반응형
거대 자기저항 효과, 巨大磁氣抵抗效果, GMR:Giant Magneto Resistive effect
자기저항효과의 특수한 경우
보통 금속의 자기저항효과 (물질의 전기저항이 자기장에 따라 변화하는 현상)은 수 %이지만, 1nm정도의 강자성박막(F층)과 비강자성박막(NF층)을 겹쳐만든 다층막에서는 수십%이상의 자기저항비가 나타나는 경우가 있다. 이러한 현상을 거대자기저항효과라 부른다.
1987년 독일의 페터 그륀베르크, 프랑스의 알베르 페르에 의해 발견되었다. 거대자기저항 효과는 다층막 자기구조가 외부자기장에 의해 변화하기 때문에 발생한다. 자기다층막 이외에도 페로브스카이트형 망간산화물에서도 발견된다.
거대자기저항효과를 응용한 자기헤드의 등장으로 인해 하드 디스크 드라이브의 용량은 비약적으로 늘어났다.
그륀베르크와 페르는 이 발견으로 2007년 노벨 물리학상을 수상했다.
작고 얇은 컴퓨터 속의 소형 하드디스크, 연료전지와 초고집적 반도체, 사람 대신 당뇨병·치매·심장병을 앓는 유전자 변형 쥐. 2007년 노벨과학상을 받은 연구업적이 초석을 놓은 산물이다. 사람을 살리고 현대문명의 이기를 탄생시킨 기초과학의 승리라 할 만한 올해 노벨과학상의 진면목을 만나보자. “슈퍼마켓에서 물건을 사며 계산대에서 컴퓨터를 이용하는 광경을 볼 때마다 속으로 생각했죠. 와, 정말 대단해. 저 사람이 내가 생각해낸 원리로 만든 기계를 이용하고 있잖아.” 컴퓨터 하드디스크의 성능을 획기적으로 개선시킨 이론을 제안한 독일 윌리히 연구소의 페터 그륀베르크(68) 박사와 올해 노벨물리학상을 공동수상한 프랑스 남(南)파리대의 알베르 페르(69) 교수는 수상발표 직후 소감을 묻는 기자에게 이렇게 답했다. 노벨상을 받은 업적이 이처럼 빨리 생활 속 기술로 자리 잡은 예가 흔치 않다는 사실을 잘 보여주는 명답이었다. 이들은 각각 독립적으로 연구를 하다가 1988년 자기화(자기모멘트의 총합) 방향이 서로 다른 얇은 박막을 겹쳐 붙인 뒤 전류를 흘려주면 매우 큰 전기저항이 생기는 이른바 ‘거대자기저항’(GMR, Giant Magnetoresistance) 현상을 거의 동시에 발견했다. GMR은 컴퓨터에 탑재된 하드디스크가 디스크의 정보를 읽는 기본 원리다. IBM은 이 현상을 응용해 자기장의 세기가 작아져도 데이터를 오류 없이 읽게 하는 하드디스크를 1997년 처음 개발했고 GMR 기술은 곧바로 업계의 표준기술로 자리 잡았다. 냉장고 크기 하드디스크에 MP3 한곡 저장? 컴퓨터 저장장치인 하드디스크는 자석처럼 자성을 띄는 물질(자성물질)이 만드는 자기장의 방향이나 세기를 이용해 데이터를 기록하고 읽는다. 정보를 처리하는데 자성물질을 이용할 수 있는 이유는 자성물질 내부 전자들의 스핀(자기모멘트) 배열에 따라 자기화의 방향이 달라지고 이를 ‘1’ 이나 ‘0’으로 나타낼 수 있기 때문이다. 자성물질을 이용해 만든 최초의 하드디스크 드라이브는 IBM이 1956년에 개발한 자기테이프 방식의 ‘RAMAC305’다. 냉장고 2대 크기에 무게가 1톤에 이르지만 저장용량은 5Mb(메가바이트)에 불과했다. 오늘날 MP3 음악 파일 한 개 밖에 저장할 수 없는 용량이다. 1970년대 말 IBM은 얇은 박막 형태의 자성물질을 이용한 헤드(하드디스크에서 디스크에 기록된 자성물질의 자기화 방향을 감지해 정보를 읽어내는 장치)를 개발했고 점차 하드디스크의 정보저장 용량도 늘었다. 1Gb(기가바이트) 하드디스크가 등장한 때는 IBM이 1991년 ‘자기저항’(MR)효과를 하드디스크 헤드에 응용하면서부터다. 자기저항이란 예를 들어 니켈과 철로 이루어진 강자성체 합금에 자기장을 가하면서 전류를 흘려주면 자기장이 없을 때보다 전기저항이 커지는 현상으로 1850년대에 영국의 물리학자 윌리엄 켈빈이 처음 발견했다. 자기장이 있을 때와 없을 때 강자성체 합금의 저항 차이는 약 2% 정도인데, IBM은 이를 자기정보를 읽는데 응용했다. 전류가 흐르는 헤드가 자성을 띠는 물질이 입혀진 디스크 위를 지나면 자기저항효과 때문에 헤드에 흐르는 전류가 감소한다. 만약 자성물질이 없는 부분을 지나면 전류가 다시 증가한다. 이 전류 차이를 ‘1’과 ‘0’으로 읽어내는 원리다. MR 하드디스크 헤드 기술덕분에 1cm2 당 1Mb였던 하드디스크의 메모리 용량은 10Mb로 증가했다. 하지만 MR하드디스크의 ‘용량확장’은 GMR 하드디스크의 ‘용량폭발’의 서곡에 불과했다. 디스크의 자기 정보에 따라 전류 열고 닫아 큰그림 보러가기 MR이 강자성체 합금에서 나타나는 현상이라면 페르 교수와 그륀베르크 박사가 처음 발견한 GMR은 철 같은 강자성체와 크롬 같은 반강자성체를 번갈아 붙인 박막에서 나타나는 현상이다. 