magnetoresistance effects · magnetoresistance effects anna s. semisalova helmholtz-zentrum...
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Dr. Anna Semisalova | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
17-22 Sept. 2017, Bad Honnef, Germany
Magnetoresistance effects
Anna S. Semisalova
Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Germany
Lomonosov Moscow State University, Faculty of Physics, Russia
Bad Honnef Physics School on
Magnetism: From Fundamentals to Spin based Nanotechnology
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Institute of Ion Beam Physics and Materials Research, HZDR
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Spintronics
(also known as spin-electronics, magneto-electronics)
Electron = charge + spin
Idea: to use the spin in the
electronic devices
• Novel functionalities
• Higher speed
• Faster performance
• Lower power consumption
• …
Control,
Manipulation, and
Detection of spin state
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Spintronics/nanomagnetism
Spintronicsapplications
Sensors
Magnetic recording
MRAM
Memristor applications
STT microwave devices
(oscillators)
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What is Magnetoresistance?
Change of electrical resistivity of the material under the application of
magnetic field
∆𝑅
𝑅% =
𝑅 𝐻 − 𝑅(0)
𝑅(0)∙ 100
MR ratio:
Magneroresistance converts magnetic signal
into electrical signal
MR ratio at RT & at low H (~1 mT) is important for device applications
From: S. Yuasa, IEEE Distinguished Lecturer
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Outline
0. Positive and negative magnetoresistance in metals
1. Anisotropic magnetoresistance – AMR
2. Giant magnetoresistance – GMR
Spin valve
GMR for hard disk drivers
3. Tunneling magnetoresistance - TMR
4. Magnetic Random Access Memory – MRAM
Amorphous vs. Epitaxial tunnel barrier
5. New twist - Magnetoresistance in antiferromagnets
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Positive magnetoresistance effect – old good one
The force on an electron:
Ԧ𝐹 = 𝑚𝑑 Ԧ𝑣
𝑑𝑡= 𝑒𝐸 + 𝑒 Ԧ𝑣 × 𝐵
Kohler’s rule
∆𝜌
𝜌= 𝜔𝑐𝜏
2 =𝑒𝐵
𝑚𝜏
2
=𝑛𝑒2𝜏
𝑚
1
𝑛𝑒𝐵
2
∆𝝆
𝝆=
𝑹𝑯
𝝆
𝟐
𝑩𝟐 ∝𝑩
𝝆
𝟐
Change of carrier trajectory due to
Lorentz force
increase of carrier path (curling of
path)
increase of scattering events and
resistance
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Negative magnetoresistance effect - introduction
Resistivity normalized to their values at TC of Ni, 631 K (from Gerritsen (1956))
Resistivity of ferromagnet:
𝜌𝑝𝑎𝑟𝑎 - const at T>TC
𝜌𝑓𝑒𝑟𝑟𝑜 ≈ 𝜌𝑝𝑎𝑟𝑎 1 −𝑀𝑠(𝑇)
𝑀𝑠(0)
2
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Negative magnetoresistance effect - introduction
Resistivity of ferromagnet:
𝜌𝑝𝑎𝑟𝑎 - const at T>TC
𝜌𝑓𝑒𝑟𝑟𝑜 ≈ 𝜌𝑝𝑎𝑟𝑎 1 −𝑀𝑠(𝑇)
𝑀𝑠(0)
2
~1-m2(T)
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Spin disorder resistivity
Resistance decreases, if magnetic field is applied,
due to splitting of d-band for majority - and minority
spin
Less scattering of s-electrons into d-band → higher
mobility
Above TC: Scattering ofboth spin orientations ofs-electrons into empty d-states possible
Below TC: No scatteringof s-electrons into emptyd-states for majority spins larger mean free path smaller resistance resistance increaseswhen approaching TC
(spin-disorder increases)
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Negative magnetoresistance
External magnetic field suppresses spin-disorder and produces relative
