IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011
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Temperature-Dependent Magnetic Properties of Magnetically Biphase Microwires Valeria Rodionova1;2 , Alexander Nikoshin1 , Jacob Torrejón3 , Giovanni A. Badini-Confalonieri3 , Nikolai Perov1 , and Manuel Vazquez3 Faculty of Physics, Lomonosov Moscow State University, 119991 Moscow, Russia Immanuel Kant State University of Russia, 236041 Kaliningrad, Russia Materials Science Institute of Madrid, CSIC, 28049 Madrid, Spain
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The magnetic behaviour of soft/hard biphase magnetic microwires has been studied as a function of temperature in the range 25 . The microwires consist of an ultrasoft CoFe-based vanishing magnetostriction amorphous core covered by insulating Pyrex to 900 coating prepared by quenching and drawing, plus an electroplated CoNi magnetically harder external shell. The magnetization process has been analyzed through the study of the hysteresis loops and their parameters like saturation magnetization and coercivity of the different phases, measured in a vibrating sample magnetometer. The magnetically biphase character has been first confirmed by roomtemperature measurements for wires with different thickness. The observed dependence of magnetization and coercivity on temperature is correlated with the overcoming of the Curie temperature and crystallization of the amorphous soft core.
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Index Terms—Ferromagnetic biphase microwire, high-temperature magnetic properties, magnetization process.
I. INTRODUCTION AGNETICALLY biphase microwires are modern and popular composite material whose attractive properties as bimagnetic systems have attracted attention for the development of multifunctional sensors and microactuators devices [1]–[3]. This composite material can consist of a magnetically hard core with magnetically soft shell, i.e., hard/soft microwires or of a magnetically soft core with magnetically hard shell, i.e., soft/hard microwires. The relative position of these magnetic phases, the diameter of the core and the thickness of the shell, the distance between them, and so on, all these parameters affect the magnetostatic and magnetoelastic interaction between the core and shell and, as a consequence, the magnetic properties of the microwires [4], [5]. As a result, the characteristics of the sensors can be changed. The magnetostatic interaction between magnetic phases is similar and even stronger to that one between two amorphous ferromagnetic microwires [6]–[8] (placed parallel to each other in close vicinity), and the presence of the magnetically hard phase can be used in new applications. For example, the small angle magnetization rotation technique can be used to determine the saturation magnetostriction of the soft-core material [9]. Moreover, the external shell phase leads to induced axial or transverse magnetoelastic anisotropy in the nucleus [4], [10]. The type of the anisotropy depends on the magnetostriction constant of the nucleus microwire alloy. Nowadays, sensors based on magnetic microwires are used far beyond the common application (e.g., for production of the cars) and occupy defense and aerospace industries. As a consequence, the improved stability of microwires properties in wide
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Manuscript received February 21, 2011; revised May 25, 2011; accepted May 25, 2011. Date of current version September 23, 2011. Corresponding author: V. Rodionova (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2158811
temperature range and better reversibility after thermal cycling are required. There are only few works devoted to investigation of the temperature dependence and thermal effects in microwires properties [11]–[13], whereas this information allows one to investigate and understand the mechanisms of the magnetization properties evolution and the interaction of the composite material components (the core and the shell). In this paper, we introduce the experimental results about the high-temperature dependence of the magnetic properties of soft/ hard biphase microwires with different shell thicknesses in the temperature range 25–900 . II. SAMPLES AND EXPERIMENTAL DETAILS The microwires under consideration consist of soft CoFebased (two different compositions) vanishing-magnetostriction at room temperature) covamorphous core ( ered by Pyrex coating prepared by quenching and drawing techexternal shell onto an nique plus an electroplated Au nanolayer, 30 nm thick, which was first sputtered onto the Pyrex. The detailed technique of biphase microwires production has been described elsewhere [2], [13], [14]. The amorphous core compositions are and , which are labeled as Co1 and Co2, respectively. The thickness of the external shell is varied as indicated in Table I after the slash of each sample label. Other details of the microwires geometry are also given in Table I. The room-temperature and the high-temperature evolution of the magnetic properties of the biphase microwires have been investigated by vibrating sample magnetometry—“LakeShore” VSM (7400 series), in the field range 10 kOe in the temperato 900 . The home-made vibrating sample ture range 25 magnetometer [15] with high magnetic field resolution of 0.02 Oe, was used to measure the room-temperature hysteresis loops in the low-field region of the single magnetically soft amorphous microwires, Co1 and Co2. The magnetization process has been studied by analysis of the hysteresis loops and their parameters at different tempera-
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TABLE I GEOMETRY PARAMETERS OF THE SAMPLES
d t
—the diameter of the magnetic amorphous core, t and —thicknesses of the glass coating and magnetic shell, correspondingly, l —the length of the samples.