만약 두 개의 강자성층을 그 자기화의 방향이 나란하도록 붙이고 전류를 흘려주면 전기저항이 작아지고 서로 반대방향이 되도록 붙이면 저항이 커진다. 이때 나타나는 저항의 변화는 MR의 수십배에 이르기 때문에 MR 앞에 ‘거대’(Giant)라는 단어를 덧붙여 ‘거대’자기저항이라 부른다. 자기장의 작은 변화에도 저항이 크게 달라진다는 사실은 그만큼 정밀한 하드디스크 헤드를 만들 수 있다는 뜻이다. 이런 원리를 이용해 만든 제품이 GMR 헤드다. GMR 헤드는 두 개의 강자성층 사이에 보통 금속층을 껴 넣은 ‘스핀밸브’ 구조를 사용한다. 헤드 안의 스핀밸브는 위쪽 강자성층은 자기화의 방향이 고정돼 있지만, 아래쪽 강자성층은 고정돼 있지 않아 디스크에 새겨진 자기정보에 따라 자기화의 방향이 달라진다. 하드디스크의 헤드가 특정한 자기장 방향을 갖는 자성물질 위를 지나는 상황을 떠올려보자. 디스크 위의 자성물질이 만드는 자기장 방향에 따라 GMR 헤드의 아래쪽 강자성층의 자기화 방향이 달라질 것이다. 이때 두 강자성층의 자기화가 같은 방향이 되면 GMR 효과 때문에 박막의 저항이 작아져 박막을 흐르는 전류에 대해 밸브를 열어 놓은 상태가 되고, 두 강자성층의 자기화 방향이 서로 반대가 되면 저항이 커져 밸브를 잠근 상태가 된다. 즉 GMR 헤드는 위아래 강자성층의 자기장 방향이 디스크에 입혀진 자성물질의 자기화 방향에 따라 오르내리는 전류값을 읽어 디지털 정보로 변환한다. 스핀밸브를 이용한 하드디스크 헤드 기술 덕분에 1cm2 당 메모리 용량이 약 100Mb로 높아졌으며 현재는 그의 100배 수준인 10Gb에 이르렀다. 유니버설 메모리를 향해 GMR 효과는 하드디스크의 기억용량을 크게 확장시켰을 뿐만 아니라 최근 하드디스크가 아예 필요치 않은 새로운 개념의 컴퓨터를 개발하는 원리로도 이용되고 있다. 하드디스크는 정보를 읽어 들이거나 쓰기 위해 디스크가 회전하고 헤드가 움직이는 기계적인 과정을 거치기 때문에 전자의 움직임만을 이용해 정보를 읽고 쓰는 메모리인 RAM에 비해 속도가 수십배 느리다. 하지만 RAM은 전원이 끊기면 정보가 모두 지워지는 단점이 있다. 과학자들은 RAM의 고속기록재생속도와 하드디스크의 큰 기억용량을 모두 가진 ‘유니버설 메모리’를 개발하고 있다. 유니버설 메모리의 후보 중 가장 유력한 것은 MRAM(Magnetic RAM)이다. MRAM은 GMR에 바탕을 둔 ‘터널링 자기저항’ 효과를 응용했다. 터널링 자기저항효과는 스핀밸브구조에서 두 개의 강자성층 사이에 있는 금속층 대신 전기가 통하지 않는 절연체층을 끼워 넣은 구조에서 나타난다. 한쪽 강자성체에 있는 전자가 양자역학적 현상인 터널링으로 절연층을 통과해 다른 쪽 강자성체로 이동할 때 나타나는 자기저항 효과를 이용해 정보를 쓰고 읽는다. 현재 모토로라와 IBM 같은 기업에서는 수십Mb 용량의 MRAM 시제품을 제작하고 있다. 전문가들은 수년 내에 실용화될 수 있다고 예측한다. 페르 교수와 그륀베르크 박사에게 노벨상이라는 거대한 선물을 안긴 GMR이 인류에게는 얼마나 더 큰 선물을 안겨줄지 기대된다. 노벨 물리학상의 숨은 공로자 이번 노벨물리학상 수상자 발표를 가장 아쉬워할 사람은 누굴까? 전문가들은 단연 미국 IBM 알마덴 연구소의 스튜어트 파킨 박사를 꼽는다. 파킨 박사는 1997년 거대자기저항효과를 응용해 ‘스핀밸브’구조를 만들고 이를 하드디스크의 헤드에 응용해 세계 최초로 GMR 하드디스크를 만든 사람이기 때문이다. 파킨 박사는 올해 수상 업적인 GMR 효과가 학계 및 산업계에서 주목받도록 했으며 GMR 효과의 발견자인 알베르 페르 교수와 페터 그륀베르크 박사가 노벨상을 수상하도록 도운 숨은 공로자다. 파킨 박사는 영국 케임브리지대 트리니티 칼리지의 물리 및 이론물리학과에서 학사와 석박사 학위를 받았지만 1982년 미국으로 건너가 지금까지 IBM에서 연구 활동을 하고 있다. 노벨상을 받지는 못했지만 파킨 박사는 현재 차세대 메모리인 MRAM 개발을 주도하며 새로운 컴퓨터 시대를 이끄는 과학자로 인정받고 있다. P r o f i l e 이재일 교수는 서울대 응용물리학과를 졸업하고 동대학원에서 이론고체물리학으로 석사·박사학위를 받았다. 현재 인하대에서 물질의 표면과 계면의 자성에 대한 이론연구를 하고 있다. 강자성체 스스로 자석이 되는 물질. 강자성체 안에 있는 전자들의 스핀이 모두 한 방향으로 자발적으로 늘어서 자석의 성질을 나타낸다. 반강자성체 이웃한 전자들의 스핀이 번갈아 반대방향으로 배열된 물질. 크롬이나 망간이 대표적인 예다.
