RKKY Exchange Coupling Research Articles

Nearly zero Poisson’s ratio and spin-state-controlled magnetoelastic response in an RKKY-mediated two-dimensional magnetic metal–organic framework

By means of first-principles calculations, the two-dimensional magnetic metal–organic framework, manganese phthalocyanine (MnPc), is proposed to exhibit nearly zero Poisson’s ratio within the elastic regime. Negative Poisson’s ratio can be reached by further increasing the uniaxial tensile strain beyond the yield point. Such an unusual mechanical property is found to rely on the spin-state of Mn atoms. Without strain, the ground state is ferrimagnetic and half-metallic. Interestingly, as the interatomic distance between Mn atoms increases under biaxial tensile strain, the alignment of magnetic moments of neighboring Mn atoms changes from antiparallel to parallel, associated with oscillatory exchange coupling strength. This not only reveals the strain-induced magnetic phase transition, but also indicates the magnetic interaction is mediated by the Ruderman–Kittel–Kasuya–Yosida exchange coupling and thus Kondo effect is likely to exist. Rather than the reported RKKY exchange coupling between supramolecular MnPc adsorbed on substrates, the predicted RKKY exchange coupling here exists in the pristine two-dimensional MnPc lattice. It would be more suitable for device applications because of the reduced size. Besides, it is noted that neighboring Mn atoms with antiparallel magnetic moments are responsible for the conducting behaviors, while those with parallel magnetic moments obstruct the flow of electrons. This property provides possibility of molding the flow of fully spin-polarized electrons through the manipulation of local magnetization by an external magnetic field, which is an attractive magnetoelectric response: the magnetic-field-controlled electron flow.

Determination of spin Hall angle, spin mixing conductance, and spin diffusion length in CoFeB/Ir for spin-orbitronic devices

Iridium is a very promising material for spintronic applications due to its interesting magnetic properties such as large RKKY exchange coupling as well as its large spin-orbit coupling value. Ir is for instance used as a spacer layer for perpendicular synthetic antiferromagnetic or ferrimagnet systems. However, only a few studies of the spintronic parameters of this material have been reported. In this paper, we present inverse spin Hall effect - spin pumping ferromagnetic resonance measurements on CoFeB/Ir based bilayers to estimate the values of the effective spin Hall angle, the spin diffusion length within iridium, and the spin mixing conductance in the CoFeB/Ir bilayer. In order to have reliable results, we performed the same experiments on CoFeB/Pt bilayers, which behavior is well known due to numerous reported studies. Our experimental results show that the spin diffusion length within iridium is 1.3 nm for resistivity of 250 n$\Omega$.m, the spin mixing conductance $g_{eff}^{\uparrow \downarrow}$ of the CoFeB/Ir interface is 30 nm$^{-2}$, and the spin Hall angle of iridium has the same sign than the one of platinum and is evaluated at 26% of the one of platinum. The value of the spin Hall angle found is 7.7% for Pt and 2% for Ir. These relevant parameters shall be useful to consider Ir in new concepts and devices combining spin-orbit torque and spin-transfer torque.

Phase-shift control of the exchange coupling between magnetic impurities

The control of magnetic interactions is one of the most relevant topics in spintronics. In this work, we propose to use electric potential barriers for tuning or even suppressing the RKKY exchange coupling between magnetic impurities in a two-dimensional electron gas. Our results show that it is possible to manipulate both the magnitude and sign of the RKKY coupling. Systems with two and three impurities are studied. In the last case, the use of two potential barriers can be employed to decouple one of the impurities to the rest. The possibility to control the interactions between magnetic atoms individually may have applications in neuromorphic and quantum computing.

CeIr3Ge7: A local moment antiferromagnetic metal with extremely low ordering temperature

CeIr$_3$Ge$_7$ is an antiferromagnetic metal with a remarkably low ordering temperature $T_{\rm N}$ = 0.63 K, while most Ce-based magnets order between 2 and 15 K. Thermodynamic and transport properties as a function of magnetic field or pressure do not show signatures of Kondo correlations, interaction competition, or frustration, as had been observed in a few antiferromagnets with comparably low or lower $T_{\rm N}$. The averaged Weiss temperature measured below 10 K is comparable to $T_{\rm N}$ suggesting that the RKKY exchange coupling is very weak in this material. The unusually low $T_{\rm N}$ in CeIr$_3$Ge$_7$ can therefore be attributed to the large Ce-Ce bond length of about 5.7 {\AA}, which is about 1.5 {\AA} larger than in the most Ce-based intermetallic systems.

