Dependência do exchange-bias e da coercividade da espessura do Cu em IrMn/Cu/Co

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Estudo da profundidade de penetração superficial em magnetometria ótica Kerr

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Synthesis of magnetic nanoparticles functionalized with histidine and nickel to immobilize His-tagged enzymes using β-galactosidase as a model

The aim of this study was to synthesize iron magnetic nanoparticles functionalized with histidine and nickel (Fe3O4-His-Ni) to be used as support materials for oriented immobilization of His-tagged recombinant enzymes of high molecular weight, using β-galactosidase as a model. The texture, morphology, magnetism, thermal stability, pH and temperature reaction conditions, and the kinetic parameters of the biocatalyst obtained were assessed. In addition, the operational stability of the biocatalyst in the lactose hydrolysis of cheese whey and skim milk by batch processes was also assessed. The load of 600 Uenzyme/gsupport showed the highest recovered activity value (~50%). After the immobilization process, the recombinant β-galactosidase (HisGal) showed increased substrate affinity and greater thermal stability (~50×) compared to the free enzyme. The immobilized β-galactosidase was employed in batch processes for lactose hydrolysis of skim milk and cheese whey, resulting in hydrolysis rates higher than 50% after 15 cycles of reuse. The support used was obtained in the present study without modifying chemical agents. The support easily recovered from the reaction medium due to its magnetic characteristics. The iron nanoparticles functionalized with histidine and nickel were efficient in the oriented immobilization of the recombinant β-galactosidase, showing its potential application in other high-molecular-weight enzymes

Estudo do Exchange Bias em filmes de IrMn/Al2O3/Co e IrMn/Cr/Co

Neste trabalho estudamos a interação de troca entre a camada antiferromagnética (AFM) de IrMn, e a ferromagnética (FM) de Co, no sistema IrMn/Co através de uma camada espaçadora. Usamos dois materiais com propriedades distintas como espaçador, ou o Cr (um material antiferromagnético com anisotropia fraca) ou a alumina (Al2O3, material isolante, não-magnético), para analisarmos o alcance dessa interação. Caracterizamos o sistema através de diferentes técnicas, ou seja, medidas de magnetização estática, ressonância ferromagnética, difratometria e refletividade de raios-X, microscopia de força atômica e microscopia eletrônica de transmissão. Tratamentos térmicos na presença de campo magnético foram feitos a fim de maximizar o efeito de exchange bias. Inicialmente, investigamos como utilizar adequadamente os diversos modelos fenomenológicos na interpretação de dados experimentais. Mostramos, através da utilização de dois modelos e de um simples experimento, a importância de considerarmos a distribuição de eixos fáceis e a anisotropia uniaxial do material FM. Negligenciar essas considerações pode ser a causa, ou pelo menos parte, das grandes diferenças relatadas em trabalhos experimentais referente à avaliação da interação FM/AFM obtida por meio de diferentes técnicas de medida. Também mostramos que, mesmo quando o modelo descreve corretamente uma bicamada que apresenta exchange bias, nenhuma das técnicas convencionais de caracterização, reversíveis ou irreversíveis, aplicadas à mesma amostra, é capaz de distinguir entre os campos de acoplamento de troca e o de anisotropia AFM, se a razão entre estes for suficientemente grande ou pequena. Demonstramos que o valor do acoplamento FM/AFM pode ser determinado a partir da variação da coercividade com a espessura da camada AFM. Dos resultados experimentais (tanto nas medidas de magnetização quanto nas de ressonância ferromagnética) obtidos para as tricamadas IrMn/Al2O3/Co, é ressaltada a variação do campo de exchange bias com o aumento da espessura da camada isolante e nãomagnética de alumina. Foi observado que a intensidade do acoplamento entre Co e IrMn diminui exponencialmente com o aumento da espessura da camada de alumina, sendo esta supressão muito abrupta, com decaimento pelo menos quatro vezes mais rápido que os anteriormente relatados. Estes resultados foram explicados em termos da perda do contato direto entre as camadas AFM e FM com o aumento da espessura da alumina. A série de IrMn/Cr/Co apresentou um comportamento oscilatório das grandezas campo de exchange bias e coercividade em função da espessura da camada de cromo, bastante peculiar. O valor do campo de anisotropia rodável, obtido a partir das simulações de FMR, tem sinal oposto ao do caso clássico, indicando que este campo é antiparalelo ao campo magnético externo aplicado. Ainda observa-se uma tendência geral de aumento do campo de anisotropia rodável com a espessura de Cr sugerindo que, nestas amostras, há uma grande contribuição proveniente da camada de Cr para esse campo.The present work reports on the exchange interaction between an antiferromagnetic (AFM) layer, i.e., IrMn, and a ferromagnetic (FM) one, Co, in IrMn/Co systems across spacer layers. Two materials with distinct properties were used as spacers, namely Cr (weak anisotropy antiferromagnet) and alumina (Al2O3, a non-magnetic insulator material), in order to investigate the intensity of this coupling. The systems were characterized using several different techniques, i.e., static magnetic characterization, ferromagnetic resonance, X-ray diffractometry and small-angle reflectivity, atomic force microscopy as well as highresolution transmission electron microscopy. Magnetic annealing was used in order to maximize the exchange-bias effect. First, it was investigated how to correctly employ different phenomenological models for experimental data interpretation. It was demonstrated, through two models and a straightforward experiment, that it could be very important to consider the easy-axis distributions and the uniaxial anisotropy of the FM material. Neglecting these distributions or underestimating the FM anisotropy could be the reason, at least in part, for the significant differences in the evaluation of the AFM/FM exchange coupling obtained via different measurement techniques reported in experimental works. It was also shown here that even when the adopted model describes correctly the EB system, none of the conventional reversible or irreversible techniques, applied to the same sample, is capable to distinguish between the exchange coupling and the AFM anisotropy fields if their ratio is sufficiently high or low. It is demonstrated that the values of AFM/FM coupling can be properly determined with the help of the variation in the coercivity with the AFM layer thickness. In the experimental results (obtained here using both static magnetic characterization and ferromagnetic resonance) of the IrMn/Al2O3/Co trilayers, special attention was paid on the EB field variation with the thickness of the non-magnetic insulator alumina spacer. It was observed that the IrMn/Co coupling strength decreases exponentially with the alumina spacer thickness, being the exponential decay length at least four times smaller than the values previously reported for exchange-bias systems. Such a rapid suppression of the coupling is explained in terms of the prompt loss of the direct contact between the antiferromagnet and the ferromagnet with the alumina layer's thickness. The IrMn/Cr/Co series presented an oscillatory behavior of both exchange bias and coercive fields as a function of the Cr layer thickness, which is rather peculiar. The value of the rotatable anisotropy field, estimated from the ferromagnetic resonance measurements, shows a sign oppose to that normally obtained, indicating that this field is antiparallel to the external applied magnetic field. Moreover, the general tendency for enhancement of the rotatable anisotropy field with the Cr spacer thickness suggests that, in these samples, there exists a considerable contribution coming from the Cr layer to this field