Giant Magnetoresistance: Basic Concepts, Microstructure, Magnetic Interactions and Applications
1. Introduction
It has been almost 30 years since one of the most fascinating advances in solid state physics occurred, the discovery of the giant magnetoresistance effect (GMR) by Grünberg and Fert in 1988 [1,2]. In thin metallic film systems, they observed that the magnetization of adjacent ferromagnetic films, separated by a thin non-magnetic interlayer, spontaneously align parallel or antiparallel, depending on the thickness of the interlayer. The orientation of the magnetization in the ferromagnetic layers strongly influences the resistance of the system. A parallel orientation is characterized by an electrical state of low resistance, while an antiparallel orientation is a state of high resistance. Due to the fact that the spacer layer thickness determines the initial configuration, an initially antiparallel orientation can be realized. The charm of this system lies in the fact that it enables a sensing of external magnetic field strengths in electrical units in between the two electric states of resistance. This discovery triggered an extensive research activity in this field in order to understand the underlying physical phenomenon as well as to exploit its technological potential. A remarkably short period, only a decade, lies between the discovery of the GMR effect and its first commercial realization in the form of magnetic field sensors and hard-disk read-heads [3]. Nowadays the spectrum of successful applications of GMR technology is impressively broad, ranging from applications in the air- and space or automotive industry, non-destructive material testing, or the compass functionality in mobile phones to biomedical techniques, like biometric measurements of eyes and biosensors, e.g., for the detection of viruses [3,4,5]. Thus, the potential of magnetoresistive technology seems to be far from being exhausted.
Nowadays the underlying physics of GMR and the interlayer exchange coupling are broadly understood. Nevertheless, when it comes to detail, discrepancies between experimental observations and theoretical models can arise: a realistic theoretical description of electron scattering at lattice discontinuities, disorder or defects is still a crucial factor [6,7].
In this review, we intend to provide an overview of different aspects of the GMR effect. The first section will focus on some of the ideas used to describe GMR effects theoretically in multilayers and to extend them into granular systems. Thereafter, we will have a look at different systems in which GMR can occur, with emphasis on the application-relevant side.
2. Theory
2.1. Giant Magnetoresistance in Magnetic Multilayered Systems
The giant magnetoresistance effect is the change of electric conductivity in a system of metallic layers when an external magnetic field changes the magnetization of the ferromagnetic layers relative to each other. A parallel alignment, like it is depicted in Figure 1a, has usually a lower resistance than an antiparallel alignment, Figure 1b. The effect size is defined as:
Figure 1. GMR double layer in Current in Plane (CIP) configuration. (a) Layer magnetization parallel; (b) antiparallel in respect to each other.
This section will introduce the Boltzmann equation approach for treating the GMR effect in multilayers in a classical picture. There are also a lot of publications presenting quantum mechanical treatments of the GMR, which will not be discussed here. The Kubo formalism [8] uses linear response theory to calculate the effect of small electric fields on currents. Examples for this ansatz are works by Camblong [9], Camblong, Levy and Zhang [10] and Levy, Zhang and Fert [11]. A detailed description and additional literature may be obtained in the extensive treatment of CPP GMR in multilayers by Gijs and Bauer [12].
Figure 3. Schematic illustration of the granular GMR (solid line) in dependence of the applied field and sample magnetization (dotted line). The granular GMR exhibits the highest resistance at the coercive field as the magnetic moments of the particles are randomly oriented there. The dashed lines are a guide to the eye.
A couple of models exist, which try to evaluate the parameter A� on a theoretical basis. Kim et al. [20] proposed a model based on the Kubo formalism. They modeled the magnetic grains as centers for potential barriers. They found their model to be in agreement with data by Xiao, Jiang and Chien [19], but as μ� approaches 1, the GMR deviated from ΔRR∝μ2(M(H)→MS)Δ��∝�2(�(�)→��). Additionally, they examined the GMR dependence on grain size compared to experiments by Xiao et al. [21] and Xiong et al. [22]. They found an optimal size for grains (compare Figure 14). The GMR effect rises rapidly until it reaches a maximum at the optimal grain size and then slowly decreases. They assumed this to be an effect of larger grains acting as conduction medium instead of only scattering centers.
Zang and Levy using a CPP like formalism they derived previously [23,24]. They found:(a)
Magnetoresistance increases with the mean free path of the electrons in the matrix material.
(b)
Magnetoresistance increases with the ratio between spin-dependent and spin-independent potentials, which they expect to be comparable to those found in multilayers.
(c)
Magnetoresistance increases with spin-dependent scattering roughness of the interfaces.
(d)
Magnetoresistance increases with decreasing grain size as long as the external magnetic field is strong enough to saturate all granules.
(e)
Magnetoresistance increases with concentration of granules as long as the granules do not form magnetic domains at high concentrations.
(f)
Magnetoresistance depends on the size distribution of the grains and needs to be precisely known to compare theory and experiment.
(g)
Magnetoresistance differs from ΔRR≈A μ2(H)Δ��≈� �2(�) when the grain size distribution is broad as μ� approaches 11.
Ferrari, da Silva and Knobel found that granular systems exhibits a behavior similar to the CPP GMR in multilayers for the case of the granule conductivity being much larger than the conductivity of the matrix [25,26].
These models all use some kind of averaging the magnetic moments of the systems, which seems to work fine as long as the concentration of grains is low enough. As soon as the distance between grains becomes small their dipole interactions lead to the assembly of ferromagnetic or antiferromagnetic domains, or more complex ordering. Teich et al. [27] used micromagnetic simulations to calculate magnetic ground states for magnetic particle assemblies, an example may be seen in Figure 4. These areas of magnetic ordering are likely to have influence on the electric conductivity of the system. To the best of our knowledge, there are to this point no studies on the influence of this. Systematic addition of differently shaped particles or the removal of particles could lead to increased GMR and tailoring of a granular system to specific needs.
Figure 4. Micromagnetic simulation of nanoparticles (20 nm) combined with a molecular dynamics simulation to model clustering of particles, see [27].
