Patent Publication Number: US-9905345-B2

Title: Magnet electroplating

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority under 35 U.S.C § 119(e) to U.S. Provisional Application No. 62/221,271, entitled “MAGNETIC ELECTROPLATING,” filed on Sep. 21, 2015, which is incorporated by reference herein in its entirety. 
    
    
     FIELD 
     The described embodiments relate generally to coatings for magnets and methods for forming the same. More particularly, the present embodiments relate to coatings that reduce or prevent the release of nickel or cobalt from an exterior surface of the coating. 
     BACKGROUND 
     Rare earth magnets are strong magnets, and are therefore used extensively in many products. Some of the characteristics of rare earth magnets, however, include a propensity for corrosion and brittleness. Therefore, many manufacturers cover surfaces of rare earth magnets with protective coatings. The protective coatings often include nickel due to nickel&#39;s high corrosion resistance. Typically, the magnets are encased within layers of nickel and copper. 
     It has been observed, however, that these nickel-containing coatings can release certain amounts of nickel when exposed to moisture. This can be a problem in consumer products that have magnets that can come into contact with a person&#39;s skin since nickel can elicit allergic skin reactions in some people. Thus, some of these protective coatings should be avoided when coating magnets used as fastening elements in wearable products such as bracelets, necklaces, watches, brooches and other jewelry, where a user&#39;s skin may be in contact with the fastening elements for prolonged time periods. What are needed therefore are coatings for magnets that reduce or prevent the release of nickel or other skin irritants to levels appropriate for wearable products. 
     SUMMARY 
     This paper describes various embodiments that relate to coatings for magnets. In particular embodiments, the coatings have multiple layers of material that cooperate to provide a durable and corrosion resistant coating that reduces or prevents the release of nickel or other potentially skin irritating agents from the coating or underlying magnet. 
     According to one embodiment, a multilayered coating for a magnet is described. The multilayered coating includes a first layer disposed on the magnet. A portion of the first layer is diffused within intergranular cracks of the magnet. The multilayered coating also includes a second layer disposed on the first layer. The second layer is characterized as having a first ductility. The multilayered coating further includes a third layer disposed on the second layer. The third layer is characterized as having a second ductility less than the first ductility. The multilayered coating additionally includes a fourth layer disposed on the third layer. The fourth layer has an exposed surface corresponding to an exterior surface of the multilayered coating. The fourth layer is substantially free of cobalt and nickel. 
     According to a further embodiment, a method of forming a multilayered coating on a magnet is described. The method includes plating a first layer on a surface of the magnet such that a portion of the first layer diffuses within intergranular cracks of the magnet. The method also includes plating a second layer on the first layer. The second layer is characterized as having a first ductility. The method further includes plating a third layer on the second layer. The third layer is characterized as having a second ductility less than the first ductility. The method additionally includes depositing a fourth layer on the third layer such that the fourth layer has an exposed surface corresponding to an exterior surface of the multilayered coating. The fourth layer is substantially free of nickel and cobalt. 
     According to an additional embodiment, a multilayered coating for a magnet is described. The multilayered coating includes a first layer disposed on a surface of the magnet. The first layer includes copper. The multilayered coating also includes a second layer disposed on the first layer. The second layer includes tin and copper. The multilayered coating further includes a third layer disposed on the second layer. The third layer corresponds to an outer layer of the multilayered coating. The third layer includes at least one of gold, rhodium, ruthenium or palladium. 
     These and other embodiments will be described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements. 
         FIG. 1  shows a photograph and a multilayered stack up for a convention magnet coating. 
         FIG. 2  shows a photograph and multilayered stack up for a magnet coating that does not include nickel. 
         FIG. 3  shows a multilayered stack up for a magnet coating that includes an initial layer of nickel. 
         FIGS. 4A and 4B  show scanning electron microscope (SEM) images of a cross-section of a magnet structure having the multilayered stack up of  FIG. 3 . 
         FIG. 5  shows a multilayered stack up for a magnet coating that includes two layers of nickel. 
         FIGS. 6A-6F  show SEM images of cross-sections of samples having the multilayered stack up of  FIG. 5  after a series of different scratch tests. 
         FIG. 7  shows a generic multilayered stack up for a magnet coating that is resistant to nickel and/or cobalt release. 
         FIG. 8  shows a number of magnetic structures having multilayered coatings in accordance with some embodiments. 
         FIG. 9  shows a number of magnetic structures having multilayered coatings with non-metal exterior layers in accordance with some embodiments. 
         FIG. 10  shows a number of magnetic structures having multilayered coatings with integrated adhesion-promoting layer and ductile layer in accordance with some embodiments. 
         FIG. 11  shows a cross-section view of magnet assembly that includes a multilayered coated magnet. 
         FIG. 12  shows a flowchart indicating a process for forming a multilayered coating on a magnet in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     The following disclosure relates to magnets and coatings for magnets, such as rare earth magnets. The coatings are designed to reduce or prevent the release of nickel and/or cobalt from the coating and/or magnet, thereby preventing allergic skin reactions to nickel/cobalt if the magnets come in contact with skin. The coated magnets described herein are useful in the manufacture of consumer products that come into contact with a person&#39;s skin, such as wearable electronic devices like watchbands. 
