Pure Antiferromagnet Research Articles

Spin Hall Nano-Oscillator Based on an Antiferromagnetic Domain Wall

We propose here a high-frequency spin-Hall nano-oscillator based on a simple magnetic texture, such as a domain wall, located in an antiferromagnet with an easy-axis anisotropy type. We show that the spin current, polarized along the anisotropy axis, excites a conical precession of the N\'eel vector in such a domain wall, which allows obtaining a robust ac output signal---contrary to the planar precession in a uniform uniaxial antiferromagnet, where ac output is hard to achieve. The frequency of the auto-oscillations is easily tunable by the applied current up to the terahertz range, and the threshold current vanishes for a pure uniaxial antiferromagnet. By micromagnetic simulations, we demonstrate that the pinning of the domain wall is crucial for the oscillator design, which can be achieved in the nanoconstriction layout of the free layer.

Current-induced N\xe9el order switching facilitated by magnetic phase transition

Terahertz (THz) spin dynamics and vanishing stray field make antiferromagnetic (AFM) materials the most promising candidate for the next-generation magnetic memory technology with revolutionary storage density and writing speed. However, owing to the extremely large exchange energy barriers, energy-efficient manipulation has been a fundamental challenge in AFM systems. Here, we report an electrical writing of antiferromagnetic orders through a record-low current density on the order of 106 A cm−2 facilitated by the unique AFM-ferromagnetic (FM) phase transition in FeRh. By introducing a transient FM state via current-induced Joule heating, the spin-orbit torque can switch the AFM order parameter by 90° with a reduced writing current density similar to ordinary FM materials. This mechanism is further verified by measuring the temperature and magnetic bias field dependences, where the X-ray magnetic linear dichroism (XMLD) results confirm the AFM switching besides the electrical transport measurement. Our findings demonstrate the exciting possibility of writing operations in AFM-based devices with a lower current density, opening a new pathway towards pure AFM memory applications.

Ferrimagnetism Induced by Off-Site Coulomb Interaction in an Itinerant Electron System

Motivated by weak ferromagnetism (FM) in a $\tau$-type molecular conductor ($\tau$-MC), we examine its mechanism using a two-band extended Hubbard model. Applying the random phase approximation, we elucidate the uniform spin and charge fluctuations between unit cells in the presence of on-site and off-site interactions. Applying the mean-field approximation, we find the ordered state mixing with antiferromagnetism (AFM), weak FM, and charge ordering (CO) components in each unit cell: we classify this state as ferrimagnetism (FIM). We reveal the phase diagrams in the interaction and interaction-temperature spaces. The former shows that the off-site interaction induces FIM from pure AFM and the latter shows that lowering the temperature stabilizes FIM. To clarify the stabilization mechanism of the phases, we focus on the microscopic nature of the ordered states, including the band structure, Fermi surface, and density of states. We find that the FIM state is obtained from mixing features of AFM, CO, and FM; therefore, the emergence of FIM requires both the on-site and off-site interactions. Then, we discuss the effect of lowering the temperature and predict that the AFM gap assists the emergence of FIM based on AFM. This FIM state is possibly related to the observation of the weak FM in $\tau$-MC.

Evolution from Ferromagnetism to Antiferromagnetism in Yb(Rh_{1-x}Co_{x})_{2}Si_{2}.

Yb(Rh_{1-x}Co_{x})_{2}Si_{2} is a model system to address two challenging problems in the field of strongly correlated electron systems. The first is the intriguing competition between ferromagnetic (FM) and antiferromagnetic (AFM) order when approaching a magnetic quantum critical point (QCP). The second is the occurrence of magnetic order along a very hard crystalline electric field (CEF) direction, i.e., along the one with the smallest available magnetic moment. Here, we present a detailed study of the evolution of the magnetic order in this system from a FM state with moments along the very hard c direction at x=0.27 towards the yet unknown magnetic state at x=0. We first observe a transition towards an AFM canted state with decreasing x and then to a pure AFM state. This confirms that the QCP in YbRh_{2}Si_{2} is AFM, but the phase diagram is very similar to those observed in some inherently FM systems like NbFe_{2} and CeRuPO, which suggests that the basic underlying instability might be FM. Despite the huge CEF anisotropy the ordered moment retains a component along the c axis also in the AFM state. The huge CEF anisotropy in Yb(Rh_{1-x}Co_{x})_{2}Si_{2} excludes that this hard-axis ordering originates from a competing exchange anisotropy as often proposed for other heavy-fermion systems. Instead, it points to an order-by-disorder based mechanism.

