Magnetic nanostructures Paolo Vavassori Ikerbasque, Basque Fundation for Science and CIC nanogune Consolider, San Sebastian, Spain. P. Vavassori nano@nanogune.eu I www.nanogune.eu 1
Outline Part I Introduction. Fabrication of artificial magnetic nano-structures. Part II Magnetism and reduced dimensionality. Micromagnetics of nano-shaped magnetic elements. Dynamic properties Part III Experimental techniques for studying the reversal of artificial magnetic nano-structures. Probing magnetic dynamic. P. Vavassori nano@nanogune.eu I www.nanogune.eu 2
Introduction Ferromagnets can form a wide variety of stable spin configurations that depend on: size, shape, material parameters and field history. Micromagnetic simulations are currently performed to investigate such effects. A wide variety of behaviors are observed that include reversal via vortex nucleation and annihilation, domain wall nucleation and motion, and coherent rotation. Micromagnetic simulations have also cataloged some of the precursor states prior to reversal: flower state, onion state, C-state, S-state,etc. One of the issues that makes magnetization reversal such a complex phenomenon is that, even for the same precursor state different particles reverse via different routes. microscopic explanation why one reversal mechanism is preferred to another when the system undergoes a transition between equilibrium states, under the action of the applied field. P. Vavassori nano@nanogune.eu I www.nanogune.eu 3
Introduction Ferromagnetic nano-structures offer a unique opportunity to investigate properties at length scales previously unattainable. Because intrinsic magnetic length-scales (e.g., exchange length or the domain wall thickness) are comparable to the sample size, novel physical properties can be expected, respect to bulkand film-like materials. Surface effects become relevant (dominant). P. Vavassori nano@nanogune.eu I www.nanogune.eu 4
Applications This field has attracted much attention because of its close ties to potential technological applications. A key issue is to understand and control the magnetic switching of small magnetic elements. Nonvolatile Magnetic Random Access Memories (MRAM). Magnetic logic devices (networks of dipolar coupled nanomagnets). Spintronics devices, in which spin-dependent transport processes are exploited leading to novel electronic functionalities (cell memories, ultrasensitive field sensors with applications in the detection of molecular recognition Lab-on-Chip applications). P. Vavassori nano@nanogune.eu I www.nanogune.eu 5
Nanomagnetism: aims and methods Objectives: Understanding magnetic phenomena on small length and fast time scales Devices/technology for practical applications Magnetization hysteresis Magnetization dynamics H Methodology: Fabrication of thin films and multilayered magnetic materials Fabrication of magnetic nanostructures Characterization of magnetic properties and dynamics (conventional and novel techniques) Modelling of magnetic properties and dynamics P. Vavassori nano@nanogune.eu I www.nanogune.eu 6
Soft nano-scale disk and rings: vortex state Magnetization configuration determined by magnetostatic and exchange energies (domain wall width bigger than size). Vortex core The energy is almost all due to exchange. Residual magnetostatic energy is confined in the core. The core is necessary to avoid the singularity in the in-plane magnetization curling. Systems developing vortes state configurations are interesting because of their reduced sensitivity to edge effects. Stabilization of the vortex state by removing the high energy core in a ring structure. Magnetic force microscopy image P. Vavassori nano@nanogune.eu I www.nanogune.eu 7
Elliptical particles H H Single vortex Double vortex P. Vavassori, et al., Phys. Rev. B. 69, 214404 (2004) P. Vavassori nano@nanogune.eu I www.nanogune.eu 8
Stripes domains pattern in Co bars. Domains pattern is not just an energy minimization issue: hysteresis Hcp Co bars with uniaxial in plane anisotropy parallel to the short edge. 40 nm thick particles 120 nm wide and lengths of 936 and 792 nm. The domains configuration is a typical stripes pattern. H H H 74 nm T. Kohda et al., IEEE Trans. Magn. 35, 3472 (1999) G. Leaf, H. Kaper, V. Novosad, P. Vavassori, and M. Grimsditch, Phys. Rev. Lett. 96, 017201 (2006) P. Vavassori nano@nanogune.eu I www.nanogune.eu 9
Fundamental studies and applications Study the accommodation of frustration through the local correlations between the moments as a function of both the strength of the interactions and the geometry of the frustration Square? E Kagomè R. F. Wang et al., Nature 439, 303 (2006) Understanding the interactions between closely spaced nanomagnets is crucial for ultrahigh density memory, data storage applications, to realize magnetic quantum-dot cellular automata systems and Nonvolatile MRAM. m j j H S +H int A. Imre, et al., Science 311, 205 (2006); R. P. Cowburn, M. E. Welland, Science 287, 1466 (2000). Assembly of interacting grains P. Vavassori nano@nanogune.eu I www.nanogune.eu 10