     In some embodiments the coatings are multilayered and have different layers of material that serve different functions. In some embodiments, the coatings are free from nickel and/or cobalt. In other embodiments, the coatings include one or more underlying layers of nickel that are covered by one or more protective layers that prevent nickel from leaching from the coating. In some embodiments, any nickel used is in a state that is non-conducive to release from the coating. The multiple layers can be deposited using any suitable technique, including different electroplating methods such as electroless plating. 
     The coatings can be tested for their ability to prevent exposure of the underlying magnet, thereby preventing corrosion of the magnet and an associated release of cobalt from the magnet. The coatings can be also tested for their robustness and ability to resist scratching such that any underlying nickel-containing layers are not exposed or minimally exposed. The coatings can be also tested for their corrosion and nickel/cobalt release resistance when exposed to moisture, such as by salt spray testing that can simulate sweaty conditions from a user&#39;s skin. 
     The magnetic coatings described herein are well suited for implementation with consumer electronic products. For example, the magnetic coatings can be used in the design and manufacture of portable electronic devices such as mobile phones, wearable electronic devices (e.g., smart watches), media players, tablet and laptop computers, electronic device accessories (e.g., covers and cases)—as well as larger electronic devices such as desktop and workstation computers, such as those manufactured by Apple Inc., based in Cupertino, Calif. 
     These and other embodiments are discussed below with reference to  FIGS. 1-12 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
     Rare earth magnets, such as neodymium magnets, can generate strong magnetic fields and are therefore used in many consumer products such as computer hard drives, motors, speakers and toys. Often, the rare earth magnets are coated with a protective coating to protect the rare earth magnet from exposure to moisture, which can quickly corrode rare earth magnets. One of the most common coatings is nickel since nickel has high corrosion resistance and can be plated onto rare earth magnets. 
     It has been found, however, that standard nickel containing coatings may not provide adequate protection against corrosion under certain conditions. This is illustrated by  FIG. 1 , which shows a photograph  100  of magnets  102  coated with a standard nickel containing multilayered stack up  104  after a salt mist test (also referred to as a salt spray test). Stack up  104 , which represents the layers of the multilayered coating on neodymium/iron/boron magnet  106 , includes first nickel layer  108 , copper layer  110 , and second nickel layer  112 . Typically, each layer of stack up  104  is successively plated onto each other. In the sample shown in  FIG. 1 , first nickel layer  108  has a thickness of about 2 micrometers, copper layer  110  has a thickness of about 3 micrometers, and second nickel layer  112  has a thickness of about 2 micrometers. 
     Photograph  100  shows significant evidence of corrosion after magnets  102  were sprayed with salt water and allowed to stand in the salt water for eight hours. In particular, dark areas  114  around magnets  102  correspond to corrosion products related to oxidized magnet material. This testing indicates that a standard nickel and copper stack up  104  may not be robust enough to protect neodymium type magnet  106  from corrosion under moisture conditions that a wearable product may be exposed. For example, a watch band will likely be exposed to sweat from a person&#39;s wrist for prolong time periods. 
     Furthermore, some people experience contact dermatitis when their skin comes in contact with nickel. Thus, magnets in products designed for direct and prolonged contact with skin, such as jewelry and watches, should not release nickel in sufficient amounts to cause allergic reactions. To quantify acceptable amounts of nickel, many manufacturers use the European Union&#39;s EN 1811 and EN 12472 guidelines and testing methods for quantifying acceptable levels of nickel release from products in order to ensure proper consumer protection. EN 1811 sets forth guidelines and procedures as to acceptable amounts of nickel release per area, per time period for a post assembly product. For articles that are intended to come in come into direct and prolonged contact with skin, some manufactures aim for compliance with No. 27 Annex XVII of Regulation (EC) No 1907/2006 of the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) regulations. EN 12472 sets forth guidelines and procedures as to an abrasion tests that simulate two years of normal use for items with nickel below an outer surface layer. 
     An additional consideration relates to the release of cobalt. Cobalt, which is generally used in rare earth magnet compositions, can also elicit allergic skin reactions. Thus, any breach of a coating on a rare earth magnet, such as evidenced by corrosion of the magnet, could also result in release of cobalt, which could also result in skin reactions. 
     One way of solving the nickel leaching problems is by avoiding the use of nickel as a magnet coating. Thus, according to some embodiments, rare earth magnets are coated with a polymer layer, such as an epoxy layer. In one embodiment, the epoxy layer was applied to a thickness of about 6 to 8 micrometers. In another embodiment, the epoxy layer was applied to a thickness of more than about 30 micrometers. Although these epoxy coatings eliminate the nickel release problem, it has been found that epoxy by itself generally does not provide adequate coverage and corrosion protection of the underlying magnet. For example, some epoxy coated magnet samples have shown evidence of magnet corrosion after an eight hour salt mist test, such as described above with reference to  FIG. 1 . Once again, this corrosion not only indicates inadequate protection of the rare earth magnet, but also an indication that cobalt is likely also released from the rare earth magnet. 