shift of spin-subbands, similar to (but much weaker than) exchange field
negative contribution
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Outline
0. Positive and negative magnetoresistance in metals
1. Anisotropic magnetoresistance – AMR
2. Giant magnetoresistance – GMR
Spin valve
GMR for hard disk drivers
3. Tunneling magnetoresistance - TMR
4. Magnetic Random Access Memory – MRAM
Amorphous vs. Epitaxial tunnel barrier
5. New twist - Magnetoresistance in antiferromagnets
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Magnetoresistance for data storage technologies
Yuasa & Djayaprawira J. Phys. D: Appl. Phys (2007)
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Anisotropic Magnetoresistance (AMR)
180°0° angle θ 90°
θ
magnetic fieldcurrent
magnetic fieldcurrentcurrent
magnetic field
Pics from lectures of Prof. Dr. M. Farle
1857: W. Thomson (Lord Kelvin) –
demonstration of AMR in FM materials
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Anisotropic Magnetoresistance (AMR)
Kohler’s rule for a ferromagnet:
∆𝜌
𝜌∝ 𝑎
𝐻
𝜌
2
+ 𝑏𝑀
𝜌
2
Ordinary magnetoresistance
Anomalous or anisotropicmagnetoresistance
Mc Guire, IEEE Trans.Magn. (1975)
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Anisotropic Magnetoresistance (AMR)
▪ ∆𝜌/𝜌= 3 to 5% in bulk NiFe at room temperature▪ AMR decreases with the film thickness and pattern size due to
additional scattering (grain boundaries, interfaces)
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Anisotropic Magnetoresistance (AMR)
Usual way of description:
𝜃 – angle between 𝑱 and 𝑴
𝜌 𝐻 =1
3𝜌∥ + 2𝜌⊥ + 𝜌∥ − 𝜌⊥ cos2 𝜃 −
1
3
𝜌av ∆𝜌
∆𝜌 𝐻 = 𝜌 𝐻 − 𝜌av
∆𝜌(𝐻)
𝜌av=∆𝜌
𝜌avcos2 𝜃 −
1
3
Resistivity in zero field for randomly demagnetized
sample
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Anisotropic Magnetoresistance (AMR)
Usual way of description:
𝜃 – angle between 𝑱 and 𝑴
𝜌 𝐻 =1
3𝜌∥ + 2𝜌⊥ + 𝜌∥ − 𝜌⊥ cos2 𝜃 −
1
3
𝜌av ∆𝜌
∆𝜌 𝐻 = 𝜌 𝐻 − 𝜌av
Resistivity in zero field for randomly demagnetized
sample
𝝆 𝜽 = 𝝆⊥ + (𝝆∥ − 𝝆⊥)𝒄𝒐𝒔𝟐𝜽
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Anisotropic Magnetoresistance (AMR) - schematic
resulting magnetic
moment
charge distribution due to
orbital motion of electrons
(no spherical symmetry for metals
with incompletely filled orbitals,
e.g. Fe (3d-metal))
rotation of magnetic moment
leads to rotation of charge distribution
(Spin-orbit coupling)
AMR is a consequence of an anisotropic mixing of spin‐up
and spin‐down conduction bands induced by the spin‐orbit
interaction
Lectures of Prof. Dr. M. Farle
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Anisotropic Magnetoresistance (AMR) - schematic
resulting magnetic
moment
charge distribution due to
orbital motion of electrons
(no spherical symmetry for metals
with incompletely filled orbitals,
e.g. Fe (3d-metal))
I
cross section small
cross section large
rotation of magnetic moment
leads to rotation of charge distribution
(Spin-orbit coupling)
Lectures of Prof. Dr. M. Farle
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AMR sensor application
Real-time monitoring of move direction,
under no-GPS coverage or rough/harsh
conditions
Lab4MEMS ©Analog Devices, Inc.
Pham et al., Sensors & Transduc. J. (2015)
Positioning
systems Mobile phones
Py AMR
- not good
Barber pole Py AMR
- good
MR heads (1992-1998)
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AMR sensor application
Real-time monitoring of move direction,
under no-GPS coverage or rough/harsh
conditions
Lab4MEMS ©Analog Devices, Inc.