Effect of surface hybridization on RKKY coupling in ferromagnet/topological insulator/ferromagnet trilayer system

We theoretically investigate the RKKY exchange coupling between two ferromagnets (FM) separated by a thin topological insulator film (TI). We find an unusual dependence of the RKKY exchange coupling Φex on the TI thickness (tTI). First, when tTI decreases, the coupling amplitude increases at first and reaches its maximum value at some critical thickness, below which the amplitude turns to diminish. This trend is attributed to the hybridization between surfaces of the TI film, which opens a gap below critical thickness and thus turns the surfaces into insulating state from semi-metal state. In insulating phase, diamagnetism induced by the gap-opening compensates paramagnetism of Dirac state, resulting in a diminishing magnetic susceptibility and RKKY coupling. For typical parameters, the critical thickness in Bi2Se3 thin film is estimated to be in the range of 3-5 nm.

Spin pumping and interlayer exchange coupling through palladium

The magnetic behaviour of ultrathin ferromagnetic films deposited on substrates is strongly affected by the properties of the substrate. We investigate the spin pumping rate, interlayer exchange coupling and dynamic exchange coupling between ultrathin ferromagnetic films through palladium, a non-magnetic substrate that displays strong Stoner enhancement. We find that the interlayer exchange coupling, both in the static and dynamic versions, is qualitatively affected by the substrate's Stoner enhancement. For instance, the oscillatory behavior that is a hallmark property of the RKKY exchange coupling is strongly suppressed by Stoner enhancement. Although the spin pumping rate of ferromagnetic films atop palladium is only mildly changed by Stoner enhancement the change is large enough to be detected experimentally. The qualitative aspects of our results for palladium are expected to remain valid for any non-magnetic substrate where Coulomb repulsion is large.

Spin-torque nano-oscillator based on a synthetic antiferromagnet free layer and perpendicular to plane polarizer

Coupled free layers systems used as active elements in spin-transfer nano-oscillators (STNOs) can have improved microwave performances in terms of linewidth or tunability. Here, we report a numerical study on the magnetization dynamics of a synthetic antiferromagnet (SAF) driven by a perpendicularly spin-polarized current and an in-plane applied magnetic field. Compared to the single free layer STNO, the current-field state diagrams, derived for strong and weak RKKY exchange coupling strength inside the SAF, show a more complex structure, with new static or dynamic states (chaotic dynamics), and also a larger zone of out-of-plane precession state (OPP) oscillations. The OPP frequency behaviour is generally similar to that of the single free layer STNO except for the zone near the chaotic dynamics and for fields larger than the spin-flop field of the SAF, where several frequency jumps have been observed.

Interplay between Nonequilibrium and Equilibrium Spin Torque Using Synthetic Ferrimagnets

We discuss the current induced magnetization dynamics of spin valves F(0)|N|SyF where the free layer is a synthetic ferrimagnet SyF made of two ferromagnetic layers F(1) and F(2) coupled by RKKY exchange coupling. When the magnetic moment of the outer layer F(2) dominates the magnetization of the SyF, the sign of the effective spin torque exerted on the layer F(1) is controlled by the coupling's strength: for weak coupling the spin torque tends to antialign F(1)'s magnetization with respect to the pinned layer F(0). At large coupling the situation is reversed and tends to align F(1) with respect to F(0). At intermediate coupling, numerical simulations reveal that the competition between these two incompatible limits leads generically to spin torque oscillator (STO) behavior. The STO is found at zero magnetic field, with very significant amplitude of oscillations and frequencies up to 50 GHz or higher.

The ferromagnetic properties of Ge magnetic quantum dots doped with Mn

We present a synthesis of Ge:Mn magnetic quantum dots (QDs) and an evaluation of their ferromagnetic properties. The QDs were grown from a GeH4/Ar mixed gas under constant flow conditions at 400°C by means of a plasma-enhanced chemical vapor deposition (PECVD) process, then doped with Mn by a magnetic sputtering technique and annealed at 600°C. The QDs, with a composition of Ge0.88Mn0.12 according to their energy spectrum, showed widely opened hysteresis loops, with large remnant magnetizations Mr of 0.14×10−4 and 0.25×10−4emu/g for the as-grown and annealed samples, respectively. Moreover, the average value of the moment per Mn atom was found to be as high as 2.36μB at room temperature, showing that the Ge1−xMnx QDs constitute an intrinsic diluted magnetic semiconductor. The unprecedented colossal moment is attributed to an effective RKKY exchange coupling interaction between the Mn ions mediated by holes.