     In some embodiments, the magnets are coated with a multilayered stack up of metals other than nickel.  FIG. 2  shows one such stack up  200  covering magnet  202 . Stack up  200  includes copper layer  204  and tin and copper layer  206 . In a particular embodiment, copper layer  204  has a thickness of about 7 micrometers, and tin and copper layer  206  has a thickness of about 8 micrometers. Since stack up  200  does not include nickel, there is no nickel release problem. However, a number of samples having the composition of stack up  200  failed the eight hour salt mist test, as described above. In addition, a number of these samples showed evidence of delamination or blistering of stack up  200  after a thermal shock test. Photograph  208  shows magnet  210  having a coating of stack up  200  after a thermal shock test where magnet  210  was heated to 250 degrees Celsius, followed by immersion in water of room temperature. As shown, the thermal shock testing resulting in blister  212  being formed, which is likely due to expansion of air or solution trapped between stack up  200  and magnet  202  during an annealing process. Blister  212  corresponds to a portion of stack up  200  that is no longer adhered to magnet  202 , and will eventually cause peeling of stack up  200  away from magnet  202 . 
     Thus, it is a goal of embodiments presented herein to provide a coating that achieves good durability and corrosion protection of an underlying magnet (e.g., as evidenced by salt mist and/or thermal shock testing) and that also releases nickel below predetermined amounts (e.g., as dictated by EN 1811 and EN 12472 testing methods). 
     Improved structural integrity and corrosion resistance was found when nickel is used as an initial layer within a coating stack up. For example,  FIG. 3  illustrates magnet structure  300  with a coating made of stack up  302  on neodymium/iron/boron magnet  304 . Stack up  302  includes nickel layer  306 , copper layer  308  and tin and copper layer  310 . In particular embodiments, nickel layer  306  has a thickness of about 5 micrometers, copper layer  308  has a thickness of about 7 micrometers, and tin and copper layer  310  has a thickness of about 8 micrometers. Samples of magnet structure  300  were generally found to pass an eight hour salt spray test (i.e., showed or very little evidence of corrosion), pass a thermal shock test (i.e., little or no blistering after heating to 250 degrees C. then immersion in water at room temperature), and pass a nickel release test as dictated by EN 1811 and EN 12472 testing methods. 
     It should be noted that the thickness of tin and copper layer  310  is thicker than standard multilayered coating. For example, some multilayered coatings use a top layer having a thickness of about 2 micrometers. Having a thicker top layer (e.g., tin and copper layer  310 ) can ensure that nickel from nickel layer  306  does not get released from stack up  302 . Thus, in some embodiments, tin and copper layer  310  has a thickness greater than about 2 micrometers, in some embodiments greater than about 5 micrometers, and in some embodiments about 8 micrometers or greater. 
     It was found that nickel from nickel layer  306  diffuses into boundaries of the neodymium/iron/boron magnet  304 . To illustrate,  FIGS. 4A and 4B  show scanning electron microscope (SEM) images of a cross-section of a boundary portion of a sample of magnet structure  300  with stack up  302  positioned over magnet  304 .  FIG. 4A  shows a 2,500× magnification and  FIG. 4B  shows a 5,000× magnification. As shown, magnet  304  includes a number of intergranular cracks  400  that are inherently formed during the manufacturing of many rare earth magnets. It has been found that some standard etching processes can exacerbate and widen intergranular cracks  400 , and therefore can be avoided. The presence of intergranular cracks  400  can cause breaching of a coating if the coating is not well adhered to magnet  304  or if stresses are not sufficiently attenuated in the coating. In particular, intergranular cracks  400  can shift, thereby causing a coating that is not well adhered to magnet  304  to blister and eventually peel away from magnet  304 . 
     The images of  FIGS. 4A and 4B , however, show that nickel  402  diffuses into a surface boundary of magnet  304  and within intergranular cracks  400 . This is confirmed by spectrum analysis at different points within intergranular cracks  400  near stack up  302 . This infusion of nickel increases surface contact with magnet, thereby improving the adhesion of stack up  302  to magnet  304 . Thus, nickel layer  306  can be referred to as an adhesion-promoting layer. The infusion of nickel  402  can also help maintain the microstructure stability of magnet  304 . 
     Nickel layer  306  can be applied onto magnet  304  using any suitable technique. In some embodiments, nickel layer  306  is plated onto magnet  304  using standard plating techniques. In other embodiments, nickel layer  306  is electrolessly plated onto magnet  304 . Electroless plating can provide a nickel layer  306  that is highly conformal and uniform in thickness. In addition, electroless plating can provide a very thin nickel layer  306 , which may be beneficial in some cases. 
