Antiferromagnetic Manganite Research Articles

Angular-dependent magnetoresistance modulated by interfacial magnetic state in Pt/LSMO heterostructures.

The interfaces between heavy metals and antiferromagnetic materials have garnered significant attention due to their interesting physical properties. La0.35Sr0.65MnO3 (LSMO), as a typical manganite, exhibits an antiferromagnetic ground state that can be controlled through epitaxial strain and interfacial spin-orbit coupling. In this work, we reported the diverse magnetoresistance, influenced by the interfacial magnetic state, in Pt (3 nm)/LSMO (6-20 nm) heterostructures. The strong spin-orbit coupling of Pt and Dzyaloshinskii-Moriya interaction alter the spin structure and enhance the electron scattering at the Pt/LSMO interface, resulting in positive magnetoresistance. The interfacial angular-dependent magnetoresistance modulated by the interfacial magnetic states was also observed in the Pt/LSMO (20 nm) heterostructures. Our findings contribute to a broader understanding of interfacial properties between heavy metals and antiferromagnetic manganites.

Modulation of Exchange Bias in La0.35Sr0.65MnO3/La0.7Sr0.3MnO3 through Volatile Polarization of P(VDF‐TrFE) Gate Dielectric

AbstractElectric‐field regulation of magnetic properties in perovskite manganites has attracted much attention for its potential in spintronics. For antiferromagnetic perovskite manganites, fewer studies are reported due to technological difficulties in probing their magnetic properties. Here, negative exchange bias (EB) is realized in epitaxial antiferromagnetic/ferromagnetic manganite bilayers with atomically flat interfaces. The low‐voltage pulse modulation of EB is demonstrated using the field‐effect device geometry with the ferroelectric copolymer, polyvinylidene fluoride with trifluoroethylene as a dielectric gating layer, antiferromagnetic La0.35Sr0.65MnO3 (AF‐LSMO) as pinning layer, and ferromagnetic La0.7Sr0.3MnO3 (FM‐LSMO) as conduction channel. Instead of using non‐volatile polarizations to control the EB, volatile polarizations in ferroelectric field effect transistors are suggested to be capable of modulating the EB. With high‐resolution electron microscopy and spectroscopy, the non‐volatile regulation of EB is attributed to the creation/annihilation of oxygen vacancies in the AF‐LSMO layer via low‐voltage pulses. This study reveals the effect of volatile electric polarizations in ferroelectric field effect devices and highlights the potential for low‐voltage pulse control of the physical properties in antiferromagnetic perovskite oxide insulators.

Ultrafast activation of the double-exchange interaction in antiferromagnetic manganites

In doped manganite systems, strong electronic correlations result in rich phase diagrams where electron delocalization strongly affects the magnetic order. Here, we employ a femtosecond all-optical pump-probe scheme to impulsively photodope the antiferromagnetic parent manganite system CaMnO3 and unveil the formation dynamics of a long-range ferromagnetic state. We resonantly target intense charge transfer electronic transitions in CaMnO3 to photodope the system and probe the subsequent dynamics of both charges and spins using a unique combination of time-resolved terahertz spectroscopy and time-resolved magneto-optical Faraday measurements. We demonstrate that photodoping promotes a long-lived population of delocalized electrons and induces a net magnetization, effectively promoting ferromagnetism resulting from light-induced carrier-mediated short-range double-exchange interactions. The picosecond set time of the magnetization, much longer than the electron timescale, and the presence of an excitation threshold are consistent with the formation of ferromagnetic patches in an antiferromagnetic background.

Coexistence of different magnetic ordering in thin films of SrMnO3 studied by spin transport

The ability to tune magnetic ordering in complex oxide based correlated antiferromagnetic insulators, due to the coupling between the charge, spin, lattice, and orbital degrees of freedom, opens a vast playground in spintronics. Here, we study a tensile strain induced coexistence of a wide range of magnetic ordering, as established from the temperature dependence of the spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) studies and complemented by structural and bulk magnetization measurements. The temperature dependence of the SMR, SSE, and bulk magnetization studies fingerprints the competition between different magnetic domains across the manganite film thickness. Our work demonstrates that strain induced spatial variation of magnetization in such nominal antiferromagnetic manganite, SrMnO3, can be tuned by orbital ordering and opens research opportunities in antiferromagnetic spintronics.

