1 A compact concentric scanning tunneling microscope for point contact investigations of magnetic nanostructures Magne Saxegaard, André Kapelrud, DeZheng Yang, Bernt M. Førre and Erik Wahlström Department of physics, NTNU, Trondheim

2 Transport properties of (magnetic) systems on a local scale. Point contacts + STM = local measurements with lateral resolution Modified commercial STM confirmed viability of method. New STM : speed, stability, resolution Modular design : simple adaptation to new techniques or procedures. A V B Motivation

3 Scanning tunneling microscopy Institut für Experimentelle und Angewandte Physic der Universität Kiel

4 Point contact measurements Nano sized constriction between metals Ballistic limit (r < l ): Non-linear resistance: scattering spectra [1] Small contacts = high current densities: Current induced spin dependent transport, [2] [1] A.G.M Jansen, Phys.Rev. C. 13 (1980) 6073 [2] M. Tsoi et.al, Nature 406 (2000)

5 Giant magnetoresistance (GMR) –Magnetic thin films with different coersivity –Resistance scale with relative magnetization –Resistance controlled with magnetic field Spin torque transfer (STT) –Electrons spin polarized in one magnetic layer –Spin angular momentum transferred to second layer –Reversal of magnetization by applied current Exchange bias (EB) –FM layer pinned by uncompensated spins in AFM layer –Coupling change with field sweeps (training) –Coupling affected by spin polarized currents Transport phenomena in magnetic nanostructures

6 Modified commercial STM Modified Omicron UHV VT STM STM operation piezo actuator tunneling current (nA) PID control PC operationtip crashed send current (mA) measure V,I Switch triggered by PID off Good: UHV, temperature control Bad:slow, unstable, noisy electronics, stray fields, commercial software Switch STM CPU (PID) Omicron Scala PCSTM DAQ LabView UHV Omicron VT STM

7 NiFe Co Cu Spin valve rings NiFe (2.5nm) / Cu (5nm) / Co (20nm). OD 1400 – 2200 nm 3 5

8 I [mA] B [mT] Spin valve rings

9 e-e- H ext = 10 4 A/m H STT = 10 8 A/m H oe = 10 4 A/m STT Oe

10 Purpose built STM t Vz PIDzero Adj max Vmax Vmin AdjustCoarsePID Contact

11 STM operation –Preamp and Vgap –Vx,Vy, Vz from FPGA –STM image from PID feedback –Image processing on PXI PCS operation –Preamp bypassed –Vgap replaced with VCC –FPGA crash tip –Current and magnetic field applied –V ts,I ts measured with digitizer PID turned off during point contacts Continous V ts, I ts allow predefined R pc Purpose built STM HV Amp Power Amp Vgap Isend Red: PCS operation Blue: STM operation V ts I ts It amp DigiDigi PXIPXI FPGAFPGA DAQDAQ

12 STM –X,Y range : 13 G = 10 –X,Y res. : G = 0.1, G = 10 –X,Y speed: nm/s –Z range: 1.7 G = 10 –Z res.: G = 0.1, 0.5 G = 10 –Z coarse range: ≈ 10 mm –Z coarse res.: ≈ 50 – 500 nm Point contacts –Applied current ≈ 1uA – 30 mA –Applied magnetic field≈ 0 – 50 mT –Resistance max ≈ mA, mA –Resistance res.limited by noise (currently 10 – 20 mΩ) –Time resolution15MHz (16bit), 500 kHz (24bit), –Durationlimited by DAQ, (currently max 100kS/s) Purpose built STM

13 Preliminary results Exchange biased spin valve Si /Cu (5nm)/ NiFe (3nm)/ Cu (3nm)/ FeMn (8nm)/ Cu (3nm) STM tip of PtIr Contact radius 5 nm Very stable GMR curves (GMR ca 1%) Next: Current induced effects? Pre-set resistance?

14 Conclusion New tool for transport measurements in nanosystems Magnetoresistance with lateral resolution Current induced effect on magnetic switching Purpose built STM : increased stability, resolution and speed Outlook Point contact methods optimized UHV and temperature control

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