Effect of impurity clustering on ferromagnetism in dilute magnetic semiconductors

We establish a model to investigate the effect of clustering of impurities on the ferromagnetism in dilute magnetic semiconductors (DMS). The Curie temperature Tc is calculated by the mean-field theory on a lattice with randomly distributed clusters of magnetic impurities which are interacting with each other by carrier mediated RKKY exchange coupling together with the nearest-neighbor (NN) direct exchange interaction. We consider different types and sizes of the clusters and find that the clustering of impurities can either enhance or reduce Tc, depending on the type and strength of the NN exchange interaction. If the NN interaction is antiferromagnetic and strong compared with the RKKY interaction, the clustering will reduce Tc. On the other hand, if it is ferromagnetic interaction or weak antiferromagnetic one, the clustering can enhance Tc. The trend of enhancing Tc is magnified if the average size of clusters increases. The clustering also changes the distribution of polarizations of impurities. The obtained results provide natural explanations on the fact that the ferromagnetism of DMS samples depends on the preparing and annealing processes even though the density of the magnetic impurities is kept the same.

Electrical and magnetic properties of nano-oxide added spin valves

The nano-oxide layer (NOL) inside the spin valve can smooth the surface topography, which results in two effects: suppressing the ferromagnetic Néel magnetostatic coupling and enhancing the RKKY exchange coupling between the free layer and the pinned layer. As a consequence, MR ratio is increased in the spin valve with NOL layer inside the pinned layer or on top of the free layer. Because of the enhancement of the specular reflectivity in the NOL added spin valves, the oscillation of the interlayer coupling field with respect to the thickness of the spacer layer and even the thickness of the cap layer has been observed. In terms of the performance of both electrical and magnetic properties of the spin valves, CoFe–O turns out to be the best materials inside the pinned layer. But, as a cap layer, ZnO is the best choice because of its crystalline growth on top of the free layer CoFe, which causes the enhancement of the MR ratio. About 4.5% of MR ratio has been achieved in a NOL added spin valve in the current-perpendicular-to-plane configuration. Large MR ratio observed in the NOL added CPP sensor is due to the increment of the interface scattering and current confined path in the NOL added pinned layer.

Numerical simulation of spin-glass transition phenomena (invited)

Results are presented from a continuing investigation of spin-glass transition effects by numerical simulation. The model is one of classical unit vectors randomly distributed on an fcc lattice, with RKKY exchange coupling and nearest-neighbor dipolar coupling in an approximate representation of dilute Mn in Cu. Results for system energy and for dipolar energy as a function of temperature show no resolvable features at the freezing temperature TG* in accord with the high-precision specific heat data of Fogle, Boyer, Phillips, and Van Curen. Additional details of shattered susceptibility behavior in the vicinity of the ground-state transition are presented. Further data on freezing temperatures as a function of dipolar coupling strength D show a trend toward macroscopic behavior (i.e., TG* independent of D) with a sample of 4928 spins. Finally, a moderately sharp onset of freezing transverse to an applied magnetic field is found, as predicted by the mean-field calculations of Toulouse and Gabay. As expected, this transition is unrelated to transverse susceptibilities.

Magnetische untersuchungen an mischkristallen der quasibinären legierungssysteme (Cd Te) 1- x (Mn Te) x und (Sn Te) 1- x (Mn Te) x

The magnetic susceptibility and magnetization are presented for polycrystalline samples of the alloy systems Cd 1- x Mn x Te 0 < x ⩽ 0.1 and Sn 1- x Mn x Te 0 < 4 ⩽ 0.4. The magnetic measurements were performed between 2.3 K and 300 K in external magnetic fields up to 11 kOe. At sufficiently high temperatures the susceptibility can be described by a Curie-Weiss law. In the system Sn 1- x Mn x Te θ p is positive. A linear dependence θ p ∞ x was found with θ p(0.4) = 49 K. In the series Cd 1- x Mn x Te θ p changes sign. For θ < x < 0.04 θ p is positive with a maximum θ p ≈ 10 K at x = 0.02. In the region x #62; 0.04 θ p becomes negative with θ p = -35 K at x = 0.1. The effective spin value of manganese is S eff #62; 5/2 for all the samples. The investigation was done to check the assumption that ferromagnetic coupling may exist in tellurides of manganese if the shortest distance d MnMn is greater than 3.4 Å. This hypothesis has been stated. In the case θ p #62; 0 the results are partly explained by the RKKY exchange coupling.