Magnetic Transitions in the Co-Modified Mn2Sb System

Mn2Sb is ferrimagnetic below its Curie temperature (TC) and passes through a spin flip transition with decreasing temperature. The Co substitution induces an additional first-order phase transition from the ferrimagnetic (FRI) to an antiferromagnetic (AFM) state. This phase transition is connected to a sizable magnetocaloric effect (MCE). To understand the underlying mechanisms, the temperature dependence of structural and magnetic changes was analyzed. At the same time, the influence of the Co substitution was explored. Three Mn2−xCoxSb (x = 0.1, 0.15, 0.2) compounds were synthesized by cold crucible induction melting. Neutron powder diffraction was performed to determine the magnetic structures and to obtain the individual magnetic moments on both symmetrically independent Mn sites. In combination with the temperature-dependent magnetization measurements, the magnetic phase transition temperatures were identified. In the low-temperature range, additional antiferromagnetic peaks were detected, which could be indexed with a propagation vector of (0 0 ½). In Mn1.9Co0.1Sb at 50 K and in Mn1.8Co0.2Sb at 200 K, a co-existence of the FRI and the AFM state was observed. The pure AFM state only occurs in Mn1.8Co0.2Sb at 50 K.

Nature of magnetic properties in R3Co1.87In4 where R = Ho, Er and Tm

ABSTRACTCrystal structure and magnetic properties of the R3Co1.87In4 compounds (R = Ho, Er, Tm) have been investigated by means of X-ray diffraction as well as dc and ac magnetic measurements. The compounds crystallize in the hexagonal Lu3Co1.87In4-type structure which is closely related to the ZrNiAl-type. Magnetic data indicate different properties at low temperatures, namely: ferromagnetism below about 10 K followed by a like spin glass behavior below 5.8 K in R = Ho, antiferromagnetic order with ferromagnetic component below 5.2 K (R = Er) and pure antiferromagnetism below 2.8 K (R = Tm).

Band-renormalization effect in coexistent state of d-wave superconducting and antiferromagnetic orders for Hubbard model

In view of cuprate superconductors, we apply a variational Monte Carlo method to a strongly correlated square-lattice Hubbard (t-t′-U) model. As a one-body part in a trial wave function, we use a coexistent state of antiferromagnetic (AF) and d-wave superconducting (SC) orders for each of which band-renormalization effect (BRE) is considered independently. We focus on t′/t and doping-rate (δ) dependence of properties of this state. It is found that the BRE for the AF order causes expansion of the AF area up to δ ∼ 0.2 for a large |t′/t|. For t′/t=0 (−0.3), the AF and SC orders are coexistent (mutually exclusive), and the state is unstable toward (stable against) phase separation. This feature of coexistence or exclusion depends on the position of the Fermi surface in the underlying pure AF state, and indicates that antinodal Fermi surfaces are crucial for forming d-wave SC.

Band-renormalization Effect on Relationship between Superconductivity and Antiferromagnetism in t-J Model

Having cuprate superconductors in mind, we study a band-renormalization effect on a coexisting state of antiferromagnetism (AF) and superconductivity (SC) in a t-J model on a square lattice with a diagonal transfer t′ and three-site hopping terms, using a variational Monte Carlo method. It is found that this t-J model with t′/t = –0.3 exhibits coexistence of AF and SC orders in lightly doped regime and robust SC for a wide range of doping rate δ. This result contrasts with the previous results of a large-U/t Hubbard model, in which the two orders are mutually exclusive and a coexisting state does not appear; the SC state appears only in heavily doped regime (δ≳ 0.2), after the pure AF state vanishes at δ ∼ 0.2.