Exchange Bias em filmes de IrMn/Cu/Co

Medidas de magnetização (MAG) e de ressonância ferromagnética (FMR) foram usadas para estudar a interação de troca entre a camada antiferromagnética (AFM) de IrMn e a ferromagnética (FM) de Co no sistema IrMn/Cu/Co em função da espessura do Cu. Parâmetros importantes extraídos destas medidas foram comparados com os respectivos valores obtidos dos ajustes dos dados experimentais através de um modelo fenomenológico. Foi observada uma concordância muito boa entre os dados experimentais e os simulados numericamente. Os valores dos campos de anisotropia e de interação, obtidos pelas duas técnicas, MAG e FMR, foram comparados. Observou-se que, apesar das variações angulares experimentais dos campos de Exchange Bias coincidirem para todas as amostras, a intensidade de acoplamento J e os campos de anisotropia das camadas de Co (obtidos via simulação numérica) HU , são diferentes. Para todas as espessuras de Cu, vimos que J FMR > J MAG e FMR U H J MAG e FMR U H < MAG . U H Another important result is that J decreases exponentially with the spacer thickness and is a short-range interaction mediated by pinholes. All these characteristics were explained in the framework of a model considering polycrystalline magnetic layers with independent easy axis distributions and taking into account the rotatable anisotropy. The role of antiferromagnetic grains at the interface with different sizes and different magnetic stability is essential for understanding the behavior of this exchange-biased system. The facts that the observed FMR eb H and MAG eb H are equal (differently from all results known till now) and that the interactions as well as the uniaxial anisotropies are different were attributed to the different processes detected by the techniques. While the static MAG measurement ‘senses’ the sample as a whole, the FMR (which depends on the frequency of the oscillation of the magnetization) detects only the magnetic moments with relaxation times less than the period of the microwave excitation. There are no differences observed between the measurements when there is no contact (and consequently, the interaction) between the IrMn and Co layers

Exchange Bias em filmes de IrMn/Cu/Co

Medidas de magnetização (MAG) e de ressonância ferromagnética (FMR) foram usadas para estudar a interação de troca entre a camada antiferromagnética (AFM) de IrMn e a ferromagnética (FM) de Co no sistema IrMn/Cu/Co em função da espessura do Cu. Parâmetros importantes extraídos destas medidas foram comparados com os respectivos valores obtidos dos ajustes dos dados experimentais através de um modelo fenomenológico. Foi observada uma concordância muito boa entre os dados experimentais e os simulados numericamente. Os valores dos campos de anisotropia e de interação, obtidos pelas duas técnicas, MAG e FMR, foram comparados. Observou-se que, apesar das variações angulares experimentais dos campos de Exchange Bias coincidirem para todas as amostras, a intensidade de acoplamento J e os campos de anisotropia das camadas de Co (obtidos via simulação numérica) HU , são diferentes. Para todas as espessuras de Cu, vimos que J FMR > J MAG e FMR U H J MAG e FMR U H < MAG . U H Another important result is that J decreases exponentially with the spacer thickness and is a short-range interaction mediated by pinholes. All these characteristics were explained in the framework of a model considering polycrystalline magnetic layers with independent easy axis distributions and taking into account the rotatable anisotropy. The role of antiferromagnetic grains at the interface with different sizes and different magnetic stability is essential for understanding the behavior of this exchange-biased system. The facts that the observed FMR eb H and MAG eb H are equal (differently from all results known till now) and that the interactions as well as the uniaxial anisotropies are different were attributed to the different processes detected by the techniques. While the static MAG measurement ‘senses’ the sample as a whole, the FMR (which depends on the frequency of the oscillation of the magnetization) detects only the magnetic moments with relaxation times less than the period of the microwave excitation. There are no differences observed between the measurements when there is no contact (and consequently, the interaction) between the IrMn and Co layers