Session F39: Magnetic Coupling and Exchange Bias

Localization of Curie Temperature in Compositionally Graded Ferromagnetic Films

Although ferromagnetism (FM) is in general a long-range collective phenomenon, it is possible to induce local spatial variations of magnetic properties in FM materials. For example, systematic variation of the exchange coupling strength can be used to create systems that behave as if they are composed of virtually independent segments that exhibit “local” Curie temperatures (Tc). While it is obvious that such localization should be possible across some lengthscale, the magnitude of that lengthscale is not so intuitive, nor is the expected behavior of a material exhibiting a nearly continuous varitation in exchange strength. We have explored these questions in real materials by using neutron scattering and mean-field simulations to study novel compositionally graded transition metal multilayer films as model systems [1-3].

Remarkably, we have found that non-local magnetic effects are significant only over distances less than 3 nm (possibly much less). Beyond that the structures behave as a continuum of decoupled layers with distinct Tc. This leads to fascinating functionality, including FM phase boundaries that can be reversibly moved up and down the thickness of a film with temperature, and modified with an applied magnetic field. Since we have demonstrated this for an itinerant metal, we assert that for virtually any modulated magnetic material system, collective effects should be suppressed to down to nanometer length scales, so that magnetic behavior overall can be well described in terms of local material properties.

[1] PRL 116, 0047203 (2016).
[2] PRB 95, 134445 (2017).
[3] PRB 98, 064404 (2018)   

Magnetization Depth Profile in a Magnetic Insulating Film Under a Thermal Gradient

By eliminating mobile electrons, insulator spintronics offer opportunities to reduce power loss and signal decay of metallic interconnects in complicated integrated circuits. As a consequence, there are no energy losses due to the heating by the conducting electrons or eddy currents. This strategy requires methods to generate, transport, and detect the spin and transfer them into a charge-based signal for further processing. A temperature gradient has been shown to generate a pure spin current inside a magnetic insulator that could be detected in an adjacent metallic film with a high spin-orbital angle (spin Seebeck effect, SSE). Little experimental evidence is known about the depth dependence of spin structure. Using a similar sample design, we have measured the polarized neutron reflectivity (PNR) from a YIG film with and without a temperature gradient. We will present the magnetization as a function of depth in the magnetic insulator extracted from the PNR along with structural, magnetic, and electric transport characterization. We have observed a larger than expected magnetization in the GGG substrate which may contribute to the size of the SSE in similar samples.   

Quantum Magnon Casimir effect

Magnons are the quanta of spin-wave excitations in magnetically ordered media. Spin-up and spin-down magnons coexist in antiferromagnets (AFs) where they act similarly as photons in many different phenomena. We find that the zero-point quantum fluctuation of magnons in AFs can play a significant role in determining the properties of magnetic thin films. When two ferromagnets are separated by an insulating AF, they can couple and exert spin torques on each other even at absolute zero temperature, thanks to the zero-point quantum fluctuation of magnons confined in the AF. This is in perfect analogy to the Casimir effect of photons confined between two plates. Moreover, we find that in a ferromagnet/AF bilayer, the magnonic Casimir effect leads to a counterintuitive temperature dependence of the overall magnetization.   

Proximity Effects in Py-Cu Composite Spin Valves with Temperature Dependent Coupling

We present results on an exchange biased spin valve whose interlayer undergoes a magnetic phase transition. The structure consisted of 3 variations of Py_xCu_1-x: Py_0.8Cu_0.2/Py_0.4Cu_0.6/Py_1.0Cu₀/IrMn. The IrMn serves to exchange bias the Py layer; the degree to which the rest of the sample exhibits exchange bias depends on ferromagnetic coupling between the Py_xCu_1-x layers. Magnetization and angle with respect to temperature (M v. T and φ_M v. T, respectively) measurements from polarized neutron reflectometry (PNR) indicate a sharp transition temperature at T = 160 K, which is the Curie temperature (TC) of the spacer layer. Below this, both the Py_0.8Cu_0.2 and Py layers exhibit exchange bias. As the temperature increases and the system approaches 160 K, the portion of the loop corresponding to the bottom free layer (Py_0.8Cu_0.2) shifts closer to zero, showing a Néel-like transition. When T > 160 K, the coupling between the top and bottom layers is broken, and EB is only observed for the Py layer in direct contact with IrMn. Additional measurements studying the dependence on layer thickness will be reported.   

Proximity effects on magnetic properties of Ni by the adjacent topological insulator

Topological insulator (TI) are a new class of quantum matter that shows a surface state with spin-momentum locking property. While this unique property was intensely investigated by using TI/ferromagnet (FM) structures, modification of magnetic properties of the FM layer by the adjacent TI has not been systematically investigated. In order to investigate the effect of TI layer on the magnetic properties of FM layer, we have chosen a Ni and a Bi₂Se₃ as FM and TI materials, respectively. We have prepared two series of Ni-based structures: one is deposited on Bi₂Se₃, and the other is deposited on ZnSe. Both Bi₂Se₃ and ZnSe layers were grown by molecular beam epitaxy on (111)B GaAs substrate. Bi₂Se₃ and ZnSe layers were mounted in an e-beam evaporator chamber at the same time, and the Ni films were deposited. The magnetization measured at 5 K with an in-plane magnetic field revealed that a coercive field of Ni on both films decrease with Ni increasing thickness up to 20 nm, but increasing again for further increase in thickness. However, when the magnetic field was applied in an out-of-plane direction, the coercive field of Ni decreased with increasing thickness in both series of samples. On the other hand, the saturation magnetization of Ni/Bi₂Se₃ was always smaller than Ni/ZnSe.   