Magnetoresistance behavior of Ni80Fe20/Ru/Ni80Fe20 nanostripes

We present a systematic investigation on the effects of interlayer coupling on the magnetoresistance (MR) behavior of Ni80Fe20/Ru(\( t_{Ru}\)/Ni80Fe20 trilayer nanostripes (NSs) as a function of temperature T . By changing \( t_{Ru}\) from 0.8 nm to 1.7 nm, either a ferromagnetic or an antiferromagnetic (AFM) type of interlayer coupling was achieved, leading to a markedly different MR behavior. We observed that the MR response is a superimposition of AMR and GMR effects, and it is significantly dependent on temperature. For AFM coupled NSs, there is a transition from a pure AFM state to a reversed spin-flop phase due to enhanced ferromagnetic pin-hole coupling at low T. Our experimental results are in good agreement with micromagnetic simulations.

Neutron diffraction studies of magnetic ordering in Ni-doped LaCoO3

Research in rare earth cobaltite has recently been intensified due to its fascinating magnetic properties. LaCoO3, an important cobaltite, exhibits two prominent susceptibility features at ~90K and 500K in low field measurement. The magnetic behavior below 100K is predominantly antiferromagnetic (AFM), but absence of pure AFM ordering and emergence of ferromagnetic coupling on further decreasing temperature made situation more intricate. The present work of studying the effect of Ni substitution at Co site in polycrystalline LaCo1−xNixO3 (0≤x≤0.3) is motivated by the interesting changes in magnetic and electronic properties. For lucid understanding, temperature dependent neutron diffraction (ND) study was carried out. ND patterns fitted with rhombohedral structure in perovskite form with R-3c space group, elucidated information on phase purity. Further temperature dependent cell parameter, Co–O bond-length and Co–O–Co bond angle were calculated for the series of Ni doped LaCoO3. The results are explained in terms of decrease in the crystal field energy which led to the transition of cobalt from low Spin (LS) state to intermediate spin state (IS).

Negative Exchange Bias in Polycrystalline Hexagonal ScMnO3, InMnO3, YMnO3, 4H-SrMnO3, and 6H-SrMnO3 and Perovskite YMnO3: Effects of Impurities

Magnetic properties of polycrystalline hexagonal ScMnO3, InMnO3, h-YMnO3, 4H-SrMnO3, and 6H-SrMnO3 and perovskite o-YMnO3 were investigated with the focus on the exchange bias phenomenon. These compounds do not have magnetic ions at the A site, they have only one type of magnetic ions at the B site, and they are very weak canted antiferromagnets or pure antiferromagnets. They were selected based on the above criteria to serve as examples of a widely-spread tendency. Negative exchange bias was observed in all the samples on isothermal magnetization curves during field-cooled measurements. Only traces of non-magnetic impurities were observed by X-ray powder diffraction (XRD) in ScMnO3, InMnO3, and h-YMnO3; other samples were single-phase based on XRD. However, dc and ac temperature-dependent magnetic measurements showed the presence of very small amounts of Mn3O4 in ScMnO3, h-YMnO3, 4H-SrMnO3, and 6H-SrMnO3 and other magnetic impurities in InMnO3. Exchange bias was suggested to originate from pinned spins t...

