6.4 Geração de voltagem em sistemas FM/AF/NM por gradiente térmico
7.1.1 Perspectivas
Organizamos as perspectivas dessa parte do trabalho nos seguintes pontos:
• Atualmente, o modelo empregado para descrever numericamente os resultados experi- mentais de PMR são baseados no modelo de monodomínio de Stoner-Wolhfarth. No entanto, os filmes simples são em geral multidomínios. Assim, é necessário implementar um tratamento teórico que leve em conta essa característica;
• Investigar o processo de nucleação dos domínios magnéticos em filmes simples através da técnica de PMR;
• Produzir uma nova série de bicamadas Si(100)/Py(10 nm)/IrMn(tIrM n) cujos campos
Heb de exchange bias sejam maiores que 100 Oe, para responder a questão sobre qual
modelo teórico melhor explica nossos resultado (SM ou Geshev);
• Ajustar numericamente os resultados experimentais de PMR angular em bicamadas FM/AF;
• Obter a partir do ajuste numérico da curva de PMR angular, os valores de Heb e Hrot.
7.2. MEDIDAS COM GRADIENTE DE TEMPERATURA 102
7.2
Medidas com gradiente de temperatura
Nossos primeiros resultados utilizando essa técnica, foram as medidas de voltagem ANE (VAN E) nas amostras Vidro / Co(34 nm) e Si(100) / Py(35 nm). Verificamos a relação
de proporcionalidade entre a amplitude de VAN E e a diferença de temperatura aplicada em
cada amostra. Observamos a inversão de VAN E devido à inversão do sentido de ~∇T . Também
foi possível constatar que VAN E possui uma dependência com o cosseno da posição angular
do campo magnético externo ~HDC aplicado na amostra (Figura 6.1).
Um fato relevante é a variação do campo coercitivo HC na amostra de Vidro / Co(34
nm) da ordem de até 80 Oe nesse tipo de medida, para diferentes valores de ∆T . Ressaltamos que para obter valores mais elevados de ∆T , foi necessário aumentar a temperatura na superfície da amostra. Portanto, há uma relação direta entre a diminuição da coercividade e o aumento da temperatura na face do filme.
A variação de HC não foi observada nas medidas feitas na amostra Si(100) / Py(35
nm), cuja coercividade manteve-se aproximadamente constante, em concordância com a me- dida de magnetização (VSM), ou seja, HC ≈ 6 Oe.
Na secção 6.2, analisamos uma série de amostras composta por bicamadas FM (iso- lante) / NM . Os resultados das Figuras 6.6 e 6.7 na amostra GGG / YIG(100 nm) / Pt(4 nm) foram importantes para o domínio da técnica. A voltagem VISHE varia linearmente em
função de ∆T (e, portanto, de ~∇T ), que é da ordem de 5 µV para ∆T = 33K. A variação angular de VISHE revela a dependência com o cosseno do ângulo entre o campo externo e a
direção onde VISHE é aferida (Figura 6.7b).
Com o objetivo de estudar VISHE e separá-la de VAN E, medimos a série GGG /
YIG(50 nm) / Cu(tCu nm) / Co(3 nm). Observamos o decaimento exponencial da voltagem
medida com a espessura tCu da camada espaçadora de Cu. Notamos ainda que o próprio pro-
cesso de medida com gradiente de temperatura é responsável por reacoplar as magnetizações do YIG e do Co dificultando a separação dos efeitos ISHE e ANE.
Realizamos também estudos na série Si(111)/Py(20 nm)/NiO(tN iO nm)/Cr(3 nm),
com tN iO assumindo os valores 1 nm, 3 nm, 5 nm, 10 nm, 15 nm, 20 nm e 25 nm. Para
7.2. MEDIDAS COM GRADIENTE DE TEMPERATURA 103
o NiO em sua fase antiferromagnética na bicamada Si (111) / Py (20 nm) / NiO (200 nm) depositada a 60 ◦C e em seguida submetida por 4 horas ao tratamento térmico a 400 ◦C. Esse processo foi adotado na produção de todas as amostras da série.
As medidas de magnetização mostraram a presença de um pequeno campo de ex- change bias, da ordem de 4 Oe para tN iO = 25 nm. Isso corrobora a natureza AF da camada
NiO. Em seguida, as medidas com aplicação de gradientes térmicos mostraram voltagens em torno de 5 µV na estrutura Si(111)/Py(20 nm)/NiO(tN iO nm)/Cr(3 nm), para todos os
valores de tN iO utilizados. Observamos a relação entre a voltagem medida e o ~∇T estimado
na camada FM geradora de corrente de spin (Py) e na camada de AF de NiO. Para isso usamos o coeficiente S que caracteriza a eficiência no processo de geração de voltagem.