Nano-opticsoptics P. Vavassori nano@nanogune.eu I www.nanogune.eu 11
Magnetoplasmonics Sub-wavelength magnetic nanoparticles Phys. Rev. B 79, 054527 (2009) Tunable porperties due to the interplay between magneto-optical activity and eccitation of plasma resonances P. Vavassori nano@nanogune.eu I www.nanogune.eu 12
Applications D. A. Allwood et al., Science 309, 1688 (2005) DW logic and racetrack memories P. Vavassori nano@nanogune.eu I www.nanogune.eu 13
Applications Spintronics effects and devices Investigation of novel physical processes due to the interplay between spin currents and magnetization at the nano-scale Manipulation of magnetization by spin polarized currents in planar nano-structures Spin torque oscillator Application: (c) nanosensors, MRAMs, spintronic devices P. Vavassori nano@nanogune.eu I www.nanogune.eu 14
Applications Biomedical applications P. Vavassori nano@nanogune.eu I www.nanogune.eu 15
Applications Tunable magnetic field landscape to pin and control the motion of superconducting vortices in superconductor-ferromagnet hybrid devices P. Vavassori nano@nanogune.eu I www.nanogune.eu 16
Applications All the above mentioned applications require a precise control of the material properties, as well as of the shape and edge roughness of the nanoelements, arrangements of the nanoelements, which, in some cases, should be homogeneous over large areas (several cm 2 ). P. Vavassori nano@nanogune.eu I www.nanogune.eu 17
Requirements for storage applications The dots should form a regular 2D matrix. A dot should behave as one entity, i.e., single domain and non-interacting with nearest neighbours. The switching field (field required for magnetization reversal) should be large enough and only two remanent states can be allowed. The dots should have a narrow switching field distribution to avoid unwanted writing of neighbouring dots. The magnetization of the dots should be thermally stable. P. Vavassori nano@nanogune.eu I www.nanogune.eu 18
Optical projection lithography. Optical interference lithography. Electron-beam lithography. X-ray lithography. Ion-beam lithography. Fabrication techniques for patterning of periodic dot arrays Main techniques Focussed Ion Beam (FIB) technique. Focused electron beam induced deposition Hole-mask colloidal nanospheres lithography P. Vavassori nano@nanogune.eu I www.nanogune.eu 19
Lithographic techniques Processing sequence a) Photoresist exposure Photoresist Substrate b) Photoresist development and reactive ion-etching O 2 O 2 + SF 6 c) Magnetic film deposition d) Lift-off P. Vavassori nano@nanogune.eu I www.nanogune.eu 20
Optical inteference lithography Ar laser Beam splitter Mirror Period p = /2sin 2 Turning mirror P. Vavassori nano@nanogune.eu I www.nanogune.eu 21
Focused Ion Beam (FIB) milling Smallest Come lateral features agisce size il achievable: ~10 nm FIB Vedi filmato Primo FIB in Italia in acquisto presso Centro di Ricerca INFM (Istituto Nazionale d i Fisica della Materia) di Modena- Ferr ara High energy Ar + ion-beam Fe Si P. Vavassori nano@nanogune.eu I www.nanogune.eu 22
Example of samples prepared with FIB P. Vavassori nano@nanogune.eu I www.nanogune.eu 23
FEBID: ultra-small nanostructure Focused electron beam induced deposition Precursor: dicobalt octacarbonyl E-beam parameters: DV= 25 kv, I=2.7 na Scanning electron microscopy image of the set of FEBID cobalt structures Single Co wire, 30 nm wide, 20 nm thick, and 5 mm long Co purity 95 at % P. Vavassori nano@nanogune.eu I www.nanogune.eu 24
FIB+FEBID: 3D nanostructuring P. Vavassori nano@nanogune.eu I www.nanogune.eu 25
FIB+FEBID: 3D nanostructuring P. Vavassori nano@nanogune.eu I www.nanogune.eu 26
Hole-mask colloidal lithography P. Vavassori nano@nanogune.eu I www.nanogune.eu 27
Smallest lateral features size achievable Optical lithography Ion-beam lithography X-ray lithography Electron-beam lithography Focused ion-beam milling FEBID 1 nm 10 nm 100 nm 1mm 10 mm P. Vavassori nano@nanogune.eu I www.nanogune.eu 28
Other techniques Ion irradiation damage (using FIB). Deposition through shadow masks. Scanning probe techniques. Nanoimprint. Self-assembling (pseudo-ordered structures). Nanotemplates (for fabricating nanowires). P. Vavassori nano@nanogune.eu I www.nanogune.eu 29
Deposition through shadow masks. Thickness 30 nm, lattice parameter 3 mm Dot diameter 2.5 mm. P. Vavassori nano@nanogune.eu I www.nanogune.eu 30