3. GMR Systems
3.1. Thin Film Systems
The first GMR multilayer stack was prepared in 1988 by Fert et al. [1]. They examined the characteristics of a {Fe/Cr}N system to explore the origin of the GMR effect. Driven by possible applications as sensors in automotive and read-head industry, numerous studies have been performed to improve the GMR characteristic since then [6,7,28]. A main goal was the improvement of layer materials and thicknesses in order to identify the optimum microstructural and magnetic features which enhances the GMR effect amplitudes in the multilayer systems and therefore, achieve higher sensitivities for sensor applications. Interface roughness is one of these microstructural characteristics that determines the GMR potential and has been intensively studied (for a review of numerous interface studies performed on Fe/Cr and Co/Cu multilayers see [6]). Furthermore the grain size has to be considered [29,30]. It has been found that neither the crystallite size nor the interface roughness alone determine the GMR of a multilayer, but the combination of both aspects. A combination of large grains with moderate interface roughness has been reported to be an ideal candidate for good GMR [29,31,32]. The interface roughness can be influenced employing a suitable buffer layer, whereas an appropriate buffer layer thickness has to be chosen depending on the materials used and the number of double layers. In Figure 5 the influence of the number of Co1.1nm/Cu2.0nm double layers on the GMR amplitude has been shown for two different Py buffer layer thicknesses. For small numbers of bilayers an increasing thickness of the buffer layer is favorable to obtain a larger GMR amplitude due to the enhancement of the antiferromagnetic coupling in the undermost bilayers.
Figure 5. GMR amplitude measured at room temperature as a function of the number of double layers N of (Co1.1 nm/Cu2.0 nm)N for two Py buffer layer thicknesses of 3.4 nm (red) and 8.1 nm (black), respectively. Data taken from [33].
This concept fails when sputtering a large number of bilayers, because the shunting of the thicker buffer or bilayer compensates or even destroys the effect of a larger antiferromagnetically coupled layer fraction [33]. However, due to the high GMR magnitude and, therefore, sensitivity for small changes of magnetic fields, GMR systems are very attractive for sensor applications in industry. In the following section we will have a closer view on different GMR applications:
3.1.1. Information Technology
The first industrial application of GMR thin film systems after the discovery of the effect was in the field of information technology: the realization of GMR based hard-disk read-heads in 1997 [3,28]. Here, the GMR sensor is used to detect the magnetization direction of the bits on the magnetic recording medium, which are assigned to a logical 0 or 1, respectively. Due to the continual improvement of storage density, and thus reduction of bit size, a good scalability and high sensitivity of the sensor element are necessary requirements. Furthermore, a linear sensor characteristic for the reliable detection of bits and long-term stability are crucial factors. To detect the transition between bits GMR spin-valve sensors are commonly used, which have been first proposed by Dieny et al. [34]. As schematically shown in Figure 6a, these spin-valves consist of three functional layers: a ferromagnetic (FM) layer with a fixed direction of magnetization (reference system), a non-magnetic (NM) interlayer and another ferromagnetic layer, which magnetization direction can freely align with external magnetic fields (free layer). To achieve a maximum stability of the reference system against external fields, it typically consists of an artificial antiferromagnet (AM) with a pinned layer and an antiferromagnetically coupled reference layer. That way, the magnetization of the reference layer can be fixed into a certain direction, employing the exchange bias effect [35]. The exchange bias field is temperature dependent and varies for different materials. In order to let the free layer follow changes in the external magnetic field, the thickness of the non-magnetic interlayer has to be chosen to ensure a minimal magnetic coupling of the magnetic layers.
Figure 6. (a) Schematic setup of the stack configuration of a GMR spin-valve sensor; (b) Conceptual operation of a GMR read head: when a spin-valve sensor moves across an interfaces between two bits with magnetic moments oriented in opposite direction (marked by “1” and “0”), the magnetic moment of the free layer is reoriented according to the orientation of the next bit.
Moving a spin-valve across the interface between two bits with opposite magnetization direction, the orientation of the magnetization of the free layer changes according to the stray field of the bits, resulting in a resistance change of the entire reading structure (compare Figure 6b). The resistance change causes a variation of current flowing through electronic circuits connected to the reading structures. This change of the current is detected and decoded to reveal the information stored on the disk.
For sensing small magnetic fields the distance between the stray field source and the sensor element is an important parameter, because the stray field strength drops strongly with increasing distance [36]. In Figure 7 the magnetic stray field strength as a function of the distance z is shown, illustrating the 1/z3 dependence. Therefore the reading head is required to maintain a constant distance to the spinning hard disk surface, which has to be as small as possible.
Figure 7. Magnetic stray field strength as a function of the distance from the layer surface, calculated for a bit structure with opposing magnetic moments as shown in the sketch. The arrows in the sketch mark the positions of the stray field calculations (black curve: middle of bits, red curve: interface between bits).
Recently, hard disk drives came onto market, which use He as filling gas between disks and read heads to reduce turbulences, and thus allowed a reduction of the distance between the disks and their read heads. Combined with e.g., the shingled magnetic recording technique for hard disks, where data tracks overlap with the adjacent tracks like shingles, GMR technology allows one to realize hard disk drives with storage capacities of up to 10 TByte [37].
3.1.2. Automotive Applications
The automotive industry offers a great field of applications for GMR sensors like sensing rotational speed, angle and position [38,39,40]. Several technical requirements have to be fulfilled to make the GMR technology compatible for automotive applications: linear and non-hysteretic GMR characteristics, high sensitivity, small temperature drift and long-term stability under application conditions. For application in rotational speed sensing for example, spin-valve sensors are commonly used (see Section 3.1.1) to ensure the desired sensor characteristics and sensitivity for small magnetic fields. For this purpose, the free layer of the spin-valve system needs to have an anisotropy axis, to which the magnetization preferably orients, if no external magnetic field is applied. This axis can be realized by using crystal anisotropy or by adjusting the geometry of the GMR structure and making use of the shape anisotropy. To obtain a high anisotropy and therefore a strong alignment, a high aspect ratio of the GMR structure has to be achieved. For example, for realization of linear transition regions in the range of several mT, the width of the GMR device has to be structured down to sizes of 1 µm and below [41,42]. A configuration which considers these aspects is the arrangement of meander shaped GMR sensors in a Wheatstone bridge [43]. This configuration minimizes the effects of temperature and disturbing magnetic fields. Furthermore, in this configuration hysteresis effects can be minimized e.g., by a slight change of the pinning directions out of the primary 90° orientation. In [43] a reduction of hysteresis by about 1/5 of the primary value has been reported. However, due to this geometry the GMR sensitivity is decreased and finally, for the optimization of GMR sensors always a compromise between sensitivity and magnetic reversal characteristic have to be found in consideration of the application of the sensor.
