Our lab is in Shrum Science Center Physics P7409 (sputter deposition cluster, molecular-beam epitaxy (MBE), sample preparation) and P7413 (fabrication cluster, magneto-optical kerr effect (MOKE), ferromagnetic resonance (FMR), giant magnetoresistance (GMR)). We make extensive use of a vibrating-sample magnetometer (VSM) in room P7419, a superconducting quantum interference device (SQUID VSM) in room P7430, and an x-ray diffractometer (XRD) in room P8405.
Control of noncollinear interlayer exchange coupling
Magnetic multilayers play a pivotal role in the development of magnetic devices for data storage (such as hard drives and magnetic random access memory) and magnetic field sensing. One of the interface phenomena discovered in magnetic multilayers is interlayer exchange coupling, which occurs between two ferromagnetic layers separated by thin nonmagnetic metallic spacer layers. This phenomenon is utilized in devices to establish antiferromagnetic coupling between magnetic layers. Recently, we discovered novel spacer layers that can significantly enhance the functionality of the existing spacer layer. These new spacer layers not only enable the control of collinear coupling between ferromagnetic layers (antiferromagnetic (180º) and ferromagnetic (0º) alignments) but also provide the capability to control the alignment of ferromagnetic layers at any angle between 0º and 180º, a phenomenon referred to as noncollinear coupling. Significance: These new spacer layers have the potential to revolutionize the majority of spintronic devices. The optimal design of such devices often requires noncollinear alignment between at least two adjacent ferromagnetic layers.
RELEVANT PAPERS:
Z. Nunn et al., Sci. Adv. 6, 1-7 (2020)
A. Claas et al., Phys. Rev. B 106, 054401 (2022)
J. Besler et al., J. Magn. Magn. Mat. 585, 171109 (2023)
Methods for measuring magnetic properties of thin films
The first method enables the measurement of the interlayer exchange coupling. The method involves depositing a film structure consisting of two magnetic layers with low damping and different resonance fields separated by a spacer layer. This structure is measured using broadband ferromagnetic resonance (FMR) to determine resonance peak positions for both acoustic and optical modes. Finally, a system of coupled Landau-Lifshitz-Gilbert equations, representing the coupled magnetic layers, is solved and employed to fit the FMR data and determine the interlayer coupling. Significance: The method can measure both ferromagnetic (up to ~4.5 mJ/m2) and antiferromagnetic interlayer exchange coupling. It is employed to measure the interlayer exchange coupling across Ta, Mo, Ru and Pt spacer layers, all of which are crucial for industrial applications. By employing this method and the unique design of ferromagnetic layers, we were able to measure ferromagnetic coupling strengths up to 4.5 mJ/m², which, to our knowledge, represents the largest ferromagnetic coupling ever measured using magnetometry techniques.
In 2011, we developed the second method for measuring the exchange stiffness and antiferromagnetic interlayer exchange coupling in magnetic thin films. This method involves depositing two antiferromagnetically coupled ferromagnetic layers, measuring their magnetization versus the field (M(H)) dependence, and fitting the M(H) dependence with a one-dimensional micromagnetic model. Recently, we developed a one-dimensional model for the case of continuous magnetization distributions across the ferromagnetic layers. Significance: The main advantage of the continuous model is that it can be numerically solved as a boundary value problem. This approach can be significantly faster than the energy minimization required for the discrete model, improving computational speed and enabling much more rapid data analysis. Measuring the exchange stiffness (which is challenging) and interlayer exchange coupling in magnetic films is essential for designing the static and dynamic magnetic properties of spintronic devices.
Both the measurement methods and the coupling and stiffness values they yield have found widespread use among international research groups.
RELEVANT PAPERS:
T. Mckinnon et al., J. Appl. Phys. 123, 223903 (2018)
P. Omelchenko et al., Appl. Phys. Lett. 113, 142401 (2018)
P. Omelchenko et al., J. Appl. Phys. 132, 173905 (2022)
C. Eyrich et al., J. Appl. Phys. 111, 07C919 (2012)
C. Eyrich et al., Phys. Rev. B 90, 235408 (2014)
E. Girt et al., J. Appl. Phys. 109, 07B765 (2011)
Spin-transfer torque magnetic random-access memory (STT-MRAM)
STT-MRAM is a promising emerging memory technology due to its non-volatile nature, high recording density, endurance, and the potential for fast write/read speeds and low power consumption. It utilizes magnetic nanopillars consisting of two magnetic layers separated by a conductive or insulating spacer to store binary information. The parallel orientation of magnetic moments of magnetic layers is the low resistance, “0”, and the antiparallel orientation is the high resistance, “1” due to the giant or tunnel magnetoresistance effect. Writing information is accomplished by passing a current through magnetic pillars, which due to the spin transfer torque (STT) effect, changes the orientation of magnetization in one of the magnetic layers, thereby altering the resistance of the nanopillar.
We have developed a novel nanopillar design for STT-MRAM in which both magnetic layers consist solely of Co/Ni magnetic multilayers. These nanopillars exhibited twice the writing efficiency of any previously reported device with a similar structure. We measured and calculated contributions from the magnetocrystalline, magnetoelastic, surface and dipolar anisotropies to the total perpendicular anisotropy of Co/Ni multilayers. Significance: This work contributes to our understanding of the origin of magnetic anisotropy in magnetic multilayers and provides insights into how to maximize the perpendicular magnetic anisotropy of Co/Ni multilayers.
RELEVANT PAPERS:
M. Arora et al., IEEE Magn. Lett. 8, 3100605 (2017)
M. Arora et al., J. Phys. D: Appl. Phys. 50 505003 (2017)
M. Arora et al., Phys. Rev. B 96, 024401 (2017)
Spin pumping
Spin pumping is a phenomenon that takes place at the interface between ferromagnetic and normal metal films. It involves the interaction between the precessing localized magnetic moments of the ferromagnet and the conduction electrons in the adjacent normal metal. This interaction results in the transfer of angular momentum from the precessing ferromagnet to the conducting electrons, generating a spin accumulation at the interface and the subsequent flow of a spin current. Spin pumping is a fundamental process in the field of spintronics, enabling the manipulation of spins for various applications in electronics and information storage.
Our group has identified a range of magnetic films that can be deposited using the sputter deposition technique and are suitable for spin pumping experiments, i.e., they have low damping and controllable ferromagnetic resonance fields. We developed and demonstrated a series of experiments to investigate spin-transport properties in important spintronic layer materials like Ta and Pt using spin pumping. Significance: These layers are used in most thin film magnetic devices. Building upon these studies, we explored spin transport in more complex normal metal structures and experimentally demonstrated a diode-like spin current effect across the Au/Pt interface.
RELEVANT PAPERS:
P. Omelchenko et al., Sci. Rep. 7, 4861 (2017)
P. Omelchenko et al., Phys. Rev. B 100, 144418 (2019)
P. Omelchenko et al., Phys. Rev. Lett. 127, 137201 (2021)