Orbital reconstruction mediated giant vertical magnetization shift and insulator-to-metal transition in superlattices based on antiferromagnetic manganites

Heterostructures made of strongly correlated oxides host various fundamentally interesting and potentially useful emergent phenomena. ${(\mathrm{LaMn}{\mathrm{O}}_{3})}_{n}/{{(\mathrm{SrMn}{\mathrm{O}}_{3})}_{n}}_{\ensuremath{-}1}$ superlattices that consist of an $A$-type antiferromagnetic insulator $\mathrm{LaMn}{\mathrm{O}}_{3}$ and a $G$-type antiferromagnetic insulator $\mathrm{SrMn}{\mathrm{O}}_{3}$ were investigated in this work. Several very intriguing effects were observed in such superlattices that include (1) the coexistence of a strong exchange bias effect and a giant vertical magnetization shift in superlattices with intermediate periods, (2) an insulator-to-metal transition associated with a change in the superlattice thickness, and (3) a large nontrivial negative magnetoresistance around the insulator-to-metal transition. To understand these phenomena, microscopic preferential orbital occupancy in different superlattices was studied through measurements of x-ray linear dichroism at Mn $L$ edges. This study facilitated the construction of a spin configuration model that takes into account the competition between interfacial ferromagnetism and underlying canted antiferromagnetism in the superlattices and can successfully explain the observed novel magnetic and transport properties. The phase transition and giant vertical magnetization shift phenomena observed in this work offer additional degrees of freedom for applications of antiferromagnetic insulator manganite-based superlattices, enabling novel device concepts.

Surface Ferromagnetism in Pr0.5Ca0.5MnO3 Nanoparticles as a Consequence of Local Imbalance in Mn3+:Mn4+ Ratio

Half-doped praseodymium manganites, Pr0.5Ca0.5MnO3, synthesized by the sol–gel method produce highly crystalline nanoparticles with no structural disorder at the surface. As for most half-doped (Mn3+:Mn4+ = 1) antiferromagnetic (AFM) charge-ordered manganites, ferromagnetism (FM) appears as particle size decreases. A possible origin of FM phase development is the lacking oxygen ligands at the surface that increases Mn3+ at the expense of Mn4+ in order to maintain electronic neutrality. In this work, we show that Mn3+:Mn4+ = 1 ratio is preserved in the whole crystalline particle with the exception of the surface, where this ratio changes to Mn3+:Mn4+ > 1. This Mn3+ excess makes a double exchange Mn3+ → O2– → Mn4+ prevail over the superexchange, giving rise to the FM interactions. We show that a shell thickness of only one unit cell, with acell = 0.38 nm, is enough to explain the onset of FM at the surface, whereas the volume remains AFM. Furthermore, the FM to AFM ratio fits to the increase of surface-to-v...

A long-range model with energy–spin coupling and critical properties of hexagonal antiferromagnetic manganites

We investigate the critical properties of a model Hamiltonian involving a four-spin term with long-range interaction together with energy–spin coupling. Employing Wilson’s momentum-shell decimation scheme at one-loop order, we perform a renormalization-group analysis and obtain the critical exponents corresponding to the non-trivial fixed point. We find that the specific heat exponent α lies in the negative range in the allowed range of a parameter ρ ocurring in the four-spin long-range coupling. This predicted theoretical range is consistent with the range observed in the thermal measurements of Oleaga et al (2012 Phys. Rev. B 85 184425) in the vicinity of the Néel temperatures of various experimental samples of hexagonal antiferromagnetic manganites.