Since a lot of automotive magnetic sensors are implemented into security-relevant functions, it is of importance that the magnetic behavior of the GMR sensors be stable under the applied conditions. Thermal stability is a main factor here due to the exposure to high temperatures in the range of 200–360 °C during manufacturing as well as temperatures up to 200 °C for extended periods during up to 40,000 h of operation, which have to be tolerated by the sensor without loss of performance. Many studies report an initial increase of the GMR magnitude, compared to the as prepared samples, after an annealing for a short time at moderate temperatures between 250 °C and 380 °C [44,45,46,47,48,49]. This increase of the GMR effect originates from an improvement of the quality of the interfaces between the magnetic/non-magnetic layers as well as defect recovery by diffusion processes [45,48,50].
The optimum temperature depends on the choice of layer materials, thicknesses, the possibly used buffer layer and substrate materials. Within the framework of this review the focus is on Co/Cu based layer systems. For example, if the thickness of the individual layers has been optimized for the first antiferromagnetic coupling (AFC) maximum an optimum temperature of about 150 °C has been reported [52], while for systems optimized for the second AFC maximum a critical temperature of about 375 °C has been observed [53]. For annealing processes above the critical temperature a breakdown of the GMR amplitude is observed. Different reasons for this deterioration of GMR in Co/Cu multilayers have been discussed in literature: Observations of Co bridges through Cu layers have been reported by means of field ion microscopy and transmission electron microscopy (TEM) [54,55]. These defects of the layered structure were observed in systems with high interface roughnesses even in the as prepared state leading to a strong ferromagnetic coupling of the adjacent Co layers. TEM studies of Co/Cu multilayer samples reported by Rätzke et al. show the transport of Cu into the Co layers along grain boundaries [47]. An alternative method for the observation of the mechanism of GMR deterioration is the atom probe tomography (APT) [51,56,57]. In Figure 8a a 3D reconstruction of a Py25nm/Cu20nm/Co10nm trilayer obtained by APT is shown. After an annealing at 350 °C for 30 min. (Figure 8b,c) it can be clearly seen that Ni atoms from the Py buffer layer segregate along grain boundaries into the Cu layer (red dots in Figure 8c). This segregation path forms the initial stage of pinhole formation and causes ferromagnetic bridges through the non-magnetic coupling layer, causing a decrease of GMR effect [51].
Figure 8. 3D reconstruction of atom probe tomography of a Fe (red) Ni (yellow)/Cu (blue)/Co (green) trilayer: (a) as prepared Co/Cu interface (upper image) as well as Cu/Py interface (lower image); (b,c) show the element distribution after annealing at 350 °C for 30 min for the marked Co/Cu interface region in (a) (adapted from [51]).
A concept how to avoid these effects and to improve the temperature stability of Cu/Co multilayer systems has been reported by Heitmann et al. [58]: For a [Py3nm/Cu6nm/Co3nm/Cu6nm]20 multilayer system it has been shown that an annealing at 500 °C for 24 h triggered a complete recrystallization of the sample from a dominating polycrystalline [111] texture in the as prepared state to a [100] quasi single crystalline state after annealing. The most striking aspect of the microstructural evolution is the preservation of the layered structure (compare Figure 9a,b). This crystallographic reorientation is triggered by the minimization of lattice mismatch elastic energy: Under equal strain the elastic energy in a [111] oriented CoCu material is higher than the energy in a [100] structure due to the elastic properties of the materials. By recrystallization in a [100] structure a reduction of elastic energy in the order of 0.8 eV per interface atom is achieved [33,59]. But it is important to note, that a prior annealing of the sample at moderate temperatures which has led to a considerable reduction of dislocations in the course of recovery, while the temperature was not high enough to activate recrystallization process, a further temperature increase not necessarily initiate a recrystallization any more. This is caused by the decrease of the driving force [60]. Therefore, recrystallization can only occur after heating up the sample directly to sufficient temperatures. The GMR measurements, given in Figure 9d, for the recrystallized Co/Cu multilayer show that the GMR effect remains stable at further heat treatment below the initial annealing temperature for 64 h.
Figure 9. Comparison of TEM images of a [Py3nm/Cu6nm/Co3nm/Cu6nm]20 multilayer in the as prepared state (a) and after annealing at 450 °C for 24 h (b). The insets show the corresponding selected area diffraction pattern. The micrographs prove that the layered structure of the sample is preserved during annealing while the microstructure changes from polycrystalline to quasi single crystalline, oriented in fcc [100] direction; (c) X-ray diffraction pattern of a Co/Cu multilayer system before and after annealing showing the recrystallization effect; (d) GMR measurements at room temperature for the recrystallized Co/Cu multilayer: the GMR effect remains stable at further heat treatment at 400 °C for 64 h [33].
3.1.3. Biosensors
Due to the ability of GMR systems to sense even small magnetic fields, the potential of GMR sensors for the detection of magnetic beads was realized and led to another growing technological field, the development of magnetic biosensors for life science applications. Only ten years after the discovery of GMR the first magnetic biosensor was presented by Baselt et al. [61].
In Figure 10 an illustration of the detection principle is shown. Specific proteins are immobilized on the sensor surface. Superparamagnetic nanoparticles or beads, which are specifically attached to a target antibody, are used for detection. In a washing step, unbound magnetic markers are removed and beads bound to antigen molecules are measured.
Figure 10. Schematic representation of a magnetic biosensor: (a) a superparamagnetic bead functionalized with a receptor molecule hybridize to the target molecule attached onto the sensor surface; (b) An external field align the magnetic moment of the bead and the magnetic stray field can be detected by the GMR sensor (adapted from [62]).