Antiferromagnet-ferromagnet transitions in cobaltites

Three series of oxygen-deficient cobaltites, La0.5Ba0.5CoO3−δ, LnBaCo2O5.5 and Sr2YCo4O10.5, have been studied. La0.5Ba0.5CoO3 is an insulating ferromagnet whereas La0.5Ba0.5CoO2.75 is a pure antiferromagnet in which the oxygen vacancies are disordered. The ordering of the oxygen vacancies leads to the appearance of a ferromagnetic component, apparently because of the formation of a noncollinear magnetic structure. The antiferromagnet-“ferromagnet” transition is accompanied by a giant magnetoresistance. It is suggested that in the ferromagnetic oxidized compounds, the Co3+ and Co4+ ions adopt intermediate spin states, whereas in the antiferromagnetic (Co4+-free) compositions, the Co3+ ions have a high-spin state (CoO5 pyramids) with a dominant low-spin state (CoO6 octahedra). In both the ferromagnetic and antiferromagnetic compounds, superexchange via oxygen plays an essential role in determining the magnetic properties.

Ferromagnetic phase transition in zinc blende (Mn,Cr)S-layers grown by molecular beam epitaxy

We studied the magnetization of zinc blende Mn1-xCrxS films embedded between diamagnetic ZnSe layers grown by molecular beam epitaxy with chromium mole fractions x ≤ 0.7. These ternary semiconductors exhibit an increasing ferromagnetic contribution with increasing x caused by competing antiferromagnetic and ferromagnetic coupling. As a result, whereas MnS in the zinc blende phase is a pure antiferromagnet, it was found that with increasing x zinc blende Mn1−xCrxS became a ferromagnet. The ferromagnetic phase transition dominates in case of x greater than about 0.5. Hence, we conclude that metastable zinc blende CrS will be a ferromagnetic material with half-metallic character in contrast to the analogous stable NiAs-structure which exhibits an antiferromagnetic phase transition.

Study on spin and charge fluctuations in τ ‐type organic conductor

AbstractWe theoretically study the possibility of the ferromagnetic behavior in a quasi‐two‐dimensional organic conductor, τ ‐(EDO‐S,S ‐DMEDT‐TTF)2(AuBr2)1+y with y ≈ 0.875. We adopt a two‐band extended Hubbard model, whose band dispersion nicely reproduces the ab initio band structure. We consider both on‐site and nearest neighbor off‐site repulsive interactions.By applying the random phase approximation, we find that both spin and charge susceptibilities are maximized at Q = (0, 0). Although this can be an indication of pure antiferromagnetism since two sites are present in a unit cell, the divergence of the susceptibilities at the origin of the k ‐space can also be related to the ferromagnetic behavior observed in this organic conductor. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-temperature redistribution of cation vacancies and irreversible magnetic transitions in the Fe1−x S nanodisks observed by the Mössbauer spectroscopy and magnetic measurements

The hexagonal pyrrhotite Fe1−x S nanodisks with the NiAs-type structure were synthesized by thermal decomposition of ferrous chloride and thiourea in oleylamine. The Mossbauer spectroscopy and magnetic measurements data indicate that a mixture of antiferromagnetic (AFM) and ferrimagnetic (FRM) phases with the NC (N ≥ 3) and 2C-type superstructures is present in the Fe1−x S compound at temperatures between 80 K and Neel temperature T N. At T 370 K, a redistribution of iron vacancies takes place, and the vacancy ordering transforms from the NC (N ≥ 3) to 2C-type which essentially increases the magnetization with maximum value at 470 K. Heating the sample above the Neel temperature 565 K leads to a random distribution of vacancies, and this state is quenched upon subsequent cooling of the sample to 300 K. This gives rise to a pure AFM structure with a zero magnetic moment due to a total compensation of the moments in neighboring iron layers. Thus, the high-temperature redistribution of cation vacancies leads to irreversible magnetic transformations in the Fe1−x S nanoparticles.