     In some cases, it has been found that plating defects in tin and copper layer  310  can compromise the integrity of tin and copper layer  310 . In general, tin and copper layer  310  is relatively difficult to corrode due to the formation of layer of tin oxide (SnOx) passivation. However, when there is a pathway (e.g., via a crack, a deep scratch or a plating defect within tin and copper layer  310 ), sweat can reach and quickly corrode copper layer  308 . When copper layer  308  becomes corroded, tin and copper layer  310  can delaminate from stack up  302  since the integrity of copper layer  308  is compromised, eventually causing corrosion of magnet  304 . It should be noted, however, that a well-plated tin and copper layer  310  can act as a sacrificial anode and limit corrosion of copper  308  and nickel  306  layers if, for example, a well-plated tin and copper layer  310  is scratched or otherwise damaged. 
     In some embodiments, an additional layer is added to the stack up  302 . For example,  FIG. 5  shows magnet structure  500 , which includes a multilayered coating comprising stack up  502  on magnet  504 . As shown, stack up  502  includes first nickel layer  506 , copper layer  508 , second nickel layer  510  and tin and copper layer  512 . As with the magnet structure  300  described above, first nickel layer  506  can be referred to as an adhesion-promoting layer since it functions to promote adhesion between stack up  502  and magnet  504 , as well as maintain a structural integrity of magnet  504 . In some embodiments, first nickel layer  506  has a thickness of about 6 micrometers. First nickel layer  506  can be deposited using a standard plating or an electroless plating technique. In some embodiments, good adhesion is found when first nickel layer  506  is deposited as a semi-bright nickel layer, which is substantially free of sulfur (e.g., less than about 0.005% sulfur by weight) to provide high corrosion resistance to first nickel layer  506 . 
     Copper layer  508  is positioned over first nickel layer  506 , and functions by deforming with shifting of the microstructure of magnet  504  caused by the presence of intergranular cracks described above. That is, copper has relatively high ductility and therefore can deform under tensile stresses due to the presence of intergranular cracks within magnet  504 . This can help to reduce stress buildup that could cause breaching of stack up  502  and exposure of magnet  504 , which can ultimately cause corrosion of magnet  504  (referred to as stress-induced corrosion). In this way, copper layer can be referred to as a ductile layer. In some embodiments, copper layer  508  has a thickness of about 5 micrometers. 
     Second nickel layer  510  is added to further attenuate stress-induced corrosion caused by cracks formed within magnet  504 . That is, second nickel layer  510  can add an additional ductile layer to stack up  502 , thereby reducing stress buildup within stack up  502 . Thus, second nickel layer  510  can be referred to as a second ductile layer. In addition, nickel is slightly more corrosion resistant than copper. Therefore, having second nickel layer  510  positioned below tin and copper layer  512  and above copper layer  508  can prevent moisture from reaching copper layer  512  if tin and copper layer  512  is compromised due to plating defects or damage (e.g., by scratching). As described above, copper layer  512  can quickly corrode when exposed to moisture, which can lead to delamination of tin and copper layer  512  and eventually lead to exposure of magnet  504 . In this way, second nickel layer  510  can act as a safeguard layer. In some embodiments, good performance is found when second nickel layer  510  is deposited as a semi-bright nickel layer to provide good corrosion resistance, as described above. Second nickel layer  510  can be relatively thin compared to each of tin and copper layer  512  and copper layer  508 . In some embodiments, second nickel layer  510  is thinner than first nickel layer  506 . In some embodiments, second nickel layer  510  has a thickness of about 3 micrometers. 
     Tin and copper layer  512  is positioned on second nickel layer  510  and corresponds to an exterior layer of stack up  502 . Tin and copper layer  512  includes an alloy of tin and copper. In some embodiments, the weight percent of copper ranges from about 15% to about 45%. Tin and copper layer  512  functions as a top protective layer that is scratch resistant—that is resistant to removal or peeling away by scratching or gouging forces. Tin and copper layer  512  functions as a replacement for an exterior nickel layer used in conventional magnet coatings. Tin and copper layer  512  has good corrosion resistance and does not release nickel that can cause skin reactions. Note that tin and copper layer  512  is generally less corrosion resistant than second nickel layer  510  and copper layer  506 . Thus, if tin and copper layer  512  is damaged (e.g., by scratch), tin and copper layer  512  could corrode instead of second nickel layer  510  or copper layer  506  (which could cause delamination and eventual exposure of magnet  504  as described above). Thus, tin and copper layer  512  can be said to act as a sacrificial anode in stack up  502 . 
     As described above, in some instances tin and copper layer  512  can have defects related to the plating process, which can quickly dissolve the copper and cause tin and copper layer  512  to peel. However, second nickel layer  510  can prevent further corrosion within stack up  500 , thereby protecting underlying copper layer  508 , first nickel layer  506  and magnet  504 . Tin and copper layer  512  is nominally relatively thick compared to conventional stack ups. In some embodiments, tin and copper layer  512  is nominally thicker than each of second nickel layer  510 , copper layer  508  and first nickel layer  506 . In some embodiments, tin and copper layer  512  has a thickness of greater than about 2 micrometers. In a particular embodiment, tin and copper layer  512  has a thickness of about 8 micrometers. 