Nossos resultados experimentais indicam uma dependência entre o valor de S e a espessura tN iO da camada AF. Observamos que para tN iO = 3 nm, ocorre um considerável
aumento no valor de S. Obtivemos um ganho acima de 100% na voltagem gerada quando tN iO = 3 nm em relação às amostras com tN iO = 1 nm e tN iO acima de 15 nm. Isso significa
que para a camada AF exerce um efeito de amplificação da voltagem gerada.
Substituímos a camada conversora de Cr por Ta. Os resultados não mostraram o efeito de amplificação de voltagem, muito embora também apresentem uma pequena variação de S em torno de tN iO = 3 nm. Também realizamos medidas na amostra Si (111) / Py (20
nm) / NiO (25 nm), ou seja, na ausência de qualquer camada conversora de Cr ou Ta. Mesmo nesse caso foi possível medir voltagem diretamente na superfície de NiO (a resistência entre contatos medida é de aproximadamente 1,3 kΩ). A causa pode estar ligada à má aderência do NiO depositado sobre o Py, o que pode ter causado buracos que são canais de condução de carga. Portanto, a voltagem observadas em todas as amostras é uma soma das contribuições do ISHE e ANE.
7.2.1 Perspectivas
Organizamos as perspectivas dessa parte do trabalho nos seguintes pontos:
• Implementar uma solução para isolar o porta amostras, e portanto o sistema medido, de correntes de ar que podem causar flutuações na temperatura local e no sinal melhorando
7.2. MEDIDAS COM GRADIENTE DE TEMPERATURA 104
a relação sinal-ruído;
• Produzir uma série de Co variando sua espessura e os substratos, para compreender com clareza o que causa o aumento da coercividade nas medidas com gradientes de temperatura. Além disso, tentar observar esse efeito em outros materiais FM;
• Substituir o YIG por ferrita de Co como material gerador de corrente de spin;
• Variar alguns parâmetros de deposição da camada de Cu na multicamada GGG / YIG (100 nm) / Cu (tCu) / Co (3 nm), visando aumentar seu grau de cristalinidade, aumen-
tando a eficiência da conversão de corrente de spin em corrente de carga na interface;
• Produzir uma nova série GGG / Py (20 nm) / NiO (tN iO) / Cr (3 nm) no novo sputtering
do LNMS da UFRN, que nos possibilitará depositar o NiO aplicando potências maiores do que 50 W na fonte RF;
• Produzir uma série de amostras variando a espessura da camada de Py na nanoestrutura citada no ítem anterior;
• Substituir o Py pelo Co.
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ANEXOS
A
ARTIGO PUBLICADO
Rev. Sci. Instrum. 89, 125115 (2018); https://doi.org/10.1063/1.5047661 89, 125115
© 2018 Author(s).
Perturbative measurement of
magnetoresistance
Cite as: Rev. Sci. Instrum. 89, 125115 (2018); https://doi.org/10.1063/1.5047661
Submitted: 09 July 2018 . Accepted: 02 December 2018 . Published Online: 21 December 2018 A. B. Oliveira , A. C. C. de Melo, R. B. da Costa , N. P. Costa, A. Azevedo , and C. Chesman
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REVIEW OF SCIENTIFIC INSTRUMENTS 89, 125115 (2018)
Perturbative measurement of magnetoresistance
A. B. Oliveira,1,2A. C. C. de Melo,1R. B. da Costa,1N. P. Costa,1A. Azevedo,3and C. Chesman1
1Departamento de F´ısica Te´orica e Experimental, Universidade Federal do Rio Grande Do Norte,
BR-59072-970 Natal, RN, Brazil
2Escola de Ciˆencias and Tecnologia Universidade Federal do Rio Grande Do Norte, BR-59072-970 Natal,
RN, Brazil
3Departamento de F´ısica, Universidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazil (Received 9 July 2018; accepted 2 December 2018; published online 21 December 2018)
In this paper, we report the development of a novel technique in which the magnetoresistance of nanos- tructures is perturbatively measured by transversally modulating the DC magnetic field. It measures the electrical transport counterpart of the transverse magnetic AC-susceptibility. The technique was devel- oped in a conventional four-probe configuration in which a small DC current flows through the sample and a small transverse AC-field perturbates the equilibrium position of the sample magnetization. Lock-in detection, in-phase with the AC-perturbation, is used to measure the voltage signal between the two inner electrodes of the four-probe station. We successfully demonstrated that this signal is proportional to the product of the first derivative of sample resistance with respect to the equilibrium position of the magnetization times the second derivative of the energy with respect to the equilibrium position of the magnetization. These dependences add new features to the technique investigated here that were not captured by the investigations previously reported on the same subject. To show the effec- tive use of the technique, we discuss its application in measuring magnetic properties of thin magnetic films in the non-saturated regime. Published by AIP Publishing.https://doi.org/10.1063/1.5047661