MEMSIC, Inc.Pham et al., Sensors & Transduc. J. (2015)
3-axis AMR
magnetometer
Positioning
systems Mobile phones
Py AMR
- not good
Barber pole Py AMR
- good
MR heads (1992-1998)
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Outline
0. Positive and negative magnetoresistance in metals
1. Anisotropic magnetoresistance – AMR
2. Giant magnetoresistance – GMR
Spin valve
GMR for hard disk drivers
3. Tunneling magnetoresistance - TMR
4. Magnetic Random Access Memory – MRAM
Amorphous vs. Epitaxial tunnel barrier
5. New twist - Magnetoresistance in antiferromagnets
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Giant Magnetoresistance
GMR
Nobel prize in physics (2007)
Baibich et al. PRL 61, 2472 (1988)Binasch et al. PRB 39, 4828 (1989)
Albert Fert
Université Paris-Sud,
Orsay, FrancePeter Grünberg
Institut für
Festkörperforschung,
Forschungszentrum
Jülich, Germany
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Giant magnetoresistance – schematic description
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Picture: Baibich et al. PRL 61, 2472 (1988) & Wikipedia
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GMR – non-magnetic layer thickness dependence
Fe/Cr system multilayer system
Parkin et al., PRL (1990)
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GMR basics
As long as spin-flip is negligible, charge current can be considered as
carried in parallel by two categories of electrons: spin up and spin
down (parallel and antiparallel to quantization axis)
Two current model (Mott 1930)
Scattering depends on the relative
orientations of the electron spins and
magnetic moments in sublattice: it is
weakest when they are parallel and
strongest when they are antiparallel
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GMR – equivalent circuits for multilayer
𝑅𝐴𝑃 =𝑅↓↑+𝑅↑↑
2𝑅𝑃 =
2𝑅↓↑𝑅↑↑𝑅↓↑ + 𝑅↑↑
GMR=𝑅AP−𝑅P
𝑅P
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“Spin-dependent scattering”
Paris, the Trendy-Shopping Street: Rue Vieille Du Temple
Pic - https://www.discoverwalks.com
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“Spin-dependent scattering”
Paris, the Trendy-Shopping Street: Rue Vieille Du Temple
Pic - https://www.discoverwalks.com
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GMR – measurements geometry
Spin up and spin down electrons are involved in conductivity
∆𝑅
𝑅𝐴𝑃=
𝛼 − 1
𝛼 + 1
2
Resistor model works if
CIP: mean-free paths of the electrons >>
thickness of the various layers
CPP: thickness << spin diffusion (flip)
length
𝛼 =𝑅↓↑
𝑅↑↑- spin scattering asymmetry
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GMR in multilayers with two switching fields
Yamamoto et al. JMMM (1991)
Co(3nm)
Cu(5nm)
Py(3nm)
Cu(5nm)
×15
Magnetic Field (Oe)
Magnetic Field (Oe)
M/M
SM
R r
atio (
%)
80 K
RT
80 K
RT
MR r
atio (
%)
M
Magnetic Field (Oe)
CoPy
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GMR in multilayers with two switching fields
Yamamoto et al. JMMM (1991)
Co(3nm)
Cu(5nm)
Py(3nm)
Cu(5nm)
×15
Magnetic Field (Oe)
Magnetic Field (Oe)
M/M
SM
R r
atio (
%)
80 K
RT
80 K
RT
A significant step towards
applications of GMR in devices was
achieved by Parkin et al.:
GMR in sputtered multilayers
Parkin et al., PRL (1990)Magnetoelectronics, ed by M. Johnson, Elsevier (2004)
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GMR - Spin Valve structure
Top type spin valve
Pics: Univ. of Leeds and here
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GMR - Spin Valve structure
Top type spin valve
Pics: Univ. of Leeds and here
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GMR - Spin Valve structure
First generation – top- or bottom-pinned spin valve
MR = 6 – 10%
AFM pinning layer
Pinned layer
Spacer
Free layer
Dieny et al. PRB (1991)Tsang et al. IEEE Trans. Magn. (1994)
Co, Co90Fe10,
Ni80Fe20/Co, Ni80Fe20/Co90Fe10
Co, Co90Fe10
Mn76Ir24, Mn50Pt50
Cu
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GMR - Spin Valve discovery
Dieny et al. PRB (1991)Dieny et al., JMMM (1991)Dieny et al., JAP (1991)
Py(15nm)
Cu(2.6nm)
Py(15nm)
FeMn(10nm)