     Tin and copper layer  512  can be relatively brittle after the plating process. Therefore, in some embodiments, an annealing process is used to strengthen tin and copper layer  512 . The annealing process can involve heating magnetic structure  500 , including magnet  504 . In particular embodiments, a slow profile annealing process was used where the temperature was raised slowly over a period of time. For example, magnet structure  500  can be heated to about 50 degrees Celsius for about 30 minutes, then about 100 degrees Celsius fro about 30 minutes, then 150 degrees Celsius for about 30 minutes, then about 200 degrees Celsius for about 1 hour, then about 220 degrees Celsius for about 1 hour. In some embodiments, an additional layer of material, such as a very thin layer of gold (e.g., about 2 micrometers thick), is deposited over tin and copper layer  512  to further prevent breaching of tin and copper layer  512 . 
     Samples of magnet structure  500  were found to consistently pass corrosion testing by salt mist testing over seven days, as well as nickel release testing as dictated by EN 1811 and EN 12472 standards. Furthermore, these samples were also found to be durable, as tested using scratch testing. To illustrate,  FIGS. 6A-6F  show SEM images of cross-sections of samples having magnet structure  500  after a series of different scratch tests. 
       FIG. 6A  shows a sample after undergoing a 1 Newton scratch test, where a tool was used to scratch the sample using 1 Newton of force. As shown, only a very small indention  602  resulted, which is barely visible to a human eye.  FIGS. 6B and 6C  show samples after undergoing a 5 Newton scratch test, resulting in indentations  602  and  604 , respectively, that may be visible but do not reach second nickel layer  510 .  FIGS. 6D and 6E  show samples after undergoing a 10 Newton and 15 Newton scratch test, respectively, resulting in indentations  606  and  608  that still do not reach second nickel layer  510 .  FIG. 6F  shows a sample after undergoing a 20 Newton scratch test, resulting in indentation  610  that does reach second nickel layer  510  to some extent. Thus, magnet structure  500  can undergo scratch testing up to at least 15 Newtons of force without breaching tin and copper layer  512  to an extent that second nickel layer  510  is reached. 
     In some embodiments, other materials are used other than nickel an adhesion-promoting layer and/or a second ductile layer. Similarly, other materials other than copper can be used as a ductile layer, and other materials other than tin and copper alloy can be used as an external layer.  FIG. 7  shows a generic magnet structure  700  having stack up  702  as a coating for magnet  704 . Stack up  702  includes adhesion-promoting layer  706 , first ductile layer  708 , second ductile layer  710  and exterior layer  712 , each of which can serve different purposes to protect and prevent nickel and cobalt release and prevent corrosion of magnet  704 . Magnet  704  can be any suitable type of rare earth magnet. In some embodiments, magnet  704  is a neodymium/iron/boron magnet. 
     Adhesion-promoting layer  706  can be made of any suitable material that provides good adhesion to magnet  704  and can provide good structural integrity to magnet  704 . As described above, nickel can infuse within intergranular cracks of magnet  704 , thereby creating good adhesive contact with and providing structural stability for magnet  704 . Zinc has also been found to diffuse within intergranular cracks of magnet  704 , and therefore can also be a good candidate for adhesion-promoting layer  706 . It should be noted that in some instances a zinc adhesion-promoting layer can cause galvanic corrosion between certain metal layers, and therefore care should be taken in choosing surrounding layers of metal. In some embodiments, palladium has been found to be a good adhesion-promoting layer  706 . In some embodiments, adhesion-promoting layer  706  includes one or more of nickel, electrolessly deposited nickel, zinc, electrolessly deposited zinc, palladium, electrolessly deposited palladium, or alloys thereof (e.g., palladium and nickel alloy or palladium and cobalt alloy). In some embodiments, adhesion-promoting layer  706  includes one or more sub-layers. For example, the sub-layers can include one or more zinc sub-layer, nickel sub-layer and/or palladium sub-layer, or alloys thereof. 
     The thickness of adhesion-promoting layer  706  can vary depending on the type of material. In a particular embodiment, adhesion-promoting layer  706  is made of nickel and has a thickness greater than about 2 micrometers. In addition, the method of deposition can vary. For example, nickel and/or zinc can be deposited using electroless plating methods in order to form a very conformal adhesion-promoting layer  706 . In some embodiments, adhesion-promoting layer  706  includes copper. In some embodiments, the copper is plated using an alkaline plating solution (instead of typical acid plating solutions) in order to from a thin conformal copper adhesion-promoting layer  706 . One advantage of using a non-nickel adhesion-promoting layer  706  is that there is no chance for nickel release from adhesion-promoting layer  706  in case there is a breach of stack up  702  down to adhesion-promoting layer  706 . 