Exchange Bias Enhancement in Ultra-Dense Arrays of Sub-100nm Co/CoO Nanodisks

We report on high stability and enhanced exchange bias (EB) of ultra-dense (Tb/inch²) arrays of sub-100 nm Co/CoO nanodisks. Exchange-biased nanodisks are grown by electron beam deposition of Co (15nm) using nanoporous anodic aluminum oxide (AAO) templates as a mask [1]. The thickness of the CoO layer, and therefore the EB of the nanodisks, can be tuned by the oxidation time of the disks. We have found that an annealing time of 10 minutes at 250°C in air produces the maximum EB (800 Oe) at 50K. A comparative study with a reference Co thin film is presented, showing that the nanodisks develop larger EB and higher blocking temperatures at each oxidation time. Although magnetic dots suffer lateral oxidation, which is not possible in the film, a simple model that takes into account this effect demonstrates that the exchange energy density in dots must be higher than that in the Co/CoO film. This is an exciting example of how nanopatterning can improve the physical properties of thin film systems.

1-Template-assisted electrodeposition of Ni and Ni/Au nanowires on planar and curved substrates.” J. Guiliani, J. Cadena, and C. Monton. Nanotechnology 29, 075301 (2017).   

Effects of field annealing on MnN/CoFeB exchange bias systems

Antiferromagnets are commonly used in magnetic tunnel junction based spintronics to fix the ferromagnetic reference layer using the exchange bias effect. The antiferromagnetic MnN Θ-phase exhibits large exchange bias fields on CoFe films on order of 1800 Oe which can be enhanced with increased annealing temperature [1,2]. The exchange bias field in MnN/CoFeB systems is observed to be dependent on annealing temperature and MnN thickness. The effect of annealing temperature (as-deposited, 325 and 525 C) and MnN thickness (30 and 48 nm) on the magnetic and structural properties of MnN/CoFeB samples have been examined with polarized neutron reflectivity (PNR), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). PNR fits show that nitrogen increasingly diffuses from MnN to the underlying seed layer as the annealing temperature increases, which leads to a disappearance of the exchange bias effect when the MnN thickness is not sufficient. Intermixing of Mn, Fe, and Co at the MnN/CoFeB interface is also observed in PNR, XAS and XMCD measurements and is correlated to observed changes in magnetization.

[1] M. Meinert, B. Büker, D. Graulich,and M. Dunz, Phys. Rev. B. 92, 144408 (2015)
[2] M. Dunz, J. Schmalhorst, and M. Meinert, AIP Adv. 8 056304 (2018)   

EXCHANGE BIAS IN IrMn3/Py BILAYERS

Recently antiferromagnets have gained increased interest as active components for spintronics due to their intrinsic high frequency dynamcis and resulting zero net magnetic moment. One such material is IrMn₃ with the magnetic moments forming a triangular structure in the (111) planes and a giant magnetocrystalline anisotropy energy of 10 meV per formula unit. We prepared films of IrMn₃ on different substrates {[001]MgO, [0001]Al₂O₃ and Si}. Since spin-orbit torques are most pronounced for [100] oriented IrMn₃ we investigated the temperature and cooling field dependence of exchange bias. X-ray diffraction shows predominantly [100] crystallographic orientations, with also a small contribution due to [111] oriented grains. Magnetometry measurements using a quantum design superconducting quantum interference device (SQUID) show that the exchange bias decreases with increasing temperature and an anisotropy of the coercivity with in-plane field direction.
  

Exchange bias induced by molecular spinterface

Molecular spins as individual are promising quantum states for coming computation technology. The “on surface” configuration of molecules in proximity to a magnetic film allows control over the orientations of molecular spins and interfacial coupling between them. The stacking of planar molecular spins could favor antiferromagnetic interlayer coupling and lead to pinning of the magnetic underlayer via exchange bias, extensively utilized in magnetic devices. Here, we show tunable molecular exchange bias and its asymmetrical magnetotransport features by varying composition of metalloporphyrin/ferromagnet bilayers. The systems exhibit a wide range of interfacial coupling (ferromagnetic or antiferromagnetic) and exchange bias. Transport measurements of the bilayers reveal the molecular exchange bias effect on a fabricated device, representing asymmetric behaviors on anisotropic and angle-dependent magnetoresistance. Theoretical calculations demonstrate the orientation of spins and interfacial magnetic coupling, and further understanding to exchange bias. A study of interfacial coupling in molecule/ferromagnet systems and its impact on magnetic and magnetotransport behaviors will extend functionalities of molecular spinterface for emerging information technology.   