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Effect of a thin layer inserted at the interfaces in spin valves
Parkin, PRL (1993)Sakakima et al., JMMM (2000)Veloso et al., APL (2000)
Non‐magnetic layers at interface and dead layers are source of strong
spin‐independent scattering and drastically reduces GMR
Additional thin FM layers (Co) at interface enhance GMR:
-> enhanced spin polarization
Specular spin valve with Nano Oxide Layers
MR = 15 – 20%
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Further improvement – synthetic layers
Synthetic antiferromagnet (SAF)Increase the exchange
field at the interface “pinned/exchange”
layers
Synthetic free layer (SF)Decrease the thickness
Guedes et al., JAP (2006)
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GMR for hard disk drives
First GMR read head – IBM, 1997
Picture: http://www.magnet.fsu.edu
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HDD head evolution
Pic - pcguide.com, education.mrsec.wisc.edu
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Outline
0. Positive and negative magnetoresistance in metals
1. Anisotropic magnetoresistance – AMR
2. Giant magnetoresistance – GMR
Spin valve
GMR for hard disk drivers
3. Tunneling magnetoresistance - TMR
4. Magnetic Random Access Memory – MRAM
Amorphous vs. Epitaxial tunnel barrier
5. New twist - Magnetoresistance in antiferromagnets
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Tunneling Magnetoresistance (TMR)
Magnetic Tunnel Junctions (MTJ)
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Julliere model of tunnel magnetoresistance (TMR)
Jullière, Phys. Lett. A (1975)Picture: http://moodera.mit.edu/Lecture of B. Dieny
TMR=𝑅AP−𝑅P
𝑅PP=
𝐷↑ 𝐸𝐹 −𝐷↓(𝐸𝐹)
𝐷↑ 𝐸𝐹 +𝐷↓(𝐸𝐹)
Spin polarization
Tunneling current in each spin channel
𝐽𝜎 ∝ 𝐷1𝜎 𝐸𝐹 × 𝐷2
𝜎 𝐸𝐹
Number of
candidates for
tunneling
Success rate
for candidate
Parallel configuration Antiparallel configuration
𝐽𝑃 ∝ 𝐷1↑𝐷2
↑ +𝐷1↓𝐷2
↓ 𝐽𝐴𝑃 ∝ 𝐷1↑𝐷2
↓ +𝐷1↓𝐷2
↑
TMR=2𝑃1𝑃2
1−𝑃1𝑃2P = 50% (Fe, Co):
TMR = 40-70%
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Magnetic tunnel junctions
Chappert et al., Nat. Mater. (2007)Picture: http://moodera.mit.edu/
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MgO-based MTJ
Fully
epitaxial
Parkin et al., Nat. Mater. (2004)Yuasa et al., APL (2005)Yuasa et al., Nature Materials (2004)Yuasa et al., APL (2006)
Highly epitaxial MgO –
giant TMR was observed
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MgO-based MTJ
Zhu & Park, Materials Today (2007)Ikeda et al., APL (2008)
Incoherent tunneling: Bloch
states are not conserved
Coherent tunneling:
Tunneling probability depends
on orbital symmetry
TMR 604% in Co20Fe60B20∕MgO∕Co20Fe60B20
Lectures of B. Dieny, S. Yuasa
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Spin filtering
Picture: http://moodera.mit.edu/
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MRAM
Photograph of the first MRAM product, a 4-Mbit
stand-alone memory (Freescale, 2006)
Magnetic
Random
Access
Memory
Chappert et al., Nat. Mater. (2007)
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MRAM - cell structure with perpendicular magnetic anisotropy
Magnetoresistive random access memory developed by Toshiba
Perpendicular Magnetic
Anisotropy:
Multilayers Co-Pt, Co-Pd, Co-Ni,
CoFe-Pt, CoFe-Pd, Co-Cr-Pt,
Alloys CoPt, FePt, CoCr, rare-
earth TM alloys
Picture - DOI: 10.5772/16539
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Outline
0. Positive and negative magnetoresistance in metals
1. Anisotropic magnetoresistance – AMR
2. Giant magnetoresistance – GMR
Spin valve
GMR for hard disk drivers
3. Tunneling magnetoresistance - TMR
4. Magnetic Random Access Memory – MRAM
Amorphous vs. Epitaxial tunnel barrier
5. New twist - Magnetoresistance in antiferromagnets
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Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Next generation – antiferromagnetic spintronics
Magneto-electronics, or spintronics, explores mainly ferromagnetic materials,
while AFM materials were given to secondary roles (exchange bias, etc.)