Correlating quasiparticle excitations with quantum femtosecond magnetism in photoexcited nonequilibrium states of insulating antiferromagnetic manganites

We describe a mechanism for insulator-to-metal transition triggered by spin canting following femtosecond laser excitation of insulating antiferromagnetic (AFM) states of colossal magnetoresistive (CMR) manganites. We show that photoexcitation of composite fermion quasiparticles dressed by spin fluctuations results in the population of a broad metallic conduction band due to canting of the AFM background spins via strong electron-spin local correlation. By inducing spin canting, photoexcitation can increase the quasiparticle energy dispersion and quench the charge excitation energy gap. This increases the critical Jahn-Teller (JT) lattice displacement required to maintain an insulating state. We present femtosecond-resolved pump-probe measurements showing biexponential relaxation of the differential reflectivity below the AFM transition temperature. We observe a nonlinear dependence of the ratio of the femtosecond and picosecond relaxation component amplitudes at the same pump fluence threshold where we observe femtosecond magnetization photoexcitation. We attribute this correlation between nonlinear femtosecond spin and charge dynamics to spin/charge/lattice coupling and population inversion between the polaronic majority carriers and metallic quasielectron minority carriers as the lattice displacement becomes smaller than the critical value required to maintain an insulating state following laser-induced spin canting.

Giant enhancement of magnetocaloric effect at room temperature by the formation of nanoparticle of La0.48Ca0.52MnO3 compound

Magnetocaloric effect (MCE) is the change in temperature or entropy of a material due to the application of external magnetic field. The temperature dependence of MCE shows maximum value near the ferromagnetic transition of the material. Material with large MCE near room temperature will make revolutionary change in cooling technology and will have large impact in human society. We have transformed antiferromagnetic bulk manganite material La0.48Ca0.52MnO3 to ferromagnetic material by the reduction of the particle size in nanometer scale. MCE, as well as the Relative cooling power (RCP) of the material at room temperature (RT) systematically increase with the reduction of the particle sizes. The RCP value for the 45 nm particle size sample at RT is about 400% larger compared with the bulk counterpart. Our detail study on the nano form of the compound highlights an effective way to enhance the MCE. It opens up a vast possibility of achieving giant enhancement of MCE of several already known materials just by the reduction of the particle size in nanometer scale.

A Minimal tight-binding model for ferromagnetic canted bilayer manganites.

Half-metallicity in materials has been a subject of extensive research due to its potential for applications in spintronics. Ferromagnetic manganites have been seen as a good candidate, and aside from a small minority-spin pocket observed in La2−2xSr1+2xMn2O7 (x = 0.38), transport measurements show that ferromagnetic manganites essentially behave like half metals. Here we develop robust tight-binding models to describe the electronic band structure of the majority as well as minority spin states of ferromagnetic, spin-canted antiferromagnetic, and fully antiferromagnetic bilayer manganites. Both the bilayer coupling between the MnO2 planes and the mixing of the |x2 − y2 > and |3z2 − r2 > Mn 3d orbitals play an important role in the subtle behavior of the bilayer splitting. Effects of kz dispersion are included.

Exchange bias and magnetic properties induced by intrinsic structural distortion in CaMn3O6 nanoribbons

Single-crystalline CaMn3O6 nanoribbons have been synthesized by a molten-salt method. To explore the origin of the magnetism of nanosized antiferromagnetic (AFM) manganites, a comparative study has been conducted for CaMn3O6 (CMO-1) and post-growth vacuum annealed (CMO-2) nanoribbons. A lattice expansion resulting from oxygen release during vacuum annealing is observed. Correspondingly, AFM ordering in CMO-2 is further suppressed, and ferromagnetism and spin-glass (SG)-like behavior are significantly enhanced, which are presumed attributable to the intrinsic structural distortions induced by oxygen vacancies. In this case, side and surface effects are not decisive factors. In addition, this study provided observations of the exchange bias effect in manganite nanoribbons with an AFM-SG-like-ferromagnetic (FM) structure, as compared with the typical AFM-core-FM-shell.