     First ductile layer  708  can be made of any suitable material that is sufficiently ductile to relieve tensile stresses encountered by stack up  702 . The stress can be due to the presence of intergranular cracks within magnet  504 , or due to external forces place on stack up  702  during normal use. The material of first ductile layer  708  can depend, in part, on the material of adhesion-promoting layer  706 . For example, first ductile layer  708  should adhere well to adhesion-promoting layer  706 . In some embodiments, first ductile layer  708  includes copper due to copper&#39;s high ductility. In particular embodiments, first ductile layer  708  includes copper and has a thickness of greater than about 2 micrometers. In some embodiments, first ductile layer  708  includes zinc. In some embodiments wherein adhesion-promoting layer  706  includes copper plated using an alkaline plating solution, first ductile layer  708  that includes copper plated using an acid plating solution was used to provide good adhesion. The thickness of first ductile layer  708  can vary. In some embodiments, first ductile layer  708  is relatively thick (e.g., thicker than adhesion-promoting layer  706 ) in order to impart good ductility to stack up  702 . 
     It should be noted that in some embodiments adhesion-promoting layer  706  and first ductile layer  708  are combined as one layer. That is, a single layer made of a material having good adhesive properties with magnet  704  and good ductility can be used. In some embodiments, the single layer is a copper layer. 
     Second ductile layer  710  can be made of any suitable material sufficient to protect exposure of underlying magnet  704  in case exterior layer  712  is breached. In some embodiments, second ductile layer  710  includes one or more of zinc, nickel, and palladium, or alloys thereof. In some embodiments, the material of second ductile layer  710  is less ductile than the material of first ductile layer  708  (e.g., a nickel second ductile layer  710  can be less ductile than a copper first ductile layer  708 ). In particular embodiments, second ductile layer  710  includes nickel and has a thickness of less than about 1 micrometer. In some embodiments, second ductile layer  710  includes one or more sub-layers. For example, the sub-layers can include one or more of nickel, electrolessly deposited nickel, zinc, electrolessly deposited zinc, palladium, electrolessly deposited palladium, or alloys thereof (e.g., palladium and nickel alloy or palladium and cobalt alloy). The thickness of second ductile layer  710  can vary. In some embodiments, second ductile layer  710  is preferably relatively thin (e.g., thinner than exterior layer  712 , first ductile layer  708  and/or adhesion-promoting layer  706 ). One advantage of using a non-nickel material is prevention of nickel release from second ductile layer  710  in case there is a breach in exterior layer  712 . 
     Exterior layer  712  can be made of any suitable material sufficient to provide good protection to stack up  702  and magnet  704  when subjected to forces such as scratching. In addition, exterior layer  712  should be durable enough to prevent release of nickel and/or cobalt from stack up  702 . In some embodiments, the thickness of exterior layer  712  should be greater than about 2 micrometers (e.g., 7 or 8 micrometers, or more). As described above tin and copper alloy is free from nickel and can provide good protection. Other candidates can include one or more layers metals such as aluminum and manganese alloy, gold and palladium alloy, palladium, rhodium, ruthenium, rhodium and ruthenium alloy, gold, zinc, and nickel. In some embodiments, exterior layer  712  includes a non-metal material such as a polymer. Some polymer candidates include epoxy and poly(p-xylylene) polymer (Parylene). In some embodiments, exterior layer  712  includes multiple metal and non-metal sub-layers. In some embodiments, exterior layer  712  includes electrolessly plated nickel since the electrolessly plated nickel can create a conformal layer that is resistant to nickel release to a certain extent. In some embodiments, the electroless nickel has a high concentration of phosphorus (high-P nickel) to create a more amorphous microstructure. 
       FIG. 8  shows a number of magnetic structures having multilayered coatings in accordance with some embodiments. Magnetic structure  800  includes stack up  802  that serves as a protective coating for magnet  801 . Stack up  802  includes nickel layer  803 , copper layer  804 , palladium layer  805  and tin and copper layer  806 . Magnetic structure  810  includes magnet  811  with stack up  812 , which includes zinc layer  813 , copper layer  814 , palladium layer  815  and tin and copper layer  816 . Magnetic structure  820  includes magnet  821  with stack up  822 , which includes zinc layer  823 , copper layer  824 , palladium layer  825  and electrolessly plated nickel layer  826 . 
     Magnetic structure  830  includes magnet  831  with stack up  832 , which includes zinc layer  833 , copper layer  834  and high phosphorus electrolessly plated nickel layer  835 . Note that high-P electrolessly plated nickel layer  835  may be less susceptible to breaching can therefore may not need a second ductile layer (e.g., second nickel layer) beneath it. Magnetic structure  840  includes magnet  841  with stack up  842 , which includes zinc layer  843  and aluminum and manganese layer  844 . Aluminum and manganese layer  844  includes an alloy of aluminum and manganese, which can serve as an exterior layer and a ductile layer that does not need a second ductile layer (e.g., nickel layer) beneath it. 
     Magnetic structure  850  includes magnet  851  with stack up  852 , which includes zinc layer  853 , copper layer  854 , palladium layer  855  and tin and copper layer  856 . Magnetic structure  860  includes magnet  861  with stack up  862 , which includes nickel layer  863 , palladium and nickel layer  864 , copper layer  865 , palladium layer  866  and tin and copper layer  867 . Note that nickel layer  863  and palladium and nickel layer  864  can cooperate to serve as adhesion-promoting layers (i.e., sub-layers of an adhesion-promoting layer). 
