Alfa/Rio Instituto do Milênio de Nanociências. Magnetometer S600X-IN Cryogenic

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1 Instituto do Milênio de Nanociências Magnetometer S600X-IN Cryogenic

2 Magnetometry: Squid magnetometry (not high field, < 8T) Based on Superconductivity, unbeatable at low fields Vibrating Sample magnetometry Accurate and sensitive at high fields Hall probe magnetometry Not absolute, but very sensitive, small samples, even at high fields Others: Extraction, Cantilever, Gradient field, AGM,...

3 SQUID Magnetometry Based on Meissner effect, Magnetic Flux quantization and Josephson effect Superconductivity review K.H.Onnes, 1911, R=0 Not only R=0! But also B=0 Meissner effect, 1933 Theory BCS, 1957, Josephson Effect

4 The Meissner Effect Perfect Diamagnet. If a conductor already had a steady magnetic field through it and was then cooled through the transition to a zero resistance state, becoming a perfect diamagnet, the magnetic field would be expected to stay the same. Alfa/Rio Superconductor Remarkably, the magnetic behavior of a superconductor is distinct from perfect diamagnetism. It will actively exclude any magnetic field present when it makes the phase change to the superconducting state.

5 A conductor will oppose any change in externally applied magnetic field. Circulating currents will be induced to oppose the buildup of magnetic field in the conductor (Lenz's law). In a solid material, this is called diamagnetism, and a perfect conductor would be a perfect diamagnet. That is, induced currents in it would meet no resistance, so they would persist in whatever magnitude necessary to perfectly cancel the external field change. A superconductor is a perfect diamagnet, but there is more than this involved in the Meissner effect.

6 SQUID Superconducting Quantum Interference Device (1970) (SQUID) Flux quantization in a ring φ 0 =(nh/2e) Josephson Effect

7 Flux Quantization In 1961 Deaver and Fairbank did experiments with a tiny superconducting cylinder made by electroplating tin on a copper wire. They found magnetic flux quantized in units of such that the flux through the cylinder was given by

8 Josephson Junction Two superconductors separated by a thin insulating layer can experience tunneling of Cooper pairs of electrons through the junction. In the DC Josephson effect, a current proportional to the phase difference of the wavefunctions can flow in the junction in the absence of a voltage. In the AC Josephson effect, a Josephson junction will oscillate with a characteristic frequency which is proportional to the voltage across the junction. Since frequencies can be measured with great accuracy, a Josephson junction device has become the standard measure of voltage. If two superconductors are separated by a thin insulating layer, then quantum mechanical tunneling can occur for the Cooper pairs without breaking up the pairs.. Under these conditions, a current will flow through the junction in the absence of an applied voltage (the DC Josephson effect).

9 Josephson Voltage Standard When a DC voltage is applied to a Josephson junction, an oscillation of frequency occurs at the junction. Since this relationship of voltage to frequency involves only fundamental constants and since frequency can be measured with extreme accuracy. The standard volt is now defined as the voltage required to produce a frequency of 483,597.9 GHz. Josephson junction standards can yield voltages with accuracies of one part in 10^10. NIST has produced a chip with series junctions to measure voltages on the order of 10 volts with this accuracy.

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11 Response to an external flux

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13 RF SQUID Electronics

14 SQUID-DC SC ring with 2 junctions

15 Gradiometers In order to be usefull one must couple a signal to the SQUID Gradiometers: measure magnetic flux diference First order Second order Planar

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20 Mecanismo para troca e movimentação da amostra. Alfa/Rio

21 Detalhe do magnetômetro Alfa/Rio

22 Características Sensibilidade 10-8 emu (momento magnético. total) Medidas DC (extração e modo oscilante) Medidas AC (0.01 ate 500 Hz) Campo magnético ± 6.5 T (0.01% homogeneo ± 2cm, estab. 0.1ppm/hora) blindagem ativa anulando campo externo resolução 0.09mT ou 0.1 µt (1mOe) na opção baixo campo Operação continua de 1.5 K ate 320 K Resolução 1mK; estabilidade ±2 mk(10k) ±3mK (100K) e ±10mK (300K) Blindagem magnética e supercondutora (campo < 1 nt) Software baseado em Labview (acesso em tempo real aos dados) Dewar com blindagem de N líquido (baixo consumo de He liquido, 4l/dia) Alfa/Rio

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24 Refrigerador de Diluição 3 He/ 4 He 1,5K 0,7K 0,1K 0,01K

25 Very low Temperature SQUID Magnetometer

26 0.01<H<1000 Oe 0.01<T<4.2 K Flux transf. Coupl. factor= x10-8 G/ Hz at sample (6x10-3 Φ 0 )

27 Aplicações do SQUID Utiliza-se o SQUID (sensibilidade <10-4 φ 0 ) acoplando-se ao sinal de interesse com um transformador de fluxo magnético. Magnetômetros de alta sensibilidade (10-9 emu) Magnetocardiografia, Magnetoencefalografia Medidas de V, I, R, L sem similares em sensibilidade Resonância Magnética Magnetometria microsquid. Medidas diretas no anel da magnetização de uma nanoparticula com 200 µ B, Microscopia de varredura fluxo magnético de alta resolução ( < 1 µm) Comparadores Criogênicos de Corrente ( 1ppB)

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30 High Temperature Superconductors SQUID

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32 MicroHall Sensors Alfa/Rio

33 MicroSQUID Technique (Grenoble, CRTBT)

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