$ phase boundary deduced. It is monotonic but displays several curvature changes, more than anticipated from certain theoretical treatments. Low temperature magnetization isotherms display inflections, the location of which varies with composition. There is also an evolution with composition of the extrapolated zero-field magnetization at sufficiently low temperatures. It is likely that incomplete cancellation of antiferromagnetic sublattice moments occurs in mixtures.

Magnetism and magnetoelasticity of a U(Ni0.91Pd0.09)2Si2 single crystal

Abstract U(Ni0.91Pd0.09)2Si2 has a critical composition in the U(Ni1−xPdx)2Si2 solid solutions, at which the ground state changes with increasing x from uncompensated antiferromagnetic (UAF) to simple antiferromagnetic. We present the results of magnetization, specific-heat measurements in magnetic fields up to 9 T and electrical resistivity under high pressures up to 0.75 GPa performed on a single crystal. At 0.45 GPa, a pressure induced anomaly has been found around 25 K in the ρ(T) curve, which indicates that the UAF phase becomes dominant at higher pressure.

High-field and high-pressure studies of UPdSi and UNiSi hydrides

Abstract High-field magnetization measurements have revealed an antiferromagnetic (AFM) ground state in UPdSiH1.0 and uncompensated AFM—in UNiSiH1.0. Subsequent magnetic phases in UNiSiH1.0 are ferromagnetic. Increase of TN in UPdSiH1.0 with pressure, and the estimate of the ordering temperatures in UNiSiH1.0 based on the bulk moduli indicate that the pressure effect on magnetism in the hydrides goes beyond the volume expansion.

Hysteresis shift in Fe-filled carbon nanotubes due to γ-Fe

The phase distribution of high aspect ratio, Fe-filled carbon nanotubes prepared by pyrolyzing a mixture of powered ferrocene and C_(60) has been determined by means of Mossbauer spectroscopy. Our results for that characterization are closely related to the observation, after field cooling processes, of a hysteresis loop shift and clearly suggest a spatial phase distribution which includes the presence of a g-Fe/α-Fe interface. The temperature ependence of the hysteresis loop shift is discussed in terms of localized regions at that interface exhibiting uncompensated antiferromagnetism within reduced dimensions.

Exact instanton solution for quantum tunneling in an uncompensated antiferromagnet

An exact instanton solution describing macroscopic quantum tunneling for a small antiferromagnetic particle with uncompensated spin and biaxial quadratic anisotropy is constructed. The solution is valid for any relation between anisotropy parameters and relative value of uncompensated spin. The obtained solution is used for calculating the tunneling amplitude taking into account the pre-exponential factor. The amplitude is characterized by a nonanalytic dependence on the ratio of small parameters of the problem, viz., anisotropy in the basal plane and the value of uncompensated spin.

Macroscopic coherent tunneling in a small particle of an uncompensated antiferromagnet in a strong magnetic field

A new macroscopic quantum tunneling effect is predicted for a particle of an uncompensated ferritin-type antiferromagnet in a noncollinear phase induced by a strong magnetic field.

Relaxometry and magnetometry of ferritin.

By combining nuclear magnetic relaxometry on 39 ferritin samples with different iron loading with magnetometry, results were obtained that suggest a new interpretation of the core structure and magnetic properties of ferritin. These studies provide evidence that, contrary to most earlier reports, the ferritin core is antiferromagnetic (AFM) even at body temperature and possesses a superparamagnetic (SPM) moment due to incomplete cancellation of antiparallel sublattices, as predicted by Néel's theory. This moment also provides a likely explanation for the anomalous T2 shortening in ferritin solution. However, the number of SPM moments derived from this model is less than the number of ferritin molecules determined chemically, and a similar discrepancy was found by retrospectively fitting previously published magnetometry data. In other words, only a fraction of the ferritin molecules seem to be SPM. The studies also provide evidence for paramagnetic (PM) Curie-Weiss iron ions at the core surface, where the local Néel temperature is lower; these ions are apparently responsible for the weaker T1 shortening. In fact, the conversion of uncompensated AFM lattice ions to PM ions could explain the small number of SPM particles. The apparent Curie Law behavior of ferritin thus appears to be a coincidental result of different temperature dependences of the PM and SPM components.

Magnetic phases and magnetoelastic phenomena in UNiGa under pressure

UNiGa exhibits several antiferromagnetic (AF) phases below T N=39 K. The related magnetic phase transitions cause pronounced thermoexpansion anomalies of opposite sign for the c- and a-axes, respectively, leaving volume effects nearly negligible. Magnetic fields applied along the c-axis induce transitions from the AF phases to an uncompensated AF and/or to a ferromagnetic (F) phase accompanied by magnetostriction effects. Our results on pressure influence on the thermoexpansion and magnetostriction anomalies allow us to propose a tentative p- T magnetic phase diagram. A new AF phase induced by pressures above 1.8 GPa has been found. Neutron-diffraction studies under hydrostatic pressure up to 0.9 GPa and in magnetic fields up to 2 T confirmed that the critical temperatures and the critical fields of transitions of UNiGa are strongly pressure dependent (some phases are even suppressed) although relevant magnetic structures themselves remain essentially unchanged.

Giant magnetoresistance and magnetic phase diagram of UNiGa

As a result of combined magnetization, specific heat, electrical resistivity, and neutron-scattering studies on single crystals, we present a complex magnetic phase diagram of UNiGa that exhibits several antiferromagnetic (AF) phases below TN=39 K. A relatively low magnetic field (∼1–1.5 T) applied along the c-axis induces metamagnetic transitions from the zero-field AF structures to an uncompensated AF and/or to a ferromagnetic (F) phase. All the magnetic structures are collinear (uranium magnetic moments parallel to the c-axis of the hexagonal structure). They consist of F basal-plane sheets which are coupled along c in various ways. The strongly anisotropic magnetism in UNiGa causes anisotropy of the electrical resistivity. The magnetic phase transitions are reflected in pronounced resistivity anomalies especially for current along the c-axis. The metamagnetic transitions are accompanied by giant magnetoresistance (GMR) effects which can be correlated with respective changes of the magnetic periodicity. This implies an important feature of the mechanism responsible for GMR phenomena in UNiGa. The strong resistance enhancement in the paramagnetic range when approaching TN is connected with AF short-range ordering with propagation along the c-axis.