The superparamagnetic nature of the beads allows to switch on their magnetic stray field by a homogeneous external magnetic field oriented perpendicular to the sensor surface, see Figure 10b. Hence, the stray field components of the magnetic markers within the sensitive sensor area can be detected by a drop in the electrical resistance of the GMR sensor. For an optimum bead detection, GMR sensors with isotropic signals and high sensitivities are needed. In [62,63] the use of a Py1.6nm [Cu1.9nm/Py1.6nm]10/Ta3nm multilayer stack for the detection of magnetic beads was reported. To prevent any influences of magnetic anisotropies of the used materials on the GMR characteristic a spiral-shaped structure has been chosen. In Figure 11a the nearly isotropic GMR characteristic for two perpendicular oriented in-plane magnetic fields are shown. For this type of sensor a sensitivity of 0.6% per kA/m for in plane magnetic fields has been achieved, resulting in a detection limit of a DNA concentration of only 16 pg/µL, which is superior to standard fluorescence detection methods [63]. The dependence of the resistance change ΔR on the particle coverage of the sensor surface is shown in Figure 11b. A nearly linear behavior of the output signal is observed for low particle concentrations [62].
Figure 11. (a) Isotropic GMR characteristic measured at room temperature for a spiral shaped GMR sensor for two in-plane fields oriented perpendicular to each other; (b) Change in the resistance of meander shaped GMR sensors, each with an area of 100 × 100 µm2, as a function of particle density. The SEM images show the particle coverage of the sensors corresponding to the measurements marked by colored circles (data taken from from [62]).
On the way from a simple bead detection to a fully integrated, easy to use, hand held “lab on a chip” device for applications in human or veterinary diagnostics, several challenges have to be mastered: (1) The magnetic core of the magnetic markers has to be stabilized to preserve their magnetic properties. Usually, this is achieved by embedding superparamagnetic magnetite nanoparticles in a polymer matrix. Chemically synthesized FeCo nanoparticles are good candidates even for single molecule detection as well, due to their superior saturation magnetization and, therefore, larger stray fields [64]; (2) the interface between chemistry and biology has to be fitted for each application, to allow a specific functionalization of the marker and sensor surface, e.g., for the detection of biotin-labeled DNA, streptavidin coated particles can be used [65,66]; (3) the GMR sensors have to be incorporated in fluidic environments, which enable the magnetic markers to pass the sensor surfaces at close distances to ensure a binding onto the surface within an acceptable time scale [67]. Due to the magnetic nature of the markers, magnetic attraction forces, created e.g., by on-chip conducting lines or magnetically structured thin films, can be employed to pull beads towards the sensors [68,69,70,71,72,73,74,75]. Another way to concentrate beads on a sensing surface uses of ultrasonic standing waves inside a microfluidic channel system [76,77] or the microfluidic system itself can be utilized to transport beads towards the sensor surface, e.g., by designing a ramp like structure [67,78].
A new concept to transport the magnetic particles in a “lab on a chip” environment without the need of external forces like microfluidic pumps, is a magnetic on-off ratchet [79,80]. Here, a combination of asymmetric magnetic potential and Brownian motion of magnetic beads moves particles through the device. The asymmetric magnetic potential is achieved by combining an external magnetic field with a spatially periodic array of conducting lines. When the asymmetric field is applied, particles move towards the minima of the potential. After switching off the diffusion process starts. Due to the asymmetric shape of the potential the particles are transported to the next minima when the field is reactivated and thus, a net transport process is achieved [79].
The realization of a lab-prototype of a “lab on a chip” device is shown in Figure 12. An array of 32 meander shaped GMR sensors combined with a suitable microfluidic design, which optimizes the bead capture rate. The measurement of individual sensor coverage can be improved by application of the guarding procedure. This procedure employs an additional amplifier which switches the voltage on the adjacent sensor rows, enabling an equal potential of the rows (see Figure 12d,e). Provided that the resistances of the matrix elements are of the same magnitude and much larger than the resistance of the supply lines, the measurement current will not expand on other paths and every resistance in the sensor matrix can be addressed individually.
Figure 12. Lab-prototype of a “lab on a chip” device: (a) SEM image of the device heart consisting of 32 GMR sensors. The marked region of four meander shaped GMR sensors is shown enlarged in (b); (c) Photograph of the connected device; (d,e) illustrate the advantage of the guarding procedure for an analysis of a matrix of 32 sensors. Only the green labeled sensor element (line 3, column D) should be measured. Possible other paths of the current are marked in red; (d) the matrix without guarding; (e) when the guarding approach is applied.
3.2. Granular Bulk Systems
Four years after the discovery of the GMR effect in multilayer structures it was shown by Berkowitz et al. and Xiao et al. that GMR is not restricted to thin film systems, but occurs in heterogeneous bulk alloys, too [19,81]. Both groups utilized magnetron sputtering or melt spinning to create ferromagnetic Co precipitates in a non-magnetic Cu matrix, respectively. Underlying physical mechanisms which can induce the formation of such granular bulk GMR structures in alloys are summarized in the schematic phase diagram shown in Figure 13. Different decomposition types for the formation of magnetic precipitates (metal A) in non-magnetic matrix materials (metal B) have been observed: (1) decomposition by classical nucleation and growth, e.g., in Ag-Co systems [21,82,83]; (2) coherent or spinodal decomposition, e.g., in Cu-Co systems [84,85] and (3) eutectic decomposition, e.g., in Au-Co alloys [86,87]. However, it is expected that Ag-Co and Cu-Co systems will behave as decomposed due to a large content of Co [88].
Figure 13. Schematic phase diagram of two metals A (magnetic) and B (non-magnetic) illustrating different types of decomposition which can lead to GMR effects in heterogeneous bulk alloys: (1) classical nucleation and growth of precipitates; (2) coherent or spinodal and (3) eutectic decomposition, which forms a lamellar microstructure similar to multilayers.