Fig. 1. (color online) Room-temperature hysteresis loops for biphase miCo ) Si B =Co Ni (Co2-series) crowires with composition (Fe and different thickness of the CoNi-shell. The insert shows the hysteresis loop for single amorphous glass-coated microwire (the magnetically soft core of the biphase microwire).
tures: saturation magnetization, coercive force, remanence, and the hysteresis loop shift.
cm , i.e., being prowith the volume of the magnetic phase portional to the area of the magnetic cross section multiplied by wire length). The saturation magnetic moments in the magnetic field 10 kOe for Co2-series are 3.6, 4.4, and 14.7 memu for Co2/4, Co2/6, and Co2/11 microwires, respectively. The hysteresis loops are characterized by two distinct processes: 1) the magnetization process for inner core of the microwire takes place at the low-field region mainly by magnetization rotation and gives rise to the step in the hysteresis loop; and 2) full magnetization reversal proceeds in a more continuous way for the harder phase at the shell. The two processes are ascribed to the presence of two different magnetic phases with soft (CoFe amorphous core) and harder (CoNi polycrystalline shell) properties, respectively. The polycrystalline structure of the magnetically hard phase was proved elsewhere [4]. and The ratio of the magnetic moments of the hard phases changes with increasing of the shell thickthe soft . Also both, ness as a consequence of the increasing of the susceptibility and the coercive force of the hard phase rise (e.g., for : from 130 to 170 Oe) with the shell thickness increasing from 4 till 11 m. That can be interpreted considering the growth of the effective demagnetizing factor of the whole system (the core plus the shell). We consider that the coercivity of the hard phase corresponds to the half height of the corresponding part of the hysteresis loops, which is much higher than the field at which the jump of the soft phase takes place. The dependence of the coercivity of the biphase system shows a low value when it is determined by the soft phase (e.g., for 4 and 6 m shell thickness, the coercivity of biphase system is mainly dominated by soft phase, whereas for 11 m the coercivity shows larger values, very close to the hard phase). So, the evolution of the magnetic properties of the soft/hard biphase microwires at room temperature strongly correlates to previous results [16]. B. Influence of the Thermal Treatments
III. EXPERIMENTAL RESULTS AND DISCUSSION A. Room-Temperature Magnetic Characterization The single amorphous Co1 and Co2 microwires show soft magnetic behavior with a typical nearly nonhysteretic behavior and high magnetic susceptibility, which is nearly constant until reaching the region of approach to magnetic saturation. As an example, the room-temperature hysteresis loop for single Co2 amorphous microwire is shown in the inset of Fig. 1. The magof sample is normalized to its saturation magnetic moment netic moment in the magnetic field of 5 Oe, and a transverse anisotropy field of 1.4 Oe is deduced. The same data treatment was done for all hysteresis loops of the biphase microwires in order to compare the magnetic character of the samples, in particular the ratio of the magnetically hard phase to the soft phase. All loops were measured in the magnetic field range 10 kOe and normalized to their saturation magnetic moment in the magnetic field 10 kOe. Similar evolution of the hysteresis loops was observed for both series of the microwires. The room-temperature hysteresis loops for biphase microwires of Co2-series are plotted in Fig. 1. The sat(emu), increases proportionally uration magnetic moment,
The hysteresis loops for both series of biphase microwires were measured at different temperatures. It is observed that with the temperature increase, the hysteresis loop shape for all samples changes in the same way. The hysteresis loops for Co2/6 sample are shown in Fig. 2 as an example. The hysteresis loops are characterized by two distinct processes in the temperature to 300–350 (the upper limit depends on range from 25 the shell thickness, but we did not find any strong dependence). At the higher temperatures, up to 900 , a single magnetization process is observed, suggesting the existence of a single magnetic phase. The magnetization under maximum applied field taken as its , as it is saturation, decreases with temperature increase, plotted in Fig. 3 for different samples. Above 300 –350 , the amorphous soft phase has overcome the Curie temperature so that a single magnetization process corresponding to the CoNi polycrystalline phase is observed. As the thickness increases from 4 to 11 m, the magnetization evolves with temperature as shown in Fig. 3. Two regions correspond to the of the low-Curie-temperature phase (the dependences amorphous core) and of the high-Curie-temperature phase (the polycrystalline shell). The Curie temperature of the CoNi phase
RODIONOVA et al.: TEMPERATURE-DEPENDENT MAGNETIC PROPERTIES OF MAGNETICALLY BIPHASE MICROWIRES
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Fig. 4. (color online) Temperature dependence of the coercive force for Co2/11 and denote the coercivities of and Co2/6 biphase microwires. Here, the hard and soft phases of the biphase microwires at low temperature, respectively, and is the coercivitiy of biphase microwire at the high-temperature region (see details in the text).