Scanning probe techniques. Voltage pulses: the AFM or STM tip, made of, or coated with, a magnetic material, is brought to within a few nanometers of the substrate; a negative voltage is applied during a few ms, inducing a material transfer from the tip to the substrate. Element size can be controlled by: applied voltage, pulse duration and tip-substrate separation. Elements as small as 10 nm can be produced. Disadvantages: only dots and poor reproducibility. STM chemical vapor deposition: a voltage pulses between the STM tip and the substrate induces the dissociation and deposition of atoms when an appropriate organometallic gas is introduced in the STM chamber. Compared to previous technique, it is more reproducible. If the substrate is immersed in an electrochemical cell, the STM tip can be used as a local counter-electrode allowing for selective nanometer electrodeposition. In general, these techniques are characterized by a small throughput and small patternable area. P. Vavassori nano@nanogune.eu I www.nanogune.eu 31
Nanoimprint. A mould, made out of a hard material by conventional electron, x-ray or interference lithography, is used to physically deform a resist. The patterning is then achieved following the same steps of lithographic techniques.. a) Resist imorint b) Reactive ion-etching O 2 O 2 + SF 6 Resist Substrate c) Magnetic film deposition d) Lift-off P. Vavassori nano@nanogune.eu I www.nanogune.eu 32
Self assembling. There is a variety of natural processes which tend to form ordered arrays of nanostructures. However, most of these processes are ordered locally, they usually do not have true long-range order. For example: Co and Ni atoms tend to nucleate at specific sites of the reconstructured Au (111) surface, forming arrays of dots. Fe tends to grow at the step edges of the Cu (111) surface, forming arrays of Fe lines or dots. Co pillars grown on Au (111) surface. P. Vavassori nano@nanogune.eu I www.nanogune.eu 33
Nanotemplates. A schematic representation of high-density nanowire fabrication in a polymer matrix. (A) An asymmetric diblock copolymer annealed above the glass transition temperature of the copolymer between two electrodes under an applied electric field, forming a hexagonal array of cylinders oriented normal to the film surface. (B) After removal of the minor component, a nanoporous film is formed. (C) By electrodeposition, nanowires can be grown in the porous template, forming an array of nanowires in a polymer matrix. P. Vavassori nano@nanogune.eu I www.nanogune.eu 34
STM With a scanning tunneling microscope, images of surfaces with atomic resolution can be readily obtained. An STM uses quantum tunneling of electrons to map the density of electrons on the surface of a sample. The STM works by bringing a metal wire with a sharp tip very close to a conducting surface. The distance is generally on the order of 1 10 9 m, a distance corresponding to a few atomic diameters P. Vavassori nano@nanogune.eu I www.nanogune.eu 35
STM P. Vavassori nano@nanogune.eu I www.nanogune.eu 36
STM P. Vavassori nano@nanogune.eu I www.nanogune.eu 37
Atomic force microscopy (AFM) P. Vavassori nano@nanogune.eu I www.nanogune.eu 38
Atomic force microscopy (AFM) P. Vavassori nano@nanogune.eu I www.nanogune.eu 39
The Interaction of a Tip and the Sample Lennard-Jones potential The forces in SFM in the absence of added magnetic or electrostatic potentials are governed by the interaction potentials between atoms. The interaction is attractive at large distances due to the van-der-waals interaction. At short distances repulsion originates between electrons when one atom tries to penetrate another. The repulsive forces have their origin in the quantum mechanical exclusion principle, which states that no two fermions can be in exactly the same state, that is to say have the same spin, angular momentum, z- component of the angular momentum and location P. Vavassori nano@nanogune.eu I www.nanogune.eu 40
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Magnetic force microscopy (MFM) P. Vavassori nano@nanogune.eu I www.nanogune.eu 44
Magnetic force microscopy (MFM) P. Vavassori nano@nanogune.eu I www.nanogune.eu 45
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Optical and electron microscopy P. Vavassori nano@nanogune.eu I www.nanogune.eu 47
Optical microscopy: Diffraction limits P. Vavassori nano@nanogune.eu I www.nanogune.eu 48
Diffraction limited optical microscopy P. Vavassori nano@nanogune.eu I www.nanogune.eu 49
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Lenses in elctron microscopes F = e vxb Cross-section of an electromagnetic lens. P. Vavassori nano@nanogune.eu I www.nanogune.eu 51
SEM resolution and contrast mechanisms P. Vavassori nano@nanogune.eu I www.nanogune.eu 52
SEM contrast mechanisms: analysis P. Vavassori nano@nanogune.eu I www.nanogune.eu 53
Defocused TEM: Lorentz force microscopy Lorentz images of vortices with opposite chirality Lorentz imaging MFM images of vortices with opposite polarity P. Vavassori nano@nanogune.eu I www.nanogune.eu 54