By applying an external magnetic field during the decomposition process (see Figure 13, case 2), elongated magnetic precipitates can be prepared. This has been demonstrated by Hütten et al. in AlNiCo5 bulk alloys [89]. It has been shown that the probability of spin scattering is two times higher, if the direction of the current is perpendicular to the direction of the particle elongation.
The GMR characteristics in these granular systems is closely correlated to the magnetic behavior of the samples, as discussed in Section 2.2. Due to TEM investigations it is known that an annealing of granular systems causes a coarsening of the magnetic precipitates and an increase of interparticle distances as it has been reported for Cu-Co [19,88] and melt-spun Au-Co [87,90]. Furthermore, in [88] it has been confirmed by Lorentz microscopy that single domain Co particles exist in as-quenched Au71.6 Co28.4 ribbons, and multidomain Co particles in annealed ribbons, respectively. The changes in grain size and the formation of multidomain particles reflect in the magnetic measurements and, therefore, in the GMR characteristics. While a constant decrease of the granular GMR with increasing particle size was observed by some groups [81,91,92], it was found in refs. [19,93,94,95], that the granular GMR first increases up to a maximum value at about the electron mean free path λ� and then decreases (see Figure 14). In both cases, the granular GMR decreases approximately with the inverse of particle size. It was concluded that the decrease of the granular GMR arises from the decreasing spin-dependent interfacial electron scattering as the surface to volume ratio decreases with increasing size [23,96]. Ge et al. concluded, that for low annealing temperatures, defects, disorder and mismatch stress are reduced [94]. Thus, the overall film resistance reduces, which leads to an increased granular GMR. At higher temperatures, particles grow fast enough compared to the curing of the film defects and the granular GMR degrades. Wang et al. noted on particle size dependence of the granular GMR, that it depends on whether the particles are superparamagnetic, single domain ferromagnetic or multidomain ferromagnetic. While their calculations show a constant decrease for superparamagnetic particles, it exhibits a maximum for single domain ferromagnetic particles [93].
Figure 14. Schematic illustration of the granular GMR effect in dependence of the particle size.
The dependence of the granular GMR on the ferromagnetic volume fraction is comparable to the particle size dependency: At low ferromagnetic volume fractions, the particles are small and few in number, therefore only a small granular GMR is measurable. With increasing ferromagnetic volume fraction, the granular GMR increases until it reaches the optimum value at a ferromagnetic volume fraction between 15% and 30%, depending on the used material system. Thereafter, it decreases with an increasing ferromagnetic volume fraction as the particles become larger and more densely packed reducing the surface to volume ratio. Furthermore, multidomain particles can arise and dipole interactions between neighboring ferromagnetic particles become more important. Finally, particles form a large connecting network with ferromagnetic domains at the percolation threshold of 55% and only an anisotropic magnetoresistance (AMR) is observable [93,94,95,97,98,99,100,101,102,103,104].
Summarizing the findings of many studies, it can be stated that the size, distribution and amount of ferromagnetic particles as well as the interface roughness determines the resulting GMR effect in granular alloys [87,88,90,105]. Therefore, it is essential to control these parameters to improve the GMR effect in granular systems.
3.3. Hybrid Structures
GMR is not restricted to thin films or bulk systems only. It also occurs in pure particular systems (see next section) and hybrid materials containing thin films as well as magnetic clusters. These hybrid structures can be prepared e.g., by heating of films or by preparing multilayers with ultrathin and therefore discontinuous magnetic layers [106,107,108,109,110]. Holody et al. showed that Co/Py hybrid systems reveal advantages for sensor applications like a lateral decoupling in the cluster layer in combination with a low coercive field [106]. Unfortunately, the above mentioned techniques for the preparation of hybrid materials typically lead to large cluster size distributions, making the investigation of influences like cluster size, distances and concentration on the resulting GMR characteristic hard to uncover. In [110] the idea to employ a “bottom-up” method by replacing a ferromagnetic electrode of a thin film trilayer by predefined magnetic nanoparticles has been presented. Here, a Co3nm/Ru0.8nm/Co4nm thin film system has been prepared by sputtering as a reference, which shows a GMR amplitude of 0.36% at room temperature (see black curve in Figure 15). The thickness of Ru interlayer has been chosen according to the best interlayer exchange coupling. For preparation of the hybrid system, Co nanoparticles with a mean diameter of 12 nm have been prepared via a wet chemical synthesis [111,112]. A monolayer of these particles have been spin coated on top of the Ru interlayer, thus replacing the 4 nm thick Co film as magnetic electrode. The corresponding GMR characteristic (see red curve in Figure 15) shows a similar behavior compared to the reference system with an effect amplitude of 0.28% at room temperature.
Figure 15. Proof of concept of the idea that Co nanoparticles can be coupled to a Co layer via a Ru spacer layer coupling. As references, the GMR characteristics of a Co3nm/Ru0.8nm/Co4nm layered sample (black curve) and a Co3nm/Ru0.8nm system (blue curve) are given. The resulting GMR curve (red) measured at room temperature of the Co3nm/Ru0.8nm/Co particles (diameter: 12 nm) hybrid system clearly shows a spin-valve character (adapted from [110]).
This indicates that the magnetic Co nanoparticles can be coupled to a Co layer utilizing the spacer layer coupling. The smaller saturation field of the hybrid structure indicates a smaller spacer layer coupling compared to the layered system of the same spacer layer thickness, but due to the larger magnetic moment of the 12 nm sized Co nanoparticles compared to the 4 nm thick Co film, the contribution of the Zeeman energy is higher, too. Thus, in the case of the hybrid system is the saturation field is smaller than for the layered structure for an assumed equal coupling strength. Nevertheless, this method seems to allow a finer tuning of GMR characteristics of hybrid systems, which is of great interest from an application point of view.