       FIG. 9  shows a number of magnetic structures having multilayered coatings with a non-metal exterior layer, accordance with some embodiments. Magnet structure  900  includes magnet  901  with stack up  902 , which includes a layer of Parylene (poly(p-xylylene))  903 . Parylene is very corrosion resistant and can have forms that are also scratch resistant. In some embodiments, the thickness of Parylene layer  903  ranges from about 5 to 10 micrometers. Magnet structure  910  includes magnet  911  with stack up  912 , which includes nickel layer  913 , copper layer  914 , nickel layer  915  and Parylene (poly(p-xylylene)) layer  916 . In some embodiments, the thickness of Parylene layer  916  ranges from about 5 to 10 micrometers. Magnet structure  920  includes magnet  921  with stack up  922 , which includes nickel layer  923 , copper layer  924 , nickel layer  925  and epoxy layer  926 . One of the advantages of using some polymers such as epoxy is that these materials have high abrasion resistance, and therefore protect underlying layers from damage from scratching forces. In some embodiments, the thickness of epoxy layer  926  ranges from about 5 to 8 micrometers. 
       FIG. 10  shows a number of magnetic structures having multilayered coatings where the adhesion-promoting layer and the ductile layer are the same layer, accordance with some embodiments. Magnet structure  1000  includes magnet  1001  with stack up  1002 , which includes copper layer  1003 , tin and copper layer  1004  and gold layer  1005 . As shown, copper layer  1003  is directly deposited onto magnet  1001  without a separate adhesion-promoting layer (e.g., nickel, zinc or palladium). In some embodiments, copper layer  1003  is deposited using an acidic plating process, while in other embodiments copper layer  1003  is deposited using an alkaline plating process. In some embodiments, copper layer  1003  includes sub-layers of copper. For example, an alkaline plating process can be used to deposit a first sub-layer of copper and an acidic plating process can be used to deposit a second sub-layer of copper. Gold layer  1005  can be used to prevent tin and copper layer  1004  from breaching, even if tin and copper layer  1004  is annealed. Gold layer  1005  can be very tin, e.g., about 2 micrometers. 
     Magnet structure  1010  includes magnet  1011  with stack up  1012 , which includes copper layer  1013 , tin and copper layer  1014  and rhodium layer  1015 . Rhodium layer  1015 , like gold layer  1005 , can prevent tin and copper layer  1014  from forming cracks. Magnet structure  1020  includes magnet  1021  with stack up  1022 , which includes copper layer  1023 , tin and copper layer  1024  and ruthenium layer  1025  (to prevent tin and copper layer  1024  from breaching). Magnet structure  1030  includes magnet  1031  with stack up  1032 , which includes copper layer  1033 , tin and copper layer  1034  and ruthenium and rhodium layer  1035  (to prevent tin and copper layer  1034  from breaching). Ruthenium and rhodium layer  1035  is an available plating process that with about 25% by weight ruthenium can be a cost savings. Magnet structure  1040  includes magnet  1041  with stack up  1042 , which includes copper layer  1043 , tin and copper layer  1044  and ruthenium and rhodium layer  1045  (to prevent tin and copper layer  1044  from breaching). Magnet structure  1050  includes magnet  1051  with stack up  1052 , which includes copper layer  1053 , tin and copper layer  1054  and gold and palladium alloy layer  1055  (to prevent tin and copper layer  1054  from breaching). Note that gold layer  1005 , rhodium layer  1015 , ruthenium layer  1025 , ruthenium and rhodium layer  1035 , palladium layer  1045  and gold and palladium alloy layer  1055  can each be very thin (e.g., about 2 micrometers). 
     It should be noted that the embodiments described above with reference to  FIGS. 8, 9 and 10  are exemplary and are not meant to limit other possible combinations. For example, any suitable combination of magnetic structures  800 ,  810 ,  820 ,  830 ,  840 ,  850 ,  860 ,  900 ,  910 ,  920 ,  1000 ,  1010 ,  1020 ,  1030  and  1040  can be used. 
       FIG. 11  illustrates a cross-section view of magnet assembly  1100 , which includes a coated magnet in accordance with some embodiments. Magnet assembly  1100  includes housing  1102 , which includes cavity  1104  that is shaped and sized to accommodate coated magnet  1106 . Coated magnet  1106  includes magnet  1108  that can correspond to a rare earth magnet (e.g., neodymium/iron/boron magnet), which is coated with metal coating  1110 . In some embodiments, metal coating is a multilayered coating that includes multiple layers of metal, such as described above. In some embodiments, metal coating  1110  covers an entirety of magnet  1108  such that none of magnet  1108  is exposed. 