H
H
H
Fig. 2. Hysteresis loops of Co2/6 biphase microwire at different temperatures.
Fig. 3. (color online) Temperature dependence of the saturation magnetic moment of the biphase microwires Co2/11, Co2/6, and Co2/4.
is higher than 900 (the highest possible temperature for this technique), apparently. The amorphous (soft) phase coercivity increases, whereas the crystalline (hard) phase coercivity decreases with temperature until around 300–350 where the two-step hysteresis loop disappears, as observed in Fig. 4. At the higher temperatures, the observed decrease of the is ascribed to the decrease coercivity of the single phase of the anisotropy of the materials with the temperature increase. The different behavior of the hysteresis loops measured at the and 435 and after cooling from these temperatures 220 is a contemperatures to room temperature sequence of the ferromagnetic-paramagnetic transition: the hysteresis loops in Fig. 5(a) show two magnetic phases in both cases and after cooling, whereas the hysteresis loops in at Fig. 5(b) shows the presence of single and two magnetic phases.
Fig. 5. (color online) (a)–(c) Hysteresis loops of Co1/4 biphase microwire measured at temperatures within the range 220–890 C, and at the room temperature after cooling of the sample. (d) Hysteresis loops at room temperature measured after cooling of the sample from the indicated temperature.
The initial hysteresis loop (before any heating) and the hysteresis loops measured at the room temperature after cooling of the heated sample for Co1/4 microwire are collected in Fig. 5(d), where, as observed, three of them exhibit very similar behavior. It indicates that they were not heated enough to reach the temis interperature of the structural phase transition. preted to be the temperature of the crystallization and recrystallization of the amorphous and polycrystalline phases. The hysand at room temperature after teresis loops measured at 890 cooling show the irreversible change of the sample properties. Since the ferromagnetic behavior remains [there are the hysteresis loops in Fig. 5(c)], we assume the achievement of the structure transition. Moreover, at the temperatures higher than the glass coating of the amorphous microwire starts to 600
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ACKNOWLEDGMENT This work was supported in part by the Russian Foundation for Basic Research under Project 11-02-00906-a and by the MES under Project 16.513.11.3073. Author V. Rodionova would like to thank the U.M.N.I.K. project. REFERENCES
Fig. 6. (color online) Dependence of the hysteresis loop shift on temperature for Co2/4 and Co2/11 biphase microwires.
melt and the observed results [see Fig. 5(c) and (d)] demonstrate the single magnetic phase even after the sample cooling. Finally, we should comment on the observed shift of the hysteresis loops. This could be explained considering the magnetostatic interaction between phases in the composite microwire at increases as the temroom temperature [4]. The shift value is shown in Fig. 6 perature increases. The dependence and can be explained by rising of the transition Au–Co layer during the microwire heating (by means of the mutual diffusion of Au and Co atoms). This layer possesses the large magnetic . anisotropy with constant IV. CONCLUSION The increase of temperature results in changing the magnetic behavior of the biphase soft/hard microwires: 1) in the tem, the hysteresis loops show biphase perature range 25–350 behavior ascribed to the two magnetic phases of the microwires, and the biphase behavior is reversible after microwire cooling; , a single magnetic 2) in the temperature range 350–500 phase behavior is observed, which allows us to assume a Curie (for the amorphous phase), temperature around and the biphase behavior is still reversible after the sample cooling; and iii) in the temperature range 500–900 , in both cases at the measured temperature and after cooling it was obtained a single phase behavior which is irreversible. The reversibility of the magnetic properties till Curie temperature for biphase microwire is thought to be relevant for microwire sensors and actuators applications. The increasing of the temperature leads to growing a diffusive intermediate layer, which results in increasing of the hysteresis loop shift.
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