Fe/Cr doping effects on the magnetism in charge-ordered manganite La0.4Ca0.6MnO3

We have investigated the Fe/Cr doping effects on the magnetic property of a charge-ordered antiferromagnetic manganite La0.4Ca0.6MnO3. Magnetic measurements reveal interesting doping effects. While all the Fe-doped samples still have antiferromagnetic ground state, strong ferromagnetic tendency can be seen in the Cr-doped samples. Meanwhile, Cr-doped samples show clear metallic transport behavior, indicating an inherent double exchange mechanism responsible for the ferromagnetic metallic state. We thus propose that the magnetic exchange interaction between Mn and dopants, and the d-orbital electronic structure of Cr/Fe are essential for the distinct doping effect of Fe and Cr.

Misfit dislocations of anisotropic magnetoresistant Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (1 1 0) substrates

Nd0.45Sr0.55MnO3 is an A-type antiferromagnetic manganite showing obvious angular-dependent magnetoresistance, which can be tuned by misfit strain. The misfit strain relaxation of Nd0.45Sr0.55MnO3 thin films is of both fundamental and technical importance. In this paper, microstructures of epitaxial Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (110) substrates by pulsed laser deposition were investigated by means of (scanning) transmission electron microscopy. The Nd0.45Sr0.55MnO3 thin films exhibit a two-layered structure: a continuous perovskite layer epitaxial grown on the substrate followed by epitaxially grown columnar nanostructures. An approximately periodic array of misfit dislocations is found along the interface with line directions of both〈111〉and [001]. High-resolution (scanning) transmission electron microscopy reveals that all the misfit dislocations possess a〈110〉-type Burgers vectors. A formation mechanism based on gliding or climbing of the dislocations is proposed to elucidate this novel misfit dislocation configuration. These misfit dislocations have complex effects on the strain relaxation and microstructure of the films, and thus their influence needs further consideration for heteroepitaxial perovskite thin film systems, especially for films grown on substrates with low-symmetry surfaces such as SrTiO3 (110) and (111), which are attracting attention for their potentially new functions.

Lattice distortion accompanied by magnetization reversal in A-type antiferromagnetic manganites

Magnetostriction was investigated for layered A-type antiferromagnetic SmMnO${}_{3}$ showing large magnetocapacitive effects around a temperature (${T}_{\mathit{TP}}$) where ferrimagnetically coupled Mn 3$d$ and Sm 4$f$ moments were reversed simultaneously. Upon sweeping temperature or a magnetic field, a significant lattice distortion was observed at ${T}_{\mathit{TP}}$ or the coercive field, respectively. This indicates that the lattice is strongly coupled with the magnetic configuration. We discuss the lattice distortion accompanied by the magnetization reversal in terms of a partial change in the orbital state of Mn ${e}_{g}$ electrons.

Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices

We demonstrate that delta doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AFM) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Theoretical consideration of these additional carriers shows that they cause a local enhancement of ferromagnetic double exchange with respect to AFM superexchange, resulting in local canting of the AFM spins. This leads to a highly modulated magnetization, as measured by polarized neutron reflectometry. The spatial modulation of the canting is related to the spreading of charge from the doped layer and establishes a fundamental length scale for charge transfer, transformation of orbital occupancy, and magnetic order in these manganites. Furthermore, we confirm the existence of the canted, AFM state as was predicted by de Gennes [Phys. Rev. 118, 141 (1960)] but had remained elusive.

Size-driven magnetic transitions in La1/3Ca2/3MnO3 nanoparticles

Magnetic properties of electron-doped La1/3Ca2/3MnO3 manganite nanoparticles with average particle size ranging from 12 to 42 nm, prepared by the glycine-nitrate method, have been investigated in temperature range 5–300 K and in magnetic fields up to 90 kOe. Reduction in the particle size suppresses antiferromagnetism and decreases the Néel temperature. In contrast to bulk crystals, the charge ordering does not occur in all studied nanoparticles, while a weak ferromagnetism appears above 200 K. Low temperature magnetic hysteresis loops indicate upon exchange bias effect displayed by horizontal and vertical shifts in field cooled processes. The spontaneous and remanent magnetization at low temperature shows a relatively complex variation with particle size. The size-induced structural/magnetic disorder drives the La1/3Ca2/3MnO3 nanoparticles to a pronounced glassy behavior for the smallest 12 nm particles, as evidenced by large difference between zero field cooled and field cooled magnetization, frequency dependent ac-susceptibility, as well as characteristic slowing down in the spin dynamics. Time evolution of magnetization recorded in magnetic fields after field cooling to low temperatures exhibits pronounced relaxation and a very noisy behavior that may be caused by formation of some collective states. Magnetic properties of the nanoparticle samples are compared with those of La0.2Ca0.8MnO3 nanoparticles. These results shed some light on the coupling between charges and spin degrees of freedom in antiferromagnetic manganite nanoparticles.

