V High frequency magnetic measurements

V High frequency magnetic measurements Rémy Lassalle-Balier What we are doing and why Ferromagnetic resonance CHIMP memory Time-resolved magneto-optic Kerr effect NISE Task 8 New materials

Spin dynamics Spin dynamics encompasses all phenomena that generate magnetisation motion. The fundamental equation that rules magnetisation motion is the so called Landau Lifshitz equation: d! M dt = µ 0 0! M ^! H eff! H eff =! H demag +! H ani +! H app +! H exc 2

Gilbert damping Equation of motion is completed with a damping term representing all the losses of the system. d! M! dt = µ 0 0M ^!H eff +! d M! M ^ M S dt 3

Examples of spin dynamics Magnetic domain wall motion M. Albert et al, J. Phys.: Condens. Matter 24 (2012) pp.024219 4

Examples of spin dynamics Magnetic domain wall motion Magnetic vortex precession K. Yamada et al, Nature Materials. 6 (2007) pp.269 4

Examples of spin dynamics Magnetic domain wall motion Magnetic vortex precession Ferromagnetic resonance http://www.physics.colostate.edu/groups/pattongroup/systems/epr_desc.html 4

Examples of spin dynamics Magnetic domain wall motion Magnetic vortex precession Ferromagnetic resonance Spin waves 4

Why spin dynamics is important? Dynamics regime and technology speed Energy loss Spin dynamics applications 5

Spin dynamics regime & technologies speed khz MHz GHz THz 6

Spin dynamics regime & technologies speed thermally assisted processes khz MHz GHz THz 6

Spin dynamics regime & technologies speed thermally assisted processes domain wall khz MHz GHz THz 6

Spin dynamics regime & technologies speed thermally assisted processes domain wall magnetic vortices khz MHz GHz THz 6

Spin dynamics regime & technologies speed thermally assisted processes domain wall magnetic vortices FMR & spin wave khz MHz GHz THz 6

Spin dynamics regime & technologies speed thermally assisted processes domain wall magnetic vortices FMR & spin wave laser induced dynamics khz MHz GHz THz 6

Spin dynamics regime & technologies speed thermally assisted processes domain wall magnetic vortices FMR & spin wave laser induced dynamics ferrimagnetic resonance khz MHz GHz THz 6

Spin dynamics regime & technologies speed thermally assisted processes domain wall magnetic vortices FMR & spin wave laser induced dynamics ferrimagnetic resonance khz MHz GHz THz SSD & USB stick memory cell 6

MRAM After GMR reading head for hard drive, the next most anticipated application of spintronics is MRAM. Bit is stored in the alignment of the magnetisation of the magnetic electrode of a GMR or TMR. The information is read by measuring the magnetoresistance of the stack. The information is written by injecting a high current in the stack. 7

Critical current density The critical current density of a GMR is the current density required to write the information in the free layer. To increase the density of information, high anisotropy material are used. j C = 2e M St ~G (! m;! p ) p H eff So to reduce the power consumed per bit written, the damping has to be reduced as much as possible. 8

Ferromagnetic resonance spectroscopy Ferromagnetic resonance Principle Broadband and cavity set-ups Going further 9

Ferromagnetic resonance Ferromagnetic resonance is the uniform precession of magnetisation across a ferromagnetic sample forced by an excitation. Excitation sources could be: a RF magnetic field a RF spin current a RF electric field a RF modulation of lattice parameter 10

Cavity FMR spectroscopy A RF source inject power in the cavity. The cavity absorbs it. Only a part of it is reflected by the sample. The reflected power is detected by a diode. The signal from the diode goes through a lock-in amplifier. This is done in continuous wave while the magnetic field is swept. 11

Cavity FMR spectroscopy A RF source inject power in the cavity. The cavity absorbs it. Only a part of it is reflected by the sample. Co40Fe40B20 The reflected power is detected by a diode. The signal from the diode goes through a lock-in amplifier. This is done in continuous wave while the magnetic field is swept. 11