Advantages of AFM:
- Absence of stray fields
(minimized cross-talk
between AFM nanocrystals)
- Abundance
- Robustness General principle formulated by Louis Néel:
-> AMR! is a promising candidate to sense the magnetic state in AFM
“Effects in antiferromagnets depending on the square of the spontaneous magnetisation should show the same
variation as in ferromagnetic substances”
Critical milestone on the way of
implementation:
- no way to manipulate or even
probe the local AFM order
Neel’s Nobel Lecture
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AMR in antiferromagnetic semiconductor
TN = 240 K
Strontium iridate Sr2IrO4
AFM semiconductorPerovskiteSingle crystal, 500 µm thick
Point contact (PC) probe
PC resistance 𝑅 = 𝜌/2𝑎𝑎~45 nm ÷ 4.2 𝜇m
Wang et al., JAP (2015)Tsoi et al., PRL (1998)
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AMR in antiferromagnetic semiconductor
TN = 240 K
Strontium iridate Sr2IrO4
AFM semiconductorPerovskiteSingle crystal, 500 µm thick
Wang et al., JAP (2015)Tsoi et al., PRL (1998)
Normalized AMR
1
0
𝑅 𝜃 − 𝑅𝑚𝑖𝑛
𝑅𝑚𝑎𝑥 − 𝑅𝑚𝑖𝑛
Max AMR: 14% at 77K
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Perovskite named after…
Gustav Rose(1798 – 1873)
President of German Geological Society
Lev Perovsky(1792 – 1856)
Duke, mineralogist, Minister in Russia
Ural mountains
… Lev PerovskyMineral Calcium titanium oxide CaTiO3 found by Gustav Rose in Ural mountains in 1839
Pics: Wiki, here and here
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Antiferromagnetic AMR in FeRh
Phase diagram of binary Fe-Rh compound
Pic - andrewsteele.co.ukHeidarian et al. PSSB (2017)
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Antiferromagnetic AMR in FeRh
Phase diagram of binary Fe-Rh compound
Heidarian et al. PSSB (2017)
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Antiferromagnetic AMR in FeRh
Marti et al. Nature Materials (2014)
Member of the Helmholtz AssociationPage 60
Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Antiferromagnetic AMR in FeRh
Marti et al. Nature Materials (2014)
Ordinary (Lorentz force ) MR
Member of the Helmholtz AssociationPage 61
Dr. Anna Semisalova | HZDR | Institute of Ion Beam Physics and Materials Research | www.hzdr.de
Summary
Spintronicsapplications
Sensors
Magnetic recording
MRAM
Memristor applications
STT microwave devices
(oscillators)
1. Anisotropic magnetoresistance – AMR
2. Giant magnetoresistance – GMR
3. Tunneling magnetoresistance - TMR
4. Magnetic Random Access Memory – MRAM
5. New twist - Magnetoresistance in antiferromagnets