I. INTRODUCTION
Measurements of the magnetic properties of nanostruc- tures are usually performed by means of a variety of tech- niques which depending on the type of the external excitation and the time scale of response can be considered as static, quasi-static, or dynamic techniques.1,2 The basic principles on which the experimental techniques are created rely on dif- ferent magnetic phenomena, such as magnetic induction,3–6 magnetic force,7,8 magneto-optical properties,9–11 magnetic resonances,12–15 light scattering by magnetic excitations,16 and magnetoresistance.17,18 To be suitable for investigation of magnetic properties of nanostructures, the development of new techniques has to consider two key aspects: (i) being able to measure weak signals and (ii) providing information on magnetic phenomena that are not easily obtained by well- established techniques. For example, the magnetic field values in which the magnetization reversal processes start in mag- netic nanostructures19 are not easily measured by existent techniques. Therefore, the development of magnetic instru- mentation dealing with weak signals, which operates in the non-saturation of the magnetization regime of field values, is of current interest.
Here we present the development of a perturbative measurement technique based on magnetoresistance effects, termed Perturbative Magnetoresistance (PMR), which can be used to characterize the magnetic properties of nanostructures in magnetic field ranges of non-saturation of the magnetization regime. The physical principle of the PMR can be understood as an extension of the AC-AMR measurements introduced by Venus et al.20 In the AC-AMR technique, the authors used a double modulation detection where the magnetization was
perturbed by means of a small AC-field (100 Hz) superim- posed with an AC-current (40 kHz) that flows through the sample. In Ref.20, the double modulation detection was car- ried out with no external DC-field. In the PMR technique, the DC-field is always applied to the sample; therefore, the magnetization direction can be slowly reversed in the pres- ence of a perpendicular weak oscillating field. The electrical excitation is obtained by applying a DC current through the sample, and the PMR signal is measured by means of a lock-in amplifier phase-locked to the reference signal. The principle of operation of the perturbative magnetoresistance (PMR) tech- nique and the required instrumentation are described in Sec.II. SectionIIIdescribes the interpretation of the data with an ana- lytical model. This section also includes the validation of the technique and comparison of the data obtained by means of PMR with data obtained by the conventional techniques of the vibrating sample magnetometer (VSM) and conventional collinear magnetoresistance (MR). Conclusions are presented in Sec.IV.
II. PERTURBATIVE MAGNETORESISTANCE:
PRINCIPLE OF OPERATION AND INSTRUMENTATION
In the PMR setup, perpendicular AC and DC magnetic fields are simultaneously applied to the sample. As shown below, this configuration makes the measured response be pro- portional to dR/dhac, where R is the sample resistance and hac is the perturbative magnetic field. On the other hand, if
the AC field is applied parallel to the DC field, the response is proportional to dR/dHdc.21 While the parallel configura-
tion has been named magnetoresistance AC-susceptibility,
125115-2 Oliveira et al. Rev. Sci. Instrum. 89, 125115 (2018)
FIG. 1. Illustration of the PMR setup. While the fixed DC-current is applied between the outer probes, the voltage is measured between the inner probes. A spurious signal is induced between the inner probes due to the magnetic flux variation. The lock-in reference signal is provided by the same signal generator used to feed the AC field.
we named the perpendicular configuration as perturbative magnetoresistance.
To avoid any contact-resistance contribution, we employed the collinear four-point probe technique.22Figure1
illustrates the experimental setup used to perform PMR mea- surements. Four gold-coated spring contact probes were tightly pressed onto the sample surface, the DC electrical current is applied between the two outer probes, and the voltage signal is measured between the two inner contacts. The cur- rent varies from hundreds of microamps to few milliamps. In some measurements, we used an additional low-noise pass-