Effect of CoFe Dusting Layer and Annealing on the Magnetic Properties of Sputtered Ta/W/CoFeB/CoFe/MgO Layer Structures

Ultrathin CoFeB films are an essential building block of most emerging spintronic applications due to the tunable perpendicular magnetic anisotropy (PMA) energy, low Gilbert damping and high annealing stability. Improvements in these magnetic properties are being explored through interface engineering. We explored the effect of a CoFe wedge inserted as a dusting layer (0.1 nm – 0.3 nm thick) at the CoFeB/MgO interface of a sputtered Ta(2 nm)/W(3 nm)/CoFeB(0.9 nm)/MgO(3 nm) film – a typical structure for spin-orbit torque devices. Films were annealed in a Rapid Thermal Annealer (RTA) at temperatures varying between 300 °C and 400 °C in an Ar environment. Ferromagnetic resonance studies were carried out to estimate the effective PMA field and the Gilbert damping as a function of the CoFe thickness and across several annealing temperatures. While as-deposited films present only easy-plane anisotropy, a transition along the wedge from easy-plane to out-of-plane was observed across several annealing temperatures. Interlaying a CoFe dusting film between CoFeB and MgO provides an alternative approach for PMA in ultrathin films.   

Hydrogen-driven switching of the magnetic surface anisotropy at the Co/Pd interface

Heterostructures exhibiting perpendicular magnetic anisotropy (PMA) have proven to be indispensable within the magnetic recording industry. By exploiting the hydrogen-induced modifications to PMA which occur exclusively at the ferromagnetic/Pd interface, an opportunity exists to expand the potential applications of PMA-based heterostructures into the realm of hydrogen sensing using ferromagnetic resonance (FMR) - an electron-spin based technology. Here, we present an interface-resolved in-operando study of a Co/Pd film which features tailorable PMA in the presence of hydrogen gas. We combine polarized neutron reflectometry with in-situ FMR to explore the nanoscopic interactions of hydrogen at the Co/Pd interface which affects the spin-resonance condition during hydrogen cycling. Key experimental data and theoretical modelling reveal that the interfacial PMA of the Co/Pd film suppresses non-reversibly upon primary exposure to hydrogen gas – highlighting a potential avenue for spintronics-based hydrogen sensing.   

Perpendicular Magnetocrystalline Anisotropy on 3d Transition-Metals Multilayers – A First-principles Study

Magnetic tunnel junctions (MTJ) with perpendicular magnetocrystalline anisotropy (MCA) have much attention for applications to high-density, high-thermal stability, nonvolatile memories. Strong perpendicular MCA appears in transition-metal films such as Co/Pt, Co/Pd and CoFeB/Pd due to the strong hybridization between the 3d and 5d orbitals at the interfaces. The remaining challenge in these systems is to understand the role of the hcp-fcc phase transition that occurs in cobalt, which leads to the change of the easy magnetization direction. To address this challenge, the mechanisms of MCA in hcp-like and fcc-like stacking of Co-based 3d transition-metal multilayers are systematically investigated by using full-potential linearized augmented plane wave calculations. The MCA energy of possible atomic-layer alignments of Co-based films including Mn, Fe, and Ni layers is presented. The results predict that large perpendicular MCA can be achieved in Co/Ni multilayers for both hcp-like and fcc-like stackings. The large perpendicular MCA arises from a second-order effect of spin-orbit coupling between occupied and unoccupied Ni d_yz,xz states near the Fermi level. A promising transition-metal film for MTJ with giant perpendicular MCA and the preferred stacking stability is demonstrated.   

Ab initio calculations of structural, optical and magnetic properties of ordered MxPt1-x, (M= Co, Ni, Fe and Mn) binary alloys

Ab initio calculations of the structural, optical and magnetic properties of ordered
M x Pt 1-x, ( M= Co, Ni, Fe and Mn) binary alloys have been performed. The optimized
parameters of L10 CoPt, FePt and MnPt systems have been calculated and found to be
(a o = 3.805, c o = 3.707 )Å, (a o = 3.88 , c o = 3.73 )Å and (a o = 4.03 , c o = 3.69 )Å,
respectively. In addition, we found that L12 Ni 3 Pt exhibits a o = 3.64 Å. Our
calculations of electronic band structure, density of states (DOS) and partial density of
states of M x Pt 1-x binary alloys indicate that the contributions to total DOS of M x Pt 1-x
come mainly from M-3d and Pt-5d electronic states, in CoPt , Ni 3 Pt, FePt and MnPt.
Furthermore, magneto-crystalline Anisotropy Energy (MAE) of CoPt, FePt and MnPt
systems have been calculated using The Force theorem and the slap approach and
found to be (5 × 10 7 erg/cm 3 ), (6.5× 10 7 erg/cm 3 ) and (1.2 × 10 7 erg/cm 3 ) respectively.
Our results reveal that ordered M x Pt 1-x exhibit attractive optical and magnetic
properties that make them potential candidates for optical-electronic and magneto-
optical devices.