Half-metallic spin dynamics at a singleLaMnO3/SrMnO3interface studied with nonlinear magneto-optical Kerr effect

Electronic and magnetic dynamics at a single interface of antiferromagnetic manganites, ${\text{LaMnO}}_{3}/{\text{SrMnO}}_{3}$, were studied with time-resolved second-harmonic generation and time-resolved nonlinear magneto-optical Kerr effect. The temporal evolution of the respective signals shows clear differences; an optical pump makes an instantaneous response of the electrons and, in contrast, induces slow demagnetization, which points to a possibility of two-dimensional half-metallic ferromagnetism for this heterointerface. Detailed analysis with various pump and probe polarizations reveals possible pathways of the optical charge-transfer excitation between $\text{Mn}\text{ }d$ orbitals, leading to a subsequent perturbation to the interface spin system.

Grain-size effects on the charge ordering and exchange bias inPr0.5Ca0.5MnO3: The role of spin configuration

The grain-size effects on the charge ordering and exchange bias in CE-type antiferromagnetic charge ordered manganite compound ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_{3}$ are explored by magnetometry. Reducing the size suppresses antiferromagnetism and charge ordering that does not occur any more when the size is smaller than 40 nm. At the same time ferromagnetic clusters appear and gradually become larger; their fraction increases to about 19.7% when the particle size is reduced to 20 nm. The magnetic hysteresis loops of the nanosized samples exhibit both horizontal and vertical shifts in the field-cooled process. The exchange bias field ${H}_{\text{eb}}$ and the coercivity ${H}_{\text{C}}$ do not depend monotonously on size but show a maximum at about 85 nm, which indicates that in antiferromagnetic charge-ordered manganites ${H}_{\text{eb}}$ can be effectively tuned by changing the grain size. These notable phenomena can be understood when the evolution of the spin configuration is considered that results from the surface-phase separation and surface effect.

Antiferromagnetic manganite La 1/3Ca 2/3MnO 3 as barrier material in superconductor/insulator/ferromagnet tunnel junctions

Current transport through thin antiferromagnetic (AF) barriers of the perovskita manganite La 1/3Ca 2/3MnO 3 (LCMO) was studied with respect to its dependence on temperature and voltage. Planar-type La 2/3Ca 1/3MnO 3(~ 80 nm)/La 1/3Ca 2/3MnO 3(~ 7 nm)/YBa 2Cu 3O 7 − δ (~ 100 nm) heterojunctions were used as basic structures. The current–voltage ( I– V) measurements were carried out on test junctions with a standard area of 20 × 40 µm 2 in a four-terminal configuration. In spite of the carefully controlled growth conditions, barriers with the same nominal thickness showed different electrical behavior varying from elastic tunneling to Mott variable range hopping (VRH) via localized states. The different transport characteristics seem to be related to intrinsic difference in microstructure as the average surface roughness of the constituent layers may already be larger than the thickness of the barrier itself.

Nonlinear optical detection of a ferromagnetic state at the single interface of an antiferromagneticLaMnO3/SrMnO3double layer

Emergence of a ferromagnetic state at the single interface of two antiferromagnetic perovskite manganites, ${\text{LaMnO}}_{3}$ and ${\text{SrMnO}}_{3}$, has been detected with nonlinear magneto-optical Kerr rotation. The samples are composed of approximately four unit cells of each material, with reversed stacking sequence. The existence and orientations of the interface dipole or that of so-called toroidal moment were determined through phase-sensitive second-harmonic detection. Characterization of multiferroic states at the single interfaces, and non-negligible effect of surfaces on the Kerr rotation are demonstrated.