     A first portion  1112  of coated magnet  1106  is positioned within adhesive  1114 , which adheres and secures coated magnet  1106  to housing  1102 . In addition, adhesive  1114  can protect first portion  1112  of coated magnet  1106  from scratching or other forces that can damage the integrity of metal coating  1110 . A second portion  1116  of coated magnet  1106  that is not positioned within adhesive  1114  can be coated with polymer coating  1118 . Polymer coating  1118  can correspond to any suitable polymer material, such as some epoxy and/or Parylene (poly(p-xylylene)) polymers. Polymer coating  1118  protects second portion  1116  from scratching or other forces that can damage the integrity of metal coating  1110 . In this way, metal coating  1110  is covered by either adhesive  1114  or polymer coating  1118 , providing scratch protection in three dimensions. 
     Magnet  1108  is configured to produce a magnetic field at fastening surface  1120  so as to couple with a magnetically attractable element (e.g., another magnet or a ferrous material). In some embodiments, the magnetic field should be sufficiently strong to couple housing  1102  to a different portion of the housing, or another housing. For example, housing  1102  can be a housing for a band for a wearable electronic device, such as a smart watch. Coated magnet  1106 , or an array of coated magnets, could be used to couple portions of the band together around a person&#39;s wrist. By covering magnet  1108  with metal coating  1110 , adhesive  1114  and polymer coating  1118 , this prevents nickel and/or cobalt from coated magnet  1106  from reaching the person&#39;s skin. The thickness of metal coating  1110  and polymer coating  1118  should be sufficiently thick to prevent release of nickel and/or cobalt (or reduce the release of nickel and/or cobalt to predetermined acceptable levels), but be thin enough for the magnetic field at fastening surface  1120  to allow adequate fastening with the corresponding magnetically attractable material. 
       FIG. 12  illustrates flowchart  1200  that indicates a process for forming a multilayered coating on a magnet in accordance with some embodiments. At  1202 , an adhesion-promoting layer is plated on a surface of the magnet. The adhesion-promoting layer is configured to adhere well to a surface of the magnet so as to create a strong foundation for the multilayered coating. In some embodiments, portions of the adhesion-promoting layer infuse within intergranular cracks of the magnet, thereby creating good adhesive contact with the magnet and also supporting the microstructure of the magnetic material of the magnet. In some embodiments, nickel was found to provide preferred adhesion characteristics. In other embodiments, zinc or palladium were found to act as a good material for adhesion-promoting layer. In some embodiments, the adhesion-promoting layer includes alloys, such as alloys of two or more of nickel, zinc and palladium. In some embodiments, the adhesion-promoting layer includes sub-layers of metals. The adhesion-promoting layer can be plated on using standard plating techniques or electroless plating. 
     At  1204 , a ductile layer is deposited on the adhesion-promoting layer. The ductile layer can serve to relieve tensile stresses within the multilayered stack up. For example, intergranular cracks within the magnet can cause stresses to build up within the multilayered coating. The ductile layer should be made of a ductile material, such as copper, that can absorb these stresses and prevent breakage of the multilayered coating. The ductile layer can be thicker than the adhesion-promoting layer. In some embodiments, the ductile layer also serves as the adhesion-promoting layer. For example, in a particular embodiment, a single copper layer serves as both the adhesion-promoting layer and ductile layer. In another embodiment, a first layer of copper deposited using alkaline plating serves as an adhesion-promoting layer and a second layer of copper deposited using acidic plating serves as a ductile layer. 
     At  1206 , an optional second ductile layer is deposited on the ductile layer. The second ductile layer can serve to prevent damage to the multilayered coating in case a subsequently deposited exterior layer is breached or to further attenuate stresses. The second ductile layer is generally very thin and made of a corrosion resistant material. In some embodiments, the second ductile layer includes one or more of zinc, nickel, palladium, and alloys thereof. Note that in some embodiments a separate second ductile layer may not be necessary if the subsequently deposited exterior layer is sufficiently resistant to breaking, such as by annealing or reinforcement. 
     At  1208 , the exterior layer is deposited on the second ductile layer, if used, or on the ductile layer if the second ductile layer is not used. The exterior layer has an exterior surface that corresponds to the exterior surface of the multilayered coating. Thus, the exterior layer can serve as a last barrier to prevent release of nickel and/or cobalt from the multilayered material. In addition, the exterior layer can be subjected to abrasion from external scratching forces, thus should be scratch resistant. In some embodiments, the exterior layer is corrosion resistant so as to maintain structural integrity when exposed to moisture. In some embodiments, the exterior layer does not include nickel and/or cobalt. For example, a tin and copper alloy layer has been found to provide good protection. In some embodiments, the tin and copper alloy is annealed to increase the tensile strength of the tin and copper alloy layer. In some embodiments, the exterior layer includes a reinforcement sub-layer, such as a gold, rhodium, ruthenium, rhodium and ruthenium alloy, palladium, or gold and palladium alloy. In other embodiments, the exterior layer can be an electrolessly deposited layer of nickel that is resistant to nickel release. In some embodiments the electrolessly deposited nickel layer has a high phosphorus content (high-P EN). In some cases, the exterior layer also includes a polymer layer, such as a layer of epoxy or a poly(p-xylylene) polymer layer. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not target to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.