Broadband FMR spectroscopy A RF stripline shorted at the end is connected to a vector network analyser (VNA). The VNA send RF power to the short that transform it in RF current. This current generates an Oersted field that excite FMR of the magnetic material laying beneath. All the power is reflected except the power absorbed by the magnetic layer. 12

Broadband FMR spectroscopy A RF stripline shorted at the end is connected to a vector network analyser (VNA). Co40Fe40B20 The VNA send RF power to the short that transform it in RF current. This current generates an Oersted field that excite FMR of the magnetic material laying beneath. All the power is reflected except the power absorbed by the magnetic layer. 12

FMR spectroscopy methods comparison Variables Measurable parameters Sample type Cavity magnetic field [-1;1]T angle [-π; π] temperature [77K;RT] Anisotropy geometry and amplitude Damping Full layer Array of lines and dots Stripline frequency [0.01;20]GHz magnetic field [-0.8;0.8]T Magnetisation Anisotropy amplitude (for low anisotropy materials) Damping Full layer Single dot 13

Damping results Co40Fe40B20 Co40Fe36Cr4B20 Co40Fe32Cr8B20 stripline 4.6 10-3 9.4 10-3 2.8 10-2 cavity 4.5 10-3 6.9 10-3 8.7 10-3 For comparison, damping of permalloy is at least 8 10-3. 14

FMR spectroscopy FMR characterisation is now integrated in our material development pipeline. But FMR can be much more than just a tool for material development. For example, we are upgrading the cavity set-up to measure the FMR of a magnetic layer deposited on top of a ferroelectric layer. The measurement will be done with variable temperature, angle, electric field (applied to the ferroelectric) and magnetic field. 15

CHIMP memory Principle Vortex dynamics simulations 16

CHIMP memory principle Magnetic vortices can be found in magnetic dots. A vortex has two parameters, chirality and polarity, each having two possible values. Then a vortex has four different states. If n vortices are stacked in a TMR structure, the full stack has 4 n states. 17

CHIMP memory principle 18

Vortex dynamics simulations Stacked vortices exhibit a complex dynamics as they are dipolarly interacting. Depending on the relative chirality and polarity, the frequency of resonance of two vortices can drastically changes in comparison with the full width at middle height. 19

Time-resolved magneto-optic Kerr effect The need Time resolved magneto-optic Kerr effect Set-up 20

Need for higher frequencies High anisotropy materials are extremely expected in a wide range of applications. For example, the hard drive disk industry increase the information density by reducing the size of the bit; therefore to keep the information stability, the anisotropy is increased. There is also a large potential for terahertz emitter. High anisotropy induces a frequency of resonance in the terahertz range which can be hardly access with electronic set-ups. 21

Magneto-optic Kerr effect The interaction between the light and a material is described by the dielectric tensor. If the material is magnetic, it breaks some of the symmetries which is reflected by non diagonal terms in the tensor. When polarised light interact with a ferromagnetic material, these non diagonal terms change its polarisation. In the case of light reflection at the surface of the material, this change in polarisation is called Kerr effect. 22

TR-MOKE A first pulse (pump) is sent to the magnetic layer to heat it above the Curie temperature. A second pulse (probe) polarised linearly is then sent with a controlled delayed. It is reflected with a different polarisation. pump probe time delay spin system The reflected probe is analysed after going through a polariser. 23

TR-MOKE A first pulse (pump) is sent to the magnetic layer to heat it above the Curie temperature. A second pulse (probe) polarised linearly is then sent with a controlled delayed. It is reflected with a different polarisation. The reflected probe is analysed after going through a polariser. B. Koopmans, Spin dynamics in confined magnetic structures 2, Springer, 2003 23

Conclusion In the last year, we have created from the scratch a spin dynamics activity. The two FMR set-ups are already versatile and are upgraded to allow more measurements such as FMR on multiferroic stacks and magnetic vortex dynamics. The main two leads followed today are: the development of a magnetic memory based on magnetic vortex dynamics the development of time resolved magneto-optic facilities for high anisotropy material characterisation. 24