Effects of Crystalline Phase and Particle Size on the Properties of Plate-Like Fe2O3 Nanoparticles during γ- to α-Phase Transformation

Plate-like Fe2O3 nanoparticles were prepared in microporous regenerated cellulose films and then completely removed from the cellulose matrix by calcination. Crystallite size and properties variations of the nanosized Fe2O3 during γ- to α-phase transformation were investigated. The γ-Fe2O3 nanoparticles with an initial particle size of 24 nm increased from 26 to 83 nm with an increase of the calcination temperature from 350 to 800 °C, and the phase transformation temperature was about 500 °C. The nanoparticles made at 350 and 450 °C exhibited superparamagnetic properties with the corresponding blocking temperature (TB) of about 83 and 80 K. The nanoparticles made at 500, 600, and 700 °C not only exhibited Curie transitions with TB's of about 77, 54, and 15 K but also had Morin transitions from a pure antiferromagnet to a ferromagnet occurring at about 238, 240, and 246 K, respectively. However, the nanoparticles made at 800 °C only gave a clear Morin transition from a pure antiferromagnet to a ferromagnet at about 250 K. Furthermore, the nanoparticles also displayed different electrochemical properties.

Superconductivity and antiferromagnetism in the phase diagram of the frustrated Hubbard model within a variational study

The interplay between antiferromagnetism (AF) and superconductivity (SC) in cuprates is studied for the two-dimensional Hubbard model with a diagonal transfer t′, using a variational Monte Carlo method. Optimizing an improved function for strongly correlated values of U/t, we construct phase diagrams in the δ (doping rate)-t′/t space. It is found that the stable state is sensitive to the value of model parameters: For the extremely large values of U/t, a coexisting state is realized for t′/t≳−0.15, whose range of doping rate extends as t′/t increases. In contrast, for t′/t=−0.3, AF and SC states are mutually exclusive, and a coexisting state does not appear. As U/t decreases, the area of pure AF extends, and that of coexisting state shrinks. As a result, the coexisting state disappears for t′/t=−0.15 and U/t=12, probable values for hole-doped cuprates. Compared with the t–J model, the Hubbard model has richer phases.

Competition and coexistence of antiferromagnetism and superconductivity in the Hubbard model

The interplay between antiferromagnetism (AF) and superconductivity (SC) in cuprate is studied for the two-dimensional Hubbard model with a diagonal transfer t′, using a variational Monte Carlo method. Optimizing an improved wave-function for strongly correlated values of U/t, we construct a phase diagram. It is found that the stable state is sensitive to the value of model parameters: For t′/t≳−0.15 a coexisting state is realized, whose range of doping rate extends as t′/t increases. In contrast, for t′/t=−0.3, AF and SC states are mutually exclusive, and the area of pure AF shrinks as U/t increases.

Stable Mean-Field Solution of a Short-Range Interacting SO(3) Quantum Heisenberg Spin Glass

We present a mean-field solution for a quantum, short-range interacting, disordered, SO(3) Heisenberg spin model, in which the Gaussian distribution of couplings is centered in an antiferromagnetic (AF) coupling J[over ]>0, and which, for weak disorder, can be treated as a perturbation of the pure AF Heisenberg system. The phase diagram contains, apart from a Néel phase at T=0, spin-glass and paramagnetic phases whose thermodynamic stability is demonstrated by an analysis of the Hessian matrix of the free-energy. The magnetic susceptibilities exhibit the typical cusp of a spin-glass transition.

Neutron Diffraction Study of Mesoporous and Bulk Hematite, α-Fe2O3

Variable temperature neutron diffraction was carried out on mesoporous α-Fe2O3 (hematite) with a mean pore diameter of 38.5 Å. Data were directly compared to measurements carried out on bulk hematite. Unlike bulk hematite, the mesoporous material does not undergo a spin−flop transition from a weak ferromagnet to a pure antiferromagnet (Morin transition, TM = 259.1(2)). Instead, the material remains a weak ferromagnet down to 2 K with the magnetic moments staying perpendicular to the R3̅c [111] ([003] in the hexagonal cell) direction rather than realigning (to) almost parallel to this direction. The angle of the magnetic Fe3+ moments to the [111] direction in the antiferromagnetic state also accurately was obtained for bulk hematite. Using magnetic hysteresis measurements, the canting angle responsible for weak ferromagnetism within the ab planes (hexagonal setting) was deduced for the mesoporous material at 12 K and compared to the angle made in bulk hematite above TM.