3.4. Nanoparticular GMR Systems
Typically, granular materials are prepared by top-down methods such as co-sputtering or co-evaporation of matrix and precipitated materials as well as by metallurgic techniques [97,113,114,115]. Particle size, volume fraction and magnetic configuration of the particles have to be controlled due to the GMR’s dependence on these parameters. These requirements can be fulfilled more easily by employing bottom-up approaches for the preparation of the granular systems like the embedment of prefabricated magnetic nanoparticles into non-magnetic matrix materials. This approach has been applied at first by Dupuis et al., who used in the gas-phase prefabricated Co and Fe particles simultaneously deposited with Ag as matrix material onto cold substrates [116]. This technique allows the independent variation of particle size and volume ratios and therefore the study of the dependence of GMR on these parameters. Furthermore, different material systems can be realized in a simple manner [116,117,118]. In 2007 Tan et al. showed that chemically synthesized ligand stabilized magnetic FeCo nanoparticles can be used for a preparation of magnetoresistive granular super-crystals [119]. In this case, the electrically isolating ligand shell acts as a tunneling barrier. Tunnel magnetoresistance effect amplitudes of up to 3000% at low temperatures have been reported for these nanoparticular systems [120]. In [121] such ligand stabilized nanoparticles have been used to create two-dimensional granular GMR structures. Therefore superparamagnetic Co nanoparticles with a mean diameter of 8 nm have been arranged in a monolayer onto a SiO substrate via a self-assembly process. The insulating ligand shells have been removed by an annealing process in a reducing gas atmosphere. Afterwards, without breaking the vacuum, a thin Cu layer has been deposited on top of the nanoparticles in order to establish an electrical contact between the particles. In Figure 16 a result of a GMR measurement at room temperature is shown. The bell shaped GMR characteristic follows mostly the expected behavior for non-interacting particles deduced from the magnetization reversal by Equation (11) (red curve, Figure 16).
Figure 16. GMR characteristic of a monolayer of 8 nm sized Co nanoparticles measured at room temperature with an in-plane external magnetic field (sample current: 1 mA, R0: 1.6 kΩ). The measurement is compared to the expected behavior of non-interacting particles (red curve). Additionally, the corresponding magnetic measurement is shown (blue curve) (adapted from [121]).
Aside from the expected magnetoresistance characteristic, additional features appear symmetrically for in- and decreasing external fields, which can be attributed to the inner magnetic arrangement in the particle assembly. Caused by a dipolar coupling of adjacent particles, magnetic domains can be formed with an antiparallel arrangement of magnetic moments which maintains a higher stability against external influences compared to the non-interacting particles [121].
Vargas et al. established a model to simulate the dipole coupling between ferromagnetic particles and its influence on the granular GMR [122]. They showed that the particles couple ferromagnetically in the near field, while in the far field an antiferromagnetic coupling is present. Considering a model system of two parallel particle chains with particle moments aligned in one direction within each chain, but opposite to the orientation of the moments of the adjacent chain, a 20% higher GMR has been expected compared to non-interacting nanoparticles. In order to realize such a nanoparticular GMR model system Meyer et al. have incorporated carbon coated Co nanoparticles into conductive agarose gels as a non-magnetic matrix [123]. These systems allow an alignment of ferromagnetic Co particles along magnetic field lines of an applied external field. The agarose gel has been heated above the melting point and the nanoparticles are spread in the liquid phase of the gel. During cooling of the gel below the gelling temperature an external magnetic field can be applied. Thus, this technique allows to trigger different particle arrangements in the conductive matrix, which are fixed after the gel’s solidification. Thereby, a variation of the GMR characteristic with every measurement caused by a change of particle positions during switching the external field, which can be observed in the case of a liquid gel matrix like a glycerin-water mixture, can be prevented [123]. Optical microscope images of a sample prepared without and with the influence of a homogenous magnetic field during the cooling process are shown in Figure 17a,b, respectively. A comparison of the corresponding GMR measurements performed at room temperature is given in Figure 17c. The impact of particle arrangement on the nanoparticular GMR effect can be seen clearly. The higher GMR amplitude in the case of the field cooled sample, compared to the sample with the randomly distributed particles, can be attributed to the larger particle volume fraction along the current path, when the current is applied parallel to the field [123].
Figure 17. Optical microscope pictures of carbon coated 18 nm sized Co particles in an agarose gel prepared without (a) and with a homogenous (b) external magnetic field applied during the cooling process. The particles clusters are randomly distributed in the sample without field, while particle chains are formed during the influence of the homogenous field. The granular GMR is measured at room temperature for both samples and shown in (c) (data taken from from [123] © IOP Publishing. Reproduced with permission. All rights reserved).
However, the particle density inside these particle superstructures is different. Hence, the dipole coupling inside these superstructures varies as shown by spin-dynamic simulations for a homogenous and a rotating field sample [27]. As a higher particle density is present in the rotating field sample, the interparticle distance is smaller and therefore, more and larger areas of ferromagnetic coupled particles are present compared to the homogenous field sample. As suggested by Vargas et al., these different dipole couplings may have an additional influence on the granular GMR effect as well [122]. To further improve the stability of the agarose gel based nanoparticular GMR characteristics, it is recommended to use an alternating current (AC) instead of a direct current (DC) (compare Figure 18). In doing so, the electrolysis of the ions in the gel and the buildup of electrical double layers are inhibited [123]. This results in an enhancement of reproducibility of nanoparticular GMR effects and therefore, opens a way to realize printable, high sensitivity sensors without the need of photo- or e-beam lithography [67].
Figure 18. (a) Comparison of a nanoparticular GMR measurements at room temperature with DC and AC at 110 Hz; (b) The GMR effect development over a number of measurements for DC and AC at 110 Hz (data taken from [124]).
4. Conclusions
We have shown that the GMR effect occurs in magnetic materials ranging from heterogeneous bulk systems over multilayered thin films to magnetic nanoparticles, synthesized by bottom-up methods. The microstructural as well as magnetic features have found to be crucial to trigger full potential of the GMR effect in all systems. For the future-oriented nanoparticular GMR systems, we have shown that an extensive control of the particle arrangement and magnetic configuration will be the key to a successful establishment of printable detection devices in industrial applications.
반응형